Through the Looking Glass.

Reflections on a mirror universe

There are more things in Heaven and Earth, Horatio, than are dreamt of in your philosophy.

"Hamlet," William Shakespeare, ca. 1600

Rabindra N. Mohapatra's office is spilling over with stuff. Crowding his narrow room at the University of Maryland in College Park are stacks of physics journals, books, and research papers, some of them overflowing the bookshelves and strewn along a metal desk.

At least, that's the material you can see. According to an otherworldly idea that Mohapatra and a few other physicists now entertain, his crowded office and the cosmos at large are far more stuffed than they appear. These scientists argue that nothing less than an entire universe of shadow matter, made of particles nearly identical to neutrons, protons, and electrons, shares our space. We just can't tell it's there.

Welcome to the mirror world, in which every particle in the known universe could have a counterpart. This cosmos would hold mirror planets, mirror stars, and even mirror life.

The concept may sound as fantastic as the world that Lewis Carroll's Alice encountered through the looking glass. But proponents of the mirror world, a notion that dates back to the 1950s, say its existence would solve a number of puzzles in physics and cosmology.

In their model of such a world, Robert Foot, Raymond R. Volkas, and Henry Lew of the University of Melbourne in Australia propose that particles would have exactly the same mass as their visible-world counterparts. In another version, suggested by Mohapatra and Vigdor L. Teplitz of Southern Methodist University in Dallas, mirror particles would dwarf their more familiar counterparts, weighing in as behemoths 15 to 20 times as massive.

In either of these mirror worlds, particles would interact with each other by mirror forces. The same gravity operating in the visible universe would exert its tug in the mirror world, but nature's other three forces--the strong, the weak, and the electromagnetic--wouldn't be exactly the same. In their own style, they would govern how mirror particles interact and build chunks of mirror matter. Since our eyes can't see mirror photons, however, all of this would remain invisible.

Because gravity straddles the boundary between the visible and mirror worlds, it opens a route for detecting the mirror universe. Mirror matter would betray its presence by exerting a gravitational attraction on the visible world.

This testability is what elevates the idea of a mirror world from mere science fantasy to a bona fide scientific theory. It may also shed new light on a problem that scientists have been grappling with for years.

For decades, cosmologists have admitted that visible types of matter simply can't explain how cosmic structure arose. They've reluctantly concluded that there's more to the cosmos than meets the eye.

For one thing, ordinary matter can't clump fast enough to have produced gargantuan clusters of galaxies seen in the universe today (SN: 8/12/00, p. 104). So, astronomers have envisioned that some kind of exotic invisible matter plays the leading role in the mystery of cosmic gravitation.

Clumps of this so-called dark matter would then have attracted the visible matter that formed the universe's structure now mapped by astronomers. Theorists say this dark matter comprises a whopping 90 percent or more of the mass of the universe.

On a much smaller scale, astronomers face another puzzle that dark matter could solve. In the late 1970s, astronomers began to find that beyond the innermost regions of spiral galaxies, the speed with which stars rotate remains the same. It's as if a spot on the outside of an old vinyl LP were rotating at the same speed as one on a groove in the middle.

A simple explanation falls into place, astronomers came to realize, if the starlit galaxies we observe are but tiny jewels awash in a great ocean of dark material. Massive halos of dark matter, extending thousands of light-years beyond a galaxy's visible outlines, would envelop the galaxy's bright parts and drag on its outer stars.

To populate these halos, astronomers have come up with a zoo of candidates. Most are as exotic as the names their inventors have given them: massive compact halo objects (MACHOs), weakly interacting massive particles (WIMPs), axions, neutralinos, and wimpzillas. To date, however, no one has found definitive evidence for any of these particles.

That's because scientists have been searching for answers on the wrong side of the looking glass, argue Mohapatra, Foot, and their various collaborators. Mirror matter could account for the cosmic conundrums more simply than dark matter can.

For starters, mirror matter would neatly explain away dark matter's invisibility since the mirror world can't be seen. Moreover, calculations show that mirror matter could easily produce objects that weigh about half the sun's mass. That's the expected heft of MACHOs, says Mohapatra.

In addition, mirror material could have built the cosmic scaffolding necessary for galaxies to form, Foot says. The abundance of elements forged in the Big Bang suggests that the mirror world would have cooled faster than the visible world, he explains. As a consequence, electrons and ions in the mirror world would combine into mirror atoms sooner than their counterparts in the visible world. This would provide ample time for galaxies and galaxy clusters to form.

Over the past 6 months, the mirror-world idea has gotten another boost, according to Mohapatra. Theorists have uncovered a flaw in their favorite type of dark matter, known as cold dark matter. Computer simulations had shown that this hypothetical material could rapidly generate large-scale structure in the universe. No problem there. But the newest simulations reveal that the cores of some of the galaxies produced in this process ought to be denser than astonomers' observations of galaxies have revealed. In addition, standard cold dark matter in these simulations makes more small galaxies than astronomers see.

To circumvent the mismatches, some theorists suggested that particles of cold dark matter interact with each other more strongly than astronomers had proposed. If cold dark matter were more sociable, to the point where its particles form a dilute gas that would slightly resist gravity, it could produce galaxies of lower density.

That's an ad hoc solution, Mohapatra maintains. He suggests that a stronger explanation can emerge from the mirror world because mirror particles, by definition, interact with each other just as much as their counterparts in the visible world.

Not surprisingly, dark matter aficionados are skeptical. "Mirror matter is a much less certain prospect and less original" than other explanations for the flaw in dark matter theory, says Paul J. Steinhardt of Princeton University. He and David N. Spergel, also at Princeton, have written several papers proposing that dark matter particles interact strongly. Steinhardt says that summoning mirror matter isn't the most promising way to solve dark matter's problems.

Another set of cosmic riddles may provide the strongest argument for mirror matter, assert Foot and Volkas. These puzzles all concern neutrinos, a ghostly class of subatomic particles that rarely interact with matter. Most neutrinos pass through the Earth unimpeded and escape detection.

Neutrinos come in three types: electron, muon, and tau. What's more, recent evidence that neutrinos have a small amount of mass leads to the likelihood that the different neutrino types transform from one to another (SN: 1/30/99, p. 76).

Consider the electron neutrino. Nuclear reactions deep in the sun create a steady supply of them, which astronomers have detected since the 1960s. However, solar physicists have faced a long-standing problem. The number of electron neutrinos detected is about half the predicted quantity.

Part of the deficit may arise because some of the electron neutrinos have transformed into mirror electron neutrinos, suggest Foot and Mohapatra. So long as they remain mirror neutrinos, they can't be detected.

Muon neutrinos pose a similar puzzle. These particles rain down on Earth when ultrahigh-energy protons and other cosmic rays smash into atoms in the upper atmosphere. Several experiments suggest there's a shortfall of muon neutrinos compared with the number predicted by theory. To account for the shortfall, Foot conjectures that some muon neutrinos may transform into tau neutrinos--as many scientists already expect. But some muon neutrinos may also have transformed into mirror muon neutrinos, he suggests.

The same scenario suggests a way to search for mirror stars, Foot adds. Suppose a mirror star explodes to become a supernova. Just as a supernova in the visible world emits a burst of neutrinos, a mirror supernova would emit a burst of mirror neutrinos. If neutrinos can oscillate between the mirror world and the visible world, then some of the mirror neutrinos will emerge from their looking glass world into ours. The mirror supernova itself couldn't be seen, but a mysterious burst of neutrinos, far from any visible star, could signify the explosion of a mirror star.

Researchers are devising a new experiment to search for signs of mirror matter. The test hinges on the true nature of positronium. This union of two elementary particles resembles a hydrogen atom--with a crucial difference Instead of an electron orbiting a proton, an electron orbits a positron, its antimatter counterpart. Antimatter, first detected in the 1930s, has the same mass as but the opposite charge of ordinary matter.

If the spin of the positron and the spin of the electron point in the same direction, the material is known as orthopositronium. Unlike a stable hydrogen atom, orthopositronium lasts only for about 140 nanoseconds, before its components annihilate each other in a burst of pure energy.

In 1986, Nobel laureate Sheldon L. Glashow of Harvard University suggested that orthopositronium could provide a sensitive way to search for the mirror universe. According to Glashow, ordinary photons may actually interact ever so slightly with mirror photons. And because electrons so readily interact with photons as well, small amounts of laboratory-made orthopositronium might transform into its looking glass counterpart, mirror orthopositronium. By the same token, the mirror form would sometimes convert into the ordinary version.

In 1990, researchers at the University of Michigan in Ann Arbor measured the lifetime of orthopositronium and found that it was slightly shorter than predicted by theory. In the May 14 PHYSICS LETTERS B, Foot and Sergei N. Gninenko of CERN, a particle-physics laboratory in Geneva, suggest that mirror orthopositronium could explain the discrepancy. If orthopositronium decays while in its mirror form, it would go undetected, and that could account for the shorter lifetime measurements.

Extraordinary claims require extraordinary proof. Previous experiments weren't sensitive enough to confirm or refute the mirror-matter explanation, Foot and Gninenko say.

To test the claim, Gninenko has proposed a new experiment at CERN. He and his colleagues would confine orthopositronium to a sensitive heat-measuring device, called a calorimeter. The device would be under a strict vacuum to isolate its contents from collisions with other matter, which could confound the findings. Under ordinary conditions, the orthopositronium constituents--an electron and a positron--produce a specific amount of energy when they annihilate each other. But that energy simply wouldn't be there if the orthopositronium had oscillated into its undetectable mirror form.

The missing energy would amount to about 1 million electron volts, which is twice the rest mass of an electron, and would be a telltale signature of the mirror universe, Gninenko asserts.

There may be other ways of detecting mirror supernovas, and thereby the mirror world, Gninenko and Foot note. During a colossal supernova explosion, pairs of mirror electrons and mirror positrons would convert into ordinary electrons and positrons. Then, they might annihilate one another, generating bursts of visible light. Observers who record a brilliant, localized glow in the heavens without being able to identify the source may have found a mirror supernova.

Similarly, a chunk of mirror matter, such as a mirror asteroid, could have a dramatic impact if it collides with Earth. Because it's made of mirror particles, the asteroid wouldn't burn up in Earth's atmosphere. It still could wreak havoc when it struck Earth's surface, however, if mirror photons transform into ordinary ones. In that case, the collision would generate a huge release of energy with nary a trace of a crater.

While it's fun to speculate about a mirror world, "it's up to experiment in the long run" to prove its existence, says Foot. "I will believe it when there is enough evidence."

RELATED ARTICLE: Restoring symmetry: They do it with mirrors 

Although mirror matter may solve several problems in astrophysics, the teams examining that concept at the University of Maryland and the University of Melbourne say they were motivated by an even deeper riddle.

Most interactions in physics appear the same when reflected in a mirror. But not all. When an atom emits a neutrino, the neutrino always spins in the same direction--counterclockwise. Reflected in a mirror, however, the neutrino would always spin clockwise.

Just by looking at the spin of a neutrino, Lewis Carroll's Alice would know immediately whether she'd stepped into the looking glass world. In contrast, electrons can spin in both directions. So, the spin of an electron wouldn't give Alice any clue to what world she was in.

The selective spin of neutrinos destroyed the symmetry that physicists had taken for granted. Physics would not necessarily be the same in a mirror world. That discovery of asymmetry, suggested in 1956 by Tsung Dao Lee and Chen Ning Yang, won them the Nobel Prize in Physics just a year later.

"Many physicists were upset by the asymmetry," recalled Robert Adair of Yale University in the February 1988 SCIENTIFIC AMERICAN. "I remember feeling that I no longer could hold anything I knew as being certain."

In their 1956 journal article, Lee and Yang noted that the symmetry could be restored if a parallel universe existed in which neutrinos rotated in the opposite sense--clockwise--to that in our world. Considered together, the two worlds would restore the symmetry that appears to be lacking in each.

--R.C.