# Can light form a black hole?

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#### chiralSPO

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##### Can light form a black hole?
« on: 11/06/2015 16:12:31 »
As I understand it, although a photon has no rest mass, it does have a gravitational field that is a function of the energy (frequency) of that photon.

Is there a frequency above which a single photon's gravitational field results in a black hole? Or could there exist a high enough concentration of energetic photons that together create a black hole? Would this black hole still travel at the speed of light?

Part of how I got to this question was thinking about the result of the impact of two black holes that are identical except that one was formed from matter, and the other from antimatter. My understanding is that the apparent masses would just add together whether or not we imagine that an annihilation event occurred. Either the degenerate matter is somehow incapable of annihilation, or if the mass of the black holes is actually completely converted into energy, it is still confined within the event horizon, and exerts the same gravitational effects as the mass before annihilation. (no information leaves the event horizon, so we cannot distinguish between the impact of two matter black holes and one matter plus one anitmatter)

#### syhprum

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##### Re: Can light form a black hole?
« Reply #1 on: 11/06/2015 17:54:05 »
The reference to an anti matter BH presumably refers to one that has formed by the collapse of an massive anti matter star, after the BH has formed is it possible to determine whether the original star was matter or anti matter ?
syhprum

#### PmbPhy

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##### Re: Can light form a black hole?
« Reply #2 on: 11/06/2015 17:57:21 »
Quote from: chiralSPO
As I understand it, although a photon has no rest mass, it does have a gravitational field that is a function of the energy (frequency) of that photon.
The following is my understanding of the related physics: Although a single photon has zero mass and thus no zero momentum frame, the same can't be said for two photons moving in opposite directions. The invariant mass of the two photons is non-zero and there is a frame of reference in which the total momentum is zero. If, in this frame, two photons of sufficient energy "collide" they will form a black hole.

Quote from: chiralSPO
Is there a frequency above which a single photon's gravitational field results in a black hole? Or could there exist a high enough concentration of energetic photons that together create a black hole? Would this black hole still travel at the speed of light?
If such a frequency did exist then it'd be possible to change to a frame of reference where the energy/frequency is too small and therefore no black hole exists in that frame. The existence of a black hole is invariant so it follows that the answer to your question is no.

#### chiralSPO

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##### Re: Can light form a black hole?
« Reply #3 on: 11/06/2015 20:32:42 »
The reference to an anti matter BH presumably refers to one that has formed by the collapse of an massive anti matter star, after the BH has formed is it possible to determine whether the original star was matter or anti matter ?

It should not be possible to tell an "antimatter black hole" from any other type of black hole. I first stated:

...two black holes that are identical except that one was formed from matter, and the other from antimatter...

from there on I lazily referred to an "antimatter black hole" and a "matter black hole."  I appologize for any confusion caused by my laziness.

#### chiralSPO

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##### Re: Can light form a black hole?
« Reply #4 on: 11/06/2015 20:36:44 »
If such a frequency did exist then it'd be possible to change to a frame of reference where the energy/frequency is too small and therefore no black hole exists in that frame. The existence of a black hole is invariant so it follows that the answer to your question is no.

Aha! That is a good way of explaining this, and certainly one I would not have thought of. Thank you.

The following is my understanding of the related physics: Although a single photon has zero mass and thus no zero momentum frame, the same can't be said for two photons moving in opposite directions. The invariant mass of the two photons is non-zero and there is a frame of reference in which the total momentum is zero. If, in this frame, two photons of sufficient energy "collide" they will form a black hole.

So, in this case, with two (or n pairs of) photons traveling in opposite directions, no frame of reference will result in a lower observed aggregate energy?

#### jeffreyH

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##### Re: Can light form a black hole?
« Reply #5 on: 11/06/2015 20:56:23 »
I think the resultant energy from the two photons would need to be equivalent to the energy of 1 Planck mass.

EDIT: So each photon  would have $$\frac{1}{2}mPc^2$$
« Last Edit: 11/06/2015 21:04:10 by jeffreyH »

#### PmbPhy

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##### Re: Can light form a black hole?
« Reply #6 on: 12/06/2015 02:19:52 »
There is no such thing as an antimatter black hole. Any matter which falls in can't get out even if there are annihilations which occur. However I can't imagine that something like that happening because the singularities will never meet according to outside observers.

#### lightarrow

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##### Re: Can light form a black hole?
« Reply #7 on: 13/06/2015 18:28:06 »
So, in this case, with two (or n pairs of) photons traveling in opposite directions, no frame of reference will result in a lower observed aggregate energy?
What does it mean?

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lightarrow

#### lightarrow

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##### Re: Can light form a black hole?
« Reply #8 on: 13/06/2015 18:35:35 »
If, in this frame, two photons of sufficient energy "collide" they will form a black hole.
It also depends on how large are the photons and I have no idea of that.

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lightarrow

#### PmbPhy

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##### Re: Can light form a black hole?
« Reply #9 on: 14/06/2015 06:13:53 »
If, in this frame, two photons of sufficient energy "collide" they will form a black hole.
It also depends on how large are the photons and I have no idea of that.

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lightarrow
Photons are point particles. However the probability of where they are is not a point function.

#### lightarrow

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##### Re: Can light form a black hole?
« Reply #10 on: 14/06/2015 09:48:25 »
If, in this frame, two photons of sufficient energy "collide" they will form a black hole.
It also depends on how large are the photons and I have no idea of that.

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lightarrow
Photons are point particles. However the probability of where they are is not a point function.
If photons really were Point particles, the Energy density would be very high and two opposite (common) beams of light should easily have the possibility to form Black holes. Or we have been so Lucky that none of the photons smashed exactly head-on yet?

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#### evan_au

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##### Re: Can light form a black hole?
« Reply #11 on: 14/06/2015 11:46:24 »
The probability of finding a photon in a particular place is related to its wavelength.
The wavelength of light gets smaller as the energy gets higher.
When the energy gets high enough to form a black hole, the photon's wavelength gets vanishingly small, so there is little likelihood that they will collide.

I suspect that long before you got to black-hole energies, the photons would turn into a shower of other particles.

#### PmbPhy

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##### Re: Can light form a black hole?
« Reply #12 on: 14/06/2015 12:03:25 »
Quote from: lightarrow
If photons really were Point particles, the Energy density would be very high and two opposite (common) beams of light should easily have the possibility to form Black holes.
That's a classical argument which you're trying to use in quantum mechanics so it's invalid argument. Electrons, photons etc. behave the laws of quantum field theory. See

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But, another reason that the electron is not considered a black hole, even assuming that its radius is infinitely small, is that it obeys the laws of quantum field theory. Normally, when one speaks about black holes, one is talking about them in terms of Einstein's theory of general relativity. No one is sure how nature merges Einstein's theory with quantum field theory. So we aren't really sure if the idea of a black hole makes sense on distance scales as small as the (possible) radius of the electron.. Our best idea to unify general relativity with quantum field theory is an idea called string theory, but string theory still appears to be a long way from being put to any experimental tests.
If I were you I'd ignore the second response. I consider it to be quite flawed.
« Last Edit: 14/06/2015 12:10:05 by PmbPhy »

#### jeffreyH

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##### Re: Can light form a black hole?
« Reply #13 on: 14/06/2015 14:08:29 »
The energy/frequency required for two photons to become a black hole are extreme and probably wouldn't occur except in extreme conditions. The most extreme being the big bang. It could be that primordial black holes were the result of collisions of photons. However, I personally doubt if two would be enough. The universe is not littered with primordial black holes as far as anyone knows so these must have been scarce events. If they were the result of light trapping itself then the equation $$\frac{1}{2}Mc^2$$ would have M equaling x number of photons. How would you ever prove this? Would you want to? The vortex around these objects would be intense. I wouldn't want to be anywhere near one.

#### lightarrow

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##### Re: Can light form a black hole?
« Reply #14 on: 14/06/2015 17:49:25 »
Quote from: lightarrow
If photons really were Point particles, the Energy density would be very high and two opposite (common) beams of light should easily have the possibility to form Black holes.
That's a classical argument which you're trying to use in quantum mechanics so it's invalid argument.
It's not used in classical mechanics only, even in QM. Let's say, rather, that we don't have a quantum theory of gravity so our reasoning is speculation (my reasoning as well as your).
Quote
Electrons, photons etc. behave the laws of quantum field theory. See
Quote
But, another reason that the electron is not considered a black hole, even assuming that its radius is infinitely small, is that it obeys the laws of quantum field theory. Normally, when one speaks about black holes, one is talking about them in terms of Einstein's theory of general relativity. No one is sure how nature merges Einstein's theory with quantum field theory. So we aren't really sure if the idea of a black hole makes sense on distance scales as small as the (possible) radius of the electron.. Our best idea to unify general relativity with quantum field theory is an idea called string theory, but string theory still appears to be a long way from being put to any experimental tests.
If I were you I'd ignore the second response. I consider it to be quite flawed.
But even that answer can have some value, it brings an interesting viewpoint.

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lightarrow

« Last Edit: 14/06/2015 17:51:12 by lightarrow »

#### lightarrow

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##### Re: Can light form a black hole?
« Reply #15 on: 14/06/2015 18:07:48 »
The probability of finding a photon in a particular place is related to its wavelength.
Wavelength is related to how light (em waves) behaves, but from this it's not possible to extrapolate a photon's dimension. If you have a plane wave, it's infinitely large, but it can have as small wavelength as you want.
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The wavelength of light gets smaller as the energy gets higher.
When the energy gets high enough to form a black hole,
So it only depends on energy? But even little masses (and so little energies) can form black holes, if the dimensions are little enough.

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lightarrow