If there’s one thing that eats up time, it’s working out what time is. It pops up in physical laws all over the place – but never quite as we expect it. In quantum theory, a “master clock” ticks away somewhere in the universe, measuring out all processes. But in Einstein’s relativity, time is distorted by motion and gravity, so clocks don’t necessarily agree on how it is passing – meaning any master clock must, somewhat implausibly, be outside the universe.
Even odder, neither theory seems to place any restriction on time going backwards. The familiar one-way flow of time is expressed in only one area of physics: thermodynamics. If time flowed both ways, sometimes your coffee would warm up while sitting forgotten on your desk. Dropped eggs might spontaneously reassemble and leap from the floor into your hand. The dead might return to life and live backwards to birth, Benjamin Button style.
In physics, things are as they are – until they’re not. Here are five cosmic impossibilities that just might turn out to be true
The culprit is entropy, essentially the thermodynamic measure of a system’s disorder. When the universe was born, matter was randomly distributed throughout its tiny dimensions and it was the same temperature everywhere. Then gravity kicked in, pulling together matter and heated it up to form galaxies, stars, planets and other ordered imperfections. Thermodynamics has been trying to re-establish disorder, increasing entropy every which way it can.
At a local level, entropy increase seems to be associated …