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I don't understand how we can accept these two sentences at the same time:

  1. Light speed is constant, therefore experiences no acceleration.

  2. On the presence of a gravitation field, light path is curved (which would mean that somehow it gets accelerated).

Qmechanic
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Iago Rodríguez
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2 Answers2

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What you are confusing here is speed and velocity. Light speed is constant, but the velocity, which takes into account the direction as well as the speed is not. As an example of how something can accelerate without changing speed, consider the case of circular motion, where the acceleration of an object moving at a speed $v$ in a circle of radius $r$ is $a=v^2/r$. In this case, the speed is constant, but the velocity is changing constantly. Gravity can also cause light to change its direction, and therefore its velocity. But the speed of light will remain constant in all reference frames. Its the law :)

tmwilson26
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The disconnect is between the first and second clauses of your first sentence:

Light speed is constant, therefore experiences no acceleration

Yes, the speed of light is a constant, but it experiences no acceleration in its direction of travel. Light definitely accelerates laterally when gravity pulls on it, which is why it curves when passing near massive objects.

Daniel Griscom
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