Are the Laws of Physics Really Universal?

Can the laws of physics change over time and space?

As far as physicists can tell, the cosmos has been playing by the same rulebook since the time of the Big Bang. But could the laws have been different in the past, and could they change in the future? Might different laws prevail in some distant corner of the cosmos?

“It’s not a completely crazy possibility,” says Sean Carroll, a theoretical physicist at Caltech, who points out that, when we ask if the laws of physics are mutable, we’re actually asking two separate questions: First, do the equations of quantum mechanics and gravity change over time and space? And second, do the numerical constants that populate those equations vary?

An artist’s impression of the quasar 3C 279. Astrophysicists use light from quasars to look for variations in the fundamental constants. Credit: ESO/M. Kornmesser, adapted under a Creative Commons license.

To see the distinction, imagine the whole universe as one big game of basketball. You can tweak certain parameters without changing the game: Raise the hoop a little higher, make the court a little bigger, change the way you score, and it’s still basketball. But if you tell the players to start running bases or kicking field goals, then you’re playing a different game.

Most of the current research into the changeability of physical laws has focused on the numerical constants. Why? It’s the easier question to answer. Physicists can make solid, testable predictions about how variations in numerical constants should affect the results of their experiments. Plus, says Carroll, it wouldn’t necessarily blow physics wide open if it turns out that constants do change over time. In fact, some constants have changed: The mass of an electron, for instance, was zero until the Higgs field turned on a tiny sliver of a second after the Big Bang. “We have lots of theories that can accommodate changing constants,” says Carroll. “All you have to do to account for time-dependent constants is to add some scalar field to the theory that moves very slowly.”

A scalar field, Carroll explains, is any quantity that has a unique value at every point in space-time. The celebrity-du-jour scalar field is the Higgs, but you can also think of less exotic quantities, like temperature, as scalar fields, too. A yet-undiscovered scalar field that changes very slowly could continue to evolve even billions of years after the Big Bang—and with it, the so-called constants of nature could evolve, too.

Luckily, the cosmos has gifted us with some handy windows through which we can peer at the constants as they were in the deep past. One such window is located in the rich uranium deposits of the Oklo region of Gabon, in Central Africa, where, in 1972, workers serendipitously discovered a group of “natural nuclear reactors”—rocks that spontaneously ignited and managed to sustain nuclear reactions for hundreds of thousands of years. The result: “A radioactive fossil of what the rules of nature looked like” two billion years ago, says Carroll. (For perspective, the Earth is about 4 billion years old, and the universe is edging toward 14 billion.)

The characteristics of that fossil depend on the value of a special number called the fine structure constant, which bundles up a handful of other constants—the speed of light, the charge on an electron, the electric constant, and Planck’s constant—into a single number, about 1/137. It’s what physicists call a “dimensionless” constant, meaning that it’s really just a number: it’s not 1/137 inches, seconds, or coulombs, it’s just plain 1/137. That makes it an ideal place to look for changes in the constants embedded within it, says Steve Lamoreaux, a physicist at Yale University. “If the constants changed in such a way that the electron mass and the electrostatic interaction energies changed in different way, it would show up in the 1/137 unambiguously, independent of measurement system.”

But interpreting that fossil isn’t easy, and over the years researchers studying Oklo have come to apparently conflicting conclusions. For decades, studies of Oklo seemed to show that the fine structure constant was absolutely steady. Then came a study suggesting that it had gotten bigger, and another that it had gotten smaller. In 2006, Lamoreaux (then at Los Alamos National Laboratory) and his colleagues published a fresh analysis that was, they wrote, “consistent with no shift.” But, they pointed out, it was still “model dependent”—that is, they had to make certain assumptions about how the fine structure constant could change.

Using atomic clocks, physicists can search for even tinier changes in the fine structure constant, but they’re limited to looking at present-day variations that happen over just a year or so. Researchers at the National Institute of Standards and Technology in Boulder, Colorado, compared time kept by atomic clocks running on aluminum and mercury to put extremely tight limits on the present-day change in the fine structure constant. Though they can’t say for certain that the fine structure constant isn’t changing, if it is, the variation is tiny: just quadrillionths of a single percent each year.

Today, the best limits on how the constants could be varying over the life of the universe come from observations of distant objects on the sky. That’s because, the farther into space you look, the farther back in time you can see. The Oklo “time machine” stops two billion years ago, but, using light from distant quasars, astronomers have dialed the cosmic time machine 11 billion years back.

Quasars are extremely bright, ancient objects that astronomers believe are probably glowing supermassive black holes. As light from these quasars travels to us, some of it gets absorbed by the gas it travels through along the way. But it doesn’t get absorbed evenly: only very particular wavelengths, or colors, get plucked out. The specific colors that are “deleted” from the spectrum depend on how photons from the quasar light interact with atoms in the gas, and those interactions depend on the fine structure constant. So, by looking at the spectrum of light from distant quasars, astrophysicists can search for changes to the fine structure constant over many billions of years.

“By the time that light has reached us here on Earth, it has collected information regarding several galaxies going back billions of years,” says Tyler Evans, who led some of the most rigorous quasar measurements to date while he was a PhD student at Swinburne University of Technology in Australia. “It is analogous to taking a core sample of ice or the Earth in order to tell how climate was behaving in previous epochs.”

Despite some tantalizing hints, the latest studies all show that changes to the fine structure constant are “consistent with zero.” That doesn’t mean that the fine structure constant absolutely isn’t changing. But if it is, it’s doing so more subtly than these experiments can detect, and that seems unlikely, says Carroll. “It’s hard to squeeze a theory into the little daylight between not changing at all, and not changing enough that we can see it.”

Astrophysicists are also looking for changes to G, the gravitational constant, which dials in the strength of gravity. In 1937, Paul Dirac, one of the pioneers of quantum mechanics, offered up the hypothesis that gravity gets weaker as the universe ages. Though the idea didn’t stick, physicists kept looking for changes in G, and today some exotic alternative theories of gravity embrace a shifting gravitational constant. While lab experiments here on Earth have returnedconfusing results, studies off Earth suggest that G isn’t changing much, if it all. Most recently, radio astronomers scoured 21 years of precise timing data from an unusually bright, stable pulsar to see if they could trace any changes in its regular “heartbeat” of radio emission to changes in the gravitational constant. The result—nothing.

But back to the second, tougher half of our original question: Could the laws of physics themselves, and not just the constants sewn into them, be changing? “That’s much harder to say,” says Carroll, who points out that there are different degrees of disruption to consider. If the rules of some “sub-theory” of quantum mechanics, like quantum electrodynamics, turned out to be fluid, maybe existing theory could accommodate that. But if the laws of quantum mechanics itself are in flux, says Carroll, “That would be very bizarre.” No theory predicts how or why such a change might happen; there is simply no framework from which to investigate the question.

As far as we can tell, the universe seems to be playing fair. But physicists will keep scouring the rulebook, looking for clues that the rules of the game could be changing at a level we haven’t yet perceived.

Go Deeper
Picks for further reading

Astrophysicist Adam Frank profiles four theorists who challenge the notion that there is one set of unchanging laws that perfectly describes the universe.

Murphy, an astrophysicist at Swinburne University of Technology, provides a general-audience overview of the search for changes in the fundamental constants, with links to related articles and video.

Natural History Magazine: On Earth As In The Heavens
In this essay, astrophysicist Neil deGrasse Tyson explains why physicists think that the same laws that apply on Earth apply throughout the cosmos, and how we may one day use this knowledge to communicate with alien civilizations.

Postingan terkait:

Belum ada tanggapan untuk "Are the Laws of Physics Really Universal?"

Post a Comment