black holes

This is a question I’ve had for a while, and no one’s been able to explain it satisfactorily… give it a shot. Why is a black hole black? because the gravitational pull is so strong not even light can escape? If that’s it, how can gravity (which affects only? mass) affect light (which has no mass). It is my understanding that ‘matter’ cannot travel the speed of light, therefore light has no mass. If I’m mistaken on this point, please let me know. It just seems to me that if its because of the gravitional pull:

a)it should have no effect on light
b)there would be a huge glowing cloud of space dust, etc… (light being reflected off of it) being sucked into it, even if its only up to the event horizon.


I’m sure more knowledgeable posters will have a better explanation shortly (probably before I finish typing this!), but in short:

  1. light doesn’t have rest-mass, but it does have relativistic mass- i.e. because it has energy, and since energy IS mass, a photon has mass that can be affected by gravity). A photon itself has a gravitational field. A photon at rest wouldn’t have mass, but you don’t find too many of those :slight_smile:

  2. Something with non-zero rest-mass can’t travel at the speed of light- it would take infinite energy to accelerate it. However, see #1 re. photons.

  3. Gravity doesn’t really “pull” mass, but merely curves space-time so that mass travels in what it thinks is a straight line, but is actually curved. Photons travel through space-time just like other objects, and if space is curved by a gravitational field, it goes with the flow :slight_smile: A gravitational lens works on that principle.

  4. There often is a huge cloud of glowing dust around a black hole, known as an accretion(sp?) disk. The matter tends to form a disk around the black hole as it falls in.


It will take me some time to sort through that to see what I have a problem with :slight_smile:

but its way better than ‘escape velocity is faster than the speed of light’

i shall return.

“escape velocity is greater than the speed of light” only holds true, as you guessed, for matter. that’s why no matter can escape from inside a black hole. keep in mind e=mc2, which tells us that energy and matter are different forms of the same thing. so it stands to reason that light (no mass mass to speak of, but muy energetic, and with a rest mass) can’t escape either.

as to Arjuna’s lens idea, imagine what would happen the more matter occupied a space. space-time itself would curve more and more, until it curved back on itself. any light traveling in a (to it) straight line would soon either spiral into the center of the hole, or orbit at the event horizon forever.

According to relativity, energy and matter and equivalent; it is equally valid to consider a packet of light to be a wave or a particle. Thus gravity can act on light. Massive bodies will in fact bend the path of light, or, in the case of a very massive body such as a black hole, attract light and absorb it. Or, if you prefer the relativistic view, the black hole bends space such that, although the light appears to be drawn into the black hole, it is still travelling in a straight path in this space curved by the black hole’s gravity.

As a somewhat related aside, Hawking published a theory that black holes are in fact not black; they radiate energy. His idea was that virtual particle/anti-particle pairs (quantum mechanical particles responsible for the transfer of energy, but are undetectable) were constantly being generated at the event horizon of the black hole. In general, these virtual particles recombine and annihilate one another, but occasionally one half of the pair will be absorbed by the black hole’s gravity while the other will radiate away from the black hole. He derrived that the rate of radiation was inversely proportional to the mass of the black hole, i.e. small black holes radiate more. I’m not familiar with the math behind the derrivation, and I wouldn’t understand the computations even if I saw them, in any case.

Arg… ignore the typos.

With regard to the OP: “there would be a huge glowing cloud of space dust, etc… (light being reflected off of it) being sucked into it, even if its only up to the event horizon.”

isn’t that what actually happens, only because of the energy levels involved, it’s in the form of X-rays rather than visible light?

Yup JTi – the OP is correct about a glowing cloud right up to the event horizon – although it wouldn’t ONLY radiate X-rays, it would radiate a large range of frequencies due to frictional heating. Otherwise we’d have no reason to think we could detect a black hole at all, at least not without Hawking radiation.

The OP makes an excellent point: the existence of black holes is not predicted by Newtonian gravity, so any detection of a black hole would be one more piece of supporting evidence for general relativity (not that the theory is in dire need of support, but every little bit helps :slight_smile: ).

Actually, black holes are predicted by Newtonian gravity, as long as you’re using Newtonian kinesmatics, as well (no Special Relativity). In that case, it’s as simple as “escape speed faster than the speed of light”. Of course, since we do have Special Relativity (which is probably the single most thoroughly-proven theory in the history of physics), we also need General Relativity or something similar to explain the existence of black holes.

Actually, the accretion disk of a black hole doesn’t reflect much light; it produces its own, by virtue of being so hot. A minor point. Anther minor point is that even if there weren’t an accretion disk, we’d still have no hope of detecting the Hawking radiation from a normal-sized hole. It’s equivalent to the radiation produced by a body at a temperature of about a millionth of a degree above absolute zero, which means that even in empty space, a hole will actually gain more from the cosmic microwave background alone than it will lose to evaporation.

Aren’t they pretty sure they have detected black holes, from the X-rays, the effect on nearby stars, and so on? For example, there’s a major X-Ray source in Cygnus that’s been touted as a black hole, and they’ve also detected a star that is apparently loosing matter to a nearby X-ray source, possibly a former companion star that collapsed into a black hole? or am I jumbling everything up?

Ahh, but in Newtonian physics, there’s no limit to velocity. Actually, the “escape velocity is greater than c” doesn’t make much sense in relativity. The escape velocity of an object is equal to what the velocity of an object dropped an infinite distance away would be when it reached that object. If you were to drop an object an infinite distance away from a black hole, what would the velocity be when it reached the black hole? Clearly not greater than c! Suppose you’re inside the event horizon, and you send a light signal to the outside. When the signal reaches the edge of the event horizon, what speed is it going? It’s a light signal, so obviously it’s going c. Mass doesn’t slow light down; light always travels at c. However, it mass does cause light to lose energy as it moves away from it, and so any light signal sent from inside a black hole would lose all of its energy before getting out, and so would be undetectable. A black hole is a place where normal Relativistic Geometry breaks down, and so terms like “escape velocity” don’t mean much.

I’m fairly sure that Cygnus X-1 was the first known black hole. But nowadays, black holes have been found in the center of pretty much all large galaxies. These are huge things, massing anywhere from a few million to a couple billion solar masses. (My guess is that they were formed in the big bang rather than by supernova, but I’m not sure if that’s the accepted theory.)

Current theory on QSOs (quasars) is that they are massive central galaxy black holes during the early era of the universe when there was lots of gas near them. The gas falling in gets heated up to very high temperatures and gives off lots of radiation. However, because of the gas and dust clouds that accumulated around their equators, quasars are only visible if we happen to see their polar regions.

But the quasars used up all the nearby gas long ago, so now galaxies have black holes in their centers, but they aren’t giving off significant radiation. They are detected by the velocities of stars in relatively close orbits.

You are confusing General Relativity with Special Relativity. In SR the speed of light is a constant, c, and nothing travels faster than c. In GR, nothing travels faster than light in the vicinity, but light does not travel at a constant speed. If you drop an object an infinite distance away from a black hole, the object is moving with a velocity of c at the event horizon and faster than that inside. “Escape velocity greater than c” does make sense in GR!

Suppose you’re inside the event horizon and you try to shine a light outward, the light would fall away from the horizon toward the singularity. The light would not reach the edge of the horizon at all.