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New experimental evidence that black holes emit a form of radiation is reported in a paper published online this week in Nature Physics. The study uses an acoustic model of a black hole — from which sound, rather than light, cannot escape — to observe Hawking-like radiation.
Access the news article and the Nature Physics paper.
SOURCE: NATURE | NEWS
 Artificial black hole creates its own version of Hawking radiation
Result could be closest thing yet to an observation of the bizarre phenomenon.
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NASA and G. Bacon (STScI)
Nothing, not even light, can escape the event horizon of a black hole.
Black holes are not actually black. Instead, these gravitational sinks are thought to emit radiation that causes them to shrink and eventually disappear. This phenomenon, one of the weirdest things about black holes, was predicted by Stephen Hawking more than 40 years ago, creating problems for theoretical physics that still convulse the field.
Now, after seven years of often solitary study, Jeff Steinhauer, an experimental physicist at the Technion-Israel Institute of Technology in Haifa, has created an artificial black hole that seems to emit such ‘Hawking radiation’ on its own, from quantum fluctuations that emerge from its experimental set-up.
It is nearly impossible to observe Hawking radiation in a real black hole, and previous artificial-black-hole experiments did not trace their radiation to spontaneous fluctuations. So the result, published on 15 August1, could be the closest thing yet to an observation of Hawking radiation.
Steinhauer says that black-hole analogues might help to solve some of the dilemmas that the phenomenon poses for other theories, including one called the black-hole information paradox, and perhaps point the way to uniting quantum mechanics with a theory of gravity.
Other physicists are impressed, but they caution that the results are not clear-cut. And some doubt whether laboratory analogues can reveal much about real black holes. “This experiment, if all statements hold, is really amazing,” says Silke Weinfurtner, a theoretical and experimental physicist at the University of Nottingham, UK. “It doesn’t prove that Hawking radiation exists around astrophysical black holes.”
It was in the mid-1970s that Hawking, a theoretical physicist at the University of Cambridge, UK, discovered that the event horizon of a black hole — the surface from which nothing, including light, can escape — should have peculiar consequences for physics.
His starting point was that the randomness of quantum theory ruled out the existence of true nothingness. Even the emptiest region of space teems with fluctuations in energy fields, causing photon pairs to appear continuously, only to immediately destroy each other. But, just as Pinocchio turned from a puppet into a boy, these ‘virtual’ photons could become real particles if the event horizon separated them before they could annihilate each other. One photon would fall inside the event horizon and the other would escape into outer space.
This, Hawking showed, causes black holes both to radiate — albeit extremely feebly — and to ultimately shrink and vanish, because the particle that falls inside always has a ‘negative energy’ that depletes the black hole. Most controversially, Hawking also suggested that a black hole’s disappearance destroys all information about objects that have fallen into it, contradicting the accepted wisdom that the total amount of information in the Universe stays constant.
In the early 1980s, physicist Bill Unruh of the University of British Columbia in Vancouver, Canada, proposed a way to test some of Hawking’s predictions2. He imagined a medium that experienced accelerated motion, such as water approaching a waterfall. Like a swimmer reaching a point where he cannot swim fast enough away to escape the waterfall, sound waves that are past the point in the medium that surpasses the speed of sound would become unable to move against the flow. Unruh predicted that this point is equivalent to an event horizon — and that it should display a sonic form of Hawking radiation.
Steinhauer implemented Unruh’s idea in a cloud of rubidium atoms that he cooled to a fraction of a degree above absolute zero. Contained in a cigar-shaped trap a few millimetres long, the atoms entered a quantum state called a Bose–Einstein condensate (BEC), in which the speed of sound was just half a millimetre per second. Steinhauer created an event horizon by accelerating the atoms until some were travelling at more than 1 mm s−1 — a supersonic speed for the condensate (see ‘Building a black hole’).
At its ultracold temperature, the BEC undergoes only weak quantum fluctuations that are similar to those in the vacuum of space. And these should produce packets of sound called phonons, just as the vacuum produces photons, Steinhauer says. The partners should separate from each other, with one partner on the supersonic side of the horizon and the other forming Hawking radiation.
On one side of his acoustical event horizon, where the atoms move at supersonic speeds, phonons became trapped. And when Steinhauer took pictures of the BEC, he found correlations between the densities of atoms that were an equal distance from the event horizon but on opposite sides. This demonstrates that pairs of phonons were entangled — a sign that they originated spontaneously from the same quantum fluctuation, he says, and that the BEC was producing Hawking radiation.
By contrast, radiation that he observed in an earlier version of the set-up had to be triggered rather than arising from the BEC itself3, whereas a previous experiment in water waves led by Unruh and Weinfurtner did not attempt to show quantum effects4.
Just as real black holes are not black, Steinhauer’s acoustical black holes are not completely quiet. Their sound, if it were audible, might resemble static noise.
“For sure, this is a pioneering paper,” says Ulf Leonhardt, a physicist at the Weizmann Institute of Science in Rehovot, Israel, who leads a different attempt to demonstrate the effect, using laser waves in an optical fibre. But he says that the evidence of entanglement seems incomplete, because Steinhauer demonstrated correlations only for phonons of relatively high energies, with lower-energy phonon pairs seemingly not correlated. He also says he’s not confident that the medium is a true BEC, which, he says, means that there could be other types of fluctuation that could mimic Hawking radiation.
Also unclear is what analogues can say about the mysteries surrounding true black holes. “I don’t believe it will illuminate the so-called information paradox,” says Leonard Susskind, a theoretical physicist at Stanford University in California. In contrast to the case of astrophysical black holes, there is no information loss in Steinhauer’s sonic black hole because the BEC does not evaporate.
Still, if Steinhauser’s results were confirmed, it would be “a triumph for Hawking, perhaps in the same sense that the expected detection of the Higgs boson was a triumph for Higgs and company”, says Susskind. Few doubted that the particle existed, but its discovery in 2012 still earned Peter Higgs and another theorist, François Englert, who predicted it, a Nobel prize.
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domingo, 24 de julio de 2016
Weird quantum effects stretch across hundreds of miles
July 19, 2016
Source:Phys.org
In the world of quantum,
infinitesimally small particles, weird and often logic-defying behaviors
abound. Perhaps the strangest of these is the idea of superposition, in which
objects can exist simultaneously in two or more seemingly counterintuitive states.
For example, according to the laws of quantum mechanics, electrons may spin
both clockwise and counter-clockwise, or be both at rest and excited, at the
same time.
The physicist Erwin
Schrödinger highlighted some strange consequences of the idea of superposition
more than 80 years ago, with a thought experiment that posed that a cat trapped
in a box with a radioactive source could be in a superposition state, considered
both alive and dead, according to the laws of quantum mechanics. Since then,
scientists have proven that particles can indeed be in superposition, at
quantum, subatomic scales. But whether such weird phenomena can be observed in
our larger, everyday world is an open, actively pursued question.
Now, MIT physicists have
found that subatomic particles called neutrinos can be in superposition, without individual identities, when
traveling hundreds of miles. Their results, to be published later this month in Physical
Review Letters, represent the longest distance over which quantum mechanics
has been tested to date.
A subatomic journey across
state lines
The team analyzed data on
the oscillations of neutrinos—subatomic particles that interact extremely
weakly with matter, passing through our bodies by the billions per second
without any effect. Neutrinos can oscillate, or change between several distinct
"flavors," as they travel through the universe at close to the speed
of light.
The researchers obtained
data from Fermilab's Main Injector Neutrino Oscillation Search, or MINOS, an
experiment in which neutrinos are produced from the scattering of other
accelerated, high-energy particles in a facility near Chicago and beamed to a
detector in Soudan, Minnesota, 735 kilometers (456 miles) away. Although the
neutrinos leave Illinois as one flavor, they may oscillate along their journey,
arriving in Minnesota as a completely different flavor.
The MIT team studied the
distribution of neutrino flavors generated in Illinois, versus those detected
in Minnesota, and found that these distributions can be explained most readily
by quantum phenomena: As neutrinos sped between the reactor and detector, they
were statistically most likely to be in a state of superposition, with no
definite flavor or identity.
What's more, the
researchers found that the data was "in high tension" with more
classical descriptions of how matter should behave. In particular, it was
statistically unlikely that the data could be explained by any model of the
sort that Einstein sought, in which objects would always embody definite
properties rather than exist in superpositions.
"What's fascinating
is, many of us tend to think of quantum mechanics applying on small
scales," says David Kaiser, the Germeshausen Professor of the History of
Science and professor of physics at MIT. "But it turns out that we can't
escape quantum mechanics, even when we describe processes that happen over
large distances. We can't stop our quantum mechanical description even when
these things leave one state and enter another, traveling hundreds of miles. I
think that's breathtaking."
Kaiser is a co-author on
the paper, which includes MIT physics professor Joseph Formaggio, junior Talia
Weiss, and former graduate student Mykola Murskyj
A flipped inequality
The team analyzed the MINOS
data by applying a slightly altered version of the Leggett-Garg inequality, a
mathematical expression named after physicists Anthony Leggett and Anupam Garg,
who derived the expression to test whether a system with two or more distinct
states acts in a quantum or classical fashion.
Leggett and Garg realized
that the measurements of such a system, and the statistical correlations
between those measurements, should be different if the system behaves according
to classical versus quantum mechanical laws.
"They realized you get
different predictions for correlations of measurements of a single system over
time, if you assume superposition versus realism," Kaiser explains, where
"realism" refers to models of the Einstein type, in which particles
should always exist in some definite state.
Formaggio had the idea to
flip the expression slightly, to apply not to repeated measurements over time
but to measurements at a range of neutrino energies. In the MINOS experiment,
huge numbers of neutrinos are created at various energies, where Kaiser says
they then "careen through the Earth, through solid rock, and a tiny
drizzle of them will be detected" 735 kilometers away.
According to Formaggio's
reworking of the Leggett-Garg inequality, the distribution of neutrino
flavors—the type of neutrino that finally arrives at the detector—should depend
on the energies at which the neutrinos were created. Furthermore, those flavor
distributions should look very different if the neutrinos assumed a definite
identity throughout their journey, versus if they were in superposition, with
no distinct flavor.
"The big world we live
in"
Applying their modified
version of the Leggett-Garg expression to neutrino oscillations, the group
predicted the distribution of neutrino flavors arriving at the detector, both
if the neutrinos were behaving classically, according to an Einstein-like
theory, and if they were acting in a quantum state, in superposition. When they compared both predicted distributions, they found there was
virtually no overlap.
More importantly, when they
compared these predictions with the actual distribution of neutrino flavors
observed from the MINOS experiment, they found that the data fit squarely
within the predicted distribution for a quantum system, meaning that the
neutrinos very likely did not have individual identities while traveling over
hundreds of miles between detectors.
But what if these particles
truly embodied distinct flavors at each moment in time, rather than being some
ghostly, neither-here-nor-there phantoms of quantum physics? What if these
neutrinos behaved according to Einstein's realism-based view of the world?
After all, there could be statistical flukes due to defects in instrumentation,
that might still generate a distribution of neutrinos that the researchers
observed. Kaiser says if that were the case and "the world truly obeyed
Einstein's intuitions," the chances of such a model accounting for the
observed data would be "something like one in a billion."
"What gives people
pause is, quantum mechanics is quantitatively precise and yet it comes with all
this conceptual baggage," Kaiser says. "That's why I like tests like
this: Let's let these things travel further than most people will drive on a
family road trip, and watch them zoom through the big world we live in, not
just the strange world of quantum mechanics, for hundreds of miles. And even
then, we can't stop using quantum mechanics. We really see quantum effects persist across macroscopic
distances."
SOURCE: PHYS .ORG
LUX Dark Matter Experiment ..............
The LUX Dark Matter
Experiment operates a mile underground at the Sanford Underground Research
Facility. It's location helps shield the detector from background radiation
that could confound a dark matter signal.
Credit: C. H. Faham
The Large Underground Xenon
(LUX) dark matter experiment, which operates beneath a mile of rock at the
Sanford Underground Research Facility in the Black Hills of South Dakota, has
completed its silent search for the missing matter of the universe.
Today at an international
dark matter conference (IDM 2016) in Sheffield, U.K., LUX scientific
collaborators presented the results from the detector's final 20-month run from
October 2014 to May 2016. The new research result is also described with
further details on the LUX Collaboration's website.
LUX's sensitivity far
exceeded the goals for the project, collaboration scientists said, but yielded
no trace of a dark matter particle. LUX's extreme sensitivity makes the team
confident that if dark matter particles had interacted with the LUX's xenon
target, the detector would almost certainly have seen it. That enables
scientists to confidently eliminate many potential models for dark matter
particles, offering critical guidance for the next generation of dark matter
experiments.
"LUX has delivered the
world's best search sensitivity since its first run in 2013," said Rick
Gaitskell, professor of physics at Brown University and co-spokesperson for the
LUX experiment. "With this final result from the 2014 to 2016 search, the
scientists of the LUX Collaboration have pushed the sensitivity of the
instrument to a final performance level that is four times better than the
original project goals. It would have been marvelous if the improved
sensitivity had also delivered a clear dark matter signal. However, what we
have observed is consistent with background alone."
Dark matter is thought to
account for more than four-fifths of the mass in the universe. Scientists are
confident of its existence because the effects of its gravity can be seen in
the rotation of galaxies and in the way light bends as it travels through the
universe, but experiments have yet to make direct contact with a dark matter
particle. The LUX experiment was designed to look for weakly interacting massive
particles, or WIMPs, the leading theoretical candidate for a dark matter
particle. If the WIMP idea is correct, billions of these particles pass through
your hand every second, and also through the Earth and everything on it. But
because WIMPs interact so weakly with ordinary matter, this ghostly traverse
goes entirely unnoticed.
The LUX detector consists
of a third-of-a-ton of cooled liquid xenon surrounded by powerful sensors
designed to detect the tiny flash of light and electrical charge emitted if a WIMP
collides with a xenon atom within the tank. The detector's location at Sanford
Lab beneath a mile of rock, and inside a 72,000-gallon, high-purity water tank,
helps shield it from cosmic rays and other radiation that would interfere with
a dark matter signal.
The 20-month run of LUX
represents one of the largest exposures ever collected by a dark matter
experiment, the researchers said. The rapid analysis of nearly a half-million
gigabytes of data produced over 20 months was made possible by the use of more
than 1,000 computer nodes at Brown University's Center for Computation and
Visualization and the advanced computer simulations at Lawrence Berkeley
National Laboratory's National Energy Research Scientific Computing Center.
Careful calibration
The exquisite sensitivity
achieved by the LUX experiment came thanks to a series of pioneering
calibration measures aimed at helping scientists tell the difference between a
dark matter signal and events created by residual background radiation that
even the elaborate construction of the experiment cannot completely block out.
"As the charge and
light signal response of the LUX experiment varied slightly over the dark
matter search period, our calibrations allowed us to consistently reject
radioactive backgrounds, maintain a well-defined dark matter signature for
which to search and compensate for a small static charge buildup on the Teflon
inner detector walls," said Dan McKinsey, professor of physics at the
University of California, Berkeley, senior faculty scientist at Lawrence
Berkeley National Laboratory, and co-spokesperson for the LUX experiment.
One calibration technique
used neutrons as stand-ins for WIMPs. By firing a beam of neutrons into the
detector, scientists were able to carefully quantify how the LUX detector
responds to the signal expected to be produced from a WIMP collision. Other
calibration techniques involved injecting radioactive gases into the detector
to help distinguish between signals produced by ambient radioactivity and a
potential dark matter signal.
These calibration measures,
used for the first time with LUX, helped scientists meticulously search through
a wide swath of potential parameter space for dark matter particles.
The LUX dark matter
detector is surrounded by light sensors that can detect the emission of just a
single photon. Those sensors are designed to capture the tiny flash of light
emitted if a dark matter particle were to interact with the …more
"These careful
background-reduction techniques and precision calibrations and modeling have
enabled us to probe dark matter candidates that would produce signals of only a
few events per century in a kilogram of xenon," said Aaron Manalaysay, the
analysis working group coordinator of the LUX experiment and a research
scientist from the University of California, Davis, who presented the new
results in Sheffield.
"We worked hard and
stayed diligent over more than a year and a half to keep the detector running
in optimal conditions and maximize useful data time," said Simon Fiorucci,
physicist at Lawrence Berkeley National Laboratory and science coordination
manager for the experiment. "The result is unambiguous data we can be
proud of and a timely result in this very competitive field—even if it is not
the positive detection we were all hoping for."
The quest
continues
While the LUX experiment
successfully eliminated a large swath of mass ranges and interaction-coupling
strengths where WIMPs might exist, the WIMP model itself, "remains alive
and viable," said Gaitskell, the Brown University physicist. And the
meticulous work of LUX scientists will aid future direct detection experiments.
"We viewed this as a
David and Goliath race between ourselves and the much larger Large Hadron
Collider (LHC) at CERN in Geneva," Gaitskell said. "LUX was racing
over the last three years to get first evidence for a dark matter signal.
We
will now have to wait and see if the new run this year at the LHC will show
evidence of dark matter particles, or if the discovery occurs in the next
generation of larger direct detectors."
Among those next generation
experiments will be the LUX-ZEPLIN (LZ) experiment, which will replace LUX at
the Sanford Underground Research Facility.
Compared to LUX's
one-third-ton of liquid xenon, LZ will have a 10-ton liquid xenon target, which
will fit inside the same 72,000-gallon tank of pure water used by LUX to help
fend off external radiation.
"The innovations of
the LUX experiment form the foundation for the LZ experiment," said Harry
Nelson, University of California, Santa Barbara, and spokesperson for LZ.
"We expect LZ to achieve 70 times the sensitivity of LUX. The LZ program
continues to pass its milestones, aided by the terrific support of the Sanford
Lab, the Department of Energy and its many collaborating institutions and
scientists. LZ should be online in 2020."
LUX, the first major
astrophysics experiment in the Davis Campus of the Sanford Underground Research
Facility (Sanford Lab), was installed in 2012. Sanford Lab is located in the
former Homestake Gold Mine in Lead, S.D. A South Dakota-owned facility, it is
managed by the South Dakota Science and Technology Authority (SDSTA), which
reopened the mine in 2007 with $40 million in funding from the South Dakota
State Legislature and a $70 million donation from philanthropist T. Denny
Sanford. The U.S. Department of Energy (DOE) supports Sanford Lab's operations.
"The global search for
dark matter aims to answer fundamental questions about the makeup of our
universe. We're proud to support the LUX collaboration and congratulate them on
reaching this higher level of sensitivity," said Mike Headley, executive
director of the SDSTA. "We're looking forward to hosting the LUX-ZEPLIN
(LZ) experiment, which will provide another major step forward in
sensitivity."
The LUX scientific
collaboration, which is supported by the DOE and National Science Foundation,
includes 20 research universities and national laboratories in the United
States, the United Kingdom and Portugal.
Over the next few months,
LUX scientists will continue to analyze the crucial data that LUX was able to
provide, in hopes of helping future experiments finally pin down a dark matter particle.
"LUX has done much
more in terms of its sensitivity and reliability than we ever expected it to
do," Gaitskell said. "We always want more time with our detectors,
but it's time to take the lessons learned from LUX and apply them to the future
search for dark matter."
Provided
by: Brown University
viernes, 22 de julio de 2016
Seeing a Black Hole’s Gravitational Vortex
Seeing a Black Hole’s Gravitational Vortex
New observations solve a 30-year-old puzzle of mysterious signals from around black holes.
An artist's conception of a supermassive black hole.
NASA/JPL-Caltech
This artist's impression depicts an accretion disk surrounding a black hole. The black hole drags spacetime with it as it spins. So X-ray-emitting plasma near the black hole, stuck in spacetime like a fly stuck in honey, precesses. The X-rays strike matter in the surrounding disk, making it to glow like a fluorescent bulb. The glow appears to rotate around the accretion disc to the right (top), to the front (middle), and to the left (bottom).
ESA / ATG medialab
ESA / ATG medialab
Strange things happen around black holes, especially spinning ones. Their strong gravitational pull means they don’t just pull in gas to munch on — they drag the very fabric of spacetime around them as they spin.
Every rotating massive body does this — even puny Earth, as measured by the Gravity Probe B. But around black holes the so-called frame-dragging effect (also known as the Lense-Thirring effect) is particularly strong. Like flies stuck in honey, anything embedded in that spacetime will get dragged along, too. And now, with new observations from the XMM-Newton and NuSTAR space telescopes, astronomers have connected the effect to long-mysterious signals seen around stellar-mass black holes.
Black Hole Beats
Adam Ingram (University of Amsterdam, The Netherlands) and colleagues set out to observe this effect directly. They pointed the XMM-Newton and NuSTAR space telescopes at the system known as H1743-322, where a black hole with a mass of about 10 Suns is drawing in gas from its companion star. Four of the five observations clearly show the iron line shifting back and forth in the spectrum over the course of 4 to 5 seconds, exactly in the way that the frame-dragging effect predicts.
“This is a very intriguing result,” says Laura Brenneman (Harvard-Smithsonian Center for Astrophysics), who was not involved with the study. “Certain types of QPOs in X-ray[-emitting black hole] binaries have long been suspected to arise from some form of precession, but this result is the closest thing I've seen to hard evidence for that.”
This result turns stellar-mass black holes into a proving ground for new physics. “If you can get to the bottom of the astrophysics, then you can really test general relativity,” Ingram said in NASA’s press release, welcome news to physicists who are searching for a deeper theory of gravity.
One of These Is Not Like the Others
Over 3 days’ worth of exposure, XMM-Newton collected five sets of data. While four of these matched beautifully, one, known as orbit 1b, didn’t conform at all to expectations. It could simply be that some gas obstructed the astronomers’ view, or it could be that the observation is telling astronomers something more fundamental.
“I am curious as to what is going on in XMM-Newton’s orbit 1b that is so anomalous compared to the others,” Brenneman adds, “but I don't think it diminishes the result at all, just adds an extra dimension and opens up more questions.”
Another intriguing aspect of QPOs is that they’ve (almost) never been seen in the supermassive variety of black holes. These active galactic nuclei (AGN) guzzle gas at the center of galaxies with the same setup as stellar-mass black holes: a black hole, a gas disk, and X-ray-emitting plasma. The only thing they’re missing is the binary companion star.
“There has only been one reputable claim of a QPO in an AGN back in 2008, and it hasn't been seen again since,” Brenneman says. “If there were QPOs-a-plenty in AGN, we would likely have detected them by now.” Why they aren’t there, no one knows.
With one mystery solved, it’s clear there are still more cases awaiting closure.
Reference:
Adam Ingram et al. "A quasi-periodic modulation of the iron line centroid energy in the black hole binary H 1743-322", Monthly Notices of the Royal Astronomical Society, 2016 May 25.(Full text)
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