# One Of Einstein's Greatest Regrets Has Turned Out To Be Useful After All

We've all been there: you figure out a problem, then second-guess yourself only to figure out later that if you'd gone with your first hunch, you'd have been right all along. Albert Einstein was no different. His general theory of relativity was a breakthrough in our understanding of the universe, but his equations predicted something he didn't think could be right. So he changed them. Turns out that his original equations were right on the money, but what he added to them — what's known as the cosmological constant — has been useful all the same.

Reining In The Universe

In 1915, Einstein published his general theory of relativity. Its earth-shattering conclusion was that space and time are not absolute, but flexible, dependent on one another, and affected by the presence of matter. At the core of this great masterpiece are what's known as the Einstein field equations. They lay out the relationship between the curvature of spacetime and the amount of matter and energy moving nearby.

When Einstein applied those equations to the entire universe, something was fishy: they predicted that the universe had to be either expanding or contracting. But Einstein, and everybody else, "knew" that they lived in a static, unchanging universe. His equation had to be wrong. To fix them, he added a symbol — Λ, or lambda — now known as the cosmological constant. It effectively put a limit on how much the universe could change by balancing the push and pull of its competing forces. Like a Vegas casino, it stacked the odds in favor of the house. But the universe doesn't care about the odds.

You're My Favorite Mistake

In 1929, Edwin Hubble announced that his study of nearby galaxies (plus some number crunching by a now-forgotten priest) showed that — oops! — the universe was actually expanding. At this point, Einstein is famously quoted as saying that adding the cosmological constant was his "biggest blunder". He probably never did (and anyway, we're willing to bet he felt a lot worse about suggesting the U.S. build an atomic bomb than adding an extra symbol to an otherwise monumental discovery) but it was certainly a big regret.

But hold on, Albert! All is not lost. Physicists haven't thrown out that little lambda; they've just repurposed it. See, the value Einstein gave the cosmological constant made it so the universe neither expanded nor contracted. But change that value, and you can show that the universe changes size again. Today, the cosmological constant describes the energy density of empty space, or basically the force that's making the universe expand. We still don't know what that force is, or why the universe's expansion is accelerating. But on our road to understanding those mysteries, the cosmological constant will be there to help — right where Einstein put it.

To get a better understanding of everybody's favorite genius, check out the biography "Einstein: His Life and Universe" by Walter Isaacson. The audiobook is free with a trial of Audible.

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