How can a photon "stop"? What does its world line look like?

Question

Einstein famously made a thought experiment: what would he see if he sat on a beam of light? His answer was -- it's impossible. Owing to him being a body with mass, he can never ever reach light speed unless fueled by an infinite amount of energy.

My question is a little different but sort of the same. At the instance of the photon's creation, it experiences no distance and no time. Then it hits something, is absorbed, and that's it. The photon "stopped."

To the photon, that took no time at all. How? From its perspective, did it simultaneously exist and not exist?

Also, I know the world line of a photon is 45 degrees on the spacetime diagram. But I only know this as a fan of science, not because I'm a physicist. So what does the complete world line of a single photon look like? Is it a line segment, with a well defined beginning and ending?

Possible duplicate of Would time freeze if you could travel at the speed of light? — ACuriousMind, Oct 21 '15 at 14:29
There is no "perspective of a photon". There is no Lorentz transformation into a frame moving at the speed of light, so it is not an admissible inertial frame. — ACuriousMind, Oct 21 '15 at 14:30
The second part of my question was, is there a "complete world line" of a photon? — markovchain, Oct 21 '15 at 14:34
What do you mean by "complete"? It has a worldline like everything else, just the tangent vector is null instead of time-like. — ACuriousMind, Oct 21 '15 at 14:35
Thanks for answering ACuriousMind. As I stated in my question above, does the worldline of a photon have a well defined beginning and ending, seeing that it is created and destroyed? — markovchain, Oct 21 '15 at 14:41

score 6 · Answer 1 · answered Oct 21 '15 at 15:37

In order to draw a spacetime diagram I need to choose some set of coordinates. Once I've done this I can start drawing the worldlines of photons and as you say they will be straight lines at 45º (in flat spacetime at least). The worldline will start at the spacetime point where the photon is created and end at the spacetime point where it is destroyed.

So the worldline will indeed be a line segment with a clearly defined start and end point.

Any other observer may choose a different set of coordinates to draw the same spacetime diagram. In different coordinates the locations of the start and end points will be different, but the worldine will still be a line segment at 45º.

What we can't do is draw the spacetime diagram in the rest frame of the photon, because photon have no rest frame. So the question you ask From its perspective, did it simultaneously exist and not exist? cannot be answered because it is meaningless. There is no photon's perspective.

How can a photon "stop"? What does its world line look like?

1 Answers1