'Quantum gravity' could help unite quantum mechanics with general relativity at last
"By understanding quantum gravity, we could solve some of the mysteries of our universe — like how it began, what happens inside black holes, or uniting all forces into one big theory."
Scientists have determined a way to measure gravity on microscopic levels, perhaps bringing them closer to forming a theory of "quantum gravity" and to solving some major cosmic mysteries.
Quantum physics offers scientists the best description of the universe on tiny scales smaller than atoms. Albert Einstein's theory of general relativity, on the other hand, brings about the best description of physics on huge, cosmic scales. Yet, something is frustratingly missing even after 100 years of both theories passing a wealth of experimental verification.
As robust and accurate as the two theories developed at the turn of the 20th century have become, they have refuse to unite.
One of the primary reasons for this dilemma is that, while three of the universe's four fundamental forces — electromagnetism, the strong nuclear force and the weak nuclear force — have quantum descriptions, there is no quantum theory of the fourth: Gravity.
Now, however, an international team has made headway in addressing this imbalance by successfully detecting a weak gravitational pull on a tiny particle using a new technique. The researchers believe this could be the first tentative step on a path that leads to a theory of "quantum gravity."
"For a century, scientists have tried and failed to understand how gravity and quantum mechanics work together," Tim Fuchs, team member and a scientist at the University of Southampton, said in a statement. "By understanding quantum gravity, we could solve some of the mysteries of our universe — like how it began, what happens inside black holes, or uniting all forces into one big theory."
Related: 'Wavy spacetime' may explain why gravity won't play by quantum rules
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Gravity gets the 'spooky' treatment
It is maybe fitting that general relativity and quantum physics don't get along; after all, Einstein was never comfortable with quantum physics. This is because while quantum physics has many counterintuitive aspects, he found one in particular very troubling.
It was the notion of entanglement. At risk of simplification, entanglement has to do with coordinating particles in such a way that changing the properties of one particle instantly alters the properties of an entangled partner particle, even if the partner is located on the opposite side of the universe. Einstein called this "spooky action at a distance" as it challenged the concept of local realism.
Local realism is the idea that objects always have defined properties and that interactions between those objects are limited by distance and the speed of light, a universal speed limit introduced by Einstein as the foundation of special relativity. Special relativity is, in fact, the theory that led to the formulation of general relativity in the first place. Yet, despite Einstein's protestations, scientists have indeed proven that entanglement and other counterintuitive aspects of quantum physics are truly factors of reality at subatomic scales.
Such proof has been achieved with a multitude of pioneering experiments. Fuchs and colleagues, for instance, are following in the footsteps of physicists such as Alain Aspect, John Clauser and Anton Zeilinger, who won the 2022 Nobel Prize in Physics for experimentally verifying the nonlocal nature of entanglement.
In their new quantum experiment, the researchers, including scientists from Southampton University, Leiden University and the Institute for Photonics and Nanotechnologies, used superconducting magnetic "traps" to measure the weak gravitational pull on the smallest mass anyone has ever attempted to investigate in this way.
The tiny particle was levitated in the superconducting trap at temperatures of around 459.4 degrees Fahrenheit (273 degrees Celsius), which is just a few hundredths of a degree above absolute zero, the hypothetical temperature at which all atomic movement would cease. This frigid temperature was needed to limit the vibrations of the particles to the very minimum. The team ultimately measured a gravitational pull of 30 "attoNewtons" on the particle.
AttoNewtons represent a measure of force; to give you an idea of how tiny the gravitational force on the studied particles was, one Newton is defined as the force needed to provide a mass of one kilogram with an acceleration of one meter per second per second. And 30 attoNewtons is equivalent to 0.00000000000000003 Newtons!
"Now we have successfully measured gravitational signals at the smallest mass ever recorded, it means we are one step closer to finally realizing how it works in tandem," Fuchs said. "From here, we will start scaling the source down using this technique until we reach the quantum world on both sides."
Team member and University of Southampton scientist Hendrik Ulbricht said this experiment paves the way for tests with even smaller masses, as well as the measurement of even smaller gravitational forces.
"We are pushing the boundaries of science that could lead to new discoveries about gravity and the quantum world. Our new technique that uses extremely cold temperatures and devices to isolate the vibration of the particle will likely prove the way forward for measuring quantum gravity," he concluded. "Unravelling these mysteries will help us unlock more secrets about the universe's very fabric, from the tiniest particles to the grandest cosmic structures."
The team's research was published on Friday (Feb. 23) in the journal Science Advances.
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Robert Lea is a science journalist in the U.K. whose articles have been published in Physics World, New Scientist, Astronomy Magazine, All About Space, Newsweek and ZME Science. He also writes about science communication for Elsevier and the European Journal of Physics. Rob holds a bachelor of science degree in physics and astronomy from the U.K.’s Open University. Follow him on Twitter @sciencef1rst.

Icepilot Much discussion about the "smallest mass" & "tiny particle", but no actual value for the mass?Reply 
bibhutibhusanpatel@gmail. The Primciple can be applied to solve such problems on quantum gravity related to entanglement;thus,no new fact.Reply 
danR entanglement has to do with coordinating particles in such a way that changing the properties of one particle instantly alters the properties of an entangled partner particle
I don't know how many times and how many ways this misapprehension of entanglement has been displayed in popscience articles. It's probably the number one reason why the lay public keeps coming up with FTL comms schemes:
' See, Bob changes the spin on his electron from up to down and Alice at Alpha Centauri has her electron changed so Bob can send binary code and Alice can receive the binary code instantly and they can... ' 
danR
430 micrograms. A lot of things were left out, including the actual means of measuring the gravitational force itself. It's like the stage was set, the extras brought in, the props placed, the lighting turned on and... where is the main actor? BBC article did the same thing.Icepilot said:Much discussion about the "smallest mass" & "tiny particle", but no actual value for the mass?
Perhaps everything is drafted by ChatGPT nowadays. 
Unclear Engineer "Quantum gravity theory" seems to be on the same schedule for completion as "commercial fusion power".Reply
We keep reading about "progress", but there have been no deliverables, and no schedules for deliverables. 
Dr.Matthé
This is an impressive technical achievement, but the whole hype about quantum gravity is without scientific basis. 1. Quantum field theory meanwhile is well possible within curved spacetime 2. Divergencies in General Relativity do not exist as people forget that time dilation automatically stops them. And how could black holes that by definition are not time reversible, ever be a solution of timeindependent GR?! 3. It is easy to argue that any possible quantum effect from gravity could never be measured (gravity is too weak, selfinteracting and since there's no negative mass, there are no dipoles to produce individual 'gravitons')Admin said:Researchers have measured the gravitational pull exhibited by the smallest mass yet, a breakthrough that could lead to a quantum theory of gravity at last.
Quantum gravity' could help unite quantum mechanics with general relativity at last : Read more 
Zdepthcharge Pathetic.Reply
Congratulations to the scientists who have managed to measure the effects of gravity at the smallest scale yet. Boo to the people bringing up the idiocy of quantum gravity. And boo to the writers of this article for continuing to perpetuate the myth that gravity is a force.
Gravity is not a force in the way that we think of forces. Unlike the particle/field forces, gravity is not transactional. In other words, there isn't a mitigating particle that controls how gravity operates. You can claim all you want that there is, but you can't prove it. The graviton is so elusive because it is simply a particle of the imagination. People may complain that I can't prove it doesn't exist, which is true, but I'm not trying to base arguments on it's nonexistence.
Particle physicists could advance scientific inquiry by fifty years if they simply stopped basing ideas on the existence of something that has not been shown to exist. The continued attempts and whining are pathetic. 
Atlan0001 There is no "singularity" to gravity physics, so there is no quantum physics' (EW) pointparticle singularity to gravity! and the many varieties of quantumcomplex singularities "to gravity"! Again: no 0point singularity to gravity's fundamental "zooms" (to the infinite and infinitesimal) element of "selfsimilar fractal zooms (infinities) universe structure" short the representative gluon of the boundaries crossing strong binding (including nuclear) force's / Casimir effect force's base2 "set and reset."Reply 
Torbjorn Larsson Unfortunately the experiment is not very convincing yet. They use a small mass and get 35 % of the modeled signal with lots of noise. It would be more convincing if they started at larger masses for both test and outside mass and run a series of experiments to see how the huge discrepancy scales.Reply
The article makes the usual pop science, unsupportable claim that gravity can't be quantized and that the two descriptions can't be unified. But a straightforward effective quantum field theory has been developed long ago and is consistent with general relativity except on highest energy Planck scales (where both theories break down).
http://www.scholarpedia.org/article/Quantum_gravity_as_a_low_energy_effective_field_theory
The potential problem with these theories seem to lie elsewhere. They are both effective and so need observations to pin down parameters, instead of being independent on observation as some would like to see. (But that is an odd requirement on scientific theories.) 
Torbjorn Larsson
The experiment constitutes a new fact, a new range of putative observation.bibhutibhusanpatel@gmail. said:The Primciple can be applied to solve such problems on quantum gravity related to entanglement;thus,no new fact.
I agree, but to be a bit fairer the author said that it was likely an oversimplification. But it is in fact an error since an observation merely reveals how the correlation should be interpreted, and the result has to be signaled to the distant location to do so  that is the meaning of nonlocal system.danR said:I don't know how many times and how many ways this misapprehension of entanglement has been displayed in popscience articles.
No property changes state, instead a state is observed. And while observation may seem odd, we have to remember that special relativity has "special" effects such as time dilation and length shortening. There is a paper that show observation "wavefunction collapse" stochastic behavior can be seen simply as a third "special" effect, of wavefunctions having quantum spin under relativistic reference frames.
et al. Answering Mermin’s challenge with conservation per no preferred reference frame. Sci Rep 10, 15771 (2020). https://doi.org/10.1038/s41598020728177 https://www.nature.com/articles/s41598020728177.pdf]
This is science, so unlike fusion power there is no master plan that we can observe delay against. But see my comment that there is a reasonable theory, but that popular science claims doesn't go into that.Unclear Engineer said:"Quantum gravity theory" seems to be on the same schedule for completion as "commercial fusion power".
Atlan0001 said:There is no "singularity" to gravity physics, so there is no quantum physics' (EW) pointparticle singularity  and the many varieties of complex singularities  to it!
Yes, there is a quantum physics, there is no problem to quantize the general relativity Lagrangian same as the other particle fields. See my own response to the article for references.
And the gravitational particle is the graviton, the force exchanging quanta of the field, same as other quantum fields, it is not the singularities of the theory. A theoretical singularity only means the theory has lost validity there, nothing more.