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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

#21

Post by irrational »

RcouF1uZ4gsC wrote: post #4

> Despite its successes, Einstein's robust theory remains mathematically irreconcilable with quantum mechanics, the scientific understanding of the subatomic world. Testing general relativity is important because the ultimate theory of the universe must encompass both gravity and quantum mechanics. Maybe the universe just has a giant if statement.

Why must the ultimate theory of the universe encompass both? Do we know that there is an ultimate theory of the universe? Could GR and QM be wholly separate with no unifying connection?

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

#22

Post by hvna »

irrational wrote: post #21
RcouF1uZ4gsC wrote: post #4

> Despite its successes, Einstein's robust theory remains mathematically irreconcilable with quantum mechanics, the scientific understanding of the subatomic world. Testing general relativity is important because the ultimate theory of the universe must encompass both gravity and quantum mechanics. Maybe the universe just has a giant if statement.

Why must the ultimate theory of the universe encompass both? Do we know that there is an ultimate theory of the universe? Could GR and QM be wholly separate with no unifying connection?

The question of if there is an ultimate theory of the universe is one of the major unsolved problems in physics.

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

#23

Post by alpha_squared »

bsaul wrote: post #18
niho wrote: post #17

Earlier quoted context omitted.

> 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 ??

Not OP, though mild experience with art history and Christian history, but I suspect OP is talking about the intersection of art and the prevailing religion in Europe at the time (Christianity). Early Christian art was intentionally unrealistic in style specifically because the intent was often symbolism and representation rather than realism (this still holds true today for most Orthodox Christian art[0]). In fact, making it realistic would even border on idolatry (see Iconoclastic Controversy[1]).

That said, up until the European middle ages, nearly all art was that which was commissioned by the church. The style of the time permeated the art that wasn't commissioned by the church. As more non-religious art was commissioned, there was little need to remain working in that style and realism was embraced. I could be wrong, and probably am, but I think Catholicism is the only large Christian denomination that adopted realistic art.

I think depth was already understood by this point, though (see sculptures[2]). Again, I'm not OP, and I'm sure OP will have a more thorough explanation.

[0] https://orthodoxartsjournal.org/topics/iconography/

[1] https://www.britannica.com/event/Iconoclastic-Controversy

[2] https://en.wikipedia.org/wiki/Victorious_Youth

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

#24

Post by drdeca »

lazyjones wrote: post #10
galaxyLogic wrote: post #7

I don't quite get it they say relativity and Quantum Mechanics are incompatible. Does that mean they give different predictions?

GR assumes reality is continuous, QM assumes it’s discrete.

QM doesn’t assume space is discrete. It does deal with a number of discrete quantities though.

Are you sure that’s where the conflict is?

I think people have done QM with a given curved spacetime, just, where the spacetime manifold isn’t decided based on the distribution of matter and whatnot?

Like, I think that’s where the ideas of Hawking radiation come from?

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

#26

Post by mrmonkeyman »

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…

Wow, your view on ancient folk is quite warped. And wrong. Someone, enlighten this arrogant youngling.

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

#27

Post by checkyoursudo »

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…

As a layperson with only basic undergraduate physics (not necessary for the book; just, I'm no expert), I'll second Carroll's The Big Picture.

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

#28

Post by imtringued »

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.

That's completely wrong. Humans were just as smart 3000 years ago as they were today. The difference between today and back then is that people were much poorer and didnt receive an education. Their jobs were farming which didnt require things we consider essential like reading or writing which meant they stay unchallenged for their entire life. When you look at the rich elite or rich cities in the past then you see how small the difference between us and the people 3000 years ago is. They were just as smart but there were fewer of them. Nowadays everyone is educated and "rich".

You can still find people who are just as "unchallenged" as a medieval peasant today. Look in rural places of countries like china.

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

#29

Post by brabel »

computerfriend wrote: post #9
mantap wrote: post #6

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

Rather it is the belief that they fundamentally must be compatible that is driving this research. Currently we don't know how to answer questions like "what is the gravitational field around a photon?".

Photons have zero mass so we know them to produce zero gravity, no?

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

#30

Post by pja »

mantap wrote: post #6

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

(What follows is a layman's understanding; I have a physics degree but never studied particle physics to the kind of depth required to give an authoritative answer to this question.)

Because any attempt to quantise gravity via the methods that worked for other forces leads to theories which aren’t any use to us. So far all such attempts have led to theories which are not renormalisable. Renormalisation is not an absolute requirement for a useful quantum theory; you can do perfectly good physics in a non renormalisable theory with a high energy cut off. But we only expect quantum gravity effects to be noticeable in high energy / curvature regimes so that's no good to us. We already have a perfectly good low energy theory of gravity that explains all observable phenomena after all: It’s called General Relativity. A non-renormalisable theory that results in infinities that cannot be eliminated in the energy regimes that you hope to probe is not a useful theory.

The other source of difficulty is that existing quantum theories assume a flat spacetime background. Reworking them to account for curved spacetime makes the mathematics much harder, even if you only assume a fixed but curved background.

So we need new ideas, because the existing ideas give infinite answers in the regime where existing theories fail, but even testing such new ideas mathematically is currently challenging & very few people have the mathematical background required.

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