Some quantum systems may have two arrows of time, one running forwards as usual and another moving backwards. This means that, at some extremely small scales, time may have the option of moving in both directions 鈥� a stunning feature that may have been overlooked across much of physics.
If you consider the most basic equations of quantum physics 鈥� those that deal with single particles 鈥� there is no reason why time should always run forwards. This means fundamental quantum laws have what is known as time reversal symmetry. But for larger systems, such as collections of particles, the laws that physicists rely on to predict and explain their behaviour do distinguish between past and present.
鈥淵ou go from a situation where you definitely have time reversal symmetry to an end result where you clearly don鈥檛 have time reversal symmetry,鈥� says at the University of Surrey in the UK. 鈥淚 was just very curious, at which particular step [of a calculation] does that happen? Where do you kill the symmetry?鈥�
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Now, he and his colleagues have shown that the symmetry can be allowed to live on.
They analysed quantum systems that are free to interact with their environment. Past calculations suggested that eventually some heat would 鈥渓eak鈥� out of such systems and enter the environment in a way that couldn鈥檛 be reversed. This could be used to designate a definite 鈥渂efore鈥� and 鈥渁fter鈥� and an arrow of time聽that points from one to the other. But all of those calculations were done under the assumption that a single arrow of time was the only option, says team member , also at the University of Surrey.
鈥淚f you start from the question of 鈥榳hat if I flip time鈥�, then the equations that describe the motion of your system of interest are slightly different from the ones that you find in [a standard] textbook,鈥� he says.
Specifically, the equations that his team derived after dropping this assumption allow there to be one arrow of time moving backwards and one moving forwards. The researchers found this to be true for several kinds of systems. These include those that are subject to randomly fluctuating forces and those that often randomly switch their states 鈥� as seen, for instance, when particles randomly change direction, as they do when they undergo Brownian motion. Though they haven鈥檛 yet developed a rigorous mathematical proof for it, the researchers believe that the emergence of the opposing time arrows could be a very general phenomenon.
at Butler University in Indiana says that the new work brings fresh light to聽the controversy over when time reversal symmetry applies. 鈥淚 think this is one of the biggest problems in physics. And they are suggesting that equations that people wrote for [time]-irreversible processes were missing one factor. Once they incorporate this factor, then these equations also do not distinguish between the future and the past anymore,鈥� he says.
Working with these new equations, which are time reversible, may also have cosmic consequences, says Ordonez. For example, they could lend credence to past ideas about there being two universes after the big bang, each evolving in an opposite direction through time, he says.
Rocco is interested in those ideas too, but cautions against overinterpreting the new work. 鈥淲e might have found something so fundamental that it comes up again and again, but we are very far from starting to understand it rigorously,鈥� he says.
Scientific Reports
