Scientists working on a joint U.S.-Russia project say they have managed to reverse time, at least on a quantum scale. But don’t start packing your bags for that trip back to your senior prom just yet. The accomplishment occurred on the tiniest of scales, and a number of leading physicists doubt that anything significant happened at all.

Time and the second law of thermodynamics

Most principles in physics work in reverse as easily as they do going forward. For instance, we all know that if you send light through a prism it will separate into its wavelengths and a rainbow will emerge on the other side. It’s right there on the cover of Pink Floyd’s Dark Side of the Moon. But, if you do it precisely enough, you can also start with the rainbow, send it back through the prism, and get the beam of light.

However, the second law of thermodynamics, which admittedly isn’t completely understood, says that the energy of our universe, which begins in small pockets, will tend to spread out over time so that rather than pockets of high energy we’re left with a kind of smear of low energy everywhere. Consider, for example, your first hot steaming cup of coffee in the morning. While it begins in high energy – heat – if you allow it to sit for any length of time, that heat will cool, loses its energy as that energy escapes into the room. This process of spreading out is called entropy. And this particular process doesn’t seem to work in reverse, at least in the macro, large-scale world we humans inhabit.

And this law seems to be intimately connected to how time behaves. Suppose, for instance, that you crack an egg into a frying pan. The liquidy egg and yolk quickly leave the broken shell and spread out over the pan. Then perhaps you scramble the egg as well. What would it take to reverse those processes, to unscramble the egg and get all the material back into the shell precisely as it was? Now imagine that problem on a scale infinitely larger and more complicated, the scale of molecules, atoms, and electrons. Think of your scrambled eggs not just as a pile of breakfast, but as millions, billions, trillions of tiny particles, all of which you’d have to deal with to get the egg back into the shell.

Now go one step further and consider that reversing time would involve not just getting that single egg to go back into its shell, but everything in the known universe, every single particle, to take a step back. The law of thermodynamics suggests that would make time-reversal infinitely difficult, or, put another way, impossible. Once the egg of our universe begins to spread out over its pan, it’s simply not possible to undo that spread.

Some question recent headlines

Scientists at Moscow’s Institute of Physics and Technology, however, believe they have managed to accomplish the feat, but only by working on the tiniest of scales. By reducing the problem down to single quantum particles, they claim they were able to reverse entropy. One of the problems of entropy is that when we break the egg we cannot predict where the particles will go as they spread. But using mathematics, the team discovered that they could make reasonable guesses about where two particles would be in the first tiniest fraction of a moment after the egg cracked, and then put those two particles back precisely where they came from.

There are a couple of catches to the team’s experiment, though. To pull off their trick, they had to work on incredibly tiny scales – two particles, which had moved for only one ten-billionth of a second. That’s hardly likely to result in any Back to the Future style trips, whether or not you can get your hands on a flux capacitor.

And other quantum physicists say what this team pulled off really couldn’t be called an actual time reversal. According to these scientists, what happened was more akin to running the rainbow back through the prism to get light. That is, they may have moved the particles back to where they originally were, certainly no small feat, and a significant breakthrough that could help us better understand quantum computing. But turning the rainbow back into light doesn’t reverse time. It simply allows us to undo what we did, recombine pieces we had previously split apart. And that, they say, certainly doesn’t constitute time travel.

So for now at least, despite the flurry of exciting headlines, we’ll have to continue to live in the only time we can access, the present.