Hacker News Board Hacker News, browsed like a classic message board
Live data from Hacker News

A physicist who bets that gravity can’t be quantized

quantamagazine.org

Post reply on HN
Print view View original

81–90 of 378 posts

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

#81

Post by javajosh »

amelius wrote: post #74
javajosh wrote: post #40

I think it would be funny if we discover that refraction is caused by the slowing of light in close proximity to mass. That is, one of the most common and observable phenomena in physics is a quantum gravity phenonema! (The usual explanation for refraction is that light as an EM wave causes sympathetic vibration in the electrons (and the protons, a little) which slows it down. But what if light's proximity to protons…

Refraction is well understood. It is caused by interactions of the incoming wave and electron clouds of atoms. https://en.wikipedia.org/wiki/Ewald%E2%80%93Oseen_extinction...

Ah, perhaps you are inclined to stop reading when you come across open parenthesis?

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

#82

Post by H8crilA »

naasking wrote: post #73
H8crilA wrote: post #66

Earlier quoted context omitted.

For the layman, what exactly is going to theoretically "explode" if spacetime is continuous?

The article discusses this question actually, about the apparent incompatibility between classical and quantum systems. Basically, there's a fundamental inconsistency where you can detect a particle's position gravitationally as it passes through a slit in the double slit experiment, which destroys the quantum properties of "passing through both slits" that leads to interference patterns.

Oh no I get that, but two comments above inciampati was suggesting that we will end up in a situation where some variable will have to have an infinite value. Like, famously, with the ultraviolet crisis: https://en.wikipedia.org/wiki/Ultraviolet_catastrophe

Detecting (something about) the particle through gravity would at best remove the interference, not create an \infty somewhere in the model, implying that the model is self inconsistent.

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

#83

Post by tsimionescu »

CraftingLinks wrote: post #22
edgyquant wrote: post #12

Earlier quoted context omitted.

Why would anyone believe such a thing?

Because that's what quantum mechanics in it's purest form tells us. Avoiding the many worlds requires tagging on extra stuff not in the equations.

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.

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

#84

Post by tgv »

captainclam wrote: post #38
tgv wrote: post #10

Earlier quoted context omitted.

Among other problems. It's just bollocks on the level of "but you didn't say it couldn't be this, nanananana."

I think because of the abuse of "many worlds" in science fiction media (especially as of late) as a convenient plot device, people develop this idea that it's an unserious proposal wrt the foundations of physics. As far as I have read, it strikes me as perhaps the most parsimonious interpretation of QM out there. Genuinely curious, what do you mean by "but you didn't say it couldn't be this, nanananana...?" The Evere…

Assuming many worlds, and many here means: enormous amounts, far exceeding the number of particles in the universe, is everything but parsimonious. There happens to be a model that fits some data, but that's it. It's a grotesque assumption to avoid a conflict in a man-made theory. It's a funny thought, but no more than that.

There's also nothing special about observing. Our consciousness isn't super-natural, so the idea is in desperate need of some other underpinning.

And probabilities: if this is one of many, many worlds, the distribution of events as we can observe them is heavily skewed. The next observations should follow a radically different pattern, unless you also assume that each split influences the probabilities of future events.

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

#85

Post by Joker_vD »

tsimionescu wrote: post #65
goatlover wrote: post #47

Earlier quoted context omitted.

Shut and calculate is putting your head in the sand and avoiding that physics should be telling us what the world is. You’re wrong about MWI in that it’s a more elegant interpretation because it adds nothing extra to the wave equation and treats the universe as fundamentally quantum with no arbitrary dividing lines for classically scaled objects.

> You’re wrong about MWI in that it’s a more elegant interpretation because it adds nothing extra to the wave equation and treats the universe as fundamentally quantum with no arbitrary dividing lines for classically scaled objects. That's how it's often presented, but this is wrong. In fact, it does add something to the theory, and that's a measure of how many "worlds" there are after a quantum measurement, which he…

> the MWI has to somehow define a formal notion of an observer/a classical world

Yes: in MWI, those things don't exist. The world is quantum all the way up and all the way down, observers are simply (other) quantum system that get to interact with the quantum system under the consideration. An observation, then, is simply an interaction between two quantum systems and follows all the usual rules so instead of the wave-function collapse leaving you with the observed system in pure state X and the observer is in pure state Y, you get a huge superposition of "the observed system in pure state Xi, the observer is in pure state Yi" states in the end. Those substates, in a sense, are multiple worlds.

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

#86

Post by H8crilA »

bowsamic wrote: post #79
H8crilA wrote: post #68

Earlier quoted context omitted.

What would such an experiment look like? If it makes it easier, perhaps an experiment that's even harder (or much, much harder) to realize but easier to explain.

It's pretty simple. You basically have two suspended mirrors on wires and you shoot lasers at each mirror. Due to gravity there will be an interaction between the mirrors. There are then two things you can probe. First, the correlation between the two reflected lasers, and if you can achieve the extreme temperature requirements, entanglement between the two reflected lasers caused by the gravity. Now, the former does…

Those reflected beams would at best be very very weakly entangled, right? I'm not sure what's the name for it, but if you arranged the quantum state of the two beams into a 2x2 matrix then the determinant would be just a tiny bit non-zero.

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

#87

Post by snowwrestler »

AnimalMuppet wrote: post #23

Somewhat off topic, and somewhat related: I don't think there actually is such a particle as a graviton. General relativity says that you can't tell the difference between being unaccelerated, and being in free fall. But in one case you have no gravitons coming in, and in the other you have gravitons. I can change whether gravitons are there or not by a (general) relativistic transformation. But that's not possible.…

Saying one theory must be wrong because the other must be right is begging the question. Where general relativity and quantum theory disagree, we don’t know which one is correct (or maybe neither is).

Highlighting a point of disagreement between the two doesn’t, by itself, resolve the disagreement.

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

#88

Post by abdullahkhalids »

naasking wrote: post #69

> But when they tried to quantize gravity, they ran into unnatural infinities that had to be sidestepped with clumsy mathematical tricks. Maybe they run into unnatural infinities because all of our formalisms in physics are still fundamentally continuous rather than discrete. Uncountable infinities are baked right into the foundations of how we use reason about these systems, so infinities will naturally result. Phys…

I can't help but suggest "Information, physics, quantum: The search for links" [1] by the one and only John Wheeler. This philosophical paper is bold, bolder than most physicists ever would be.

> Abstract: This report reviews what quantum physics and information theory have to tell us about the age-old question, How come existence? No escape is evident from four conclusions: (1) The world cannot be a giant machine, ruled by any preestablished continuum physical law. (2) There is no such thing at the microscopic level as space or time or spacetime continuum. (3) The familiar probability function or functional, and wave equation or functional wave equation, of standard quantum theory provide mere continuum idealizations and by reason of this circumstance conceal the information-theoretic source from which they derive. (4) No element in the description of physics shows itself as closer to primordial than the elementary quantum phenomenon, that is, the elementary device-intermediated act of posing a yes-no physical question and eliciting an answer or, in brief, the elementary act of observer-participancy. Otherwise stated, every physical quantity, every it, derives its ultimate significance from bits, binary yes-or-no indications, a conclusion which we epitomize in the phrase, it from bit.

[1] https://philarchive.org/archive/WHEIPQ

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

#89

Post by CuriouslyC »

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.

People talk about how space time can't be quantized as if the universe was made up of tiny little cells like minecraft because there is some evidence against that hypothesis.

An alternate hypothesis, that a particle is "virtual" and its position/momentum "vector" contains a finite amount of information, is actually both extremely plausible and explains odd paradoxes like the Heisenberg uncertainty principle. That would result in "quantized" spacetime for the same reason floating point numbers are imprecise.

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

#90

Post by jacquesm »

contravariant wrote: post #39
eterevsky wrote: post #8

(Not a quantum physicist. Please correct me if I am misunderstanding.) From what this article says, his assumption is that gravity is classical, but "fuzzy" or probabilistic: you can't precisely measure the gravitational field of a sufficiently small object. In the last years we've seen progressively bigger objects being put in quantum superposition. The theory from this article is incompatible with this process cont…

> 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...
Post reply on HN