Squeezed
between two pieces of diamond, hydrogen has been transformed into a
metallic form believed to exist inside giant planets like Jupiter, scientists reported on Thursday.
“You can see it becomes a lustrous, shiny material, which is what you expect for a metal,” said Isaac F. Silvera, a professor of physics at Harvard.
If
some theoretical predictions turn out to be true, the new state of
hydrogen could even be a solid metal that is metastable — remaining
solid even after the crushing pressure is removed — and a
superconductor, able to conduct electricity without resistance, Dr.
Silvera said.
Dr. Silvera and Ranga P. Dias, a postdoctoral researcher, published the findings on Thursday in the journal Science.
But
in the small but contentious field of high-pressure physics, some
scientists who perform similar experiments were harshly skeptical and
wondered how the research passed peer review at a top journal like
Science.
“It’s
— how should I put it? — the product of Ike’s imagination from the
title to the end,” said Eugene Gregoryanz, a physicist at the University
of Edinburgh in Scotland.
Paul
Loubeyre, a physicist at France’s Atomic Energy Commission, wrote in an
email, “The fact that the paper went through illustrates the fact that
the reviewing process has some flaws.”
Editors
at Science declined to discuss the paper, but in a statement, Jeremy
Berg, the editor in chief, said that submissions must pass rigorous
review by experts and that only about 7 percent are published.
Dr. Silvera defended his work. “If we did it again, we’d get the same result, I’m certain,” he said.
Hydrogen
is the lightest of elements; each atom consists only of one proton and
one electron. In the ordinary conditions at Earth’s surface, where the
weight of air presses at 14.7 pounds per square inch, hydrogen atoms
pair up into simple molecules.
At
supercold temperatures, hydrogen molecules first condense into a
liquid, then a solid with the molecules intact. When squeezed together,
the molecules stack up into a solid form. More than 80 years ago, the
physicists Eugene Wigner and Hillard Bell Huntington predicted that
hydrogen, under high enough pressures, would turn metallic, with the
hydrogen molecules broken apart and the electrons squeezed loose.
Scientists
have momentarily transformed hydrogen into a liquid metallic state with
violent shock wave experiments. Planetary scientists are convinced
metallic must exist inside Jupiter, to generate that planet’s powerful
magnetic fields.
But so far, no one has convincingly demonstrated the solid metallic hydrogen state predicted by Wigner and Huntington.
In
the Harvard experiment, a small amount of hydrogen was placed between
two diamond tips about one 850th of an inch in diameter and cooled to
-433 degrees Fahrenheit. Initially transparent, the hydrogen became dark
as it was squeezed. Then, at nearly 72 million pounds per square inch,
the hydrogen became shiny, reflecting 90 percent of light shining on it,
the scientists reported.
However,
they did not make additional measurements to confirm other metallic
properties, like the ability to conduct electricity as metals do, and
they did not show that it is solid.
Only
a handful of research teams around the world have been attempting to
squeeze hydrogen into a metal. While the experiments are not large — the
Harvard apparatus is about the size of a soda bottle — the techniques
require care and precision. Imperfect diamonds often shatter under
pressure. Hydrogen can leak out. Precise measurements are tricky.
Dr.
Silvera said his group had developed techniques to polish, etch and
coat the diamond surfaces with aluminum oxide to enable the diamonds to
squeeze together without breaking.
Still,
Dr. Gregoryanz was incredulous that the Harvard scientists could have
achieved the ultrahigh pressure they report, which is 20 percent higher
than their competitors, on the first try, in October, and that they have
not yet attempted to duplicate the feat.
“You
make such a big and bold claim,” said Dr. Gregoryanz, who said he
typically has one success out of 10 to 15 tries. “Why didn’t you repeat
the experiment?”
The
pressure estimates are also indirect estimates. “That is probably the
weakest point in this paper,” said Reinhard Boehler, a high-pressure
scientist at the Carnegie Institution for Science in Washington, D.C.,
and Oak Ridge National Laboratory in Tennessee.
“Literally
there was a screw turned, and a pressure was estimated,” he said. He
said the optical measurements were also weak. “If true, fantastic, but
there’s a lot of ifs and buts in this publication.”
Dr.
Silvera said that he will be performing additional measurements called
Raman scattering — shining laser light on it and seeing how the atoms
are excited—- and then take it to Argonne National Laboratory in
Illinois to probe it with a bright beam of X-rays. Those measurements,
if successful, could tell more about the state of the hydrogen.
If
the sample survives without breaking, Dr. Silvera said he wanted to
then open it up, relieving it of pressure, and see if there is a speck
of solid metallic hydrogen still inside. Some theorists have predicted
that solid metallic hydrogen, if it can be produced, would remain in
that state, in much the same way a diamond stays a diamond instead of
reshaping itself into graphite, a lower-energy configuration of carbon
atoms.
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