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I Why is speed of light constant in all frames of reference?

  1. Apr 24, 2016 #1
    Why is speed of light constant in all frames of reference.
    I don't understand.
  2. jcsd
  3. Apr 24, 2016 #2


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    Maxwell's equations produced a source-independent, frame-independent speed of light. Everyone assumed that there was something subtly wrong or incomplete about the equations. Einstein took the apparently silly idea at face value and showed that he could explain all the confusing experimental results people had been getting for half a century by doing so.

    That's why we believe light speed is constant in all frames. Theories that accept this make accurate predictions, those that don't, don't. As to why it should be that way, our current understanding is just that it is this way. That light speed is invariant (at least locally) is a cornerstone of modern physics. Why it should be that way is not a question physics can answer, at least not today. Ultimately all science relies on some (very small set of) propositions that we simply assert are true, and justify because the theories based on those assumptions work.
  4. Apr 24, 2016 #3
    All scientists must believe that magic exists. The speed of light is definite, so an object travelling at more 0.5c must spend a lot of time with light than the object travelling at 0.2c. But if we say speed of light is the same in all inertial frames, this suggests no matter how fast you travel, but all objects spend equal amount of time with a selected photon since photon travels at c.
  5. Apr 24, 2016 #4


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    I have no idea what you are trying to communicate.

    The speed of light is constant in all frames. Time dilation, length contraction, and the relativity of simultaneity are consequences of the constancy of the speed of light, and will yield a coherent reason for this measurement made in any frame, as I also said in the other thread in which you are currently posting.
  6. Apr 24, 2016 #5


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    No, there is no magic involved. Please don't be flippant about your lack of understanding. Do you seriously think that "all scientists" as you say must be wrong? Think about the likelihood of that vs the likelihood that you just don't understand something.

    It has already been explained to you in two different threads. Do you get it now? I know it's counter intuitive, but one of the first things you HAVE to learn if you are going to study Quantum Mechanics or cosmology is that human "intuition" and "common sense" are worse that useless, they just lead you down blind alleys. Go with the evidence.
  7. Apr 24, 2016 #6


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    The speed of a moving object such as a pulse of light is defined as a ratio:

    speed = [itex]\frac{\delta x}{\delta t}[/itex]

    where [itex]\delta x[/itex] is the distance traveled, and [itex]\delta t[/itex] is the time it took. When you switch to a difference reference frame, [itex]\delta x[/itex] of course changes. But according to Special Relativity, [itex]\delta t[/itex] changes as well. Those two changes work together to insure that the ratio [itex]\frac{\delta x}{\delta t}[/itex] stays the same, in the case of a pulse of light.
  8. Apr 24, 2016 #7


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    What does "spend a lot of time with light" mean? This doesn't correspond to anything in the actual theory of SR that I can see.
  9. Apr 25, 2016 #8
    Light moves away from the 0.5c object at c. Light also moves away from the 0.2c object at c. But with respect to what reference frame are the objects moving? If you want to examine a real problem you need to define the motion of your reference frames with respect to each other.

    As for how everyone can agree on the measurement of c, it's easy when they DON'T all agree on the measurements of either space or time. A loose example of something similar is a fraction. 4/8 = 1/2, correct? What about 16/32? That also is 1/2. Let's say all fractions in a particular set are required to be 1/2. If I change an input in a numerator, the only way for the fraction to remain one half is for a corresponding change to occur in a denominator. Is that something you disagree with?

    In special relativity, inertial observers must agree upon the measured speed of light. They do NOT have to agree on either distance or time they measure. Your (sorry to say poorly written) example seems to be assuming length measurement is also invariant. It is NOT.

    Let's say I'm on the road at point A and you're in a very fast car that can somehow move at 25% of the speed of light. You drive to point B, which before you leave is 10 miles away. At the moment you leave, a beam of light is shot to a mirror at point B and reflects back. For me, the beam of light takes about 107.3 microseconds to return, which means the beam hit B at about 53.7 microseconds. However, for you, since you are moving at .25c, the distance is NOT 10 miles. The distance you actually experience traveling is contracted to about 9.68 miles. Which means for you the light hits back at A in 103.9 microseconds, and so it hits B in 51.9 microseconds.

    Now do the fractions. Speed = distance/time.
    For me, the speed of light is ten miles/ 53.7 microseconds which is about 186, 000 miles per second, and is in agreement with the known speed of light.

    For you, the speed the light had to have is 9.68 miles/ 51.9 microseconds, which is about 186, 000 miles per second, which exactly the same as the above. (Note: I had rounding errors but to within 3 significant figures they are the same. Look up significant figures)

    We BOTH measure c for the speed of light. We do NOT both measure the same distance you traveled nor do we agree on the time the trip took. This is actually a logical inevitability of every inertial frame agreeing on c.

    Is this really a hard concept to grasp? How is it any more difficult to understand than the concept of two fractions with different numbers in their numerators and denominators being equal? All you have to do is stop insisting on absolute length and time. As for where the numbers I use come from its the Lorentz factor and just a function of speed. Look it up.
  10. Apr 26, 2016 #9
    If the lightsource is in one frame of reference and i measure the spedd of the light moving in that same frame it has a velocity of approx. 300.000 km per second. Now if i am in another frame of reference moving away from the lightsource and its frame I still messure the spedd of the light comong from the source as 300.000 km per second. So how can that bee? Well the light I messure if I am going away from a lightsource is changed to longer wavelengths and shorter frequences. Speed of a wave is the same as the waves wavelength multiplied by the waves frequency. The spedd of light in all frames of reference is allways 300.00 km/sec. What changes in the different frames is the lights wavelengt and its frequency to make that statement true, that light allways have the same velocity regardless of frame of reference and their different velocitys en relation to each other.
  11. Apr 26, 2016 #10


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    It's probably worth adding to @EnergyTheory's comment that in Newtonian physics it's only frequency that changes due to a change of frame. The wavelength does not change. Hence the speed of waves is not invariant.

    You do need both wavelength and frequency to change if you want an invariant wave speed, but you also need them to change in reciprocal ways so that their product is constant. And if you choose to explain the constancy of the speed of light this way you simply produce a different question, namely, why did we postulate that the wavelength and frequency happen to change in this way? And the answer to that is "because that way theory is consistent with experiment", just as the answer to why Einstein postulated an invariant speed of light was "because it works".

    In other words, postulating a specific form for Doppler frequency and wavelength shifts lets you derive the invariance of light speed. Postulating the invariance of light speed lets you derive a specific form for Doppler frequency and wavelength shifts. In both cases, the postulates can only be justified by the success of the theory.
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