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From: Jonathan Groves on 27 Feb 2010 19:56
I'm no physicist, so I've wondered after learning some about special
relativity why the speed of light is constant in all reference frames.
It makes sense that the speed of a traveling object is relative to
reference frame. For example, people sitting in the same car we are
sitting in appear to us to be at rest, and we ourselves appear to be at rest
as well because their positions relative to ours don't change. However, to a person
standing still on the side of the road, all of us are moving at some nonzero speed
because our position relative to that person is changing. If it weren't for the
car and the surrounding environment, we might think we are at rest and the person on
the side of the road is moving. In other words, it is possible to be in a
situation where we would have no reason to believe that we are moving and
something else is at rest but also have no reason to believe that we are at
rest and something else is moving.

However, this idea apparently doesn't work for light, but every source I've
seen that explains relative speeds doesn't explain why. These sources say that
the speed of light is an absolute constant as if it were completely obvious
that this should be true. But it is not obvious why this should be true.
It is physically impossible for me to travel at the speed of light. But
suppose I could, and a beam of light is shot in the same direction I'm
traveling in. Since the light's position relative to mine never changes
(I take it that this must be the case; otherwise, wouldn't the light and I
be traveling at different speeds if we are moving in the same direction?),
shouldn't the light appear to me to be at rest? If I could travel at
half the speed of light, shouldn't the speed of light appear to be (1/2)c?
I know that if a train were moving at 40 mph and I were traveling in a
car in the same direction as the train but at 20 mph, the train would
appear to me to be traveling at 20 mph rather than at 40 mph. So why
doesn't this work for light?

Is this counterintuitive notion one reason that Einstein's ideas on
special relativity were not accepted universally (or at least nearly universally)
for a long time by physicists?


Jonathan Groves
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