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Arrow of Time

February 16, 2016

A popular phrase these days is “going forward” or “moving forward.” “Going forward, our team must play better,” or “Moving forward, I think I’ll have the pancakes.” It implies that the speaker somehow has a choice — that he or she could move backward instead of forward. But the universe doesn’t work that way. As far as we know, time moves in only one direction — it follows the “arrow of time.”

That’s something we see in both our everyday lives and in the universe as a whole. We can pour milk, flour, and eggs in a bowl, then mix them up to make pancake batter. But we can’t unmix the batter to make milk, flour, and eggs. In the universe, a massive star explodes as a supernova, but its remnants don’t spontaneously come back together to make a massive star.

That concept is known as entropy, which says that everything evolves from neat and orderly to messy and disorderly. So a set of billiard balls starts out in a neat arrangement, then scatters around the table as you play the game — never the other way around.

Scientists spend a lot of time trying to explain why time’s arrow moves in only one direction. Most of the laws of physics work fine whether time moves forward or backward. And in most cases, when the laws of physics allow something, it happens. There are several possible explanations, but, so far, nothing that’s stuck.

So going forward, there’s still a lot of work to do to explain the one-way arrow of time.

More about time tomorrow.

Script by Damond Benningfield, Copyright 2015

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