UTA scientists test for quantum nature of gravity
Research at the south pole studied the mysterious quantum structure of space and time
UNIVERSITY OF TEXAS AT ARLINGTON
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ICECUBE LAB UNDER THE STARS IN ANTARCTICA. CREDIT: MARTIN WOLF, ICECUBE/NSF
view moreCREDIT: CREDIT: MARTIN WOLF, ICECUBE/NSF
Einstein’s theory of general relativity explains that gravity is caused by a curvature of the directions of space and time. The most familiar manifestation of this is the Earth’s gravity, which keeps us on the ground and explains why balls fall to the floor and individuals have weight when stepping on a scale.
In the field of high-energy physics, on the other hand, scientists study tiny invisible objects that obey the laws of quantum mechanics—characterized by random fluctuations that create uncertainty in the positions and energies of particles like electrons, protons and neutrons. Understanding the randomness of quantum mechanics is required to explain the behavior of matter and light on a subatomic scale.
For decades, scientists have been trying to unite those two fields of study to achieve a quantum description of gravity. This would combine the physics of curvature associated with general relativity with the mysterious random fluctuations associated with quantum mechanics.
A new study in Nature Physics from physicists at The University of Texas at Arlington reports on a deep new probe into the interface between these two theories, using ultra-high energy neutrino particles detected by a particle detector set deep into the Antarctic glacier at the south pole.
“The challenge of unifying quantum mechanics with the theory of gravitation remains one of the most pressing unsolved problems in physics,” said co-author Benjamin Jones, associate professor of physics. “If the gravitational field behaves in a similar way to the other fields in nature, its curvature should exhibit random quantum fluctuations.”
Jones and UTA graduate students Akshima Negi and Grant Parker were part of an international IceCube Collaboration team that included more than 300 scientists from around the U.S., as well as Australia, Belgium, Canada, Denmark, Germany, Italy, Japan, New Zealand, Korea, Sweden, Switzerland, Taiwan and the United Kingdom.
To search for signatures of quantum gravity, the team placed thousands of sensors throughout one square kilometer near the south pole in Antarctica that monitored neutrinos, unusual but abundant subatomic particles that are neutral in charge and have no mass. The team was able to study more than 300,000 neutrinos. They were looking to see whether these ultra-high-energy particles were bothered by random quantum fluctuations in spacetime that would be expected if gravity were quantum mechanical, as they travel long distances across the Earth.
“We searched for those fluctuations by studying the flavors of neutrinos detected by the IceCube Observatory,” Negi said. “Our work resulted in a measurement that was far more sensitive than previous ones (over a million times more, for some of the models), but it did not find evidence of the expected quantum gravitational effects.”
This non-observation of a quantum geometry of spacetime is a powerful statement about the still-unknown physics that operate at the interface of quantum physics and general relativity.
“This analysis represents the final chapter in UTA’s nearly decade-long contribution to the IceCube Observatory,” said Jones. “My group is now pursuing new experiments that aim to understand the origin and value of the neutrinos mass using atomic, molecular and optical physics techniques.”
JOURNAL
Nature Physics
METHOD OF RESEARCH
Observational study
SUBJECT OF RESEARCH
Not applicable
ARTICLE TITLE
The IceCube Collaboration. Search for decoherence from quantum gravity with atmospheric neutrinos
Revealing the quantumness of gravity
Proposed experiment shows that quantum entanglement is not the only way to test whether gravity has a quantum nature
Gravity is part of our everyday life. Still, the gravitational force remains mysterious: to this day we do not understand whether its ultimate nature is geometrical, as Einstein envisaged, or governed by the laws of quantum mechanics. Until now, all experimental proposals to answer this question have relied on creating the quantum phenomenon of entanglement between heavy, macroscopic masses. But the heavier an object is, the more it tends to shed its quantum features and become ‘classical’, making it incredibly challenging to make a heavy mass behave as a quantum particle. In a study published in Physical Review X this week, researchers from Amsterdam and Ulm propose an experiment that circumvents these issues.
Classical or quantum?
Successfully combining quantum mechanics and gravitational physics is one of the main challenges of modern science. Generally speaking, progress in this area is hindered by the fact that we cannot yet perform experiments in regimes where both quantum and gravitational effects are relevant. At a more fundamental level, as Nobel Prize laureate Roger Penrose once put it, we do not even know whether a combined theory of gravity and quantum mechanics will require a ‘quantisation of gravity’ or a ‘gravitisation of quantum mechanics’. In other words: is gravity fundamentally a quantum force, its properties being determined at the smallest possible scales, or is it a ‘classical’ force for which a large-scale geometrical description suffices? Or is it something different yet?
It has always seemed that to answer these questions, a central role would be played by the typically quantum phenomenon of entanglement. As Ludovico Lami, mathematical physicist at the University of Amsterdam and QuSoft, puts it: “The central question, initially posed by Richard Feynman in 1957, is to understand whether the gravitational field of a massive object can enter a so-called quantum superposition, where it would be in several states at the same time. Prior to our work, the main idea to decide this question experimentally was to look for gravitationally induced entanglement – a way in which distant but related masses could share quantum information. The existence of such entanglement would falsify the hypothesis that the gravitational field is purely local and classical.”
A different angle
The main problem with the previous proposals is that distant but related massive objects – known as delocalised states – are very challenging to create. The heaviest object for which quantum delocalisation has been observed to date is a large molecule, much lighter than the smallest source mass whose gravitational field has been detected, which is just below 100 mg – more than a billion billion times heavier. This pushed any hope of an experimental realisation decades away.
In the new work, Lami and his colleagues from Amsterdam and Ulm – interestingly, the place where Einstein was born – present a possible way out of this deadlock. They propose an experiment that would reveal the quantumness of gravity without generating any entanglement. Lami: “We design and investigate a class of experiments involving a system of massive ‘harmonic oscillators’ – for example, torsion pendula, essentially like the one that Cavendish used in his famous 1797 experiment to measure the strength of the gravitational force. We establish mathematically rigorous bounds on certain experimental signals for quantumness that a local classical gravity should not be able to overcome. We have carefully analysed the experimental requirements needed to implement our proposal in an actual experiments, and find that even though some degree of technological progress is still needed, such experiments could really be within reach soon.”
A shadow of entanglement
Surprisingly, to analyse the experiment, the researchers still need the mathematical machinery of entanglement theory in quantum information science. How is that possible? Lami: “The reason is that, although entanglement is not physically there, it is still there in spirit — in a precise mathematical sense. It is enough that entanglement could have been generated.”
The paper in which Lami and colleagues explain their findings was published in Physical Review X this week. The researchers hope that their paper is only the beginning, and that their proposal will help design experiments that may answer the fundamental question about the quantumness of gravity much earlier than expected.
Publication
Testing the quantum nature of gravity without entanglement, Ludovico Lami, Julen S. Pedernales and Martin B. Plenio, Physical Review X.
JOURNAL
Physical Review X
METHOD OF RESEARCH
Experimental study
SUBJECT OF RESEARCH
Not applicable
ARTICLE TITLE
Testing the quantumness of gravity without entanglement
ARTICLE PUBLICATION DATE
1-May-2024
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