# The Naked Scientists Forum

### Author Topic: Why doesn't the light get out of a black hole?  (Read 8337 times)

#### JohnDuffield

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##### Why doesn't the light get out of a black hole?
« on: 02/02/2015 22:36:52 »
You're standing on a gedanken planet holding a laser pointer straight up. The light doesn't curve round, or slow down as it ascends, or fall down. It goes straight up. Now we make the planet denser and more massive. The light still doesn't curve round, or slow down as it ascends, or fall down. We make the planet even denser and more massive. The light still doesn't curve round, or slow down as it ascends, or fall down. We make the planet even denser and more massive, and take it to the limit such that it's a black hole. At no point did the light ever curve round, or slow down as it ascends, or fall down.

So why doesn't the light get out?

#### Ethos_

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##### Re: Why doesn't the light get out of a black hole?
« Reply #1 on: 02/02/2015 23:10:15 »
You're standing on a gedanken planet holding a laser pointer straight up. The light doesn't curve round, or slow down as it ascends, or fall down. It goes straight up. Now we make the planet denser and more massive. The light still doesn't curve round, or slow down as it ascends, or fall down. We make the planet even denser and more massive. The light still doesn't curve round, or slow down as it ascends, or fall down. We make the planet even denser and more massive, and take it to the limit such that it's a black hole. At no point did the light ever curve round, or slow down as it ascends, or fall down.

So why doesn't the light get out?
Discounting other possible limitations and or other physical details, the source of the light is falling toward the center at light speed. With the light source falling toward the center at light speed, would this freeze the beam at the event horizon or would it appear that the beam was traveling in the direction toward the singularity. I'm frankly not sure which is the case. This may or not be the answer you're seeking but it's the only one I can come up with.
« Last Edit: 02/02/2015 23:24:46 by Ethos_ »

#### yor_on

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##### Re: Why doesn't the light get out of a black hole?
« Reply #2 on: 03/02/2015 03:18:47 »
It's a strange question John. Are you assuming a atmosphere showing us the lights path inside that black hole? And then a observer able to follow that path up to some 'event horizon' to then 'curve' back to some center? It's a geodesic, that is formed by gravity. And it's pretty hard seeing the light 'move', you only see it 'interact'. Everything in SpaceTime (uniformly moving) follows geodesics, light is no exception. Our Earth does it too.

#### JohnDuffield

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##### Re: Why doesn't the light get out of a black hole?
« Reply #3 on: 03/02/2015 11:50:34 »
Discounting other possible limitations and or other physical details, the source of the light is falling toward the center at light speed. With the light source falling toward the center at light speed, would this freeze the beam at the event horizon or would it appear that the beam was traveling in the direction toward the singularity. I'm frankly not sure which is the case. This may or not be the answer you're seeking but it's the only one I can come up with.
No, the light source isn't falling. And IMHO what's interesting about this question, is that most people can't give a satisfactory answer even though they feel they're knowledgeable about black holes.

Quote from: yor_on
It's a strange question John.
It's one of those naďve simple little questions that really makes you think. Or at least I hope it is.

Quote from: yor_on
Are you assuming a atmosphere showing us the lights path inside that black hole?
No, but we could sprinkle some dust around if that helps.

Quote from: yor_on
And then a observer able to follow that path up to some 'event horizon' to then 'curve' back to some center? It's a geodesic, that is formed by gravity. And it's pretty hard seeing the light 'move', you only see it 'interact'. Everything in SpaceTime (uniformly moving) follows geodesics, light is no exception. Our Earth does it too.
The point is that the vertical light beam doesn't curve round. It goes straight up. So why doesn't the light get out?

#### Ethos_

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##### Re: Why doesn't the light get out of a black hole?
« Reply #4 on: 03/02/2015 13:43:54 »
No, the light source isn't falling.
If the light source isn't falling, then another explanation is; The beam is in a perpetual orbit around the black hole disallowing it's escape.

Another scenario would be that the intense gravitational force has slowed the passage of time to such a degree that the light hasn't had sufficient time to escape? With the passage of time at a virtual standstill within this gravity well, light would appear to be frozen in place to the outside observer. The state of affairs within the black hole would be vastly different for the fellow with the flash light however.
« Last Edit: 03/02/2015 14:07:12 by Ethos_ »

#### JohnDuffield

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##### Re: Why doesn't the light get out of a black hole?
« Reply #5 on: 03/02/2015 14:12:33 »
If the light source isn't falling, then one other other explanation is; The beam is in a perpetual orbit around the black hole disallowing it's escape.
No. The beam starts off vertical, we increase the gedanken mass/density, and the beam stays vertical. It never strays from vertical. We don't see it curving a little when we're getting close to black-hole density.

Or could it be that the intense gravitational force has slowed the passage of time to such a degree that the light hasn't had sufficient time to escape? With the passage of time at a virtual standstill within this gravity well, light would appear to be frozen in place to the outside observer.
Now you're talking. Note though that "the passage of time" is a figure of speech. There is no actual time passing inside an optical clock, or inside any other clock. See this Einstein quote.

The state of affairs within the black hole would be vastly different for the fellow with the flash light however.
That's what people say. And it's another can of worms.

#### Ethos_

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##### Re: Why doesn't the light get out of a black hole?
« Reply #6 on: 03/02/2015 14:33:34 »
Note though that "the passage of time" is a figure of speech. There is no actual time passing inside an optical clock, or inside any other clock.
The subject of "Time" is a very debatable topic. Being a conscious abstraction we personally relate to change, time is of course difficult to quantify. Only when we have uniform metrics agreeable to every observer does time take on a meaningful quality. Even then, time is still only a personalized conscious experience.

"Time is different everywhere because "change" is what makes the difference.".............................Ethos

#### JohnDuffield

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##### Re: Why doesn't the light get out of a black hole?
« Reply #7 on: 03/02/2015 17:57:40 »
Time is the key. I don't know if you've ever seen this. But once you appreciate that a clock "clocks up" some kind of regular cyclical motion rather than the literal flow of time, IMHO it's rather liberating. You then appreciate that when an optical clock goes slower when it's slower, it's because light goes slower. Hence you appreciate what Einstein was saying in the second paragraph here:

« Last Edit: 03/02/2015 18:03:15 by JohnDuffield »

#### Ethos_

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##### Re: Why doesn't the light get out of a black hole?
« Reply #8 on: 03/02/2015 19:27:33 »
Time is the key.
Might I have your consent to respectfully rephrase that line with the following?........... "Change is the key"
Quote from: JohnDuffield
I don't know if you've ever seen this.

Haven't read the book but I agree that the use of the word "time" usually only clouds the issues about reality.

I'll withhold further opinions about how I understand "time" so as not to guide this thread off topic. Just allow me to say three things about time and I'll shut up about it.

1. The past is not a point in time, it was a place which has changed to our present location.

2. The future is not a point in time, it is a location which we have yet to visit.

3. And the present will continuously morph into new realities leaving us with only the memories of it. The present is lost before it's realized.

« Last Edit: 03/02/2015 19:30:48 by Ethos_ »

#### JohnDuffield

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##### Re: Why doesn't the light get out of a black hole?
« Reply #9 on: 03/02/2015 20:32:02 »
Might I have your consent to respectfully rephrase that line with the following?........... "Change is the key"
Yep. And my blessing.

Haven't read the book but I agree that the use of the word "time" usually only clouds the issues about reality.

I'll withhold further opinions about how I understand "time" so as not to guide this thread off topic. Just allow me to say three things about time and I'll shut up about it.

1. The past is not a point in time, it was a place which has changed to our present location.

2. The future is not a point in time, it is a location which we have yet to visit.

3. And the present will continuously morph into new realities leaving us with only the memories of it. The present is lost before it's realized.
All good stuff. OK, to get back on topic, get a load of this:

Optical clocks go slower when they're lower, because light goes slower when its lower. Check out the coordinate speed of light. Einstein didn't call it that, so I won't. I'll just call it the speed of light. Anyway, in the normal course of events, when you're standing on a planet rather than a black hole...

the vertical light beam speeds up

#### jeffreyH

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##### Re: Why doesn't the light get out of a black hole?
« Reply #10 on: 03/02/2015 21:07:20 »
Without subjecting a photon to traveling through some super cooled medium or by other means you will never be able to detect a change in the speed of light. It will change between frames where the properties of the fields in those frames differ from each other. However EVERYTHING changes proportionally with the photon. If a photon slows down by 0.01% then an electron slows down by 0.01% and a proton and a neutron. That is why, in a vacuum to remove air resistence, dropping an elephant and a mouse from the same height they will both hit the ground at the same time. Oh my god john, I am so sorry, I mentioned the T word.

#### JohnDuffield

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##### Re: Why doesn't the light get out of a black hole?
« Reply #11 on: 03/02/2015 21:20:58 »
However EVERYTHING changes proportionally with the photon. If a photon slows down by 0.01% then an electron slows down by 0.01% and a proton and a neutron.
You got it. Everything slows down, including you and your clocks, so you can't tell that the light has slowed down. We call it gravitational time dilation, but at no juncture can we ever see this thing called time which is said to slow down.

Oh my god john, I am so sorry, I mentioned the T word.
We should play a little game where we talk physics, but aren't allowed to say "time". I tell you, it isn't easy.

Oh, and Ethos, cross my heart and hope to die, but when the vertical light beam is pointed down rather than up, it slows down. Is that counterintuitive or what? Nowhere will you ever read that in any textbook. You will read about the coordinate speed of light varying with gravitational potential, but the true ramifications of that don't get mentioned. Not that I'm aware of anyway. Anyway, this is how Professor Tom Moore answered the question:

"...As the planet's mass approaches the black hole limit, the signal emitted from the surface will seem to move more and more slowly away from the surface (and will also be seen to be increasingly red-shifted as observed from infinity). When the surface of the planet coincides with the black hole's event horizon, the signal will stop moving outward from the surface (and the redshift observed at infinity will go to infinity). So light no longer escapes..."

The light doesn't get out because it's stopped.
« Last Edit: 03/02/2015 21:24:55 by JohnDuffield »

#### jeffreyH

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##### Re: Why doesn't the light get out of a black hole?
« Reply #12 on: 03/02/2015 21:58:43 »
Wow I got it. Do I get a medal? Sorry about the sarcasm John but do you really think I have just started thinking about this stuff? I have spent 25 years thinking about it. It is only now that I have had the time to get serious about following it up.
« Last Edit: 03/02/2015 22:03:11 by jeffreyH »

#### Ethos_

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##### Re: Why doesn't the light get out of a black hole?
« Reply #13 on: 03/02/2015 22:37:05 »
Wow I got it. Do I get a medal? Sorry about the sarcasm John but do you really think I have just started thinking about this stuff? I have spent 25 years thinking about it. It is only now that I have had the time to get serious about following it up.
Don't feel lonesome there Jeff, I'm 72 years young and have been interested in Black Holes for the majority of my adult years. I remember discussing this phenomenon with a friend of mine 50 years ago. I suggested to him then that I felt that a Black Hole was lurking at the center of just about every galaxy in the universe. He exclaimed to me that he didn't believe in Black Holes and that was the end of our conversation. Needless to say, whether they are called Black Holes or described as some other freak of nature, research has now proven with little opposition that such a scenario presently exists. I haven't met up with this fellow for many years but if I ever see him again, there's going to be an "I told you so" in our conversation.

« Last Edit: 04/02/2015 02:37:45 by Ethos_ »

#### yor_on

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##### Re: Why doesn't the light get out of a black hole?
« Reply #14 on: 04/02/2015 03:01:26 »
Your problem is twofold John. One is your presumption of watching that 'light path', finding it to go 'straight'. The other is the idea of a geodesic being something that doesn't exist. It does, that's what all uniform motion is about. And geodesics are a result of the idea on no 'resistance' and a result of gravity interacting with 'mass and energy' or momentum if you like. A black hole is to its definition a place where noting can pass the event horizon due to gravity. So assuming this to be non-existent you also redefine a black hole to 'something else', what it should be I don't know? You also put into question the way Einstein proved GR https://en.wikipedia.org/wiki/Tests_of_general_relativity as all of those is about the same thing you question? Namely how light 'curves' following those 'geodesics'.

#### JohnDuffield

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##### Re: Why doesn't the light get out of a black hole?
« Reply #15 on: 04/02/2015 14:37:09 »
I haven't got a problem, yor_on. The vertical or radial light beam goes straight. It just doesn't curve. It's like the "geodesic" that passes between two stars. It's a straight line. There's still a Shapiro delay, but there is no curvature of the light beam. And the Shapiro delay is there because "the speed of a light wave depends on the strength of the gravitational potential along its path". I don't redefine a black hole to be something else. It's small, it's dense, it's black, and light doesn't get out. Nor do I put into question the  way Einstein proved GR. I'm referring to Einstein here. Light doesn't curve because it follows those geodesics. Einstein never said that. What he said is light curves because the speed of light varies with position. See for example this quote. Note though that the translation employs the word velocity rather than speed, which is a pity. However you can tell it should be speed because he refers to one of the two fundamental assumptions in the special theory of relativity.
« Last Edit: 04/02/2015 14:39:40 by JohnDuffield »

#### JohnDuffield

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##### Re: Why doesn't the light get out of a black hole?
« Reply #16 on: 04/02/2015 14:38:39 »
Wow I got it. Do I get a medal? Sorry about the sarcasm John but do you really think I have just started thinking about this stuff? I have spent 25 years thinking about it. It is only now that I have had the time to get serious about following it up.
Sorry Jeffrey. But this is so simple, and yet so few people know about it. Try asking the question somewhere else, and you'll see what I mean.

#### yor_on

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##### Re: Why doesn't the light get out of a black hole?
« Reply #17 on: 05/02/2015 12:32:30 »
Let me see if I get you straight? You define gravity to a varying of the speed of light, as defined from a 'container model' in where we all will agree to different 'time densities' due to this variation? Or would you separate time from gravity, how? And then we come to uniform motion, how do you define the 'densities' there?

#### ClaesN

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##### Re: Why doesn't the light get out of a black hole?
« Reply #18 on: 05/02/2015 13:22:12 »
Maybe it gets knocked back down by other particles (photons)?

Is there something one may call squeezed light?
« Last Edit: 05/02/2015 13:32:03 by ClaesN »

#### yor_on

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##### Re: Why doesn't the light get out of a black hole?
« Reply #19 on: 05/02/2015 19:20:52 »
As if photons interacted with other photons?
Now you added something to Johns idea :)
The only definition I've seen there is the idea of two 'photon interactions'

"From quantum electrodynamics it can be found that photons cannot couple directly to each other, since they carry no charge, but they can interact through higher-order processes. A photon can, within the bounds of the uncertainty principle, fluctuate into a charged fermion–antifermion pair, to either of which the other photon can couple. This fermion pair can be leptons or quarks. Thus, two-photon physics experiments can be used as ways to study the photon structure, or what is "inside" the photon."

that's more or less about a regime where we had a 'hot' constricted universe. It's what I would call a 'container model'

#### ClaesN

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##### Re: Why doesn't the light get out of a black hole?
« Reply #20 on: 05/02/2015 23:34:17 »
As if photons interacted with other photons?
Now you added something to Johns idea :)
The only definition I've seen there is the idea of two 'photon interactions'

"From quantum electrodynamics it can be found that photons cannot couple directly to each other, since they carry no charge, but they can interact through higher-order processes. A photon can, within the bounds of the uncertainty principle, fluctuate into a charged fermion–antifermion pair, to either of which the other photon can couple. This fermion pair can be leptons or quarks. Thus, two-photon physics experiments can be used as ways to study the photon structure, or what is "inside" the photon."

that's more or less about a regime where we had a 'hot' constricted universe. It's what I would call a 'container model'

When speaking of Radiation pressure photons gently push matter even though it’s said photons have no mass, like for solar sails and in optical tweezers and the Casimir effect. If this matter was leptons or quarks, would it be wrong to say that photons may interact with each other, with some extra energy from the quantum vacuum? Maybe that is what happens if they hit each other on a sweet spot? It’s an elastic or inelastic impulse (“+vacuum energy”) which set particles out of path. Is it possible that the probably energetic dense field where this is happening somehow has the characteristics of setting all particles trying to get out from the black hole pushed back?

Just two humble questions: Since light is bent by gravity, do we need to redefine the definition of mass? What may not fluctuate into something else within the boundaries of the uncertainty principal if Big bang was such a fluctuation?

#### diethyl

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##### Re: Why doesn't the light get out of a black hole?
« Reply #21 on: 06/02/2015 03:35:06 »
The law of universal gravitation
Even light, the estimate cannot escape this theory.
This is the most unreliable but possible conclusion that I can think of.

#### ClaesN

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##### Re: Why doesn't the light get out of a black hole?
« Reply #22 on: 06/02/2015 09:34:24 »
I´ve been told to not see universe intuitively since universe is not intuitive. Others may have been told just to continue the calculations without thinking intuitively. Is our force of intuition inherited from universe, or is it the other way around? Maybe we should stop counting, at least for a moment or two, and start to think, intuitively? Maybe we will find if we seek? Maybe there is something north of the North Pole after all? Maybe God is somewhat of a gambler? We are. Have we created God?

Was it an infinitely small volume that set of Big bang trough a quantum fluctuation? Does the volume of space itself expand? Is this volume expanding into nothing? Do we need gravitons, antigravitons and/or gravitational waves when we try to explain forces of universe, or do the Standard model give us what we need? Maybe we should use our intuition when we answer those questions, or is there a widening abyss between the two camps, QM and Relativity theory, that makes it impossible for individuals to think out of the box without risking everything, as easy targets of sharks? What are researchers that partly supported Le Sage's theory of gravitation in the 20th century doing today?
« Last Edit: 06/02/2015 09:55:24 by ClaesN »

#### JohnDuffield

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##### Re: Why doesn't the light get out of a black hole?
« Reply #23 on: 07/02/2015 17:41:28 »
Let me see if I get you straight? You define gravity to a varying of the speed of light, as defined from a 'container model' in where we all will agree to different 'time densities' due to this variation? Or would you separate time from gravity, how? And then we come to uniform motion, how do you define the 'densities' there?
I'm sorry yor_on, I'm not clear what you mean here. Einstein said light curves because the speed of light varies with position. That's simple enough. Then think of the wave nature of matter, and think of an electron as some kind of light-like wave in a closed path, then simplify it to a square path. The horizontals curve downwards and the electron falls down. And since only the horizontals curve downwards, light is affected twice as much as matter.

As if photons interacted with other photons? Now you added something to Johns idea :) The only definition I've seen there is the idea of two 'photon interactions'

"From quantum electrodynamics it can be found that photons cannot couple directly to each other, since they carry no charge, but they can interact through higher-order processes. A photon can, within the bounds of the uncertainty principle, fluctuate into a charged fermion–antifermion pair, to either of which the other photon can couple. This fermion pair can be leptons or quarks. Thus, two-photon physics experiments can be used as ways to study the photon structure, or what is "inside" the photon."

that's more or less about a regime where we had a 'hot' constricted universe. It's what I would call a 'container model'
That's wrong I'm afraid. Look carefully at what the article is saying. It's saying one 511keV photon somehow spontaneously "fluctuates" into a 511keV electron and a 511keV positron. And that if pair production doesn't occur, they somehow fluctuate back into one 511keV photon in defiance of conservation of momentum and conservation of energy. And that this photon somehow managed to keep on propagating at c! It's a fairy tale, and a tautology. Gamma-gamma pair production doesn't occur because pair production occurred, spontaneously, like worms from mud. It occurs because photons interact with photons.

#### JohnDuffield

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##### Re: Why doesn't the light get out of a black hole?
« Reply #24 on: 07/02/2015 17:46:30 »
When speaking of Radiation pressure photons gently push matter even though it’s said photons have no mass, like for solar sails and in optical tweezers and the Casimir effect. If this matter was leptons or quarks, would it be wrong to say that photons may interact with each other, with some extra energy from the quantum vacuum? Maybe that is what happens if they hit each other on a sweet spot? It’s an elastic or inelastic impulse (“+vacuum energy”) which set particles out of path. Is it possible that the probably energetic dense field where this is happening somehow has the characteristics of setting all particles trying to get out from the black hole pushed back?
When you read the original material by Einstein, you can find answers to most of the questions. Like, the photon has no rest mass, but it does have an inertial mass.

Just two humble questions: Since light is bent by gravity, do we need to redefine the definition of mass?
No. The mass of a body is a measure of its energy-content. Trap a massless photon in a mirror-box, and it adds to the mass of that body.

What may not fluctuate into something else within the boundaries of the uncertainty principal if Big bang was such a fluctuation?
IMHO this "fluctuation" stuff is a non-answer pretending to be an answer.

#### The Naked Scientists Forum

##### Re: Why doesn't the light get out of a black hole?
« Reply #24 on: 07/02/2015 17:46:30 »