7 ms·
Physicists measure the gravitational force between the smallest masses yet
- politelemon 5y agoI couldn't find a link to the original paper, if any. Could someone assist?
- aelhaji 5y agoHere you go! https://www.nature.com/articles/s41586-021-03250-7 https://www.nature.com/articles/s41586-021-03250-7
- soheil 5y agoOriginal paper without paywall https://arxiv.org/pdf/2009.09546.pdf https://arxiv.org/pdf/2009.09546.pdf
- timeinput 5y agoThanks! I wanted to know what masses they were using ~90 mg, and that had the answer. It was surprising to me it wasn't in the article.
- was_a_dev 5y agotwo tiny gold spheres, each about the size of a sesame seed and weighing as much as four grains of rice This was the most infuriating part of the article. How hard is it to use SI units in a popular science magazine?
- wwarner 5y agoI love this experiment. I wonder if G can be measured optomechanically, as Nergis Mavalvala explains in this video https://youtu.be/j-6BQPTmWeA https://youtu.be/j-6BQPTmWeA
- kurthr 5y agoThose are really for coherent gravitational waves (the same across locations across the world) not for interactions between individual small objects. Since they're looking for any defect in the inverse square law (e.g. due to extra dimensions of string theory) these would screw it up. Light actually exerts a force (even like tweezers) so it's not a good choice for this kind of measurement. They must have done an amazing job of removing any electrostatic or magnetic effects since those so quickly dominate at these dimensions. The gold ball isn't so bad to discharge, but typically glass fibers are used, which can build up charges. My expectation is that they used radically different frequencies on the two ball supports and very high oscillator Q (they're pendulums in a vacuum) to separate them mechanically since they have to be in the same chamber. People have tried placing "screening materials" between the masses to reduce electro-magnetic coupling, but those tended to cause their own non-gravitational effects.
- kkotak 5y agoThey should do this experiment in space.
- bckr 5y agoExcellent idea! Also this whole experiment is really exciting. I'd heard of quantum entanglement being demonstrated with larger objects, but this is the first I've heard of measuring gravity between small objects. It seems like this must eventually lead to quantum gravity, or. . . something else!
- krastanov 5y agoThe issue is that gravity and quantum mechanics contradict each other only when the curvature of spacetime is significant, which is not the case here. Still, it would be amazing to measure gravitational field from an entangled object... But I do not expect to see this during my natural lifetime.
- tasty_freeze 5y agoI don't believe that is true. For example, if this experiment can be carried out with much smaller masses, they could find that the attractive force isn't smooth but is instead stair-stepped, which would imply some kind of quantization in space or gravity.
- simiones 5y agoI believe QM and GR contradict each other for any curvature, but measuring the extremely small curvature/gravitational effects caused by a single particle would require enormous amounts of energy. Specifically the problem is the way that the uncertainty principle interacts with GR's assumptions.
- krastanov 5y agoIn flat space time you have QFT, which does not have severe problems until you make the gravitational fields very strong.
- bashmelek 5y agoI'm sure this is a dumb question, but are there any conceivable use cases for a gravity-based communication system?
- mikewarot 5y agoNo, but it is used for Navigation in submarines. https://en.wikipedia.org/wiki/Gravity_gradiometry#Gravity_gradiometers https://en.wikipedia.org/wiki/Gravity_gradiometry#Gravity_gr...
- contravariant 5y agoWell you've got a virtually unblockable signal that travels at the speed of light, but other than that there are literally no upsides. I reckon you might as well use neutrino's, they're easier to detect and easier to produce.
- loteck 5y agoWouldn't neutrino communication have to be highly targeted, though? Whereas gravity could "broadcast"?
- umvi 5y agoSo what is this really measuring? The curvature of spacetime between two points?
- contravariant 5y agoYou're measuring a force. There's no real reason not to interpret it as one.
- 8note 5y agoIf you're measuring a force, you're measuring the force required to keep the two masses from coming together
- kurthr 5y agoActually, I agree... and while it's not clear that spacetime is a necessary complexity in this case, it is explicitly looking for quantum effects (e.g. the extra dimensions of string theory).
- contravariant 5y agoWell yeah as is usually the case with inertial forces you're basically just measuring how difficult it is to keep something in place.
- steve76 5y agoI think of it as internally, where space and time converge to the same medium, each grain of gold has it's own filmstrip with very similar, yet still different frame rates. For example, the Sun is on a slower frame rate than the Earth. The Earth needs to do something in those frames yet still conserve spacetime. So the Earth orbits it. You floating in space are on a much faster frame rate than Earth. You might look like you are going to float by it. But you need to fill those frames while the Earth doesn't. You crash in, sync frame rates, and feel that impact. Those two grains are orbiting each other at really really small angles. It doesn't happen though as convection or electrical conduction fills the frames. There could be gravity applications, as in using a tremendous amount of energy, all the energy that exists, to generate a particle. That could be considered communication.
- libraryofbabel 5y agoIt’s impressive, but for some perspective Henry Cavendish was able to measure the gravitational attraction between two (rather larger) spheres in a lab in the 18th century![0] He called it “weighing the world”, because it allowed him to calculate the mass of the earth. [0] https://en.m.wikipedia.org/wiki/Cavendish_experiment https://en.m.wikipedia.org/wiki/Cavendish_experiment
- EastLondonCoder 5y agoAnd it can be replicated in a school setting! https://www.youtube.com/watch?v=MbucRPiL92Q https://www.youtube.com/watch?v=MbucRPiL92Q
- imglorp 5y agoThat was an amazing demonstration. Showing the sped up video was genius: the mass clearly reversed its direction in mid oscillation and then bounced down. I felt a tiny bit sad he didn't go 5 minutes longer and estimate F and G with a little math from the video frames. He'd already demonstrated the noise amplitude nicely.
- Someone 5y agoFrom that Wikipedia page: “The motion of the rod was only about 0.16 inches (4.1 mm)” At first sight, I find that surprisingly large. Thinking of it, he used heavy objects (300kg in total), it isn’t hard to twist a wire, and falling down even a meter hurts, so perhaps it isn’t that surprising.
- mdturnerphys 5y agoI did my PhD in the Eöt-Wash group [0] at the University of Washington, where many of the "world's best" torsion balance experiments have be done: tests of the equivalence principle, measurement of gravity at short distances, measurement of the gravitational constant, and others. The 3 professors in the group won the Breakthrough Prize in Physics last year. When this article came out a few months ago I was surprised it got so much press. If I'd had any idea it would get so much attention (and merit a paper in Nature) I would have pushed for us to do it :) The authors did a great job doing the experiment and addressing the possible systematics--doing torsion balance experiments the right way is difficult--but there doesn't seem to be anything too new here. [0] https://www.npl.washington.edu/eotwash/ https://www.npl.washington.edu/eotwash/ [1] https://www.washington.edu/news/2020/09/10/four-uw-professors-win-2021-breakthrough-prize-so-called-oscars-of-science/ https://www.washington.edu/news/2020/09/10/four-uw-professor...
- lamontcg 5y agoFor more on the possible measurement of quantum entanglement in gravity see: https://www.sciencedirect.com/science/article/pii/S0370269319301698 https://www.sciencedirect.com/science/article/pii/S037026931... If the BMV effect is observed it would falsify Penrose's conjecture that something unusual and nonlinear happens around the Planck mass (conversely the experiment could also begin to probe and measure that regime and find Penrose was on to something).
- trhway 5y agowrt. smallest mass for one of the gravitationally interacting objects - a neutron position quantization in the Earth gravitational field https://physicsworld.com/a/neutrons-reveal-quantum-effects-of-gravity/ https://physicsworld.com/a/neutrons-reveal-quantum-effects-o... A domain review paper https://iopscience.iop.org/article/10.1088/1367-2630/14/5/055010/pdf https://iopscience.iop.org/article/10.1088/1367-2630/14/5/05... "Abstract. This paper describes gravity experiments, where the outcome depends upon both the gravitational acceleration g and the Planck constant h¯ . We focus on the work performed with an elementary particle, the neutron"
- pantulis 5y agoCould any experiment like this ever measure how the dark energy driven expansion of the universe affects gravitationally bound systems? Of course I understand it's beyond any reasonable measurement capability --masses should be infinitesimal and the system be isolated from any other interactions, the question is about if it's theoretically possible. Like, "it's impossible because masses should be below the Planck mass"
- osigurdson 5y agoThe following link discusses the same experiment and has more specifics (does not discuss mass only in terms grains of rice or eyelashes - TL;DR 92mg). It also includes a clearer discussion of how well the results match Newton's equation (F=Gm1m2/r^2) vs the uncertainty of the experimental setup (TL;DR - this was inconclusive). https://physicsworld.com/a/physicists-measure-smallest-gravitational-field-yet/ https://physicsworld.com/a/physicists-measure-smallest-gravi...
- beefman 5y agoPrevious discussion: https://news.ycombinator.com/item?id=26420692 https://news.ycombinator.com/item?id=26420692