quick relativity question (1 Viewer)

[underthesun]

Consider this exceprt from excel book:

"Consider a spacecraft moving at speed of c/2 (half speed of light), towards another planet. An astronaut in the spacecraft now flashes a light beam in the direction of the motion of the spacecraft. What is the speed of the light relative to the planet? Prior to einstein we would have said 3c/2 but now we know it's c."

And so it says, relative to the planet it's c. So, does that mean, that the spacecraft is seeing the light going away from it at c/2 (portion of light towards the planet)?

And does it mean that if the spacecraft was flashing a light onto the opposite direction, then the speed of the light relative to the spacecraft would be 3c/2?

 

[kini mini]

Originally posted by underthesun
And so it says, relative to the planet it's c. So, does that mean, that the spacecraft is seeing the light going away from it at c/2 (portion of light towards the planet)?

I think you are extremely confused about the idea of frames of reference . The speed of light is constant, it is c relative to everything . That is the point of Einstein's theory of Relativity. There is no absolute point of reference. The spacecraft sees the light going away at c.

And does it mean that if the spacecraft was flashing a light onto the opposite direction, then the speed of the light relative to the spacecraft would be 3c/2?

No, it'd still be c.

 

[...]

light travels at the speed of 'c' inside a vacuum

nothing in this world travels faster than 'c'

 

[Ragerunner]

i do

 

[...]

ur nothing but just an iimagination god created

 

[underthesun]

oh.. i get it now

been like 7 months of relativity wondering, and now i understand the whole picture after a short answer..

this is why the education system sucks..

thanks a lot anyways

 

[kini mini]

No problem

Feel free to post anything you're confused or even just a little doubtful about, it's much better than staying ignorant .

 

[ND]

Originally posted by ...
no thing in this world travels faster than 'c'

*cough*GTR*cough*

 

[...]

sore throat there son????

 

[Mathematician]

Relativity Q

A Spacecraft is moving at 0.999c in one direction and another at 0.6c in the other.

Does that mean Velocity relative to each other is greater than c ?

Thats impossible!

 

[mommba]

yo underthesun....is ur name gayse?

 

[McLake]

Re: Relativity Q

Originally posted by Mathematician
A Spacecraft is moving at 0.999c in one direction and another at 0.6c in the other.

Does that mean Velocity relative to each other is greater than c ?

Thats impossible!

RELATIVE velocity > "c" IS possible

 

[underthesun]

dickhead ridwan.. i never expected i'd see you here..
you noob

 

[Constip8edSkunk]

Re: Re: Relativity Q

Originally posted by McLake
RELATIVE velocity > "c" IS possible

how? if they are both non accererating bodies, they both would be in inertial frames of references. so what would a person in 1 space ship see the other travelling at?

if relative velocity can be > c, isnt that like saying earth is moving relative to another reference pt at constant v near c and another galaxy is moving in opposite from reference pt at a constant v near c, and we could c that other galaxy 2 be moving faster than earth (ignoring circular motions and so on .... or does this matter SIGNIFICANTLY enuf to say a situation like this is impossible in reality)

 

[McLake]

Originally posted by Constip8edSkunk
how? if they are both non accererating bodies, they both would be in inertial frames of references. so what would a person in 1 space ship see the other travelling at?

Speed of ship A + Speed of ship B

if relative velocity can be > c, isnt that like saying earth is moving relative to another reference pt at constant v near c and another galaxy is moving in opposite from reference pt at a constant v near c, and we could c that other galaxy 2 be moving faster than earth (ignoring circular motions and so on .... or does this matter SIGNIFICANTLY enuf to say a situation like this is impossible in reality)

I'm not sure I understand your question ...

 

[Dangar]

RELATIVE velocity > "c" IS possible

really?? I don't think it is. Because light cannot travel faster than itself, so how could you, who rely on light to see possibly observe anything faster than c?


isn't this where mass dilation comes in?

A Spacecraft is moving at 0.999c in one direction and another at 0.6c in the other.

Each spacecraft would see the other spacecraft grow in mass, because you can't see anything moving faster than c, so to compensate the extra kinetic energy is turned into mass by e=mc2. Or at least that is how it looks to the observer from each spacecraft?

 

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but the kid is asking the relative velocity....

 

[Dangar]

I was talking about relative velocity... how the people in each spacecraft would see the other spacecraft. Isn't that what relative velocity is??

 

[mrbassman]

This is the wonderful area where classical physics and relativistic physics part ways. Where you add velocities vectorially in classical physics, you have to use different methods for velocities close to c. Differentiating Lorentz position relations with respect to time.

Example: Two spacecraft A and B are moving in opposite directions. An observer on earth measures the speed of A to be 0.75c and the speed of B to be -0.85c. Find the velocity of B w.r.t. A.

Soln. The Earth is in a separate frame of reference to the spaceships

u'(b) = (u(b) - v(a))/(1 - v(a).u(b)/c^2)

= (-0.85c - 0.75c)/(1 - (-0.85c)(0.75c)/c^2)

= -0.98c

Therefore spaceship B is moving away from A at 0.98c. Not a veloctiy greater than c as you would expect from classical calculations.

 

[Mathematician]

..

yeah i was hoping for a different answer. I saw this on a site , and was unhappy with this formula cause even i can derive this formula to make c remain fastest speed(in relative velocity too).
Seems like an excuse. So i was thinkin of that light effect when u pass light through cesium gas . What happens is the light effect travels faster than the speed of light . Somwthing to do with relativity still staying true and something else with causality.

So could u say relative velocity is just an effect?
Cause his would make it possible.