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alagulovesall
2008-Dec-30, 11:22 AM
Is there anything which can be made to travel at the speed of light?

grant hutchison
2008-Dec-30, 11:27 AM
Welcome.
Particles with zero rest mass, like photons, travel at the speed of light.
To accelerate anything with rest mass to lightspeed requires infinite energy: so familiar matter (composed of electrons, neutrons and protons) can never reach lightspeed, no matter how much energy you pump in.

Grant Hutchison

alagulovesall
2008-Dec-30, 11:35 AM
Thanks hatch
does a light has mass?if yes what is the mass of light?

grant hutchison
2008-Dec-30, 12:10 PM
does a light has mass?if yes what is the mass of light?Light has energy, and energy is equivalent to mass. The energy depends on the wavelength of light, and how many photons you have. The equivalent mass is undetectably tiny in the sort of situations we normally encounter.

Grant Hutchison

swansont
2008-Dec-30, 01:52 PM
In the more common expression of mass, it refers to the invariant mass, aka rest mass, and in this light (as it were) the photon does not have mass.

The equation governing this is

E^2 = p^2c^2 + m^2c^4

The familiar E = mc^2 comes from this, but only for an object at rest, which is not the case for a photon. The kinetic term accounts for energy of the entity's motion, and the mass term is unchanged, and in the photon's case, zero.

grant hutchison
2008-Dec-30, 02:24 PM
Yes, I was trying to maintain that distinction above, but probably not doing very well.

If we get some photons into a situation where we could, in theory, measure their mass, for instance by storing them inside a perfectly reflective box, we'd find that box+photons had a greater mass than the box alone: the box containing the photons would accelerate more slowly under an applied force, for instance. And the measured increase in mass would depend on the energy and number of photons.
If we filled the box with very energetic photons which were eventually absorbed by the box walls, that increased mass would still be present, in the form of the extra energy of oscillation in the heated molecules of the box's structure.
And the increment in mass would then decay as the box emitted thermal photons to the outside environment: the extra mass-energy would depart in the energy of the emitted thermal photons.

All of the above is distinct from the concept of "zero rest mass", as you say.

Grant Hutchison

Ara Pacis
2008-Dec-30, 04:15 PM
Shouldn't this be in Q&A?

grant hutchison
2008-Dec-30, 04:52 PM
Shouldn't this be in Q&A?I guess it depends on whether you think it is a "space/astronomy question", or a question about "general science".

Grant Hutchison

Ara Pacis
2008-Dec-30, 05:35 PM
Good point, GH.

The Answer to the OP is sorta. It depends on the medium through which the light travels. Charged particles in nuclear reactors travel faster through the water moderator than light, thus giving rise to Cherenkov Radiation (http://en.wikipedia.org/wiki/Cherenkov_Radiation), a bluish glow.

No massy particle can travel at light in a vacuum. However, there are some theories that claim that there are particles, Tachyons (http://en.wikipedia.org/wiki/Tachyon) that can travel faster than C by virtue of a loophole. IIRC, these particles would have a lower speed limit of c and can't travel slower than it.

grant hutchison
2008-Dec-30, 05:41 PM
However, there are some theories that claim that there are particles, Tachyons (http://en.wikipedia.org/wiki/Tachyon) that can travel faster than C by virtue of a loophole. IIRC, these particles would have a lower speed limit of c and can't travel slower than it.Yeah, the loophole is that they have imaginary rest mass (whatever that might be). Thus equipped, under the conventional equations of SR, they move always faster than the speed of light and get slower the more energy they have: they likewise require an infinite amount of energy to go at the speed of light.

Grant Hutchison

Kebsis
2008-Dec-30, 09:13 PM
Welcome.
Particles with zero rest mass, like photons, travel at the speed of light.
To accelerate anything with rest mass to lightspeed requires infinite energy: so familiar matter (composed of electrons, neutrons and protons) can never reach lightspeed, no matter how much energy you pump in.

Grant Hutchison

Hi Grant

It's my understanding that when a singularity is mentioned in general relativity, it is not actually a point that is infinite, but a point where the mathematics of the object is undefined. So, a black holes center is not nessesarilly infinitely dense, it's just that the GE equations do not function inside the event horizon and so indicate that that point is infinite.

I do not know if I'm understanding that correctly. But, if I do, does that mean that when Einstein says it takes an infinite amount of energy to move a particle to the speed of light, is it a definite statement or is it more like the singularity example, where the equations cannot be used to decide how much energy it would take?

swansont
2008-Dec-30, 10:05 PM
The actual value of the energy is undefined, but the manner in which the equation diverges is that the energy tends to infinity. As you get arbitrarily close to c, the energy is arbitrarily large.

Disinfo Agent
2008-Dec-31, 10:38 AM
In other words, Kebsis...


It's my understanding that when a singularity is mentioned in general relativity, it is not actually a point that is infinite, but a point where the mathematics of the object is undefined. So, a black holes center is not nessesarilly infinitely dense, it's just that the GE equations do not function inside the event horizon and so indicate that that point is infinite....It's both. It's undefined because it would have to be infinite, if we were to define it.

Now, naturally physicists have a hard time believing in physical infinities. So, when an equation tends to infinity at some point, they refuse to take it literally in the vicinity of that point, and assume that something else is missing from the picture that produced the equation. In the case of black holes, what's (likely) missing are the quantum effects that would have to be taken into account at such high pressures as you get when you come near to a black hole.

Which doesn't mean that the classical equations, though probably "wrong", can't give a damn good approximation to the reality. And the approximation, if it is accurate as it seems to be, is already intriguing by itself.

Kebsis
2008-Dec-31, 04:08 PM
In other words, Kebsis...

...It's both. It's undefined because it would have to be infinite, if we were to define it.

Now, naturally physicists have a hard time believing in physical infinities. So, when an equation tends to infinity at some point, they refuse to take it literally in the vicinity of that point, and assume that something else is missing from the picture that produced the equation. In the case of black holes, what's (likely) missing are the quantum effects that would have to be taken into account at such high pressures as you get when you come near to a black hole.

Which doesn't mean that the classical equations, though probably "wrong", can't give a damn good approximation to the reality. And the approximation, if it is accurate as it seems to be, is already intriguing by itself.


Hmm...I'm not sure I understand. How can something approximate infinity? It is either infinite or it is not. A very large number, as large as it may be, has nothing to do with infinity.

I know little about these things, but it seems like an important distinction. In the case of moving something at the speed of light, if the energy required truely is infinite (or that certain physical roadblocks make it impossible to accomplish regardless of the amount of energy invested), than it most certainly is impossible to accomplish. But if the energy required is a very large, finite amount, then the energy requirement could perhaps be circumvented or artificially lowered or something to accomplish the feat.

I'm not trying to argue a theory here or anything, I'm just trying to describe where my minds at with this...if I am in fact in the wrong ballpark, directions to the proper one are welcome :)

grant hutchison
2008-Dec-31, 04:14 PM
I know little about these things, but it seems like an important distinction. In the case of moving something at the speed of light, if the energy required truely is infinite (or that certain physical roadblocks make it impossible to accomplish regardless of the amount of energy invested), than it most certainly is impossible to accomplish. But if the energy required is a very large, finite amount, then the energy requirement could perhaps be circumvented or artificially lowered or something to accomplish the feat.Any finite amount of energy results in a velocity less than lightspeed. We therefore cannot define how much energy would be required to achieve lightspeed. So the amount of energy is both infinite (= not finite) and undefined (= cannot be defined).

Disinfo Agent is making a rather different point about approximation, I believe.
When infinities show up in our theories, it makes us wonder if the current theory is only an approximation: we wonder if some additional theory will be required in the vicinity of the singular behaviour, which will eliminate the infinities. In the case of accelerating towards lightspeed, we are "protected" from the infinity by the fact that we can't get there with any finite energy. But in the case of a black hole singularity, we could potentially interact with this locus of infinity: so we wonder if some new theory, combining general relativity and quantum mechanics, will actually become strongly significant in that region, eliminating the infinity we see in the pure GR equations.

Grant Hutchison

cosmocrazy
2008-Dec-31, 04:18 PM
Hmm...I'm not sure I understand. How can something approximate infinity? It is either infinite or it is not. A very large number, as large as it may be, has nothing to do with infinity.

I know little about these things, but it seems like an important distinction. In the case of moving something at the speed of light, if the energy required truly is infinite (or that certain physical roadblocks make it impossible to accomplish regardless of the amount of energy invested), than it most certainly is impossible to accomplish. But if the energy required is a very large, finite amount, then the energy requirement could perhaps be circumvented or artificially lowered or something to accomplish the feat.

I'm not trying to argue a theory here or anything, I'm just trying to describe where my minds at with this...if I am in fact in the wrong ballpark, directions to the proper one are welcome :)

I'm with you on this. I was under the impression that to reach "C" any rest mass particle regardless how small, would require infinite energy to reach the mark. :(

cosmocrazy
2008-Dec-31, 04:19 PM
Any finite amount of energy results in a velocity less than lightspeed. We therefore cannot define how much energy would be required to achieve lightspeed. So the amount of energy is both infinite (= not finite) and undefined (= cannot be defined).

Grant Hutchison

good point.

tommac
2008-Dec-31, 09:40 PM
but something could travel from one part of the universe to the other in a millisecond right?

cosmocrazy
2008-Dec-31, 09:44 PM
but something could travel from one part of the universe to the other in a millisecond right?

Yes, in that "something's" own reference frame. Any relative observer would measure it to be travelling close to "C" (assuming you are talking about a large distance) over that given distance measured by the observer.

Kebsis
2008-Dec-31, 10:38 PM
but something could travel from one part of the universe to the other in a millisecond right?



Well I can travel from one point in the universe to another in a millisecond too. Not very far apart points, of course, but I can do it :)