If any of you haven't seen it, you will likely be quite interesting the The
Edge's list of responses to this year's question, "What scientific idea is
ready for retirement?" Some of the answers are fascinating, some are
absurd, and some are confusing. Take a look!
My favorite comes from Amanda Gefter. I'll reproduce it below. (Hopefully
that counts as fair use.)
Consultant, New Scientist; Founding Editor, CultureLab
Physics has a time-honored tradition of laughing in the face of our most
basic intuitions. Einstein's relativity forced us to retire our notions of
absolute space and time, while quantum mechanics forced us to retire our
notions of pretty much everything else. Still, one stubborn idea has stood
steadfast through it all: the universe.
Sure, our picture of the universe has evolved over the years—its history
dynamic, its origin inflating, its expansion accelerating. It has even been
downgraded to just one in a multiverse of infinite universes forever
divided by event horizons. But still we've clung to the belief that here,
as residents in the Milky Way, we all live in a single spacetime, our
shared corner of the cosmos—our universe.
In recent years, however, the concept of a single, shared spacetime has
sent physics spiraling into paradox. The first sign that something was
amiss came from Stephen Hawking's landmark work in the 1970s showing that
black holes radiate and evaporate, disappearing from the universe and
purportedly taking some quantum information with them. Quantum mechanics,
however, is predicated upon the principle that information can never be
Here was the conundrum. Once information falls into a black hole, it can't
climb back out without traveling faster than light and violating
relativity. Therefore, the only way to save it is to show that it never
fell into the black hole in the first place. From the point of view of an
accelerated observer who remains outside the black hole, that's not hard to
do. Thanks to relativistic effects, from his vantage point, the information
stretches and slows as it approaches the black hole, then burns to
scrambled ash in the heat of the Hawking radiation before it ever crosses
the horizon. It's a different story, however, for the inertial, infalling
observer, who plunges into the black hole, passing through the horizon
without noticing any weird relativistic effects or Hawking radiation,
courtesy of Einstein's equivalence principle. For him, information better
fall into the black hole, or relativity is in trouble. In other words, in
order to uphold all the laws of physics, one copy of the bit of information
has to remain outside the black hole while its clone falls inside. Oh, and
one last thing—quantum mechanics forbids cloning.
Leonard Susskind eventually solved the information paradox by insisting
that we restrict our description of the world to either the region of
spacetime outside the black hole's horizon or to the interior of the black
hole. Either one is consistent—it's only when you talk about both that you
violate the laws of physics. This "horizon complementarity," as it became
known, tells us that the inside and outside of the black hole are not part
and parcel of a single universe. They are *two* universes, but not in the
Horizon complementarity kept paradox at bay until last year, when the
physics community was shaken up by a new conundrum more harrowing still—
the so-called firewall paradox. Here, our two observers find themselves
with contradictory quantum descriptions of a single bit of information, but
now the contradiction occurs while both observers are still outside the
horizon, before the inertial observer falls in. That is, it occurs while
they're still supposedly in the same universe.
Physicists are beginning to think that the best solution to the firewall
paradox may be to adopt "strong complementarity"—that is, to restrict our
descriptions not merely to spacetime regions separated by horizons, but to
the reference frames of individual observers, wherever they are. As if each
observer has his or her own universe*.*
Ordinary horizon complementarity had already undermined the possibility of
a multiverse. If you violate physics by describing two regions separated by
a horizon, imagine what happens when you describe *infinite* regions
separated by *infinite *horizons! Now, strong complementarity is
undermining the possibility of a single, shared universe. On glance, you'd
think it would create its own kind of multiverse, but it doesn't. Yes,
there are multiple observers, and yes, any observer's universe is as good
as any other. But if you want to stay on the right side of the laws of
physics, you can only talk about one at a time. Which means, really, that
only one *exists* at a time. It's cosmic solipsism.
Sending the universe into early retirement is a pretty radical move, so it
better buy us something pretty in the way of scientific advancement. I
think it does. For one, it might shed some light on the disconcerting low
quadrupole coincidence—the fact that the cosmic microwave background
radiation shows no temperature fluctuations at scales larger than 60
degrees on the sky, capping the size of space at precisely the size of our
observable universe – as if reality abruptly stops at the edge of an
observer's reference frame.
More importantly, it could offer us a better conceptual grasp of quantum
mechanics. Quantum mechanics defies understanding because it allows things
to hover in superpositions of mutually exclusive states, like when a photon
goes through this slit *and* that slit, or when a cat is simultaneously
dead *and* alive. It balks at our Boolean logic, it laughs at the law of
the excluded middle. Worse, when we actually observe something, the
superposition vanishes and a single reality miraculously unfurls.
In light of the universe's retirement, this all looks slightly less
miraculous. After all, superpositions are really superpositions of
reference frames. In any single reference frame, an animal's vitals are
well defined. Cats are only alive *and* dead when you try to piece together
multiple frames under the false assumption that they're all part of the
Finally, the universe's retirement might offer some guidance as physicists
push forward with the program of quantum gravity. For instance, if each
observer has his or her own universe, then each observer has his or her own
Hilbert space, his or her own cosmic horizon and his or her own version of
holography, in which case what we need from a theory of quantum gravity is
a set of consistency conditions that can relate what different observers
can operationally measure.
Adjusting our intuitions and adapting to the strange truths uncovered by
physics is never easy. But we may just have to come around to the notion
that there's my universe, and there's your universe—but there's no such
thing as *the *universe.
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