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(Erica Stefano / The McGill Tribune)

The universe at odds: Quantum mechanics versus general relativity

Science & Technology by

For over a century, the field of theoretical physics has been in a perpetual state of quandary. In recent weeks, following noted physicist Stephen Hawking’s death, popular media has turned the spotlight onto the unsolved mysteries of physics. With physicists searching for the next steps to advance the field, the question of “Where do we go from here?”persists.

Until the revolutionary discoveries of quantum mechanics and relativity that occurred at the turn of the 20th century—primarily through the work of physicists like Max Planck and Albert Einstein—human understanding of the guiding forces of the universe were limited to classical, or pre-1900, models. Classical mechanics deals with the forces that influence motion, and is based largely on Newtonian principles.  Einstein’s Nobel Prize-winning paper, written in 1905 and awarded in 1921, discussed how light is emitted and transformed. His discoveries marked a major paradigm shift that advanced knowledge beyond the scope of classical physics. From there, the field of quantum mechanics, the branch of physics that deals with the behaviour of atomic particles, was born.

Alongside quantum mechanics, a second modern, or post-1900, physical principle that Einstein called “general relativity” emerged. This theory provided a unified description of gravity, describing natural phenomena on a much larger scale than quantum mechanics at the level of orbiting planets and galaxies. General relativity gives an accurate description to space-time, postulating that large objects distort both space and time, creating the effect that we feel as gravity.

As one of the large objects that distort space-time, the sun, as theorized by Einstein, creates a ‘dip’ in the universal membrane of space, pulling Earth, along with the rest of the solar system, into the void it creates.

“[Einstein] reinterpreted the theory of gravity in a very radical way,” McGill Professor of Physics Alexander Maloney said in an interview with The McGill Tribune. “He proposed that rather than thinking of gravity as originating from a gravitational field, one should attribute the existence of gravity to the curvature of space-time itself.”

Einstein took a fundamental principle, challenged it, and provided a theory that was radically divergent from Newtonian mechanics, yet remains the most accurate model considered by physicists.

His second theory of quantum mechanics describes the behaviour of light as both particles and as waves, applying physics at the smallest of scales, which in today’s world has broad applications, such as in quantum computing and modern electronics.

Einstein’s legacy left two completely justified and scientifically provable theories. However, when viewed together, the quantum mechanical model does not completely align with general relativity. This is where the fundamental dilemma arises: A major disagreement exists between physics’ two most important frameworks.

Relativity treats objects as point particles that exist as independent masses in time and space. Quantum mechanics, however, treats matter as wave functions that do not possess positions as point particles do, but are probability distributions. Relativity’s predictions produce definite outcomes, whereas quantum mechanics’ predictions produce probabilistic ones. As a result, applying relativity to objects of the scale at which quantum mechanics operates fails to produce sensible answers.

Robert Brandenberger, a professor in the department of Physics at McGill, completed his post-doctoral research under Hawking. Brandenberger now works on the cosmological aspects of string theory, a postmodern theory that thinks of particles not as definite points, but as one-dimensional ‘strings’ which propagate through space-time in constant interaction with one another.

“For matter to be described quantum mechanically, then gravity must be described quantum mechanically as well.” Brandenberger said.

If both quantum mechanics and relativity work independently, then they also have to work in unison. Here lies the scientific grey area physicists face when trying to integrate quantum mechanics with relativity: They simply do not function properly.

“It is not that they oppose one another, but general relativity has a limited range of applicability,” Brandenberger said. “Newtonian [classical] mechanics describes point particles very well except if you go down to very small scales.”

The same drawback applies to quantum mechanics on a larger scale: It describes, with great accuracy, the inner workings of subatomic particles, but fails to precisely address particle properties in the grand scheme.

“When you include quantum mechanics, you get corrections to Einstein’s original equation,” Professor of Theoretical Cosmology Jim Cline told the Tribune. “These corrections are very small when talking about the everyday applications of gravity, but at short distances and high energies the corrections that come from quantum mechanics become very big and are infinitely many.”

He explained that the data required to discern a unified theory are incredibly, and maybe impossibly, hard to collect using current research methods.

“The theory itself becomes un-predictive,” Cline said. “Scientists do not like that.”

However, black holes in the outer-reaches of the universe may provide the answer to unifying these two theories, an argument Hawking himself supported in his hypothetical ‘theory of everything.’

“If you want to consider the physics of something very massive, that is also very small, you would need to understand both general relativity and quantum mechanics simultaneously,” Maloney said.

The density of black holes is so great that nothing, not even light, can escape their immense pull. If a black hole can exert gravitational effects on large masses like planets in the same way that it can pull in light—which has a mass of almost zero—then an explanation of the phenomena of black holes would, in theory, reveal how large, macroscopic particles can interact with tiny, nanoscopic ones.

Hawking came extremely close to breaking through the quantum mechanic-relativity barrier with his work on string theory and Hawking radiation, a type of radiation that is emitted from black holes, proving that if a black hole doesn’t gain mass over time, it will shrink and disappear. Although Hawking radiation is too small to be observed, it remains an important discovery that allows physicists to peer into what unification may look like.

“The unification of general relativity with quantum mechanics would allow us to make [great] progress in our understanding [of the early universe],” Brandenberger said.  

Since the universe is constantly expanding, physicists believe that in the distant past it was extremely small in size.

“It would allow us to understand the beginning of the universe and would also allow us to probe black holes, to see what they look like on the inside,” Brandenberger said.  

Currently, Canadian research on string theory and quantum-gravity is centred around McGill, the University of British Columbia (UBC), and the Perimeter Institute in Waterloo, Ontario, where Hawking was a Visiting Research Chair. Other initiatives like the Large Hadron Collider (LHC) near Geneva, Switzerland, which can strike particles together at immense speeds, is managed by the European Organization for Nuclear Research (CERN). Institutes such as these are beacons of the hope that, one day, even the most baffling of physical principles may be uncovered.

It was not until 2016 that a group of scientists at the California Institute of Technology (Caltech) were able to record two black holes colliding, which affirmed Einstein’s supposedly ‘unprovable’ theory 100 years after its original proposal.

The scientific community may still be decades away from unearthing the secrets of the distant universe. As of today, there remains no conclusive answer to whether or not relativity and quantum mechanics can be unified, but there exists a firm beginning to finding what that solution may be.

While a unified theory is the end goal, the best scientific theories are the ones that don’t just explain phenomena, but provoke further questions: Questions that can continue scientific discovery on a path that may be as infinite as space-time itself.

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