Traveling faster than the speed of light?

[Mentz114]

Magnetron:

Now I know no object object as yet observed by science can move faster that the speed of light (SOL), you did a very good job of explaining that.

It isn't just a case of observation - if an object were to move at LS relative to us, any light from it would be red-shifted to nothing so we could not see it in principle.

And what do we observe as the ship accelerates above the SOL, and then decelerates below the SOL?

According to special relativity this absolutely cannot happen. If you had nearly infinitely powerful rocket engines you can get closer and closer to LS, but never reach it. And this is true from every observers point of view.

 

[tiny-tim]

It isn't just a case of observation - if an object were to move at LS relative to us, any light from it would be red-shifted to nothing so we could not see it in principle.

Hi Mentz114!

But wouldn't we be able to see it against the background?

If we were slightly offset from its track, it would hide light coming from distant stars, so we could tell, visually, where it was, and when?

According to special relativity this absolutely cannot happen. If you had nearly infinitely powerful rocket engines you can get closer and closer to LS, but never reach it. And this is true from every observers point of view.

Yes. STL has to stay STL. FTL has to stay FTL. No crossing LS.

 

[JesseM]

The relativistic doppler shift formula is for a source that's emitting radiation at some set frequency in its own rest frame--the reason redshift goes to infinity (i.e. frequency goes to zero) as you approach c is because of time dilation, so if the source is emitting peaks at a frequency of one peak/microsecond in its own frame, in our frame the time between peaks being emitted (not seen) gets longer and longer as the source approaches c, since the time between microseconds on a clock moving along with the object is getting longer in our frame because of time dilation. I'm pretty sure the doppler shift formula doesn't imply that the frequency of light which is reflected off a moving object must go to zero as its speed approaches c (for example, assume the frequency of the incoming light before it hits the object is held constant in our frame, and only the speed of the object is varied).

 

[Mentz114]

tiny-tim:

But wouldn't we be able to see it against the background?

If we were slightly offset from its track, it would hide light coming from distant stars, so we could tell, visually, where it was, and when?

Very likely. I don't see why it shouldn't cast a shadow.

I should add that my remarks about red-shift only apply to receeding sources.

JesseM:

the doppler shift formula doesn't imply that the frequency of light which is reflected off a moving object

Isn't reflected light being absorbed and re-emitted ?
Interesting point. How do doppler speed traps work ? I thought the velocity of the car changed the frequency of pulses.

 

[Hernik]

But what would happen if we could see a spaceship moving faster than light?

According to SR an object moving at the speed of light travels an infinite distance at 0 time. (That is maybe what light "experiences".) So who needs to go any faster than c?

I've seen considerations that anything going faster than c would be traveling backwards in time. Tachyons are believed to go that fast ... but no one has found a tachyon

- Henrik

 

[Hernik]

sorry.. this was an old dead end I just revived. Realized that too late.

 

[Austin0]

The relativistic doppler shift formula is for a source that's emitting radiation at some set frequency in its own rest frame--the reason redshift goes to infinity (i.e. frequency goes to zero) as you approach c is because of time dilation, so if the source is emitting peaks at a frequency of one peak/microsecond in its own frame, in our frame the time between peaks being emitted (not seen) gets longer and longer as the source approaches c, since the time between microseconds on a clock moving along with the object is getting longer in our frame because of time dilation. I'm pretty sure the doppler shift formula doesn't imply that the frequency of light which is reflected off a moving object must go to zero as its speed approaches c (for example, assume the frequency of the incoming light before it hits the object is held constant in our frame, and only the speed of the object is varied).

Hi You may of course be right about reflected light but it also appears that there is some reason to assume the opposite.
Whether or not you consider reflection to be a re-emmission or a direct reflected waveform , wouldn't both cases suggest the doppler effect would take place.
If the reflecting surface is moving in the time between the incidence of phase peaks, wouldn't this result in an expansion of the waveform [or contraction depending on direction of motion] of the reflected wave??
If it is a case of re-emmission , then the light would seem to be subject to the normal doppler shift ?

 

[JesseM]

Hi You may of course be right about reflected light but it also appears that there is some reason to assume the opposite.
Whether or not you consider reflection to be a re-emmission or a direct reflected waveform , wouldn't both cases suggest the doppler effect would take place.
If the reflecting surface is moving in the time between the incidence of phase peaks, wouldn't this result in an expansion of the waveform [or contraction depending on direction of motion] of the reflected wave??
If it is a case of re-emmission , then the light would seem to be subject to the normal doppler shift ?

Yes, you're right, even in the case of reflection the peaks would get shifted because immediately after one peak is reflected, then the next peak won't be reflected until it catches up with the object which is moving away from it, so the space between peaks will be greater than if the wave had been reflected by an object at rest in our frame. Although unless I'm thinking about it wrong, it seems to me like the shift would not be the same as the relativistic Doppler shift in this case, since time dilation doesn't seem to play any role--you can calculate the shift without worrying about multiple frames, so it's just a matter of kinematics your own frame and the shift shouldn't be any different than for a wave moving at c reflected off a moving object in classical physics.

 

[Austin0]

Yes, you're right, even in the case of reflection the peaks would get shifted because immediately after one peak is reflected, then the next peak won't be reflected until it catches up with the object which is moving away from it, so the space between peaks will be greater than if the wave had been reflected by an object at rest in our frame. Although unless I'm thinking about it wrong, it seems to me like the shift would not be the same as the relativistic Doppler shift in this case, since time dilation doesn't seem to play any role--you can calculate the shift without worrying about multiple frames, so it's just a matter of kinematics your own frame and the shift shouldn't be any different than for a wave moving at c reflected off a moving object in classical physics.

Actually you have touched on something I have been wondering about.
Does time dilation play a role in relativistic doppler shift?
It seems like the effect is a direct result of relative velocity and as such, the difference between approach and recessional velocities is equal and opposite, whereas the time dilation would be exactly the same in both cases.
Does it consider time dilation regarding electron resonance frequencies and the effect on emitted and absorbed frequencies which apply in GR , but wouldn't really make any sense in this context where the dilation is assumed to be reciprocal at emitter and receiver?
But then I am still having a hard time forming a consistent picture of the analogous situation in GR, where there are two different effects, the time dilation effecting the emission and reception electrons, and the frequency shift attributed to the photon translation though the gravitational gradient. Either one separately makes sense and seems to completely explain the observed phenomena , but taken together, it seems like one effect too many, unless the actual measurements were greater than expected for either one by itself.
If that makes any sense??
Thanks