It is easy to take time's arrow for granted - yet the gears of physics work just as smoothly in reverse. Perhaps that time machine is possible after all?
An experiment earlier in 2019 shows just how much wiggle room we can expect regarding distinguishing the past from the future. That, at least on a quantum scale. It may not allow us to relive the '60s, but it could help us better understand why not.
Researchers from Russia and the United States teamed up to find a way to break, or at least bend, one of the most fundamental laws of Physics on energy.
The second law of thermodynamics is less a hard rule and more of a guiding principle for the Universe. It says that hot things get colder over time as energy transforms and spreads out from areas where it's most intense.
It is a principle that explains why your coffee will not stay hot in a cold room, why it is easier to scramble an egg than unscramble it, and why no one will ever let you patent a perpetual motion machine.
It is also the closest we can get to a rule that tells us why we can remember what we had for dinner last night, yet have no memory of next Christmas.
Virtually every rule in physics can be flipped and still make sense. For instance, you could zoom in on a game of pool, and a single collision between any two balls will not look weird if you happened to see it in reverse.
However, if you watched balls roll out of pockets and reform the starting pyramid, it'd be a sobering experience. This is the second law at work for you.
On the macro scale of omelets and games of pool, we should not expect a lot of give in the laws of thermodynamics. As we focus in on the tiny gears of reality, though, loopholes appear.
Electrons are not like tiny billiard balls, but more akin to information which occupies a space. Their details are defined by the Schrödinger equation, which represents the possibilities of an electron's characteristics as a wave of chance.
If that's a bit confusing, let's go back to imagining a game of pool, but this time the lights are off. You begin with the information in your hand, and then send it rolling across the table.
The Schrödinger equation tells you that ball is somewhere on the pool table moving around at a certain speed. In quantum terms, the ball is everywhere at a bunch of speeds … some just more likely than others.
You can stick your hand out and grab it to pinpoint its location, but now you're not sure of how fast it was going. You could also gently brush your finger against it and confidently know its velocity, but where it went... who knows?
There is one other trick you could use, however. A split second after you send that ball rolling, you can be reasonably sure it is still near your hand moving at a high rate.
The Schrödinger equation predicts the same thing for quantum particles in one sense. According to Science Alert, the possibilities of a particle's positions and velocities expand over time.
It is as if your cue ball was no longer spreading out in a wave of infinite possible positions across the dark table, but rewinding into your hand.
In theory, nothing is stopping it from occurring spontaneously. You'd need to stare at ten billion electron-sized pool tables every second and the lifetime of our Universe to see it happen once, however.
Instead of patiently waiting around and watch funding trickle away, the team used the undetermined states of particles in a quantum computer as their pool ball, and some smart manipulation of the computer as their 'time machine'.
Each of those states, or qubits, was arranged into a simple state which corresponded to a hand holding the ball. Once the quantum computer was set into action, these states rolled out into a range of possibilities.
By tweaking particular conditions in the computer's setup, these possibilities were confined in a way that effectively rewound the Schrödinger equation deliberately.
To test that, the group relaunched the set-up, as if kicking a pool table and watching the scattered balls rearrange into the original pyramid shape. In about 85% of trials based on just two qubits, that's exactly what happened.
The algorithms used to manipulate the Schrödinger equation into rewinding in this way could practically help improve the accuracy of quantum computers.
Finding ways to push the limits of such physical laws on the quantum scale may help us understand why the Universe 'flows' like it does.
The research was published in Scientific Reports.
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