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Why cant forces travel faster than light speed?

  1. Jan 20, 2007 #1
    Why cant "forces" travel faster than light speed?

    As always, I'll start with a disclaimer that I know next to nothing about physics, just that I have an interest in the subject (actually, relativity kinda scares the hell out of me for some odd reason).

    Anyways, Im reading through this book called "The Elegant Universe" and the author is talking about the dilemma between relativity and Newtonian gravity. As I understand the concept, under the principles of relativity, nothing can travel faster than the speed of light as an objects mass will increase and meet up with an infinite resistance.

    However, why should this hold true for "forces" (i..e gravity, magnetism)? Is it because these forces are comprised of mass?

    Just wondering. Thanks.
  2. jcsd
  3. Jan 20, 2007 #2


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    According to relativity no information can be sent faster than the speed of light. this means that i can't influence anything that's far away from me faster than light could - if forces "traveled" faster than the speed of light then i could transfer information at that speed to.
    Also, in QM forces are caused by force particles (photons, mesons..) and those particles can't go faster than light either.

    Edited by HallsofIvy. Daniel, use or , not [Bold].
    Last edited by a moderator: Jan 20, 2007
  4. Jan 20, 2007 #3
    Relativity says that what I call "faster than light", you may call "backwards in time", and you mightn't like it if I can start causing effects to your past.
  5. Jan 20, 2007 #4

    Chris Hillman

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    Danger Will Robinson!

    Hi, Ulnarian,

    I trust I have not previously encountered you in any forum, but please be aware that I (and others) have learned the hard way to be wary of being "ambushed" in fora like PF by persons using "anonymous handles". So please forgive me if I start off with an act of self-defense.

    In view of other current threads on this board, I feel that I need to reiterate a ground rule for my own participation in PF threads: I am not going to discuss a certain crank http://www.math.ucr.edu/home/baez/RelWWW/wrong.html#speed [Broken] here at PF, nor am I willing to "discuss" physics with him or his supporters at PF (or elsewhere). It should not be neccessary for me to explain why not, and I will not attempt to do so. Similar remarks apply to other cranks and "fringe physics" figures.

    [EDIT: regarding the previous two paragraphs: Ulnarian, I am struggling to concoct some kind of self-defensive boilerplate, and it seems that further tweaking will be needed. Just to be clear I have seen no evidence that you are "socking" for someone I have previously encountered, I am just trying to minimize the chances of that happening.]

    OK, with that said, let's proceed:

    Well, this is helpful to know in terms of framing an answer. I trust you agree that it follows that you probably cannot expect to attain a very deep understanding of these issues; you might have to be satisfied with simply hearing an unfamiliar way of trying to describe in natural language concepts which can be properly stated only in mathematical language. This inevitably involves considerable loss in even the best of translations.

    It would be nice to mention his name; fortunately, I happen to know that the author you have in mind is Brian Greene (Physics, Columbia).

    I have not read this book myself, but it has been well reviewed. As a general rule I deprecate popular science books because I find they tend to confuse more than they enlighten; however in recent years a number of physicists (e.g. Greene, Lisa Randall) have written popular books, and from past experience (e.g. Geroch, Wald, Weinberg) popular books by leading experts have a much better chance of conveying some meaning, so perhaps the trend is toward better books for the public. I hope so!

    Since I haven't read the book, I don't know whether your description accurately reflects what he really said--- I hope not, because I don't like the sound of that one bit! (no pun intended)

    Be this as it may, as others already told you, you should replace "nothing" with "energy/information" and "mass" with "kinetic energy" and you should line out "and meet up with an infinite resistance" and replace it with "and ultimately tend to infinity as the velocity of the object, measured over an infinitesimal region, increases toward the speed of light". Even this verbal formulation is not quite right, but it is much less misleading.

    I would hope that one point you would take away from Greene's book is that physicists have experienced enormous difficulty in unifying quantum physics with relativistic classical field theories of gravitation such as general relativity; some proponents of string theory do claim that this subject provides a quantum theory of gravity but most (including, I believe, Greene) would say that string theory may turn out to provide a workable quantum theory of gravity, but we aren't there it yet.

    At any rate (npi), I think you have a much better chance of getting a not terribly misleading answer to your question if you place it in the context of classical field theories such as Maxwell's theory of electromagnetism or Einstein's theory of gravitation (aka his general theory of relativity, or gtr for short). In such theories, a "force" can act on a bit of matter, and if so it has the effect of changing the momentum and energy of that matter, but force is not something which is transmitted in the sense you probably intend.

    Rather, momentum and energy are physical quantities which can be transmitted by various means, including radiation, such as electromagnetic radiation (radio waves, light waves) and gravitational radiation. For example, in Maxwell's theory of electromagnetism, it turns out that the electromagnetic field itself possesses energy and momentum, and an electromagnetic wave can transfer energy and momentum from one system, from which radiation is being emitted (roughly speaking, as the result of charged matter "wiggling" inside that system, due to some physical process), to another, where it might "wiggle" some charged matter--- this changes the energy and momentum of that charged matter, and it also produces some new radiation. This is essentially what happens in the interaction between a radio transmitter and your car radio, for example.

    Something similar holds for gtr and gravitational radiation, except that in this case the stuff which "wiggles" can be any form of mass-energy (and there are some important differences between gravitational and EM radiation--- in the context of Brian Greene's book we could say this has to do with the difference between "spin one" and "spin two" interactions, but this distinction doesn't affect the issue of the characteristic speed with which radiation propagates in a vacuum region).

    The point is that what we call "the speed of light" is really a sort of universal characteristic speed in physics, and (roughly speaking ) all kinds of fundamental "massless" radiation (including EM radiation and gravitational radiation) will, according to physical theories like the two mentioned above, travel at this characteristic speed through a "vacuum region" (one not filled with matter, which can slow down such radiation).

    Indeed, as others already told you, it turns out that roughly speaking, as a matter of fundamental principle, no form of energy or information can be transmitted faster than the speed of light (as measured in an infinitesimal region of spacetime). One way to think about this is that, as I tried to suggest above, in a local relativistic classical field theory, radiation is the fundamental means by which changes in the field near one event are transmitted to other events. We might say that radiation carries "news" or "field updating instructions", and this news cannot be transmitted faster than the speed of light. But when there is news and this is transmitted by radiation, it can have physical effects because radiation carries energy and momentum and thus can transfer energy and momentum from one place to another.

    An additional wrinkle here is that it turns out that "distance" and "speed" are not well defined without substantial qualifications for accelerating observers or in curved spacetimes, except over infinitesimal regions of spacetime. One can certainly define relative velocities and distances over larger regions, but there will be multiple distinct possible definitions and you must specify one of these alternatives.

    Another wrinkle is that it gets trickier to formulate a "speed limit" over larger regions. Roughly speaking, this can however be done and then what I said above does hold in various technical senses in general. Let's state it informally like this: according to physically reasonable notions of "distance in the large", "velocity in the large", "physical field", "energy", and so on, even in curved spacetimes it should not be possible to transmit energy or information ("news") faster than "lightlike signals" (or at least, not faster than all lightlike signals--- in curved spacetimes there can be multiple paths taken from one place to another by such signals). (More advanced readers can see papers like http://www.arxiv.org/abs/gr-qc/9812067 to get some idea of what I have in mind.)

    Lastly, if you are asking "what is radiation?", very roughly speaking, in this context, in a local field theory, you can think of radiation as that part of the field which can vary most rapidly and which transmits "field updating information". In a sense, the cumulative effect of lots of past events (in particular, the gradual concentration of charge in some compact region) can over time build up further contributions to the field, such as "Coulomb components", which vary much more slowly. See "How does Gravity Escape from a Black Hole?" at http://www.math.ucr.edu/home/baez/RelWWW/group.html [Broken] Roughly speaking, under some circumstances, you can think of the slowly varying "Coulomb part" of the field (in either Maxwell's theory of EM or in gtr) as representing the "nonrelativistic part" which behaves approximately in Newtonian fashion, while the rapidly varying "radiative part" of the field represents "relativistic corrections". The crank I hope no-one plans to mention here is, in essence, confusing radiative and nonradiative parts of the field (more advanced readers can see Feynman's Lecture on Physics, vol. II, for the EM field, or http://www.math.ucr.edu/home/baez/PUB/debate [Broken], for the gravitational field treated according to gtr).

    If your real interest is in the propagation of energy/information in highly curved spacetimes such near black holes, I recommend the books by Geroch or Wald cited http://www.math.ucr.edu/home/baez/RelWWW/reading.html#pop; [Broken] if you are interested in cosmology, I'd recommend the book by Weinberg listed there. But if you want to go much further into these matters, you will really need to study math and physics, I think. (The same page contains some recommendations for those wishing to delve deeper.)
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  6. Jan 20, 2007 #5

    Gib Z

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    Such a small post Chris :p Not very exhaustive either :D
  7. Jan 21, 2007 #6
    There is a difference between continuous effects such as the force between two magnets - or electrons - or masses - which doesn't readily get explained by wave propagation. In the latter, something wiggles at one place and later that wiggle is detected elsewhere - most folks are convinced that such phenomena (whether it be electromagnetic or gravitational wave) travels at c where c is a universal constant. But in the case of a static situation that involves for example the repulsion of two electrons spaced apart by a distance d, we don't really have a perfect analogy to something wiggling, and while some theories seek to explain static fields as being consequent to traveling particles such as virtual photons or gravitons, these may turn out to be more metaphorical than real. Physicists of the main stream typically fall back upon Einstein's postulate that whatever the mechanism that is responsible for static forces, it cannot convey information at a velocity greater than c.

    As far as an introductory treatise, I like Ed Harrison's book "Cosmology, the Science of the Universe" Very good explanations and not difficult math.
    Last edited: Jan 21, 2007
  8. Jan 21, 2007 #7
    I noticed that you are not in favor of popular scientific books. Let me put it this way: if the real scientists do not bother to explain and educate the public and laymen, someone else would. When that happens, your career will end.
  9. Jan 21, 2007 #8


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    quantum123, it might be good to review this excerpt:

    IMHO, I agree that "popular books by leading experts have a much better chance of conveying some meaning". (I'll admit a bias: Some of these experts have been my professors.)
  10. Jan 22, 2007 #9

    Thanks for all of the answers, sorry for the delay. I've been very busy lately with things and I havent really had time to sort through the answers to give them the analysis they deserve.

    Starting now....:)
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