# Faster than light.

## Main Question or Discussion Point

Well, it's an interesting thought, but one that I can't quite believe, or maybe more I can't understand how, something can travel faster than light?

I've read this wikipedia, and although a few people don't trust what is written on wikipedia, I have heard of it before.

Could anyone explain quite simply how this can happen? Or maybe just some information that will make it a bit easier to understand, I'm in year 12, so alot of the information on that page, I cannot understand.

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So things can go faster than light, but as humans we cannot pick up information travelling at those speeds?

dst
So things can go faster than light, but as humans we cannot pick up information travelling at those speeds?
Anything that does move faster than light, will carry no information. Information being, two or more 'things' that differ from each other in a way that allows you to recognize either one and note clearly that one is not the other.

The classical example is a rotating laser and the moon - the dot on the moon WILL travel faster than light (in the direction of the circumference) but WILL NOT allow information transmission faster than light, because it still takes time for the light to get there. So by the time your beam dot has 'moved faster than light', it would take the same amount of time to transmit information at light speed anyway (i.e. using the laser itself to encode a signal). The beam dot would be a virtual object, and is more or less meaningless.

Danger
Gold Member
How can anything travel faster than light? It would have infinite mass and energy according to relativity.
There are a couple of caveats to that whole situation. Einstein never said that nothing can travel faster than light. Nothing with mass can travel at the speed of light in vacuum, nor accelerate to it from either direction. That's why people postulated tachyons; (theoretically, they travel faster than light, but can never slow down enough to cross the threshold). At the same time, Cherenkov radiation is produced by fast neutrons exceeding the speed of light in water. (The individual photons still travel at c, but the propogation of the light as a whole is retarded by the medium.)

Right. So there is no evidence that anything travels faster than light. Certainly not a laser pointer projected at the moon.

And if I remember correctly, c is sort of like the spacetime speed limit, so that traveling faster through space makes you travel slower in time so that your overall velocity is still c. I may be mistaking an analogy for actual theory though.

Fundamentally nothing we know of that has mass can travel faster than light. It would require infinite amounts of energy with infinite amounts of time, and I think its obtains infinite mass too(not sure though). But in theory if your talking about going from one point to another faster than it takes light to get there it is possible. Although we do not yet have the ability to do so yet, its possible to bend two points like two points of a long twig bent to touch each other. Where it would take light a year to reach, you could possibly just walk there. All in theory and conjecture of course.

DaveC426913
Gold Member
The classical example is a rotating laser and the moon - the dot on the moon WILL travel faster than light
The thing about this example is that it's not valid, it's simply an illusion. This "dot" does not exist - it is entirely a fabrication of our minds.

Imagine if, as you swept it across the Moon, you pulsed the light, or dimmed it so that only a few photons were coming out of the laser. Instead of getting a "dot crossing the Moon", you'd get a couple of discrete, stationary dots appearing one after another across the Moon. No one would claim that those discrete, stationary dots are the single, same dot. Now speed it up again. At some point they blur in our perception and we pretend it's one dot.

Sweeping light beams of any sort do not really demonstrate a superluminous event.

That's one down. Any other examples of superluminal events we can debunk?

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OK, so nothing that we know can travel faster than the speed of light in a vacuum? Just like nothing known to us has completely massless, in terms of true mass like 0. I've heard that a photon has no invariable mass, and that is why it travels at the speed of light.

I know there is a sticky on something related but, it'd be nice to get another version of it.

dst
The thing about this example is that it's not valid, it's simply an illusion. This "dot" does not exist - it is entirely a fabrication of our minds.

Imagine if, as you swept it across the Moon, you pulsed the light, or dimmed it so that only a few photons were coming out of the laser. Instead of getting a "dot crossing the Moon", you'd get a couple of discrete, stationary dots appearing one after another across the Moon. No one would claim that those discrete, stationary dots are the single, same dot. Now speed it up again. At some point they blur in our perception and we pretend it's one dot.

Sweeping light beams of any sort do not really demonstrate a superluminous event.

That's one down. Any other examples of superluminal events we can debunk?
That is, sort of the whole point. Everything that genuinely travels faster than light is usually a 'virtual object' of that type.

Unless the OP would be meaning things that really do travel faster than light? But anyway, it's not just the speed limit, it's the law. Tough.

Danger
Gold Member
I've heard that a photon has no invariable mass, and that is why it travels at the speed of light.
A photon is an individual particle that composes light; therefore, it ,by definition, travels at the speed of light. There's no way around that one.

The way I see it, holding c constant is just as arbitrary as making time universal.

I think it's silly to say that two objects, each 1/2 a light-year away from earth, one traveling at 2c/3 in one direction towards earth, and another traveling at 2c/3 in the other direction towards earth would have an effect on each other only after a year. Another thing to note is that if everything followed Newtonian physics, if we sent a light beam to one of these objects traveling towards us to measure its distance, the light traveling away from us at c would be traveling towards the object at 5c/3 relative to the object, and would bounce off a mirror and travel 5c/3 away from the object relative to the object, or 7c/3 relative to us. If the light reaches the object exactly as it is 1/2 a light year away from us, it would have traveled for 1/2 a year on the trip there, and would take 3/14 of a year to return. This would mean our measured distance assuming the speed of light is always c would be 5/14 of a light year. The object would take 3/4 of a year to reach us, but because our measured distance value was less, we would think it had traveled less of a distance in that time, so we would assume it had a speed of 20c/42 or 10c/21.

Another way to think about it is by looking at red shift and blue shift. If the frequency is increasing as I am moving towards the object the faster I move towards it, then I am receiving the information faster. If I am on a rocket ship 1 light-year away from earth on January 1st, 2009 and a radio signal is sent to me, it would take one year to first reach me if I were to stay still. If I were to travel towards earth, the frequency of that radio signal would increase, and I would get it in less time. So, it would seem information can travel between two objects 1 light year apart in less than a year.

ZapperZ
Staff Emeritus
Another way to think about it is by looking at red shift and blue shift. If the frequency is increasing as I am moving towards the object the faster I move towards it, then I am receiving the information faster. If I am on a rocket ship 1 light-year away from earth on January 1st, 2009 and a radio signal is sent to me, it would take one year to first reach me if I were to stay still. If I were to travel towards earth, the frequency of that radio signal would increase, and I would get it in less time. So, it would seem information can travel between two objects 1 light year apart in less than a year.
There's a logical mistake in what you just wrote. If you are moving towards the light source, then you are no longer "1 light year apart"! So of course as you are moving towards the source, the signal gets to you faster than the signal between that source and an object that stayed at 1 light year apart!

The constancy of "c" isn't arbitrary. The "value" we get may be arbitrary depending on our units of length and time, but no matter what scale you use, it is always a constant in that scale.

Zz.

Theoretically, superluminal travel would require an extradimensional space if you're going to go with the SR belief that nothing can travel faster than light. Most of the current hypotheses in unification postulate a large number of dimensions.

Then again, what happens when you hit the singularity of a black hole? The refractive index of all materials when they interact with x-rays is less than one, could a similar idea be put into the movement of photons in a vacuum to get c/n which is greater than c? There's a lot more questions like that out there, but thinking of some of them makes my brain hurt.

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There's a logical mistake in what you just wrote. If you are moving towards the light source, then you are no longer "1 light year apart"! So of course as you are moving towards the source, the signal gets to you faster than the signal between that source and an object that stayed at 1 light year apart!

The constancy of "c" isn't arbitrary. The "value" we get may be arbitrary depending on our units of length and time, but no matter what scale you use, it is always a constant in that scale.

Zz.

Now, we could just as easily have a planet going very close to c towards us (only slightly less) instead of the light. Okay, so now if two planets 1 light year apart were traveling as close to c as possible towards eachother, they could meet in only a little more than half of a year. Let's say one of those planets is Earth. So we are sitting on Earth looking at a planet exactly one light year apart. Simultaneously, we use ultra-powerful rockets to get us going as close to c as possible towards the midpoint between the two planets, as to aliens on the other planets. We reach there in a little over 1/2 a year, right?

Now, lets say the earth is already moving very close to c towards the other planet, and then the other planet still fires their rockets. This situation would be no different from the first, correct?

Now lets say there is a third planet that is also traveling along with the earth at the same speed, but 1/2 a light year away in the opposite direction as the second planet. Looking at only this third planet and the earth and ignoring the second planet, it would violate no rules for this third planet to put on it's rocket boosters and start traveling at as close to c as possible towards Earth. In a little more than 1/2 a year, this third planet and the Earth would collide. In addition, during that same time, the second planet and the earth would collide. If the Earth were to suddenly disappear before this instant, two planets that were 1.5 lightyears apart would collide with eachother after a little over 1/2 a year. This is an impossibility unless they are each traveling at greater than c towards the midpoint between the two planets.

ZapperZ
Staff Emeritus
Er.... what? What this has anything to do with what you wrote earlier is beyond me.

You have 2 planets, A and B. A is stuck at 1 ly away from earth. B is some distance away moving towards earth. Earth is continually emitting light.

At a certain time, say T1, B happens to be exactly 1 ly away from earth, the same distance as A. If Earth sent some signal earlier, at T1, that signal will be received at the same instant by both A and B, no sooner, no later. At a later time, B will start receiving more signals from earth but earlier than A, because it is moving towards the earth.

There's no relativity here. It is all basic mechanics. I have no idea what you are making this anymore complicated than it is.

Zz.

Let me rephrase what I said:

Scenario 1:
Planet A is at location 0
Planet B is at location -1ly
Planet A sends a beam of light at c towards Planet B
Planet B moves towards Planet A at close to c
================
Planet B will receive the light in a little over 1/2 year

Scenario 2:
Planet A is at location 0
Planet B is at location -1ly
Planet A moves towards Planet B at close to c (instead of firing light at c)
Planet B moves towards Planet A at close to c
================
Planet A will collide with Planet B in a little over 1/2 year
They will collide at the midpoint between the original locations of the planets

Scenario 3:
Planet A is at location 0
Planet B is at location -1ly and already moving at close to c towards Planet A
Planet A moves towards Planet B at close to c (instead of firing light at c)
================
Planet A will collide with Planet B in a little over 1/2 year
They will collide at the midpoint between the original locations of the planets

Scenario 4:
Planet B is at location -1ly
Planet C is at location -1.5ly
Planet C moves towards Planet B at close to c
================
Planet C will collide with Planet B in a little over 1/2 year

Scenario 5:
Planet A is at location 0
Planet B is at location -1ly moving at close to c towards Planet A
Planet C is at location -1.5y moving at close to c towards Planet B (as in scenario 4)
Planet A moves towards Planet B at close to c
================
Planet A will collide with Planet B in a little over 1/2 year
Planet C will collide with Planet B in a little over 1/2 year
A and B will collide at the midpoint between the original locations of the planets
If Planet B were not there, Planets A and C would collide in the same amount of time (1/2 year)
Planets A and C were initially 1.5ly apart
After about 1/2 year, Planets A and C are at a location 0.5ly away from A
Planet C must have traveled at greater than c to get 1ly away from its original position in a little over 1/2 year

ZapperZ
Staff Emeritus
This is nonsense. Why don't you do a lorentz transformation, and put in the value of what each planet sees the speed of the other planets? Let all of them move at 0.9c to some arbitrary reference. Now find the velocity of planet B, and C with respect to A, and do the same for B and C. Then tell me that you get some planet going faster than c.

Zz.

This is nonsense. Why don't you do a lorentz transformation, and put in the value of what each planet sees the speed of the other planets? Let all of them move at 0.9c to some arbitrary reference. Now find the velocity of planet B, and C with respect to A, and do the same for B and C. Then tell me that you get some planet going faster than c.

Zz.
Of course you don't with the Lorrentz transformations. This is because each planet sees the other planet as being closer than it really is. Notice how in this example below, the perceived speed is lower than the actual speed:

greeniguana00 said:
I think it's silly to say that two objects, each 1/2 a light-year away from earth, one traveling at 2c/3 in one direction towards earth, and another traveling at 2c/3 in the other direction towards earth would have an effect on each other only after a year. Another thing to note is that if everything followed Newtonian physics, if we sent a light beam to one of these objects traveling towards us to measure its distance, the light traveling away from us at c would be traveling towards the object at 5c/3 relative to the object, and would bounce off a mirror and travel 5c/3 away from the object relative to the object, or 7c/3 relative to us. If the light reaches the object exactly as it is 1/2 a light year away from us, it would have traveled for 1/2 a year on the trip there, and would take 3/14 of a year to return. This would mean our measured distance assuming the speed of light is always c would be 5/14 of a light year. The object would take 3/4 of a year to reach us, but because our measured distance value was less, we would think it had traveled less of a distance in that time, so we would assume it had a speed of 20c/42 or 10c/21.

ZapperZ
Staff Emeritus
Of course you don't with the Lorrentz transformations. This is because each planet sees the other planet as being closer than it really is. Notice how in this example below, the perceived speed is lower than the actual speed:
First of all, what does "being closer than it really is" have anything to do with computing the speed that each planet sees of the other planet? Where does "distance" come in in the computation of the velocity here?

Secondly, if the Lorentz transformation produces no velocity greater than c, then what is the point in all of this? It appears that since such transformation produces no such thing, then all you've claimed so far is nonsense and irrelevant to anything going faster than c. Then what is your motive in introducing such a thing? To illustrate the flaw in your "logic"?

Zz.

Why? Because then the distance between two events in space-time would not be the same for all observers. The theory of special relativity postulates that c is constant (it did not prove it) to make the math work out nicely.

Doc Al
Mentor
Let me rephrase what I said:

Scenario 1:
Planet A is at location 0
Planet B is at location -1ly
Planet A sends a beam of light at c towards Planet B
Planet B moves towards Planet A at close to c
================
Planet B will receive the light in a little over 1/2 year
Let's rephrase:

Planet B moves towards Planet A at 0.99c. At the instant--according to Planet A--that Planet B is 1 ly away, Planet A sends a light beam towards Planet B. Yes--according to Planet A observers--it will take about 1/2 year for the light to reach Planet B. So? (Planet B will not agree, of course.)

According to Planet A observers, Planet B is 1 ly away and moving towards Planet A at 0.99c.

Scenario 2:
Planet A is at location 0
Planet B is at location -1ly
Planet A moves towards Planet B at close to c (instead of firing light at c)
Planet B moves towards Planet A at close to c
================
Planet A will collide with Planet B in a little over 1/2 year
They will collide at the midpoint between the original locations of the planets
You messed this one up beyond repair. If Planet A moves towards Planet B at 0.99c, then of course Planet B moves toward Planet A at the same speed. Of course they will disagree as to when they were at a distance of 1 ly apart. According to Planet A, it will take Planet B about 1 year to cover the 1 ly distance (of course).

What you probably meant to say was that both Planet A and Planet B are moving with respect to some third frame at speeds of 0.99c towards each other. In that case, according to that third frame, they will meet in about 1/2 year. So what?

Etc., etc....

ZapperZ
Staff Emeritus
Why? Because then the distance between two events in space-time would not be the same for all observers. The theory of special relativity postulates that c is constant (it did not prove it) to make the math work out nicely.
You still haven't answered my question. Look at the velocity transformation equation. Where is there a "distance"?

I didn't say anything about the distance being the same. Why should I since I was the one who said that this makes no difference. I asked for the VELOCITY. And I thought that is what we are talking about.

Please show me where there is a distance in the velocity transformation.

Zz.

You still haven't answered my question. Look at the velocity transformation equation. Where is there a "distance"?

I didn't say anything about the distance being the same. Why should I since I was the one who said that this makes no difference. I asked for the VELOCITY. And I thought that is what we are talking about.

Please show me where there is a distance in the velocity transformation.

Zz.
Tell me, how do you measure velocity?

EDIT: This is the main purpose of Lorentz transformations: http://en.wikipedia.org/wiki/Lorentz_transformation#Spacetime_interval

The idea is that the Lorentz transformations allow you to convert measurements from one observer to another when time is not universal. This allows you to take into account the kinds of things that cause redshift. So, two people on two different planets could measure the distance between two events, communicate their answer, and find their measurements are of the same event.

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Doc Al
Mentor
Tell me, how do you measure velocity?
Get a grip. Of course, to derive the velocity transformations you start with the Lorentz transformations for distance and time. But the velocity transformations themselves do not reference distance.