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The world becomes a fuzzy, surreal place at its smallest levels,
according to quantum physics. It has long been a mystery why strange
quantum behavior is not seen at larger scales in everyday life. Now
researchers find that the way Earth warps time could help explain this
division.
One mind-boggling consequence of quantum physics is that atoms and
subatomic particles can actually exist in states known as
“superpositions,” meaning they could literally be located in two or more
places at once, for instance, until “observed” — that is, until they
interact with surrounding particles in some way. This concept is often
illustrated using an analogy called Schrödinger’s cat, in which a cat is
both dead and alive until beheld.
Superpositions are very fragile. Once disturbed in some way, they
collapse or “decohere” to just a single outcome. As such, they often
involve objects just a few particles large at most — the bigger an
object in superposition is, the more difficult it is to keep it
unperturbed. However, it is a mystery at what scale the realm of quantum
physics ended and the one of classical physics begins, and why such a
boundary exists in the first place.
Now researchers suggest that Einstein’s theory of space and time
could help explain this shift from quantum to classical physics.
The
scientists detailed their findings online June 15 in the journal Nature Physics.
A century ago, Einstein’s theory of relativity explained that gravity
occurred when mass curved the fabric of space and time. If one
envisions a bunch of balls on a rubber sheet, the more massive a ball
is, the more it dents the rubber around it and pulls other balls closer.
One curious consequence of the theory of relativity was time
dilation, which means that time moves slower the closer one is to a
massive object. Although this effect is small on Earth, it is
noticeable. “If you live in a two-story house on the top floor, you will
age faster than your neighbor below by about 10 nanoseconds in one
year,” said quantum physicist Igor Pikovski
at the Harvard-Smithsonian Center for Astrophysics. “This tiny effect
has actually been confirmed in many experiments with very precise
clocks.”
Pikovski and his colleagues calculated that once small building
blocks form bigger composite objects, such as molecules and eventually
microbes or dust motes, the time dilation experienced on Earth’s surface
can suppress superpositions. “It was very exciting to find that
Einstein’s time dilation can matter,” Pikovski said.
An object’s energy makes it vibrate. According to the laws of quantum
physics, when an object is in superposition, all its parts vibrate in
synchrony. However, time dilation will cause parts of that object that
are at slightly higher altitudes to jitter at different frequencies than
parts at slightly lower altitudes, just as a neighbor living in a floor
below you will age more slowly than you. The greater the difference in
altitude between parts, the greater the mismatch. For a big enough
object, the mismatch grows big enough to disrupt superpositions.
“I am overjoyed to see any new ideas on the influence of gravity in quantum objects,” said experimental physicist Holger Müller at the University of California at Berkeley, who did not take part in this research.
To prove this effect occurs, scientists need to create large
superposed objects in which they have suppressed every other possible
source of decoherence, such as heat. If time dilation can collapse
superpositions, then the size limit for a superposed item on Earth
should be about a millimeter.
Müller said that the creation of such large superposed objects would
be very unlikely, since there are many potential sources of decoherence
besides time dilation.
Pikovski was more optimistic. “In principle, it should be possible to
overcome these limitations,” he said. “It’s just a matter of
technology.” He pointed out that, thanks to technological advancements
for creating superpositions, scientists can now create superpositions in
objects that weigh as much as thousands of atoms.
These findings suggest that outside of Earth’s gravitational field,
scientists may be able to see superpositions in objects even larger than
a millimeter. “So far, we don’t have any good reason to believe that
quantum theory breaks down,” Pikovski said.
Charles
Q. Choi is a freelance science writer based in New York City who has
written for The New York Times, Scientific American, Wired, Science,
Nature, and many other news outlets. He tweets at @cqchoi.