Tuesday, March 25, 2014

Physics with a Bang

Modern physics is on a roll. Less than two years ago: the discovery of the Higgs boson. Providing evidence long sought for the mechanism through which (some) particles exhibit the property known as mass, this discovery led to a Nobel prize the very next year. That must be in record time.

National Ignition Facility
Last month: the quest for a sustainable fusion reaction reached an important -- if interim -- milestone: a fuel gain greater than one. Deep within the National Ignition Facility, tiny fusible pellets, blasted by 172 synchronized laser pulses, yielded more energy than had been input. Meaningful nuclear reactions are happening after each pellet is imploded/crushed so as to raise its internal temperature to 50 million degrees Celsius.

(Alas, we remain far from the Holy Grail of fusion R&D: exceeding end-to-end energy breakeven. The input of the recent milestone is very narrowly defined as that part of the incident laser-beam energy absorbed through the pellet surface -- because a fair chunk of the beam energy gets reflected. To reach end-to-end breakeven, the energy produced will have to exceed all energy pumped into the lasers. Ditto, there must be an allowance for the fraction of fusion energy that goes unrecovered -- because no process is 100% efficient. The quest continues.)

But the truly magnificent news, reported just last week by the Harvard-Smithsonian Center for Astrophysics: "First Direct Evidence of Cosmic Inflation." A Nobel in the making, almost certainly.

Where to begin? The first detection of gravity waves: a prediction of Einstein's General Relativity that went unconfirmed even longer than Peter Higgs's prediction of the eponymous boson.

And not just any gravity waves, but waves from the very dawn of time: a trillionth of a trillionth of a trillionth of a second after the Big Bang. That is a very special time in the history of the universe ...

The big picture
Big Bang theory explains much about the observed nature of the cosmos, such as the Cosmic Microwave Background Radiation (first detected in 1964, coincidentally the year the Higgs field was first postulated) and the observed cosmic concentrations of hydrogen and helium. But Big Bang theory alone can't describe everything. For many years, the large-scale uniformity of the universe (galaxies distributed in much the same way, no matter where one looks) remained puzzling. Classic Big Bang theory didn't provide enough time for thorough mixing of the newborn cosmos before its expansion.

Enter inflation theory. In 1980, Alan Guth proposed a mathematical fix: between the very early universe, slowly expanding, in which thorough mixing occurred, and the modern universe, with its comparatively stately rate of expansion, was a super-fast, extremely brief era of super-expansion. In a tiny fraction of a second, the universe inflated at many times light speed. (While nothing can move faster than light, space-time -- the fabric of the universe -- is no thing. And expanding space-time can carry things with it.)

Crazy, no? It would seem not. The gravity waves newly detected match the predictions of inflation theory.

But wait, there's more!

Rather than one inflation theory, there have been -- in the absence of physical data from that early epoch -- many. With last week's exciting news, the range of options is already beginning to narrow. From Nature, see "Gravitational-wave finding causes 'spring cleaning' in physics."

Nor are the implications limited to "just" inflation theories ...

Theoretical physicists have long tried to imagine what was before and behind the Big Bang, so it's fascinating to read in that Nature article how "Big Bang findings would strengthen case for multiverse and all but rule out a 'cyclic Universe'."

A froth of bubble universes
Multiverse? That's a concept in which our familiar universe is but one among many. The spontaneous spawning of universes is a natural consequence of the small fraction of inflation theories that are consistent with the gravity-wave measurements.

Cyclic universe? That's a concept (arising out of a string-theory generalization called (mem)brane theory) in which space-time involves up to eleven dimensions. The familiar four dimensions of our universe are a four-dimensional island afloat in the higher-dimensional space-time -- and other such islands are out there. In a cyclic universe, the Big Bang -- and hence, our universe -- arose from an inter-island collision -- to be followed by a Big Crunch as gravity brings islands back together -- to be followed by a new Big Bang ....

If the possibility of a Big Crunch in 11-space has kept you up at night, be of good cheer. The gravity waves just detected do not match models for this cyclic universe.

And there's still more! General Relativity is wildly successful in predicting the large-scale behavior of familiar matter across the world, solar system, galaxy, and universe. Quantum mechanics is wildly successful at predicting the behavior of energy and particles at the subatomic scale. Alas, the two theories don't mesh. Not at all. GR and QM build upon wildly different assumptions, as basic as whether space-time is continuous or quantized/discontinuous. The recent gravity-wave findings may offer new insight into unifying the two foundational theories of modern physics.

In short: three cheers for modern physics! It doesn't get more mind-blowing than this.


Anonymous said...

Amazing stuff and it keeps coming. The irony to me is that everything discovered raises many more questions. So the more we know absolutely, the less we know relative to the number of questions.

Edward M. Lerner said...

How boring it would be to know everything!