“Anyone who has never made a mistake has never tried anything new.” –Albert Einstein
Back when Einstein first proposed his theory of General Relativity, his revolutionary picture of the Universe was met with a mix of curiosity, awe, and intense skepticism. It isn’t every day that your most cherished of all physical theories — the theory of Newtonian Gravity that had ruled the cosmos for nearly two-and-a-half centuries — gets challenged by a newcomer.
And yet, that’s exactly what Einstein did when he proposed General Relativity at the end of 1915, nearly a century ago. Newtonian gravity, according to Einstein, was just an illusion. Objects didn’t really exert gravitational forces on one another, which in turn caused accelerations/changes in momentum, but rather the entire Universe existed in a framework known as spacetime, and the presence of matter-and-energy curved the fabric of that spacetime, causing objects to move as they do.
Einstein’s theory not only reduced to Newtonian gravity when gravitational fields were weak, it also predicted the orbital anomaly of Mercury, something that had puzzled astronomers and physicists alike for nearly 50 years. When the 1919 eclipse was observed, and distant starlight was observed to have bent in agreement with General Relativity (and not in agreement with any interpretation of Newton’s laws), our picture of the Universe was revolutionized.
Before any of this happened, however, Einstein was very much bothered by an aspect of his theory. You see, it was assumed at the time that the Universe was made up of stars, whose distribution was relatively uniform throughout space. This was furthermore assumed to be stable, and not something that had either changed much with time or that was likely to change into the far future. The stars were assumed to be long-lived, and evenly distributed around us in all directions.
In general, this type of solution presented a grave problem for Einstein: it is an unstable solution! If you have a roughly (but not perfectly) uniform distribution of matter, then spacetime is going to curve due to the presence of that matter. And once spacetime is curved, those regions with slightly more matter than others are going to preferentially attract more and more matter, and will grow over time!
What’s even worse is that the fate of all such configurations of mass like this, regardless of what shape they start off in, wind up creating a black hole!
This clearly isn’t the case for our Universe! And Einstein knew this wasn’t the case for our Universe, so what was actually happening?
The laws of gravity weren’t lying, but there must’ve been something that wasn’t properly accounted for. As far as Einstein could tell, stars pretty much stayed where they were over time, and extended out maybe on the order of thousands of light-years in all directions. Because they weren’t all collapsing towards a point or region, Einstein reasoned that there had to be something fighting gravity on these large, interstellar scales.
He proposed that there was an intrinsic energy to space itself, a cosmological constant, responsible for this. This cosmological constant would push back with exactly the force needed to counteract gravity on these large scales, and would lead to the Universe being static.
Now, we can fast-forward almost 100 years, to our modern picture of the Universe.
The Universe is not, in fact, static, but has been expanding for billions of years. What Einstein missed is that our Universe extends far beyond our own galaxy, and in fact contains many hundreds of billions of galaxies comparable to our own. This wasn’t discovered observationally until years after General Relativity was proposed, so Einstein could hardly be faulted, and yet he was frustrated at himself for not finding the solution in General Relativity that admits an expanding Universe. Perhaps apocryphally, he’s credited with calling his introduction of the cosmological constant his “greatest blunder.”
Had he found the solutions later found by Friedmann, Lemaitre, Robertson and Walker, he might have proposed that the Universe was expanding, and never suggested the ad hoc cosmological constant at all.
And yet, since the late 1990s, we’ve realized that the Universe does in fact have a non-zero cosmological constant: that’s what we call dark energy, and use to explain the accelerated expansion of the Universe!
You might think that, because the cosmological constant does turn out to exist, and be non-zero, and because there is an intrinsic energy to space itself, that perhaps Einstein didn’t make a mistake after all.
Nothing could be further from the truth. In physics, we propose novel theoretical mechanisms to both explain observed phenomena and to predict new, hitherto unobserved phenomena. That’s what theoretical physics is all about.
And I hate to break it to you, but Einstein’s cosmological constant utterly failed on both of those counts.
Not only did he not successfully explain why the stars in our galaxy remain in a roughly stable configuration — because they’re in quasi-stable orbits around the galaxy — but he also failed to predict the phenomena of the expanding Universe.
Had he gone with the expanding Universe solution instead of the cosmological constant solution to the problem of a Universe that hadn’t yet collapsed into a black hole, that would’ve been correct.
Einstein, to his great credit, was smart enough to admit to himself, and to the world, that his solution was not the right one.
Even today, looking back and recognizing that there is, in fact, a cosmological constant / dark energy component to the Universe, Einstein was still wrong!
It isn’t enough to get the right answer in physics, or in science in general. You need to get the right answer for the right reasons, otherwise you are doomed to lead yourself astray.
The cosmological constant may have come back, but it has nothing to do with the reasons Einstein proposed for its existence, nor is it of anywhere near the same magnitude that Einstein suggested. Sometimes old ideas come back in new forms to solve new puzzles.
Why do I tell you this? Because it’s tempting to revise history, to make our heroes even more heroic and to give them credit for discoveries that they themselves did not make. It’s also all too easy to fool ourselves, and to discount our own actual mistakes because there was a somewhat-related success down the road.
It’s okay to be wrong; being wrong is evidence that you were trying, and also evidence that you were honest with yourself. The important thing is to get it right in the end. We’re going to be wrong about an awful lot of things going forward; of that I’m certain.
What will separate those of us who are good scientists about it will be our willingness to let go of ideas that no longer agree with the data, admit we were wrong, and embrace the theoretical ideas that are in accord with what we observe. We may even wind up reviving old ideas and finding new ways that they apply to our Universe as we learn more about it. (It doesn’t mean the old ideas were right all along, though!)
This is science, where every day we come a little closer to getting it right. Thanks for coming along on the journey with me.