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The grand design

Does the universe still need Einstein, asks Dennis Overbye
Einstein couldn't explain our Universe.

New York Times News Service   |     |   Published 31.03.19, 11:08 AM

Is Albert Einstein finally dead? Yes. The old sage took his last breath and muttered his last indecipherable words, in German, on April 18, 1955. But lately he has been dying a second death, if one believes a new spate of articles and papers bemoaning the state of contemporary physics.

Never mind the recent, staggering discovery of gravitational waves: ripples in space-time that Einstein predicted a century ago, and which indicate the universe is peppered with black holes that shred and swallow stars.

No, something much deeper than gravity or quantum theory, Einstein’s other misbegotten legacy, is at stake.

It was Einstein who set the goal for modern science: the search for a final theory of everything, a “unified theory,” he said, that would explain why there was no other way to put together the universe than the one we seem to live in. Or, as he famously put it, “What interests me is whether God had any choice in the creation of the world.”

Roll over, Albert.

“There are no laws of physics,” read the headline on an article in Quanta, the online science magazine, last summer by Robbert Dijkgraaf, director of the Institute for Advanced Study, where Einstein spent his last 22 years.

Instead, Dijkgraaf wrote, there is a frighteningly complex “landscape” of possibilities, a nearly infinite, subtly connected network of complementary versions of reality. There exists a universe for every good or bad dream you’ve ever had, each with its own set of fundamental particles, forces, laws and dimensions. This landscape, also known as the multiverse, is the vision of string theorists who have vaulted past Einstein in the current scientific imagination.

String theory unites gravity, which curves the cosmos, with quantum mechanics, which describes the randomness that lives inside it, by envisaging the fundamental constituents of nature as tiny strings of energy vibrating in 11 dimensions. The theory has been described as a piece of 21st-century physics that fell into the 20th century by accident — and which might require 22nd-century mathematics to understand.

The result is a mathematical labyrinth with 10^500 solutions, each one a different potential universe. In principle, one of those universes is ours — but nobody knows, because the math and physics are so horrendously complex.

Or so the story goes. “If our world is but one of many, how do we deal with the alternatives?” Dijkgraaf wrote. “The current point of view can be seen as the polar opposite of Einstein’s dream of a unique cosmos.” Reached in Princeton, Dijkgraaf said that the article’s headline, which he hadn’t written, perhaps was an overstatement. Probably there is some fundamental principle, he said, perhaps whatever it is that lies behind string theory.

But nobody, not even the founders of string theory, can say what that might be.

Scientists were drawn to this vision by the discovery, two decades ago, that a mysterious force — “dark energy” — is accelerating the expansion of the universe, making the galaxies retreat from each other faster and faster as cosmic time goes by. This dark energy bears all the earmarks of a fudge factor, called the cosmological constant, that Einstein inserted into his equations a century ago, and later rejected as a blunder. But the amount of this dark energy is smaller than the predicted value of the cosmological constant by a factor of 10^60.

Physicists can only explain the discrepancy by assuming that the value of Einstein’s constant is random across all potential universes; we live in one with the right amount of dark energy to allow stars and galaxies to form.

In short, we live where we can.

The discovery in 2012 of the long-sought Higgs boson confirmed the last outstanding piece of an ambitious mathematical edifice known as the Standard Model, which details all the forms of matter and energy that can be measured in a lab. The Standard Model explains why your computer boots up and why a gardenia smells so sweet.

But the model works too well. Particle physicists have now sifted the debris from trillions upon trillions of subatomic collisions in the Large Hadron Collider, the immense machine in which the Higgs was discovered. So far they have confirmed that the Higgs behaves as the Standard Model predicted.

That is a great intellectual achievement, but it fails to reveal any discrepancies that could lead to a deeper theory. Researchers have found no trace of a much-hoped-for phenomenon called supersymmetry, which would tie together the individual physical forces and supply a whole new menu of elementary particles, including, perhaps, the stuff of dark matter.

But supersymmetry might always have been an illusion, according to Sabine Hossenfelder, a theorist at the Frankfurt Institute for Advanced Study in Germany. She emerged last year as one of the most vocal critics of modern physics, with a provocative new book, Lost in Math: How Beauty Leads Physics Astray.

Hossenfelder argues that physicists have gone off course by exalting mathematical elegance. “They believed that Mother Nature was elegant, simple and kind about providing clues,” she wrote. “They thought they could hear her whispering when they were talking to themselves.”

Particle physicists contend that they merely have been following time-honored and successful principles. They chased the Higgs boson for half a century, and nearly gave up before nature finally coughed it up. Meanwhile, the cosmologists, a notably fractious group, have agreed on their own standard model of our particular universe.

According to them, atoms — the stuff of you, me and the stars — account for only 5 per cent of the cosmos by weight. Dark matter, of which we know nothing except that its collective gravity sculpts and holds the galaxies together, amounts to 25 per cent.

The remaining 70 per cent is dark energy, pushing everything apart; we don’t know anything about that, either. We only know that this “dark sector” exists because of the effect of its gravity on the luminous universe, the motions of stars and galaxies.

A theory that leaves 95 per cent of the universe unidentified is hardly a sign that science is over.

Maybe we don’t understand gravity after all, some astronomers say. “I worry that we deify Einstein too much,” Stacy McGaugh, an astronomer at Case Western Reserve University, told Gizmodo in June.

Hossenfelder, for all her skepticism, ends her book on a hopeful note. “The next breakthrough in physics will occur in this century,” she wrote. “It will be beautiful.”


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