It's a classic underdog story: Working in a disused tunnel with a couple of lasers and a few mirrors, a plucky band of physicists dreamed up a way to test one of the wildest ideas in theoretical physics—a notion from the nearly inscrutable realm of "string theory" that our universe may be like an enormous hologram. However, science doesn't indulge sentimental favorites. After years of probing the fabric of spacetime for a signal of the "holographic principle," researchers at Fermi National Accelerator Laboratory (Fermilab) in Batavia, Illinois, have come up empty, as they will report tomorrow at the lab.
The null result won't surprise many people, as some of the inventors of the principle had complained that the experiment, the $2.5 million Fermilab Holometer, couldn't test it. But Yanbei Chen, a theorist at the California Institute of Technology in Pasadena, says the experiment and its inventor, Fermilab theorist Craig Hogan, deserve some credit for trying. "At least he's making some effort to make an experimental test," Chen says. "I think we should do more of this, and if the string theorists complain that this is not testing what they're doing, well, they can come up with their own tests."
The holographic principle springs from the theoretical study of black holes, spherical regions where gravity is so intense that not even light can escape. Theorists realized that a black hole has an amount of disorder, or entropy, that is proportional to its surface area. As entropy is related to information content, some theorists suggested that an information-area connection might be extended to any properly defined volume of space and time, or spacetime. Thus, crudely speaking, the maximum amount of information contained in a 3D region of space would be proportional its 2D surface area. The universe would then work a bit like a hologram, in which a 2D pattern captures a 3D image.
If true, the principle might guide string theorists in their grand quest to meld the theories of gravity and quantum mechanics. And it would imply, rather astonishingly, that the total amount of information in the observable universe is finite.
In 2009 Hogan dreamed up a way to test the idea. One way the holographic principle might come about, he reasoned, is if coordinates in different directions—up-down, forward-backward, right-left—obey a quantum mechanical uncertainty relationship a bit like the famous Heisenberg uncertainty principle, which states that you cannot simultaneously know both the position and momentum of a particle such as an electron. If so, then it should be impossible to precisely define a 3D position, at least on very small scales of 10-35 meters.
Hogan figured he could spot the effect using L-shaped optical devices known as interferometers, in which laser light is used to measure the relative length of a device's two arms to within a fraction of an atom's width. If it were impossible to exactly define position, then "holographic noise" should cause the output of an interferometer to jiggle at a frequency of millions of cycles per second, he argued. If two interferometers were placed back to back, they would sample distinct volumes of spacetime, and their holographic noise would be uncorrelated. But if they were nestled one inside the other, the interferometers would probe the same volume of spacetime and the holographic noise would be correlated. And if the interferometers were big enough, that correlated holographic noise should be effectively amplified to observable scales.
Now, Hogan, Fermilab experimenter Aaron Chou, and colleagues have done the measurement with interferometers with 39-meter-long arms. Unfortunately for them, they find no evidence of holographic noise. "A correlation that you would attribute to novel physics effects is not seen," says Lee McCuller, a graduate student at the University of Chicago in Illinois, who will present the result in a talk at the lab.
Just what the null result means remains unclear, however. Chen says he has never fully understood neither exactly how the experiment works nor Hogan's theory of how the holographic principle originates. What's really needed is some sort of general analysis of what types of theories the experiment can and cannot test, he says.
For his part, Hogan says that the experiment reached the sensitivity it aimed for, showing that the technique has the potential to make further measurements. "For me, the big news is that we have a technique for measuring spacetime at this level," he says.
In fact, he says, the holometer can be reconfigured to look not for an inherent uncertainty in positions, but rather for a jitter in angular orientation in spacetime—in his view another possible sign of holographic noise. Maybe the underdogs still have a chance, after all.