6 ms·

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

73 points by another 9y ago

noobermin 9y ago
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.119.240402 https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.11... https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.119.240401 https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.11...
- ianai 9y ago
  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 9y ago
    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 9y ago
      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 9y ago
        They have teams working on it now, but experiment is notoriously more expensive than theory.
pizza 9y ago
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 9y ago
  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/
- dang 9y ago
  We changed the url from https://www.theatlantic.com/science/archive/2018/03/quantum-gravity/555107/?single_page=true 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 https://news.ycombinator.com/newsguidelines.html
exabrial 9y ago
Is gravity, fundamentally quantum? I thought the jury was still out
- Santosh83 9y ago
  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 9y ago
    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.
  - SAI_Peregrinus 9y ago
    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.
Santosh83 9y ago
Just a dumb layman question: can gravitons (assuming they exist) escape a black hole?
- krastanov 9y ago
  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 9y ago
    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.
    - qubex 9y ago
      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.
      - Santosh83 9y ago
        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?
ShorsHammer 9y ago
https://en.wikipedia.org/wiki/Betteridge%27s_law_of_headlines https://en.wikipedia.org/wiki/Betteridge%27s_law_of_headline...
beefman 9y ago
Original source: https://www.quantamagazine.org/physicists-find-a-way-to-see-the-grin-of-quantum-gravity-20180306/ 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=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 9y ago
  We changed the URL to that from https://www.theatlantic.com/science/archive/2018/03/quantum-gravity/555107/?single_page=true 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 https://news.ycombinator.com/newsfaq.html.
placebo 9y ago
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
smallhands 9y ago
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

73 points by another 9y ago

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.119.240402 https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.11... https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.119.240401 https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.11...

ianai 9y ago
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 9y ago
  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 9y ago
    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 9y ago
      They have teams working on it now, but experiment is notoriously more expensive than theory.

pizza 9y ago

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 9y ago
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/
dang 9y ago
We changed the url from https://www.theatlantic.com/science/archive/2018/03/quantum-gravity/555107/?single_page=true 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 https://news.ycombinator.com/newsguidelines.html

exabrial 9y ago

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

Santosh83 9y ago
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 9y ago
  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.
- SAI_Peregrinus 9y ago
  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.

Santosh83 9y ago

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

krastanov 9y ago
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 9y ago
  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.
  - qubex 9y ago
    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.
    - Santosh83 9y ago
      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?

ShorsHammer 9y ago

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

beefman 9y ago

Original source: https://www.quantamagazine.org/physicists-find-a-way-to-see-the-grin-of-quantum-gravity-20180306/ 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=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 9y ago
We changed the URL to that from https://www.theatlantic.com/science/archive/2018/03/quantum-gravity/555107/?single_page=true 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 https://news.ycombinator.com/newsfaq.html.

placebo 9y ago

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

smallhands 9y ago

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