'nothing, not even light'

[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.

[Divinus Arma]

The gravitational field is so strong that the escape velocity near it exceeds the speed of light. This implies that nothing, not even light, can escape its gravity, hence the word "black."

http://www.crystalinks.com/black_holes.html

Formed in one of three main processes, they exert so much gravitational force that nothing - not even light - can escape their pull.

http://home.cwru.edu/~sjr16/stars_blackhole.html

Black holes are places in space where the force of gravity is so strong that nothing, not even light, can escape

http://news.nationalgeographic.com/n...lackholes.html

By a score of 135 to zero, scientists using NASA's Rossi X-ray Timing Explorer have compared suspected neutron stars and black holes and found that the black holes behaved as if each one has an event horizon, the theoretical border from beyond which nothing, not even light, can escape.

http://www.physorg.com/news9693.html

If a star, for example, collapses beyond some critical radius, Rc, the gravitational forces at the surface will be so great that the escape velocity of any object will be greater than the speed of light. According to Einstein, then, once this critical radius is reached, nothing, not even light will be able to escape from the surface of the star.

http://theory.uwinnipeg.ca/mod_tech/node61.html

A black hole's immense gravity accelerates nearby matter and energy, which spirals inward and eventually reaches the speed of light. At that speed, the incoming stuff crosses a so-called "event horizon," a sphere beyond which events cannot be seen because nothing, not even light, comes back out.

http://www.space.com/scienceastronom...le_010123.html

A team of astronomers from MIT and Harvard have observed that a certain kind of explosion seen on neutron stars is never seen coming from black holes. This is strong evidence for the existence of a black hole's event horizon; the point at which nothing, not even light, can escape from its grasp.

http://www.universetoday.com/am/publ..._measured.html

Black holes are celestial objects with gravity so intense that nothing, not even light, can escape from them once past their boundary, called the event horizon.

http://www.gsfc.nasa.gov/topstory/20020626bhlight.html

One way to make a black hole, theorists agree, is to explode a very massive star. In one of these supernova events, outer portions of the star are flung into space. Some material falls back, however, and collapses into a sphere so dense that nothing, not even light, can escape.

http://www.space.com/scienceastronom...th_030319.html

It had to be done. The list of google returns creates an event horizon and goes on for eternity until nothing, not even light, can escape.

LOL

Custom user title worthy!!!

[Duke of Gloucester]

Thanks Pape. You turn your back for two decades and suddenly your degree is out of date. Sounds as if I was a bit short with DJ as well. My apologies. Saying it is ok to regard photons as having mass sounds generous in view of Pape's links. Mind you I think that it takes a theortical physicist to say that a photon has momentum, inertia and experiences gravity but does not have any mass. This probably explains why Stephen Hawking uses mass to mean relativistic mass in his popular works. Just goes to show are never to old to learn something new.

[Papewaio]

Thanks Pape. You turn your back for two decades and suddenly your degree is out of date. Sounds as if I was a bit short with DJ as well. My apologies. Saying it is ok to regard photons as having mass sounds generous in view of Pape's links. Mind you I think that it takes a theortical physicist to say that a photon has momentum, inertia and experiences gravity but does not have any mass. This probably explains why Stephen Hawking uses mass to mean relativistic mass in his popular works. Just goes to show are never to old to learn something new.

As for the second bit about the hammer + feather analogy you are correct, the mass of the orbiting object cancels out when calculating the accelaration of it.

[Louis VI the Fat]

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.

Yes, I was about to bring that up.

Spoiler Alert, click show to read: 

Ok, so I was not.

So which is it. Does light escape or not?

[Papewaio]

Photons not being able to escape black holes is a consequence of relativity. Photons being massless means that gravity has to warp space... if it was just a force of attraction between two masses then photons would ignore gravity.

The very definition of a black hole is a star which has such a huge gravity that light cannot escape. Making something that would otherwise produce massive amounts of light a black hole. The escape velocity for a black hole is faster then the speed of light. As light is the maximum speed that objects can travel by saying that not even light can escape means that nothing can escape from within the event horizon at all.

The concept was first talked about by John Mitchell in 1784. Where he discussed the size of a star needed to be greater then the escape velocity of light.

[Uesugi Kenshin]

So why do we have a term for tachyonic particles? (faster than light)

Is there absolutely no factual basis for such things and it is mere sci-fi with not even a dedicated lunatic fringe talking about them in as real?

Just curious.