Relativity at 1c

[Zacharine]

Hiya.

A friend of mine posed an interesting question, one to which neither of us could find an immediate answer as it has been far too many years since either of us actually dealt with the subject at hand.

The question is about what the theory of relativity says about objects moving near and at the speed of light.

Suppose for a moment, that a spaceship is travelling at exactly at 1.0c. Suppose also, that it fires a laser beam from a laser-cannon towards some distant target.

What happens to the laser-light? Does it ever actually exit the barrel (supposing there is no change to the speed of the spaceship), and if yes then at what speed relative to the spaceship? Would the situation change, if for example the spaceship is moving 'only' at 0.8c?

And what would an outside observer have to say about all this, both when at rest (relative to the spaceship) as well as when moving at substancial fractional c (up to 1.0c) velocities in the exact opposite direction of the spaceship with the laser-weapon?

For discussion: do even halfway accurate descriptions of hard sci-fi matter to you in books or movies or tv-series? Do you notice mistakes and do you care?

 

[Serenegoose]

The problem to begin with is that a spaceship cannot move at c as you would require infinite amounts of energy to move at that speed, as the ships mass would become infinite. Therefore you'd never know. Also, if you fired a laser from a ship moving at 0.99c,the laser would look to be moving at c, as far as I'm aware, as photons don't work like particles with mass, and so don't take any of the speed of the object in addition to their own velocity.

Halfway -plausible- descriptions matter to me in sci-fi, but whether they're accurate I couldn't give a monkeys about. I only care about science mistakes if they're trying to make the science accurate to begin with. 'Red matter' in star trek didn't bug me, for example.

 

[Zacharine]

The problem to begin with is that a spaceship cannot move at c as you would require infinite amounts of energy to move at that speed, as the ships mass would become infinite.

I could have restated the question as something along the lines of 'a photon is travelling trough space and experiences a hitherto unknown quantum event that sends out another photon along the same linear axis also at the speed of light...' but I figured a spaceship, while impossible to travel at 1.0c, is a nice enough concrete example and the principles are same for both.

 

[Serenegoose]

The problem to begin with is that a spaceship cannot move at c as you would require infinite amounts of energy to move at that speed, as the ships mass would become infinite.

I could have restated the question as something along the lines of 'a photon is travelling trough space and experiences a hitherto unknown quantum event that sends out another photon along the same linear axis also at the speed of light...' but I figured a spaceship, while impossible to travel at 1.0c, is a nice enough concrete example and the principles are same for both.

I think my answer is still the same, however, as objects in space are not stationary - that is, we can already see what happens when a photon is emitted by a moving object - it moves at c. Therefore anything moving at c emitting a photon would emit a photon moving at c. That it's moving at c is no more special than if it was moving at any other speed - the resultant emitted photon always seems to move at c. In a vacuum, at least. There are certain man-made devices that can cause light to briefly travel far, far slower.

 

[Zacharine]

The problem to begin with is that a spaceship cannot move at c as you would require infinite amounts of energy to move at that speed, as the ships mass would become infinite.

I could have restated the question as something along the lines of 'a photon is travelling trough space and experiences a hitherto unknown quantum event that sends out another photon along the same linear axis also at the speed of light...' but I figured a spaceship, while impossible to travel at 1.0c, is a nice enough concrete example and the principles are same for both.

I think my answer is still the same, however, as objects in space are not stationary - that is, we can already see what happens when a photon is emitted by a moving object - it moves at c. Therefore anything moving at c emitting a photon would emit a photon moving at c. That it's moving at c is no more special than if it was moving at any other speed - the resultant emitted photon always seems to move at c. In a vacuum, at least. There are certain man-made devices that can cause light to briefly travel far, far slower.

But would the distance between the first and the second photon ever increase, if we assume any outside interference happens simultaneously to both?

 

[Xhu]

Regardless of the point of observation or relative velocity of the observer, it will be seen to be moving at c. That's relativity for you. Even if you, in a spaceship, are travelling at the speed of light, light will always appear to move at the same speed whether your ship emitted it or not. Hence differences in perceived time [time dilation] and so forth.

On the other hand, accelerating towards the speed of light is said to be impossible, so...

Edit: the above assumes a vacuum.

 

[theComposer]

Light always moves at the speed of light, c, relative to your reference frame. So it doesn't matter if you're standing still or moving at c, light still moves at a speed of c.

 

[Zacharine]

Light always moves at the speed of light, c, relative to your reference frame. So it doesn't matter if you're standing still or moving at c, light still moves at a speed of c.

So, would the people in the spaceship see the light coming out of their laser-cannon?

 

[MikailCaboose]

The problem to begin with is that a spaceship cannot move at c as you would require infinite amounts of energy to move at that speed, as the ships mass would become infinite. Therefore you'd never know. Also, if you fired a laser from a ship moving at 0.99c,the laser would look to be moving at c, as far as I'm aware, as photons don't work like particles with mass, and so don't take any of the speed of the object in addition to their own velocity.

Halfway -plausible- descriptions matter to me in sci-fi, but whether they're accurate I couldn't give a monkeys about. I only care about science mistakes if they're trying to make the science accurate to begin with. 'Red matter' in star trek didn't bug me, for example.

Saying for the sake of argument that the ship did travel at 1c, and it's mass thusly did become infinite, wouldn't that make the laser bend right back to the ship as a result of the inevitable gravitational pull from it's infinite mass?

 

[Scabadus]

But would the distance between the first and the second photon ever increase, if we assume any outside interference happens simultaneously to both?

I don't believe the distance would increase, the speed of light is a maximum speed so logically the second photon could not gain any ground. This is of course unless one were to enter a medium while the other did not, at which point the speed of light slows (though interestingly the "maximum velocity" C stays the same, meaning it is actually possible to move faster than light, as long as you are within a medium. If you do, something called Cherenkov Radiation (Wikipedia it) is produced.

As for your spaceship, it is actually possible for one to be traveling at the speed of light: it is impossible to accelerate a normal spaceship up to the speed of light, however it is theoretically possible that via spontaneous creation a spasehip "spawns" in the universe already traveling at the speed of light. The chances of this happening are really too small to calculate, after all we don't exactly have peperoni pizzas spawning around us (which would be fantastic, by the way) never mind spaceships, but it's an inteesting thought exersise. Now, if this spaceship were to fire a laser what I think would happen is that the energy would build up into a sort of 'light ball' and shoot out if the spaceship ever dissapeared again with trmendous force! Pew! However... as mentioned earlier, there is also a theory that due to relativity the people on the spaceship (by the way, there are people on the spaceship) would see themselves as stationary and the light speeds away from them at... well, the speed of light.

Light flies away, minds are bent, universe is broken. Perhaps it's best if this spaceship never does appear, it probably won't end very well.

 

[theComposer]

Light always moves at the speed of light, c, relative to your reference frame. So it doesn't matter if you're standing still or moving at c, light still moves at a speed of c.

So, would the people in the spaceship see the light coming out of their laser-cannon?

Yes, the laser would look the same to the people in the spaceship as if the spaceship was not moving at all.

 

[Zacharine]

Light always moves at the speed of light, c, relative to your reference frame. So it doesn't matter if you're standing still or moving at c, light still moves at a speed of c.

So, would the people in the spaceship see the light coming out of their laser-cannon?

Yes, the laser would look the same to the people in the spaceship as if the spaceship was not moving at all.

Interesting, because it instantly leads to the mindbending of what an outside observer would see: a ship travelling at 1c, firing a laser-beam that moves at 1c, with the distance between the two increasing, despite both travelling at the same speed. Is this so?

 

[Zacharine]

Perhaps it's best if this spaceship never does appear, it probably won't end very well.

Indeed. And if a functional spaceship travelling at c could appear, what is to say Cthulhu wouldn't?

Or already hasn't?

umm, Ph'nglui mglw'nafh Cthulhu R'lyeh wgah'nagl fhtagn?

 

[theComposer]

Light always moves at the speed of light, c, relative to your reference frame. So it doesn't matter if you're standing still or moving at c, light still moves at a speed of c.

So, would the people in the spaceship see the light coming out of their laser-cannon?

Yes, the laser would look the same to the people in the spaceship as if the spaceship was not moving at all.

Interesting, because it instantly leads to the mindbending of what an outside observer would see: a ship travelling at 1c, firing a laser-beam that moves at 1c, with the distance between the two increasing, despite both travelling at the same speed. Is this so?

Yeah, that's special relativity for you. It's a bit crazy to think about, but the math is correct. Fun stuff [http://en.wikipedia.org/wiki/Special_relativity#Consequences] happens as a result.

 

[zfactor]

Light always moves at the speed of light, c, relative to your reference frame. So it doesn't matter if you're standing still or moving at c, light still moves at a speed of c.

So, would the people in the spaceship see the light coming out of their laser-cannon?

Yes, the laser would look the same to the people in the spaceship as if the spaceship was not moving at all.

Interesting, because it instantly leads to the mindbending of what an outside observer would see: a ship travelling at 1c, firing a laser-beam that moves at 1c, with the distance between the two increasing, despite both travelling at the same speed. Is this so?

-sigh-

If a spaceship fires a laser (which moves at c) while it is moving at c, the spaceship will see the laser move away from it at c. An outside observer, however, woud see them both moving at c.

It is similar to a guy on a motorcycle is moving at c with a beam of light next to him. He sees the beam of light move away from him at c, but an outside observer sees them bother moving at the same speed.

EDIT: I think this mind-fuck is a result of space being able to travel at light speed, the ship moves at c, and so does light, but light is energy (or really really really fast mass, but that is a different arguement) so it goes at c relative to all reference frames regaurdless of their speed...

 

[Zacharine]

Light always moves at the speed of light, c, relative to your reference frame. So it doesn't matter if you're standing still or moving at c, light still moves at a speed of c.

So, would the people in the spaceship see the light coming out of their laser-cannon?

Yes, the laser would look the same to the people in the spaceship as if the spaceship was not moving at all.

Interesting, because it instantly leads to the mindbending of what an outside observer would see: a ship travelling at 1c, firing a laser-beam that moves at 1c, with the distance between the two increasing, despite both travelling at the same speed. Is this so?

Yeah, that's special relativity for you. It's a bit crazy to think about, but the math is correct. Fun stuff [http://en.wikipedia.org/wiki/Special_relativity#Consequences] happens as a result.

And this is why I have to keep telling myself to leave common sense out of physics questions - common sense is all to often wrong.

Well, thanks for the answer.

 

[zfactor]

For discussion: do even halfway accurate descriptions of hard sci-fi matter to you in books or movies or tv-series? Do you notice mistakes and do you care?

I notice many of these mistakes and point them out to all around me, whereupon they reply "IT'S ONLY A MOVIE!" There are a bunch of screw-ups in crime dramas too.

 

[Zacharine]

Light always moves at the speed of light, c, relative to your reference frame. So it doesn't matter if you're standing still or moving at c, light still moves at a speed of c.

So, would the people in the spaceship see the light coming out of their laser-cannon?

Yes, the laser would look the same to the people in the spaceship as if the spaceship was not moving at all.

Interesting, because it instantly leads to the mindbending of what an outside observer would see: a ship travelling at 1c, firing a laser-beam that moves at 1c, with the distance between the two increasing, despite both travelling at the same speed. Is this so?

-sigh-

If a spaceship fires a laser (which moves at c) while it is moving at c, the spaceship will see the laser move away from it at c. An outside observer, however, woud see them both moving at c.

It is similar to a guy on a motorcycle is moving at c with a beam of light next to him. He sees the beam of light move away from him at c, but an outside observer sees them bother moving at the same speed.

Thanks for continuing to answer, and I hope you can appreciate why I have a bit of a hard time wrapping my head around it again after all these years. If only we were equipped to deal with fractional c velocities as instinctively as we handle throwing a ball or cathing a falling object.

 

[theComposer]

snip

And this is why I have to keep telling myself to leave common sense out of physics questions - common sense is all to often wrong.

Well, thanks for the answer.

No problem. Also, what you're talking about is only half of relativity as a whole. This is special relativity. There's also general relativity, which is equally cool and confusing. Check it out when your head is done asploding.

 

[zfactor]

Nothing can go faster than 1c so if you ever do get something with mass to go 1c, don't go firing weapons during your travels.

And you can't see a laser in space since you need air for it to be visible.

Where the balls did you get that idea? Light (and lasers) need no medium to be visible, it is the reason we can see the sun. Besides, you don't "see" a laser while it is flying through the air, you see it when it reflects off the object it hits.