Summary: Seeking a grander theory, rebel physicists break a cosmic speed limit. As we near the end of our first century in a relative universe, challenges to Einstein's theory are in the air.
Seeking a grander theory, rebel physicists break a cosmic speed limit
By Robert Kunzig
Even if João Magueijo had not advocated castrating an editor at a prestigious science journal, and even if he hadn't suggested that rival physicists, ostensibly brilliant like Magueijo, were just posers--in short, even if Magueijo were a less colorful rebel, his ideas would have attracted attention. After all, he is taking on Einstein, and ever since we placed Einstein on a pedestal, we have been titillated by the idea of knocking him off. Magazine editors know that putting the antic-haired genius on the cover, perhaps over the words Faster Than the Speed of Light--the somewhat misleading title of Magueijo's recent book--all but guarantees sales.
And yet Magueijo, a physicist at Imperial College, London, is for real, and he is not alone. As we near the end of our first century in a relative universe, challenges to Einstein's theory are in the air. They are respectful challenges; even Magueijo isn't proposing to throw relativity out the window, any more than Einstein junked Newton. "We get E-mail and letters all the time from amateurs who think they have found a mistake in Einstein's theory," says Lee Smolin, a physicist at the Perimeter Institute in Waterloo, Ontario. "That's not what is going on here." If relativity is wrong, it is wrong by such tiny amounts or in such particular circumstances that you have to go to great lengths to find the error.
But there is optimism these days that, by studying light from the distant universe, researchers may soon be able to measure such smaller-than-nano deviations. They may need to find them, if they are ever to create a unified theory of all the forces of nature. To fulfill that dream, which obsessed Einstein after he turned 40 or so, physicists may have to tinker with the theory he invented when he, too, was a young revolutionary, disguised as a patent clerk. They may have to bend his best-known principle: the one that says the speed of light is an absolute, always and everywhere the same, and faster than anything else.
C is special. That principle was born in 1905, when Einstein wedded space and time into something called spacetime. Until then, in Newtonian physics, space and time were separate, independent of each other and of the things in them. Time flowed at the same rate for everyone, and space was a fixed stage on which the universe played out its history. Nineteenth-century physicists filled that stage with a mysterious, invisible "aether," a medium that transmitted light waves the way air transmits sound waves. The aether was assumed to be at absolute rest, a fixed "reference frame" for all motions.
No one could ever detect the aether, though; in the 1880s, Albert Michelson and Edward Morley tried ingeniously and failed. And after Einstein proposed his theory of special relativity two decades later, physicists realized they should give up on absolute space and time. Einstein postulated, first, that the laws of physics don't prefer one reference frame over another, as long as each is moving at a constant velocity. Second, he said that c, the speed of light, will appear exactly the same to every observer, in every frame of reference.
A century later, that second postulate still defies common sense. It says that if you're driving down the highway at a quarter the speed of light, you'll still see the photons from your headlights racing ahead of you at light speed--not three-quarters light speed. If I'm coming from the opposite direction at half light speed, I'll still see your photons approaching at c--not 1.5 times c. Since speed is just space divided by time, and we both agree about the speed of light, we can't possibly agree about space and time. You say my clock is too slow and my yardstick has shrunk (not to mention my whole car). Maddeningly, I say the same about you. The one thing we agree on, aside from c itself, is the distance covered by the photons in the weird new reference frame of four-dimensional spacetime.
It might be a relief to learn that physicists were talking about chucking this deeply strange theory. But just as Einstein made only minute corrections to Newton in everyday life--to really feel the effects of special relativity, you have to move at a large fraction of light speed--the proposed changes to relativity would have only subtle, hard-to-detect effects. Yet the stakes are big: the quest for a single theory that would unite general relativity, Einstein's later theory describing gravity, with quantum mechanics, the theory describing the forces inside the atom.
Physicists are taking many paths to this "quantum gravity" grail, but in all of them spacetime itself, instead of being continuous, is made of quantum bits. "It's like the difference between sand and water," says Giovanni Amelino-Camelia of La Sapienza University in Rome--except that the spacetime grains could be around a hundred billion billionth the size of an atomic nucleus. At this "Planck length," named after the father of quantum physics, gravity would no longer be described by general relativity but by the new theory.
It's also where you run into a conflict with special relativity, Amelino-Camelia found a few years ago. Because measurements of length depend on the velocity of the observer, two observers could end up disagreeing about whether a physical process was taking place at the Planck scale or not, and which laws applied. One would say the process was governed by quantum gravity; the other would say general relativity.
After a long struggle--"three years of frustration and a couple of evenings of success"--Amelino-Camelia came up with a way around this absurdity: "doubly special relativity." The math is not simple, but the basic idea is. To the maximum speed limit that Einstein set, Amelino-Camelia would add a minimum length, below which space could not contract, no matter how fast the observer moved. That limit would be the size of the spacetime quanta.
That minimum length comes at a cost, however: The speed of light would no longer be a constant. The more energetic a photon, Amelino-Camelia's calculations indicate, the faster it would navigate the minefield of quantized spacetime. If you were to pit blue photons against red photons in a race across the Atlantic, Amelino-Camelia says, the blue ones--being slightly higher in energy-- would win by about 0.000000000000000000000000001 of a second.
The effect on light speed would be much more dramatic at the fantastic energies that prevailed in the primordial fireball of the big bang--and that's where Amelino-Camelia's ideas intersect with the very different approach of cosmologist João Magueijo. On a rainy winter morning a few years back, Magueijo was brooding about some of the most nagging problems of cosmology. As he walked across an athletic field at Cambridge University, he writes in his book, "the answer seemed to drop from the sky." (He later learned that a Canadian physicist named John Moffat had beat him to the epiphany.) Just allow light to travel much faster in the first fraction of a second after the big bang--quadrillions upon quadrillions of times faster--and the problems would be solved.
Take the so-called horizon problem. From beyond the farthest galaxies comes a faint emanation called the cosmic microwave background, the afterglow of the big bang. It looks almost exactly the same in every direction, meaning the hot gases it came from must have had almost exactly the same temperature. When the background was emitted, around 300,000 years after the big bang, light had had only 300,000 years to travel. Yet the universe was already tens of millions of light-years across. Its opposite sides could not have exchanged light or heat. Short of an incredible fluke, how could they have evened out their temperature?
The widely accepted but still unproven solution is the inflationary-universe theory. It says that the opposite sides of the universe were originally in contact, because the universe was much smaller in its first instants than the standard Big Bang theory suggests; it then underwent an infinitesimally brief but exponential inflation that swept its parts far out of contact with each other. Walking in the rain, Magueijo discovered what to him seemed a more elegant solution: The different regions of the early universe were in contact not because it was smaller but because light traveled much faster--fast enough to connect them. The laws of physics must have changed since then, drastically slowing light.
Lately, though, he and Smolin have proposed something a little more moderate: that this varying speed of light might be grounded in doubly special relativity. Amelino-Camelia's theory that the speed of light depends on its energy doesn't require the laws of physics to change, unlike Magueijo's original idea. Light might have traveled vastly faster in the hot early universe simply because its energy was so much higher then. "That you could be comfortable with," says Smolin.
Real world. Well, maybe. Many physicists aren't ready to take any of these ideas too seriously for now. But real-world tests could change that. Amelino-Camelia and Magueijo can both point to puzzling astrophysical observations that their theories may be able to explain. Ultrahigh-energy cosmic rays that special relativity says should never be able to make it to Earth--but that have been showing up at cosmic-ray observatories in recent years--might make sense in doubly special relativity. "For us to be mentioned in astrophysics reviews as one of the possible explanations--you have no idea what that means," says Amelino-Camelia.
Similarly, though no one has reported a direct observation of the speed of light changing over cosmic history, a tantalizing hint comes from something called the fine-structure constant. The fine-structure constant is a number that determines the size of the energy jumps that atoms make when they absorb photons and get excited, and it depends on c, among other things. For years now, a team of astronomers led by John Webb of the University of New South Wales, Australia, and his colleagues Victor Flambaum and Michael Murphy have been compiling observations of how atoms in gas clouds billions of light-years away absorb light. From these observations they can calculate the value that the fine-structure constant had billions of years ago and compare it with measurements today. They find evidence that it has increased by around a thousandth of a percent over the past 10 billion years. If other researchers confirm the effect, a gradual decrease in c will be a plausible explanation.
But perhaps the most direct evidence that something is amiss with c could come from GLAST, an orbiting gamma-ray telescope that NASA is planning to launch in 2006 or 2007. GLAST will observe bursts of gamma rays--ultrahigh-energy light--from far-off galaxies. If doubly special relativity is correct, the more energetic gamma rays should travel faster and should tend to arrive at the telescope first--by about a millisecond, Amelino-Camelia says, after a billion-year race. That would be strong evidence that Einstein was wrong. But also that he was pretty darn close.