Relativity Question

[ConstipatedVigilante]

Given two spaceships that can somehow travel at .9C (don't ask how), what would happen if they traveled in opposite directions?　Intuitively, they would see each other traveling at 1.8c, but that's not possible, right? So would they both slow down to just under .5c, or would only the observing spaceship, or what?

 

[KIAman]

They would both see the other ship moving away at .92c.

 

[Ben90]

Redshift or Blueshift depending on the direction of travel.

 

[totalnoob]

They would both see the other ship moving away at .92c.

Umm..Isn't it more likely that they just wouldn't see the other ship? There is no possible way for the light from the other ship to reach them given the OP's scenario..so they would have no way to measure relative velocity.

 

[ConstipatedVigilante]

Umm..Isn't it more likely that they just wouldn't see the other ship? There is no possible way for the light from the other ship to reach them given the OP's scenario..so they would have no way to measure relative velocity.

I suppose that's true, but wouldn't they still be traveling at 1.8c relative to each other, neglecting observation? And isn't any speed beyond c, relative or not, impossible to reach?

 

[totalnoob]

A stationary observer on the outside would see them each moving at .9c in opposite directions, so yes..relative to each other they would be moving apart at 1.8c

A more tricky question would be what would happen if they were moving TOWARDS each other at a relative velocity of 1.8c What would that look like?

*brain explodes*

 

[TastesLikeChicken]

Umm..Isn't it more likely that they just wouldn't see the other ship? There is no possible way for the light from the other ship to reach them given the OP's scenario..so they would have no way to measure relative velocity.

Light from each ship would still be traveling at C which is faster than either ship is moving. Assuming both started their opposite voyage from the same general area, both ships would be able to observe the other with powerful enough telescopes (since we're already talking about spaceships traveling 9/10ths of the speed of light, may as well include them). Even if both started their voyage a light-year apart, within 36.5 days each would be able to observe the other.

 

[TecHNooB]

it hurts to think about relativity.

 

[TastesLikeChicken]

Oops. Thinking about it, I was incorrect with my last statement. It would actually be closer to 39 days because each ship would have traveled an additional .09 of a light year before the light reached them. Also, after that 39 days, each ship would see the other just leaving, assuming both left at the same time.

It does hurt to think about relativity and the speed of light, but in a good way.

 

[Ninjahedge]

Make it something where time dialation is less of an issue. If they were both going at .6c they WOULD see each other moving apart at 1.2c. Moving apart and absolute speed are two different things.

The light will travel out fom the back of the ship at c, not c relative to their own speed, just c. So one ship travels out at .6c, the light coming out the back of the ship will travel at c from the rear of the ship, it would be observed moving away at 1.6c (it IS additive) but not MOVING faster than c....

Time dialation will actually make everything seem to be moving FASTER, so.....


Think of it this way, no object in space can be seen as moving faster than light when observed by a STATIONARY OBSERVER. Relativity does not say that two objects speeding away from each other will not see the other apparently moving faster than light. Seeing something like that automatically tells you that you are not stationary!

 

[Paul98]

A stationary observer on the outside would see them each moving at .9c in opposite directions, so yes..relative to each other they would be moving apart at 1.8c

A more tricky question would be what would happen if they were moving TOWARDS each other at a relative velocity of 1.8c What would that look like?

*brain explodes*

Speed isn't absolute, it depends on the frame of reference. If from the outside observer each ship is moving away from it at .9c. Then in the frame of reference of the outside observer they will be moving away from each other at 1.8c.

This does NOT mean that they are moving faster than C. In each of the ships frame of reference the other ship is moving away from them at ~.9945c.

http://en.wikipedia.org/wiki/Velocity-addition_formula

 

[Paul98]

Make it something where time dialation is less of an issue. If they were both going at .6c they WOULD see each other moving apart at 1.2c. Moving apart and absolute speed are two different things.

That is just false, that isn't possible. First since velocity can only be known when you compare it to something else. So if they are both moving away from some center point at .6c, to that center point they would be moving away from each other at 1.2c. BUT from the frame of reference of ether ship you need to use the relativity velocity addition equation.

1.2c/(1+.36) = .882c.

The light will travel out fom the back of the ship at c, not c relative to their own speed, just c. So one ship travels out at .6c, the light coming out the back of the ship will travel at c from the rear of the ship, it would be observed moving away at 1.6c (it IS additive) but not MOVING faster than c....

the light WILL travel at C relative to there own speed, this is the very basic part of relativity. The speed of light coming away from the ship would be c NOT 1.6c.

Time dialation will actually make everything seem to be moving FASTER, so.....


Think of it this way, no object in space can be seen as moving faster than light when observed by a STATIONARY OBSERVER. Relativity does not say that two objects speeding away from each other will not see the other apparently moving faster than light. Seeing something like that automatically tells you that you are not stationary!

A stationary observer can by anyone who is moving at a constant velocity. That is another basis of relativity.

http://en.wikipedia.org/wiki/Inertial_frame_of_reference

also here is video that is rather interesting about things happening at different times depending on the frame of reference.

http://www.youtube.com/watch?v=wteiuxyqtoM

 

[Paul98]

Given two spaceships that can somehow travel at .9C (don't ask how), what would happen if they traveled in opposite directions?　Intuitively, they would see each other traveling at 1.8c, but that's not possible, right? So would they both slow down to just under .5c, or would only the observing spaceship, or what?

Some basics about relativity, first there is no absolute speed, in one frame of reference you may not be moving at all. In another one you might be moving at .4c, another one at .995c. So to figure out how fast you are moving you need to figure out with respect to what.

Also the speed of light moves at c in a vacuum. This doesn't matter which frame of reference you use. Light will always be moving in your frame of reference at C.

 

[Ninjahedge]

Paul, I do not think you are right on that.

Observing something that seems to be going faster than the speed of light will NOT mean that they are travelling faster than it.

Two things ruin the relativity argument:

1. If they were traveling along a giant ruler, both starting at "0". they would both be able to tell where the other was at any gioven time in relation to where THEY were. They would also be able to do the math bsed on the travel time of the signal (light) itself to see how long it would take.

2. Time dialation would make them seem to be going EVEN FASTER.

The only thing relativity says is that light travels at a constant speed and that nothing can travel faster than light. It does not say that you cannt see another moving away from you at faster than that if you are both moving.

Relativity has a weird relation to light though. I don't think I have ever heard of anyone saying anything about traveling at .99c and firing a beam backwards. Supposedly they will not be able to see it go faster than light speed regardless of their own speed, but how would this be possible without a duality?

 

[Throckmorton]

There's something I don't understand... If you're in a space ship going at a high speed, time compresses for you and dilates for everybody else. So you travel for a few years and when you return to earth, you're hundreds of years in the future.

So shouldn't the same time differential exist with the two spaceships? Time goes much slower for you because you're traveling at a high speed. But shouldn't the same thing happen for the other spaceship?? Who compresses and who dilates time? What supreme being decides that?

 

[TastesLikeChicken]

There's something I don't understand... If you're in a space ship going at a high speed, time compresses for you and dilates for everybody else. So you travel for a few years and when you return to earth, you're hundreds of years in the future.

So shouldn't the same time differential exist with the two spaceships? Time goes much slower for you because you're traveling at a high speed. But shouldn't the same thing happen for the other spaceship?? Who compresses and who dilates time? What supreme being decides that?

If both ships were traveling at identical speed the time dilation effects would be the same for both. Given a large enough distance and a speed fairly close to C (since time dilation doesn't have a huge effect until velocity is near C) they would return well into our future.

Nobody compresses or dilates time. It's simply an attribute of the physics in our universe. It's been measured and verified. I imagine it's a necessary component. Without it we probably wouldn't be around because if it didn't happen there'd be all kinds of issues with particle decay. Think about it. A photon lasts so long because by traveling at the speed of light time essentially stops for it so it never ages.

 

[Paul98]

Paul, I do not think you are right on that.

Observing something that seems to be going faster than the speed of light will NOT mean that they are travelling faster than it.

First please look at those links as it may help out.

I not that sure what you are trying to get at here.

Two things ruin the relativity argument:

1. If they were traveling along a giant ruler, both starting at "0". they would both be able to tell where the other was at any gioven time in relation to where THEY were. They would also be able to do the math bsed on the travel time of the signal (light) itself to see how long it would take.

It depends on the frame of reference, in an outside observer who sees the ships moving away from each other. The distance between each ruler mark would be a certain distance. But for those ships moving down the ruler the marks are closer together. So if a ship has a ruler that measures 1 foot. Then they are moving at .9c with respect to this giant ruler, and they check to see how long there ruler is compared to each foot on the giant ruler. each foot on the giant ruler will be shorter than there 1 foot ruler. Here is a link to what is happening with that.
http://en.wikipedia.org/wiki/Lorentz_contraction

Depending on the frame of reference the time it takes for them to move a certain distance will be different.

2. Time dialation would make them seem to be going EVEN FASTER.

The only thing relativity says is that light travels at a constant speed and that nothing can travel faster than light. It does not say that you cannt see another moving away from you at faster than that if you are both moving.

Relativity says that the speed of light is the same in all inertial frames of reference. Only way you get velocity is by figuring out how fast you are moving in relation to something else. Velocity isn't absolute, there isn't an absolute rest frame that you can say on my speed is this against. And if you have a frame of reference seeing two objects moving away from each other at a speed faster than C. It only means that in there frame of reference this is happening. If you go to the frame of reference of ether of the objects they will NOT be moving away from each other faster than c.

Look at the link for velocity addition that I posted. If you don't believe me why would there even be this equation? or the link below might help also. just remember the "rest" observer they could have just as easily called Observer B the rest observer. Also remember that it's not just that these are observed but it's actually what is happening.

http://hyperphysics.phy-astr.gsu.edu/hbase/relativ/einvel.html

Edited: In the frame of reference for each space ship time moves slower in the other space ship.

Relativity has a weird relation to light though. I don't think I have ever heard of anyone saying anything about traveling at .99c and firing a beam backwards. Supposedly they will not be able to see it go faster than light speed regardless of their own speed, but how would this be possible without a duality?

If two spaceships lets call them A and B are moving away from each other at .99c. Spaceship A fires a beam of light at spaceship B. On spaceship A the light moves away from it at c. On spaceship B the light moves away from spaceship A at 1.99c.

as for duality I think you should look up Relativity of Simultaneity. or here
http://en.wikipedia.org/wiki/Relativity_of_simultaneity

it's also in that video I posted earlier.

 

[Paul98]

There's something I don't understand... If you're in a space ship going at a high speed, time compresses for you and dilates for everybody else. So you travel for a few years and when you return to earth, you're hundreds of years in the future.

So shouldn't the same time differential exist with the two spaceships? Time goes much slower for you because you're traveling at a high speed. But shouldn't the same thing happen for the other spaceship?? Who compresses and who dilates time? What supreme being decides that?

http://en.wikipedia.org/wiki/Twin_paradox#Resolution_of_the_paradox_in_special_relativity

and a quote from it and in this example they used two twins one who stays on earth and another who leaves earth then returns.

Special relativity does not claim that all observers are equivalent, only that all observers at rest in inertial reference frames are equivalent. But the space ship jumps frames (accelerates) when it performs a U-turn. In contrast, the twin who stays home remains in the same inertial frame for the whole duration of his brother's flight. No accelerating or decelerating forces apply to the homebound twin.

 

[Paul98]

But a really good to get answers to the questions that you guys are having is

http://www.physicsforums.com/forumdisplay.php?f=70
http://www.physicsforums.com

 

[DrPizza]

If you think that two ships moving apart at .6c each would see the other receding from them at 1.2c, then you're using Newtonian physics, not relativistic physics. Sorry, you're wrong.

 

[Ninjahedge]

The only thing that turns this on its ear is if you put a clock on the back of each ship so they can reference the speeds of the vehicles involved.

Since you are using Light to "see" what the other ship is doing, the time it takes for light to get from one to the other will make it appear that the ship is travelling slower than it is, but that is countered when you have both a distance and a time meter OR if you have some other means of observing.

So although it may be physically possible to have two things seperating faster than the speed of light, you cannot USE light to "see" it...


Ah well.

The only thing that throws a monkey wrench into the whole thing is still time compression and dialation.

If that is linear, then you eliminate the viewer lag that happens when you are running away from the light that is coming to you......

What is the compression at .6c?

 

[Ninjahedge]

Funky, with the Lorentz factor, they see each other moving AT c.

Both moving away from each other at .6c

at 1 second they are both at 111.6K miles from the start. At 2 seconds, 223.2K mi.

But one ship only sees the other at those two points at 3 and 6 seconds, where it is at 334.8K and 669.6K miles.

So it sees, in 3 standard seconds (stationary) a traveling of 111.6K+334.8K miles = 446.4K miles.

The Lorentz factor (1/(1-v^2/c^2)^0.5) comes out to 1.25. So, 3/1.25 = 2.4 perceived seconds on the ship.

446.4K mi/2.4 sec = 186K mi/sec = Light Speed.

Was the Lorentz factor derived using this supposition to begin with? Am I just calculating around in circles, or was that formula tested and proven?

 

[TastesLikeChicken]

You are mixing reference frames, which is one reason why your calculation is incorrect. An independent observer on earth would see them at those distances of separation. On each ship, however, they wouldn't appear to be that far apart. If you want to know the speed at which each ship sees the other moving away you have to calculate from their reference frame, not an independent one.

 

[Paul98]

The only thing that turns this on its ear is if you put a clock on the back of each ship so they can reference the speeds of the vehicles involved.

Since you are using Light to "see" what the other ship is doing, the time it takes for light to get from one to the other will make it appear that the ship is travelling slower than it is, but that is countered when you have both a distance and a time meter OR if you have some other means of observing.

So although it may be physically possible to have two things seperating faster than the speed of light, you cannot USE light to "see" it...


Ah well.

The only thing that throws a monkey wrench into the whole thing is still time compression and dialation.

If that is linear, then you eliminate the viewer lag that happens when you are running away from the light that is coming to you......

What is the compression at .6c?

It's not just what the observer sees, you don't need to actually see the ship. We are talking about what actually is happening on these ships not just what you see. Those lengths are actually smaller it's NOT that you see smaller lengths by some optical illusion.

Really please go and actually read all those links. Maybe head over to that forum I posted and ask your questions there. Right now you are trying to use some of your notions that are incorrect.

Like if you say you have two spaceships A and B and they are moving away from each other at .6c. This means that spaceship A is moving away from spaceship B at .6c. Or if you wanted to you could say spaceship B is moving away from spaceship A at .6c.

Now if you say you have spaceship A moving in one direction at .6c, and another spaceship B moving in the other direction at .6c. You have already added a third frame of reference, it's the one you have these two spaceships moving away from. If you want to figure out how fast these ships are moving away from each other in ether one of there frame of reference you use the velocity addition formula.

 

[Ninjahedge]

You are mixing reference frames, which is one reason why your calculation is incorrect. An independent observer on earth would see them at those distances of separation. On each ship, however, they wouldn't appear to be that far apart. If you want to know the speed at which each ship sees the other moving away you have to calculate from their reference frame, not an independent one.

I am not saying that. I am telling it as it is.

My example does take it from the POV of a guy on the ship. Read through it. The places each ship appears to be based on how long it would take the light to get from one ship to another when it passes a particular point.

The 3 and 6 second points on the observers ship would be the places where the other would appear to be at 1 and 2 seconds (with 2 and 4 seconds lag due to travel time of the light). They will see, using light as the transmission media, those distances at those times. IOW, at T=3 seconds, the light from the other ship from T=1 would just be reaching it....

Now add the time slowing effect...


And paul, no. I am saying they are each travelling away from an absolute stationary point in space at .6c. The rate that they are seperating is 1.2c. According to my calcs, if they were lookng out their rear view they would see the other moving away from them at the speed of light due to the lag from the time it takes light to catch up, and then taking that and applying Lorentz.


IOW, they ARE moving away from each other at 1.2c.

If they went ONLY on looking out the back, they would appear to be seperating at c.

If there was NO time slowing on the craft, they would see the other seperating at 0.8c (Lorentz = 1.25)

If they were all physicists and knew the color of the other ship or were aware of how fast they were actually travelling, they would "see" the other moving away at 1.2c (applying red shift to the calc, Lorentz, or otehr factors...)



The question is posed again, is Lorentz actually proven (did not google) or is it simply a formula used to try and make everything fit together?

Or did I just happen to choose the perfect speed that when all factors are applied that they appear to be seperating exactly at c?