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A physicist who bets that gravity can’t be quantized

quantamagazine.org

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Re: A physicist who bets that gravity can’t be quantized

#171

Post by jfengel »

Gordonjcp wrote: post #101
inciampati wrote: post #61

> So if gravity is quantized, that means space-time is also quantized. But that doesn’t work, But... How could space-time not be quantized? That would imply the existence of infinities in the structure of the universe. It is like the ultraviolet catastrophe but in space-time.

But space is quantised, that's what Planck's Constant is. The oddity we see with things at relativistic energies is simply because - as with all discretised representations of continuous-time systems - things get a bit fucky close to Nyquist, and the maths breaks down.

That's not what Planck's Constant is. Planck's Constant connects energy to length (in space or time). It's just a way of shifting units around.

You're thinking of the Planck Length, which is the result of combining the Planck Constant with the speed of light. It also happens to be the wavelength of a photon so short that its energy makes it a black hole, connecting it to gravity.

It's often presented as if it were the fundamental unit of length, but that's a guess at best. There's no specific reason to believe it, other than the fact that it happens to be a way to connect length, charge, and gravity. That's evocative, but hardly proof. There's neither evidence for it nor a well-supported theory behind it.

Re: A physicist who bets that gravity can’t be quantized

#172

Post by brianpan »

jacquesm wrote: post #90
contravariant wrote: post #39

Earlier quoted context omitted.

> possibly even without creating a particle accelerator the size of the solar system. Sure, you just need to entangle two objects big enough to exert a noticeable amount of gravity on one another but somehow do not interact gravitationally with the rest of the set up. Anyway, let's try a cat sized object first, then we'll finally know if Schroedinger had a point.

Assume a point sized spherical cat...

I think if it's point sized, you're allowed to imagine any shape of cat. :D

Re: A physicist who bets that gravity can’t be quantized

#173

Post by consilient »

FollowingTheDao wrote: post #153
eigenket wrote: post #136

Earlier quoted context omitted.

There is absolutely no evidence that the Planck length is anything like the smallest possible length or wavelength. Its "just" a length scale that pops out when you combine several physical constants. That combination of constants means that at the Planck scale quantum effects and gravitational effects are likely to both be relevant, which is cool, but other than that it doesn't really have any fundamental meaning.

I’m giving you a hypothesis about the reason why the Planck length exists as a constant. Below Planck length there is no length, time, mass, or temperature. All of those things are given “reality” by our mental process.

It's not a constant, any more than the meter or the foot. It's just part of a unit system that happens to be convenient for carrying out certain calculations.

Re: A physicist who bets that gravity can’t be quantized

#174

Post by eigenket »

bowsamic wrote: post #161
eigenket wrote: post #146

Earlier quoted context omitted.

I attended a talk by Jonathan Oppenheim a while back on this subject, and my understanding based on that is that in his model something (very) roughly like this happens. You put your massive object in superposition, it interacts with space-time and "tries" to put space-time in superposition but since space-time is fundamentally classical (in his model) what happens is space-time ends up in a probabalistic mixture of…

> the interaction with space-time acts as some decoherence process. Well, the question is, can we use gravity in the same way that we use quantum processes, i.e., in a coherent way? If we can, e.g. using it to establish entanglement, then the theory you describe cannot hold.

Thats definitely a good question, and one that people have thought about before. I think its amazingly difficult to test it experimentally, even compared to something like directly measuring to see whether a massive object in superposition is forced to decohere faster than we expect, which is already incredibly difficult.

Re: A physicist who bets that gravity can’t be quantized

#175

Post by hughesjj »

codethief wrote: post #105

> It’s become dogma. All the other fields in nature are quantized. There’s a sense that there’s nothing special about gravity — it’s just a field like any other — and therefore we should quantize it. I keep on citing Stephen Hawking here on HN, but it again seems very appropriate: > It would be rather boring if this were the case. Gravity would be just like any other field. But I believe it is distinctively different…

I'm definitely into the "it's an emergent phenomena" camp. I think it's inherently relational, as that's a more efficient way to encode geometry rather than space itself being a quantized "thing".

Afaik this theory is the "leading"/imho most promising theory of "quantum gravity", that being the "gravity = entanglement" conjecture and the related ideas of "Complexity= action/volume/whatever" that susskind and many others have been developing for the past 20 so years.

All that said I'm nowhere near a physicist and am probably just spewing a total misunderstanding of the situation from my armchair.

That said, I've been incessantly watching lectures in this space to try to beat an understanding into my dumb dumb brain because it's super, super fucking cool.

- https://youtu.be/6_7aKoEx_kk

- https://youtu.be/6OpAreb779U

- https://youtu.be/9crggox5rbc

- https://youtu.be/OBPpRqxY8Uw

- https://youtube.com/@isqg423

I'm also fascinated by the idea of phase transitions, which seems to be how the laws of physics have "evolved" so far. It's crazy how much quantum computation is coming into play with this stuff, ex last year's Nobel prize with the bell inequality. That said I'm sure being a programmer I'm biased to think the universe is inherently computation/math.

Re: A physicist who bets that gravity can’t be quantized

#176

Post by meroes »

edgyquant wrote: post #154
tsimionescu wrote: post #83

Earlier quoted context omitted.

Quantum mechanics tells us that, in order to predict the outcome of a measurement, we have to compute a specific probability based on the amplitude of the wavefunction. We can explain this probability as some kind of collapse, or we can explain it as some measure of the number of observers making the measurement in parallel "worlds". Neither is inherently closer to the math.

Why should we not just take it as a probability based abstraction for something we don’t have a true understanding of and move on? Anything else seems like weird theology to me.

1) Because physics has had an ontology, what there really is, since like Aristotle. Even if ontology changes, posing one has worked for roughly 2500 years to partially guide science. It really does inform all kinds of experiments, even thought experiments. BUT, you might be right the trade off you propose is more worth it

2) Reichenbach's principle says any correlation must have a cause. Even without an idea of an ontology like in 1) this looser principle is hard to give up as well. It explains the mystery behind Simpson's for example. And why without the causal model behind correlations, you could easily take the wrong drug for you specifically.

Again though, these are guiding principles and trade offs I am curious to see how we could progress without them

Re: A physicist who bets that gravity can’t be quantized

#177

Post by srlowe »

consilient wrote: post #138
srlowe wrote: post #117

Earlier quoted context omitted.

Oh, really? I totally misunderstood then. I thought I knew physics a little better than that! Thanks. EDIT: Hmm, so doesn't the fact that this particular value makes the math simplified not also imply some kind of meaning?

> Hmm, so doesn't the fact that this particular value makes the math simplified not also imply some kind of meaning? No. There are plenty of other sets of constants you can choose to set to 1, inducing other length scales. Only dimensionless constants have physical meaning in isolation; dimensioned quantities are meaningful only with respect to each other.

Thanks for the explanation. I had to google "dimensionless constant" lol. Maybe it is all just math at the foundation after all!

Re: A physicist who bets that gravity can’t be quantized

#178

Post by consilient »

FollowingTheDao wrote: post #150
317070 wrote: post #140

Earlier quoted context omitted.

> our minds collapse the wave function While I know there are many sources available of people saying this exact thing, this is a common misunderstanding of the Copenhagen interpretation of quantum physics. There is nothing special about our minds.

First, thank you for responding. I’m not saying there’s anything special about our mind, it’s just a function of our mind. And is it misinterpretation or is it my interpretation that you disagree with? How can one rationalize the fact that a photon can be a wave and a particle at the same time? I do not believe, for example, that the moon exist, only because I look at it. I think it exists in a probabilistic state in…

> How can one rationalize the fact that a photon can be a wave and a particle at the same time?

Photons are not particles or waves. They are similar to classical particles in certain respects and similar to classical waves in other respects, but what they're really like is a localized excitation of the electromagnetic field. Because that's what they are.

Physics by analogy simply does not work. If you understand the math, analogies can occasionally be a helpful signpost. If you don't, they're worse than useless.

Re: A physicist who bets that gravity can’t be quantized

#179

Post by jacquesm »

brianpan wrote: post #172
jacquesm wrote: post #90

Earlier quoted context omitted.

Assume a point sized spherical cat...

I think if it's point sized, you're allowed to imagine any shape of cat. :D

For a uniform gravity field it is essential that the cat is spherical.

Re: A physicist who bets that gravity can’t be quantized

#180

Post by FollowingTheDao »

consilient wrote: post #173
FollowingTheDao wrote: post #153

Earlier quoted context omitted.

I’m giving you a hypothesis about the reason why the Planck length exists as a constant. Below Planck length there is no length, time, mass, or temperature. All of those things are given “reality” by our mental process.

It's not a constant, any more than the meter or the foot. It's just part of a unit system that happens to be convenient for carrying out certain calculations.

I have questions about your understanding. Or maybe it is a problem with my communication. There is a planck constant.

https://en.m.wikipedia.org/wiki/Planck_constant

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