What is time?

“What’s done cannot be undone.” Look to Shakespeare for a great quote. He had Lady Macbeth murmur these simple but profound words to herself. Who does not wish they had done something differently? But the past is past. A broken teacup will not put itself back together. A dissolved sugar cube will not reassemble itself.

Physics at its deepest level is about understanding space, time, matter and energy. Four distinctly different things? Einstein showed that matter is a form of energy. Mass can be converted into energy in the form of heat in the sun or a nuclear reactor, and kinetic energy can be converted to new matter particles in an accelerator. In the same theory, the theory of relativity, Einstein showed that space and time form a unity: space-time. Different, because we can move forward and backward in space – in all three directions – but only forward in time.

via GIPHY. What’s done cannot be undone — unless you are a subatomic particle (or your consciousness runs backward)!

In his wonderful story “The Time Machine,” published in 1895 when Einstein was a schoolboy, H.G. Wells wrote, “There are really four dimensions, three which we call the three planes of Space, and a fourth, Time. There is, however, a tendency to draw an unreal distinction between the former three dimensions and the latter, because it happens that our consciousness moves intermittently in one direction along the latter from the beginning to the end of our lives.”

Wells anticipated Hermann Minkowski who, praising Einstein’s theory in 1908, said, “From henceforth space in itself and time in itself sink to mere shadows, and only a kind of union of the two preserves an independent existence.” This is our stage: space-time, and 300,000 kilometers of space equals one second of time. That’s the speed of light in empty space, and when particles approach that speed, space and time get mixed up.

In the tiny world of particles, time is can even seem to go backward. Physicist Richard Feynman said that an antielectron (a positron) can be treated like an electron going backward in time. A movie of two particles scattering looks the same in reverse.

A particle called a muon decays into an electron with an average lifetime of about two microseconds. But in a particle accelerator at Fermilab, muons travel at 0.9994 times the speed of light and live 30 times longer (by our clocks). This is called “time dilation.”

Sorry, I didn’t answer the question “What is time?” But time is different for particles, teacups and consciousness. Food for (deep) thought.

This is a version of an article that appeared in Positively Naperville.