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This image shows an experiment in which a heavy particle (illustrated as a moon) causes an interference pattern (a quantum effect) and simultaneously bends space-time. The hanging pendulum represents the measurement of space and time. Actual experiments are usually performed using carbon-60, one of the largest molecules known. The UCL calculations indicate that the experiment should be performed using denser atoms, such as gold. Two of his other images represent his two experiments proposed by the UCL group, both of which constrain theories in which spacetime is classically treated. One is mass measurement and the other is interference experiment.Credit: Isaac Young
A radical theory that consistently integrates gravitational mechanics and quantum mechanics while preserving Einstein’s classical space-time concepts has been published in two papers published simultaneously by physicists at UCL (University College London). Announced.
Modern physics is built on two pillars. One is quantum theory, which governs the smallest particles in the universe, and the other is Einstein’s general theory of relativity, which explains gravity through the bending of space-time. However, these two theories of his contradicted each other, and reconciliation could not be achieved for more than a century of his life.
It is a common assumption that Einstein’s theory of gravity needs to be modified, or “quantized,” to make it compatible with quantum theory. This is an approach based on his two leading candidates for quantum theory of gravity: string theory and loop quantum gravity.
However, a new theory developed by Professor Jonathan Oppenheim (UCL Physics and Astronomy) and published in a 2016 paper, Physical Review X, They challenge consensus and alternative approaches by suggesting that spacetime may be classical, that is, not governed at all by quantum theory.
This theory, called a “post-quantum theory of classical gravity,” does not modify spacetime, but rather quantum theory, predicting an essential breakdown in the predictability mediated by spacetime itself. The result is random, violent fluctuations in spacetime that are larger than expected in quantum theory, and when measured accurately enough, the apparent weight of an object becomes unpredictable.
The second paper, published at the same time nature communications Led by Professor Oppenheim’s former Ph.D. Students consider some of the results of the theory and propose experiments to test it. It’s about measuring mass very precisely to see if that weight changes over time.
For example, France’s International Bureau of Weights and Measures routinely weighs masses of 1 kg, where 1 kg was previously the standard. If the variation in the measurements of this 1 kg mass is less than what is required for mathematical consistency, then the theory can be ruled out.
The results of an experiment, or new evidence supporting the quantum and classical nature of space-time, are subject to a wager of 5000:1 odds between the two. Professor Oppenheim, Professor Carlo Rovelli, Dr. Jeff Pennington—Major proponents of quantum loop gravity theory and string theory, respectively.
For the past five years, the UCL research group has been stress-testing the theory and investigating the results.
Professor Oppenheim said, “Quantum theory and Einstein’s general theory of relativity are mathematically inconsistent with each other, so it is important to understand how this contradiction is resolved. Or should we revise quantum theory, or is it something else entirely?” Now that we have a consistent basic theory that spacetime is not quantized, no one can guess it. ”
Co-author Zach Weller-Davis is a Ph.D. The UCL student helped develop the experimental proposal and made an important contribution to the theory itself, he said, adding: “This discovery not only questions our understanding of the fundamental nature of gravity, but also challenges its potential. It also provides an avenue to investigate quantum properties.”
“We showed that if spacetime does not have quantum properties, then there should be random fluctuations in the curvature of spacetime with certain features that can be verified experimentally.
“In both quantum and classical gravity, space-time must be experiencing violent random fluctuations around us, the magnitude of which we have not yet detected. But if space-time is classical , the fluctuation must be larger than a second.” This scale can be determined by another experiment that tests how long heavy atoms can be left superimposed in two different locations. . ”
Co-authors Dr. Carlo Sparaciari and Dr. Barbara Shoda say that analytical and numerical calculations are helping guide the project, and that these experiments can determine whether pursuing quantum gravitational theory is the right approach. He expressed his expectations.
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Mass metric – The experiment proposed by the UCL group constrains theories in which spacetime is treated classically.Credit: Isaac Young
Dr Shoda, formerly a UCL Physics and Astronomer and now at the Perimeter Institute for Theoretical Physics in Canada, said: The flow of time has a quantum or classical property.
“And testing this is almost as easy as testing whether the weight of a mass is constant or appears to vary in a certain way.”
Dr Spalasciari (UCL Physics and Astronomy) said: ‘Although the concept of the experiment is simple, the weighing of the object has to be carried out with great precision.
“But what I find interesting, starting from very general assumptions, is the magnitude of space-time fluctuations and how long an object such as an atom or an apple can be placed in a quantum superposition of two different locations. It is the ability to prove a definite relationship between two measurable quantities. . . . You can then determine these two quantities experimentally.”
Professor Weller-Davis continued, “If quantum particles such as atoms can bend classical spacetime, then subtle interactions must exist.The wave nature of atoms and the magnitude of random fluctuations in spacetime There must be a fundamental trade-off between how large the size has to be.” . ”
The proposal to test whether spacetime is classical by looking for random fluctuations in mass was aimed at verifying the quantum nature of spacetime by looking for something called “gravitationally mediated entanglement.” It complements another experimental proposal.
Professor Sougat Bose (UCL Physics and Astronomy), who was not involved in today’s presentation but was one of the first to propose the entanglement experiment, said: “Experiments that test the properties of spacetime require large-scale efforts.” But “this is very important from the point of view of understanding the fundamental laws of nature. I believe that these experiments are within reach, even though it is difficult to predict these things. , we will probably know the answer within the next 20 years.”
Post-quantum theory has implications beyond gravity. Quantum superpositions are necessarily localized through interaction with classical spacetime, so there is no need for quantum theory’s notoriously problematic “measurement postulate.”
This theory was motivated by Professor Oppenheim’s attempt to solve the black hole information problem. According to standard quantum theory, an object that enters a black hole cannot destroy information, so it should somehow be radiated back out, but this violates general relativity and the black hole’s event horizon It is said that we can never know about objects that cross the New theories can destroy information because predictability is fundamentally broken.
For more information:
A post-quantum theory of classical gravity? Physical Review X (2023). Journal.aps.org/prx/accepted/ … 584bc2567e68f9f76c1e.upon arXiv: DOI: 10.48550/arxiv.1811.03116
Jonathan Oppenheim et al., Gravity-induced decoherence versus space-time diffusion: A test of the quantum nature of gravity, nature communications (2023). DOI: 10.1038/s41467-023-43348-2. www.nature.com/articles/s41467-023-43348-2