Two Questions about Black Holes

[Speed of Light]

Question One -

Various answers have been given, as to why Gravity is able to excape from a black hole - but none have addressed the fact that gravity has now been shown to travel at speed c.

Question Two

At speed c, time has dilated to zero -
Therefore, from light's frame of reference, time does not exist.
Light therefore travels instantaneously - So should easily be able to excape from a black hole.

Just because we see light as travelling at speed c, does not alter the fact that from light's point of view, nothing can ever change -
Another way of looking at it, is to say that light travels instantaneously, but we simply see it in slow motion

 

[Alareth]

Light consists of photons which have mass. Mass is subject to the forces of gravity.

 

[Schneibster]

Question One -

Various answers have been given, as to why Gravity is able to excape from a black hole - but none have addressed the fact that gravity has now been shown to travel at speed c.

Well, it's not that it has now been shown- it was shown a long time ago. That's what general relativity is about.

But the question is an interesting one.

However, this question assumes that gravity is like light- that is, it is transmitted by a particle that has mass. Although photons have mass (they just don't have rest mass), gravitons may not, and if they don't, then they would be unaffected by gravity. That is, their paths wouldn't be affected by a gravity field. Therefore they would escape a black hole unimpeded. Note that despite being the virtual quantum of the electric field, the path of photons is unaffected by the presence of an electric field- they are uncharged. So this is not entirely surprising.

Furthermore, gravitons are only theoretical at this time- no one has detected a graviton. All attempts to quantize gravity (which is what is required to describe the characteristics of a graviton) have failed miserably. The closest anyone has come is string physics, loop quantum gravity, and twistors, and none of these theories make predictions that can be tested, and although they describe gravitons, they do not do so in a manner that can be tested.

I'll wait to hear what some of the other folks on this forum who have experience in physics have to say before I go any further. I'd be curious to know whether the field equations of gravity from GR include a self-interaction, and whether that means that a gravity field should interact, either with itself or another field. The comments on this (assuming anyone with detailed knowledge in this area chooses to comment) should be interesting.

 

[strathmeyer]

Can you please put question marks at the end of your questions so that I can find them?

 

[Corsair 115]

However, this question assumes that gravity is like light- that is, it is transmitted by a particle that has mass.

How does that work exactly? I mean, how can it have a mass when moving at c but not at rest? Has the mass of a photon of light been determined and what is it in comparison to, say, an electron or proton?

I'd always thought that light had no mass whatsoever and that's why it was able to move at c.

 

[Schneibster]

How does that work exactly? I mean, how can it have a mass when moving at c but not at rest? Has the mass of a photon of light been determined and what is it in comparison to, say, an electron or proton?

I'd always thought that light had no mass whatsoever and that's why it was able to move at c.

First, a photon is never at rest. It is always moving at the speed of light. When it is not anymore, it ceases to exist. For instance, when it is absorbed by an electron.

Energy has mass. Photons have an energy that is related to the frequency of the light they make up by a simple equation; it's the one that contains Planck's constant. Therefore, photons have mass. And this mass has been measured.

It's not that photons have no mass that enables them to move at the speed of light; it's that they have no rest mass.

 

[Speed of Light]

Can you please put question marks at the end of your questions so that I can find them?

No, for two reasons

Reason One - It is too late to edit it

Reason Two - 'Questioning' Something doesn't need question marks

 

[Speed of Light]

First, a photon is never at rest. It is always moving at the speed of light.

At the 'Speed of Light', time has dilated to zero.
Therefore for light, time does not exist.
If time does not exist for light, it must be at rest.
Thus, a Photon can only ever be at rest

 

[Corsair 115]

First, a photon is never at rest. It is always moving at the speed of light. When it is not anymore, it ceases to exist. For instance, when it is absorbed by an electron.

Energy has mass. Photons have an energy that is related to the frequency of the light they make up by a simple equation; it's the one that contains Planck's constant. Therefore, photons have mass. And this mass has been measured.

It's not that photons have no mass that enables them to move at the speed of light; it's that they have no rest mass.

Thanks for the explanation. I remember now the part about photons only moving at the speed of light but I got confused by the mention of mass. I will readily admit my understanding and knowledge of physics is not particularly great.

Another, perhaps odd, question: what is it exactly about the nature of a photon which allows it to only exist at the speed of light? Or is it simply that's the nature of the particle?

 

[vexed]

No, for two reasons

Reason One - It is too late to edit it

Reason Two - 'Questioning' Something doesn't need question marks

Neither are questions, only statements. Not that it matters, but when asking a 'question', question marks are necessary. Sorry to sound like the grammar police, interesting statements though!

 

[Schneibster]

At the 'Speed of Light', time has dilated to zero.
Therefore for light, time does not exist.
If time does not exist for light, it must be at rest.
Thus, a Photon can only ever be at rest

Not in our frame. And if time doesn't exist for a photon, it can't exist in its own frame- you can't see anything that exists for a duration of zero.

 

[Schneibster]

Thanks for the explanation. I remember now the part about photons only moving at the speed of light but I got confused by the mention of mass. I will readily admit my understanding and knowledge of physics is not particularly great.

You're doin' OK. Don't think you have to know all the math to understand physics- just to do physics. If you just want to understand it, you can at least partly do that without having to have all the math.

Another, perhaps odd, question: what is it exactly about the nature of a photon which allows it to only exist at the speed of light? Or is it simply that's the nature of the particle?

Yes, it's just the nature of the particle. It's as intrinsic a part of being a photon as moving at less than the speed of light is a part of being an electron or proton.

 

[yairhol]

Various answers have been given, as to why Gravity is able to excape from a black hole - but none have addressed the fact that gravity has now been shown to travel at speed c.

This is an interesting question indeed. What you're actually asking ia how can gravity (assuming it is made up gravitons (which have not been discovered yet in an experiment)) can escape gravity. All matter has gravity associated with it. The core of the black hole 'feels' most of the gravity since it is surrounded by very condensed matter from all around it. The slightly upper layer of matter from the core feels slightly less and so on until we reach the surface of the black hole which is also made up of matter and thus also has gravity. But the gravity on the surface is much much less than that of the core. We know that one property of gravity is that it weakens as a function of distance (like EM fields). That gravity 'escapes' the black hole's gravity and (propagates?) out to space.

If time does not exist for light, it must be at rest.

Why? The photon is ONLY existant at the speed of light. There is no photon if not travelling at the speed of light.

At speed c, time has dilated to zero -
Therefore, from light's frame of reference, time does not exist.
Light therefore travels instantaneously - So should easily be able to excape from a black hole.

Since Einstein's equations take into account a body's rest mass in order to predict its properties at different velocities, light does not follow these equations and so time, from a photon's prespective is not defined. But, since time is defined for everything else in the universe then light is affected by timely phenomena such as travelling distance (Light is at point x at time 1 and at point x+delta at time 2).


Regards,
Yair

 

[Fredrik]

Question One -

Various answers have been given, as to why Gravity is able to excape from a black hole - but none have addressed the fact that gravity has now been shown to travel at speed c.

This seems to be the wrong question to ask. In a way the black hole is gravity (it's a geometrical property of space-time), so it seems strange to ask how gravity (a geometrical property of space-time) can escape from it.

I don't see how the speed of gravitational wave propagation is relevant. It would be if the space-time geometry on the outside of the event horizon had been generated by gravitational waves from a source on the inside, but no one says it is.

Question Two

At speed c, time has dilated to zero -
Therefore, from light's frame of reference, time does not exist.
Light therefore travels instantaneously - So should easily be able to excape from a black hole.

Just because we see light as travelling at speed c, does not alter the fact that from light's point of view, nothing can ever change -
Another way of looking at it, is to say that light travels instantaneously, but we simply see it in slow motion

I don't think it's possible to answer this without going into the details of the solutions of Einstein's equation that describe black holes. It just happens to be a property of these space-time geometries that all time-like and null geodesics (representing massive particles and light rays) inside the event horizon "hit" the singularity instead of reaching the outside. There's a change of variables that makes this relatively easy to see, but I don't think it's possible to explain it using only concepts that are familiar to someone who has studied special relativity but not general relativity.

 

[Ziggurat]

However, this question assumes that gravity is like light- that is, it is transmitted by a particle that has mass. Although photons have mass (they just don't have rest mass), gravitons may not, and if they don't, then they would be unaffected by gravity.

The answer should have nothing to do with quantum mechanics, because the problem arrises within a non-quantum GR theory, and unless the theory is inconsistent with itself (which I don't believe it is), the answer cannot appeal to something outside the theory. And it doesn't: no such assumptions about carrier particles is necessary. Electric fields will "escape" the event horizon too: what doesn't escape is electromagnetic radiation. And that's because Gauss' law still applies. Something similar applies for gravitational fields as well.

One way of thinking about this is that both the total charge and the total mass produce fields before formation of the black hole, and the continued existence of those fields after collapse doesn't trasmit information from within the black hole, since that information already existed outside the black hole's event horizon (again, via Gauss' law).

 

[Ziggurat]

I'd always thought that light had no mass whatsoever and that's why it was able to move at c.

I'm not sure why Schneibster is so attached to relativistic mass, but there's really no need for it. It's a completely superfluous and redundant concept (it's exactly the same thing as total energy, except for a scale factor). You can do everything correctly considering relativistic mass, so he's not wrong, but there's really no point. Which is why, for the most part, it's been abandoned. Rest mass, or invariant mass, is a more fundamental concept, and that is what physicists generally mean when they use "mass" without any qualifiers. And photons have zero rest mass. So I think you're better off continuing to think of photons as massless, and yes, massless particles not only can travel at c, they can only travel at c.

 

[Schneibster]

Electric fields will "escape" the event horizon too: what doesn't escape is electromagnetic radiation.

Good point.

 

[Stellafane]

Re gravity: I believe you can think of gravity not as something radiating from the black hole, but rather as the local curvature of space-time around a massive object. Things fall into the black hole not because some unseen radiation "pulls" them in, but because they are following the "shortest" path (which in this case isn't a straight line, because they are following curved space-time into the black hole). It's faintly reminicent of jumping into a pond -- the waves don't radiate out of your body, they're caused by your body interacting with the surface of the water.

As to your question concerning light; I'd have to think about that. I suspect it's too far beyond my comprehension to answer properly.

 

[casebro]

So if light has matter (photons) and light is an electro-magnetic phenomenon, then do radio waves also have...radons? no, I guess photons too. Just at such a low rate that we can't see them. Perhaps they are visible in the low infra red, at a very low 'volume'? Lowest in that continuum would be gravity? So, then I guess gravity would have photons too?

 

[MortFurd]

So if light has matter (photons) and light is an electro-magnetic phenomenon, then do radio waves also have...radons? no, I guess photons too. Just at such a low rate that we can't see them. Perhaps they are visible in the low infra red, at a very low 'volume'? Lowest in that continuum would be gravity? So, then I guess gravity would have photons too?

All electromagnetic waves are photons. That's probably phrased wrong for the physics purists. The only difference between visible light and radio is that the frequency of the waves for radio is FAR lower than the frequency of the waves for light.

It's called the electromagnetic spectrum, and ranges from radio waves down at the low end up to gamma rays at the high end.

Gravity is a separate beast, not in the electromagnetic spectrum at all.