The Speed of Light: Relative to Us?

[Chronos]

Try to think of space and time as being stretched between two mass possessing objects in motion. Light ignores it, other than doppler shifting. So, the two spaceships stretch the medium between them, but, all light does is get a bit redder.

 

[Chronos]

Relativity is hard to conceptualize. Ed, the rope you imagined tying to the two spaceships also gets stretched in the process. Any time you stretch time, you also stretch space. There is no way to separate the two. There are no absolute reference frames. That is the fundamental concept of relativity. It works and has survived every test ever devised to prove it wrong.

 

[russ_watters]

Is this question basically that 2 objects can't separate from each other at 400,000 k/s? What if you had 2 rockets that you have tested and know go 200,000k/s and tied a rope to them that was 400,000km long then set the rockets back to back and simultaniously sent then out opposite directions in 1sec would they would pull the rope tight?

Yes, they would. And an observer on Earth would see the ships separating at 400,000 km/sec. But that does not mean the ships are traveling 400,000 k/sec with respect to each other.

 

[Adam]

So if there are three stars in a line, A-B-C, separated by 0.8 lightyears each way from B, and the spaceships leave B... one heads for A, one heads for C, each traveling at 0.8C...

How long does it take for each to reach its destination?
How far did each travel?
What is that time multiplied by that distance?

 

[Doc Al]

So if there are three stars in a line, A-B-C, separated by 0.8 lightyears each way from B, and the spaceships leave B... one heads for A, one heads for C, each traveling at 0.8C...

Once again, we have to be clear as to what frame is claiming that they are separated by 0.8 lightyears. I will assume that those distances are as measured in the rest frame of the stars. And that the speed of the spaceships is with respect to that frame.

Recall that different frames will observe different lengths and times.

How long does it take for each to reach its destination?

According to the star frame: t = D/v = 0.8/0.8 = 1 year.
According to the ship frame: t' = D'/v = (0.48)/0.8 = 0.6 year.

How far did each travel?

According to the star frame: D = 0.8 lightyears.
According to the ship frame: D' = D \sqrt{(1-v^2/c^2)} = D (0.6) = 0.48 lightyears.

What is that time multiplied by that distance?

Why would you do that? Perhaps you mean: what is the distance divided by the time? Answer: the speed, which equals 0.8c.

 

[Nenad]

How long does it take for each to reach its destination?

t = \frac{d}{V}

t = \frac{d \sqrt{1 - v^{2}/c^{2}}}{0.8c}

t = \frac{0.8ly \sqrt{1 - (0.8c)^{2}/c^{2}}}{0.8c}

t = 0.6y

How far did each travel?

d_o = d\sqrt{1 - v^{2}/c^{2}}

d_o = 0.8ly \sqrt{1 - (0.8c)^{2}/c^{2}}

d_o = 0.48ly

What is that time multiplied by that distance?

I see no point to multiplying time and distance.

 

[Nenad]

damn, Doc Al keeps beating me to answers, BLAST!

 

[Adam]

I'm just trying to get my head around the maths. I'm crap at maths. I want to see all the bits and pieces.

 

[aekanshchumber]

look, you first of all photon is nothing as you are thinking. you are treating a photon as a particle to explain for which higher dimensions of dimensions of universe are required.

 

[Nenad]

I'm just trying to get my head around the maths. I'm crap at maths. I want to see all the bits and pieces.

you don't understand what I did in my higher post?

 

[JasonRox]

Anyone know the link for the David Hoggs SR Book?

This might help him out.

It sure has shed some light on me.

I'll post it later if I find it.

 

[Chronos]

A journalist once asked Sir Arthur Eddington... I hear that only three people on Earth understand relativity... After repeating the question, Eddington replied, hmm, I am still tying to figure out who the third person is.. a physics joke, but a good one.

 

[Adam]

you don't understand what I did in my higher post?

I never studied maths in high school. I quit school early to join the military. I basically have to play catch-up with everyone else.

 

[Doc Al]

David Hogg's SR Book

Anyone know the link for the David Hoggs SR Book?

http://www.physics.nyu.edu/hogg/sr/sr.pdf

An excellent book! Short, clear, accurate.

 

[Nenad]

A journalist once asked Sir Arthur Eddington... I hear that only three people on Earth understand relativity... After repeating the question, Eddington replied, hmm, I am still tying to figure out who the third person is.. a physics joke, but a good one.

they were actually talking about General Relativity. The joke is irrelivant on a Special relativity thread, which is quite easy to understand.