Concerning reference frames

In summary, a question is raised about two spacecraft firing laser beams simultaneously towards a planet at different speeds. The questioner believes the beams will reach the planet at the same time, but is looking for an explanation. Others confirm that this is indeed the case, as the speed of light is constant regardless of the frame of reference. The question is deemed to be ambiguously worded and may be a textbook problem. One poster suggests that the speed of light is simply a part of the definitions of theories like Special and General Relativity, and that any further explanation would require a better theory of gravity.
  • #1
ombudsmansect
29
0
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?
 
Physics news on Phys.org
  • #2
ombudsmansect said:
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.
 
  • #3
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
 
  • #4
ombudsmansect said:
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!
 
  • #5
ombudsmansect said:
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?
 
  • #6
ombudsmansect said:
yeah perhaps they r looking for a bit of an explanation as to why this would be so.

I'll quote myself:
Fredrik said:
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.
 

What is a reference frame?

A reference frame is a coordinate system used to describe the position and motion of objects. It is a frame of reference against which measurements can be made.

Why are reference frames important in science?

Reference frames are important in science because they provide a consistent and standardized way to measure and describe the physical world. They allow scientists to make accurate observations and predictions about the behavior of objects.

What are the different types of reference frames?

There are two main types of reference frames: inertial and non-inertial. Inertial reference frames are those that are not accelerating or rotating, while non-inertial reference frames are those that are accelerating or rotating.

How do reference frames relate to the laws of physics?

The laws of physics are based on the principle of relativity, which states that the laws of physics should be the same in all inertial reference frames. This means that the laws of physics should not change depending on the observer's perspective.

Why do we need to consider multiple reference frames in certain situations?

In certain situations, such as when dealing with objects in motion or in different gravitational fields, it is necessary to consider multiple reference frames to accurately describe and predict the behavior of the objects. This allows for a more complete understanding of physical phenomena.

Similar threads

  • Special and General Relativity
Replies
11
Views
940
  • Special and General Relativity
2
Replies
35
Views
3K
  • Special and General Relativity
Replies
5
Views
998
  • Special and General Relativity
2
Replies
66
Views
4K
  • Special and General Relativity
Replies
16
Views
1K
  • Special and General Relativity
Replies
15
Views
1K
  • Special and General Relativity
Replies
3
Views
1K
  • Special and General Relativity
2
Replies
36
Views
3K
  • Special and General Relativity
Replies
34
Views
2K
  • Special and General Relativity
2
Replies
62
Views
5K
Back
Top