If you believe what you read on the internet, it’s been an exciting 24 hours for quantum physics.
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The headlines have been incredible. Newsweek (Scientists Have Reversed Time in a Quantum Computer), Discover (Scientists Used IBM’s Quantum Computer to Reverse Time, Possibly Breaking a Law of Physics) and the UK’s Independent newspaper (Scientists ‘Reverse Time’ With Quantum Computer in Breakthrough Study). Cosmopolitan magazine also chimed in: Scientists just turned back time and it’s like Back to the Future is coming true. There are many, many more.
The trigger for all of these was a Scientific Reports paper with the provocative title “Arrow of time and its reversal on the IBM quantum computer.” In it, the authors claimed to have performed an experiment that opens up lines of research, in their words, toward “investigating time reversal and the backward time flow.”
If you had difficulty understanding how scientists accomplished such a counterintuitive feat, don’t worry. They didn’t.
Some simple physical models are symmetric in time. Think of an idealized version of the Earth orbiting the sun, where each is a perfect sphere. Look at that system going forward in time, and the Earth orbits in a clockwise direction. “Reverse” time and instead the Earth will travel in a counterclockwise orbit. Both are equally realistic. Or think of two billiard balls colliding. You can run the video in either direction and it still seems physically plausible.
The real world is not that way. Things look different going forward in time from how they would were time reversed—in a number of different ways, among them that entropy (very loosely speaking, a measure of disorder) increases. This is a law both of physics and of common sense. (For a fun and sad exploration of how strange reversing the flow of time would make things, check out Time’s Arrow by Martin Amis. And if you really want to get into the weeds on the physics of time travel, try here.)
So if they didn’t invent time travel, what did these scientists actually do?
Think about pressing rewind on a video. That “reverses the flow of time,” in a way. If you’ve never seen it before, it’s kind of neat. It might let you see things—like steam flowing back into a tea kettle or Humpty Dumpty spontaneously assembling from a jumble of broken pieces—that appear to “reverse the arrow of time.” The paper in question describes a quantum-computing version of such a video running in reverse.
A closer analogy is a lens, like what one would find in a telescope, a microscope, or eyeglasses. A lens can be used to focus light—“reversing” the dispersal of light that had gone out of focus. The authors of the paper, from the Moscow Institute of Physics and Technology, Argonne National Laboratory in Illinois, and ETH Zurich, say their technique might be useful for testing quantum programs. This is correct. But it’s a lot less interesting than a time machine.
As Scott Aaronson, director of the Quantum Information Center at the University of Texas at Austin, says, “If you’re simulating a time-reversible process on your computer, then you can ‘reverse the direction of time’ by simply reversing the direction of your simulation. From a quick look at the paper, I confess that I didn’t understand how this becomes more profound if the simulation is being done on IBM’s quantum computer.”
Other quantum computing experts we spoke to agreed. One, who did not wish to be named, said: “I don’t know how useful this is … it doesn’t mean that these guys made a time machine. They certainly didn’t violate the laws of thermodynamics or the laws of physics.” He added: “This is the type of hype that is going to give quantum computing a bad name.”
He’s right. Wild headlines don’t just give quantum computing a bad name. They do damage to science as a whole by convincing the public that science is so bewildering it’s beyond their comprehension. It’s tough enough to explain the paradoxes that actually exist in quantum mechanics without sensationalist embellishment. Time, whether any of us likes it or not, marches on.
This article was originally published under the headline: "No, IBM didn’t just 'reverse time' with a quantum computer." While this is accurate, the headline was modified to reflect the fact that though the researchers in question used publicly-available IBM quantum computing facilities, they are not affiliated with the company.
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June 11-12, 2019