Travelling faster than light...

[Peppers]

I was under the impression that nothing can travel at speeds that are faster than light, but I recently read a small "blurb" in a textbook that made reference to Cerenkov radiation. It said charged particles in water travelled faster than light and gave off a blu glow. I also read that this was because the particles somehow outran their electric fields and travelled at faster than light speeds. I still don't understand how this is possible, as I thought the whole "rule" of nothing being able to travel faster than light was an absolute rule that could not be broken under any circumstance. Can someone explain this to me? (not just sending a wikipedia link , because I still don't get how it is possible)

[guardian]

The high energy charged particles (electrons) appear to travel faster than light and in the Cerenkov radiation will beat light from A to B but this is due to light appearing to travel slower in any medium (air, glass, water etc.). Light travelling through a medium like water still travels at c but the photons of the light are being absorbed and re-emitted as they travel - this takes additional time and makes the light appear to travel slower. The highly energetic charged particles (in your reference) do not undergo this absorption/re-emission like the light does and with enough energy can actually beat the photons of the light. Someone may want to expand more (if needed) or correct me if I've garbled something up.

 

Cheers

[swansont]

There are a few subtleties here. "Faster than light" and "faster than c" do not necessarily mean the same thing.

 

c is the limit that can't be broken by massive particles or information. Light in a medium, though, travels at c/n, where n is the index of refraction. As guardian points out, the photons still travel at c, but are being absorbed and reemitted, so the overall speed decreases.

 

Cerenkov radiation is emitted under the condition of c/n < v < c

 

- faster than light (in the medium) but not faster than c

[insane_alien]

light travelling through a medium is only the average speed of the light by speed=distance/time but at any instance it is either going at c or is stored in the increased energy of an electron orbiting an atom.

[Mart]

insane_alien wrote

light travelling through a medium is only the average speed of the light by speed=distance/time but at any instance it is either going at c or is stored in the increased energy of an electron orbiting an atom.

Does this have any relevance to the finite speed of light in "empty" space?

[swansont]

Does this have any relevance to the finite speed of light in "empty" space?

 

c is related to the permittivity and permeability of space, which changes in a medium. In the wave picture, it changes the ability of the electric and magnetic fields to oscillate.

[Mart]

I was thinking of photons interacting with virtual particles.

[insane_alien]

i was just meaning that it could be thought of that way if you were new to the concept.

[Peppers]

Thank you for your replies, I definately understand the errro in my thinking.

Does this mean that light only travels at c in a vacuum? Or is Mart correct in saying that the photons would still be absorbed and re emitted by virtual particles, thus slowing its average speed in a vacuum as well?

[Mart]

Originally Posted by insane_alien

i was just meaning that it could be thought of that way

It seems a useful way to think about the problem. But you seem to be hinting that there are other ways.

[Daecon]

It means faster than the light will travel through the water, not faster then the light will travel through the vacuum of space.

 

[BobbyJoeCool]

But isn't is actually possible to travel faster than the speed of light, in theory? Black Hole? Once past the event horizon, gravety is exerting such a force on you that it pulls you to the singuarity at speeds faster than the speed of light. Of course, the sheering effects would tear you a new one before you got to the event horizon unless it was one REALLY BIG black hole. And escaping to tell anyone about it would prove impossible... unless of course you believe in the book Sphere. Because theoretically, traveling faster than light would send you back in time (but not across space) to a time where the black hole wasn't there... of course, being put close the the "surface" of a star would be a lot better than the inside of a black hole... Obviously because you'd get a tan before you burned into oblivion.

[Daecon]

I'm sure that the gravity of a black hole can't pull anything faster than light, it's just a reference to the escape velocity neded to break free of the gravity of a black hole, is higher than light speed.

 

That same way that the escape velocity of earth is quite high, but not too fast that a rocket can't match it.

[BobbyJoeCool]

but if the escape velocity is higher than the speed of light, and you aren't fighting against it, aren't you moving faster than light? Or does a black hole not pull any stronger once past the event horizon because it would break the laws of physics by making things move faster than light. Of course... by this theory, you don't have to be under the EH in order to be moving faster than light... If you are moving directly at the singularity as a speed of .1c, it only needs to pull you at .9c in order to make you travel at c. Right? So, if it pulls you at .90000001c, you are traveling faster than light.

[Daecon]

I think it's the same kind of thing like if you were to drop a 1g weight and a 10Kg weight, they'd both travel towards the ground at the same speed, which is dependant to the mass of the earth. I forget what KPH that is, but bear with me.

 

Even with a mass as superdense as a black hole/singularity, the speed it which it pulls anything towards it can't be faster than light speed, surely?

 

(EDIT: I hope that makes sense, I know what I'm trying to say but don't know how to word it.)

[insane_alien]

No gravity only provides an acceleration. the acceleration on earth is roughly 9.8m/s/s. that is anything dropped on the surface of earth will accelerate towards the ground at 9.8m/s/s. the only speed limit is c. a dropped object will not just accelerate to a velocity v where v is proportional to the force of gravitational attraction.

The dropped object if dropped from infinity will hit the gravitational body with the escape velocity but it does not mean that that is a speed limit.

[BobbyJoeCool]

No gravity only provides an acceleration. the acceleration on earth is roughly 9.8m/s/s. that is anything dropped on the surface of earth will accelerate towards the ground at 9.8m/s/s. the only speed limit is c. a dropped object will not just accelerate to a velocity v where v is proportional to the force of gravitational attraction.

The dropped object if dropped from infinity will hit the gravitational body with the escape velocity but it does not mean that that is a speed limit.

 

I see what your saying. your saying that no matter what the acceleration created by a the gravety of a black hole (for example, say it's c/s (which would be the forces at the event horizon)), after the second it takes to reach c, it reaches terminal velocity and cannot go any faster. And now that the acceloration is k*c/s, where k>1 (so the acceleration is faster than c/s), in order to escape, you have to be able to accelerate FASTER than gravety pulls on you. right? In order to escape Earth's gravety, you don'thave to reach a certain speed, you just have to accelerate up at a speed greater than 9.8m/s/s, right?

 

So, is it theoretically possible that if one could accelereate faster than c/s (even though you can't go faster than c), that you could escape a black hole, even once inside the event horizon (assuming that the shearing effects don't tear you a new one)?

[Spyman]

I see what your saying. your saying that no matter what the acceleration created by a the gravety of a black hole (for example, say it's c/s (which would be the forces at the event horizon)), after the second it takes to reach c, it reaches terminal velocity and cannot go any faster. And now that the acceloration is k*c/s, where k>1 (so the acceleration is faster than c/s), in order to escape, you have to be able to accelerate FASTER than gravety pulls on you. right? In order to escape Earth's gravety, you don'thave to reach a certain speed, you just have to accelerate up at a speed greater than 9.8m/s/s, right?

No, to escape You need to reach the Escape Velocity.

 

As soon as You stop accelerating, below EV, the gravity will catch You again.

In physics, for a given gravitational field and a given position, the escape velocity is the minimum speed an object without propulsion, at that position, needs to have to move away indefinitely from the source of the field, as opposed to falling back or staying in an orbit within a bounded distance from the source.

http://en.wikipedia.org/wiki/Escape_velocity

 

So, is it theoretically possible that if one could accelereate faster than c/s (even though you can't go faster than c), that you could escape a black hole, even once inside the event horizon (assuming that the shearing effects don't tear you a new one)?

No, in theory gravity and acceleration is the same.

 

Thus such an mighty acceleration should turn You into a Black Hole.

 

After defining his theory of special relativity, Albert Einstein realized that forces felt by objects undergoing constant acceleration are indistinguishable from those in a gravitational field, and thus defined general relativity.

http://en.wikipedia.org/wiki/Acceleration

 

EDIT:

Also according to theory there is no path out from a Black Hole.

 

Which direction are You going to accelerate in when all directions leads to the Singularity ?

 

Spacetime inside the event horizon is peculiar in that the singularity is in every observer's future, so all particles within the event horizon move inexorably towards it (Penrose and Hawking).

In Michell's theory' date=' the escape velocity equals the speed of light, but it would still, for example, be theoretically possible to hoist an object out of a black hole using a rope. General relativity eliminates such loopholes, because once an object is inside the event horizon, its time-line contains an end-point to time itself, and no possible world-lines come back out through the event horizon.[/quote']http://en.wikipedia.org/wiki/Black_hole

[Severian]

Really it would be clearer if we used the term 'limiting speed' or somesuch rather than 'speed of light'. The fact the light usually travels at this 'limiting speed' is rather besides the point.

[BobbyJoeCool]

No' date=' in theory gravity and acceleration is the same.

 

Thus such an mighty acceleration should turn You into a Black Hole.

 

http://en.wikipedia.org/wiki/Acceleration[/quote']

 

so, when the Enterprise goes from a standstill to .5c in .25 seconds (and thus accelerating at about 2c/s), it's created it's own black hole and is impossible because having done that should have destroyed the ship because of the immence gravity created by its own acceleration?

 

(I'm of course not trying to find a scientific flaw in a TV series...)

 

but seriously (in theory of course). If I was accelerating at 2c/s, the gravetational forces created by myself would make me colapse on myself and pull enough "stuff" in with me and actually create a black hole?

 

in otherwords, acceleration creates gravity?

[insane_alien]

Acceleration DOES NOT create a gravitational field!

[Spyman]

If I was accelerating at 2c/s' date=' the gravetational forces created by myself would make me colapse on myself and pull enough "stuff" in with me and actually create a black hole?

 

in otherwords, acceleration creates gravity?[/quote']No, forces of acceleration = forces of gravity.

 

The forces from the acceleration needed would be in the same magnitude as the gravity of the Black Hole, trying to escape from.

 

The forces of the acceleration would colapse You to a Singularity.

[BobbyJoeCool]

The forces of the acceleration would colapse You to a Singularity.

 

Theoretically then, since I could no longer produce the acceleration, would I then explode out of the singularity (because I can't really be accelerating without the power to accelerate, and since my mass isn't great enough to keep up the acceleration needed to force many atoms (or cells) to occupy a point, would I then explode, or once a singularity exists, does it just keep existing?

[Spyman]

Theoretically then, since I could no longer produce the acceleration, would I then explode out of the singularity (because I can't really be accelerating without the power to accelerate, and since my mass isn't great enough to keep up the acceleration needed to force many atoms (or cells) to occupy a point, would I then explode, or once a singularity exists, does it just keep existing?

Your mass will be great enough if the density grows high enough.

 

But Hawking Radiation will cause small Black Holes to evaporate very fast.

 

http://www.scienceforums.net/forums/showthread.php?t=9500

 

http://en.wikipedia.org/wiki/Hawking_radiation

[BobbyJoeCool]

I see now then. Thanks for all the help (and the referances)!