like an innocent enough question, right? Absolute zero is 0 on the
Kelvin scale, or about minus 460 F. You can't get colder than that; it
would be like trying to go south from the South Pole. Is there a
corresponding maximum possible temperature?
the answer, depending on which theoretical physicist you ask, is yes,
no, or maybe. Huh? you ask. Yeah, that's how I felt. And the question
doesn't just mess with the minds of physics dummies like me. Several
physicists begged off of trying to answer it, referring me to
colleagues. Even ones who did talk about it said things like "It's a
little bit out of my comfort zone" and "I think I'd like to ruminate
I posed it to one cosmologist, there was dead silence on the other end
of the line for long enough that I wondered if we had a dropped call.
had touched a nerve, because, unbeknownst to me, the
highest-temperature question gets to the heart of current inquiries and
proposed theories in cosmology and theoretical physics. Indeed,
scientists who work in these fields are zealously trying to answer that
question. Why? Because, in some sense, nothing less than the future
course of physics rests on the answer.
Contender #1—1032 K
cosmological models, including the one that has held sway for decades,
the Standard Model, posit a theoretical highest temperature. It's
called the Planck temperature, after the Austrian physicist Max Planck,
and it equals about 100 million million million million million
degrees, or 1032 Kelvin. "It's ridiculous is what it is," said Columbia
physicist Arlin Crotts when I asked him if he could please put that
number in perspective for me. "It's a billion billion times the largest
temperature that we have to think about" (in gamma-ray bursts and
quasars, for instance). Oh, that helped.
when contemplating the Planck temperature, you can forget perspective.
All the usual terms for very hot—scorching, broiling, hellish, insert
your favorite here—prove ludicrously inadequate. In short, saying 1032
K is hot is like saying the universe occupies some space. (For a game
attempt at perspective, see A Sense of Scale.)
Whatever the highest temperature is, it might be essentially equivalent to the coldest temperature.
conventional physics—that is, the kind that relies on Einstein's theory
of general relativity to describe the very large and quantum mechanics
to describe the very small—the Planck temperature was reached 10-43
seconds after the Big Bang got under way. At that instant, known as one
Planck time, the entire universe is thought to have been the Planck
length, or 10-35 meters. (In physics, Max Planck is the king of the
eponymous.) An awfully high temperature in an awfully small space in an
awfully short time after … well, after what? That's arguably an even
bigger question—how did the universe begin?—and we won't go there.
A brick wall
Planck temperature is the highest temperature in conventional physics
because conventional physics breaks down at that temperature. Above
1032 K—that is, earlier than one Planck time—calculations show that
strange things, unknown things, begin to happen to phenomena we hold
near and dear, like space and time. Theory predicts that particle
energies become so large that the gravitational forces between them
become as strong as any other forces. That is, gravity and the other
three fundamental forces of the universe—electromagnetism and the
strong and weak nuclear forces—become a single unified force. Knowing
how that happens, the so-called "theory of everything," is the holy
grail of theoretical physics today.
do not know enough about the quantum nature of gravitation even to
speculate intelligently about the history of the universe before this
time," writes Nobel laureate Steven Weinberg about this
up-against-a-brick-wall instant in his book The First Three Minutes.
"Thus, whatever other veils may have been lifted, there is one veil, at
a temperature of 1032 K, that still obscures our view of the earliest
times." Until someone comes up with a widely accepted quantum theory of
gravity, the Planck temperature, for conventional physicists like
Steven Weinberg, will remain the highest temperature. Read