
Physicists propose experiment to see the ‘grin’ of quantum gravity - another
https://www.quantamagazine.org/physicists-find-a-way-to-see-the-grin-of-quantum-gravity-20180306/
======
noobermin
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...](https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.119.240402)

[https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.11...](https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.119.240401)

~~~
ianai
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.”

~~~
kurthr
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.

~~~
ianai
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.

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

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placebo
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

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Santosh83
Just a dumb layman question: can gravitons (assuming they exist) escape a
black hole?

~~~
krastanov
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.

~~~
ianai
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.

~~~
rotorblade
> 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.

~~~
ianai
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.

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

~~~
Santosh83
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)...

~~~
sulam
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.

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ShorsHammer
[https://en.wikipedia.org/wiki/Betteridge%27s_law_of_headline...](https://en.wikipedia.org/wiki/Betteridge%27s_law_of_headlines)

------
beefman
Original source: [https://www.quantamagazine.org/physicists-find-a-way-to-
see-...](https://www.quantamagazine.org/physicists-find-a-way-to-see-the-grin-
of-quantum-gravity-20180306/)

Previous postings/comments:

[https://news.ycombinator.com/item?id=16534057](https://news.ycombinator.com/item?id=16534057)

[https://news.ycombinator.com/item?id=16536208](https://news.ycombinator.com/item?id=16536208)

[https://news.ycombinator.com/item?id=16531016](https://news.ycombinator.com/item?id=16531016)

~~~
dang
We changed the URL to that from
[https://www.theatlantic.com/science/archive/2018/03/quantum-...](https://www.theatlantic.com/science/archive/2018/03/quantum-
gravity/555107/?single_page=true).

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](https://news.ycombinator.com/newsfaq.html).

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pizza
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?

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

[https://www.quantamagazine.org/about/](https://www.quantamagazine.org/about/)

------
smallhands
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

