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May be a silly and simple question, but I've been wondering if:

The speed of light is constant, and

  1. When we're moving in the same direction (where both the emitter and the receiver move with the light direction) we would be making it take more time for the light to reach the other end.
  2. Conversely when moving in the opposite direction we'd be shortening the time it takes.

Why do we not see a non-uniform speed of light caused by solar/galactic movement? Or do we?

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    $\begingroup$ see this hyperphysics.phy-astr.gsu.edu/hbase/Relativ/ltrans.html#c5 and the measurement of this speed hyperphysics.phy-astr.gsu.edu/hbase/Relativ/lighthist.html . The Lorentz transformations have been validated by a large number of experiments in accelerators. $\endgroup$
    – anna v
    Commented Oct 16, 2021 at 8:30
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    $\begingroup$ It is because you are thinking in Galilean transformations, and light follows Lorentz tranformations $\endgroup$
    – anna v
    Commented Oct 16, 2021 at 8:59
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    $\begingroup$ See the second line of your question! $\endgroup$
    – m4r35n357
    Commented Oct 16, 2021 at 11:50
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    $\begingroup$ You are in good company. Michelson and Morley wondered the same thing. $\endgroup$
    – mmesser314
    Commented Oct 16, 2021 at 16:18
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    $\begingroup$ As with many "why" questions, the answer is "because that's the way the universe works". If there's some fundamental reason for it, or for many other things (like quantum :-)), we certainly haven't discovered it. We just determine by observation that it works the way it does. then construct theories that explain the observations, and perhaps make new predictions. $\endgroup$
    – jamesqf
    Commented Oct 17, 2021 at 4:46

5 Answers 5

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It is not a silly question at all, and posed a problem until Einstein explained the results of Michelson–Morley experiment using special relativity in 1905. This experiment sought to measure this exact observed difference in the speed of light that you would imagine would result from the Earth's motion. They found that it remained invariant, and Einstein resolved this discrepancy by saying that the speed of light must be held constant for any observer. It’s not the easiest thing to imagine, but it has yet to be disproven and it makes many useful predictions.

What does happen as a result of relative movement is called a Doppler shift. Similar to classical sound waves, the observed frequency increases as you move towards a source, and decreases as you move away. Instead of this shift corresponding to a higher/lower pitch, it results in bluer/redder light. You can then use this information to gauge the motion of celestial bodies.

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    $\begingroup$ Actually, we do not know whether the speed of light is constant. We know that if speed goes a roundtrip, it will always finish that in the same amount of time. It is just convenient to think/believe that it goes both directions the same speed, but it is currently not provable. See this video for a thorough explanation: youtu.be/pTn6Ewhb27k $\endgroup$
    – D. Kovács
    Commented Oct 19, 2021 at 4:45
  • $\begingroup$ @D.Kovács And refreshingly that situation also solves a lot of problems with defining simultaneity along the lightcone. There is no simultaneity. Or, the best assumption of simultaneity becomes at observation. For once the intuitive answer and the maths agree. $\endgroup$
    – Stian
    Commented Oct 19, 2021 at 12:01
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The speed of light is constant

is one of Einstein's postulates of relativity and says something profound that can take time to realise.

It means that the light is measured to have the same speed independent of the motion of the source.

So for example if a light was shone from a stationary spaceship we would receive it at $3\times 10^8$ m/s. If the same spaceship is moving towards or away from us at half the speed of light, and shines the same light towards us - we would still receive it at $3\times 10^8$ m/s.

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    $\begingroup$ @ Gensys LTD Yes, there would be redshift or blueshift, but according to relativity the speed is always the same. It leads to mind-boggling consequences like time dilation and length contraction. It's not understandable from classical mechanics, but relativity theory is different and says that the received light is always received at the same speed. $\endgroup$
    – John Hunter
    Commented Oct 16, 2021 at 8:39
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    $\begingroup$ Wait, do You mean that if we're moving we still see speed of light as exactly the same relative to us? Meaning regardless of how fast you're moving your perception of the speed of light is always the same? Would this then mean a third persons perspective would measure a higher-than-speed-of-light? This seems contradictory to me so I must be mistaken here somewhere. $\endgroup$
    – Gensys LTD
    Commented Oct 16, 2021 at 8:42
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    $\begingroup$ @ Gensys LTD Yes it seems contradictory. Many people have probably been through this, thinking it has to be wrong, (myself too). That's what makes it a great theory, it was a new insight. Relativity isn't contradictory, all contradictions can be resolved when time dilation and length contraction of objects moving relative to the observer are included. $\endgroup$
    – John Hunter
    Commented Oct 16, 2021 at 8:46
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    $\begingroup$ @ Gensys LTD perhaps a popular book on relativity will deal with some of the many questions that relativity theory brings up - all the best with it. $\endgroup$
    – John Hunter
    Commented Oct 16, 2021 at 8:52
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    $\begingroup$ @GensysLTD have a look at this, how velocity addition is different for relativistic motion hyperphysics.phy-astr.gsu.edu/hbase/Relativ/einvel2.html $\endgroup$
    – anna v
    Commented Oct 16, 2021 at 8:54
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Judging by your comments, the point you are missing is that the speed of light is constant relative to everything. Suppose you have a friend who is standing still, and you pass her at half the speed of light at the same instant that I overtake you at three quarters of the speed of light, a light ray will pass all of us at the speed c. That means your stationary friend will consider that the light is moving away from her at a speed c, you will think it is moving away from you at a speed c, and I will think it is moving away from me at a speed c.

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    $\begingroup$ So if light has the same speed in any frame, what has to give? Simultaneity for one thing. Two events that appear simultaneous to my friend will not appear to be simultaneous to me. $\endgroup$
    – Neil_UK
    Commented Oct 18, 2021 at 12:57
  • $\begingroup$ Wait ... I'm newb at physics, so if you move at 3/4 c and light passes you, why would you think the light is passing you at c rather than 1/4th c? -- If I pass you at 100mph while ur going 75, I'd surely appear as passing you with 25mph from your perspective, correct? $\endgroup$
    – csstudent1418
    Commented Oct 18, 2021 at 17:53
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    $\begingroup$ @csstudent1418: It would actually be just slightly greater than 25mph, but close enough that after measurement error in all of 100mph, 75mph, and 25mph you couldn't detect the difference. Get up to .0100c vs .0075c, and you're going to see the the relative speed is not exactly .0025c. Get up to 1c overtaking .50c, and the relative speed is now 1c $\endgroup$
    – Ben Voigt
    Commented Oct 18, 2021 at 18:59
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    $\begingroup$ @csstudent1418 If Objects A and B are moving at given speeds relative to a reference frame, then the relativistic velocity formula will tell you what speed Objects A and B are moving at relative to each other. For speeds that are very low compared with the speed of light, the formula approximates to a simple subtraction- so if you are going at 75mph and I am going at 100mph, we are moving almost exactly, but not quite, 25mph relative to each other. At high speeds, however, you cannot just subtract the velocities. $\endgroup$
    – Professor Sushing
    Commented Oct 18, 2021 at 19:26
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    $\begingroup$ @csstudent1418 For normal things that travel less than the speed of light like cars and rockets yes. You will appear to be passing me at a slower speed. But not for light. Light will always pass you at the speed of light no matter how fast you travel. This is how nature behaves and it took Einstein to make sense of it (basically if light always appear to travel at the speed of light no matter how fast you travel it must mean that one second for you is not the same as one second for me (speed is distance per time) so TIME must be different). This is relativity $\endgroup$
    – slebetman
    Commented Oct 19, 2021 at 8:23
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The central postulate of relativity is that the speed of light is constant in every frame of reference. In Newtonian physics, this is impossible; if light is traveling at $c$ in one frame of reference, then someone traveling at velocity $v$ in the opposite direction would see light traveling at $c+v$. This led Einstein to conclude that Newtonian physics had foundational assumptions that were incorrect, namely that temporal and spatial distances are constant in every frame of reference. In relativity, if Alice and Bob are moving relative to each other, Bob will measure that Alice's clocks as going more slowly, and her distances to be shortened.

Clearly, even in Newtonian physics, the two people will disagree as to what happens constitutes the "same place"; since Bob views Alice as moving, what she considers the same place, Bob considers to be different places. For instance, suppose Alice is on a train. If she gets up from her seat, walks around, then sits back down in her seat, then from her perspective, she ended up in the "same" place. But to Bob, the whole train moved while Alice was walking around, so she did not end up in the same place. Relativity says that not only will they disagree about what is the "same place", but also what is the "same time".

So suppose Alice is on Earth, traveling at rest with respect to Earth, and Bob is out in space at rest with respect to the Cosmic Microwave Background. Alice observes three events: E1 is light be emitted in all direction, E2 is a photon being absorbed after traveling the same direction Earth is traveling, and E3 is a photon being absorbed after traveling in the direction opposite to Earth's direction. Alice observes E2 and E3 as being the same distance and same elapsed time relative to E2. Bob measures E3 to be less elapsed time, but he also measures E3 to be less distance, so he still measures the photon to be traveling at $c$.

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The speed of a wave is constant regardless of the source velocity. Only the frequency of the wave is affected by the source velocity. See this answer for sound waves: Does sound waves pick up the speed of its source?

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  • $\begingroup$ But the speed of a wave relative to an observer is dependent on the velocity of the observer. $\endgroup$
    – Acccumulation
    Commented Oct 19, 2021 at 2:55

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