There’s been quite a lot of noise about an alleged discovery of time travel recently. Many news sites have come up with articles with big titles about the wonderful Russian time machine. The contents tend to be more accurate, telling what their device actually could do – reversing the state of a quantum system a very short period of time back in the time axis, but who cares about the contents, titles are what people read.
Why it’s not time travel
In a typical science fiction story about time travel, the hero uses a time machine, usually explained of using some technobabble, to transport himself into a past version of the universe around him. The time machine usually also allows to travel into the future as well, although that isn’t very different from some kind of artificial sleep without the possibility to travel back.
The critical ability of a typical time machine is to transport matter into a past version of the universe, turning the world into a system where present state isn’t only determined by past states, but also by future states.
A famous property of a time machine is that it generates paradoxes. What happens if you travel into the past to kill your grandfather long enough before your father was born, will you die or disappear (as you have no cause), will you become a person with no past or will you fail no matter what you try and find out you accidentally caused him to meet your grandmother and fall in love with her. There was also an idea that you become something like the Schrödinger’s cat, both alive and unborn, and one of those possibilities will be selected at random
Does the Russian device have this ability? No. It could perfectly restore a few particles into their past state. In other words, they could undo some changes the system had undergone over time. That’s very cool, but it has nothing to do with time travel. The time machine would have to do this with everything except the time traveller, which is somewhat an unsolvable issue of scale.
Does the Russian device generate paradoxes? No. The step that was rolled back took place in an enclosed system. If you were inside it (there’s quite a difference between a few particles and a human, but let’s put that aside), others would have observed having done something with you but you would not. Cool, but not time travel.
Why is it cool anyway
All small scale physical processes are reversible in time. Many equations depend on time, but none requires the time to go forwards. Schrödinger’s equation is a sum of three terms, one depends on position (or speed, speed is a derivative of position anyway), second depends on time, third depends on potential. If you reverse the potential (possible) and the position (also possible) and multiply the equation by minus one, you get an equation describing a system that develops in time similarly to the previous one, but moving as if the time was flowing backwards.
But there is one law that usually prevents this. Enthtopy. Systems never become more organised over time, only less organised. If you mix hot coffee with cold coffee, the result has the expected temperature and will not randomly filter out the particles of the hot water from the particles of cold water. You can cause warm particles to receive heat from cold particles using some kind of fridge or air conditioner, but it requires an energy source.
Enthropy is often used to describe the state of a system’s disorder. And the second law of thermodynamics says that enthropy in a closed system can only increase (it must be closed, otherwise a mere house with an air conditioner would violate this law. Enthropy is often used to define what is a positive change in time and what is a negative one.
Because enthropy cannot decrease, heat (which is a typical case of disorder on molecular scale) cannot be destroyed, only taken away. Energy can be spent on making objects warmer, but it cannot be transformed back to usable energy. Making usable energy from the difference of temperatures is possible, that’s what makes combustion engines in cars work.
And this experiment has decreased the enthropy in a system. It has, in some way, converted heat back to usable energy. And that could have great applications if it can be expanded to more common systems. And that is awesome.