One way to do this is to use a network of amplifiers to switch signals as desired. This is pictured below: a signal entering port A of the device should be directed to port B a signal entering at B should go to port C and a signal entering port C should be directed to port A, clockwise around the device.Ī simple representation of a circulator. There are various applications where engineers want electromagnetic signals (such as light or radio waves) in a circuit to behave a bit like cars around a roundabout. It complicates the diversion of electronic and radio signals around a circuit. Microscopic reversibility presents an important technological challenge. How does this irreversibility emerge from microscopic laws that are reversible? This remains a mystery. This is obvious in our everyday experience: a scrambled egg is not reversible. At the macroscopic scale, the entropy of the universe – a measure of disorder or randomness – always increases, so that there is an arrow of time. Unlike Doc Brown, we can’t make the clock tick backwards. Since the microscopic laws of physics appear to be unchanged under this mathematical transformation, we say the universe possesses time-reversal symmetry, even though we cannot actually reverse time in reality. This mathematical transformation reverses the flow of time in our equations. Indeed, the basic equations of physics look essentially the same if we replace time with its negative. A beam of light travelling in one direction obeys exactly the same laws of physics as a beam of light travelling in the opposite direction. The collision of two billiard balls looks pretty similar in reverse even more so for the collision of two atoms. Credit: Thomas Stace/ The Conversationīut at a microscopic level, the story is not that clear. Golf doesn’t look so convincing in reverse. This might seem obvious: people don’t usually walk or talk backwards spilt milk doesn’t spontaneously jump back into its carton a golf ball doesn’t miraculously launch backwards from the fairway, landing perfectly balanced on the tee at the same moment as the club catches it. You could then ask: “If I edited the movie to run backwards, and showed it to my friends, could they tell?” Suppose you make a movie of an event occurring. Time reversal symmetry is a complex sort of symmetry that physicists like to think about, and relies on the imaginary as much as the real. There are many kinds of symmetry in science, including one that deals with time reversal. How did we do it? Well, it’s all to do with symmetry. While we can’t send a DeLorean car back in time, we hope it will have important applications in communication technology and quantum computing. We’ve now developed our own kind of flux capacitor, as detailed recently in Physical Review Letters. The technology that allowed Marty McFly to travel back in time in the 1985 movie Back to the Future was the mythical flux capacitor, designed by inventor Doc Brown.
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