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We know the speed of a photon is constant. Yet when any other object changes direction, its velocity must go to zero. Thus, my question is how does a photon avoid this? I assume that to change direction, a photon must be absorbed and then re-emmitted?

K. W. Cooper
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    What makes you say that any other object's velocity must go to zero when it turns? That is simply not true. Do you perhaps mean reflected? – Jim Oct 01 '15 at 13:53
  • My understanding is that the earth is always turning. Is its velocity always zero? – WillO Oct 01 '15 at 14:39
  • Please see the edit, turning wasn't exactly what i had meant – K. W. Cooper Oct 01 '15 at 14:59
  • "Change direction" is more ambiguous than "turn". Wait no it isn't. Taken literally, changing direction means the magnitude of the velocity vector is constant. Which means if it starts as non-zero, it will always be non-zero – Jim Oct 01 '15 at 15:00
  • My understanding is that the earth is always changing direction. If this isn't so, we're going to have to rewrite a lot of textbooks. – WillO Oct 01 '15 at 21:03

2 Answers2

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A photon is a quantum mechanical elementary particle and follows quantum mechanical formulae, not classical ones.

In quantum mechanics the only way an elementary particle can change direction is through an interaction with another elementary particle or field. The interaction is shown with feynman diagrams which give the integrals that have to be calculated in order to predict the four momenta of the elementary particles, and these predictions are probabilistic, so distributions are given.

gamma electron

This is what happens when a photon interacts with an electron and changes direction. The integrated formula gives the outgoing angular distribution. The photon has speed c always.

MildBubby
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anna v
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This comes down to the difference between speed and velocity. Velocity is a vector quantity which takes into account both speed and direction. So, if an object turns, the velocity will change between the different vector components of the velocity. The speed however doesn't necessarily change. For instance, in a circular orbit, the speed of the object remains fixed while the velocity is constantly changing. So, when you say that when an object turns, its velocity must go to zero, this is not necessarily true. What you probably mean is that a particular velocity component must go to zero, and even in this case, this is only true in cases where the turn is by more than 90 degrees.

For light, its trajectory can be redirected under the influence of gravity, so it will 'turn' as you have suggested. The speed of light however remains fixed. Things get more complicated when you are traveling through mediums and other things, but I think the misunderstanding that you have comes from the difference between the definitions of speed and velocity.

tmwilson26
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