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relativity thought experiment
HalberdBlue
Registered User regular
Registered User regular
I was reading The Elegant Universe the other day and had a question about some physics stuff. I've read plenty about relativity and thought I had a pretty good grasp of what it was but now I need some stuff cleared up. I'd appreciate knowing where my thought process is going wrong:
1. Any object's movement is indistinguishable from not moving if it isn't in reference to another object. All movement is relative.
2. If an object is moving at 0.5c away from Earth relative to Earth, and another object is moving at 0.6c in the opposite direction away from Earth relative to Earth, the objects are both moving at 1.1c relative to each other.
3. You could consider either object as being stationary and the other one as moving, and both viewpoints would be equally valid.
4. So, with that all said, by what standard are we measuring something's speed to determine whether or not its going faster than light speed? Obviously, according to relativity, nothing can travel faster than light, because it would require an infinite amount of energy to do so. We can easily pick two stars and find that they are moving away from each other at faster than the speed of light. Then you look at one of their planets and find that they are pretty much stationary compared to each other (on a grand scale that is, obviously there is an orbit). There is no such thing as a point in the universe that is considered stationary because movement is only relevent in relation to other objects, so the light-speed-barrier isn't relative to some imaginary "stationary point." So if all speed is relative to other objects, what is something that is trying to get past light speed but can't relative to?
1. Any object's movement is indistinguishable from not moving if it isn't in reference to another object. All movement is relative.
2. If an object is moving at 0.5c away from Earth relative to Earth, and another object is moving at 0.6c in the opposite direction away from Earth relative to Earth, the objects are both moving at 1.1c relative to each other.
3. You could consider either object as being stationary and the other one as moving, and both viewpoints would be equally valid.
4. So, with that all said, by what standard are we measuring something's speed to determine whether or not its going faster than light speed? Obviously, according to relativity, nothing can travel faster than light, because it would require an infinite amount of energy to do so. We can easily pick two stars and find that they are moving away from each other at faster than the speed of light. Then you look at one of their planets and find that they are pretty much stationary compared to each other (on a grand scale that is, obviously there is an orbit). There is no such thing as a point in the universe that is considered stationary because movement is only relevent in relation to other objects, so the light-speed-barrier isn't relative to some imaginary "stationary point." So if all speed is relative to other objects, what is something that is trying to get past light speed but can't relative to?
HalberdBlue on
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(Please do not gift. My game bank is already full.)
I think the weird part that you're missing is that in this situation, 0.5 + 0.6 = 1, not 1.1 -- from the position of being on either earth or the other object.
If you're at a third point watching, then you'd see that (from that perspective) 0.5 + 0.6 do indeed = 1.1.
This is where the time-dilation effects of relativity come from. Because nothing can move faster than light (according to relativity), you basically have to rewrite every other rule to ensure that nothing does move faster than light.
It's been a while since I studied any of this, though, so I could be wrong. There's a book called Einstein's Dreams which is basically a collection of relativistic thought experiments. You might find it interesting reading.
All movement/non-movement is indistinguishable in a non-intertial frame of reference (or is it the other way around, I can never remember which is the accelerating and which is the constant velocity one).
And everything is relative to the speed of light, not just entirely relative.
So, these things add up to - you need to do Einstein Velocity Addition, so get those Lorentz transformations ready...
As far as I can recall, the two bodies will be travelling apart at about 0.8c from what I can recall from similar numbers and physics class...
Ummm...
look here
http://hyperphysics.phy-astr.gsu.edu/hbase/relativ/einvel.html#c1