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I Why was the speed of light even considered to be infinite?

  1. Jul 16, 2017 #1
    Wouldn't the first thought be that we would be burned to a crisp if we where hit with all the light(in our path) across the span of the entire universe simultaneously? It seems there could be paradoxes even before the advent of relativity.

    Or maybe it was shown mathematically that the energy that reaches us is finite using arguments with falling off of heat across greater distances from the source.( i.e inverse proportionality to surface area.)
     
  2. jcsd
  3. Jul 16, 2017 #2
    That's the Olbers' paradox.
     
  4. Jul 16, 2017 #3

    CWatters

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    I don't understand what you mean by "all the light (in our path)" but..

    Lets say photons of light leave stars at a certain rate. Changing the speed of light would change how long it takes for them to get to us not the rate at which they leave/arrive.

    The speed of light was proved to be finite in 1676.
    https://en.wikipedia.org/wiki/Rømer's_determination_of_the_speed_of_light

    How much was known about stars back then? In the early 1700's Herschel thought that the sun was cool and habitable.
     
  5. Jul 16, 2017 #4

    CWatters

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    That was 100 years after the speed of light was shown to be finite.
     
  6. Jul 16, 2017 #5
    That doesn't matter because the paradox is not limited to infinite speed of light.
     
  7. Jul 16, 2017 #6

    CWatters

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    Agreed. All I meant was Olbers' paradox couldn't be "the first thought" mentioned by the OP because it wouldn't even be proposed for 100 years after the speed of light was known.
     
  8. Jul 16, 2017 #7
    If Olbers' paradox would have been proposed by somebody else 100 years before, nothing would change (except that it would be called somebody else's paradox). A finit speed of light still wouldn't solve it.
     
  9. Jul 16, 2017 #8
    "In the path", I'm referring to if light were just shooting out of a star, a bunch of it wouldn't reach us. For example, the light from the other side of the sun doesn't reach us.

    But if you imagine the universe as being very large, composed of a lattice of grid points of a bunch of stars,(each grid in space could contain a star) it is conceivable that even if there is a finite rate that every star produces light at, the stars could be arranged so that an incredible about of light would pile up upon us. For example, star1 emits at rate 1. Star 2 emits at rate 2, just slightly off of rate1(slightly out of sync), star 3, rate3.... and so on. Further, assume that star 1, star2, ...star n.. are sequentially further away from earth monotonically.

    We can see that much more light would hit us, with much more intensity. If rate1, rate2, rate3.... are extremely close such that it is almost continuous, then there could be an indefintely large(depending on arrangement of the lattice of stars) amount of light that converges in on us at an instant.
     
  10. Jul 16, 2017 #9
    That would also happen with a limited speed of light.
     
  11. Jul 16, 2017 #10

    Vanadium 50

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    One car per day leaves Boston for Providence at 30 mph. How many cars per day arrive in Providence?
    One car per day leaves Boston for Providence at 45 mph. How many cars per day arrive in Providence?
    One car per day leaves Boston for Providence at 60 mph. How many cars per day arrive in Providence?
     
  12. Jul 16, 2017 #11
    I see your point. Perhaps, I should have included red shifting due to expansion of space.

    If the universe is infinite, then it's possible that we can burn up even if the speed of light is finite, since light from infinitely far way could hit us. But dark energy saves us.
     
  13. Jul 16, 2017 #12
    Nobody was thinking about an expanding universe when the speed of light has been considered to be infinite.

    A better question would be why the universe has been considered to be static.
     
  14. Jul 16, 2017 #13

    How about an infinite amount of cars leave for Providence from every direction. With each direction of approach coorespponding to a to a rate that is unsynchronized, not necessarily different. How many cars would arrive in Providence? Infinite.
     
  15. Jul 16, 2017 #14
    Yeah that is true, thinking about an expanding universe may be too premature at that point.

    Well, thinking again about the issue, I think I meant to say that the rates are unsynchronized, but doesn't have to be different, though it could be.

    Surround a point in space with an imaginary 2d sphere. I

    f every point on the surface shines inwards towards the center at different times, then there would be a continuous beam hitting the center since every moment in time corresponds to a light particle launched towards the center under two conditions. 1)Infinite universe in age and size. 2) Infinite speed of light with a very large universe.

    1)the universe is infinite in age and size, assuming stars always existed, then every point on that sphere would truly map towards the center, even if light was finite in speed because just by chance out of infinite possibilities of arraignment of infinite stars.

    2) But if light is infinite in speed, then I don't think an infinitely old universe is required, just one infinite in size or very large, then the same would happen. All points on that 2d sphere would converge towards the center. In fact, if the universe here is infinite in size, then since light here is also infinite in speed, then you would have infinite overlapping particles of light hitting the center of the sphere at the same time.

    In modern parlance, this would reduce everything to a kugelblitz blackhole, leaving the universe in a state of maximum and constant entropy, which we know is not true.

    But back then, they should still suspect that points in space everywhere would burn up, since light is bright and in general, bright things have heat, fire is hot, sun is hot, molten metal shines and is hot etc, so it is not unreasonable to assume that light itself could be associated with thermal properties.
     
  16. Jul 16, 2017 #15

    russ_watters

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    That isn't how light works though, regardless of its speed.
     
  17. Jul 16, 2017 #16
    And why should that rule out the infinite speed of light and not the infinite homogeneous distribution of stars?
     
  18. Jul 16, 2017 #17
    My argument is combinatoric. Not about the actual process of generating light per se.

    If there are a large enough amount of stars, many of those stars shine 360 all around. Then some is aimed at us. So if the universe is infinite, which Newton thought at the time, then I could argue that there are infinite stars, each one shining in every directions. Therefore, each one is aiming light directly at us. And infinite universe means an infinite amount of stars that have no obstacles in getting their light towards us. Of course, this also means that there are infinite stars that could be blocked from getting their light to us. But in an infinite universe proposed by Newton, who cares?
     
  19. Jul 16, 2017 #18

    russ_watters

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    But you have made an assumption that light is emitted at an infinite rate. You appear to be confusing that with emitting light at an infinite speed. You seem to have one question and one error.

    Just to be clear; even if it were right, Olber's paradox does not result in an infinitely bright universe.
     
  20. Jul 16, 2017 #19
    An infinite homogenous distribution of stars everywhere is more unlikely. They can use astronomical observation to note that the stars are not homogenous, which implies the existence of possible local deviations from homogeneity. Moreover, even if on average the the stars are homogenous, just by having an infinite amount of them, there is a finite probability of some configuration at some distance where nearly and infinite amount of light just hits us at once due to their infinite speed.

    You could have 100 trillion stars shining upon us such that they are timed just right to hit the earth all at once. And these stars can be googles of light years far from each other, it doesn't matter since the universe is infinite. So by combinatorics, infinite space means any configuration could happen, no matter how improbable. This is assuming that the universe is on average, homogenous in stars.
     
  21. Jul 16, 2017 #20
    No, I have assumed each star shines light at a finite rate of their own. But with infinite stars, each one is shining a light particle at different moments(slightly different or just completely different, all possibilities happen due to the infinitude of stars) then every, moment in time, a light particle is emitted somewhere. Assuming that each star has their own rate that is slightly different. Then yes, the such a universe would have an infinite rate when considering all the stars in total, which is infinite amount of stars, according to the physics of the time.
     
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