'nothing, not even light'

[Duke of Gloucester]

To escape the gravitational field of an object such as a planet or star, you need to be travelling at a certain minimum speed, or eacape velocity. IIRC the escape velocity at the surface of the earth is about 11 km/s. The escape velocity of a black hole is greater than the speed of light. Since nothing can move faster than light nothing can escape from a black hole.

According to the Theory of Relativity, light does have mass and is affected by gravitational fields. Light cannot escape from a black hole. It would actually be more accurate to say no electromagnetic radiation can escape. Hawking radiation is emitted from the space around a black hole.

[Ironside]

According to the Theory of Relativity, light does have mass and is affected by gravitational fields. Light cannot escape from a black hole. It would actually be more accurate to say no electromagnetic radiation can escape. Hawking radiation is emitted from the space around a black hole.

From what I know light doesn't have a mass but does act as it had one in several aspects. If light does have mass, then the Theory of Relativity says that this mass is unlimited and that would create a huge black hole that consumes the hole universe.

And IIRC Hawking radiation never really escapes the black hole. It's a radom creation of 2 particles on the exact edge of the black hole. One escapes and the other is getting drawn into the black hole. It also prevents the particles to destroy eachother as they usually do.

And I've found a source that didn't use 'nothing, not even light, can escape' in Swedish

[Ronin]

From what I know light doesn't have a mass but does act as it had one in several aspects. If light does have mass, then the Theory of Relativity says that this mass is unlimited and that would create a huge black hole that consumes the hole universe.

And IIRC Hawking radiation never really escapes the black hole. It's a radom creation of 2 particles on the exact edge of the black hole. One escapes and the other is getting drawn into the black hole. It also prevents the particles to destroy eachother as they usually do.

And I've found a source that didn't use 'nothing, not even light, can escape' in Swedish

I remember an old experient in physics class back in high-school that would prove that light has mass.....

it consisted of a glass container that was in a vacuum situation, in this container there was this device that kinda looked like a top, it was set in an axil that allowed it to spin, now...one of the sides of this thing was reflective, with the oposite side was black......when sun light shined on this device the little top thing inside spun around.....I can´t remember the name of the bloody thing in portuguese let alone in english...but if i can find a picture i´ll post it.

[Duke of Gloucester]

From what I know light doesn't have a mass but does act as it had one in several aspects. If light does have mass, then the Theory of Relativity says that this mass is unlimited and that would create a huge black hole that consumes the hole universe.

Light does have mass. The mass of a given photon is hf/c^2 where h is Plancks constant, c is the speed of light and f is the frequency of the photon. E=mc^2 tells us that mass and energy are basically the same thing, and since photons have energy, they must have mass.

The equipment Ronin describes is called a Crooke's radiometer, but in spite of what some school teachers tell you, it does not rotate because it is being knocked by photons. It rotates because the black sides are hotter than the silver sides. See the link:
http://www.physics.brown.edu/physics...emo/4d2010.htm

[Big_John]

photons are considered to have no "rest mass".. but since they never rest, it's ok to treat them as massive particles.

also, there's a bit more to hawking radiation than disassociated particle pairs.. QM allows for particles "within" the black hole to momentarily acquire enough energy to overcome the event horizon. iirc, it's a situation similar to quantum tunneling. i think that's still a rather speculative theory, and i'm not sure if QM is even supposed to hold at event horizons.. but i think that is thought to be another way that we can learn about the inner workings of black holes (along with the quantum entanglement of particle pairs).

[Duke of Gloucester]

photons are considered to have no "rest mass".. but since they never rest, it's ok to treat them as massive particles.

It is "ok" if "ok" can be used as a synonym for "correct"

[Seamus Fermanagh]

Is this yet another phrase for which we should exhume Carl Sagan and beat the bejeebers out of him -- or does it predate that eminent MC of physics?

[Big_John]

It is "ok" if "ok" can be used as a synonym for "correct"

#5


http://dictionary.reference.com/search?q=ok

[Papewaio]

Light does have mass. The mass of a given photon is hf/c^2 where h is Plancks constant, c is the speed of light and f is the frequency of the photon. E=mc^2 tells us that mass and energy are basically the same thing, and since photons have energy, they must have mass.

Photons do not have a mass, all the mass is in the form of energy. They do however have momentum.

If they had any mass then they would have infinite mass at the speed of light.

As for the light being bent by gravity, it is the very fabric of space being warped that causes that. The effect would be stronger if photons had a tiny mass... and the amount that the light bends due to gravity would change dependent on the mass... since they all have zero rest mass, the light is all bent by the same amount.

If photons had different masses then gravitational lensing would cause chromatic dispersion.

[Duke of Gloucester]

Sounds convincing, Pape, but I don't think any of your post is correct.

Photons do not have a mass, all the mass is in the form of energy. They do however have momentum.

Special relativity tells us that mass and energy are the same thing, so if photons have energy, they must have mass. Momentum is the product of mass and velocity, so if they have momentum, they must have mass. However, as Big John says, they have zero rest mass. Of course they cannot exist at rest so whenever photons exist, they have mass.

If they had any mass then they would have infinite mass at the speed of light.

This is not quite right. If they had any rest mass, they would have infinite mass at the speed of light.

As for the light being bent by gravity, it is the very fabric of space being warped that causes that. The effect would be stronger if photons had a tiny mass... and the amount that the light bends due to gravity would change dependent on the mass... since they all have zero rest mass, the light is all bent by the same amount.

You seem to be distinguishing between gravity and the bending of space/time, but gravity is the bending of space time. The bending of space time is what causes you to return to earth and the planets to orbit the sun as well as bending the paths of photons. As for the interesting notion of chromatic aberation with gravitational lensing effects, a moment's consideration will tell you that this is not correct. A red photon has less mass than a blue one, but it also has less inertia, and therefore its deflection by a gravtiational field is the same. The same argument applies to the hammer and feather dropped on the moon. The curvature of space/time is a different way of explaining the same thing both for the blue and red photons and the hammer and feather.

[Papewaio]

Special relativity tells us that mass and energy are the same thing, so if photons have energy, they must have mass. Momentum is the product of mass and velocity, so if they have momentum, they must have mass. However, as Big John says, they have zero rest mass. Of course they cannot exist at rest so whenever photons exist, they have mass.

http://math.ucr.edu/home/baez/physic...oton_mass.html

If the rest mass of the photon was non-zero, the theory of quantum electrodynamics would be "in trouble" primarily through loss of gauge invariance, which would make it non-renormalizable; also, charge-conservation would no longer be absolutely guaranteed, as it is if photons have vanishing rest-mass. However, whatever theory says, it is still necessary to check theory against experiment.

http://www.everything2.com/index.pl?node=photon

Photon Mass
Mass is a confusingly misused term. We all know Einstein's rather famous mass-energy equivalence, E=mc², and it's fairly obvious that photons have energy, so they must have mass, right? Well, no. The correct way to interpret E=mc² is to use it define the energy of an object when it is not moving, or rest energy, E0, in terms of a fixed quantity, it's mass. This is sometimes emphasised by calling this value the rest mass, but this isn't helpful, a particle only has one mass, it may have variable energy depending on how fast it is travelling, but mass is constant.

When a particle is moving the total energy is given correctly by, E² = m²c4 + p²c², where p is momentum. You can see if the object is at rest then p²c²=0, and the equation reduces back to E0=mc². In the case of a massless, but moving, particle then it reduces to E=pc. This means that a particle can have energy without mass. You can't stop photons so they always have momentum.

Theoretically, if photons did have mass we would see deviations from the Coulomb inverse square law. It is photons that transfer the electromagnetic force, they are gauge bosons. If they are massless then they can have infinite range and the 1/r² law holds true, if they have mass, they become limited in their range so the 1/r² rule will not hold anymore. Experimental tests for photon mass concentrate on finding such deviations. The upper limit for photon mass so far stands at 3x10-27 eV, which is about 10-46 kg.

[The_Mark]

I remember an old experient in physics class back in high-school that would prove that light has mass.....

it consisted of a glass container that was in a vacuum situation, in this container there was this device that kinda looked like a top, it was set in an axil that allowed it to spin, now...one of the sides of this thing was reflective, with the oposite side was black......when sun light shined on this device the little top thing inside spun around.....I can´t remember the name of the bloody thing in portuguese let alone in english...but if i can find a picture i´ll post it.

It's a radiometer, but it doesn't prove that light has a mass. The experiment proves that the gas molecules inside (the thing doesn't contain a complete vacuum) take heat off the black side, expanding the gas on the black side of the squares, resulting in differences of pressure (pushing the black side a bit) and resulting in a flow of gas molecules around the little squares from the colder side to the warmer, giving the spinning thing more speed.

[Sardo]

This thread has put so much pressure on my brain, that nothing, not even light can get into it anymore.