By David K. Johnson, Ph.D., King’s College
Time travel is a staple of science fiction, and it’s been conceived of in many different ways. Given that science fiction has inspired so many real technological advances, one can legitimately wonder if it is actually possible. Read on to learn about the idea of quantum entanglement and the quantum eraser that implies time travel.
Quantum Leap and Travel to the Past
In the show called Quantum Leap, Dr. Samuel Beckett is able to travel in time, but only into the years of his own past. He trades places with past persons, like Lee Harvey Oswald, but is seen by others as that person. Once he has set right what once went wrong—for example, according to the show, Sam was the Secret Service agent who saved Jackie Kennedy—he ‘leaps’ into another body.
Although the show does not explain much about why he leaped into the bodies he did, there are handwaving explanations to explain how. As the title suggests, it has something to do with quantum mechanics. But it probably has nothing to do with actual quantum leaping. That’s when an electron moves from one orbit of an atom to another without passing in-between—it just appears. And that’s cool, but that’s not going to make time travel.
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But there are elements of quantum mechanics that might. Take quantum entanglement, for example. If two particles are entangled, their properties must always match—or, in the case of spin, be exactly opposite. If one is a wave, the other must be a wave. If one particle becomes spin up, its entangled particle must become spin down. And this happens even if one particle takes on a property while the other is light-years away. It’s as if they can send instantaneous signals to each other.
Now, people can’t actually send information via these signals because, although particles can be made to take on properties by measuring them, what properties they take can’t be controlled. For example, an electron can be forced to take on a spin by making it pass through a detector, but whether it takes an ‘up’ spin or a ‘down’ spin, as a result, can’t be dictated. According to quantum mechanics, a particle’s spin is assigned at random.
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But, suppose that aspect of quantum mechanics is wrong, and it could be controlled? Well, then the properties of the paired particle could be controlled too, even though it is light-years away. For an ‘up’ spin, the nearer particle would be made to have a property of ‘down’ spin.
So, for a person to communicate with, say, their mother across light-years—they’d simply need to do the following: first, work out a system of communication. They could, for example, use Morse code, and use ‘up’ for dot and ‘down’ for dash.
Second, arrange for a stream of particles near them to be entangled with a stream of particles near their mother; and then third, make the particles near them take on the up or down properties necessary to send their message.
Now, that’s a lot of ifs, but science fiction is built on ifs. And if they are granted, and with a little bit of squinting, it might be possible to see how the time travel in Quantum Leap could work. Because, a few years after Quantum Leap went off the air, something called a quantum eraser showed that entanglement not only holds across long distances but also backward in time.
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The Quantum Eraser
The quantum eraser was first tested by Yoon-Ho Kim. Roughly speaking, in a quantum eraser, two entangled photons are sent through a device: photon A to a detector, and photon B through a series of 2-way mirrors. Now, photons are waves but can collapse down into particles. In the eraser, if photon B randomly takes a certain path, it becomes a particle at the end of its journey. If it randomly takes another path, it stays a wave.
If this is done over and over, and all the A photons that were entangled with photons that stayed waves were taken, those A photons will be seen to hit their detector as a wave—they create an interference pattern.
However, the A photons paired with B photons that became particles will hit their detector as a particle. That’s what should happen; quantum mechanics demands that paired particles must have the same properties.
The kicker is this: Each A photon hits its detector long before its B partner finishes its journey. Even before the B photon reaches its first 2-way mirror, the A photon has already hit its detector. It is as if the A photons know what path their partner will take and how they will end up before they get there. Or, at least in some interpretations of quantum mechanics, it is as if, once the B photon finishes its journey, it sends a signal back in time to let its partner A photon know whether it should be a wave or particle.
Entanglement in Quantum Leap
So, if Dr. Sam Beckett has developed a way to dictate what properties quantum particles take on, and then quantum entangles the particles of his brain with the brain of someone in the past, then he could potentially control their actions. Perhaps that’s why he can only travel to a past date within his own lifetime; there must be an earlier version of his brain nearby at that past time to facilitate the entanglement.
That something like this is going on is suggested by the fact that Sam’s trusty companion, Al, appears to Sam as a hologram that only he can see and hear. Al is transmitting himself into the visual and auditory centers of the brain that Sam controls. Now, that explanation is not entirely scientifically sound, but it sounds fun.
In any case, it seems that quantum entanglement is a possible method of time travel, at least at the level of photons.
Common Questions about Quantum Entanglement and Quantum Erasers
If two particles are entangled, their properties must always match—or, in the case of spin, be exactly opposite. If one is a wave, the other must be a wave. If one particle becomes spin up, its entangled particle must become spin down.
According to current scientific knowledge, the properties of quantum particles can only be observed; the change in properties of quantum particles is random and can’t be controlled.
A quantum eraser is an experiment where two entangled photons are sent through a device. It appears to demonstrate that entangled particles can send information back in time.