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## Gravity and Light

Since a photon has no mass, gravity doenst affect light, but rather the space that the light travels through. Is this correct? I was just thinking because i always hear people say that light cant escape from a black hole, but... i dont know. maybe im just confusing myself more. So does gravity affect light?

2. Yes it does. A photon has no rest mass, but a moving photon does have energy, which is equivalent to mass.

Plank's Law
E = hf

Einstein's equation for mass-energy equivalence
E = mc²

hf = mc²

Therefore, the mass of a photon is given by

m = hf/c²

Of course, you have to bear in mind that h is very small and c² is very large, so m is very very small.

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Two intrepid explorers (Joe and Ali) unwittingly approach a very large black hole event horizon. This black hole is large enough that there aren't those nasty ripping-you-apart forces at the EH. Ali and Joe are separated by, say, 3 meters. Joe crosses the EH first and turns to signal Ali with his light pen. What does Joe see? What does Ali see?

Similarly, if Joe and Ali enter at the same time do they see each other's light?

Apologies to earthman if this hijacks your topic, but it seems related.

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Ali would just see Joe suspended at the EH for the rest of eternity. Im not sure about the rest

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Originally Posted by earthman2110
Ali would just see Joe suspended at the EH for the rest of eternity. Im not sure about the rest
How is that going to happen? Explain the details of how Ali would see a continuous image of Joe suspended in space forever.

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Time slows in a gravitational field. At the event horizon of a black hole, time slows to a stop. So, anything falling into the black hole will appear to have frozen in time at the event horizon.

Now, actually it's just a bit more complicated than that. The thing falling into the black hole will continually be falling in (according to the person at some distance from the event horizon) but will never actually hit the event horizon because time slows down too fast to allow that. It's pretty odd.

Anyway, neither observer ever sees him/herself within the event horizon. In fact, Joe, who is falling into the black hole (from Ali's perspective) will actually see the black hole shrinking away from him. By the time he actually percieves himself as falling into the black hole, it's a point (in which case, all of those nasty little tidal forces kick back in), and one can't ever be "inside" a point. This is all allowed by the space distorting aspects of gravity.

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Originally Posted by Pi Man
Time slows in a gravitational field. At the event horizon of a black hole, time slows to a stop. So, anything falling into the black hole will appear to have frozen in time at the event horizon.

Now, actually it's just a bit more complicated than that. The thing falling into the black hole will continually be falling in (according to the person at some distance from the event horizon) but will never actually hit the event horizon because time slows down too fast to allow that. It's pretty odd.
Oh, that’s a bunch of relativity baloney.

How is the guy outside the hole going to get a continuous flow of light waves forever, from something that just passes through the event horizon but doesn’t linger in one spot forever? This is typical relativity nonsense. It is not “time itself” that slows down at the event horizon, it is the oscillation rates of the atoms that pass through the horizon. This reduces their internal vibration rate and also lowers the frequency of the light they emit. Thus, they become invisible.

This is like the myth about the Bern clock-tower thought experiment. The myth says that if you travel away from the clock tower at the speed of light, you’ll always see the clock read the same time. This thought experiment is based on a misunderstanding and misinterpretation of the classical Doppler effect, and it reveals a basic paradox and flaw in the SR theory.

In the first place, the Bern tower clock will tick at the rate determined by the laws of physics at the clock, that govern the tick rate of the clock. That does not change, no matter who or what is moving “relative” to it.

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In the first place, the Bern tower clock will tick at the rate determined by the laws of physics at the clock, that govern the tick rate of the clock. That does not change, no matter who or what is moving “relative” to it.
Well if you get on your magic bike, and ride away from the clock at .99c, and look back at the clock, you will see the clock moving very slowly. The people standing right under the clock will see the clock moving at its normal rate. right?

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Originally Posted by earthman2110
In the first place, the Bern tower clock will tick at the rate determined by the laws of physics at the clock, that govern the tick rate of the clock. That does not change, no matter who or what is moving “relative” to it.
Well if you get on your magic bike, and ride away from the clock at .99c, and look back at the clock, you will see the clock moving very slowly. The people standing right under the clock will see the clock moving at its normal rate. right?
If you travel away from the clock rapidly, you will experience a Doppler shift in the light waves coming to you from the clock. However, if you travel rapidly near the surface of the earth you will probably notice some other effects too, and you will probably experience some effects that you don’t notice.

The whole story about what would happen and what would be observed hasn’t been decided or agreed upon yet, not regarding light signals. In fact, it’s still not completely understood yet.

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Originally Posted by ChaosInc
Two intrepid explorers (Joe and Ali) unwittingly approach a very large black hole event horizon. This black hole is large enough that there aren't those nasty ripping-you-apart forces at the EH. Ali and Joe are separated by, say, 3 meters. Joe crosses the EH first and turns to signal Ali with his light pen. What does Joe see? What does Ali see?
Actually, I disagree with both earthman2110 and Sam5 on this one. The trick is keeping track of where Joe and Ali are relative to each other. Joe crosses the horizon first and doesn't start waving the light pen around until he's inside the hole. As such, Ali will never see the signal from the pen even if she crosses the horizon. I don't think Ali will see time stop for Joe as he approaches the horizon, since the solution earthman is referring to only applies to an observer very far away from the horizon. Either way, anything she sees from Joe will be severely redshifted.

As for Joe, he'll see stranger effects. If Ali remains outside the hole, he'll see her becoming more and more distorted as he approaches the singularity. In fact, just before he reaches the center, he'll see Ali distorted into a ring shape, circling the sky. Presumably, he will be similar, if less extreme, distortion if Ali follows him into the hole, but I haven't done the math for that so don't quote me.

Originally Posted by ChaosInc
Similarly, if Joe and Ali enter at the same time do they see each other's light?
Yes. They will be in the same reference frame so should be able to send signals back and forth.

11. Originally Posted by Sam5
[Snip!]Oh, that’s a bunch of relativity baloney.
How do you know? Can you do the math?
Originally Posted by Sam5
This [Snip!] lowers the frequency of the light they emit. Thus, they become invisible.
You are right about this. The one who falls in becomes invisible because all light he emits or reflects gets redshifted so that even any gamma-emitting isotopes he carries are redshifted beyond radio frequencies and rendered invisible. Likewise, the one who remains outside will ultimately disappear from the view of the unfortunate that falls in because all of the light gets blueshifted into gamma rays. Of course the unfortunate one is probably torn apart by tidal forces before being fried by gamma rays.

12. In fact, it’s still not completely understood yet.
By you! Over and over, on thread after thread, you dismiss anything that you do not understand as not being understood (or even understandable) by anyone!

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I don't understand how the image of something is forever frozen on the even horizon to an outside observer... If this is the case, why don't we see frozen images of all kinds of things around black holes rather than the radiation we do see?

14. Originally Posted by thefish7
I don't understand how the image of something is forever frozen on the even horizon to an outside observer... If this is the case, why don't we see frozen images of all kinds of things around black holes rather than the radiation we do see?
Well, nobody said it would be digital quality.

15. A light cone diagram would be helpful, if I could only find a way to post the one I've drawn... I can't just send it, I have to put it in as an http address... hmmm...

The world line of any photon crossing the EH of a black hole would be 'tipped' toward the singularity. I don't know about this whole 'red-shift' stuff. The minkowskian representation of this was totally helpful in the 'visualization'...

[edited to take away smar-castic-remarks]

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Originally Posted by Glom
Well, nobody said it would be digital quality.
Hhahaha, that was hilarious. Thank you for making a slow day at work pass a little more quickly.

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Originally Posted by thefish7
I don't understand how the image of something is forever frozen on the even horizon to an outside observer... If this is the case, why don't we see frozen images of all kinds of things around black holes rather than the radiation we do see?
Of course this is not correct. One can not see an image of an object “frozen” forever unless there is a stream of light emanating from or being reflected by the object forever. And if an object is “frozen in time” it cannot emit a stream of light forever, since the atoms of the object have to be moving in order to emit the light, so the object is not “frozen in time”.

18. Would it be painful?

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Okay then help me out with this: Joe and Ali can signal each other if they are the same distance from the center, assuming this thing is spherical, but if they are within the EH but at different levels then would there effectively be no communication between them?

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Originally Posted by Sam5
Of course this is not correct. One can not see an image of an object “frozen” forever unless there is a stream of light emanating from or being reflected by the object forever. And if an object is “frozen in time” it cannot emit a stream of light forever, since the atoms of the object have to be moving in order to emit the light, so the object is not “frozen in time”.
[nit]
Atoms dont have to move to emit light. electrons move.[/nithasbeenpicked]

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Originally Posted by thefish7
I don't understand how the image of something is forever frozen on the even horizon to an outside observer... If this is the case, why don't we see frozen images of all kinds of things around black holes rather than the radiation we do see?
It will be there, just more and more red shifted, I think. Anyway, I don't think any human have seen a black hole up close enough to see anything like this even if it was visible.

I would think the person flying into the hole will always see the event horizon in front of them, light can't climb out. Backwards s/he could still see the stars and stuff(blue shifted), though the gravity would likely cause it to be distorted, I guess it would look compressed, it would maybe be similar to the aberration that would be seen from a craft accelerating to near light speed.

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Originally Posted by ChaosInc
Okay then help me out with this: Joe and Ali can signal each other if they are the same distance from the center, assuming this thing is spherical, but if they are within the EH but at different levels then would there effectively be no communication between them?
Well, the way GR works out, the two would never be able to find themselves inside the event horizon. In fact, they could both agree (in the situation where Joe was falling towards the event horizon) that neither one actually fell beyond the event horizon. Ali (who is not falling toward the BH) would see Joe as being plastered to the EH. Joe would see the black hole shrinking away from him. There's no way to really fall past the EH, so there's no way to really get inside a black hole.

However, if time runs forward outside a black hole, and freezes at the EH, it must go backwards inside the black hole. So, would it be possible to communicate inside a black hole? Only if one had already communicated before the black hole had formed.[/quote]

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Originally Posted by TrAI
It will be there, just more and more red shifted, I think. Anyway, I don't think any human have seen a black hole up close enough to see anything like this even if it was visible.
Yup. He's right. In fact, nobody has even experimentally or observationally proven the existance of black holes. That's why Stephen Hawking hasn't yet gotten a Nobel prize in physics.

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Originally Posted by Pi Man
Well, the way GR works out, the two would never be able to find themselves inside the event horizon. In fact, they could both agree (in the situation where Joe was falling towards the event horizon) that neither one actually fell beyond the event horizon. Ali (who is not falling toward the BH) would see Joe as being plastered to the EH. Joe would see the black hole shrinking away from him. There's no way to really fall past the EH, so there's no way to really get inside a black hole.

However, if time runs forward outside a black hole, and freezes at the EH, it must go backwards inside the black hole. So, would it be possible to communicate inside a black hole? Only if one had already communicated before the black hole had formed.
I don't think it run backwards, I think it is exchanged with space, so that you are not falling a certain distance, but a certain time, traveling towards the time you are going to hit, not the place. Though I have heard that rotating and charged black hole could have a second event horizon where time and space are swapped again some time before you hit or miss(a rotating singularity could be missed if you come from the right angle, it makes a kind of ring along the plane, this is where the hypothesized wormhole thingy would be) the singularity

25. I too recall reading that time-coordinates become space-like and space-coordinates become time-like. I swear the light cone depiction would be helpful.
I'm going to revamp my massivespacetime site so I can at least post some images to it.

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Originally Posted by earthman2110
[nit]
Atoms dont have to move to emit light. electrons move.[/nithasbeenpicked]
Ok, you’re right. I should have said an atom must “oscillate internally”, rather than just “move”.

I think the concept of “time dilation” has not been explained properly in many pop-sci books.

Although it has been commonly called “time dilation” for many years, it basically involves the internal oscillation rates of atoms. Rapidly oscillating atoms emit high frequency light, and slowly oscillating atoms emit lower frequency light.

The idea of an “atomic clock” and “atomic time” goes back to Maxwell’s concept that was first published in 1873 in this form:

“In astronomy the mean distance of the earth from the sun is
sometimes taken as a unit of length.

In the present state of science the most universal standard of
length which we could assume would be the wave length in
vacuum of a particular kind of light, emitted by some widely
diffused substance such as sodium, which has well-defined lines
in its spectrum. Such a standard would be independent of any
changes in the dimensions of the earth, and should be adopted
by those who expect their writings to be more permanent than
that body.

In treating of the dimensions of units we shall call the unit of
length L. If l is the numerical value of a length, it is under-
stood to be expressed in terms of the concrete unit L, so that
the actual length would be fully expressed by 1 L.

4.] (2) Time. The standard unit of time in all civilized
countries is deduced from the time of rotation of the earth
about its axis. The sidereal day, or the true period of rotation
of the earth, can be ascertained with great exactness by the
ordinary observations of astronomers; and the mean solar day
can be deduced from this by our knowledge of the length of
the year.

The unit of time adopted in all physical researches is one
second of mean solar time.

In astronomy a year is sometimes used as a unit of time. A
more universal unit of time might be found by taking the
periodic time of vibration of the particular kind of light whose
wave length is the unit of length.

We shall call the concrete unit of time T, and the numerical
measure of time t.”

It was this concept that Lorentz used in developing his time dilation theory in his 1895 electrodynamics relativity book, “Versuch Einer Theorie Der Elektrischen Und Optischen Erscheinungen In Bewegten Körpern”. Lorentz called the time dilation “local time”. It was this concept that Einstein used in his 1905 SR theory, but he mistook the “local time” to be true time or all of “time itself” in the vicinity of Lorentz’s slowly oscillating atoms, and so Einstein assumed that all clocks would act the same in the vicinity of those atoms. That’s why he used “balance wheel” clocks in the 1905 theory, rather than atomic clocks. He didn’t add atomic clocks to the SR theory until 1918. So, it was this misunderstanding that gave us the incorrect pop-sci concept that “time itself” stops at the “event horizon”, thus whatever we “see” there must be “frozen in time” and we must “see” it there forever. But this is wrong. Things don’t go banging into the event horizon and they don’t get stuck there, “frozen in time”. They go through it and we never see them again.

Based on the Maxwell and Lorentz concepts, what happens at black holes is that atoms slow down their internal oscillation rates so much, they emit light of a lower and lower frequency the closer they get, and eventually they just stop emitting light, so we don’t see them anymore. This is interpreted in some pop-sci books as “time standing still inside a black hole”. What it really is (in my opinion) is atoms no longer oscillating and no longer emitting light.

I don’t know where the idea of “time running backwards” inside a black hole comes from, but I guess someone just made it up and published it in some book or on some website.

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Originally Posted by Pi Man
Originally Posted by ChaosInc
Okay then help me out with this: Joe and Ali can signal each other if they are the same distance from the center, assuming this thing is spherical, but if they are within the EH but at different levels then would there effectively be no communication between them?
Well, the way GR works out, the two would never be able to find themselves inside the event horizon. In fact, they could both agree (in the situation where Joe was falling towards the event horizon) that neither one actually fell beyond the event horizon. Ali (who is not falling toward the BH) would see Joe as being plastered to the EH. Joe would see the black hole shrinking away from him. There's no way to really fall past the EH, so there's no way to really get inside a black hole.
Again, I disagree. Joe would absolutely see himself moving into the black hole. He might not necessarily know he's crossed the horizon, but sooner or later, he'll realise he's in freefall, heading for the singularity. From Joe's point of view, he's in a perfectly normal, freely falling reference framce. Only an observer *outside* the black hole would see an infalling object assymptotically stop at the horizon. With that in mind, that might only apply to an observer watching from an extreme distance - and I'm not convinced if an observer close to the horizon would see the same thing.

Originally Posted by ChaosInc
However, if time runs forward outside a black hole, and freezes at the EH, it must go backwards inside the black hole. So, would it be possible to communicate inside a black hole? Only if one had already communicated before the black hole had formed.
We don't know if time can run backwards inside a black hole or not. My understanding is that it can't, based on Eddington-Finkelstein coordinates.

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Originally Posted by Sam5
I don’t know where the idea of “time running backwards” inside a black hole comes from, but I guess someone just made it up and published it in some book or on some website.
Solve the Scwarzschild line element for r &lt; 2GM/c^2 and you'll see where the idea comes from.

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Originally Posted by Taibak
Originally Posted by Sam5
I don’t know where the idea of “time running backwards” inside a black hole comes from, but I guess someone just made it up and published it in some book or on some website.
Solve the Scwarzschild line element for r &lt; 2GM/c^2 and you'll see where the idea comes from.
When the oscillation and vibration rates slow down to zero, that’s it. They don’t speed up again in a negative way so that “time flows backwards”.

What you are suggesting is like saying there are “negative temperatures” that are of a “negative coldness” that is less than Absolute Zero.

If you have time “going backwards” then you have to have an “effect and cause” universe rather than a “cause and effect” universe. That only happens on TV shows and in movies.

There are things you can do with math equations that don’t actually happen in real life.

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Originally Posted by Sam5
When the oscillation and vibration rates slow down to zero, that’s it.
As measured by whom? If you were to fall into a black hole, what would *you* experience as you crossed the horizon? Would that be different than what I would see if I were at rest relative to the black hole and watching from some incredibly huge distance?

Originally Posted by Sam5
They don’t speed up again in a negative way so that “time flows backwards”.
Time exists regardless of how fast a particle is vibrating.

Originally Posted by Sam5
What you are suggesting is like saying there are “negative temperatures” that are of a “negative coldness” that is less than Absolute Zero.
You're comparing apples and Volkswagons. Time and temperature are two totally different phenomena.

Originally Posted by Sam5
If you have time “going backwards” then you have to have an “effect and cause” universe rather than a “cause and effect” universe. That only happens on TV shows and in movies.
Agreed. I have yet to see any evidence - or even a convincing theory - that says this can happen. I was merely pointing out to you where the idea comes from.

Originally Posted by Sam5
There are things you can do with math equations that don’t actually happen in real life.
Have you actually done the math for the Scwarzschild equation? If not, how can you be so sure that it doesn't describe 'real life?'

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