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Hawking's speed of light

  1. Aug 6, 2013 #1
    In Stephane Hawking's book-A Brief History of Time-Stephane says that in the past they used to believe thatthe speed of light is constant only relative to ether and it's variable acc. to the speed of the observer , but now acc. to Enistein: he says that the speed of light is constant whatever the speed of the observer was.
    So, why the speed is constant whatever the speed of the observer was?
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  3. Aug 6, 2013 #2


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    A more precise way of stating this is that in the past they used to believe that the speed of light is constant only relative to the Inertial Reference Frame (IRF) in which the ether was at rest and that all of us observers would be subject to Time Dilation and Length Contraction (by some unknown amount) but Einstein turned that around and said that we could pick any IRF moving with respect to the supposed ether and consider any observer at rest in the ether to be subject to Time Dilation and Length Contraction. Since no one can tell which IRF the ether is at rest in, Einstein's idea was so much simpler.
  4. Aug 6, 2013 #3


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    I don't know who this Stephane Hawking fellow is, but Stephen Hawking is a pretty good theoretical physicist, much like Sheldon Cooper.
  5. Aug 6, 2013 #4


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    Stephane Hawking is Stephen Hawking's beautiful twin sister- another similarity to Sheldon Cooper!
  6. Aug 6, 2013 #5
    That's a postulate of relativity. As for the mechanism it's described by Lorentz transformation equations which tell us how lorentz contraction and time dilation occurs.
    Last edited: Aug 6, 2013
  7. Aug 6, 2013 #6
    I believe that the Michelson-Morley
    Experiment tried to do that and failed phenomenally . Though it did verify lorentz contraction.
    Last edited: Aug 7, 2013
  8. Aug 6, 2013 #7


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    WHY is not really a question physics is very good at. When you answer one WHY question, you are usually just left with another why question and it goes on and on.
  9. Aug 6, 2013 #8
    And when you don't answer the questions or even bother asking one you are left in stone age....

    And if physics is not good enough it just means that we need to make it better. ..
    I repeat keep asking.
  10. Aug 6, 2013 #9
    If I didn't use WHY so what I should use.By the way, Newton used WHY when "the apple fell down" and then his famous theory about gravity was formed.
    I don't think that there is science without WHY.
  11. Aug 6, 2013 #10
    Newton actually never explained why the apple fell down. Why the apple fell down is actually an unsolved problem, and I don't think we'll ever be able to solve it.

    Of course, Newton invented the concept of gravity. But that's just a name for the phenomenon, it doesn't answer why.


    Newton derives some formulas and proved them to quantify gravity. In particular, he showed the famous formula ##F = G\frac{m_1m_2}{r^2}##, which was a genius thing to do. But it doesn't answer why.

    See this:

  12. Aug 6, 2013 #11

    Simon Bridge

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    The objection is not about the word but about a style of question.
    "Why" is a kind of question and leads to answers in terms of some sort of reason ("why did you hit your sister?") or something like "God did it". Seldom helpful.
    i.e. the speed of light is constant to all observers because none of us deserve to see anything else. See?

    You will hear "why" being used as a shorthand where another kind of question is actually intended ... usually it works out fine, like if you ask "why is the night sky dark" you are expecting a certain sort of reply ... in terms of mechanisms and what it tells us about the structure that gives us the night sky we observe.
    That is actually a "how" question... the listener understands this.

    If you ask: "how is the speed of light the same for all observers?" you get an answer in terms of the mathematical transformation that make this work.
    I suspect that this is not the sort of reply you are after.

    "why is the speed of light the same for all observers?"
    ... well, that's a property of the Universe.
    Science cannot tell us why something is a property of the Universe because of "empiricism" - look it up.
    It can tell us what is, and how it got to be that way (at least, in principle) but not why it is.

    After a lot of data was collected to the effect that the speed of light looks pretty much the same to everyone so far, Einstein postulated that this was a law of nature: the speed of light is the same for absolutely everyone. It seems to have worked out so far but we cannot prove that there is not an inertial observer someplace who sees some different speed for light in a vacuum. This is an example of an issue called the "problem of induction". You can read about that too, and the solutions offered by Popper and co.

    You got the comment about why questions because it was starting to look like you wanted an answer in terms of some ultimate "reason for everything".
    It looks like there isn't one. We don't even have an overall model/theory of everything and even if we did it would only show that the invariance of the speed of light is consistent with that model ... you can still ask "why is that?" Why doesn't the Universe have different physics? Well... because it doesn't.
    Last edited: Aug 6, 2013
  13. Aug 6, 2013 #12
    "Millions saw the apple fall,
    but Newton was the one who asked
    Baruch, Bernard M.(the guy who invented the word cold war.)
    No, Newton never answered the why, but he did start by asking why and only then did he explain the how...

    "Why the apple fell down is actually an unsolved problem, and I don't think we'll ever be able to solve it."
    You seem to be in a bit too pessimistic mood for my taste. I would generally expect you to launch into a lengthy discourse on virtual photons, gravitons and whatnots, and finish it off with a youtube link...:(
  14. Aug 6, 2013 #13
    I could start talkig about virtual photons and gravitons and whatever (if I knew anything about them). But then I would describe a mathematical model. All science does is giving mathematical models for reality. Then experiments show whether they indeed approximate reality and to what degree.

    So suppose that we could describe a mathematical model that approximates the universe perfectly. Would that answer the why question? No, because we do not and can not know for certain whether the universe actually is the model. All we will know is how to calculate phenomena perfectly and we will be able to answer why-questions about our model. We still don't know whether the model is reality.

    For example (as described in the video), I could say that when I accelarate in my car, I get pressed to the back of the car. Why is this? The Classical Mechanics Theory says that it is the first law of Newton. That doesn't explain anything, does it? It just gives a name to the phenomenon and then works from there.
  15. Aug 6, 2013 #14


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    Yes I agree, and I would add that when doing so, it's a good idea to ask questions that have meaningful answers. WHY is generally not one of those, as has now been explained. Did you get it?
  16. Aug 6, 2013 #15
    But all of it does start with asking why. The how is only the first step in answering the why...
    Technically yes, we should start with the how but I am sure you will agree that to entirely give up on the why is rather pessimistic...

    Thanks for agreeing. Though I can't say the same for you.
    We should only ask questions which have an answer. Then it logically follows that we know if the question does have an answer... what if we don't?
    Do we then only ask the questions that we aldready know whose answer exists.
    I don't think Newton, Galileo, Einstein or any others knew that if their questions had any answers ;they just took it on faith....
    Last edited: Aug 6, 2013
  17. Aug 6, 2013 #16
    This is a whole different philosophical debate on whether mathematics are invented or discovered. I like to think they are discovered, simply because I like the idea of the universe having some kind of ultimate structure/order to it, even if we can't fully perceive it.

    I feel that (especially if you're getting your physics through popular books and such, like myself), you eventually get to an idea that you may just have to accept as true. There may be simplified analogies and ways to explain it, but to truly understand would take a lot of hard work and mathematics.

    Finally, if physics is an approximation of reality, then, engineering is an approximation of an approximation (a copy of a copy of a copy), haha.
  18. Aug 6, 2013 #17
    Sorry about the double posts. But I sense we are getting rather far from the original question as we delve into philosophy of questioning.
    Will anyone please post the lorentz transformation equations and explaination as to the how?
    [I would do it but I am posting from the app and I really don't know how to use LaTeX on my phone; heck I can't really even type on this small screen without getting finger cramps]
  19. Aug 7, 2013 #18


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    Einstein's second postulate is about the propagation of light which cannot be measured, observed or seen apart from asserting a postulate or something equivalent. The measurement comes after the postulate, not before. It's not about a lot of data being collected and asserting that there will never be a deviation of similar collections of data in the future.
  20. Aug 7, 2013 #19


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    There's an ongoing discussion about the the Lorentz Transformation equations and how to use them in this thread. Do we need to repeat it here all over again?
  21. Aug 7, 2013 #20
    No sorry, (yikes!)
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