reach a maximum, becoming just like its timeless de Sitter
space parent. And then it could give birth to baby universes
of its own.
challenged as vigorously as health care reform and Arizona’s
“As time evolves, you pop universes into existence — a
baby universe comes into existence, expands and cools, and
for a moment, there’s an arrow of time,” Carroll said. “The
moment is several trillion years.”
Best of all, that scenario can happen in both directions
of time, because de Sitter space can spawn many spacetime
bubbles. Any one bubble has an arrow of
time going in only one direction, but another
bubble’s arrow could point the opposite way.
Overall, time symmetry would be preserved.
In his 2002 book A New Kind of Science, for instance,
mathematician Stephen Wolfram cited computer simulations that he claimed were evidence that the second law is
simply wrong. Others have sought loopholes in the law by
examining various scenarios, especially where quantum
effects come into play.
“Over the last 10–15 years an unparalleled number of chal-
Occupants of any given bubble would always
believe the Big Bang creating their bubble was
in their past. From the view of a superobserver
outside of it all, though, it would be clear that
time moves in both directions, just as the laws
of physics always indicated.
lenges has been proposed against the status
“If and when we of the Second Law of Thermodynamics,”
get a really, really Italian physicist Germano D’Abramo notes
in a recent issue of Physics Letters A. During
that time, more than 50 such papers have
appeared in the refereed literature, he writes.
relativity say, But regardless of the second law’s ultimate
or quantum fate, to all appearances time’s fuse continues
mechanics ... then
a large ingredient
to burn in one direction, and considerable
expenditure of scientific effort continues
“The point is that the whole shebang, the
whole multiverse, is symmetric with respect
to time overall,” Carroll said.
in an attempt to explain it. Considering all
in that revolution
the time that has passed without a foolproof
will be radical
solution, it seems highly likely that some
pieces of the time-direction puzzle remain
Beyond the law missing. And that’s not so shocking. Phys-He cautions that this idea is just a proposal; ics has, to put it bluntly, failed in its efforts
revision of our
ideas about the
arrow of time.”
rigorous proof awaits considerable progress to explain several serious mysteries despite
in cosmologists’ calculational prowess. And decades of effort.
it is only one of many ideas for reconciling In the 20th century, the great scientific
timeless physical laws with time’s directional arrow. Physi- revolutions of relativity theory and quantum mechanics
cist Lorenzo Maccone of MIT, for instance, points out that opened enormous windows into the intricate inner workings
the second law actually does allow both directions of time. of nature. But many of the deepest questions still remain
But anything happening in the reverse direction would unanswered. What is the universe mostly made of? A type of
leave no trace, no record or even a memory. (If you scram- matter of unknown identity plus an equally mysterious form
bled some eggs and then time reversed itself, not only of energy present in all space with an unexplainable level
would the eggs unscramble, but the nerve cells in your of density. Are the large-scale laws (basically gravity) ulti-
brain would be restored to their previous condition, wiping mately incompatible with the quantum laws of the micro-
out your knowledge of the original scrambling.) So if time world? Nobody wants to believe that, but efforts to combine
did flow backward, nobody would be aware of it anyway, gravity and quantum physics never quite succeed.
Maccone proposed in a paper published last year in Simmering in these issues is a sense among some scien-
Physical Review Letters. tists that the 21st century will produce another revolution
Still, even this approach, Maccone has acknowledged, does of Einsteinian magnitude — it’s just a matter of time. And
not explain why the level of entropy in what scientists per- some suggest that solving the mystery of time’s arrow might
ceive to be the past started out so much lower than what the require (or enable) such a revolution. Among this group
entropy is today. is Nobel laureate Anthony Leggett, a University of Illinois
To add yet another complication, solutions to time’s riddle physicist who spoke at the AAAS meeting.
invoking the second law of thermodynamics might depend “If and when we get a really, really major revolution,”
on the validity of the second law itself. Explaining time with Leggett says, “comparable to relativity say, or quantum
the aid of the second law might not work so well if that law mechanics, sometime in the next few decades in physics,
turns out to be wrong. then a large ingredient in that revolution will be radical revi-
It is widely considered unforgivable heresy to doubt the sion of our ideas about the arrow of time.”
second law, thanks in part to a famous passage declaring its
infallibility by the British astrophysicist Sir Arthur Eddington
s For more resources about how physicists and philosophers
understand time, visit www.sciencenews.org/arrowoftime