Solving the Mystery of Relativity: Can You Travel Faster than c?

[KrisOhn]

I've been thinking of this for the past couple days and I can't find an answer myself, so I figured I would come here.

The theory of relativity states that the speed of light, c, is the highest speed something can possibly travel at with respect to an observer.

To me this means that from a stationary observer, you cannot travel faster than the speed of light, but say you were traveling at .9c and there was another observer traveling next to you at 0.9 your speed. In my mind you would be able to accelerate to near the speed of light from that observer traveling next to you, but at that point your speed with respect to the original observer would be much greater than c.

A way of thinking of this is using solar sails. The speed of light is constant from all reference frames so using a solar sail you should be able to continually accelerate since the speed of light never slows down no matter how fast you go.

In my mind this is possible, it's just that at the second speed, with respect to the original observer, you wouldn't be able to measure your velocity because the information cannot transfer between the two. This essentially is the thought that a speed greater than c is possible, but cannot be measured, so it's a moot point.

My question is, am I correct in thinking this? If not, why? And, if I am, as you accelerate up to near c for the secondary observer (accelerating past c for the stationary one) what would happen to the moving source relative to the stationary observer? Would it just disappear? Or would it look to continue getting closer and closer to c?

 

[totentanz]

You now my friend,when you say something is moving with respect to x,y,... you presupose that some frame of refrence is special,and this is (I think) is the core of S.Relativity,there is now special frame of reference...I think you should read about the Tachyon...and I am soory that I can not guide you

 

[tiny-tim]

Hi KrisOhn!

see http://en.wikipedia.org/wiki/Addition_of_Velocities_Formula"

 

[KrisOhn]

Hi KrisOhn!

see http://en.wikipedia.org/wiki/Addition_of_Velocities_Formula"

Well damn!

That is something else

Thank you.

 

[harrylin]

I've been thinking of this for the past couple days and I can't find an answer myself, so I figured I would come here.

The theory of relativity states that the speed of light, c, is the highest speed something can possibly travel at with respect to an observer.

To me this means that from a stationary observer, you cannot travel faster than the speed of light, but say you were traveling at .9c and there was another observer traveling next to you at 0.9 your speed. In my mind you would be able to accelerate to near the speed of light from that observer traveling next to you, but at that point your speed with respect to the original observer would be much greater than c.

A way of thinking of this is using solar sails. The speed of light is constant from all reference frames so using a solar sail you should be able to continually accelerate since the speed of light never slows down no matter how fast you go.

In my mind this is possible, it's just that at the second speed, with respect to the original observer, you wouldn't be able to measure your velocity because the information cannot transfer between the two. This essentially is the thought that a speed greater than c is possible, but cannot be measured, so it's a moot point.

My question is, am I correct in thinking this? If not, why? And, if I am, as you accelerate up to near c for the secondary observer (accelerating past c for the stationary one) what would happen to the moving source relative to the stationary observer? Would it just disappear? Or would it look to continue getting closer and closer to c?

Are you sure that you checked out the recent threads?

- https://www.physicsforums.com/showthread.php?t=482318
- https://www.physicsforums.com/showthread.php?t=482294

In addition, a speed greater than c can surely be measured between two points A and B:

A ---------------> B
t1 ... L ... t2

v=L/(t2-t1)

However, this strongly depends on clock synchronization. More fundamental is the measurement at a single point over a distance. Take for example a light source and detector, with the light reflected from a mirror (all objects fixed to an inertial platform that you use as reference system):

A -----------------> M
<------------------
t1,t2 ... L

v=2L/(t2-t1)

The speed of light in vacuum that you thus will measure is always c, independent of the velocity of your system.

- section 1 of http://www.fourmilab.ch/etexts/einstein/specrel/www/

 

[tiny-tim]

Everything is something else!

(unless you go round in circles! )​

 

[bobc2]

...In my mind this is possible, it's just that at the second speed, with respect to the original observer, you wouldn't be able to measure your velocity because the information cannot transfer between the two. This essentially is the thought that a speed greater than c is possible, but cannot be measured, so it's a moot point.

My question is, am I correct in thinking this? If not, why? And, if I am, as you accelerate up to near c for the secondary observer (accelerating past c for the stationary one) what would happen to the moving source relative to the stationary observer? Would it just disappear? Or would it look to continue getting closer and closer to c?

Hi, KrisOhn

Here is a sequence of spacetime diagrams that kind of picture the situation. First, in the upper left box of diagrams, I just try to emphasize the way in which a pair of x1 and x4 coordinates are oriented for someone moving at relativistic speed with respect to the black rest system. The sequence in the left box shows blue systems corresponding to moving with higher and higher speeds relative to black.

But now we show in the right side sequence of sketches that you can simply present the situation from the point of view of one of the blue guys (from sketch f), that is, you now show the situation in the rest system of the sketch f) blue guy and add in another red guy who is moving even faster with respect to the original black coordinates than the blue guy was (see sketch h).

But, even after that, you can show the situation in the red guy's rest system and add in yet another faster moving purple guy (see sketch i). And you could then show the situation with the purple guy's rest system, then add in someone going even faster with respect to him---and on and on ad infinitum (as long as you don't try to have one of them going at light speed).