Hey, I'm just wondering why light can go the speed of light, but yet it's impossible to go the speed of light. This probably sounds like a stupid question, but if you know the answer, please tell me. thanks :-D

Because light is not matter its energy of an elemental force (gravity, magnetic, strong and weak force) and all those energies move at ~299,000km a sec (aka the speed of light) the universal speed limit is that. Why? I don't know, I’m not a physicist... in fact physics does not yet know why!

Good point!!

It's one of those things you never really think about, I'll have to ponder that tonight.

Massless particles move at the speed of light. Particles with mass cannot.

But I've heard that light does have mass. Or does it change? And how does that happen?

Light has virtual mass, this is that impact from light can be transferred as kinetic energy, like getting hit by a tiny BB traveling REALY fast.

So therefore, heat would have virtual mass too, right? Or does heat need a medium of mass to have a kinetic affect on an object? But thanks for your last post!

Heat isn't actually energy as you're thinking of it, unless you are referring to infrared in the electromagnetic spectrum (heat vision) in which case it is exactly like light except a different frequency.

Heat as we normally think about it, is a macroscopic way of measuring the average kinetic energy of a group of particles. This does not imply that heat is a particle in any sense, and if it is not a particle, it can't have virtual or real mass. It simply doesn't make sense to think of heat in that way I don't think.

-AntonK

Yeah, I just thought about that. There is heat in the form of radiation, but of course, that would be the same as a light wave, so nevermind about my last post. :o

Heat in the form of IR radiation… which is light!, Heat as in moving particles has mass as well: the mass of the particles and the increase in mass from movement predicted by the theory of relativity (this increase is very VERY small unless a particle is moving a near light speeds relative to a observer).

Well, in the form of IR radiation for heat, does it actually have mass (like, is it an electron that's travelling, or is it actually light?). If it's just light, then it should have the same properties right? And if it's light, then it should have no mass (except for virtual mass), and therefore wouldn't increase mass as it travels.

Originally posted by strategicman
Well, in the form of IR radiation for heat, does it actually have mass (like, is it an electron that's travelling, or is it actually light?). If it's just light, then it should have the same properties right? And if it's light, then it should have no mass (except for virtual mass), and therefore wouldn't increase mass as it travels. Infrared radiation is just another wavelength of light, and is just like all other wavelengths of light as far as physical properties. Infrared photons have no mass.

Its like asking what happens if you drive a car at the speed of light and switch the headlights on. You just can't is a pretty good answer. If you could you wouldn't see anything. Time slows to a stop at the speed of light. Everything is relative as Einstein used to say. Even if you stayed a smidgen under the light barrier the universe would die before you saw any movement. Slower and you defeat the purpose of the question you see. You are no longer driving at light speed. It makes sense.

Trust me.

Do people still think light is still composed of photon packets or has that idea changed while I wasn't looking (I haven't looked for about ten years).

the light/photon/wave theory has remained largley the same for the past couple decades. the underlying details (quarks, gluons, etc) have changed slightly, but the basic ideas are the same. We still ahve a wave/particle duality thing going on which no one can quite explain with proof. only theories.

Does anybody know why the speed of light is limited to 186,000 mps. What drives a photon to that speed anyway and why is it limited to that speed?

Originally posted by Greco
Does anybody know why the speed of light is limited to 186,000 mps. What drives a photon to that speed anyway and why is it limited to that speed?

It's a product of the permittivity and permeability of free space, ie, how well the medium of the vacuum (Paul Dirac Electron Sea Anybody ;-) ?) allows electrmagnetic waves to pass through it.

Here, we have to distinguish between a vacuum, and a void. Space is a vacuum, and a vacuum is an area where matter and energy can exist, and as such, can be thought of as a viscous medium. And Viscous mediums have speed limits, an analogy would be the speed of sound in air.

Formula for speed of light is;

<img src="http://www.fnal.gov/pub/inquiring/more/light/images/c_vac.gif">

IE, the reciprocal of the square root of the product of permeability and permittivity of free space.

Of course, the same formula applies with different parameters for any medium, water, glass, whatever. Weird huh?

Nothing can accelrate to the speed of light, light on the other hand has always been going at the speed of light (ignore the possibilty of varying values of c there not relevant), so there is no reason why it shouldn't travel at the speed of light.

[QUOTE]Originally posted by phlogistician
[B]It's a product of the permittivity and permeability of free space, ie, how well the medium of the vacuum (Paul Dirac Electron Sea Anybody ;-) ?) allows electrmagnetic waves to pass through it.

That leads to the question "is the vacuum of space constant"? If the vacuum varies then the speed of light must varry. If space has permeability and you refer to it as a medium then how come we can not detect this "ether"? Werent the Michelson experiments trying to prove just that? With no luck.

That leads to the question "is the vacuum of space constant"? If the vacuum varies then the speed of light must varry. If space has permeability and you refer to it as a medium then how come we can not detect this "ether"? Werent the Michelson experiments trying to prove just that? With no luck. [/B][/QUOTE]

How can a vacume vary and still remain a vacume.

I think he was referring to the properties of the vacuum (epsilon and mu).

Anyway, the speed of light is just that, the speed that photons happen to travel. The real interesting question is why the rest of nature seems to travel at this speed (and also, why does all light, and why is it the same for everyone). The particular value of c, in terms of light, is c because a different value would actually require a rest mass, or what you probably think of as "actual" mass.

A comment about the "virtual" mass. This is still NOT mass. If you consult special relativity (the most widely accepted method is to use tensor analysis), then you have a more general quantity called mass-energy. If a particle is not moving in your rest-frame, then it has a mass-energy of mc^2, and we usually drop the c^2 and just call it mass. The closer a particle is to the speed of light, the more this fixed amount of energy rotates into the form of what we conventionally call energy. Photons are particles that are travelling exactly at the speed of light (by definition), and so all of their mass-energy is in the form of what you would probably call just energy. This never appears as what you would call mass. Why, that is an interesting question.

If you sware that you heard someone say that photons have mass, it is probably a confusion between having mass, and having the ability to transfer momentum, since we conventionally define momentum as the mass times the velocity. However, we should really say that the photon has a momentum, not a mass. It can lose this momentum in a collision and cause an object with mass to move, but then it disappears.

Is there such thing as a natural void (as opposed to a vacuum)?

Black holes are like voids in the sense that there is some minimum distance from the center where space-time ceases to exist observably. In terms of relativity, this is a pocket, or bubble, in spacetime that does not contain any spacetime (in this universe).

When I was in High School, somebody 'presented' me a word "anti-matter", and since then I have heard that:

1) All kind of matter travel at the maximum speed of c
2) Energy travels at c
3) Anti matters travel at the minimum speed of c

What say you?
And by the way, has there been any thread discussing bout mass?

You may be thinking of "tachyons," as opposed to "antimatter." Antimatter is just matter produced with a charge/flavor/whatever conjugate value such that the composite has a neutral value for the charge/flavor/whatever. Mass is not like these properties, it is a component of a second rank tensor that is common to all matter/antimatter.

For instance: a gamma particle can interact with a heavy nucleas to form an electron-positron pair. The positron is an element of antimatter, but it has positive mass and "moves" in the same way that the electron does, just with opposite charge. One evidence of this is annihilation. An electron-positron pair attract each other and annihilate (they are converted into energy). The resulting gamma particles (there are two) have the same energy as the energy of the corresponding electron. So, most likely, the positron has the same energy.

Tachyons are particles (of what, I'm not exactly sure, but I don't think that any such thing has been observed anyway; they are merely abstractions that also avoid the singularity of relativity like matter does) that travel faster than light because they have always been going faster than light, and therefore don't have to cross the light cone. I am a bit speculative about their existence, though. I don't really understand them, but it seems like a symmetry argument, and I have not yet come to acceptence of the symmetry argument.

The thing about Tachyons is they travel backwards in time and have negitive energy :bugeye:

If u drove a car at close to the speed of light, according to Einstein, would u not see a beam of light from the headlights move away from u at exactly the speed of light (c)?

John Connellan,

nope everything would look normal to you (inside the ship), your time would slow in compensation.

Originally posted by John Connellan
If u drove a car at close to the speed of light, according to Einstein, would u not see a beam of light from the headlights move away from u at exactly the speed of light (c)?

summing the two velocities c and c in special relativity gives you a value of c, so yes, but the as you can't travel at the speed of c the validty of this is a moot point.

He said "close to the speed of light."

The beam of light would appear to be travelling at the speed of light (like normal), You don't have to do any math to arrive at this conclusion; it is one of the two postulates of the theory. So, what it sounds like you're asking is, "Is the special theory of relativity correct?"

Have a look back - really I was asking the rest of them do they think the special theory of relativity is correct?

quote: "WOULD u not see a beam of light from the headlights move away from u at exactly the speed of light?!"

Of course I know the answer (who ISN'T familiar with this theory, come on!) but its just that somebody said u wouldn't see anything when u switch the headlights on!!!

Originally posted by John Connellan
Have a look back - really I was asking the rest of them do they think the special theory of relativity is correct?Actually, I don't see you asking the question in either one of your previous two posts. Did you imply it? In what way?

By the way, my last thread was directed towards <i>you</i>. I thought that you <i>were</i> asking about the validity of SR. Now you seem to have affirmed my conjecture. So, were you testing "the rest of them," or something, or just making rhetoric?