Faster than light

Main Question or Discussion Point

I know that relativity doesn't permits a motion at speed of light or fast, as the Lorentz factor becomes infinite and we get absurd results, but what about wormholes or something, say wrap drive and what about http://en.wikipedia.org/wiki/Alcubierre_drive" [Broken] and
tachyons.
Also I've heard that in quantum chromodynamics that electrons normally travel at speed greater than c, ans some less than c, so that average speed is c.
So does this all means that we can go faster than light, if so than what will happen to casualty?

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I know that relativity doesn't permits a motion at speed of light or fast, as the Lorentz factor becomes infinite and we get absurd results, but what about wormholes or something, say wrap drive and what about http://en.wikipedia.org/wiki/Alcubierre_drive" [Broken] and
tachyons.
Also I've heard that in quantum chromodynamics that electrons normally travel at speed greater than c, ans some less than c, so that average speed is c.
So does this all means that we can go faster than light, if so than what will happen to casualty?
Special relativity explains that an information cannot travel at speed greater than c, because the Lorentz factor would become a complex number ; but there are situations where "something" goes faster than c. For example, imagine a laser pointed forward a planet ; then, you turn the laser forward another planet ; this action during one second. If there is a distance of one light-year between the two planets, the spot will have a speed of one light-year per second.

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HallsofIvy
Homework Helper
"The spot" not being an actual physical entity, of course.

Dale
Mentor
but what about wormholes or something
The whole point about wormholes etc. is that you always have a timelike worldline (ie v<c always). Wormholes are about taking a shortcut, not about going fast.
if so than what will happen to casualty?
You can have any two of the three: relativity, causality, and FTL. Right now it looks like relativity and causality, not FTL.

Special relativity explains that an information cannot travel at speed greater than c, because the Lorentz factor would become a complex number ; but there are situations where "something" goes faster than c. For example, imagine a laser pointed forward a planet ; then, you turn the laser forward another planet ; this action during one second. If there is a distance of one light-year between the two planets, the spot will have a speed of one light-year per second.
No, light will reach the spot only at c so the speed of the spot will prove to be c The light would be like a string rotated in the air.

Doc Al
Mentor
No, light will reach the spot only at c so the speed of the spot will prove to be c The light would be like a string rotated in the air.
No, there's nothing preventing a "spot" from moving faster than c. As has been pointed out, the spot is not a physical entity, so there's no issue with relativity.

diazona
Homework Helper
No, light will reach the spot only at c so the speed of the spot will prove to be c The light would be like a string rotated in the air.
The spot will appear to travel at 1 ly/s, since at one time the laser is shining on one planet, and 1 second later the laser is shining on the other planet. But all this will only be seen on the planets some time after the person holding the laser moves it, because of the finite speed of light.

No, the spot does not move at>c, because you do not see it to be moving at>c as light takes longer to reach the spot and reflect back

Doc Al
Mentor
No, the spot does not move at>c, because you do not see it to be moving at>c as light takes longer to reach the spot and reflect back
The travel time of the light to the surface is irrelevant.

The point here is that he said the spot moves at >c and all agreed.That is what is clearly wrong to me you cannot say the spot has moved unless you see it

Doc Al
Mentor
sylas
The point here is that he said the spot moves at >c and all agreed.That is what is clearly wrong to me you cannot say the spot has moved unless you see it
Doc Al is correct. The time it takes for the light to get from the spot to you is irrelevant.

For example, consider a laser beam swept across the face of the moon; assume that it is bright enough that you can see it; and that it takes 1 microsecond to sweep from one side to the other. The dot moves faster than c.

From Earth, you see the dot sweeping across the face of the moon a bit over a second afterwards. But you still see the dot sweeping over the face of the moon in a microsecond.

If you are closer, or further away, you'll change the time between when you see the spot moving and when it actually swept across the moon; but that makes no difference to how fast the spot is moving or how fast you see it moving.

Cheers -- sylas

PS. For pedants in the audience; yeah, I know there will be tiny differences due to different angles at one side of the sweep or the other. It's still seen to be moving at almost the same velocity it actually did sweep across the surface, and that speed is much greater than lightspeed.

The scissor Gedanken exp is wrong The theory essentially says no mass can move at speeds greater than c Mass elements farther than approximately 3*10^7 away are not free to move in(neglect curvature)
Also, there is shadow only if there is light, so the speed of shadow is restricted to be equal to the speed of light

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sylas
The scissor Gedanken exp is wrong The theory essentially says no mass can move at speeds greater than c Mass elements farther than approximately 3*10^7 away are not free to move in(neglect curvature)
"scissor"?

I'm guessing you mean a case where you close scissors sufficiently fast and the cutting edge where the two arms meet moves faster than light.

This edge is not a "mass element". The sweeping dot is not a "mass element". There is no violation of relativity or the limit of light speed on transmission of information or particles.

Added in edit: Found it. You are referring to The Superluminal Scissors. It is indeed precisely as I guessed, and it is not "wrong". In the specific example given, the contact point moves faster than the speed of light, but there is no matter moving faster than light speed.

This edge is not a "mass element".
Yes?Then what is it?
The sweeping dot is not a "mass element"..
It is not, but if you think of my answer you will find it not to be greater than c

. But you still see the dot sweeping over the face of the moon in a microsecond.
No no no

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sylas
Yes?Then what is it?
The point in space where the two blades meet of course. The particles involved in that point keep changing as the scissors close.

It is not, but if you think of my answer you will find it not to be greater than c
Your answer is flatly wrong. This isn't hard. Abstract points like the meeting point of two scissor blades or a dot of light sweeping over the face of the moon easily go much faster than c. There's no particles moving with the dot, or the cutting point on scissors.

I have no idea why you don't follow this. It isn't hard.

The point in space where the two blades meet of course. The particles involved in that point keep changing as the scissors close.

Your answer is flatly wrong. This isn't hard. Abstract points like the meeting point of two scissor blades or a dot of light sweeping over the face of the moon easily go much faster than c. There's no particles moving with the dot, or the cutting point on scissors.

I have no idea why you don't follow this. It isn't hard.
The explanation is right, but it takes much much longer to close the scissor as written there,it is written that it takes longer for the tips to come close, and note that I had initially written the light travels as a string rotated in the air, analogous to bending of blades here and from the numeric mention in the previous post the contact point moves at c towards the tip, not greater than c

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I'll explain. In the gedanken expt, the scissor is closed in 1/10 of a sec. If the edges were not to bend, the scissor had to be of length c/10 meters the farthest mass element has velocity c.If in 1/10 sec the edges overlap, the velocity of the farthest element and the contact point are both same=c.Even if they were bent, the velocity of the farthest element(at 1 ly meters,original lt)is c and the velocity of cp is c
The blades are said to be bent because it takes apparently longer time for the farther elements to come close
The scissors flex at c/10 meters

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Abstract points like the meeting point of two scissor blades or a dot of light sweeping over the face of the moon easily go much faster than c. There's no particles moving with the dot, or the cutting point on scissors.

.
There's no particles moving with the dot, but that does not mean you can see the universe rolling around you at superluminal speeds.You perceive it as moving slower than supposed, because it takes time for light from such large distances to reach you

Shadows do not travel faster than light as mentioned in post#13,
If you still say contact point can move faster than light, it is mentioned in the explanation of gedanken exp that it takes longer for the scissor edges to come close completely(it would be possible to say how much if the angle were mentioned) so the contact point moving faster than light would surpass the tip of the scisor

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Hello all,

Is the following reasoning valid?

The events of the laser beam stopping its illumination of planet A and the event of the laser beam first illuminating planet B are events in spacetime. Events in spacetime do not move and so the question of how fast an event travels to another event is meaningless. It is of course true that, if the spatial separation of the planets is large enough (this is presumably intended by the questioner) it would require a superluminal signal to leave the first event, beam stopping illuminating planet A, and be present at the second event, beam first hitting planet B, in the frame in which they are all assumed to be at rest with respect to each other.

Matheinste.

DrGreg
Gold Member
vin300

Consider this. You are on earth and you aim a laser beam at a target that is 2 light-seconds wide and one light-minute distant. Your laser is aimed with the help of a telescopic sight with crosshairs.

Up until 12:00:00 the laser is aimed at the left edge of the target. You can see both the crosshairs and the laser spot on the left of the target.

Between 12:00:00 and 12:00:01, you change the aim to the right edge of the target. From 12:00:01, you see the crosshairs on the right edge of the target, but you still see the spot on the left edge, because it takes a minute for the light to reach the target.

The light aimed at the left edge continues to reach the target until 12:01:00. The light aimed at the right edge reaches the target from 12:01:01 onwards. This is because each photon of light travels in a straight line in whatever direction you aim it, and takes exactly one minute to get there.

You, on Earth, still have to wait another minute for the reflected light to get back to earth. Up until 12:02:00 you see the laser spot on the left side of the target. From 12:02:01 you see the laser spot on the right side of the target.

Between 12:02:00 and 12:02:01, you see the spot move 2 light-seconds in one second. And you know the spot actually moved between 12:01:00 and 12:01:01. Whichever way you look at it, the spot moved 2 light-seconds in one second, which is twice the speed of light.

If you still think that's impossible, which step in the above argument do you think is wrong?

(All times and distances measured relative to Earth. Gravitational effects ignored.)

russ_watters
Mentor
Shadows do not travel faster than light as mentioned in post#13,
If you can make a point of light travel at faster than C (and you can), you can make a shadow travel faster than C. The principle is exactly the same.
If you still say contact point can move faster than light, it is mentioned in the explanation of gedanken exp that it takes 1 ly for the scissor edges to come close completely so the contact point moving faster than light would surpass the tip of the scisor
The speed of the contact point is not constant, it accelerates as the scissors close.

i think this is explained somewhere else. the individual photon does not travel > c.

The illusion that the laser travels faster than c is because once you move the laser to another spot, it is different photons that hits the new spot.