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Is there an acceleration limit? (self.askscience)

submitted ago by yourstand

There is a speed limit (speed of light) but is there a limit of acceleration?

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[–]jfpowellTheoretical Physics|Magnetic Resonance 898 points899 points ago*

Here is my go at a top level comment because there is an awful lot of unscientific noise in here.

There is no current compelling evidence (or consensus) to suggest that space-time is quantised. Current generally accepted theories such as GR, SR and QFT do not have any limit on acceleration.

All other answers involving Planck's constant are complete speculation.

[–][deleted] 372 points373 points ago

Does a photon accelerate to C, or does it come into existence at C? (Or is this just a nonsense question because I am an idiot?)

[–]jfpowellTheoretical Physics|Magnetic Resonance 446 points447 points ago

It's a good question. Massless particles like photons can only travel at c according to relativity. So they can't undergo acceleration, they come into the world fully formed and travel at c until they are destroyed in an interaction.

[–]Rhombinator 94 points95 points ago

Can photons change direction so long as their speed is still at c, or do they only move in one direction (I have forgotten a LOT of physics)?

[–]Ambirational 123 points124 points ago*

They follow the geodesic depending on the 'spacetime' through which they are travelling. (a straight line in flat space...a circle on a sphere...) until they interact with matter.

This is what validated Einstein's GR.

[–]Ph0X 68 points69 points ago

But what happens when a photon hits a reflective surface and bounces back?

[–]Van_Occupanther 207 points208 points ago*

Generally speaking, they aren't the same photons. One will be absorbed, promoting an electron to a higher energy level, then another one will be emitted a tiny amount of time later. (Going in the opposite direction).

EDIT: for additional science. As other folks have pointed out, the question of identity at such scales is a funny one. Really photons and the like are not "distinguishable". What you can say is that to a first approximation, an electron absorbed a photon then emitted another.

[–]Gaether 42 points43 points ago

So this allows photons to "change their direction" without incurring into acceleration, right? Because a change of direction means a change of velocity? Sorry if this is obvious, BTW

[–]Van_Occupanther 49 points50 points ago

Yes, velocity is a vector, so a change of direction is necessarily an acceleration. Hey, while it's nice to work things out it can be good to ask, and I'd not say it's necessarily obvious. Like, a car going around a roundabout constantly will show a constant speed on the speedometer, but the car is always accelerating. This is obviously getting offtopic now.

There are some other good comments in this thread about the warping of spacetime and how it affects photons. There, you need to extend the idea of what a straight line is; the result is that it moves along a kind of straight line in 4D but this might appear to be curved in 3D. I think this is rather going beyond the scope of your question (and beyond the limits of my knowledge).

[–]harry_hastur 9 points10 points ago

Like, a car going around a roundabout constantly will show a constant speed on the speedometer, but the car is always accelerating. This is obviously getting offtopic now.

Hi, would you mind going into detail about this comment? Thanks!

[–]Gaether 1 point2 points ago

Thank you for your well-thought answer, and I'm just more pleased by the fact that you add information not directly related to the question.

I'd like to ask, are there other ways in which this absorption and reemission fits into the general model (I mean equations and laws that describe it) or is it just a way to "bypass" acceleration? Actually I'm not sure if there can be a "bypass" in Physics, because every phenomenon has to answer to laws which in turn form a cohesive structure with the precision and perfection of mathematical equations. Maybe in the case of different interpretations and disconnected theories which have not been integrated yet, but I guess this matter of photons and their velocity must be a somewhat settled ground?

[–]mwengler 25 points26 points ago

This is not true. Consider reflection from a superconducting metal surface. The only way to promote an electron to a higher energy level in a superconductor is to break a pair. This does NOT happen on reflection. The superconducting "fluid" of electrons carries the currents required to turn the photon around, to reflect it.

This is some very nerdy stuff, but i happened to work on superconducting submillimeter wave receivers and looked at the quantum interaction A LOT in order to work out issues of the noise in these recievers. The point is, the photon is not absorbed and reemitted because it is never absorbed in the first place. (This applies to radio-frequency photons hitting superconducting surface where "radio frequency" here means significantly lower frequency (photon energy) than is required to break a pair of superconducting electrons (gap energy)). This is the super nerdy part put here in case someone who knows about superconductors and pair-breaking in to quasiparticles reads this and thinks I missed that.

[–]ajeprogThin Film Deposition|Applied Superconductivity 7 points8 points ago

So what IS the mechanism for reflection?

[–]mwengler 0 points1 point ago

Excellent question. We usually think about the superconductor as a boundary condition and the reflecting cavity as a "mode" and the photons as "stationary" standing waves, this is more linear algebra than physics. But indeed, there is physics and a moving photon in space that hits a superconducting wall and bounces off.

For sure the photon is accelerated. photons have momentum in the direction they are traveling, and that momentum is turned around by the reflection. Indeed, we know about "photon pressure" which has been verified experimentally: the photons hitting the superconductor (or any reflector) and bouncing off do apply a force to the superconductor (or any reflector).

so a = dp/dt (acceleration vector is the change in momentum vector divided by the amount of time it takes for that momentum to change). How much time did it take to change the momentum of the photon? I think this has been worked out by looking at photon group velocity and the details of interactions with hitting the reflector. For at least some intents and purposes, the reflector is like hitting in to a spring which compresses and then decompresses returning all the energy it picked up back to the thing that hit in to it. I might guess that the "reflection time" is something like inverse of the plasma frequency of the reflector, as the plasma frequency is sort of the resonant frequency of the spring the photon bounces off of. http://en.wikipedia.org/wiki/Plasma_frequency for more on that. I suspect that there is an upper limit to a plasma frequency made from electrons at temperatures less than that of the sun, for example, but for hotter denser collections of electrons or more basic charged particles, there may be no easily derived physical upper limit. I don't know.

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    [–]Van_Occupanther 11 points12 points ago

    Well, that's vastly out of the area of things I know. And I think it's a little beyond the scope of this question. Good answer though! Do you have a paper or something I might take a look at?

    [–]mwengler 0 points1 point ago

    Other comments i've made refer to radiation pressure on wikipedia to at least assure you it happens. Maybe there are references from there. This is tricky stuff and I did some reading to check myself before setting about contradicting anything here. Wave particle duality is wacky but the upshot is there are a lot of truths on both sides of the duality: that is everything you can think of a particle doing, a thing you mostly think of as a wave can probably also do.

    [–]theocarina 2 points3 points ago

    This is fantastically interesting. I'm a physics undergrad (senior), and I'm curious if you know of any papers that would discuss this particular phenomenon.

    [–]mwengler 0 points1 point ago

    start with radiation pressure on wikipedia. Also look for the momentum of a photon, also on wikipedia. If you get totally in to it, you want Yariv's book and/or Marcuse's book on quantum electronics. (There publication dates will show you how old I am and there may be better or at least more modern books since then). I am SO GLAD you find this fantastically interesting. I totally agree, I LOVED my physics education! Keep it up!

    [–]FuckYouImAPorygon 2 points3 points ago

    Perhaps unrelated, but how does one get a job working on superconducting submillimeter wave receivers? This is so beyond the scope of anything you hear about in day to day conversation.

    [–]mwengler 0 points1 point ago

    I was a graduate student in Physics at Caltech. Radioastronomy uses superconducting receivers, the ones I work on are called SIS (superconductor-insulator-superconductor) receivers. They are worked on in perhaps 15 university programs around the country.

    Hypres and TRW and Westinghouse and before them IBM and Bell Labs had programs using SIS devices (and other Josephon Junctions) for various other applications, if you prefer more applied stuff. Look up the program of the Applied Superconductivity Converence and look at the companies the papers come from (along with the titles of the papers) to get a fuller idea.

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      [–]McMonty 3 points4 points ago

      Isn't there a half wavelength phase shift that happens during external reflection? How is it that the absorption-reemission consistently causes a half wavelength phase shift? Also, if I remember correctly, internal reflection does not have a phase shift. Does this mean that the absorption-reemission mechanism is altered?

      [–]thebloodygrinch 0 points1 point ago

      This blows my mind. The fact that photons do this really screwed up the picture of thin films that I had in my head.

      [–]thosethatwere 2 points3 points ago

      This doesn't make sense to me. Electrons have discrete energy states in matter, which I understand in the context of colours, but that makes no sense in reflection. As some surfaces are, from what I can see, perfectly reflective in the visible spectrum, wouldn't that mean that the particles that make up say a mirror would have every visible-light-energy-amount state?

      [–]Van_Occupanther 6 points7 points ago

      Yes! :) individual atoms do have discrete energy levels, and the energy level is quantised, but when you get lots of them together (say 1023 metal atoms) then the electrons at the outer energy levels slightly dissociate from the atoms. They then actually occupy a continuous spectrum of energy levels which is why mirrors actually work :)

      [–]thosethatwere 4 points5 points ago

      Ah, thanks for this. Totally forgot about metals, was only thinking in simple crystal structures and getting confused!

      [–]SillyTralfamadorian 3 points4 points ago

      Then why do mirrors reflect light at certain angles? why doesn't the newly emitted light have an equal probability of going off in any direction?

      i always thought it was because it was the same photon traveling in a new direction

      [–]Van_Occupanther 1 point2 points ago

      Well... I guess a counterpoint to your question is, if it's the same photon why isn't it scattered in a random direction? That, to me, is independent of whether the photon is absorbed or not.

      In an actual attempt to answer your question, I'm going to cop out slightly, because I am not sure how to explain it in a semi-classical fashion. If you view light as a wave - the classical view - you can explain it using rules and geometry. However, once you bring in the QM view, I don't really know what is going on. The answer might actually be rather complicated (and I'm hoping that it's not something I've ever been taught, as it'd indicate a severe gap in my knowledge).

      This is, of course, /r/AskScience; why not submit? It's a decent question, this is far enough down the chain of replies that it might not be so visible, so I think you should ask it of our wonderful panellists :) I'll keep an eye out for it as I'd like to know myself (and see if it's something basic that I should know, I'll be kicking myself).

      [–]SillyTralfamadorian 0 points1 point ago

      Done

      we'll see what AskScience says

      [–]Goatsnarfer 1 point2 points ago

      So with a lens or a prism is light still absorbed and then re-emitted?

      [–]Van_Occupanther 3 points4 points ago

      Yes, repeatedly, as the light travels through the material.

      [–]btxtsf 2 points3 points ago

      The same with air then(?) i.e. the photos coming from my computer screen aren't the same photos hitting my retina, they have been absorbed & reemitted many times.

      [–]shaurz 1 point2 points ago

      That's interesting. I always assumed reflected photons were the "same" photons.

      [–]EqualPeaceComp 0 points1 point ago

      Does that mean that each new photon contains less Energy than the previously absorbed one?

      [–]T00LMaker 0 points1 point ago

      So, in Michelson Morley experiment, which involved mirrors, the (finite) time for this photon-electron interaction followed by excitation and liberation of new photon was all accounted for?

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        [–]redreds 9 points10 points ago

        Light has both wave and particle properties, and when considering different situations, you often have to treat it in one domain or the other. In the case of reflection, I think it is easier to understand what is going on when considering the photon to be a localized electromagnetic wave packet that is subjected to boundary conditions determined by Maxwell's equations in matter. I don't think it is of much meaning to discuss whether it is "the same photon" or not.

        [–]spazmodic- 4 points5 points ago

        I wanted to say something like this, but I couldn't quite put it as eloquently. To me, the idea of "same" photon is meaningless; maybe an inadequacy/assumption of the human mind or human language when it comes to quantum-scale phenomenon. We like to think of particles like tennis balls; while it works in some cases, there are many cases where it is not a good analogy. I guess the moral is that we should avoid using an analogy to describe (or predict) things on a quantum scale because that world can just be immensely counter-intuitive at times.

        [–]billofrighteous 3 points4 points ago

        It isn't the same photon that bounces back. The photon is absorbed by an atom of the surface and then re-emitted.

        [–]sullyz0r 2 points3 points ago

        The same photon is not reflected back. I'm not sure of the proof of this, but I believe the photon is always absorbed, and a new photon of lower energy is emitted.

        [–]craigdubyah 36 points37 points ago

        Yes and no.

        If the photon changes direction by interacting via electromagnetic forces (mirror, prism, etc.) then the redirected photon isn't the same photon you started with. The first one was absorbed by an electron, which released another photon.

        If the photon changes direction by gravity, as in gravitational lensing, then it is the original photon. Whether it truly 'changed direction' is a matter of semantics. Einstein would say that it went straight along its vector through spacetime, but spacetime itself was bent.

        [–]papasee 9 points10 points ago

        If photons are massless how does gravity affect them?

        [–]craigdubyah 37 points38 points ago

        Because in general relativity, gravity isn't a force. It is the bending of spacetime itself.

        [–]papasee 6 points7 points ago

        I'd like to get my head round this. could you explain in more detail about gravity bending spacetime?

        [–]jfpowellTheoretical Physics|Magnetic Resonance 21 points22 points ago

        It's a very good question. But this really isn't the place to get a good in depth answer about curved spacetime.

        I would recommend you look at the wikipedia article for general relativity for a good overview. http://en.wikipedia.org/wiki/General_relativity

        There are plenty of good resources already available for understanding from a layman's perspective the theory of GR, but if you are still having issues with a particular concept then it would be a good topic for it's own askscience question.

        [–]CantWearHats 21 points22 points ago

        Short answer: Gravity doesn't bend space-time, it is the bending of space-time. Mass is what really bends space-time, according to general relativity.

        [–]Kaghuros 12 points13 points ago

        Or, rather, gravity is a measure of the effect of a given mass on the fabric of space-time.

        [–]craigdubyah 5 points6 points ago

        http://en.wikipedia.org/wiki/General_relativity

        Alternatively,

        http://simple.wikipedia.org/wiki/General_relativity

        [–]Occasionally_Right 1 point2 points ago

        See most of these questions (even most of the ones that aren't precisely your question will still have discussion on spacetime).

        [–]fixyergrammer 0 points1 point ago

        This really cannot be understood without the mathematics. Mathematics is basically just a lot of abbreviations for very clearly-defined concepts that produce beautiful structures when considered in relation to one another, and GR is one of the examples where the conclusions are themselves only meaningful as such structures.

        Einstein himself wrote a "layman's" book on GR (and SR) that you might try. However, I would argue that it will give you, at best, an inadequate (and at worst an incorrect) understanding of the theory, whose claims are inherently mathematical in nature.

        The book of nature is written in mathematics. Sometimes we can simply expand a few of the abbreviations into words and get something meaningful, but at this level of physics, that is rarely the case.

        [–]aaomalley 16 points17 points ago

        First off, I am not a physicist nor an expert in gravitational field theory or any type of physics for that matter. I am purely, in this case, a layman. The following is what I have been told about gravity, from multiple reputable scientists, as well as my synthesis of information that I have read on my own. It is in incomplete answer so it is meant to be taken with a grain of salt and an awareness that it does not give a 100% accurate depiction of the theory of gravity.

        The easiest (if ultimately incorrect as all analogies are) was to describe this is to stop thinking of gravity as a thing. Gravity is a description of events, it is like a car crash, the crash isn't a thing it is the term used to describe the events of a car impacting an object.

        My favorite, and most intuitive (though again, ultimately inaccurate) analogy is to think of a large sheet of foam 100 meters by 100 meters. Now, on this sheet you place multiple spherical objects of different mass, like a bowling ball, some ball bearings, a tennis ball, a large spherical steel weight 1 meter in diameter and some other flotsam. All of these objects create indentations in the foam, distorting it to different degrees depending on their weight. That distortion, the depression made bay the mass, IS gravity, it describes what occurs when a massive object creates a distortion in spacetime.

        Now, to the photon being impacted by gravity. Imagine you take a marble and you roll it with enough force to cross the entire sheet of foam. What happens when it passes close to one of the object? It falls into the depression and if its velocity is great enough escapes that depression with a different vector because it has rolled around the depression slightly. Now Einstein would say it is still traveling in a straight line and that it is space-time which distorted around it. That is basically what gravity is, sattelites orbiting Earth are simple falling into the gravity well, but they have enough velocity and are at the right height that they never actually leave the well or fall into the Earth itself. Gravity is falling.

        Now, before any of the serious physics people jump on me, i will say clearly THIS IS MILES AWAY FROM AN ACCURATE DEPICTION OF GRAVITY AND ITS EFFECTS AND FAILS TO DESCRIBE MANY SPECIFIC SITUATIONS AND INTRICACIES OF GRAVITY'S INTERACTION WITH SPACE-TIME. This analogy is only meant to give you a basic concept of what gravity is describing and allow you to visualize the theory when reading the more exact explanations.

        [–]jfpowellTheoretical Physics|Magnetic Resonance 7 points8 points ago

        Hey, the rubber sheet/foam analogy is a good one! Clearly and concisely explains gravity as a geometrical phenomena.

        [–]lnava 9 points10 points ago

        but ultimately gravity is like gravity: http://xkcd.com/895/

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          [–]oblimo_2K12 2 points3 points ago*

          A professor of mine (I was taking General Relativity for English Majors) suggested making the foam rubber sheet into giant cubes to explain effects of gravity: imagine an empty square metal crate. Now, imagine a bowling ball suspended in the middle of the cube. Got it? Okay, now fill the empty space in the crate (around the ball) with Nerf foam rubber. So you've got a 3-dimensional nerf-space with a bowling ball in the middle. With me so far?

          Okay, now we get to the fun part: Imagine that you make the bowling ball disintegrate and vanish, leaving nothing but a bowling ball sized hollow. Now, imagine pinching the nerf from the inside of that hollow, pulling it in from all directions into the very center of the empty space, filling the once empty hollow with nerf. Nerf is spongy, right, so instead of the cube getting smaller, the nerf stretches inward. The nerf closer to the hollow is stretched out more than the nerf at the edges.

          The nerf is spacetime, the stretching is gravity, and the bowling ball is mass, like a planet. This analogy breaks down because it's not the size of the bowling ball that determines the size of the hollow spacetime stretches to fill, it's the mass.

          General Relativity is the topology of the inside of a 4-dimensional nerf football.

          [–]brinx64 0 points1 point ago

          this is probably the best explanation i've heard, thanks!

          [–]aaomalley 0 points1 point ago

          That is a much better, though less visual analogy. I like the sheet analogy because, though imperfect, it allows you to visualize the impact that gravity has on an object traveling through space-time. While the nerd cube is more accurate (though also imperfect) it is still not intuitive to see the interactions on an object traveling in a straight line through that space.

          [–]I_read_a_lot 1 point2 points ago

          So, where does the graviton fall into this ? since gravity is not a force, there should be no force carrier, right ?

          [–]aaomalley 1 point2 points ago

          The graviton, as I understand it, is a very fringe theory with absolutely no popular support among major physicists within the field. Everything I have read about the graviton has been paper after paper that demonstrated how GR and QM didn't work if the graviton existed, and there had been no experiment completed ever which resulted in the outcomes the existence of the graviton would suggest.

          To answer your question, yes, because it is not a force it doesn't need a fundamental particle.

          [–]papasee 0 points1 point ago

          Thank you that was a good analogy

          [–]Felicia_Svilling 2 points3 points ago

          Gravity affects everything. Massive or not, everything must follow the curvature of spacetime.

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            [–]ignatiusloyolaHigh Energy Physics|Theoretical Particle Physics 2 points3 points ago

            Depends on what you mean by "same". Clearly the properties of all particles involved have changed. Can you really say it is identically the same particle? An electron may interact with a photon and an electron comes out (tree level diagram of e+ e- gamma interaction), but there is no reason to make a philosophical claim as to the identity of the electron.

            [–]FamousMortimer 0 points1 point ago

            Yes, it's provable that electrons don't have identity. There's no such thing as THIS electron or THAT electron.

            [–]user2196 2 points3 points ago

            This isn't quite right. Yes, when a photon hits a mirror, it might be absorbed and then a new photon emitted. However, there are also electromagnetic scattering processes that change the direction of a photon without absorption or emission.

            [–]craigdubyah 1 point2 points ago

            Which scattering processes are you referring to?

            [–]user2196 3 points4 points ago

            The easiest one to understand is Thomson scattering (photon scattering off an electron.) An electron cannot absorb a photon while conserving both energy and momentum, so the process can clearly not be interpreted as an absorption and reemission.

            [–]machsmitPlasma Physics|Magnetic-Confinement Fusion 9 points10 points ago

            This isn't quite right, actually. Thomson scattering is a classical effect (wave interaction). The correct generalization to allow for single-photon interactions is Compton scattering (it's straightforward to show that Compton scattering cross-sections reduce to Thomson scattering in the limit of photon energies much less than the electron rest energy). So, a Thomson/Compton scattered photon does alter the momentum of the electron (just in the classical limit where Thomson is applicable the change in electron momentum is negligibly small). Moreover, Compton scattering is reliably treated as an absorption/re-emission process.

            [–]user2196 1 point2 points ago

            I am aware of the fact that Thomson scattering is a low energy limit of Compton scattering, but I also thought Compton scattering could not be treated as an absorption/re-emission process. Could you refer me to a source on the treatment or something? My knowledge of Compton scattering comes primarily from Radiative Processes by Rybicki and Lightman, which I don't recall treating it as an absorption and re-emission process. I thought there was still the issue that if you have a photon with momentum p and an electron at rest, there is no way for the electron to absorb the photon since it cannot have both a momentum of p and a change in energy of p*c. Where am I going wrong in this?

            [–]Gaether 0 points1 point ago

            But how can its acceleration be explained if it's always the same photon? Or is this only when considering light as a wave?

            [–]Kill_Welly 1 point2 points ago*

            They do that every time they hit a mirror, right? (unless that's actually the destruction of one photon and the creation of a new one...)

            EDIT: Apparently not.

            [–]featherfooted 6 points7 points ago

            Your second idea was slightly more accurate.

            [–]user2196 0 points1 point ago

            Many of the answers here are telling you that a photon travels along a geodesic in terms of gravity and that all electromagnetic interactions are the photon being absorbed and the reemitted. This is wrong. There are electromagnetic scattering processes that change the direction of the photon without any absorption.

            [–]jfpowellTheoretical Physics|Magnetic Resonance 12 points13 points ago

            Not according to Quantum Electrodynamics. All quantum interactions of photons with charged matter involve the destruction of one photon and creation of a new one.

            [–]zeugRelativistic Nuclear Collisions 5 points6 points ago

            Isn't this all sort of a cop out?

            I mean, all photons are indistinguishable in principle. In a Feynman diagram for Compton scattering, the lowest-order diagram may look like a photon getting absorbed and another being emitted, but the diagram is not what really happens, the sum of all diagrams gives one the probability of what actually happens.

            I don't see the value in calling the outgoing photon in a Compton scattering a 'different' photon. Before the scattering the photon field is in an excited state, and it is in an excited state after the scattering.

            Furthermore, the 'photon' that the experimentalist looks at is only a theoretical photon in approximation. The photon in perturbative QED is a true momentum eigenstate of the field, and so that theoretical photon is literally everywhere in space. In reality, the experimental 'photon' is just a superposition of photon states smeared over some narrow volume of momentum space.

            When a 'photon' passes through a dielectric, it does not zig-zag from one scattering center to another. What happens is the superposition of all possible scatterings, which interfere with each other to produce the outcome expected by the classical laws of refraction with a high probability. Why do we insist on calling this annihilation and creation when there is no such singular chain of events?

            So what is the point of repeating this mantra of photons only travel at c, when the experimental photon is not really a true momentum eigenstate of the photon field?

            An actual, true, exact momentum eigenstate of the photon field is everywhere. It has a perfectly well defined momentum, but it really has no sense of a classical velocity as it remains everywhere in space indefinitely.

            I have seen a lot of misconceptions persist by embracing this perturbative metaphor too literally. I guess I don't see the value in it.

            [–]jfpowellTheoretical Physics|Magnetic Resonance 2 points3 points ago

            Ah yes. How silly of me. Clearly photons are everywhere and have no velocity :|

            [–]zeugRelativistic Nuclear Collisions 0 points1 point ago

            In the theoretical formalism, yes - go look in any QFT book. An electron is a plane wave solution to the Dirac equation. A photon is a plane wave solution to the quantum interpretation of Maxwell's equations. Where is a plane wave? Everywhere in the volume that you are considering.

            An experimentalist's photon is just a superposition of many photon states all narrowly peaked around some reasonably precise momentum. So then the superposition of plane waves actually has an approximate position and can be said to be going somewhere at c. This is close enough to the actual plane wave solution that S-matrix calculation actually yields realistic predictions.

            What I don't like is this blanket statement that a photon is destroyed and a new one is created. Technically, yes, they are different photons by virtue of the fact that they have different momenta. However, one can argue the exact same thing for an electron. There is an incoming electron state with one momentum. It is 'destroyed', i.e. there is a destruction operator, and an electron with a new momentum is 'created', i.e. there is a creation operator. So one ought to at least be consistent and say that everything is destroyed and new things created in every scattering event.

            The only reason I can see that people use this language of creation and destruction for photons but not electrons is that fermion lines form an unbroken path. That is just an artifact of the metaphor of the Feynman diagram, and taking these things so incredibly literally lends itself to error.

            In any case, I find it ridiculous to insist that we call Compton scattering processes absorption, as you imply in your reply to user2196. If we are going to do that then the term absorption has no useful meaning, and is completely synonymous with the term interaction.

            [–]niugnep24 1 point2 points ago

            Why do we insist on calling this annihilation and creation when there is no such singular chain of events?

            Probably because our brains refuse to drop the classical pictures of how the world works, where particular things are in particular places and interact in definite ways. The idea of wave functions evolving in this strange manner and us observing one probabilistic eigenstate at the end, which just happens to approximate the classical picture most of the time when you look at a large sample size, continues to be a really hard one for us to wrap our heads around. Even high-profile particle physicists keep talking about individual particles doing definite things in interviews and TV shows and the like -- it's just a really hard paradigm to shake!

            So what is the point of repeating this mantra of photons only travel at c, when the experimental photon is not really a true momentum eigenstate of the photon field?

            Well the group velocity of any photon wavefunction is c, even if it's not an eigenstate, so it does work in that sense.

            [–]ididnoteatyourcatExperimental Particle Physics 2 points3 points ago

            Strictly speaking, that's not according to QED, but to perturbation theory. It's not clear in non-perturbative QFT whether there is a meaningful difference from what user2196 said and what you say. The question becomes philosophical; is a photon whose direction/wavelength is perturbed the same photon, or a different photon?

            [–]asking_science 0 points1 point ago

            that change the direction of the photon

            How would the Feynman diagram of such an event be drawn?

            [–]ididnoteatyourcatExperimental Particle Physics 1 point2 points ago

            To be fair, a Feynman diagram is but one term in a perturbation series involving integration over nonphysical momenta. I'm not sure it's fair to put too much weight on an a virtual photon ontology, one whose description is not invariant with respect to chosen formalism, gauge, basis states, regularization technique, or mathematical approach to perturbation series. Once you sum over all paths/momenta/diagrams, or just study the situation non-perturbatively, it's not so clear anymore what ontology is meaningful.

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              [–]Plancus 4 points5 points ago*

              What about when light changes mediums such as vacuum to glass? The speed changes according to: V(n)=C/n, or is this instantaneous? Or is this an illusion?

              EDIT: I just realized that the photons are exchanged between atoms and the rate of the exchange is what then becomes the speed of light in that medium.

              EDIT2: I'm an idiot.

              [–]jfpowellTheoretical Physics|Magnetic Resonance 2 points3 points ago

              You are asking questions already answered elsewhere in this thread.

              http://www.reddit.com/r/askscience/comments/oqatv/is_there_an_acceleration_limit/c3j7wpw

              [–]reventropy2003 2 points3 points ago

              Not true. Photons can have a phase velocity that is c or slower which complicates the question. Does this not imply instantaneous deceleration through a varying medium? or is this not accurate since the mechanism is absorption and re-emission? I'll have to track down some papers on this.

              [–]jfpowellTheoretical Physics|Magnetic Resonance 11 points12 points ago

              To get highly technical, when photons travel in matter they interact with excitations of vibrational modes of that matter called phonons. They then form composite particles called polaritons that have a mass and can hence travel slower than c.

              Basically photons in matter in a sense are no longer photons, and so they haven't decelerated, they were destroyed in an interaction which produced a new quasi-particle.

              [–]Scurry 1 point2 points ago

              Basically photons in matter in a sense are no longer photons, and so they haven't decelerated, they were destroyed in an interaction which produced a new quasi-particle.

              Then what is it that bounces off an object and enters our eyes? And are they travelling at c?

              [–]mugibugi 0 points1 point ago

              I'm sorry, but how much of this is experimentally observed and "known" and how much of it is theoretical and/or mathematical? Or is that a question that doesn't matter to physicists?

              I'm still trying to wrap my head around this, even going back to whether mathematics is simply a collection of numbers that describe reality or whether mathematics is reality - is truth. Sorry of it sounds confusing, I'm confused myself.

              [–]jfpowellTheoretical Physics|Magnetic Resonance 2 points3 points ago

              It's all theoretical, but the predictions of the theory match our observed measurements.

              In science the only important question is "do the predictions match observation"? How you then interpret the mathematical objects that constitute your theory is as much philosophy as science.

              [–]phl0x 5 points6 points ago

              I love your responses. They are clear and meaningful to the physics-naive (me). Thank you.

              [–]jfpowellTheoretical Physics|Magnetic Resonance 2 points3 points ago

              Thanks. I often think my answers are barely coherent rambles.

              [–]drockers 1 point2 points ago

              can't you slow light down in a medium though?

              [–]shaveraStrong Force|Quark-Gluon Plasma|Particle Jets 7 points8 points ago

              So, in a medium what happens is that the photons still travel at c, but the medium responds with photons of its own such that the bulk propagation of light is less than c.

              [–]GhostOfJulesVerne 1 point2 points ago

              I thought that photons only travelled at C in a perfect vacuum?

              [–]shaveraStrong Force|Quark-Gluon Plasma|Particle Jets 9 points10 points ago

              So, in a medium what happens is that the photons still travel at c, but the medium responds with photons of its own such that the bulk propagation of light is less than c.

              [–]jroth 0 points1 point ago

              Can you explain this more please?

              [–]shaveraStrong Force|Quark-Gluon Plasma|Particle Jets 12 points13 points ago

              jfpowell does it better a few posts below. But treat it classically before you try to handle it in a quantum mechanics sense. Light, electromagnetic radiation, enters a material made of charged particles. These electromagnetic waves push and pull on the charged particles that make up the material. And those charged particles, as they get pushed and pulled, create electromagnetic disturbances of their own. So as the electromagnetic radiation enters the material, the material response is such that the overall propagation of light is slower.

              jfpowell's more quantum argument is that when the photon enters the medium, we can treat the photon and its interaction with the medium as a "quasi-particle." Ie, it's not a particle we think of as building nature, but it's a convenient mathematical tool for solving the equations in the right way; a particle we mathematically construct to exist that behaves in the proper way. So in this way, a photon in a material isn't exactly like a photon in a vacuum, it's a new thing, a material+EM disturbance propagation.

              [–]sittingGiantTheoretical Elementary Particles 1 point2 points ago

              very well explained

              [–]thedoh 1 point2 points ago

              they come into the world fully formed and travel at c

              This seriously hurts my brain.

              Can you explain the process by which a photon comes into existence and comes to be moving at c?

              [–]zepp3lin 0 points1 point ago

              How could anything just come into existiance instantaneously? Even in those pictures of particle collisions the particles have slight gaps where they don't leave a signature.

              [–]shaveraStrong Force|Quark-Gluon Plasma|Particle Jets 5 points6 points ago

              Because you can't always be measuring every single instant in time. You measure no photon, and the next time you measure the system, there's a photon, travelling at c. The way the math in between those two measurements works out is that the photon always travels at c.

              [–]ignatiusloyolaHigh Energy Physics|Theoretical Particle Physics 0 points1 point ago

              Acceleration, from a particle physics perspective, is just a (continuous) series of interactions with virtual photons. Acceleration doesn't really enter into the equations - an interaction occurs and causes a change in momentum, and there is no description regarding the time it takes for that to happen, as far as I have seen.

              [–][deleted] ago

              [deleted]

              [–]jfpowellTheoretical Physics|Magnetic Resonance 2 points3 points ago

              Again. This has been answered elsewhere.

              http://www.reddit.com/r/askscience/comments/oqatv/is_there_an_acceleration_limit/c3j7wpw

              In all cases where "light travels slower than c" what is happening is a different quasi-particle is travelling slower than c, no contradiction with special relativity.

              [–]Tiyumba 0 points1 point ago

              What about when light is artificially slowed? Does it ever speed up again? If it can be decelerated how is that it cannot accelerate? See, Slow Light

              [–]jfpowellTheoretical Physics|Magnetic Resonance 0 points1 point ago

              Please read the other comments in the thread.

              [–]kaizenallthethings 0 points1 point ago

              Why would that not be considered infinite acceleration? For example, in an electron/positron collision, the mass of the previous particles is converted to photons going c.

              I got downvoted off the map previously for this comment, so I am probably wrong, but would someone tell me why?

              [–]jfpowellTheoretical Physics|Magnetic Resonance 2 points3 points ago

              This isn't a bad question per se. It just assumes that before a particle exists it's velocity is zero. The problem lies in trying to assign a value to a physical quantity for a particle that doesn't exist!

              If you want to, you can claim that photons undergo an infinite acceleration at their moment of creation. But that doesn't really add anything to the theory, is fairly mathematically inconsistent with our treatment of infinite quantities, and is just plain confusing.

              [–]kaizenallthethings 0 points1 point ago

              I did not mean that the photons themselves undergo an infinite acceleration, but that the mass/energy that created them does.

              [–]wadeNYC 0 points1 point ago

              But when they hit an object they bounce right? So changing the direction of travel would mean they have accelerated somehow.

              [–]kabbinet 0 points1 point ago

              Is there any way to slow it down?

              [–]bop_ad 0 points1 point ago

              What about a photon traveling through a substance? It then travels at a lower speed. Are you saying that a one photon can't travel through, say, a diamond?

              [–]rmxz 0 points1 point ago

              Massless particles ... can't undergo acceleration,

              So when a massive particle emits a photon -- would conservation of momentum dictate that the massive particle's momentum changes instantaneously when the photon was created?

              And if that massive particle's momentum changed instantaneously, that means infinite acceleration.

              If so - it seems that means that all acceleration involving electromagnetism is instantaneous acceleration.

              Or did I mis something.

              [–]margirtakk 0 points1 point ago

              So if two photons struck each other with forces opposing each other, so the particles stopped completely, would the photons cease to exist?

              [–]ranon20 0 points1 point ago

              What is the deceleration of a photon when they are absorbed.

              Are they absorbed instantaneously?

              [–]Diels_Alder 0 points1 point ago

              Slightly off topic but when a photon is absorbed by an electron, my understanding is that its energy is absorbed only when it exactly matches the energy difference in the electron levels. But what are the chances that these are completely exact?

              [–]SarahC 0 points1 point ago

              But they slow down in glass, and speed back up when they leave, how do they do that?

              Also - I remember something were photons were slowed down to a meter a second by lasers... again... I wonder how that worked.

              [–]jfpowellTheoretical Physics|Magnetic Resonance 1 point2 points ago

              Read the replies to the other questions that ask exactly the same thing :)

              load more comments (12 replies)

                [–]retrogamer500 21 points22 points ago

                Photons always travel at c.

                And to the inevitable guy who brings up the refractive index, photons always travel at c. The slower speed of light is only an illusion caused by the photon being absorbed by the atoms in the material, only to be released after a slight delay.

                [–]84_sheepdog 7 points8 points ago

                Interesting. Is it absorbed, delayed, and released, or is it actually absorbed and destroyed, and then after a delay a brand new photon is created and released? It would seem like it would have to be the second in order to obey the "always travelling at c" rule, no? (or I could be way off, please enlighten!)

                [–]canopener 4 points5 points ago

                I don't think questions like that about identity have clear answers at the quantum scale. Photons are a lot like waves, after all, and identity conditions don't apply to waves at all.

                [–]gyarrr 1 point2 points ago

                A photon is just a form of energy, so it's the same energy that's being transferred through the material (with some of the energy being absorbed by the material) but technically yes it would be a different photon

                [–]ihateyouguys 0 points1 point ago

                How technically if identity stuff just doesn't matter at that scale?

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                  [–]ididnoteatyourcatExperimental Particle Physics 4 points5 points ago

                  As a complement to what jfpowell said, a photon is not just some isolated object, but a vibrational mode of the electromagnetic field. When you talk about vibrations of fields, those vibrations don't "accelerate" in the sense you are familiar with. For example you can have interference effects that make it "look like" an object is accelerating, but the underlying field is just vibrating up and down.

                  [–]rainytig1 2 points3 points ago

                  So, a photon is just the interaction of one particle with another separated by distance and a time delay?

                  [–]ididnoteatyourcatExperimental Particle Physics 3 points4 points ago

                  Well, the theory is a lot less elegant if we try to get away with removing the electromagnetic field as an independently existing entity and only talking in terms of interactions. Things hang together a lot more cleanly if we just counterfactually definitively accept the existence of the electromagnetic field, and talk about photons and interactions in terms of quantized interacting fields.

                  [–]wczwe 4 points5 points ago

                  Nope--photons travel at c, always. This is just the speed that electromagnetic waves propagate; in the absence of gravity (where it can take significantly longer than (coordinate distance)/c for light signals to reach a far away observer from near the event horizon of a black hole---but that's more of a problem with space just being curved, as opposed to the photons actually travelling slower. Someone near the photon will still always see it move at c. Reference frames are important in GR, but I digress) this is the same always.

                  The issue is complicated somewhat by special relativity, but a simple example is water waves far from the shore (where they'll drag across the ground and stuff). They always travel at the same speed. Same with sound waves (assuming fixed pressure) and so on. The beginning bit is complicated--say if a hand pushes a slinky faster than the velocity that waves travel through it--but if you look close enough you'll notice that even during the creation of the wave, no part of the wave travels faster than the medium's fixed propagation velocity.

                  [–]ihateyouguys 0 points1 point ago

                  What's that propagation velocity determined by?

                  [–]wczwe 1 point2 points ago

                  Sorry for late reply; didn't see this till now. (It turned into a pretty big hootenany, so apologies.)

                  For light, as far as I know, it simply is what it is. This is why some physicists (cosmologists, particle physicists) prefer natural units in which our measuring system is chosen to make c=1; the fact that the speed of light is the tremendously silly value it is in m/s is because our measuring sticks (and clocks) are based on human things.

                  For other media, it has to do with the material; how easily forces are transmitted through it, etc. Of course, when you get down to it, that depends on the arrangement of atoms in the material, and THAT's due to the electromagnetic interaction again. So, photons.

                  Unfortunately I don't have a more satisfying answer for why c is what it is. One can certainly conceive of a universe exactly like ours except light travels at 3.1e8 m/s instead of 2.99(ish)e8. So why this and not that? Some physicists will tell you that it's a pointless question. I know of one way of explaining it with less hand waving, so I'll try to give a quick rundown here...better presentation of ideas can be found at this SciAm article by Max Tegmark or this slightly more technical (but still accesible) paper on the same topic.

                  The basic idea is, we have the multiverse (which I will take in this paragraph to mean "the totality of everything that exists including regions of spacetime we can't ever get to," as opposed to "universe" = "the region of spacetime we can get to." Others do not like this definition. But they are just words.) and the theory of "chaotic inflation," which says that the multiverse inflates at different rates throughout its volume. Due to quantum interactions, some regions will suddenly start inflating extremely quickly (think bubble forming in boiling water); I'm fuzzy on the details here (I'm undergrad physics, apologies, the details are grad cosmology) but the energy from this sudden inflation causes particles to start coalescing out of the vacuum. (Think E=mc2 but that is nowhere near precise enough for anything more than a faint idea of what is going on.) This, so the theory goes, was the big bang.

                  Anyway, the way that that bubble inflates is supposed to influence how the gauge fields (such as the electromagnetic field, or the strong/weak fields for nuclear/atomic interactions) interact. In particular, things like propagation speeds for perturbations on these fields (for electromagnetism, these are photons). How this happens has to do with 27 (or something)-dimensional spacetime and how the inflation influences the geometry of the curled up dimensions, but once again, I'm straying into regions beyond my current skill set.

                  [–]Lai90 2 points3 points ago

                  It come into existence at C.

                  [–]xaeru 0 points1 point ago

                  Read this incredible post of Neil deGrasse Tyson on photons... http://www.reddit.com/r/IAmA/comments/mateq/i_am_neil_degrasse_tyson_ama/c2zg8gi

                  [–]drchaos2000 0 points1 point ago

                  even after reading all the answers here, this is one of those questions that totally shattered my review of reality. i will have to evaluate that on my mind for some weeks now.

                  [–]beason4251 0 points1 point ago

                  Photons always travel at c to all reference frames able to observe the photons.

                  While photons do not "accelerate" in the traditional sense of changing velocity, they do change momentum (Energy = plank's constant * frequency, momentum = plank's constant / wavelength). For example, light traveling away from a black hole (from outside the event horizon) will be red-shifted because its wavelength increases. Light emitted from anything moving towards or away from us will to us appear to have different wavelengths - and thus energies and momentums - than when they were emitted.

                  [–]frostbyte001 6 points7 points ago

                  So basically, in theory, you could go from 0 to c in literally no time at all and have infinite acceleration?

                  [–]adamsolomonTheoretical Cosmology|General Relativity 37 points38 points ago

                  In theory you could go from 0 to arbitrarily close to c. It's impossible for a massive particle to go at c, and similarly impossible for a massless particle to go at any speed besides c, so there's no 0 to c in physics.

                  [–]jfpowellTheoretical Physics|Magnetic Resonance 31 points32 points ago

                  Not to c (that's not possible). But all the focusing on the speed of light has missed a very simple point. If I want to go from rest to even 1 m/s in an arbitrarily small time period, I would require an arbitrarily large force and acceleration.

                  Just look at the classical physics definition of acceleration a = dv/t, all you need to do is make the time period over which you perform your acceleration as small as you can, and there is no inherent law of physics that prevents this. It's a practical and engineering limit.

                  [–]ReinSci 3 points4 points ago

                  Well, close enough to C. I understand the limitation. I will be more careful in the future.

                  My point is that theoretically a uniform gravitation field has no theoretical limit in the amount of work it can do on an object, and what is interesting to me is that the object would not be damaged by extremely high accelerations.

                  Your answer is the most appropriate direct response. Going from 0 m/s to 1 m/s in 1.0 x 10-20 seconds would be frightening, but this is allowed given sufficient energy and space-time is not quantized, which puts no limit on the acceleration. (You said it better).

                  I am curious if you think a direct answer is better or an answer that does deal with 'sexy' things like space travel and the speed of light. There is focus on it because it is fun to think about for a majority of people. I think what happens in a few femtoseconds is equally interesting, but I don't expect as many people to share my opinion that. So, I would tend toward answers that include 'exciting' things. Thoughts?

                  [–]jfpowellTheoretical Physics|Magnetic Resonance 8 points9 points ago

                  For the purposes of this subreddit, I think it's most appropriate for the most up-voted top level answer to be a clear and concise answer to the best of our current scientific understanding.

                  The comment threads below those answers should have on-topic speculation and discussion. But it's best not to try and construct a top level answer if you don't have the appropriate knowledge to disseminate correct scientific answers.

                  If you can correctly answer a question and also "sex" it up, all the better. But an incorrect answer that contains pseduoscientific buzzwords is not helpful to anyone.

                  [–]ReinSci 2 points3 points ago

                  Understood. Thank you.

                  [–]StankyMung 1 point2 points ago

                  I know by doing this I'm breaking the rules, but:

                  I'd just like to say, that you have done an admirable job policing this post. It shouldn't be necessary, but those of us looking for answers and discussion greatly appreciate your effort. Thank you.

                  [–]adamsolomonTheoretical Cosmology|General Relativity 0 points1 point ago

                  Yep.

                  [–]khiron 0 points1 point ago

                  Forgive my stupid question, but I got curious.

                  So, if the time was 0, going from 0 to X speed (I'm not saying that we could achieve it, it is just for the sake of my question) we'd be returning to the behaviour of the photons? I know that sounds obvious, I'm just trying to confirm what I believe to be understanding.

                  [–]jfpowellTheoretical Physics|Magnetic Resonance 2 points3 points ago

                  I don't understand the question. Sorry. What does "returning to the behaviour of the photons" mean?

                  [–][deleted] 2 points3 points ago

                  Ahhh, there is no acceleration because there is no time at which the massless particle has a velocity of 0. Thanks! Physics always makes me feel like a pre-monolith hominid in 2001.

                  [–]GalacticWhale 0 points1 point ago

                  Okay, i see this is derailing almost instantly every time. Yes or no, is there a limit to acceleration besides hitting the speed limit? If it needs explanation please explain, but simply put Yes or No?

                  I won't sleep tonight not knowing.

                  [–]dariusj18 2 points3 points ago

                  I am wondering why general_relative's post was downvoted, though GR etc may not have any mathematical limitations on acceleration, would there not be a practical limit based on there not being enough energy in the universe to accelerate at some upper limit?

                  [–][deleted] ago

                  [deleted]

                  [–][deleted] ago

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                  [–]adamsolomonTheoretical Cosmology|General Relativity 5 points6 points ago

                  GFT?

                  [–]elbeem 2 points3 points ago*

                  Group field theory? Or a typo of QFT.

                  [–]adamsolomonTheoretical Cosmology|General Relativity 5 points6 points ago

                  Oh?

                  I figured it was a typo of QFT. This "group field theory" certainly doesn't fall under the umbrella of "currently generally accepted theory."

                  [–]elbeem 1 point2 points ago

                  Yes, you are probably right.

                  [–]jfpowellTheoretical Physics|Magnetic Resonance 3 points4 points ago

                  Good spot, fixed my typo :)

                  [–]Sparling 1 point2 points ago*

                  So hypothetically we could imagine an object that could linearly accelerate at n*c for x seconds (n >1, x>1)? Clearly this is wrong however I'm not sure I have a full grasp on why at a meaningful level. I imagine that one would first grapple with the object not breaking apart at such acceleration, and then say that it would require that we input an infinite amount of energy to make such an action occur but I'm not sure why there isn't a limiting case where we say that such linear acceleration has a supremum/upper bound?

                  edit: spelling

                  Edit 2: I think I got this figured out on why that's a silly hypothetical. If we say object undergoes n*c the limit is that it can only under go that for at most 1/n sec.

                  [–]jfpowellTheoretical Physics|Magnetic Resonance 4 points5 points ago

                  c does not have units of acceleration.

                  It is not meaningful to talk of the acceleration of light, or to construct a meaningful acceleration out of just the numerical value of c.

                  [–]rekk14 1 point2 points ago

                  Far be it for me to disagree with what you've said, but I always feel it necessary to point out that a "consensus" in the scientific world is meaningless. If every physicist agreed that there was some sort of acceleration cap, that would not make it true. We cannot vote on the laws of the universe.

                  [–]jfpowellTheoretical Physics|Magnetic Resonance 3 points4 points ago

                  I absolutely agree. However the burden of proof is definitely upon the extraordinary claim. Why are the currently accepted theories accepted? Because all the available evidence points to their validity. Once one wants to posit the necessity of extra quantised variables they need to provide valid reasons to do so.

                  Consensus doesn't in any way imply scientific truth, but there is a reason why we don't give equal weight to every theory. Phrenology should not be given equal time to neuroscience.

                  [–]atomfullereneAnimal Behavior/Marine Biology 2 points3 points ago

                  On the other hand, if every physicist agreed that there was some sort of acceleration cap, after years of study and a deep understanding of the underlying theory, then that would be a good sign that, within the realm of our understanding, there is probably an acceleration cap. We don't vote on the laws of the universe but we do observe them. If everyone who looks closely sees the same thing, then even if they aren't quite right they are probably on to something

                  [–][deleted] 0 points1 point ago

                  My apologies if this is a dumb question but what if a particle at rest speeds up to the speed of light in 1 second? Wouldn't its acceleration be equal to the speed of light? Therefore, unless particles can travel faster than the speed of light, it is physically impossible to have an acceleration higher than the speed of light?

                  [–]jfpowellTheoretical Physics|Magnetic Resonance 7 points8 points ago

                  Ignoring the fact that it couldn't accelerate to c. You are describing an acceleration of 3x108 m/s2. This has the same numerical value of the physical constant c, which is the quantity 3x108 m/s.

                  Physical quantities have numerical values coupled with units of measurement. If you change your units, i.e. from kg to lbs you change the numerical value. But the mass of an object itself does not change if you change your units.

                  To illustrate, the statement "an acceleration of c" is as meaningless as the statement "Bob has a mass of 20 feet".

                  load more comments (1 reply)

                    [–]spazmodic- 0 points1 point ago

                    Finally! I saw this thread earlier and the non-science made my brain hurt. None of it sat right since you can accelerate indefinitely in your frame, but you never get to c (approach an asymptote).

                    [–]mostirrelephant 0 points1 point ago

                    What about the problem of mass dilation with acceleration? There may not be a hard and fast limit, but doesn't an object reach an infinite mass before at a certain acceleration, making the force needed to accelerate it infinite? I also haven't done any physics in a while, so I may be mistaken.

                    [–]jfpowellTheoretical Physics|Magnetic Resonance 1 point2 points ago

                    You are thinking of the increase of relativistic mass as velocity approaches c.

                    With an arbitrarily large force you can obtain arbitrarily large accelerations. These arbitrarily large accelerations will not equate to arbitrarily large velocities, as you cannot sustain an arbitrarily large acceleration for any period of time that would cause something to travel faster than c.

                    In non relativistic physics a constant force creates a constant acceleration. So a particle with a constant force will accelerate without limit up to an arbitrarily large velocity.

                    In special relativity, a constant force does not create a constant acceleration. As the velocity of the particle increases the acceleration decreases for a constant force. When you calculate the force required to sustain an arbitrarily large acceleration you discover it diverges (becomes infinite) before the particle reaches the speed of light.

                    Does that make sense?

                    [–]mostirrelephant 0 points1 point ago

                    Pretty much

                    [–]sniperx99 0 points1 point ago

                    Could one theoretically come up with a limit based on the time/energy uncertainty principle? I was under the impressions that on the ~10-13 order timescale, time sort of stopped working in the conventional sense. Would an acceleration or (3E8) / (this incredibly small amount of time) be impossible?

                    [–]kinurabi 0 points1 point ago

                    I didnt understand shit of what you said back there dude, but it sounds scientific and cool so you'll get an uppvote for the comment

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                      [–]reidzenHeavy Industrial Construction|Environmental Law 16 points17 points ago

                      And a followup question: Is there an upper limit to change in acceleration (absolute jerk?)

                      [–]someone13 9 points10 points ago

                      Quick addendum for those of us that weren't sure what "jerk" referred to: in this case, it's the derivative of acceleration with respect to time. Or, in short, you have:

                      Position --> change over time --> velocity
                      Velocity --> change over time --> acceleration
                      Acceleration --> change over time --> jerk

                      Jerk can also be called jolt, surge and lurch, and after that you have jounce (aka. snap), crackle and pop. Yes, I'm serious.

                      [–]toasters_are_great 2 points3 points ago

                      3.932e51m/s2, which Christoph Schiller derived in 2004.

                      The way to interpret this is that attempts to create greater acceleration will necessarily create an event horizon, thus cutting off the mechanism of acceleration from the accelerated item.

                      [–]Shipyaad 106 points107 points ago

                      THIS IS NOT ASKREDDIT, PEOPLE. THIS IS WHERE THE EXPERTS ANSWER THE QUESTIONS, NOT THE LAYMEN. IF YOU DON'T HAVE SPECIAL EXPERTISE IN THE TOPIC AT HAND, DON'T ANSWER THE QUESTION.

                      I'm unsubscribing from r/askscience. I just can't deal with this shit anymore - this thread is a perfect example of the problems everyone knew would occur when r/askscience became a default subreddit. I'm sorry to leave, because I loved reading the responses from the scientists, but the other commenters are driving me crazy.

                      This was one of my favorite places on reddit and it has become just too much effort to wade through all the garbage left by idiots who won't police themselves. The mods and admins can't do it all.

                      Sorry for the off-topic, non-scientific, top-tier comment. Just had to get that off my chest. See ya.

                      [–]GeoManCamTectonic Geology|Geophysics|Basin Analysis[M] 138 points139 points ago

                      Hello, mod here: Please, if you see stuff that you don't agree with, report it. Report report report, it helps us out tremendously. It's usually a bit longer before we get to some of the more questionable answers on the weekend, but don't let it discourage you from learning on the forum. Report report!

                      [–][deleted] ago

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                      [–]BrainSturgeonMicrofluidics|Chemical Engineering[M] 5 points6 points ago

                      The mods have discussed it before. We're open to suggestions - feel free to modmail us with ideas/graphics/color schemes and well consider them!

                      [–]hero2bash 0 points1 point ago

                      In my opinion, it's good as it is.

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                        [–]Subhazard 18 points19 points ago

                        You guys should just start banning people who don't follow the rules.

                        There's nowhere else to go from here. There is no 'trueaskscience'

                        Keep this place about science -please-.

                        Have the balls to ban these people

                        [–]Tarqon 8 points9 points ago

                        Aside from questionable answers, could you please do something about bad questions? I've been seeing so many lately that are just speculative, non-falsifiable or unscientific in nature lately. Also questions like "in how much time can we do X" and questions that are very narrow/factual/googleable.

                        [–]flounder19 0 points1 point ago

                        I mean at that point it just sounds like you're manufacturing your own questions to be answered by you.

                        [–]ryeguy 2 points3 points ago

                        To help with this, why not change "report" to something like "this isn't science" or something that stands out more? Reporting is something that has a more purposeful meaning in this subreddit, and I think it should stand out.

                        [–]Zimaben 1 point2 points ago*

                        We report and downvote, and then we don't see anything happen so we assume any cleaning up that manages to still happen is a result of the users and not the moderators. This is not a good incentive to keep reporting comments in violation.

                        Downvotes used to mitigate this problem but now the community is too diffuse to act according to its' own guidelines and this subreddit has just become another extension of reddit instead of an extension of the scientific community: source:flagrant and a weekday

                        /*sorry, obvious meta post. Feel free to delete, as this is mostly a complaint to moderators anyway and not adding any value.

                        load more comments (1 reply)

                          [–]ducttapedude 38 points39 points ago

                          Give the moderators a little time to sort through the unqualified answers. This thread is only 2 hours old, and the best answers are just now getting the visibility (upvotes) they deserve, while other "answers" are being deleted.

                          Reddit is a community engine and will take care of itself when there is time to adjust. You can't stop all the stupids on here instantly, though.

                          [–]jfpowellTheoretical Physics|Magnetic Resonance 26 points27 points ago

                          Looking at the remnants of the thread I think it's quite clear the mods are doing an excellent job.

                          Three cheers for the AskScience moderators!

                          [–]selfintersection 8 points9 points ago

                          I think the argument is that the number of stupids is so high now that the "time to adjust" is longer than is desirable.

                          [–]rjc34 9 points10 points ago

                          Mature AskScience threads are the best ones to read through. Why? Because the moderators have come through to clean up all of the garbage. Anything below the first few top-level comments is pretty much a wasteland.

                          [–][deleted] ago*

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                          [–]erocysp 3 points4 points ago

                          I agree, I still find it remarkable how many commenters can't read... Well, that's the only conclusion i can come to since the rules blurb is right next to the comment box yet they still choose to post drivel here.

                          [–]Lignoba 0 points1 point ago

                          Internet is serious business afterall...

                          I would take anything said in here with a grain of salt. If you really want the answer, go ask the experts. Don't ask someone claiming to be the expert on the Internet.

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                            [–]eelvex 27 points28 points ago*

                            No, there is no acceleration limit according to general relativity (and most quantum gravity theories).

                            IF space has a minimum length then maximum acceleration will be defined by this length.

                            edit: typo

                            [–]clyspe 9 points10 points ago*

                            Since it's popped up a lot here, can someone clarify what quantising space time means and why it's important that we don't?

                            edit: did as suggested here

                            [–]s-mores 8 points9 points ago

                            You should make a post yourself asking that very question. Provide examples where it pops up and you'll get an answer better and in an easier-to-find form.

                            [–]g33n 1 point2 points ago

                            Quick and dirty answer, but you should probably consider following s-mores' advice...

                            Because of a number of physical limits (speed of light, uncertainty principle), there's a smallest possible unit of meaningful distance, which is known as the Planck length. Some people take this to indicate that there are discrete points in space-time where particles can exist, and there are no intermediate points (i.e. continuous space-time). While quantization can be useful for certain computations, we have no reason to believe that it's an accurate model of the universe.

                            [–]Occasionally_Right 1 point2 points ago

                            Because of a number of physical limits (speed of light, uncertainty principle), there's a smallest possible unit of meaningful distance, which is known as the Planck length

                            There is nothing in the standard model of particle physics or the general theory of relativity that would suggest the Planck length is "the smallest possible unit of distance". There is reason to believe that the Planck length represents the scale at which both general relativistic and quantum effects are important, and there are some models beyond the standard model that interpret the length as being a physical limit, but for the most part that's purely speculative at this point.

                            [–]g33n 0 points1 point ago

                            You left off a word when you quoted me. I think "meaningful distance" is critical.

                            And anyway, in giving my "quick and dirty answer" I was mostly offering pathways for further inquiry by throwing out terms to a five-hour old question that had not yet had any significant reply.

                            [–]Occasionally_Right 1 point2 points ago

                            I think "meaningful distance" is critical.

                            I did leave off "meaningful", but that doesn't change my point which is that there's no reason to believe distances smaller than the Planck length aren't meaningful. In fact, they could be interpreted as being very meaningful in that they represent precisely the scale at which interactions between gravity and quantum effects are both significant.

                            [–]GratefulTonyRadiation-Matter Interaction 4 points5 points ago

                            Is there a limit of force?

                            In order to speculate infinite acceleration, one would need to also speculate an infinite force... (f=ma, a = f/m) which is sort of a sour concept. One could also have vanishingly small mass to achieve near infinite acceleration.

                            Massless particles always travel at c-- they do not accelerate, and it appears that matter tends to appear fundamentally as particles with finite, discrete masses.

                            So unless you have a particle of non-zero, vanishingly small mass or an infinite magnitude force, you will have some finite maximum acceleration. I don't think I can tell you what it is.

                            [–]RuinerParticles 0 points1 point ago

                            Suppose that you have a delta type potential. Then the interaction is made "instantaneously", in the sense that the particle only "feels" the potential barrier once it's on top of it and there is no timescale for the interaction, so the force can be said to be infinite.

                            Anyway, this whole discussion is anyway meaningless to QFT, since we only deal with asymptotic states.

                            [–]breakN_leaveflowers 2 points3 points ago

                            The answer is yes, but not for the same reason as with the speed of light.

                            In order to accelerate, you must apply a force, which means you have to exchange momentum. This exchange of momentum, dp, must result from collisions of some kind between fundamental particles. Now if the momentum exchange is large enough, it will not only result in a scattering of the initial object, but also the creation of new particles (due to Einstein's energy/mass relation). Now these new particles will take away most of the momentum of the initial interaction, and the harder the impact, the more particles appear, taking away more energy with them.

                            Conversely, if you try to bombard a particle with a large number of slowly moving particles so that each momentum exchange, dp, is too small to create new particles, you run into another kind of upper bound. This time there is a limit to how many particles you can squeeze into a space without creating a blackhole.

                            Therefore, you run into a problem at both ends. There are only so many slow moving particles you can squeeze into a space in order to accelerate on object. And a small number of fast moving particles will create particle-antiparticle pairs which absorb most of the momentum.

                            [–]thoughtso 4 points5 points ago*

                            There is are limits on non-gravitational acceleration for anything built out of smaller parts set by the strength of the binding forces holding the object's parts together. Beyond that, special relativity sets a limit on the non-gravitational acceleration of a composite object in that the object will 'come apart' at any acceleration where the separate parts have to communicate faster-than-light to accelerate as a single whole.

                            That's the practical limit for anything made of smaller parts (including protons and neutrons).

                            So, let's consider the case of the non-gravitational acceleration of something without smaller parts: An electron.

                            To the best of our ability to measure electrons are truely elementary particles with no 'parts' and apparently with 0 dimension.

                            So is there a limit to how much acceleration you can apply to an electron?

                            Ignoring for the moment issues such as the Unruh effect which tend to oppose any acceleration of a particle having a charge, lets just consider the conditions you have to have for a particle to accelerate at all in the first place.

                            First, you have to have a potential difference. In the case of an electron we are talking about an electrical potential. When you establish an electrical potential gradient then electrons will accelerate.

                            At first glance that doesn't seem a problem - just a description.

                            But on a deeper look it is actually the limiting factor. We normally treat the vacuum as literally "nothing". But under the conditions we are talking about that is no longer a good-enough description.

                            That's because the potential gradient polarizes the vacuum. And beyond a certain limit, you actually start pulling particle-antiparticle pairs right out of the vacuum that tend to cancel out your potential gradient. The vacuum 'breaks down'.

                            This limit is about 1.3 * 1016 volts/centimeter. The vacuum will not let you create an electrical potential gradient higher than that. This gives us our acceleration limit: It is the acceleration of an electron (the lighest electrically charged particle) through a potential gradient of about 1.3 * 1016 volts/centimeter.

                            You can't achieve a higher acceleration than that because the vacuum breaks down first.

                            Edit: Some reading on the topic (PDF warning): Electron-positron pairs in physics and astrophysics.

                            [–]light_mnemonic 5 points6 points ago

                            This is actually wrong. Such a potential limit is present in the vacuum, but not in occupied space. There's quite a bit of research currently underway to find plasmas for use in accelerators that would allow a greater potential gradient while minimizing obstruction to the beam. Cool stuff really, would allow old accelerators to be used at new energy levels and would generally cut down on the MASSIVE amount of space they require.

                            [–]thoughtso 4 points5 points ago

                            Reference please?

                            From what I can see plasma accelerators are currently targetting order of 5 * 108 V/cm (42 GeV over 85 cm) - or about 7 orders of magnitude lower than the vacuum break down limit.

                            [–]antonivs 0 points1 point ago

                            Such a potential limit is present in the vacuum, but not in occupied space.

                            Why would that be? I'd love a reference.

                            [–]quantumripple 0 points1 point ago

                            I thought the Unruh effect occured for all bodies regardless of their charge? I suppose you could say that at an acceleration of 1053 m/s2, the Unruh temperature reaches the Planck temperature, but not necessarily a limit... more likely, we just need new theories at this point.

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                              [–]NeedsEvidence 0 points1 point ago

                              No, there is no principle limit. There is no physical law, no naturalistic symmetry principle I am aware of that would limit the change of speed with time (aka acceleration) in the same fashion as the speed of a particle is limited.

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