Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

B Speed of light in the universe

  1. Jun 27, 2016 #1

    wolram

    User Avatar
    Gold Member

    Is it true that the speed of light is only dependent on the size of the universe, if the universe were much smaller would the speed of light be faster?
     
  2. jcsd
  3. Jun 27, 2016 #2
    How speed of light is dependent on size of universe ?
     
  4. Jun 27, 2016 #3

    BiGyElLoWhAt

    User Avatar
    Gold Member

  5. Jun 27, 2016 #4

    PeterDonis

    Staff: Mentor

    Not according to our best current measurements. As far as we can tell, the speed of light is constant and does not depend on the size of the universe.
     
  6. Jun 27, 2016 #5

    PeterDonis

    Staff: Mentor

  7. Jun 27, 2016 #6

    BiGyElLoWhAt

    User Avatar
    Gold Member

    Yea, I just found them, and hadn't heard of them before. OP inspired me to look into it, as I didn't know much about it.
    That is what I'm gathering, however.
     
  8. Jul 1, 2016 #7

    Drakkith

    User Avatar
    Staff Emeritus
    Science Advisor

    Where did you read this?
     
  9. Jul 2, 2016 #8

    Chronos

    User Avatar
    Science Advisor
    Gold Member

    Ranks right up there with the green cheese moon theory.
     
  10. Jul 3, 2016 #9
    This would mean that special and general relativity are based on a false assumption, (that 'c' is constant)
    So somebody would need to propose an alternative to those theories which lead to the same predictions and explanations.
    It won't be me.
     
    Last edited: Jul 3, 2016
  11. Jul 3, 2016 #10

    Chronos

    User Avatar
    Science Advisor
    Gold Member

    The basis for your premise is unclear.
     
  12. Jul 3, 2016 #11
    This particular theory was debunked by Bill Anders.
    "Is the Moon made out of green cheese?
    No, it's American cheese".

    -Bills Anders, Apollo 8 Commander,

    Two lines from the Wiki article.
    "refers to a family of hypotheses"
    "many of them non-mainstream."
     
  13. Jul 3, 2016 #12
    Surely the cheese could be both American and green without violating any known laws?
     
  14. Jul 3, 2016 #13
    Has the wavefunction collapsed yet? o_O
     
  15. Jul 3, 2016 #14
    It will in about 10 minutes, my girl friend has advised me so. Goodnight.
     
  16. Jul 4, 2016 #15

    BiGyElLoWhAt

    User Avatar
    Gold Member

    I don't know if that would actually violate sr, or gr for that matter. Sure, there are based on the constancy of light, however, in the sense that any observer would measure the same speed of light no matter their direction or speed of motion. If the value of c changed, but was the same throughout the universe over small ranges of time, meaning any experiment I could do would return 'c' as the speed of light, we would end up with the same lorentz transformation and same consequences, just with a different value for gamma.
     
  17. Jul 4, 2016 #16

    PeterDonis

    Staff: Mentor

    This doesn't make sense as you state it, because the "consequences" of a Lorentz transformation depend on the value of ##\gamma##. Different ##\gamma##, different consequences.

    The real issue here is that ##c## is not a fundamental dimensionless physical constant; you can change its value by changing your choice of units. What we really need to look for is evidence that fundamental dimensionless physical constants, whose values are independent of the choice of units, have changed. The one most closely related to ##c## is the fine structure constant ##\alpha##. So far we have found no evidence that ##\alpha## has changed over the life of the universe (or any other fundamental dimensionless constant).
     
  18. Jul 4, 2016 #17

    BiGyElLoWhAt

    User Avatar
    Gold Member

    I didn't mean the consequences of the lorentz transformation, I meant the consequences of SR, which is (IMO) slightly different. The consequences of SR are the lorentz transformations, which warrant that t/t' \neq 1. The same with l/l', assuming a non zero velocity between frames. If c changes, then so will the numerical value of gamma, but time dilation between frames is still a thing. That is what I meant.

    Interestingly, though, is that this implies a "god frame" so to speak. An "absolutely stationary" frame in which the size of the universe is measured in order to determine c.
     
  19. Jul 4, 2016 #18

    PeterDonis

    Staff: Mentor

    Unless you have an actual acceptable reference describing a theory that shows this, I would be very careful making such claims. Certainly standard SR (or GR) is not such a theory.

    Same comment as above. Please bear in mind the PF rules about personal theories.

    Also, you don't seem to have grasped the other point I made in my previous post, regarding which constants are actually "fundamental".
     
  20. Jul 5, 2016 #19

    BiGyElLoWhAt

    User Avatar
    Gold Member

    I'm not trying to make my own theory, I linked to "variable speed of light theories" previously, and that is what this thread is about, no?
    SR assumes that all observers (at least the ones that are capable of observing each other, all of the derivations I have seen utilize this concept) that measure "c" will agree on it's value. Generally, it assumes that the experiments are "close togwther in time". I.e. in my frame, they are happening simultaneously. Again, all of the derivations that I have seen are setup as such. Despite the value of c, you will end up with the same transformations. If the changea were small enough for some mystical c (t), then we would never even notice it.
    In order for c to be a function of the size of the universe, and for all observers to agree on c, then either a) all observers would have to agree on the size of the universe, or b) there would have to be an absolute size to the universe. Is there a 3rd option that can keep these consistent that I'm overlooking? I'm not advocating for the theories, I just find them interesting as they seem to shake things up, but they would still have to be consistent with observations.
     
  21. Jul 5, 2016 #20

    PeterDonis

    Staff: Mentor

    The Wikipedia article you linked to earlier talks about speculative models predicting a variable speed of light (more precisely a variable fine structure constant). But we don't know if those are what the OP was asking about; he hasn't said.

    But more importantly, the speculations you are making have nothing to do, as far as I can see, with the speculations referred to in the Wikipedia article. See below.

    You're not even talking about them, as far as I can see. You're just waving your hands about what you think some imaginary theory in which ##c## varies might look like. What you should be doing, if you're really interested in the (speculative) theories along these lines already advanced, is to read the actual papers proposing them and see what they say, and then base your discussion on that (with references).
     
  22. Jul 5, 2016 #21

    Drakkith

    User Avatar
    Staff Emeritus
    Science Advisor

    Reminds me of a "game" some college students made where the value of c was much, much smaller. Something like 10 mph or something. The player controlled a character who had to walk/run around and perform some (usually) trivial tasks. But with the reduced speed of light, relativistic effects were very noticeable, making these trivial tasks not so trivial anymore.
     
  23. Jul 5, 2016 #22

    BiGyElLoWhAt

    User Avatar
    Gold Member

    You're right, I just assumed and since I wasn't ever corrected, assumed that to be the case.
    I suppose it is a little bit out there. I'm assuming that the observational consequences of SR still need to be intact. In the ways that I know how to derive the Lorentz transformations via light clocks and the like, at any value of t in some frame, if an observer measures c in that frame and then in another, nearly-simultaneously for the observer in question, they will notice that time and length are both distorted. The postulates of SR, in fact, do assume that "c is constant". However, in practice, it doesn't seem to be utilized as strongly. "c is constant" means, to me, constant through both space and time. It seems to be used meaning constant through space. This may or may not be the case with GR. I'm not very far into GR as of right now.
    I might be spouting off again. I'll go look through some papers that I find and see what they say.
     
  24. Jul 5, 2016 #23

    PeterDonis

    Staff: Mentor

    Yes, you are. Please look at the actual papers.
     
  25. Jul 5, 2016 #24

    BiGyElLoWhAt

    User Avatar
    Gold Member

    In progress...
     
  26. Jul 5, 2016 #25

    BiGyElLoWhAt

    User Avatar
    Gold Member

    https://arxiv.org/abs/astro-ph/0305457
    Here, this isn't necessarily about a specific theory, at least not yet. Just some general things about VSL theories, and apparently later he analyzes some of them.
    He seems to be making comments about alpha similar to what you were doing. However, his argument which resembles what I interpreted you to be saying earlier, "A varying speed of light = a varying set of units" doesn't seem to hold much water. He says
    ...
    However, in his example, the meter was defined in terms of c. Of course c is constant if c is defined as a function of c!
    If we define the meter in terms of c, so ##1 \text{meter} = \frac{1}{299,***,***}\times c \times N_{\text{Cs}=1s} \times ( \text{duration of one excitation of cs} )##
    What do you end up with? ##1=1## ? These quantities are linearly dependent on each other (the meter and c), and you will always get a constant c if you define the meter in terms of c. However, if you were to go old school, grab a meter stick, and define a meter to be that long, would you experience the same thing? Now we have value, c, which is linearly independent of both the definition of time (using current SI), and the definition of length. Does the same argument hold?
    My hunch says no, but I might be missing something. I will continue to go through this article, but 2 sections later, he still hasn't clarified this.
     
    Last edited: Jul 5, 2016
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted