# The Naked Scientists Forum

### Author Topic: Red shift  (Read 5596 times)

#### DoctorBeaver

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##### Red shift
« on: 23/11/2005 10:27:04 »
I've just been reading about how light from the proximity of a black hole gets red-shifted from visible light to infra-red to radio waves. How far down the spectrum can this process go? Is it possible to red-shift light so far that it has zero frequency? If so, what exactly would that mean? Would it entail the photon having zero energy? Would it cease to exist? Or would it transform into something else?

#### rakarthxii

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##### Re: Red shift
« Reply #1 on: 24/11/2005 18:35:32 »
I don't think a photon can have zero energy. If it did it would be still, that means we can determine exactly where is it, which according to the Heisenburg uncertainty principle, is not possible.

I haven't looked into the math, but is it possible that the speeds required to make such a large shift, are speeds greater than the speed of light? If so then is it not impossible on 2 counts?

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« Last Edit: 24/11/2005 18:52:54 by rakarthxii »

#### another_someone

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##### Re: Red shift
« Reply #2 on: 25/11/2005 10:28:54 »
quote:
Originally posted by DoctorBeaver

I've just been reading about how light from the proximity of a black hole gets red-shifted from visible light to infra-red to radio waves. How far down the spectrum can this process go? Is it possible to red-shift light so far that it has zero frequency? If so, what exactly would that mean? Would it entail the photon having zero energy? Would it cease to exist? Or would it transform into something else?

If light has zero frequency, then you are talking about constant electric charge (i.e. no fluctuation in the field).  By the time this would happen in a black hole, I suspect the particle emitting the photon  would have reached the event horizon of the black hole, so you'd never get to see this happening.

#### DoctorBeaver

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##### Re: Red shift
« Reply #3 on: 25/11/2005 18:27:08 »
quote:
If light has zero frequency, then you are talking about constant electric charge (i.e. no fluctuation in the field). By the time this would happen in a black hole, I suspect the particle emitting the photon would have reached the event horizon of the black hole, so you'd never get to see this happening.

What about at the instant the whatever-it-is crosses the event horizon? Would a photon emitted at that exact moment be red-shifted to such an extent?

#### Dr. Praetoria

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##### Re: Red shift
« Reply #4 on: 25/11/2005 21:43:38 »
"I don't think a photon can have zero energy. If it did it would be still, that means we can determine exactly where is it, which according to the Heisenburg uncertainty principle, is not possible."

Would it be possible for such a photon to have variable degrees of velocity reduction as it's temperature was being diminished to absolute zero and therefore, indicating discrepancy to Heisenberg's Uncertainty Principle?
Doc

#### another_someone

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##### Re: Red shift
« Reply #5 on: 26/11/2005 00:07:45 »
quote:
Originally posted by DoctorBeaver
What about at the instant the whatever-it-is crosses the event horizon? Would a photon emitted at that exact moment be red-shifted to such an extent?

I would guess possibly so, but since the light would take an infinite time to reach you, you would (in practice) never see the event, and thus no visible violation of any of the laws of physics would occur.

#### Solvay_1927

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##### Re: Red shift
« Reply #6 on: 26/11/2005 01:56:30 »
Er, isn't frequency = no. of peaks of a wave that pass a certain point per unit time?  So zero frequency would mean the wave isn't moving anymore - i.e. in effect it's ceased to exist?

(Or is my understanding of physics just too basic?)

#### DoctorBeaver

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##### Re: Red shift
« Reply #7 on: 26/11/2005 15:13:23 »
quote:
I don't think a photon can have zero energy. If it did it would be still, that means we can determine exactly where is it, which according to the Heisenburg uncertainty principle, is not possible.

But if the photon is stationary, there is no speed to measure. Heisenberg's principle states that it is impossible to know both the speed & the position of a particle. But if the particle is stationary there is no speed to measure, so that part of the principle surely becomes invalid. OR does stationary just = 0v?

#### Soul Surfer

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##### Re: Red shift
« Reply #8 on: 26/11/2005 18:16:38 »
This gravitational red shift down to zero frequency is precisely what the event horizon on a black hole means ie you can't see out.  In practice zero frequency is just a nominal point.  All light and radio waves travel at the speed of light however low their frequency. The lowest frequecies used for  eletromagnetic signalling are a few cycles ber second altough the sun generates lower frequencies that are experienced as iduced power surges during electromagnetric storms.

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#### Dr. Praetoria

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##### Re: Red shift
« Reply #9 on: 26/11/2005 21:54:37 »
".... and thus no visible violation of any of the laws of physics would occur."

But, wouldn't all the laws of physics be violated within a blackhole which was one of Einstein's abhorrenced to such a concept?
Doc

#### Soul Surfer

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##### Re: Red shift
« Reply #10 on: 26/11/2005 22:44:44 »
Most texts and descriptions skip round what happens inside a black hole except to say that they can't see any good reason why everything shouldn't collapse into a singularity that's where I feel that they've missed something by forgetting the angular momentum (see my comments today elsewhere in this forum)

My feeling is that nature abhors a singularity.

Just outside the event horizon the laws of physics apply and are well understood, although precisely all the complex processes have not been analysed or observed yet.  Just inside the event horizon things are much the same as just outside.  In fact if it was a really big hole like those expected to be at the centre of some large elliptical galaxies the gravity gradient would not be excessive and it would in theory be possible for living things cross the event horizon without really noticing it.

Now we get to the bit about where the question about relativistic orbits is leading.  the material crossing the event horizon will have angular momentum and will be in orbit around the centre of mass and will have to lose angular momentu to fall further towards the black hole.  so the big question is does angular momentum prevent the formation of a point singularity?  From my reading on rotating black holes the xperts suggest a disk singularity might form but I would like to throw another rock in the pond.

I have already said that there is no reason why the laws of phyisics shouldnt be much the same inside a black hole as outside so let is consider this collapsing material inside this event horizon capsule containing normal space.  as it collapses the gravity gradient increases and as Stephen Hawking showed a grvity gradient causes the emission of energy in the form of hawking radiation. as the collapse progresses this gradient gets steeper and the radiation gets more and more energetic.  OK this radiatio zooms out to eventually fall back ont the hole but that doesnt matter it still causes a loss of energy from the rest of the collapsing material eventually the radiation into the event horizon cavity should balance the emission from the gradient surface and a stable state is reached.

Without angular momentum this will happen quite quickly but angular momentum will delay this considerably.

This is where my question on relativistic orbits is leading because the energy loss processes and speed of reaching equilibrium depend on the answer.

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#### another_someone

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##### Re: Red shift
« Reply #11 on: 27/11/2005 06:17:59 »
quote:
Originally posted by Soul Surfer

I have already said that there is no reason why the laws of phyisics shouldnt be much the same inside a black hole as outside so let is consider this collapsing material inside this event horizon capsule containing normal space.  as it collapses the gravity gradient increases and as Stephen Hawking showed a grvity gradient causes the emission of energy in the form of hawking radiation. as the collapse progresses this gradient gets steeper and the radiation gets more and more energetic.  OK this radiatio zooms out to eventually fall back ont the hole but that doesnt matter it still causes a loss of energy from the rest of the collapsing material eventually the radiation into the event horizon cavity should balance the emission from the gradient surface and a stable state is reached.

I am sure that I am getting way out of my depth here, and would be totally lost chasing around the mathematics of this matter; but I will ask a few questions purely from a superficial logical perspective.

Another interesting (if naïve) question is if we are having all this hot matter being created by Hawkins radiation within a black hole, how close are these conditions to those of the original Big Bang – would we be seeing such anomalous behaviour as cosmic inflation?

#### Soul Surfer

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##### Re: Red shift
« Reply #12 on: 27/11/2005 11:03:06 »
You've got part of it but you're thinking slightly the wrong way.

Firstly, forget about the singularity for the moment.  That is a term that people have used because they can't find an explanation why one doesn't exist.

Think about what happens right in the middle when a large star goes supernova and forms a black hole.  The moment a load of stuff goes inside an event horizon it doesn't stop obeying the normal laws of physics and although conditions are pretty extreme they are well within the regions of earth bound experimentation.  It is possible to look at what might happen using them.  What I am saying is that there are at least a couple of reasons why we don't have to give up hope that solutions that avoid a singularity may be found.

The first and simpliest is the hawking (and thermal) radiation from the gravity gradient at the "surface" as the material is collapsing towards a "singularity"  can reach a stable radiative state and secondly the need to conserve angular momentum.

The solutions that the text books give ate very specific and mathematical. They do not take normal thermodynamics into account.  They are a bit like saying that the random motion of all the molecules in the glass of water on my desk could suddenly decide to go up and the water jump out of my glass! this we know does not happen however a few molecules do jump out all the time as the water evaporates.

Remember also that right at the very centre of the collapsing object there is initally no strong gravitational field or gradient becasuse inside a spherical object of "nominally" uniform density only the material inside the sphere centered on the object contributes to the gravitational field  the material outside integrates out.  This can be easily proved by showing that gravity falls off linearly as you head down towards the centre of the earth ie objects fall slower down a deep mine.

So rigt in the middle there is no irrisistable force dragginng the material to it just a load of very hot and dense material radiating like mad and pushing the material away by normal pressure.

I could be convinced that the thermal radiation might not be enough to stabilise the object under gravitational collapse but when the Hawking radiation cuts in because of the gravity gradient at the surface that might just remove enough energy for long enough to halt the gravitational collapse.

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#### Soul Surfer

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##### Re: Red shift
« Reply #13 on: 27/11/2005 11:12:04 »
I agree that this could look something like conditions just before the big bang and I m reasonably confident that our universe is probably an example of the sort of thing that happens inside a black hole but there's a long way to go before we can say that.

Look at Lee Smolin's "life in the cosmos"  Lee is a respected cosmologist who has developed an idea that the laws of physics come from an evolutionary process that ensures the universe generates maximum possible number of black holes

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#### another_someone

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##### Re: Red shift
« Reply #14 on: 27/11/2005 16:23:32 »
As I said, my expertise in this is non-existent, so it is very possible that I am making many false assumptions.

I asked whether Hawkins radiation was non-linear in the rate it was generated in response to a gravitational gradient because this would have made the mere existence of a singularity unstable.  You had postulated that singularities did not exist.  Merely to say that the commonly held mechanism by which singularities are thought to be created would not create a singularity does not inherently prove that such singularities cannot exist, it only demonstrates that we do not yet know how they might be formed.  That is merely negative evidence against their formation, not positive proof they cannot exist.

The specific issue you mention regarding a super nova does not take into account the inertia of the implosion.  Just as the exploding outer shell will have inertia that it carries away from the star, I would have expected the remnant imploding core will also have inertia that will force it beyond the point of equilibrium.  Exactly how significant this might be I do not know, but at very least I would expect it might set up oscillations within the core.  Normally, one would think these oscillations would die down as energy is carried away from the system, but if all of this is happening within a black hole, then I cannot see how energy could be carried out of the oscillating core of the black hole.

The other factor that you point out is that the steepest gravitational gradient, and thus the most intense Hawkins radiation, is at the the surface of the core of the imploding star.  Might this not then provide external pressure that might drive the core (albeit, an ever smaller core) to compress ever further?

#### another_someone

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##### Re: Red shift
« Reply #15 on: 27/11/2005 17:25:38 »
quote:
Originally posted by Soul Surfer

I agree that this could look something like conditions just before the big bang and I m reasonably confident that our universe is probably an example of the sort of thing that happens inside a black hole but there's a long way to go before we can say that.

I was not thinking so much about the situation before the Big Bang, as the situation during and after.

Firstly, as I understand it, the Big Bang itself is supposedly initiated from a singularity.  If your notion that a singularity is impossible is correct, then this must itself impose changes to the notion of the Big Bang.  This is why it is important to distinguish between merely demonstrating that super nova will not collapse into a singularity and demonstrating that singularities cannot exist in any context.

Secondly, if one follows your argument that anything approaching a singularity (whether it be a true singularity, or merely a close approximation) would generate vast amounts of Hawkins radiation, it would imply that the situation immediately after the Big Bang must have created vast amounts of Hawkins radiation.  Furthermore, since Hawkins radiation can escape a black hole, something that light itself cannot do, it would imply that the propagation of Hawkins radiation is not constrained by the speed of light.  Could this possibly explain the apparent inflationary period in the immediate post Big Bang era?

#### Soul Surfer

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##### Re: Red shift
« Reply #16 on: 27/11/2005 23:28:32 »
There's quite a lot to come back to here so lets take things in order.

I The reason I don't liks singularities is that they are a mathematical construct like a point or a straight line and don't have any sensible physical reality and I am a physicist who in the past has proved physics is more reliable than mathematics when it comes to predicting complex things.
OK quantum mechanics is truly weird but it does result in physical structures.

Because I did not mention specifically what you call the inertia of the progressing collapse does not mean it was not taken into account.

When I was talking about "hawking" radiation from the surface of the collapsing material inside a black hole I do not mean that this escapes from the black hole, it cannot cross the event horizon.  all the photons and particles radiated from the surface of the collapsing matter within the hole will eventually fall back onto the surface but they will take time while they are in their orbits thus removing energy from the surface for a while.

Maybe I should have used another term.

Let me describe the process in a different way. consider a load of material iside a black hole that has just formed collapsing to form a "singularity"

One way of describing an event horizon is when the photons go into orbit arount the concentrayion of mass

The laws of physics are contiuous at the evrent horizon and as it collapses the gravity gradient becomes even greater and the orbits of the photons get smaller so there is in effect a second event horizon inside the first one collapsing with the material it is this contracting event horizon that atarts to emit "hawking" radiation into the cavity that is the hole.

Maybe I'm wrong but I feel that most theroeticians seem to have given up using the ordinarly laws of physics to think about what might be going on inside black holes and could be missing out on sime important insights.

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#### Jenesis

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##### Re: Red shift
« Reply #17 on: 28/11/2005 09:51:38 »
i think my knowledge is a bit basic, but a particla can only have zero energy at absolute zero (-273K or thereabouts). If a black hole reduced the energy of photons to nothing, that would mean the event horizon has a temperature of absolute zero. Isn't this impossible? Also, frequency is related to this, as if a light wave has zero frequency then, like solvay says, it is still, and in effect ceases to exist - a phenomenon similar to that of particles at absolute zero.

Astronauts used to ride rockets with a million litres of hydrogen burning under their seats. D'you think they said "Gee, I'd love to go to the Moon today, but it seems a little risky"?

#### Soul Surfer

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##### Re: Red shift
« Reply #18 on: 28/11/2005 23:11:54 »
Jenesis you are thinking ov an event horizon as a material object. It isn't it's just a location on the quantum mechanical vacuum.  It doesn't have a temperature except in the sense that it originates hawking radiation.

The simplest way of visualising this is as follows.  The quantum mechanical vacuum is full of objects appearing and dissapearing under the laws of uncertainty which says you can have small energy discrepancies for a longish time and the bigger the discrepancy are the shorter time you can have them.  You can think of these energy discrepancies as the creation and destruction of particles and antiparticles, however if this creation provcess happens and one of them pops over the horizon before it has time to be destroyed one of the particles escapes.  For normal star sized black holes the only particles for which this can happen are very low energy photons of electromagnetic radiation (a photon happens to be its own antiparticle) an antiphoton is just the same photon with a 180degree phase shif so when the mee they null each other out. so we see an object radiating photons related to a particular temerature.  It is thought that very small black holes (a few million tons say)where the gradient is very intense can originate other sorts of particles.

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#### The Naked Scientists Forum

##### Re: Red shift
« Reply #18 on: 28/11/2005 23:11:54 »