A new theory suggests that the unification between quantum physics and general relativity has eluded scientists for a hundred years because huge ‘fluctuations’ in space and time mean gravity doesn’t follow quantum rules.
Since the early 20th century, two revolutionary theories have defined our fundamental understanding of the physics that governs the universe. Quantum physics describes the physics of the small, on a scale smaller than the atom, and tells us what fundamental particles look like electrons and photons interact and are controlled. General relativityon the other hand, describes the universe on a huge scale and tells us how planets move around stars, how stars can die and collapse until birth black holesand how galaxies cluster together to build the largest structures in the cosmos.
Since their development, these two theories have become more robust and have strengthened science with their enormous success. Quantum mechanics has shown that the quantum world is full of counterintuitive aspects, such as the existence of systems that are in contradictory states at the same time or of particles that influence each other instantly, even at opposite ends of the universe. General relativity has now revealed that the fabric of spacetime is formed by the matter on it, and that violent interactions between large-mass bodies can cause ripples in space-time, known as gravitational waves who can travel for billions light years to wash over Soil.
Yet there is a problem: a dark cloud hangs over these disciplines: now that these two pillars of physics have been perfected, scientists are still unable to bridge the gap between them.
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“The two pillars of modern physics are inconsistent with each other, meaning there is a fundamental contradiction underlying our laws of physics,” Professor Jonathan Oppenheim, University College London (UCL), told Space.com via email .
Oppenheim is the pioneer of a new and radical theory that could finally bring these two concepts together, a reconciliation that has defied the greatest scientific minds for more than 100 years.
Why would gravity be ‘quantum’ at all?
Previously, reconciling general relativity with quantum physics meant taking space-time – the three dimensions of room and the one dimension of time, unified as a single 4D entity that underlies general relativity – breaking it down into discrete units, or ‘quanta’.
This requires space-time to be a passive stage on which the ‘action’ takes place the universe is set. However, general relativity depends on space-time not being a static stage, but rather a dynamic player in the cosmic ballet of the universe, shaped by the presence of matter and energy and then telling matter and energy how to move via the curvature and gravity arising from It.
Oppenheim’s idea of ’waving space-time’ is based on the question why gravity should have a quantum nature as discovered for the others of the universe fundamental forces — electromagnetism, and the strong and weak nuclear forces. Gravity, he argues, is not like these other forces. After all, it is the only one of the four that can define the geometry of space-time itself, and the fields of quantum physics evolve based on this geometry.
“We feel gravity because matter causes space-time to bend. Time flows at different locations at unequal speeds,” Oppenheim writes in an article discussing his theory, published in the journal Physical Assessment X. “The speed at which time flows and its causal structure [the fact that cause always precedes effect] it may provide a classical description necessary to properly formulate quantum theory.”
This means, according to his theory, also called a ‘post-quantum theory of classical gravity’, that space-time, and therefore gravity, has no quantum description. This is due to random fluctuations in space-time, which cause changes in the passage of time, breaking the concept of predictability.
“In both quantum gravity and classical gravity must cause space-time to undergo violent and random fluctuations all around us, but on a scale we have not yet been able to detect,” says Zach Weller-Davies, a researcher at the Perimeter Institute of Theoretical Physics in Canada. and a former doctoral student of Oppenheim, said in a statement.
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Betting on ‘wavy space-time’ versus string theory and quantum loop gravity
Of course, Oppenheim’s idea is far from the first theory proposed to bridge the gap between quantum physics and general relativity.
String theory is one of the most well-known concepts proposed to unite these two pillars of physics. In short, it views the particles that fill the universe as manifestations of one-dimensional vibrating objects called strings. String theory describes how these strings propagate through space and interact with each other, creating a particle called a graviton, which carries gravity, thus providing a quantum account of this fundamental force.
A popular alternative unification theory to string theory is gravity in the quantum loopwhich changes the formulation of general relativity to quantify space-time into chunks.
Quantum loop gravity proponent Carlo Rovelli and string theory proponent Geoff Penington doubt Oppenheim’s post-quantum theory of classical gravity. Their skepticism forms the basis of a 5,000-to-1 bet with Oppenheim, which would be paid out if an experiment devised by Weller-Davies and other former PhD students failed. students at the UCL scientist to measure a mass very precisely to see if it appears to fluctuate over time – confirming the post-quantum theory of classical gravity.
This skepticism is a two-way street for Oppenheim, as he doubts the validity of string theory and quantum loop gravity as accurate solutions to the problem of quantum physics and general relativity. This is because these other ideas require adding additional ingredients to the universe, including extra dimensions, that the post-quantum theory of classical gravity does not require.
“It is unclear to me whether any of the current approaches consistently reconcile quantum theory and general relativity,” Oppenheim said. ‘If string theory succeeds, extra dimensions are needed supersymmetryneither of which we have observed in nature, although it is still possible that they are there.”
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He added that unitary theories, such as string theory and loop quantum gravity, also seem to require breaking Einstein’s famous equivalence principle. This refers to the equality between two forms of mass: gravitational mass, experienced when standing on a body like the Earth, and inertial mass, experienced in an accelerating frame of reference. Oppenheim explained that this breaking of equivalence is necessary to reconcile these theories with the ‘information paradox about a black hole”, which asks where the information carried by matter swallowed by black holes goes.
In addition to testing the fluctuation of a solid mass due to the post-quantum theory of classical gravity, Oppenheim and his former students are looking for aspects of nature that could emerge and further support this new theory.
“We have shown that if space-time does not have a quantum character, then there must be random fluctuations in the curvature of space-time that have some signature that can be verified experimentally,” said Zach Weller-Davies. ‘If space-time is classical, the fluctuations must be larger than a certain scale, and this scale can be determined by another experiment where we test how long we can sustain a heavy load. atom in superposition of being in two different locations.”