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Einstein's description of gravity just got much harder to beat

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Re: Einstein's description of gravity just got much harder to beat

#11

Post by brianpan »

mantap wrote: post #6

Can some knowledgable person articulate why physicists are so sure that QM and GR are incompatible?

Quantum mechanics deals with very small particles interacting with very strong forces. Gravity is so weak it can be ignored.

Relativity deals with so much gravity that spacetime is warped.

Neither is appropriate for the other and they are on opposite sides of the spectrum. Classical physics is useful in the middle at "human" scale.

What would be nice is a simple theory that covers it all. Nothing we currently have is able to stretch to be useful in all situations.

Sean Carroll's _The Big Picture_ was useful for me, as was the older _A Brief History Of Time_ by Stephen Hawking

Re: Einstein's description of gravity just got much harder to beat

#12

Post by ry454 »

jb775 wrote: post #5

During brainstorming sessions, I often think about why/how medieval artists couldn't quite grasp how to depict depth in their paintings (as a lateral thinking technique). Being humans living in the world, they obviously experienced and understood that depth existed all around them, but had a hard time grasping the concept as a whole. I feel like we're kind of in a "medieval depth" phase of gravity/spacetime understan…

Here's a possible explanation that I'd seen somewhere. Up until very recently, humans had had weak ability to think, especially abstract thoughts, and the phrase common today "just think about it", would be meaningless just a few centuries ago. Perhaps the concept of depth in those times would be similar in complexity to 4-dimensional structures today.

Re: Einstein's description of gravity just got much harder to beat

#13

Post by cygx »

mantap wrote: post #6

Can some knowledgable person articulate why physicists are so sure that QM and GR are incompatible?

Energy gravitates. QM tells us systems can be in superposition of energy eigenstates, and we'd like to know what happens to the gravitational field in such cases. For that, we need a quantum theory of gravity.

We have some recipes to go from a classical to a quantum field theory, but they fail for general relativity. For example, GR is not perturbatively renormalizable: For electromagnetism, we only need to determine a finite number of parameters experimentally to (presumably) fix the theory at all energy scales. That doesn't work for general relativity for technical reasons, and we'd have to measure an infinite number of parameters.

Then, there are a couple of conceptual problems, such as GR doing away with a fixed background spacetime, or the so-called problem of time. Different approaches to quantum gravity approach this in different ways (as far as I'm aware, the background normally gets fixed asymptotically in string theory - though don't quote me on that one - whereas loop quantum gravity tackles the issue head on).

Re: Einstein's description of gravity just got much harder to beat

#14

Post by CJefferson »

mantap wrote: post #6

Can some knowledgable person articulate why physicists are so sure that QM and GR are incompatible?

By "QM" and "GM", physicists mean our current mathematical models of these theories.

They just don't line up -- QM doesn't have anytging in it which would create relativistic gravity, and GM doesn't contain any weird quantum effects.

Previous attempts to just "glue them together" have always failed, as the maths ends up falling apart and you get stupid results, which clearly don't align with the real world.

Re: Einstein's description of gravity just got much harder to beat

#15

Post by roomey »

ry454 wrote: post #12
jb775 wrote: post #5

During brainstorming sessions, I often think about why/how medieval artists couldn't quite grasp how to depict depth in their paintings (as a lateral thinking technique). Being humans living in the world, they obviously experienced and understood that depth existed all around them, but had a hard time grasping the concept as a whole. I feel like we're kind of in a "medieval depth" phase of gravity/spacetime understan…

Here's a possible explanation that I'd seen somewhere. Up until very recently, humans had had weak ability to think, especially abstract thoughts, and the phrase common today "just think about it", would be meaningless just a few centuries ago. Perhaps the concept of depth in those times would be similar in complexity to 4-dimensional structures today.

Plato? Socrates? Pythagoras?

I was an artist for a while. It wasn't that artists couldn't depict depth. Fashion, religious instructon, and to a lesser extent materials, is what decided which paintings and frescoes lasted the test of time.

Re: Einstein's description of gravity just got much harder to beat

#16

Post by ry454 »

CJefferson wrote: post #14
mantap wrote: post #6

Can some knowledgable person articulate why physicists are so sure that QM and GR are incompatible?

By "QM" and "GM", physicists mean our current mathematical models of these theories. They just don't line up -- QM doesn't have anytging in it which would create relativistic gravity, and GM doesn't contain any weird quantum effects. Previous attempts to just "glue them together" have always failed, as the maths ends up falling apart and you get stupid results, which clearly don't align with the real world.

I've seen somewhere a clever trick that derives some QM with a small tweak in GR. I can't judge whether the trick makes sense, though. So the trick was to modify the ds2=c2dt2-dx2-dy2-dz2 equation to exp(a cos(wt))ds2=... That extra exp(...) modulation, when combined with Lorentz transforms, produced some basic QM formula. With more wizardry, the author derived the time-dependent Schrodinger equation. Someone who knows GR well could probably tell if GR has "room" for the exp(...) modulation, i.e. that something in GR doesn't fall apart after this addition.

Re: Einstein's description of gravity just got much harder to beat

#17

Post by niho »

jb775 wrote: post #5

During brainstorming sessions, I often think about why/how medieval artists couldn't quite grasp how to depict depth in their paintings (as a lateral thinking technique). Being humans living in the world, they obviously experienced and understood that depth existed all around them, but had a hard time grasping the concept as a whole. I feel like we're kind of in a "medieval depth" phase of gravity/spacetime understan…

> I often think about why/how medieval artists couldn't quite grasp how to depict depth in their paintings (as a lateral thinking technique).

The explanation for that is generally agreed on to have more to do with the aesthetics of early Christianity influencing Medieval practices, more than changes to human perception (source: I have a masters degree in Art History).

Re: Einstein's description of gravity just got much harder to beat

#18

Post by bsaul »

niho wrote: post #17
jb775 wrote: post #5

During brainstorming sessions, I often think about why/how medieval artists couldn't quite grasp how to depict depth in their paintings (as a lateral thinking technique). Being humans living in the world, they obviously experienced and understood that depth existed all around them, but had a hard time grasping the concept as a whole. I feel like we're kind of in a "medieval depth" phase of gravity/spacetime understan…

> I often think about why/how medieval artists couldn't quite grasp how to depict depth in their paintings (as a lateral thinking technique). The explanation for that is generally agreed on to have more to do with the aesthetics of early Christianity influencing Medieval practices, more than changes to human perception (source: I have a masters degree in Art History).

could you give more on that ? i can imagine how religious dogmas can influence a lot of things, but representation of depth ??

Re: Einstein's description of gravity just got much harder to beat

#20

Post by irrational »

brianpan wrote: post #11
mantap wrote: post #6

Can some knowledgable person articulate why physicists are so sure that QM and GR are incompatible?

Quantum mechanics deals with very small particles interacting with very strong forces. Gravity is so weak it can be ignored. Relativity deals with so much gravity that spacetime is warped. Neither is appropriate for the other and they are on opposite sides of the spectrum. Classical physics is useful in the middle at "human" scale. What would be nice is a simple theory that covers it all. Nothing we currently have is…

Is it possible that there is no overarching theory that unifies both of them? Could they both be true yet not connected in any way?
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