Space and time have been cynosures of science at least since Einstein published his theory of general relativity in 1915, transforming them from a passive stage for the play of matter into a riveting headliner of the entire production. From the Off-Broadway venue of science, they leaped into headline news in 1919 with Eddington's confirmation during a solar eclipse that they bend, stretch and twist, taking matter and light along for the ride. The New York Times headline of November 10 read: "Lights all askew in the heavens: Men of science more or less agog over results of eclipse observations."
Space and time capture the imagination precisely because they engender, and also imprison, our imagination. Imagine a holiday in Hawaii or a new design for a car, recall the wedding of a dear friend, contemplate the last moments of Custer's last stand, and in each case space and time are your helpful, even essential, partners. But then try to imagine a world of four dimensions—up/down, forward/backward, left/right and, say, nim/zur. No one succeeds. Our partner turns jailor and straightjackets the imagination. Now try two dimensions of time, or no time at all. The straightjacket tightens.
In 1926 a brash talent debuted. Quantum theory can, in special cases, get on well with space and time, and the result of their collaboration is the standard model of particle physics that successfully describes the electromagnetic, weak and strong nuclear interactions and their associated subatomic particles. But when the density of matter is too large or the distance of interaction is too small the collaboration breaks down and quantum theory, it now appears, can upstage its costar.
Hints of the breakdown surfaced in 1935 when Einstein, Podolski and Rosen observed that, according to quantum theory, measurement of the quantum state of one particle can instantly change the state of another particle entangled with it, no matter how distant in space. Entanglement cannot transmit information faster than light. Nevertheless its insouciance about space and time deeply troubled Einstein.
The breakdown splashed front and center in string theory. Nobel Laureate David Gross observed, “Everyone in string theory is convinced...that spacetime is doomed. But we don't know what it's replaced by.” Fields medalist Edward Witten also thought that space and time may be “doomed.” Nathan Seiberg of the Institute for Advanced Study at Princeton said, "I am almost certain that space and time are illusions. These are primitive notions that will be replaced by something more sophisticated."
The good news is that sophisticated replacements might be on the way.
One new candidate is entanglement itself. Brian Swingle and Mark Van Raamsdonk found that curved space-times obeying Einstein’s theory of general relativity can emerge from tensor networks of entangled quantum bits. On this scenario, the insouciance of entanglement is feigned. Entanglement itself is somehow the fabric that holds space-time together.
Another new candidate is a class of geometric constructions outside of space and time, including the amplituhedron discovered by Nima Arkani-Hamed and Jaroslav Trnka. Subatomic particles collide and scatter in a multitude of ways, and physicists have for decades had formulas for computing their probabilities, formulas that assume physical processes which evolve locally in space and time. But, as it happens, these formulas are unnecessarily complex and hide deep symmetries of nature. The amplituhedron simplifies the formulas, exposes the symmetries hidden by space-time and, in the process, abandons the assumption that space and time are fundamental.
What is fundamental, if not space and time? No one is yet sure. The prime suspect is quantum information—quantum bits and quantum gates. But quantum information viewed abstractly, not as embedded in space-time. Space-time and objects somehow emerge from non-spatial and non-temporal dynamics of quantum information. As John Wheeler put it, “It from bit.” But this raises its own questions. Why should information, quantum or otherwise, be the bedrock of reality? And in what sense is it information?
It may be premature to write the obituary of space and time. The report of their death might be an exaggeration. But either way, dead or alive, it will be news that is important and lasting. Whether space and time prove fundamental or not, the proof itself will bring in its wake new and deep insights into the nature of reality, and perhaps also into the nature of our own imagination.
I suspect that the report of their death is not an exaggeration. This will raise new questions for researchers in perceptual psychology. Why have our perceptual systems evolved to present us a world in the format of space and time if, as Seiberg says, space and time are illusions, primitive notions that will be replaced by something more sophisticated? What selection pressures favored the ascendancy of this primitive format? What fitness advantages does it confer?
The standard assumption in perceptual psychology is that evolution favors veridical perceptions, those that accurately describe those aspects of the environment that are crucial to the fitness of an organism. It is not standard to assume that the very space-time format of our perceptions is itself non-veridical, primitive and illusory. How will this field have to change if space and time are themselves illusions? And how will our notions of physical causality have to change? Will these changes affect how we approach the classic mind-body problem, the question of how our conscious experiences are related to our physical bodies and, in particular, to the activity of our brains?
Such questions make clear that the stakes are high. The grand entrance of space and time a century ago made world headlines. Their denouement will be no less riveting.