I'm sure you guys get this question all the time, but how come light can't escape a black hole?

It can't go fast enough? So if gravity is a particle, or whatever, is it going faster than the speed of light?

If you managed to achieve a speed of about 25,000 miles per hour (about 7 miles per second), you would leave the Earth and never return. At a lesser speed, Earth gravity would pull you back to the Earth. Therefore, 25,00 miles per hour is called the escape velocity for the Earth. For more massive object, the escape velocity is high.

At the event horizon of a black hole, the escape velocity is the speed of light. Nothing, not even light, can escape from a black hole due to the high escape velocity. There is a process which causes energy to escape from a black hole, but for large Black holes this takes a long, long time. Do a search for “Hawking radiation” if you want to learn more about black hole evaporation.

We do not yet have a quantum theory of gravity, so nobody really knows about gravity particles (or if they exist), although there have been a lot of articles describing various theories about them. There are good reasons to believe that gravitation effects propagate at the speed of light. If there are gravity particles this indicates that they travel at the speed of light.

As of now, General Relativity provides the best model for gravity. It is probably an incomplete (though not flawed) theory. It models gravity using a 4D curved space. In that model, a moving object is a static 4D path called a World Line. The geometry of the 4D space determines the shape of the World Line for objects in a gravitational field. In this model, there are no gravity particles. If a quantum theory of gravity is ever devised, it will have something to say about gravity particles.

The World Line concept can be understood by considering a 3D space-time continuum. Imagine an ordinary XYZ coordinate system with Z being time instead of a third direction in space. In this model, there is no third spatial dimension. Now think of the solar system as being in the XYplaneThe World Line of the Sun is the Z (or time) axis.

The World Line of the Earth is a helix.

If an object fell or moved under power directly toward the sun starting at time zero, its World Line would be a straight line starting in XYplane and angled away from the plane toward the Z (or Time) axis. The faster the object moved, the smaller the angle between its path and the XYplane.

A decaying orbit would be a spiral around the Z (or Time) axis. Some springs are spirals rather than helixes.Note that the above 3D space curves model the motions of a solar system if all of the objects are in one plane. Note that the motions are modeled by static curves. Similarly, General Relativity models motion in a 3D space via 4D World Lines in a 4D continuum.

In the GR model, there are no gravity particles, forces, or motion. Everything is modeled by curves in a 4D space. Is the model real? Who knows? It provides an accurate description of what happens, which is all that is required of a mathematical model.

... The speed of light always remains constant, so there no way light cannot escape Event horizon.

If light is beamed out from inside the BH still the speed of light cannot change so the light will just pop out ?

If it cannot escape the event horizon then why ?

you have to keep in mind, both space, AND time are warped here.

The speed of light always remains constant, so there no way light cannot escape Event horizon.

If light is beamed out from inside the BH still the speed of light cannot change so the light will just pop out ?

If it cannot escape the event horizon then why ?

Space is deformed inside of the event horizon in such a way that the light cannot escape, I don't believe that it has anything to due with gravity. Light (photons are not affected by gravity). I am sure someone else can add some more imput.

http://en.wikipedia.org/wiki/Black_hole

http://antwrp.gsfc.nasa.gov/htmltest/gifcity/bh_pub_faq.html

Hawking radiation is NOT particles coming out of the black hole.

"Everywhere in the vacuum of space virtual particle pairs are created and annihilate quickly. Near an event horizon, they can be separated. Effectively, a particle or photon will be emitted from the horizon, the so-called Hawking radiation." (http://en.wikipedia.org/wiki/Event_horizon)

Hope this helps alittle :eek:

you have to keep in mind, both space, AND time are warped here.

Why is it that as you approach the speed of light, time slows down? I remember back in high school there was an experiment done with an atomic clock on an airplane that demonstrated this fact. Anyone have any theory?

LOL, hahahahahahaha

Why is it that as you approach the speed of light, time slows down? I remember back in high school there was an experiment done with an atomic clock on an airplane that demonstrated this fact. Anyone have any theory?

there are only about 100 threads here that deal with this problem.

LOL, hahahahahahaha
there are only about 100 threads here that deal with this problem.
Sorry cato, I was getting bored with the religion threads so I am just now checking out the other threads. Didn't know :eek:

I'm sure you guys get this question all the time, but how come light can't escape a black hole?

Because space is so curved inside the event horizon of a black hole that the only direction light (and anything else) can travel inside the horizon is towards the singularity at the centre of the hole. Outwards travel is not an option.

So if gravity is a particle, or whatever, is it going faster than the speed of light?

Gravity is a field, which determines the local curvature of spacetime at every point in space. Objects sit or move in that field, which is pre-existing, and so react to the field instantaneously.

James R: It is my understanding that the Newtonian gravitational equations conform to your following statement, but I am not sure about the General Relativity equations. Gravity is a field, which determines the local curvature of spacetime at every point in space. Objects sit or move in that field, which is pre-existing, and so react to the field instantaneously.When simulating the Solar system using Newtonian equations, there is no adjustment for the distance between the various bodies. For example you do not consider the fact that Sol is about 8.5 light minutes from Earth and several light hours from Pluto. You calculate the forces using the current positions, not the positions 8.5 light minutes or several light hours ago. It is assumed that the fields move instantaneously as the corresponding objects move.

Since the Newtonian equations are very close approximations to the GR equations, the above must be close to valid for GR Solar system calculations, but I am not sure that more intense fields conform to the above. It also seems contrary to local causality for the field (or space-time curvature) to change instantaneously as the corresponding object move.s I have a vague memory of some GR effect which corrects for the field not moving instantaneously, allowing the above statement to be very close to precise. I am not at all confident of this memory. It has been a long time since I did any serious study or reading on the subject.

Hey James R, I am hoping you can clear this up for me. What is the correct interpretation for why photons are effected by gravitational fields?

Because the photons travel through space by the shortest path possible. When the gravity of a massive body curves the space, then the shortest path for a photon passing close by will be also curved. So the photons are not affected directly by the gravitational field, but by the space curved by the field. Atleast thats how I understand it. You might wanna wait for someone smarter's point of view aswell.

Gravitiational waves are waves through space time. They are not limited by other gravitiational waves. Just as the field due to a charge isn't directly affected by a field due to another charge.

James,
Outwards travel is not an option.

So, by definition, there's only one velocity when it comes to a black hole? And that would be towards its center. So light still moves light speeds, it is only prevented from moving in a different direction? Doesn't that make light impossible to move at certain velocities, and thus... I dunno. C is constant coming out of a black hole, or non-existant, or what?

I'm not sure what you mean, but if you're talking about light coming out normally (to the event horizon, or the surface), and hence not being deflected, there is another solution. Gravity red shifts light. So I think, light coming normally will have all it's energy red shifted away, so that zero (light) energy will get out of the black hole.

If this thread is going to turn into a discussion on what comes out of black holes (universal balance and all), then I suggest that everyone read up of hawkings radiation.

I'm not sure what you mean, but if you're talking about light coming out normally (to the event horizon, or the surface), and hence not being deflected, there is another solution. Gravity red shifts light. So I think, light coming normally will have all it's energy red shifted away, so that zero (light) energy will get out of the black hole.

I don't think that gravity does anything to light. Light goes into the blackhole and and cannot escape due to the curvature of space within the event horizon. This is from our perspective by the way. Right before the photons enter the blackhole they are ripped apart from there "twin" (again read up on Hawking radiation). The x-rays that we see coming out of the blackhole are one of the twin photon that entered the hole.

http://en.wikipedia.org/wiki/Event_horizon

The Legendary Jamer R.

First of all , when U say space is bend or wraped; it dosent mean that space is cutoff from the universe, OK, hence somw part of that continious light should allow light back out.

Secondly, Light always travels in a STRAIGHT line, dont forget that dear; Hence the path from which light goes in will be the path from which light should come out ????????????????????

it dosent mean that space is cutoff from the universe
really? I was under the impression it did.

Light always travels in a STRAIGHT line, dont forget that dear; Hence the path from which light goes in will be the path from which light should come out
a black hole is not a thing within spacetime, it is spacetime (with mass in there somewhere) and light follows straight lines through spacetime. so a black hole can be so warped that light, when it takes the shortest path, will never get out.

as a side note, I don't think it is a good idea to talk about what goes on inside a black hole, because all known laws if physics break down.

Because the photons travel through space by the shortest path possible. When the gravity of a massive body curves the space, then the shortest path for a photon passing close by will be also curved. So the photons are not affected directly by the gravitational field, but by the space curved by the field. Atleast thats how I understand it. You might wanna wait for someone smarter's point of view aswell. Ha. I offer this exact explaination on physicsforums and I am effectively called a crackpot. They claim a photon has relativistic mass which causes it to be effected by the gravitational field... I suppose that is one way to look at it - although, I am not sure it is fundamentally correct. James R, thoughts?

I've heard that since a photon has zero mass, traveling at light speeds is the only way to give it mass (what with going c giving an infinite amount of mass; Infinity • 0 = 1).

Infinity * 0 = 1 is a misconception, especially in theoretical applications. In real application, it all depends on the order of magnitude of infinity and the order of magnitude of 0.

Roman:

So, by definition, there's only one velocity when it comes to a black hole? And that would be towards its center. So light still moves light speeds, it is only prevented from moving in a different direction?

Yes. Light always moves at c locally. Inside a black hole it just can't move in the outward direction, because of the spacetime curvature.


Anomalous:

First of all , when U say space is bend or wraped; it dosent mean that space is cutoff from the universe, OK, hence somw part of that continious light should allow light back out.

In the case of a black hole, the inside of a black hole is cut off from the outside, in that nothing which goes in can come out again, and no information can propagate from inside the event horizon to the outside.*

Secondly, Light always travels in a STRAIGHT line, dont forget that dear; Hence the path from which light goes in will be the path from which light should come out ?

A straight line in curved space may not look like a straight line in flat space, darling.

Aer:

They claim a photon has relativistic mass which causes it to be effected by the gravitational field... I suppose that is one way to look at it - although, I am not sure it is fundamentally correct. James R, thoughts?

It's better to look at photons as following geodesics in curved spacetime, rather than as having some kind of mass.

Infinity * 0 = 1 is a misconception, especially in theoretical applications. In real application, it all depends on the order of magnitude of infinity and the order of magnitude of 0.

That's essentially right. The value of Infinity * 0 depends on how the infinite quanitity approaches infinity, and how the zero quantity approaches zero. Infinity * zero as a raw statement about numbers is undefined.

(* Caveat: Hawking's recent work qualifies this somewhat.)

(* Caveat: Hawking's recent work qualifies this somewhat.) Is there any information on this "qualification" on the internet?

The originator of this thread did not seem to need a high level explanation: He seemed to need a low level explanation and a starting point for further education. Esoteric notions are very worthwhile, but a bit off topic.

There is some erroneous notions being posted. to this thread.The known laws of physics do not break down inside the event horizon, although there is no way to experimentally verify that they apply. The GR equations breakdown at the singularity at the center of the black hole, since they predict infinite density and zero volume. As far as I know, nobody believes in the actual existence of such a singularity. The problem at the center of a black hole is far more complex, but it is analogous to the problem at the North Pole of a sphere. Because the tangent of 90 degrees (pi/2 radians) is infinite, various equations are not applicable at the North Pole and certain theorems are more difficult to prove on a sphere than on a torus (EG: The map coloring conjecture). At Latitude 90, Longitude is undefined, but a simple change of coordinates shows that the geometry at the North Pole is the same as for any other point on a sphere. Some quantum gravity theorem or improvement to GR is likely to provide equations without a singularity. If it could be shown that there is a limit to the compressibility of a proton/quark/electron mixture, the singularity would go away because you would have neither infinite density nor zero volume.


Light travels along 4D geodesic paths, not straight lines.


GR describes world lines inside the event horizon as analogous to ballistic trajectories in a lesser gravitational field for objects with less than escape velocity. Just as an artillery shell can move away from Earth before being pulled back by gravity, an object or light ray inside the event horizon could move away from the center of the black hole before being pulled back. Observational evidence indicates that gravitational collapse of our universe will not occur. However, GR allows for the possibility of a such a universe, which could appear to be a black hole from the outside. GR allows for the possibility of all sorts of motion within the event horizon prior to the eventual collapse to the center, and even allows for a very large black hole, which (for a while) has a low density (say less than that of water) everywhere. If it were possible for some process to create it, I would like to know what GR would predict for a black hole containing nothing but electrons.


The X-rays and other radiation we see from the vicinity of a black hole is not Hawking radiation. It comes from processes external to the event horizon due to the extreme gradient of the gravitational field. It is analogous to what would happen if something caused the moon to enter a decaying orbit. The entire moon would never collide with the Earth. Tidal effects due to the gravitational gradient would break the moon into many fragments. Similarly matter falling toward the event horizon is ripped apart due to the gradient. The shredding of molecules and atoms results in X-rays and other types of radiation. Hawking radiation is something else.

Is there any information on this "qualification" on the internet?
I assume he's talking about Hawking radiation?

http://math.ucr.edu/home/baez/physics/Relativity/BlackHoles/hawking.html

I assume he's talking about Hawking radiation?

http://math.ucr.edu/home/baez/physics/Relativity/BlackHoles/hawking.html
While interesting, the page doesn't even mention zero or infinity.

Aer:

Is there any information on this "qualification" on the internet?

Yes, plenty. There has been quite a big fuss about Hawking revising his ideas about information and black holes, recently. A brief web search should find you lots of stuff.

OK - skimming the internet, the quickest summary I can come up with is as follows:

A black hole is not actually a singularity, but very close. If a singularity were actually to happen, then this "black hole singularity" would explode and possibly/probably cause a big bang event.

All the energy/mass of a black hole is not therefore contained in a singularity, rather most of it exists at the center while some can exist at the outer limits within the event horizon.

This means that since a black hole is not actually a singularity, some things can escape as the escape velocity is not exactly the speed of light.

Is any of this information bad/wrong?

And how is infinity/zero qualified? It seems that a singularity/zero can still exist (e.g. big bang).

I guess my middle school math teacher was correct about dividing by zero. Everything asplodes.

Aer:

I don't claim to be an expert on black holes.

A black hole is not actually a singularity, but very close. If a singularity were actually to happen, then this "black hole singularity" would explode and possibly/probably cause a big bang event.

I haven't heard that. Where did you get it from?

All the energy/mass of a black hole is not therefore contained in a singularity, rather most of it exists at the center while some can exist at the outer limits within the event horizon.

That cannot be true if the Schwarzschild metric applies.

This means that since a black hole is not actually a singularity, some things can escape as the escape velocity is not exactly the speed of light.

Does a black hole have an event horizon, according to your sources?

Is any of this information bad/wrong?

I'm not sure, but it looks a bit dodgy to me at this stage.

And how is infinity/zero qualified? It seems that a singularity/zero can still exist (e.g. big bang).

I don't understand the question.

Ugh, I closed all the windows from which I extracted information. Anyway, my google search was: stephen hawking "black holes" zero infinity

I'll compile a more comprehensive and referenced summary. My curiosity gets the best of me.

Edit: OK - I am not going to compile a more comprehensive summary. Not because I cannot, but because Hawking's opinion seems to change with the wind and it's impossible to get any true consensus of what is going on inside his head.

A black hole is just a thing that has so much gravity that if you get too close you will get sucked in even if you are moving as fast as it is possible to go. So how then does something like this "evaporate?" I have heard that it is because matter and anti matter particles that would otherwise pair up and disappear pop up near the event horizon and one of them gets pulled in while the other one escapes. But how does this result in a loss of mass?

I do not think that any knowledgeable physicist believes that there is a singularity at the center of a black hole. I think the term is used to indicate that General Relativity breaks done near the center, and cannot be trusted to provide reasonable data about the conditions there.

I also do not think that anyone knowledgeable makes statements about what would happen if there was a singularity. Perhaps there would be a big bang, but I doubt it. Maybe a little bang due to the lessor amount of mass than I assume existed when the really big bang occurred.

BTW: I am not sure it is reasonable to extrapolate the expanding universe data back as far as modern cosomologists seem willing to do. I wonder if it is valid to extrapolate beyond the time when the universe was opaque and radiation dominated. Since I do not know enough about this subject to understand much of what the experts say, I would not argue this issue.

When it comes to black holes, physicists statements can be all over the map. While a singularity is the traditional view of a black hole, it has been said that string theory describes the inner structure of a black hole differently.

See here for a discussion on the issue of the interior or inside the event horizon of black holes according to string theory: Document (http://www.arxiv.org/PS_cache/hep-th/pdf/0311/0311092.pdf)

We do not yet have a quantum theory of gravity, so nobody really knows about gravity particles (or if they exist), although there have been a lot of articles describing various theories about them. There are good reasons to believe that gravitation effects propagate at the speed of light. If there are gravity particles this indicates that they travel at the speed of light.

Which is an excellent argument for gravity being an externally induced force and not an inherent property of mass.

Howelse could gravity traveling at the speed of light escape the Black Hole.? :D

If on the other hand the source of gravity is external and has a finite energy capacity once a Black Hole accumulated sufficient mass it would become a 100% shield or absorber and added mass would no longer increase gravity.

There would be no singularities.

Just my 2 cents.

Another way to think about the event horizon (for those who get a headache trying to hold 4D curves in their mind) is in terms of energy. This is closely related to the escape velocity explanation above. As light goes up a gravity well it loses energy just like a particle would. Since light can't travel at slower than c the energy loss is in terms of redshift. The event horizon is just where light redshifts to 0 Hz. 0 Hz is zero energy for light.

-Dale

MacM: Excellent point about gravitons at c ( not ) escaping a hole.

DaleSpam: This implies that the photon effectively vanishes. This is very interesting.