[Kauffmann (OP)]

All of us who are even slightly educated in modern physics know that General Relativity and Quantum Physics both are our best tools to describe physical reality, but they're known to contradict each other.

My question to my physicist pals is this: Exactly why and where do they disagree?

[PmbPhy]

All of us who are even slightly educated in modern physics know that General Relativity and Quantum Physics both are our best tools to describe physical reality, but they're known to contradict each other.

My question to my physicist pals is this: Exactly why and where do they disagree?

Dear Kauffmann,

I haven't said this before so I'll say it now. Welcome to the forum!

Thanks for posting this. I wasn't aware that physicists thought that there were incompatibilities. I did a search and found this: http://www.askamathematician.com/2009/12/q-howwhy-are-quantum-mechanics-and-relativity-incompatible/

Frankly I found each of the incompatibilities to be problematic in themselves. The so-called incompatibilities are

Assertion: Smooth vs. Chunky: General relativity needs space to be “smooth”, or at the very least continuous.  QM requires everything to be “quantized”, or show up in discrete pieces. 

Problem: This isn't really true. Without a theory of quantum gravity we can't really say whether such a theory would require space to be quantized. Another problem that I have with this assertion is that QM doesn't really require everything to be quantized. For example: The energy of a free particle is not required to be quantized. That only holds for bound particles. So that assertion is just plain wrong in my eyes.

2) The Information Paradox: According to general relativity when stuff falls into a black hole everything about it’s existence (with the exception of mass, charge, and momentum) is completely erased. However, if all the information about something is destroyed, then you lose time-reversibility. QM requires that time-reversibility (or “unitarity”, to a professional) holds. So QM requires that blackholes cannot destroy information.

Problem: The article states that quantum entanglement might be a way out of this. It also once again says what QM states rather than what a quantum theory of gravity would state. I think there is something along those lines which might help resolve it. However I'm far from being knowledgeable in this area, at least as of yet. Let's see what time holds.

[evan_au]

If you assume that the Graviton exists (a hypothetical spin=2 particle), this automatically produces a theory that looks like our Solar System, with Newton's theory of planetary motions, and Einsteins' theory of relativity.

There are a few problems with this:

• No-one has yet detected a graviton - or even huge bunches of them (eg from colliding black holes). However, most physicists believe that the graviton exists, even if we currently see no way to detect individual gravitons here on Earth.
  
• At extreme energies such as near a black hole, it is expected that quantum effects will appear that are not predicted by relativity alone. Since we don't currently have any black holes to study, we can only guess at what these effects might be. (Some physicists still hope that the LHC might produce some micro black holes to study.)
  
• Attempts to produce a theory that incorporates both relativity and gravitons at high energies often come up with infinities. It has proven possible to eliminate infinities in some other theories, but nobody has managed to do this in a convincing way for a theory of quantum gravity.
  
• Two of the popular hypotheses are "String Theory" and "Quantum Loop Gravity", but they are still very much a work in progress. 

For more details, see here.

[PmbPhy]

There are a few problems with this:

No-one has yet detected a graviton - or even huge bunches of them (eg from colliding black holes). However, most physicists believe that the graviton exists, even if we currently see no way to detect individual gravitons here on Earth.

I wouldn't say that this is a problem since we haven't even looked for them yet.

At extreme energies such as near a black hole, ...

I know of no such energy near black hole. What energy are you referring to?

..., it is expected that quantum effects will appear that are not predicted by relativity alone. Since we don't currently have any black holes to study, we can only guess at what these effects might be.

That's not a problem though. To be a problem means to be a contradiction or something of that nature.

Attempts to produce a theory that incorporates both relativity and gravitons at high energies often come up with infinities.

Ah! See? This is exactly what I mean by a problem. Thanks!

Two of the popular hypotheses are "String Theory" and "Quantum Loop Gravity", but they are still very much a work in progress.

I don't understand. Why do you list this as being a problem?

Thanks evan![/list]

[lightarrow]

Thanks for posting this. I wasn't aware that physicists thought that there were incompatibilities.

Don't take this as a critic because it's not.
You really didn't know it? This is astonishing to me, considered your high level knowledge of GR and QM.
That GR and QM don't "mix" well is well known even in high schools, probably!  []

--
lightarrow

[PmbPhy]

You really didn't know it?

Why do you ask? It's not exactly something one would joke about.

That GR and QM don't "mix" well is well known even in high schools, probably!  []

Hardly.

[jeffreyH]

Why is this astonishing? People specialize. No one can know everything. The jack of all trades, as the saying goes, is master of none. Those who state they have a grasp of lots of things probably don't. I prefer someone who admits to not knowing than one who pretends to know. That way I know that the person is an honest critic because they can admit their own limitations.

[PmbPhy]

Why is this astonishing? People specialize. No one can know everything. The jack of all trades, as the saying goes, is master of none. Those who state they have a grasp of lots of things probably don't. I prefer someone who admits to not knowing than one who pretends to know. That way I know that the person is an honest critic because they can admit their own limitations.

That is exactly the way I feel and that's the reason I'm not up on quantum gravity (QG) to any extent. QG, like string theory and quantum loop gravity, are fields that I've decided not to learn anything about until I'm able to learn them full bore. I've see how much physicists distort theories in books written for the public and how much layman have to unlearn in order to understand it correctly. My motivation is not to learn anything that I'll have to unlearn later.

After all I have a ton on my plate right now. I'm spending the next 12 months preparing to start tutoring. That means that during those months I'll be relearning (in the sense that I'll be brushing up on them to the graduate level) classical mechanics, quantum mechanics, electrodynamics, thermodynamics, SR, GR, nuclear physics, particle physics, astrophysics and cosmology. I have no time to spend on things I'm not interested in.

[JohnDuffield]

All of us who are even slightly educated in modern physics know that General Relativity and Quantum Physics both are our best tools to describe physical reality, but they're known to contradict each other. My question to my physicist pals is this: Exactly why and where do they disagree?

I'd say general relativity describes gravity in terms of geometry, whilst quantum physics describes light and electromagnetism and more in terms of particles and probabilities. IMHO there are severe interpretational problems with the latter. For example electromagnetic forces are said to be mediated by messenger particles, but hydrogen atoms don't twinkle, and magnets don't shine. In addition there's no geometry at all in QM, even though electromagnetic geometry is known about. If cornered I'd say GR is right and QM is wrong.   

[Kauffmann (OP)]

I haven't said this before so I'll say it now. Welcome to the forum!

heh, thanks a lot bro!

Well I myself just finished my major in mechanical engineering and read Einstein's Special and General Relativity, I was surprised to see how simple it is, the guy just made pretty damn good and pertinent observations. Sad thing, I always dreamed of superluminal travel and communications, now I see just what kind insurmountable wall we have before us... prob will have to escape spacetime alltogether to achieve such feats.

Now im not that versed in quantum mechanics, the only concept i truly grasp is the double slit experiment, which is mind shattering, though I think that the current interpretation of probability waves to be just silly. It's like when you're facing a problem that you can't solve with the current data and just give the most extravagant explanation just because it "seems" to solve it. I have no problem with such explanations, as long as they stay in their place, if you want to give them such a high profile, then you must provide mroe solid evidence.

Here's an forgotten intepretation i find more likely: newbielink:http://en.wikipedia.org/wiki/Pilot_wave [nonactive]

Also, somewhere i found another explanation for the DS experiment, it said that you just have to greatly reduce the speed of the electrons shot at the slits to eliminate the interference pattern, and that the interference pattern was caused by some "ghost" behavior on particles traveling at near light speeds. Sadly i dont remember where i found that thing.

[evan_au]

No-one has yet detected a graviton - or even huge bunches of them (eg from colliding black holes)

I wouldn't say that this is a problem since we haven't even looked for them yet.

The LIGO project spent over $US360M up to 2010 looking for gravitational waves from orbiting neutron stars and similar sources; the biggest-ever project funded by NSF sounds like scientists have been looking for huge bunches of gravitons (in the form of disturbances in the gravitational field). They haven't found any, which I am sure would drive an accountant wild.

At extreme energies such as near a black hole, ...

I know of no such energy near black hole. What energy are you referring to?

The tidal forces near a black hole can tear apart stars and planets; closer in they can grind rocks to atoms; even closer in the intense X-rays can rip all the electrons off the atoms; Magnetic forces within the accretion disk seem to accelerate matter to almost the speed of light away from the black hole along the polar axis; even closer to the event horizon, Stephen Hawking speculates that normally-undetectable particles can become detectable. Overall, it is thought that up to 30% of Einstein's E=MC² can be released as various forms of energy, producing far higher energies than can be produced by the LHC.
So, overall, the vicinity of an active black hole is an extremely energetic environment, producing relativistic, quantum and gravitational effects that we cannot reproduce on Earth at this time.
Despite the nice graphics in the movie "Interstellar", this is not a place you would expect to find a habitable planet!

..., it is expected that quantum effects will appear that are not predicted by relativity alone. Since we don't currently have any black holes to study, we can only guess at what these effects might be.

That's not a problem though. To be a problem means to be a contradiction or something of that nature.

However, there are other kinds of problems. One kind of problem is that a theory has not yet been tested, so to some extent it just speculation, and we cannot confidently answer questions about the agreement between Relativity & Gravity.

Attempts to produce a theory that incorporates both relativity and gravitons at high energies often come up with infinities.

Ah! See? This is exactly what I mean by a problem. Thanks!

Two of the popular hypotheses are "String Theory" and "Quantum Loop Gravity", but they are still very much a work in progress.

I don't understand. Why do you list this as being a problem?

I have heard comments that String Theory has a large number of undefined parameters, which allow up to 10⁵⁰⁰ possible String Theories. Until such time as we can narrow down these parameters to something which approximates the universe in which we find ourselves, we can't use it to make usable predictions. Predicting future findings (such as the behaviour of gravity close to a black hole), is the essence of a useful theory.
I understand that Quantum Loop Gravity is also an incomplete theory.

For now, we have to say that neither String Theory nor Quantum Loop Gravity has aligned Relativity and Gravity (a source of continued frustration for Sheldon on "The Big Bang Theory").

[PmbPhy]

The LIGO project spent over $US360M up to 2010 looking for gravitational waves from orbiting neutron stars and similar sources; the biggest-ever project funded by NSF sounds like scientists have been looking for huge bunches of gravitons (in the form of disturbances in the gravitational field). They haven't found any, which I am sure would drive an accountant wild.

My understanding of LIGO

http://adamgetchell.blogspot.com/2008/11/can-ligo-detect-graviton.html

is that it can't detect gravitons as it is currently constructed.

[dlorde]

... even closer to the event horizon, Stephen Hawking speculates that normally-undetectable particles can become detectable...

Did you really mean the event horizon, or the singularity? As I understand it, now that the firewall problem appears to be resolved, the EH is back to being ordinary spacetime. Have things moved on?

[JohnDuffield]

What's that about the firewall problem being resolved?

[jeffreyH]

IMHO The firewall issue hasn't gone away. The event horizon is the wrong place to look.

[dlorde]

What's that about the firewall problem being resolved?

I heard that  Sabine Hossenfelder had shown that Hawking radiation isn't entangled locally or over time across the horizon, so it isn't in a pure state - which was required for the firewall idea to hold.

I'm only going by what I've read, so I'm happy to be corrected.

[yor_on]

Never thought it would be correct either. And Bee is a ingenious lady indeed.

[evan_au]

... even closer to the event horizon, Stephen Hawking speculates that normally-undetectable particles can become detectable...

Did you really mean the event horizon, or the singularity? As I understand it, now that the firewall problem appears to be resolved, the EH is back to being ordinary spacetime. Have things moved on?

I was thinking of Hawking Radiation, which is thought to be produced just outside the event horizon, not at the singularity.

The behavior of matter and spacetime just outside the event horizon is a matter of conjecture, but the behavior inside the event horizon remains pure speculation at this time.

[dlorde]

I was thinking of Hawking Radiation, which is thought to be produced just outside the event horizon, not at the singularity.

OIC! Yes, fair enough. It just didn't occur to me that was what you meant...

The behavior of matter and spacetime just outside the event horizon is a matter of conjecture, but the behavior inside the event horizon remains pure speculation at this time.

Ah... from what I've read, the event horizon should have no special properties, and spacetime immediately inside it should be no different from that immediately outside it - except that the escape velocity exceeds c. Close to the singularity, things will go off-scale into speculative realms, but for a large BH, you might not even notice crossing the EH. Am I mistaken?

[JohnDuffield]

IMHO The firewall issue hasn't gone away. The event horizon is the wrong place to look.

Sounds good to me.

I heard that  Sabine Hossenfelder had shown that Hawking radiation isn't entangled locally or over time across the horizon, so it isn't in a pure state - which was required for the firewall idea to hold. I'm only going by what I've read, so I'm happy to be corrected.

I've had some exchanges with Sabine, and to be blunt I don't think she understands gravity at all. I think you should take what she says with a bucket of popscience-blogger salt. That's not to say that the AMPS firewall is right either. But take a look at an old version of the Wikipedia article and you can see mention of Friedwardt Winterberg's firewall. It's to do with gamma-ray bursters, and I'm pretty sure it's correct. You see where you said the escape velocity exceeds c? In Winterberg's scenario the speed of an infalling body would exceed the local speed of light, so it gets destroyed.