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Why doesn't light go faster than c?

  1. Dec 22, 2011 #1
    it's my first post in this forum so I hope I'm in the right section. I've asked three different physics teachers in my high school an explanation for this to no avail. if acceleration is inversely proportional to mass, and photons have no mass, why don't they have infinite acceleration? in other words, what poses a boundary to the speed of light?
  2. jcsd
  3. Dec 22, 2011 #2


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    You're assuming [itex] F = ma[/itex] is applicable to massless particles, which it is not. The quick explanation as to why light has a fixed speed is basically that the laws which govern electricity and magnetism, Maxwell's Equations, show that electromagnetic waves travel at a fixed speed in vacuum. A better explanation would require knowing your level of mathematical understanding.
  4. Dec 23, 2011 #3
    In fact it was thought that light travelled at infinite speed once and then came Maxwell's equations as just mentioned, then Einstein pondered what this meant and consequently upset a lot of people.
  5. Dec 23, 2011 #4
    @jamietmoore, as said in #2 by Penguwino, to understand the border imposed on a ray of light, a person must acquire the discipline of math first, then understanding comes after.
  6. Dec 23, 2011 #5
    I couldn't disagree more with that. Seems like complete nonsense.

    In no way does someone have to know, or use math, to understand the limit of c.

    even the simple equation p=mv has quite a depth to it.

    It is the concepts behind the p, m and v that is equal to "understand" math.

    From the perspective of describing a limit like c using math is merely the language, an extremly well defined one. If you don't understand the definitions (i.e. p, m, v) then you don't understand why c is the limit.

    Other wise e=mc2 is an answer to the question. Or rearranged so c= whatever, I don't know how to rearrange equations.

    An answer that maybe as vague as an equation is to the poster. It's because the axis of dimensions can't cross each other, that's just the way taking measurements works.
    Last edited: Dec 23, 2011
  7. Dec 23, 2011 #6
    one answer: nobody really knows.

    But, in a way they do have "infinite acceleration": immediately upon production, say as a result of an electron energy level transition in an atom for example, all photons are measured at "c"....[they are emitted with different energies (frequencies) but not different speeds.]

    not that I'd describe it that way: a better explanation, from quantum mechanics, would note that a photon is not a "particle" in the classic sense, it's a wave quanta...and such electromagnetic waves travel at "c" and "only c"....

    So you are really asking about the behavior of electromagnetic waves: why are they always observed at "c"? So another view would be that "Maxwell's equations describe electromagnetic radiation and they show a fixed speed of light", not acceleration from one speed to another. [Not that this mathematics describes photons explicitly.]

    Another perspective would be that I don't think anyone understands exactly why an energy level transition results in the electromagnetic form of energy transfer,,,like many things in physics, it's what we observe, what we explain, and we are stuck with it. The electromagnetic wave is one form of energy carrier [field] in this universe, along with the strong, weak, and gravitational 'forces'. As to WHY any of those or all of those are observed, I'm not yet sure anyone has a concrete answer.
  8. Dec 23, 2011 #7


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    As photons are always going at the same speed then is there any way that they could 'accelerate'? When they are created, they are instantly at speed c.
    And before you start talking about photons being slowed down within matter, afaik, this can be 'explained' in terms of photons moving in small steps at c within the matter and pausing between their journeys.
  9. Dec 23, 2011 #8
    If you're still around, how about this:

    Your question is based on F=ma, and it doesn't apply. If you want to accelerate a car or an atom you have to push on something. The force has to have something to to push against. There is no "something" there in the sense that the formula applies to only things with mass. Bottom line F=ma doesn't apply to electromagnetic fields which is what light is.

    It usually helps to reduce concepts and thinking to basics.

    Now, why do electromagnetic waves travel at the velocity C, no one knows.

  10. Dec 23, 2011 #9


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    To take the OP's question literally, you actually get the right answer.

    Massless particles do have infinite acceleration. When a photon is emitted, it immediately has a velocity of c.

    But that's a pretty tortuous and misleading path.

    And it has nothing to do with why they don't have infinite velocity.
  11. Dec 23, 2011 #10
    I have a hard time accepting the no one knows why c is the limit.

    How is something as simple as, "because that is what is observed" not acceptable?

    I think an acceptable answer would start with dimensions being at right angles to each other, throw in the concept of relative motion, SR postulates and observations and I'd guess that's well on the way to explaining why c is the "limit", or as I read in another thread, specifically why c is invariant (the speed is arbitrary, I think the OP is fundamentaly questioning why c is finite)

    I hope to someday understand SR well enough to actualy provide an answer along those lines lol :shy:, diagrams diagrams diagrams :smile:

    I noticed the infinite acceleration thing to, but refrained from pointing it because I don't think you can consider acceleration to be "infinite", particularly in this case because it implies that the photon had accelerated. Which I dont think they do.
    Last edited: Dec 23, 2011
  12. Dec 23, 2011 #11
    The question was posed, ...if acceleration is inversely proportional to mass...? To my limited knowledge there is no formula for the acceleration of EM waves.

    Last edited: Dec 23, 2011
  13. Dec 23, 2011 #12
    Why? There are many things, in a similar vein that no one knows and probably never will. I dare say all the universal constants are like this.
  14. Dec 23, 2011 #13
    I think I get it. But then, is light a wave or a particle, or both?
  15. Dec 23, 2011 #14
    Either or neither depending on your point of view. It is a quantum object, or a 'wavicle' if you like. However if your question relates to its behaviour, then it behaves according to what the experiment is attempting to detect.

    I was listening to one of Feynman's university of Auckland lectures last night and he said that light always comes in lumps. It's a good habit to try and remember all the time that when you are visualising any quantum object, it's only for the purposes of understanding some other characteristic of its behaviour, otherwise it's best to take Heisenberg's advice and not even attempt to visualise it at all.

    The reason is that you necessarily only have experience visualising classical objects. Similarly relativity theory is only strange because you have no instinctive understanding or experience of moving at very high velocities.
  16. Dec 23, 2011 #15
    It's a touchy subject, but it's really just a matter of semantics; either word, in this case, describes the same behavior. QED(Quantum Electro-Dynamics) and SR(Special Relativity) are weird almost beyond comprehension, but if you really want to understand light, that's where you'll have to look.

    To give you something of an answer to why light travels at c, you can think of it like it is the fundamental maximum velocity at which space-time can be traversed. I could throw a dozen formulas at you that show how nothing can ever be observed traveling faster than c, because any massive particles will asymptotically approach it, and mass-less particles simply propagate at it(c). I'm not sure there's really an answer out there that will satisfy you; science isn't really in the buisness of providing those. Science is great for telling you how things work; the why is slightly beyond it's scope.
  17. Dec 23, 2011 #16
    Dare I say all universal constants are well defined, logically and mathematically.
  18. Dec 23, 2011 #17
    But I am referring to the 'why', and no one knows, why.
  19. Dec 23, 2011 #18


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    Except special relativity is based on the postulate of the speed of light being constant. The speed of light being constant is completely an observation of nature. Relativity and spacetime is built up on the idea that the speed of light (technically, any massless particle) is constant. Without a kind of universal speed limit, you have no special relativity nor do you have spacetime in its current form. It is totally an observation (in the sense that Maxwell's Equations were simply observed to be true).

    In QED, and quantum mechanics in general, forces make little sense and holding onto them as a tool to explain things like photons and light will cause far more problems than solve.

    It has properties of both. Again, holding onto the idea that something must be either/or will get you into trouble. A lot of modern physics is simply accepting that the world isn't confined to the ideas present in Newtonian physics.

    What does this mean? Most universal constants (with a few exceptions such as the fine structure constant) are just man-made numbers. Hell, knowing mathematicians, they'd be happy to set everything equal to 1 and to hell with what reality says :cool: Not to worry, we don't let mathematicians handle the dangerous toys, thankfully.
  20. Dec 23, 2011 #19
    With your first comment, I'm not sure how your point of view is different from mine. My point is; why isn't the answer "Because it's what is observed." not satisfying enough. I go on to describe what I think would be another acceptable answer (which is fundamentaly exactly the same as the first, just more complicated/descriptive).

    My second comment is similar to "Relativity and spacetime is built up on the idea that the speed of light (technically, any massless particle) is constant.". Yes, the logic of SR is built up on the postulates. Those postulates are well defined mathematically by the theory itself. In addition, my comment was worded specifically to be similar to what Yummyfur used (for reader specific context :) I only know the constant c.

    I don't know what this means. -> "Most universal constants (with a few exceptions such as the fine structure constant)? are just man-made numbers?."

    Re-reading the first comment, It might be that your point is the constancy of c is merely observed (measured + calculated). The postulate addresses this. What ever questions come before this postulate maybe for a different forum, idk.

    by the way congratulations on the '11 Best Humor award!
    Last edited: Dec 23, 2011
  21. Dec 23, 2011 #20


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    You're trying to suggest the use of SR to explain why the speed of light is constant when SR exists because the speed of light is constant. In fact, as you stated, saying it is what we observe is pretty much the answer.

    As far as constants, yes, they're all just numbers we use as humans. Hell, HEP guys love setting all the constants to 1 which shows that these constants, while they still have physical meaning (although some would argue different), are things we use to simply keep our system of units in place.
  22. Dec 23, 2011 #21
    there's my new signature
  23. Dec 23, 2011 #22
    I can't reason what you say.

    What we observe is the answer on one hand, well on the other what we observe is described with SR. So yes I am trying to suggest SR explains why the speed of light is constant.
    Last edited: Dec 23, 2011
  24. Dec 23, 2011 #23
    Oh, I understood it as the units (here i guess seconds and meters) are already "in place" (note we're talking dimensions here). from there the math is derived from quantitative measurements. 1 is nice and all, but saying time is equivalent to length is way cooler. Ha there is another answer to the OP.
    Last edited: Dec 23, 2011
  25. Dec 23, 2011 #24
    Particle or wave? It seems to me that what one finds overall is, when you impede a "photon" of light, such as with a detector screen or tube, it appears like a particle. When unmolested it behaves like a wave. The connection appears to be when the radiation's energy is absorbed.

    The name photon bothers me some because it conveys a sense of a little sphere that would travel, for example, from a star through space to the earth, but it isn't and doesn't. In transit light is a wave, but the waves CAN be divided into "chunks," for example by a single-photon light source.

    When the "wave packet" hits something it behaves like a unit of energy and is therefore particle-like, even though the analogy is inadequate. The amount of energy of each "packet" depends on the wavelength/frequency of the light.

    The deeper one digs into this, the more interesting it gets.

  26. Dec 24, 2011 #25
    Aren't you making the same basic error in your description that you are trying to eliminate?
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