The University of Pennsylvania is part of a collaboration of physicists and computer scientists from the United States, Canada, the United Kingdom, Israel, Argentina and Japan to investigate a radical new idea about the fundamental nature of reality.
The collaboration, which is funded by the Simons Foundation, is called “It from Qubit,” and aims to find out if a subtle property of quantum information, measured by “quantum bits” or “qubits,” gives rise to the structure of space and gravity.
Vijay Balasubramanian, a professor of physics in the School of Arts & Sciences at Penn, is a principal investigator in the collaboration, which is directed by Patrick Hayden of Stanford University.
“Over the last half-dozen years,” Balasubramanian said, “there has been a motley crew of people from all over the world who have been pursuing an idea that the nature of space is determined by an underlying quantum mechanical reality that shared quantum information leads to connections in physical space and that gravity can emerge from the dynamics of information.”
The name of the collaboration is an allusion to an essay by physicist John Archibald Wheeler who coined the term “black hole.” In an essay written in 1989, Wheeler asked the question, “How come existence?” and proposed that “…every physical quantity, every it, derives its ultimate significance from bits, binary yes-or-no indications, a conclusion which we epitomize in the phrase, it from bit.”
It from Qubit is striving towards what has been the Holy Grail of modern physics for decades: a theory of quantum gravity.
In the early part of the 20th century, Albert Einstein revolutionized the field of physics with his theory of general relativity. Gravity, Einstein explained, was not simply a force as had previously been thought but actually resulted from a warping of space and time.
Around the same time, there was another revolution in physics. Physicists realized that at the shortest distances, the fundamental nature of reality is quantum mechanical, which means that it is full of laws that allow strange, counterintuitive behaviors, such as a particle existing in multiple places at the same time or randomly fluctuating between locations.
One of the strangest phenomena in quantum mechanics is something called entanglement. On a macroscopic scale, entanglement can be thought of in terms of correlation, the tendency of things to happen together. For example, as Balasubramanian explained, the colder it is outside, the more likely a person is to wear a jacket. In other words, temperature and wearing a jacket are correlated.
When two systems are entangled in quantum mechanics, it means that they are correlated in a very special way so that if there is manipulation or measurement of one of the systems, it can have the property of measuring the other system at the same time. This is not just a theoretical idea, scientists are actually working on harnessing this phenomenon in order to share quantum bits of information, or qubits, in a practical way between separated receivers.
During the last few years, Balasubramanian said, physicists have proposed “a beautiful new idea that large amounts of quantum entanglement can lead to physical connections between different locations in space and that gravity can arise from the warping of this entanglement by flows of energy.”
This idea originally started with the realization that one can sometimes consider a physical theory in a certain number of dimensions – say, in three dimensions – and show that it is exactly equivalent to the description of a universe with more dimensions.
“There can be physical situations where some of the dimensions of space are fictions,” Balasubramanian said. “They're emergent; they're not ‘really there.’ That raises the question of what it means for a dimension of space to be really there.”
In these so-called dualities between lower dimensional and higher dimensional theories, he said, it appears that the extra dimensions of space and gravity emerge from the effects of quantum entanglement.
Examples that theoretical physicists have solved suggest a dramatic general idea: Reality itself may be constructed from qubits, and what is actually there is simply the information about things, so that thinking about continuous space and its organization is merely a way of conceptualizing the information relationships between quantum events.
“A really sharp question along these lines is the so-called black hole information paradox that Stephen Hawking articulated,” says Onkar Parrikar, a Penn postdoc also involved in the collaboration. “Hawking proposed that any information that enters a black hole is basically destroyed forever, something that violates the rules of quantum mechanics. For many years now people have been trying to figure out whether information can be recovered from inside black holes. We now think that the resolution of this paradox may lie in the quantum entanglement of the interior and exterior of black holes and the way in which this entanglement effectively connects the inside and the outside.”
Balasubramanian agrees that understanding this problem will lead to a fuller understanding of where the universe came from and to the fundamental nature of reality.
“Great civilizations seek to understand the place of human beings in the universe and to organize thinking about who we are, where we are and what the universe is like,” he said. “Questions about quantum gravity are really questions about the nature of the universe, and all humans since the beginning of sentience have cared about this kind of thing. These are the great questions of existence.”
According to Parrikar, one the most exciting aspects of this collaboration “is the sheer number of people who are talking to each other.”
In addition to Penn, there are researchers from Brandeis University, the Institute for Advanced Study, University of Maryland-College Park, University College London, the Hebrew University of Jerusalem, the Perimeter Institute, the University of British Columbia, McGill University, the Yukawa Insitute of Kyoto University, the Instituto Balseiro, the University of California, Santa Barbara, the University of Texas at Austin, Princeton University, Stanford University, the Massachusetts Institute of Technology and the California Institute of Technology. The larger the number of people working together, Parrikar said, the larger the pool of ideas.
Also, the “It from Qubit” project is a collaboration of people from two different scientific communities: computer science and physics.
“It's a really amazing confluence of intellectual directions,” Balasubramanian said. “There are people who want to think about the nature of space and time and the origin of the universe and people who want to think about the nature of information. It's a really interesting and beautiful idea that being ‘it’ comes out of information ‘qubit.’ And it’s astonishing to see this idea incarnated as a physical, mathematical theory that may have a concrete realization in the world.”