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Basis for the postulate that all physics is the same for all inertial reference frame

  1. Sep 10, 2006 #1
    SR is based on the postulate that all physics experiments yield the same results regardless of inertial reference frame? What was Einstein's basis for this assumption? What thought process did he go through to come to that conclusion? How could he know the speed of light was the same regardless of inertial reference frame when he did not have the experimental apparatus at the time to know? People have said that the measurement of e0 and mu0 are the same in all reference frames, so therefore the speed of light must be the same in all reference frames. I am not entirely convinced by this statement.
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  3. Sep 10, 2006 #2


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    The Michelson-Morley experiment was performed in 1887, so Einstein was aware well before he wrote his SR paper in 1905 about the problems with the idea of an "ether".

    Einstein realized that assuming that the speed of light was constant for all observers would explain the M.M. experiment results. I'm not really clear on how many replications of the MM experiment had been performed in Einstein's time period, but the result was well-accepted, being 18 years old in 1905.
  4. Sep 10, 2006 #3


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    This article has a lot of good info on what the historical information suggests about Einstein's process of discovering special relativity:


    It seems that he was aware that experiments to try to detect changes in the speed of light due to different motion through the ether, like the Michelson-Morley experiment, had failed to find such changes; and he was also aware of Lorentz's hypothesis of Lorentz contraction which was supposed to account for this and save the ether, but in a seemingly contrived and unmotivated way. Some other puzzles, such as the fact that the relative motion of a magnet and a conducting wire loop will be the same regardless of whether the magnet was at rest or moving relative to the ether, yet the ether theory would have to explain these cases in completely different ways, might also have influenced him. Once he had begun moving towards rejecting the ether, he may also have been influenced by the philosophy of Ernst Mach (who also is known to have influenced him in his discover of general relativity), which said that motion can only be defined in relative terms. Finally, at the same time he was thinking about the photoelectric effect and how light could be considered as a particle, which does not seem to fit well with the ether theory of light as a pure wave.
  5. Sep 10, 2006 #4
    Also, I am aware that Lorentz formulated the time dilation and length contraction formulas using the assumption that there was an ether, and measurements of the speed of light are only ~300,000 m/s wrt the ether frame. However, why is time dilation and length contraction necessary under an ether theory? I do not completely understand where the time dilation and length contraction formulas come from in Lorentz's original ether theory.
  6. Sep 10, 2006 #5
    I still feel that if light is to be the same speed in all inertial frames, then we can take all inertial frames to be in the frame of the "ether". I am much more capable to accept an ether theory than SR. I will have to sleep on it for a few years before I truly come to terms with this subject.

    I like the idea that all different inertial frames are perhaps in the same "frame", as time and space are shifted accordingly to allow for this.
  7. Sep 10, 2006 #6


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    If the ether was an actual physical substance, how could it be at rest in multiple frames simultaneously?
  8. Sep 10, 2006 #7
    Perhaps we are not really in motion wrt one another...it just seems that way?
  9. Sep 10, 2006 #8


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    But physics only deals with what we measure, not with what "really is" in some ultimate philosophical sense. If you set up a coordinate system out of physical rulers and clocks, and you find that my coordinate position changes with coordinate time while yours does not, that's all it means to say we are in motion wrt one another.
  10. Sep 10, 2006 #9

    It boils down to Maxwel's equations being unchanged between inertial frames. Magnetic and electric fields change, but the real effects of these fields (induced currents) do not.

    Time-dialtion and length contraction were contrived by lorentz to make ether theory agree with MM.

    It just so happened that you get these same effects when you make the two basic assumptions, that c is the same in all inertial frames, and that all observers agree on events (i.e. the laws of physics are the same for all intertial observers).
  11. Sep 10, 2006 #10
    If I look at Maxwell's equations it appears as though they are not invariant, but this may just be the fields that are variant, but the actual effects are invariant. But how exactly are the field representations in MAxwell's equations variant, but the resulting effects NOT variant??
  12. Sep 10, 2006 #11
    You can either do the brute force approach of changing from one reference frame to another, and seeing what Maxwell's Eqns. look like, and their effects in the new frame, or you can formulate things in tensor notation, which shows automatically, that Maxwell's Eqns. are covariant (which means they change, but in the expected way, so as to produce effects that don't change).
  13. Sep 10, 2006 #12

    Quoted for saving me from having to say the exact same thing.
  14. Sep 10, 2006 #13
    Well, I have seen modified MAxwell's equations that take into account relativistic effects, but those just correct the field expressions, but the end result does not change?
  15. Sep 10, 2006 #14
    Honestly, I think I lack the mathematical skill to perform these traansformations....I've never taken a rigorous relativity class. I am in modern physics right now, and it just spends a few weeks going over relativity (SR and GR) in a fairly non-rigorous way. I've taken an electromagnetics class, but again, relativistic effects were not covered.
  16. Sep 10, 2006 #15


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    Do you mean, "we can take any inertial frame to be an ether frame"? That would be correct.
    SR and Lorentz ether theory are the same physical theory (SR) cast in different coordinate systems. The "two postulates" do not define SR per se, but only one formulation of it.
    Not sure what this means, but it is the space-time interval that is invariant.
    That's right, if you're talking about 3-velocity; 4-velocity is what's real.
    Last edited: Sep 10, 2006
  17. Sep 10, 2006 #16
    I think we should not neglect Galileo's invitation in the cabin of a ship isolated from the surrounding, trying to detect if the ship is in a state of rest or in uniform motion.
    sine ira et studio
  18. Sep 11, 2006 #17
    But that is Galilean relativity - everyone was already pretty much convinced that you could not detect a difference in the actions between mechanical components when all of the elements had the same velocity (e.g., everything moving uniformly in the same direction). What SR did was extend the physics to em phenomena - light which was thought to have a velocity relative to the ether would not be expected to propagate at the same velocity when the ship velocity changed relative to space - that was the shocking result that MMx revealed - but even Michelson never really accepted the result.
  19. Sep 11, 2006 #18
    I have considered a modern Galileo, equipped with devices which allow experiments with light signals.
    sine ira et studio
  20. Sep 11, 2006 #19
    These are not modifed equations in the sense that any new physics has to be added. They are just written in a manifestly covariant fashion using the Faraday tensor. If you actually substitute in the E and B components that make up the Faraday tensor into these covariant equations, you just get back the usual 4 Maxwell's equations for E and B. There is no extra information in these covariant equations (apart from their obvious covariance under Lorentz transformations.)
  21. Sep 11, 2006 #20


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    In what way did you "consider" that? The point is that light signals are "electro-magnetic" signals and it was known in the 19th century that a magnetic field affected a charged particle proportional to their relative speeds. What part of your "consideration" shows that relativity still applies to magnetic phenomena?
  22. Sep 11, 2006 #21
    Consider that the modern Galileo invites you to perform a Bucherer like experiment measurring the radius of the circle along which a charged particle moves in an uniform magnetic field confined under the deck of the ship and to decide if it is in a state of rest or in a state of uniform motion. Besides that if you consider the Lorentz force then magnetic field B and charge speed v should be measured in the same inertial reference frame a fact textbooks miss to mention Thank you for your answer to my thread.
    sine ira et studio
  23. Sep 12, 2006 #22
    The way I approached it (may be wrong) was to assume ether and constant speed of light with regard to it. Then try to think how to built a clock in a most basic way, so that you can make use of the two assumptions in deriving its behaviour. The natural thing, I think, is to let a photon bounce between two mirrors and count the bounces. Thats your clock.

    Now imagine you start moving in the direction perpendicular to the line between the mirrors: the photon will have to go zickzack with regard to the ether. Consequently the path is longer and because the photon cannot go faster, it will take longer to go back and forth. But you don't know about your speed relative to the ether. All you can do is count, but obviously now you are counting less often --- hence your clock runs slower than when at rest.

    Side note: If you now consider that this "longer" path not only pertains to the photon, but to all force mediating particles, inside yourself and inside the matter around you, you could imagine that the activity of the respective forces is slowed down in sync with the clock you built. Your whole little system moving along with you is in "slow motion" so to say.

    Once you've described how to measure time, you want to continue with length. How do you measure length, based on the only two assumptions you made? Well, take your clock as a reference and say the length of 1 is defined by the distance a photon travels during 1 bounce.-) Suddenly time and length are nearly the same. Now you measure length in the direction of your travel as well as perpendicular to it. And you compare with what these lengths would be in the assumed ether. Voila, length contraction in the direction of travel. But careful, the calculations can easily go wrong.
  24. Sep 12, 2006 #23


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    it's because we can't tell the difference between a "stationary" vacuum and a "moving" vacuum that is whizzing past our faces at a speed of c/2. if we can't tell any difference, perhaps their really is no difference. that the concept of a moving vacuum is meaningless.

    now consider Maxwell's equations and radiation of an E&M wave that is observed by two different oveservers moving relative to each other. they look at the very same piece of light (an EM wave). this wave propagates because a changing E field causes a changing B field which causes a changing E field, etc. but this phenomenon is exactly the same for both observers moving relative to each other. so they see the EM wave propagate at the same speed even if one observer is "stationary" and the other is whizzing past the first along with the flashlight that generated the EM wave.
  25. Sep 12, 2006 #24

    This one always gets me. If we say 'the laws of physics are the same for all intertial observers' therefore c must be constant in both can't we just as easily say "therefore 'the speed of sound' must be constant in both ?? If you replace 'observers' with 'listeners' and 'observed events' with 'heard events', what's the difference ?
  26. Sep 12, 2006 #25


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    Well the speed of sound, by the laws of physics, varies with pressure and temperature, and if the observers in their different frames set up the same conditions, they will observe the same speed of sound, locally. In particular if there is no material medium, sound cannot travel, so in the frequent examples of different frames separated by a vacuum, the question of sound between them does not arise. On the other hand, guess what? Light can pass through a vacuum!
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