Ask any question you want about Physics

originally posted by: Thill
why is there a need for a graviton ?

I think this is not the right question. The right question is, "how does gravity really work?" and if it doesn't use gravitons there's no need for them. There are some difficulties with trying to integrate gravitons into the standard model, and so the standard model doesn't include gravitons. "Extended versions" of the standard model do but they aren't really "standard".

I suppose it would be strange if everything else was quantized but not gravity, however Nature doesn't have to meet our expectations. Since everything else does seem to be quantized, doesn't it make sense to consider that gravity may also be quantized?

This is a difficult question and if you want to read a longer and more involved answer you can see it here, but the tl;dr answer is, to use Phage's expression "I dunno" how to solve the graviton puzzle, and apparently nobody knows, at least nobody who has been able to prove their idea so far. To complicate matters the hypothesized graviton is nearly impossible to detect in the experiments we've thought of so far to detect them, so gravitons truly present a case that absence of evidence is not evidence of absence (but certainly not evidence of presence either).

By the way one reason we would like to have a theory of quantum gravity is to solve the puzzle discussed in the posts before yours, where the equations of relativity present an "undefined" condition at the center of a black hole, so it fails to make a reliable prediction about what happens to the density of matter there. We hope a theory of quantum gravity could make some predictions about this, so if the question is "why do we need a theory of quantum gravity?", that's one reason.

originally posted by: Arbitrageur
Yes of course, the radius of matter in the black hole is zero, so the volume is zero, which is why it's called a "singularity". When you calculate density it's mass per unit volume, so mass divided by zero volume gives you the density of the mass in the black hole singularity.

What factor of the theory dictates that 'the radius of matter in the black hole is zero'? According to theory, what makes that statement necessary to be made, and thought to be true?

I ask that first, because that seems like the first assumption, which then produces the following ones (volume, density, dividing by zero).

If you politely answer my questions, we can get to the bottom of this and locate where the error/s is.

a reply to: Arbitrageur

Great reply and would just like to further support the reply by saying that, it is basically a placeholder.

The standard model of particle physics does not contain the graviton, but, because a new particle interacting in a similar way as the other force carriers, it would be incorrect to basically include nothing since, best part of scientific history we have been thinking about gravity.

It is just the place holder saying "Well, if it is a field much like the higgs field or other quantum field theory, then typically the consequence of it is to have a particle that is the physical excitation state of that field. Thats it... nothing more...

It is in a way to say "hey look, we still need to put gravity in here..."

Gravity is horrendous to study beyond simple free fall experiments. The current experiments looking for gravitational waves are quire amazing and technologically some of the most precise setups in the world. It is because of this that when I see people trying to explain / make statements as to what gravity is, or is not, because of some very crude experiment they performed, it makes me err on the side of caution.

originally posted by: ImaFungi
What factor of the theory dictates that 'the radius of matter in the black hole is zero'? According to theory, what makes that statement necessary to be made, and thought to be true?

Roger Penrose published a paper about this in 1965. His point of view was that what you stated is a prediction of general relativity, and he listed these four ideas on on how the problem of that prediction might be resolved, which to me shows he didn't find the prediction plausible, though he used the qualifier "If, as seems justifiable,....":

Gravitational Collapse and Space-Time Singularities

The implication of (d) is of course what I just mentioned at the top of page 231, that perhaps a theory of quantum gravity would make a more accurate prediction, with no singularity.

He also explains why even if the mass is not spherically symmetrical before the collapse, a singularity will still result from Einstein's equations, so you can see his math in that paper.

originally posted by: ErosA433
a reply to: Arbitrageur

Great reply and would just like to further support the reply by saying that, it is basically a placeholder.

Thank you, and yes a placeholder is a good description!

a reply to: Arbitrageur

Thanks;

Can you say, what precisely about these particular Einstein equations, results in the singularity?

There is space time, there is mass;

Where and why does the positing come in, that with a large enough mass, 'space time' results in matters radius being 0?

I presume it may be built off of the realization of the potential configuration of mass and space time which results in lights inability to escape a particular configuration of mass and space time;

But even if just that is so, I at this point do not see how such a concept would necessarily be related to the nature of non light matter, and where the insistence that non light matter would have 0 radius is compelled to be made.

Are you sure you mean to say the theory states the radius of matter is 0? Or do you mean to say the believed center of a black hole has a radius of 0?

originally posted by: ImaFungi
Can you say, what precisely about these particular Einstein equations, results in the singularity?

I can't explain the math any better than Roger Penrose and probably not as well. It's a consequence of the math of general relativity. Did you read his paper?

Are you sure you mean to say the theory states the radius of matter is 0? Or do you mean to say the believed center of a black hole has a radius of 0?

Einstein's original theory of general relativity doesn't mention black holes, but if you apply his equations as Roger Penrose did that's the result that comes from doing the math and yes it's the mass. Penrose and probably most other physicists are skeptical about this zero radius prediction and as Penrose said, maybe "Einstein's equations are violated" so the math of general relativity no longer applies at such high space-time curvatures, meaning maybe the radius isn't really zero and there isn't really a singularity. When I read between the lines in his paper, I get the impression he thinks the singularity probably doesn't really happen so the radius isn't really zero, for the reasons I stated in my previous post.

I always kind of imagine something else happens for blackholes, so really we have two very compact objects defined by quantum mechanics.

White dwarfs in which the collapse is halted by electron degeneracy

Neutron stars in which the collapse is halted by neutron degeneracy

I cant help that think that in black holes something else happens, quark/gluon pressure holds things from collapsing? Not sure really

originally posted by: ErosA433

White dwarfs in which the collapse is halted by electron degeneracy

What is meant by electron degeneracy?

Electrons leave their host atoms;

Or, electrons cease to be electrons?

a reply to: ImaFungi

It goes a bit beyond simple definition of the words degenerate and electron

en.wikipedia.org...

a reply to: Arbitrageur

You cannot generally relatively say what about Einstein concept of space time leads to mass radius 0?

Any chance the reason for such conception, has to do with the volume in question being a 'relative sphere' (likely closer to sphere, than pyramid or cube); my point being; all 3d shapes, or 3d volumes, have a relative exact center; and so potentially distinguished with the 1d pointillistic math theory and graphing, was there a potential confusion about the potential importance of the exact 1/0 dimensional conceptual exact center of a sphere, and the thought that continual quantities of mass and energy must potentially be able to end up there?

a reply to: ErosA433

Can you tell me a simple explanation of the theory as to how a massive object collapses? Is that, by atoms radiating away their motive energy and then without that excess motion, electrons begin to go to lower energy levels, and then electrons of neighbors start leaking to others, but the theory is that there is all the common 'pull' toward the center; is the idea that electrons leak to the center of the collective mass? Yeah, I could check out the wiki, but if you could just easily say some related statements from your back pocket that could be swell.

a reply to: ErosA433
That train of thought/logic makes sense to me, but I've also read the idea that in a black hole the fermions which are subjected to degeneracy pressure might be converted into bosons which are not, so nobody really knows, but yes that's likely true about white dwarfs and neutron stars.

originally posted by: ImaFungi
What is meant by electron degeneracy?

It's primarily electron degeneracy pressure that keeps your feet above the ground when you're standing up, in a non-intuitive quantum mechanical effect.

originally posted by: ImaFungi
You cannot generally relatively say what about Einstein concept of space time leads to mass radius 0?

The space-time curvature gets so curved that mass no longer takes up any space according to the equations though Einstein himself didn't predict this and I don't think he believed in black holes. This comes from others solving his equations for black hole conditions.

Any chance the reason for such conception, has to do with the volume in question being a 'relative sphere' (likely closer to sphere, than pyramid or cube); my point being; all 3d shapes, or 3d volumes, have a relative exact center; and so potentially distinguished with the 1d pointillistic math theory and graphing, was there a potential confusion about the potential importance of the exact 1/0 dimensional conceptual exact center of a sphere, and the thought that continual quantities of mass and energy must potentially be able to end up there?

There are some times when you just need the math and this is one of them, not necessarily to understand what really happens since Penrose doesn't believe the prediction inside a black hole, but to understand how the math makes this prediction.

In a star, the system is a dynamic equilibrium between outward pressure and inward gravity. The outward pressure is supplied by Fusion and the subsequent scattering of high energy gammas with electrons that rattle around with lots of kinetic energy.

When fusion ceases however, the energy source keeping the material hot, ceases, thus the material cools and gravitational forces take over, causing the core of the star to compress.

Now, depending upon the star, this compression can re-ignite the core achieving fusion but with heavier elements than hydrogen. But what if the inward pressure and heating does not re-ignite the core? Does it just collapose and become a singularity?

Well it turns out not to be the case. When the plasma of a stellar core is compressed you eventually reach a state in which you are compressing material and it is changing the electron configuration of the material. You begin due to physical space limitations (probability distributions really) develop a high level of interaction between electron shells of the material at the quantum level... that means that you fill up all the available energy levels up to the fermi-energy.

Essentially the pressure that is generated is pressure due to all states being filled up to the fermi-energy and still you are trying to compress it and add more electrons, it generates a quantum mechanical pressure that holds the core from any more appreciable collapse.... that is unless you have so much more matter and with gravity you are able to compress the degenerate material more until the inner electrons begin to intersect with the nucleus (again within the probability distribution that just means that the electrons pass within the nucleus at a higher rate)

Neutron degeneracy is a similar effect but occurs in the cores of neutron stars

In a high-energy photon collision a positron electron pair is formed.
if it is possible to create mass from something massless by adding energu, by removing energy is it possible to reduce mass?

originally posted by: Arbitrageur


The space-time curvature gets so curved that mass no longer takes up any space according to the equations though Einstein himself didn't predict this and I don't think he believed in black holes. This comes from others solving his equations for black hole conditions.

Well I will back up Einstein here, and say that this; 'The space-time curvature gets so curved that mass no longer takes up any space' cannot be a meaningful statement in any real universe.

What is said there, does not add up.

Space-time. Mass. Lets say infinite curvature. There is no reason to conclude that mass, would cease to exist amidst curvature approaching 'infinity' (even if, curvature approaching infinity, was not a meaningless concept)(insert, nature doesnt have to conform to your understanding of meaning, statement; here for an automatic thoughtless win).

So, those others, are wrong.

But anyway, lets move past that and let me ask this;

Considering some of the thoughts regarding black holes are correct, but lets assume it is false that mass has radius of 0 (false in reality, and false according to a better interpretation of Einstein);

Then what else would be an issue? Where else is there a hang up?

Someone should solve the equations, with the distinction that; mass which enters a black hole cannot = 0 radius; and see what that pull and tug does to the rest of the equations (which interpreted one way, compelled some to say that 0 radius is possible), to see where down the line in the equations the other direction (direction like, starting from the axioms and assumptions and equations result in this 0 radius, start from the definite no 0 radius, and work backward, and see how that effects the axioms and assumptions).

I know the generally principles as to why there are troubles, I just want to hear some steady step by step so I can point them out.

Cool thanks I think I got some of that.

originally posted by: ErosA433

When fusion ceases however, the energy source keeping the material hot, ceases, thus the material cools and gravitational forces take over, causing the core of the star to compress.

Why would the material cooling result in greater effectiveness of gravity?

But what if the inward pressure and heating does not re-ignite the core? Does it just collapose and become a singularity?

Funny and/or ironic, we with arb are talking about 'singularity' potentially being an unfounded concept.

a reply to: ImaFungi

Why does a balloon filled with air expand when heated? or more over, why does the same balloon collapse when cooled?


I dont really see the irony there... it was a question posed with an answer as to why it doesn't occur in two very extreme examples.

originally posted by: ImaFungi
Why would the material cooling result in greater effectiveness of gravity?

It's not necessarily the cooling, but the cessation of the nuclear fusion.

That nuclear fusion process was providing an outward pressure that was counteracting the natural tendency for the force of gravity to collapse the star material inwards. Once the fusion stops (along with the outward pressure created by that fusion process), then the force of gravity takes over, collapsing the star.

originally posted by: dashen
In a high-energy photon collision a positron electron pair is formed.
if it is possible to create mass from something massless by adding energu, by removing energy is it possible to reduce mass?

Yes indeed and that's the whole idea behind Stephen Hawking's concept of Hawking radiation, that the mass of a black hole can be reduced because some radiation is emitted for the black hole. Of course to have a net loss in mass, the temperature of the black hole needs to be higher than the temperature of the Cosmic Microwave background (CMB), which would only happen with a relatively small black hole, with less mass than the moon I think. Larger black holes still gain mass overall because of absorbing CMB radiation for the time being. Eventually this will change as the CMB cools further.

originally posted by: ImaFungi
There is no reason to conclude that mass, would cease to exist amidst curvature approaching 'infinity'

Nobody ever said that the mass would cease to exist, just that Einstein's equations say it would stop taking up space which as I said is not widely believed to be what really happens.

But anyway, lets move past that and let me ask this;

Considering some of the thoughts regarding black holes are correct, but lets assume it is false that mass has radius of 0 (false in reality, and false according to a better interpretation of Einstein);

Then what else would be an issue? Where else is there a hang up?

For me personally a conceptual problem is that relativity implies time would continue to pass normally to an observer falling into a black hole even after they pass the event horizon (let's assume supermassive black hole at this point so no need to discuss spaghettification near the event horizon). So how do you correlate this with the passage of time outside the event horizon? You can't. I have no problem conceiving how an observer on earth sees time slowing down near the event horizon outside it, but what about inside the event horizon? Is it true that there's no correlation between time inside the event horizon and the time of an external observer?

Someone should solve the equations,

Be my guest, but I'll bet you didn't even read Roger Penrose's paper on how he solved the equations and you have nothing constructive to add. No, we don't need someone else solving the equations differently in my opinion, what we need are other equations for this circumstance. To me your comment is like suggesting that someone needed a better solution of Newton's math to predict the precession of Mercury and that was never going to happen, because we needed more equations, not a different interpretation of existing equations.

a reply to: Arbitrageur

By that token would supercooling something to absolute zero or somewhere near that reduce its mass also?