The Physics of Time

Posted on August 09, 2013 by Richard Hoptroff

Borough Market, London, 9th Aug 2013

A Monmouth coffee, seated on the market steps in the sunshine.  An apple falls off a trader's cart.  Why did it fall?

We can't define beauty, but we know it when we see it.  We can’t say what consciousness is, but we know what we mean by it.  Time is just as hard to define, and just as easy to grasp, though quite useful, day-to-day.  It helps us to describe change. 

Time measures change.  An apple changes position as it flies through the air.  A seed changes to sprout branches, then leaves, then fruit.  A pendulum swings back and forth.  We can judge how fast things change by comparing one change with another.  We can create standard measures - seconds based on the pendulum or the rattling of a caesium atom, years by the height of the sun, just as rulers measure length.

Unlike other measures, time has a curious sense of direction.  We seem to pace forward along its ruler at a constant rate and we can't go back.  Or maybe it’s simpler say the present is a window that future events queue up to plunge through to get to the past.  But in any event, it’s a one-way street, a real thing you can’t fight against.  You can add cream to your coffee easily enough, but try separating them again afterwards.

The arrow of time has its advantages, the most important being working out what caused what.  If I had a cold before I left the house today, I can’t have caught it after I left.  That may sound obvious, but when you try to study complex interactions such as the causes of global warming, it is often the most powerful tool we have.  If we were in the habit of zipping back in time and changing the past, causality would fall apart, and physics would be in even more of a mess than it is. 

Don't get me started on loop quantum gravity.

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