Doctor Who’s blue box that is bigger on the inside has a myriad of buttons and levers that could take you forwards, backwards, left right and just about any other direction in time. As much as everyone would want to park one of those in their driveway, time only goes one way in this universe.
Thank the second law of thermodynamics for our traveling ever onward into the future. This is the reason why you can’t press the rewind button on life, because it states that systems will always succumb to more disorder — entropy — as time passes. But someone just pressed it. A team of researchers using a quantum computer have recently been able to perform some sci-fi calculations that reversed the effects of time for subatomic particles.
Quantum mechanics bend the rules of physics as we know them because they apply to the subatomic scale, where things that normally don’t happen (and sometimes shouldn't) do. Systems exist in a state known as a wave function, which expresses every possible state of a system, even where it is if you’re dealing with infinitesimal particles, and how likely it is that the system is in any one of those states at any given time. It explains how another team of scientists were able to prove that a particle can exist in two states at once.
The thing about wave functions is, they spread out more and more as more time elapses, meaning a particle could wander off further and further away. Trying to reverse that is like un-spilling the Doctor’s tea at an unfortunate moment when the buttons that go back in time aren’t functioning.
You need an extreme amount of control to get the tea back in the cup. Quantum computers provide that kind of control.
Using a quantum computer, the researchers simulated a single subatomic particle and then wrote an algorithm that reversed all components of its wave function, kind of like rewinding a scene in a movie where something that spilled going forwards appears to magically clean up when the digital stream goes backwards. What is really incredible is that they managed to do this without increasing entropy anywhere or in anything surrounding the particle. Could this be the smallest TARDIS ever?
So it’s not exactly a Gallifreyan time machine, but this experiment still didn’t break the rules of physics, because there is a loophole in the second law of thermodynamics. While it does say that entropy will inevitably happen over time, it never mentions that disorder keeps creeping up on us nonstop. But you can’t get much more complicated with it. Introducing another particle only screwed the system up.
Looks like time machines aren’t anywhere in our near future, so just try to convince people that the odd blue thing sitting around in your garage actually works.