Faster than light

[Numsgil]

Relativity always made my head hurt.

The reason cestial bodies can move aparty at speeds greater than the speed of light is that the actual fabric of space is expanding.  Either celestial body isn't moving greater than light speed in its frame of reference, but compared to each other they look like it.

The end effect is a doplar shift in the light's wavelength that either body recieves.  Usually a redshift (I think).

[Endy]

I think I read that you have to either do extremly complicated equations or measure the speed from a nonmoving frame of reference, to tell the actual speeds(all of which are less than light).

What exactly would an object going backwards in time look like to us? I heard some talk about antimattter being matter going backwards in time, but that it still can't be used to send a message since we can only observe reactions taking place in a forward direction. My guess is that time travel is possible as long as the law of conservation holds. Since matter/anti-matter are essencially opposites it should balance out. But I'm not exactly planning on volunteering to be turned into explosive anti-matter anytime soon :)

Maybe, like PY says, we're really in a giant computer and all we need to do is to find the savegame file somewhere. Who knows?

Endy :blueblob:

[Zelos]

when tis about the special relativity and 2 ships moving at 0,9c and are moving toward each other, there is a formula which tells us they will only see that the other ship is moving whit 0,99C or something like that. nothing can move whit the speed of light relative to anything. and the speed of light is always the same relative to everythingand endy, antimatter is like ordinary matter, but it have the opposite charge, a proton is -1 and a electron (positron) is +1, you get it?

[PurpleYouko]

I think I read that you have to either do extremly complicated equations or measure the speed from a nonmoving frame of reference, to tell the actual speeds(all of which are less than light).

Trouble is there is no such thing as a non-moving frame of reference.

I remember a simple example that my college physics proffessor showed us once to demonstrate relativity and why time is the variable rather than light speed. You have to have a pretty good imagination for this.

There is an observer on an asteroid who watches a spaceship fly past at high speed.

On board the spaceship is another observer who carries out an experiment to measure the speed of light. he fires a light beam from one side of the ship to the other, where it hits a mirror and returns to a detector at the source. He knows the distance travelled and he measures the time taken so he can easily calculate the velocity. Distance = twice the width of the ship. Time = t. Go figure.

The observer on the asteroid is able to watch the beam of light that traverses the spaceship and makes his own measurements of it. To him the light beam doesn't simply cross the ship and back in a straight line, It travels diagonally as the spaceship is moving. Let's say the ship is moving at the correct speed for the light beam to be moving at a 45 degree angle to the perpendicular.

From the asteroid observer's frame of reference, the light beam travels a much greater distance than it does for the observer on the spaceship. he measures that distance as 2 * the square root of (double) the square of  the width of the spaceship using simple trigonometry.

He calculates the speed of light from his time and distance measurements.

According to Einstein, both observers will measure the speed of light to be exactly the same.

We know that the light beam must actually be moving in the way that the observer on the asteroid sees it since the ship is in motion. It is logical that the light beam must be moving with the direction of the spaceship's travel and not just side to side since it takes a finite time to complete its trip.

Therefore, in order for the observer on the spaceship to measure the speed of light to be the same universal constant, his time must be moving slower than that of the observer on the asteroid.  (relatively)

Everybody follow this still?

I will leave it at this point before carrying on. More later.

 :D  PY  :D

[shvarz]

Makes perfect sense to me.  I guess the question I have here is how these two guys know which one is moving and which one is sitting still?  It might as well be that the ship is stationary and the asteroid is passing by :)
Also, on the practical side of things: these two dudes seem to observe and measure the speed of light without any special equipment.  In real life they'd be somewhat distanced from the beam of light in question and will use some physical phenomena to a) measure the distance the light travels and B) measure the time.  These measurements would introduce another level of calculations and make the problem really complicated!  I'm glad I'm not a physicist to figure all this stuff out :)

[PurpleYouko]

Makes perfect sense to me.  I guess the question I have here is how these two guys know which one is moving and which one is sitting still?  It might as well be that the ship is stationary and the asteroid is passing by :)
Also, on the practical side of things: these two dudes seem to observe and measure the speed of light without any special equipment.  In real life they'd be somewhat distanced from the beam of light in question and will use some physical phenomena to a) measure the distance the light travels and B) measure the time.  These measurements would introduce another level of calculations and make the problem really complicated!  I'm glad I'm not a physicist to figure all this stuff out :)

Exactly right.

It all makes perfect sense within th eprecepts of relativity.

And there is no known way that these observers could actually measure the stuff in this way. You have to assume a few things here but they shouldn't change the fundamentals of the equations.

Your point about the relative speed of the spaceship and the asteroid actually lead on to my next point which is where a lot of this logic falls apart for me.

Let's imagine that the asteroid is actually a second spaceship going the other way. It too has a little guy doing a light beam experiment as well as watching the other guy's light experiment through some kind of subspace observation module or something.

In this scenario both observers will see time slowing down for the other observer while their own stays the same.

If both ships travel in a wide circular path and eventually meet again while constantly observing each other (again with the imaginary subspace instant communications module), each one should see the other as being much younger than he is himself. Eventually, each observer will see the other one remain young while he himself dies of old age.

This is somewhat of a paradox that I just can't get my head around.

I realize that a wide circular path means an acceleration is applied which could change things according to special relativity so let's assume that this path is an orbit of a large black hole or something so that the gravity will exactly cancel the acceleration of travelling in a circle.

How do you square these observations with each other?

 :blink:  PY  :blink:

[Zelos]

talking about relativity? sweet I love how this evolve. well when it comes to the timedilation that happen when you begin to reach speed close to light can easly be calcylated whit this formula:
T2=T1/root(1-(V/C)²)
where T2=the time for us
T1= the time on the ship
V=velocity of the ship
C=speed of light
so lets say they are moving at 99% of the light, their time will then be slowed down so that 1 second for them is 7 for us. but both is still moving whit the same speed relative to the blackhole on question they are orbeting. but to reach this speeds they need to be very close to the blackhole. So now of a suddenly the timedilation from gravity become important, the formula for that is:
T2=T1/root(1-(R1/R2))
where T1/T2 is the same as whit speed
and R1=Schwarzschild radius (the place where the escape velocity is C, nothing can from here return)
and R2 is the distance from the center of the blackhole.
but both cant be older for each other and still be young to it self. where have you heard that? I would like to read it, but I dont think that is possible, coz what happen then when both stop moving? they are younger compared to each other, and still older compared to each toher, its not possible to be older/younger compared to both. but if you can show me where you read it I´ll gladly read it
 :lecture: zelos :lecture:

[PurpleYouko]

its not possible to be older/younger compared to both. but if you can show me where you read it I´ll gladly read it

I didn't read it. it is an exercise in logic. just think it through.

Also these observers don't need to be anywhere close to the speed of light and they could be orbiting the entire universe rather than a black hole. It doesn't matter.
Each observer sees his own time as normal and measures light speed at C.
Each observer also sees light moving further on the other spaceship than the observer on that spaceship does.
Therefore each observer sees time moving more slowly on the opposite ship than it does on his own.
Inference: The observer will grow old and die while the other won't.

This clearly can't be correct.

I can't see anything wrong with the logic so what is the deal here?

 :wacko:  PY  :wacko:

[Zelos]

well, if they have the same speed compared to the same thing they will never become older compared to each other. they have the same flow of time. if they dont have the same speed or are compared to different thing one will have a slower flow of time and will be younger when they reach the same speed to compared to the same thing
 :lecture: zelos :lecture:

[shvarz]

PY, I found a web-site that explains special thoery of relativity in very simple terms: http://www.phys.vt.edu/~takeuchi/relativity/notes/

You should check it out.  The question that you posed is a variant of the old "twin paradox" and it is explained right here: http://www.phys.vt.edu/~takeuchi/relativit.../section15.html

[Botsareus]

In the (x,t) frame, the apple is hit after the arrow was shot. In the (x',t') frame, the arrow being hit and the shooting of the arrow occur simultaneously. But in the (x'',t'') frame the apple is hit before arrow is shot!. How can this possibly happen? This contradiction occurs because we allow the arrow to travel along the line connecting A and B which requires the arrow to travel at a speed exceeding the speed of light.

Bs , Even if arrow travels faster then light, The abserver will see apple being hit after he launched the arrow: why? Simple , because light is slower then the arrow it talkes more time for light to travel from the apple to the abserver then for the arrow to the apple.

aye , lets look at another example: wth does instantaneous communication have to do with faster then light travel? And even if there was sutch a thing then distance and speed of objects wont matter.

[MightyPenguin]

PY, one gets old and dies because time from his perspective and that of his body is actually going faster, as I understand it.

[Botsareus]

But do I have a point here or what? Or is my english so bad no one can read it? I mean who is going to argue with 50 years of physics? right?

[MightyPenguin]

Depends how much faster the arrow is going than light, I suppose. And you can only communicate instantaeneously if the carrier medium can move the information from point to point instantaneously; I.e. faster than light.

[Botsareus]

Depends how much faster the arrow is going than light, I suppose.

Lets do a picy:

 :shoot:  -------------------------------------------------------------------  :help:

light reached here in <-- 20^202 years because its too far <-----------

----------> too fast arrow -----> reached the target in 1/10^10 a milisecond

means the guy had to wait a nice 20^202 years - 1/10^10 milisecond witch still about 20^202 years.