Physicists propose experiment to see the ‘grin’ of quantum gravity | Hacker News Reader

Physicists propose experiment to see the ‘grin’ of quantum gravity

quantamagazine.org

73 pointsanother30 comments

To save time and dredging through pop-sci journalism for those who have some background knowledge already:

https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.11...

And to rip off the abstract “We show that despite the weakness of gravity, the phase evolution induced by the gravitational interaction of two micron size test masses in adjacent matter-wave interferometers can detectably entangle them even when they are placed far apart enough to keep Casimir-Polder forces at bay. We provide a prescription for witnessing this entanglement, which certifies gravity as a quantum coherent mediator, through simple spin correlation measurements.”

A key point is that they have not measured the effect yet so it's not known whether the entanglement will be measured. Both papers describe a gravitational entanglement detection through measurement of evolution of a separate quantum phenomenon (e.g. nitrogen spin in a diamond lattice).

One challenge, which is commonly a problem for laboratory gravitational experiments, is that many other force interactions dominate over short distance scales (electric, magnetic, or even vibrational phonons) so excluding those is critical to showing the entanglement is gravitational. This means near perfect electrical, magnetic, and vibrational isolation... and probably measuring their coupling amplitude them to show that it is perfect.

As usual in experimental physics, the problem is not in detecting the effect, but in not detecting all the other interfering effects. It borders on proving a negative.

Having not seen this sort of theory before, it felt as though the sort of thing that may be a widespread hypothesis but the data are prohibitive to collect.

They have teams working on it now, but experiment is notoriously more expensive than theory.

I find the subject fascinating, but the main impact I got from reading this was learning how a young, curious, extraordinarily intelligent person with immense potential and who already contributed so much is murdered by human stupidity. I'm reminded of a line from Elton John's song about John Lennon's death, "It's funny how one insect can damage so much grain" and wonder about the amount of grain destroyed by insects such as Hitler, Stalin and so many others. Don't have much of a point to make, just some reflections about the human condition

Just a dumb layman question: can gravitons (assuming they exist) escape a black hole?

No.

I imagine there is some context for this question, but I can not really give a better answer without knowing the question. My guess is that the question stems from a misunderstanding what a graviton is, so I will grossly simplify and say: graviton is to gravitational field what photon is to electrostatic field.

True but the photon and graviton both transmit fields. For a black hole to pull matter in it must push out gravitons. Unless there really is no graviton particle and the Higgs tells space which way to curve. Ie gravity is an emergent property instead of a force.

I’ve never heard of the Higgs being postulated as having a ’curving’ effect (it’s role is more akin to being a kind of molasses through which particles with weak hypercharge need to ’push’).

Gravity at black holes’s event horizon is not quite coming from where you think it is: it isn’t ”emitted by” or ”emanating from” the singularity. Rather, as gravity is a form of energy, and mass-energy creates gravity, the gravitational field at the border, within our observable universe, is intense enough to whip itself up into a self-maintaining recursive but sustainable loop. No communication from what lies within the event horizon is neither possible nor required.

This is what I wanted to know too. Speaking as a non-physicist and a layman, that's almost the first conundrum that occurs to you when reading about black holes from popular science literature. How is it that gravitons can seemingly be exchanged by the pair of masses for any body to feel the pull of a black hole?

I still don't quite get "the gravitational field at the border, within our observable universe, is intense enough to whip itself up into a self-maintaining recursive but sustainable loop" (especially the self-maintaining, recursive part. How?), but I guess one would need a good understanding of GR to go beyond this level?

Since black-holes interact gravitationally with other particles, there must be a mediation of a graviton between them (assuming quantum properties of gravity). However, force mediation is different from a physical particle escaping one object than interacting with another (e.g. physical graviton escaping a BH, interacting with an other particle). That is why we have the concept of virtual particles [0]:

"[...] the electromagnetic repulsion or attraction between two charges—can be thought of as due to the exchange of many virtual photons between the charges."

so when the electromagnetic force mediation takes place, it exchanges virtual photons, and this is different from an object exchanging physical photons with another (which we would more think of as one object shining (if the photon is in the visible spectrum) on another).

How a virtual graviton would interact between BHs and other particles are up to any suggested theory for quantum gravity to explain.

[0] https://en.wikipedia.org/wiki/Virtual_particle

Thanks. That makes it somewhat clearer. Also including both links below as they are relevant to this misconception:

http://math.ucr.edu/home/baez/physics/Relativity/BlackHoles/... https://www.quora.com/in/If-both-the-photon-and-the-graviton...

I’m not a physicist but I have a background in applied mathematics.

What I am trying to describe is the inherent nonlinearity of general relativity, wherein compound solutions cannot be obtained simply summing up or overlaying simpler situations: the gravity filed itself generates more gravity, so it can recursively reach and sustain a critical value.

Black holes also evaporate. What’s happening here to make the field get smaller and ultimately disperse if the field is self-sustaining?

> graviton particle and the Higgs tells space which way to curve

The Higgs does not play that kind of role. It is a common misconception, since Higgs it often referred to as "giving mass to the other particles", which is true, and called the Higgs-mechanism, it does not influence the curvature of space-time that way. That is all due to the graviton (assuming, which it is also commonly believed, that gravity is quantised).

Mass is one thing, interactions between masses another.

You took out part of my comment and only commented on the part you took out. In my original statement I made that statement as a hypothetical and in a context supposing there isn’t a graviton particle. My desire was to communicate that for gravitons to fill their hypothesized role for gravity they must somehow transmit the gravity from a BH.

> For a black hole to pull matter in it must push out gravitons.

This is a non sequitur, presumably due to a misunderstanding caused from some pop sci article. You really do not need gravitons to describe any of the dynamics of "pulling matter in" (at least for "usual" black holes), and there is no "pushing out gravitons" involved.

tl;dr: no, and the issue here is that gravitons probably aren't like what you're thinking. (and it's not a dumb question).

In perturbatively quantized gravity, we start with a background metric (e.g. Minkowski flat spacetime or Schwarzschild black hole spacetime) and then "perturb" it with other moving matter. With a careful choice of slicing of the 4-spacetime into 3+1 space+time, we can see the (leading order) deviations from the background metric as a plane wave propagating lightlike from one spacelike slice to the next. Quantizing those waves in much the same way as we quantize classical electromagnetic planar waves leads to the graviton as a massless boson (but with spin-2 instead of spin-1).

A spherically symmetric non-rotating black hole (and having factored out changes in coordinate position and linear momentum by choice of coordinates) sources exactly the Schwarzschild metric, so there is no perturbation from a Schwarzschild background (and thus no gravitational waves to quantize, and thus no gravitons in the vincinity). If we throw something into it, there will be a perturbation of the exterior metric, and thus gravitons in principle observable outside the horizon.

There are theories that define a graviton, and the result ought to be the same: you generally only get (real rather than virtual) gravitons when something is falling into a (maximally symmetric) black hole (and very shortly afterwards, during "balding"), or:

If you break the Schwarzschild symmetries by adding angular momentum and/or charge to the black hole, you can also get perturbations of a well-chosen background metric, although extremely little gravitational radiation (and thus few gravitons) unless something massive is sufficiently near the black hole.

But just to repeat: an isolated non-rotating black hole emits no gravitons. You need to perturb it out of its highly symmetrical state in order to get gravitons. [cf. https://en.wikipedia.org/wiki/Gravitational_wave?oldformat=t... -- for a black hole the relevant configurations are a significant "bump" on an otherwise highly symmetric black hole or a significant mass creating a "barbell" style binary]

Is gravity, fundamentally quantum? I thought the jury was still out

From what I understand of the article, it is still totally undecided, and that is what this experiment aims to address, although it still won't provide any insight as to the exact quantum nature of gravity. It will merely decide if it is indeed quantised. And even there, Freeman Dyson appears to disagree (according to the article)...

Dyson seems to be making a more abstract argument — that if the domains where a graviton could be detected are inaccessible to us, then they effectively do not exist (if a tree falls in a forest...). At least that’s what I took away from the description in the article and his dismissal of the proposed experiment as not having a bearing on his objections.

It won't even necessarily decide if gravity is quantized. If it gets a positive result then it will show that gravity is quantized, but a negative result doesn't mean that gravity isn't quantized, merely that the experiment failed.

https://en.wikipedia.org/wiki/Betteridge%27s_law_of_headline...

Original source: https://www.quantamagazine.org/physicists-find-a-way-to-see-...

Previous postings/comments:

https://news.ycombinator.com/item?id=16534057

https://news.ycombinator.com/item?id=16536208

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We changed the URL to that from https://www.theatlantic.com/science/archive/2018/03/quantum-....

Those previous HN submissions don't count as dupes because they didn't get significant attention. This is in the FAQ: https://news.ycombinator.com/newsfaq.html.

off topic, but; before clicking the link I guess I was kinda surprised the Atlantic would get in on a story like this but then I saw that its a reprint from Quanta Magazine. Anyone know what their partnership is?

Seems that The Atlantic is a syndication partner with Quanta along with a few others.

https://www.quantamagazine.org/about/

We changed the url from https://www.theatlantic.com/science/archive/2018/03/quantum-..., in keeping with the site guidelines' call for original sources.

https://news.ycombinator.com/newsguidelines.html

it could take a decade or more to pull it off.

My question why is it getting so long to get result How do software step in to help streamline research ? I can not help thinking this is steve job 'next' company time in the sun

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