Time’s arrow may make U-turns in universe The new millennium, or at least the new

year, begins. Yet somewhere in our uni- verse, the year 2000 may have just reced- ed into the future, new research suggests.

Imagine a renegade world where in Re- versed-Times Square, the ball moved up the tower last night instead of down and where calendars flipped back to Dec. 31, 1999, instead of ahead. Such time incon- sistency from one world to another could occur even in our own galaxy, proposes Lawrence S. Schulman of Clarkson Uni- versity in Potsdam, N.Y.

“The actual appearance of this [oppo- site-directed time] in our universe is something we would have to see by ob- servation. The only thing I’m saying, the- oretically, is it’s not ruled out,” Schulman explains.

“Time is an extremely important but very difficult field of research,” notes David T. Pegg of Griffith University in Brisbane, Australia. Although Schulman investigates concepts that may “seem like total science fiction,” Pegg says, he “makes some real progress.”

In his studies, Schulman assumes that the universe expands and then con- tracts-a Big Bang followed by a Big Crunch. He finds that celestial bodies with reversed time would have originat- ed in our distant future and would al-

ready have experienced the cosmic turn- around. To us, these worlds would look like very old objects. Spectroscopic sur- veys looking for elemental composition typical of such old objects might reveal their presence.

Unlikely to be luminous anymore be- cause of their enormous age, the bodies would still exert gravitational attraction. Detecting them might help clear up a pro- found mystery in cosmology-the identi- ty of the invisible, or dark, matter that provides most of the gravitational glue of the universe, Schulman asserts. His ex- ploration of time reversal appears in the Dec. 27,1999 PHYSICAL REVIEW LEITERS.

For the most part, equations describ- ing the behavior of subatomic particles and the propagation of electromagnetic waves look the same if the flow of time is reversed. A year ago, however, scientists found experimental evidence that certain very rare interactions of particles called kaons imply a distinction between the forward and backward directions of time (SN: 10/31/98, p. 277).

In everyday experience, time appears to flow one way. Scientists have pro- posed that the second law of thermody- namics may orient time’s arrow. It holds that ordinary processes cause some irre- versible loss of energy to randomness

Stem cells repair rat spinal cord damage Among the challenges of medicine,

spinal cord injury ranks high. Nerve cells in the spine don’t regenerate natu- rally, and attempts to revive or repair a damaged cord have met with frustra- tion. To bypass this problem, re- searchers have recently tried animal ex- periments replacing ruined nerve cells in animals with transplants of fetal cells. This technique has shown promise, but only when experimenters perform the transplant within a few days of an injury.

Researchers at Washington University School of Medicine in St. Louis now r e port that they have restored leg move- ment in injured rats by transplanting cells into the injury site 9 days after the rats re- ceived a crushing blow to the spine. The scientists used mouseembryo stem cells modified to ensure they would grow into basic nerve cells and associated cells.

When the spine is severely bruised, some nerve cells die off immediately. A second wave of programmed cell death called apoptosis follows. Most of this carnage occurs within 24 hours, shut- ting off nerve signals traveling the spine, says study coauthor John W. McDonald, a physician and neuroscientist at Wash- ington University.

The center of the bruised spine fills with fluid, becoming a cyst. Later, scar

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tissue piles up, preventing recovery. Neurons at the injury site stop function- ing, as do their elongated extensions, called axons. Even if a neuron remains intact, it often dies quickly if the trauma has stripped the protective sheath, a fat- ty protein called myelin, off its axon.

The blunt injury that the researchers used in the rat experiment “simulates the majority of [traumas] seen in people who have spinal cord injuries,” McDon- ald says. He and his colleagues studied 62 rats whose spines were bruised and that could not support weight on their back legs. Nine days after injury, 28 of the rats each received injections of roughly 1 million embryonic stem cells pretreated with retinoic acid to induce their growth into nervous system cells. Coauthor David 1. Cottlieb, a neurobiolo- gist also at Washington University, de- vised the pretreatment.

Rats given stem cell injections re- gained the ability to stand on four legs and walk, albeit not perfectly, within 2 weeks, McDonald says. In 34 rats re- ceiving no cell transplants, the hind legs remained crippled, the researchers re- port in the December 1999 NATURE MEDI- CINE. All rats received drugs typically used to prevent rejection of transplants.

Examination of the rats after 2 and 5

SCIENCE NEWS, VOL. 157

and disorder. If time reversed, improba- ble events would occur, such as broken eggs spontaneously reassembling.

Thomas Gold of Cornell University ar- gued in the 1960s that the universe’s ex- pansion dictates that time points in a particular direction. As time passes, the universe must grow bigger.

In Schulman’s bang-crunch universe, space expands until it reaches a maxi- mum volume. Then, as it starts to con- tract, the arrow of time flips, A la Cold. Schulman contends that, by virtue of be- ing isolated, a particular star or galaxy might survive through the universe’s re- bound from expansion to contraction with time arrows intact and oppositely oriented to those of the rest of matter.

To explore the interactions of such a survivor, Schulman uses a computer model of two side-by-side boxes of gas molecules that exert a mild influence on each other as they compress or expand. He finds that the gases can communicate without destroying their direction of vol- ume change, the time arrows.

The new theory could self-destruct, however, if opposed time streams inter- act in such a way that leads to unresolv- able paradoxes, such as putting effects before causes, Schulman acknowledges. Also, recent evidence of accelerating ex- pansion of the universe (SN: 10/31/98, p. 277) makes a Big Crunch seem less likely, he notes. -P Weiss

weeks showed that most of the trans- planted stem cells had died off, but that enough had survived for the animals to have a growing supply of new nervous system cells, McDonald says. In the treated rats, the researchers observed some new neurons with axons that ex- tended up to 1 centimeter away from the injection site.

The researchers also found two other kinds of nervous system cells-oligo- dendrocytes and astrocytes-thriving at the injury site in the treated rats. Oligo- dendrocytes form the myelin sheaths that protect axons, much like plastic coating insulates electric wires, and speed the signals that travel along ax- ons. Astrocytes are star-shaped cells that provide the scaffolding upon which neurons can grow.

It’s unclear how the transplant re- stored leg movement.

In the rats, about 60 percent of the daughter cells of the injected stem cells were oligodendrocytes, 20 percent were astrocytes, 10 percent were neurons, and 10 percent were various other types of cells, McDonald says.

“That sounds like a good mix, be- cause you will need oligodendrocytes . . . to remyelinate the tissue,” says Wolf- gang J. Streit, a neuroscientist at the University of Florida Brain Institute in Cainesville. -N Seppa

JANUARY 1,2000