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My teacher told that speed of light is constant. The speed of moving object appears 0 when seen by another moving object( of same speed as that of first) but she told that light speed is constant whether observed from rest or in motion. Why is that so? Wont the speed of light seem less/more in terms of another object in motion?

Prakhar Soni
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    Welcome to PSE and Special Relativity. – Frobenius Oct 16 '21 at 05:00
  • That's how special relativity works. You shouldn't try to use your intuition to understand these things (at least in the beginning). You see, our natural intuition has evolved over many millions of years for human-size scales and slow speeds. It is not built for quantum scales and relativistic speeds. This is precisely why in these regimes, we need to rely on mathematical models and scientific experiments. Our senses are easy to deceive. – Prahar Oct 16 '21 at 11:28
  • Of course, once you spend enough time learning SR and QM, you can modify and tune your intuition after which you can go back to trusting it. – Prahar Oct 16 '21 at 11:29

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Because it does. No, really. That's it. That's the answer.

We know light speed is constant in this way because we measured it, and it turns out to be constant. The Michaelson-Morley experiment is by far the most famous.

Science doesn't really answer the "why" question. It models behaviors, permits us to make predictions, and provides ways to test them. We observed that the speed of light was constant, so our models of how light works were updated accordingly.

The accepted way to make such predictions today is with Einstein's theory of relativity, which defines exactly how different objects will appear to move in different frames. It has been validated by an astonishing number of experiments, all of which have been in line with Einstein's predictions. But relativity isn't "why" the speed of light is constant. To claim relativity says why the speed of light is constant is to engage in circular reasoning. Relativity was constructed to fit the fact that the speed of light is constant, not the other way around.

Cort Ammon
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Here is my slightly different take on this.

If the velocity of a moving object were to be added to that of the light it is emitting in the direction of its travel, then there will exist an inertial frame (that itself is moving relative to the moving object) from which that moving object can be observed to behave as if its time were running backwards i.e., we would see its past occurring before its present.

This means that from that moving frame, we would observe some effects preceding their causes, while other (different) events observed from that same moving frame would show their causes preceding their effects.

Therefore, in a universe in which velocity addition holds for light beams, causality would depend on your state of motion relative to the events being observed and the universe overall would be acausal.

niels nielsen
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