1
$\begingroup$

Please help me understand my mistake(s) in this reasoning. I will write as if I knew exactly what I'm talking about for the sake of clarity, but I of course don't.

Consider observer A and its inertial frame, in which it is at rest.

Now, consider a light source B, moving at nearly the speed of light (with respect to the reference frame A), directly towards the observer. Let's call this velocity $v$. Finally, let A and B initially be a distance of one light-year apart, as seen in frame A.

By the time the very first photons (image) emitted by B arrive at A's eyes, B will have travelled the fraction $\frac{v}{c}$ of the distance. Since A is seeing B's first photons, it sees B's image from when it was the entire light year away.

From that point, it will take less than a year for B to reach A, assuming v is big enough. While A will still see the whole of B's trip, accelerated by a Doppler shift, it will also see B coming towards him at a velocity greater than that of light, based on the images he is creating from the photons he receives.

To be extra clear, I am not referring to the velocity of B itself, or the velocity of the photons emited by B, in frame A. I am referring to the velocity at which the observer sees B approach him, based on the timing photons hit his eyes. In other words, I am thinking about the velocity of B's image, which by my understanding can be greater than $c$.

What am I missing?

$\endgroup$
2

1 Answer 1

4
$\begingroup$

Since A is seeing B's first photons, it sees B's image from when it was the entire light year away.

Right.

From that point, it will take less than a year for B to reach A

Sure.

see B coming towards him at a velocity greater than that of light

Yup. They will see the entire trip take a short time, say a few months to cross the light year.

What am I missing?

Nothing at all!

What one person sees vs. another is precisely the whole issue of relativity. You can have very different ideas of the same physical events.

$\endgroup$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge that you have read and understand our privacy policy and code of conduct.

Not the answer you're looking for? Browse other questions tagged or ask your own question.