Do Laser Beams from Spacecrafts Travel at Different Speeds?

[ombudsmansect]

Hey guys I don't think i have the correct solution to this question. I would say tht the laser beams would bothe reach the planet at the same time, but this just seems way too easy. Any hints/thoughts/suggestions on solutions to this question? Thanks


Two spacecraft are flying towards a planet at 0.9c and 0.3c relative to the planet, respectively. If they fire their laser beams simultaneously from a range of 109 m, which will strike the planet first and what will be the delay before the other beam strikes?

 

[Petr Mugver]

I would say tht the laser beams would bothe reach the planet at the same time, but this just seems way too easy.

It might be too easy, but it's true. The laser beams travel at the speed of light, which is independent of coordinate frame.

 

[ombudsmansect]

yeah perhaps they r looking for a bit of an explanation as to why this would be so. thanks for helpin me out man :D

 

[TheRealTL]

Hey guys I don't think i have the correct solution to this question. I would say tht the laser beams would bothe reach the planet at the same time, but this just seems way too easy. Any hints/thoughts/suggestions on solutions to this question? ThanksTwo spacecraft are flying towards a planet at 0.9c and 0.3c relative to the planet, respectively. If they fire their laser beams simultaneously from a range of 109 m, which will strike the planet first and what will be the delay before the other beam strikes?

Oh this ones easy. And experimentally verified a million times over.

First if they are both equal distance from the planet, the signals will reach the Earth at the exact same time. They both however will blueshift (which I hope links to wiki) with the 0.9c craft blue shifting more than the 0.3c. f+=sqrt((1+v2/c2)/(1-v2/c2)). f-=sqrt((1-v2/c2)/(1+v2/c2)). And that's from memory!

 

[Doc Al]

Hey guys I don't think i have the correct solution to this question. I would say tht the laser beams would bothe reach the planet at the same time, but this just seems way too easy. Any hints/thoughts/suggestions on solutions to this question? Thanks


Two spacecraft are flying towards a planet at 0.9c and 0.3c relative to the planet, respectively. If they fire their laser beams simultaneously from a range of 109 m, which will strike the planet first and what will be the delay before the other beam strikes?

This problem must be making the rounds. (See: https://www.physicsforums.com/showthread.php?t=425249)

As I said in the other thread, the problem seems ambiguously worded. (Unless they just want to test if you realize that the speed of the beams is independent of the speed of the ships.) If this is a textbook problem, what textbook?

 

[Fredrik]

yeah perhaps they r looking for a bit of an explanation as to why this would be so.

I'll quote myself:

A question about something in the real world can only be answered by a theory. When you're asking "why is the speed of light invariant?", you're probably wondering how SR and GR answers that question. The answer is that they don't. The invariant speed of light is part of the definitions of those theories. You could ask the follow-up question "why do those theories predict results of experiments so accurately?", but the only thing that can answer that is another theory, and what theory would that be? It would have to be a better theory of gravity than GR. There are a few candidates (e.g. string theory), but none of them is developed to the point where they can make predictions about experiments.

Some of the other posts in that thread might be useful too. The > above the quote is a link.