Relativity 1 - the experimental imperative

Physics geekness for my own enjoyment. If that's not your thing, you've been warned.

My physics playlist: http://www.youtube.com/view_play_list?p=7B0D5AF68906CEFF

Category:

Education

Tags:

• Relativity
• science
• physics

[Trygve12345678987654]

You seem like you're great at explaining! I wish I was better at English or that you spoke my language so I could understand this :D

[ozmoroid]

@Trygve12345678987654 Well, my mom's mom spoke Norwegian, but none of her kids learned and therefore none of her grandkids, although I used to know the names of the Norwegian Christmas pastries. ;-)

[WhiteDragon103]

Question-

If there is no "absolute rest velocity", then how is it impossible to travel faster than the speed of light, since such a condition would require a reference point to begin with.

Suppose our planet (the sun, the galaxy containing it, however far back you want to go out) is moving close to the speed of light. We could measure absolute velocity through how difficult it is to accelerate ourselves in different directions. (Do we move faster or get heavier?)

[WhiteDragon103]

@WhiteDragon103 Cont...

Imagine two observers are moving in opposite directions away from a third observer, and the third observer would perceive the first two as moving away at 2/3rds the speed of light.

However, the first and second observer would perceive each other to be moving away from one another at twice that - 4/3rds. That's 1.3x the speed of light. Since neither right nor wrong about who is moving at what speed, isn't there a problem here?

[ozmoroid]

@WhiteDragon103 In relativity velocities don't simply add. If I'm A and B is moving away from me at speed u and C is moving away from B at speed v then C is moving away from me at speed (u+v)/(1+u*v) which is always less than 1.

[ozmoroid]

@WhiteDragon103 It's impossible to travel faster than the speed of light relative to other nearby objects. Specifically you can never "outrun" a beam of light. In principle distant objects in the universe are receding away from us at faster than the speed of light due to the expansion of the universe.

[nelmonster1]

Both amusing and informative. Thanks ozmiroid.

[CptnMatruz]

@ozmoroid Very clever, I knew there was something of that, the train transmits its energy to the earth but since earth is so massive we don't notice its acceleration, the train on the other hand has less mass so by the equation a = F / m we would have a bigger acceleration.

[zartek1337]

gues what! im only 10 and i under stand this stuff! woopeee for me!

[ozmoroid]

@CptnMatruz Earth is definitely much more massive than the train, so when equal and opposite forces act between them the acceleration of the earth will be negligible. If you had a really big train things would be more interesting. Of course the surface of Earth is not at rest, as shown by a Foucault pendulum.

[ozmoroid]

@ozmoroid It's just that the equations of motion are so complicated as to be impossible to work out. That's why when NephilimFree was claiming Earth is at rest I asked him to provide the equations of motion.

[ozmoroid]

@iFreeThinker Classically it would be difficult to explain a geostationary orbit without having the earth rotating. If the earth rotated in the ether then the Michelson-Morley experiment would have detected it. People tried assuming Earth dragged part of the ether with it, but that led to bizarre predictions that would be readily observed in telescopes but weren't. In the end, General Relativity say that you can consider Earth at rest if you want to.

[CptnMatruz]

When I'm on a train in movement I see the ground moving relative to the train, and when I'm standing on the ground I see the train moving relative to the ground. However, when I'm on the train I can feel the acceleration of it relative to the ground, whereas in the ground I cannot feel its acceleration relative to the train, because of this I know that regardless of the frame of reference the truth is that the train is moving relative to the ground and not the other way around.