Tuesday, October 4, 2011

The red queen of cosmology and astrophysics

 Charles Dodgson -- aka Lewis Carroll -- was a very clever guy. I could give many examples, but today I'm thinking of the Red Queen's race in Through the Looking Glass:
"Well, in our country," said Alice, still panting a little, "you'd generally get to somewhere else — if you run very fast for a long time, as we've been doing."

"A slow sort of country!" said the Queen. "Now, here, you see, it takes all the running you can do, to keep in the same place. If you want to get somewhere else, you must run at least twice as fast as that!"
 Why think about Alice's adventures?

Because of the Physics Nobel just awarded to Adam Riess, Saul Perlmutter and Brian Schmidt for discovery of the accelerating expansion of the universe.

Why is the universe expanding at an increasing rate, despite the pull of gravity to slow things down? No one knows, but we have a name for our ignorance: dark energy.

Einstein sort-of predicted this effect. Before Hubble's Law and Big Bang Theory, the universe was believed to be static and eternal. So: to explain why the universe had not collapsed on itself, Einstein's Theory of General Relativity includes a term called the cosmological constant that works against gravity.

Mathematically this factor was simply a constant of integration of unknown value -- and so it looked like a kludge. After the discovery of the expanding universe and the rise of Big Bang Theory, it became common to set the cosmological constant to zero. Einstein called the cosmological constant his "biggest blunder."

The cosmological constant (which is a mathematical description, not a physical explanation) is now a leading contender for the nature of dark energy. Even Einstein should not bet against Einstein.

Lapsing from physics to philosophy ...  humanity is constantly reminded by science how little we know. In less than a century cosmology and astrophysics have gone from trying to explain a static, eternal universe, to trying to explain how the expanding universe born in a Big Bang might escape either flying apart or collapsing again, to trying to explain why the universe is accelerating in its expansion.

Along the way, we lost our smug assurance (or should have) that we understand the nature of the universe. The stuff that stars and humans are made of represents only a few percent of the universe's total mass/energy. Dark energy and dark matter (a label for another aspect of our ignorance) predominate.

And what is familiar matter, that modest few percent of the universe that is like us? Do we even understand that?

It's been only a century since Ernest Rutherford postulated that the atom -- previously considered to be the smallest, and indivisible, component of the chemical elements -- consisted of electrons and a small, positively charged nucleus. (He first split the atom in 1917.) In due time, physicists further learned that nuclei are comprised of proton and neutrons, and that protons and neutrons are comprised of quarks held together by gluons.

Can we be sure that quarks aren't made of something yet smaller? String theory (without a shred of proof, to be sure, merely beautiful math) supposes that apparently pointlike electrons and quarks are vibrational modes of one-dimensional strings too small to be detected using the most powerful particle accelerator.

Bringing me back to the Red Queen race. The more science learns, the more we should understand just how little we know.

And likewise bringing me to Clarke's First (and unjustly lesser known) Law:
When a distinguished but elderly scientist states that something is possible, he is almost certainly right. When he states that something is impossible, he is very probably wrong.
Consider the exciting news that scientists at CERN measured neutrinos moving faster than light. A future Physics Nobel, I do not hesitate to predict -- if the experiment is substantiated. Because if it is substantiated, physicists will -- yet again -- have overturned a core conclusion of a century's standing.

Which will prevail this time? Clarke's First Law or Einsteinian relativity?


Erik said...

Regardless of the truth of the matter, I agree with Einstein when he says that the cosmological constant was a blunder. He invented the constant to explain something that didn't need explaining. We had a physics that predicted a non-static universe, which conflicted with our previous notions. He modified the model to produce a static universe, simply because a static universe is what he/we wanted.
Since this was before we knew the universe was accelerating in its expansion, there was no reason for a cosmological constant at the time.
I think it's worth remembering that making a correct prediction does not mean that you made the smart prediction. Even psychics and horoscopes are right sometimes.

Regarding the neutrinos

So do you think we will acquire a complete theory of physics in the next 50 years?

Gary Baker said...

Fascinating post. My teenage daughter asks, "Could the dark energy counteracting the pull of gravity be life itself?"
"Because," she adds, "life sucks."

Edward M. Lerner said...

Hi Erik,

First of all: that's a great cartoon.

And thanks for your thoughtful comment about the origins of the cosmological constant. My interpretation of the CC differs from yours. I believe that the CC emerges naturally from solving the equations of general relativity; the debate over the decades was whether the constant's unknown value could be defined as zero. That the (then current) evidence for a static universe pointed Einstein in the right direction is (IMO) immaterial. His "worst blunder" comment was, in fact, the blunder -- uncharacteristically, he lost faith in his own insight.

And to your question: I'd be astonished if physics in 50 years isn't radically different in some way(s) than what is understood today. (Though not nearly as surprised as that I'm around in 50 years to be surprised.)

- Ed

Edward M. Lerner said...


Your daughter is wise ...

- Ed