Holometer rules out first theory of space-time correlations

Controversial experiment sees no evidence that the universe is a hologram

Our common sense and the laws of physics assume that space and time are continuous. The Holometer, an experiment based at the US Department of Energy’s Fermi National Accelerator Laboratory, challenges this assumption.

We know that energy on the atomic level, for instance, is not continuous and comes in small, indivisible amounts. The Holometer was built to test if space and time behave the same way.

In a new result released this week after a year of data-taking, the Holometer collaboration has announced that it has ruled out one theory of a pixelated universe to a high level of statistical significance.

If space and time were not continuous, everything would be pixelated, like a digital image.

When you zoom in far enough, you see that a digital image is not smooth, but made up of individual pixels. An image can only store as much data as the number of pixels allows. If the universe were similarly segmented, then there would be a limit to the amount of information space-time could contain.

The main theory the Holometer was built to test was posited by Craig Hogan, a professor of astronomy and physics at the University of Chicago and the head of Fermilab’s Center for Particle Astrophysics. The Holometer did not detect the amount of correlated holographic noise—quantum jitter—that this particular model of space-time predicts.

But as Hogan emphasizes, it's just one theory, and with the Holometer, this team of scientists has proven that space-time can be probed at an unprecedented level.

“This is just the beginning of the story,” Hogan says. “We’ve developed a new way of studying space and time that we didn’t have before. We weren’t even sure we could attain the sensitivity we did.”

The Holometer isn’t much to look at. It’s a small array of lasers and mirrors with a trailer for a control room. But the low-tech look of the device belies the fact that it is an unprecedentedly sensitive instrument, able to measure movements that last only a millionth of a second and distances that are a billionth of a billionth of a meter—a thousand times smaller than a single proton.

The Holometer uses a pair of laser interferometers placed close to one another, each sending a 1-kilowatt beam of light through a beam splitter and down two perpendicular arms, 40 meters each. The light is then reflected back into the beam splitter where the two beams recombine.

If no motion has occurred, then the recombined beam will be the same as the original beam. But if fluctuations in brightness are observed, researchers will then analyze these fluctuations to see if the splitter is moving in a certain way, being carried along on a jitter of space itself.

According to Fermilab’s Aaron Chou, project manager of the Holometer experiment, the collaboration looked to the work done to design other, similar instruments, such as the one used in the Laser Interferometer Gravitational-Wave Observatory experiment. Chou says that once the Holometer team realized that this technology could be used to study the quantum fluctuation they were after, the work of other collaborations using laser interferometers (including LIGO) was invaluable.

“No one has ever applied this technology in this way before,” Chou says. “A small team, mostly students, built an instrument nearly as sensitive as LIGO’s to look for something completely different.”

The challenge for researchers using the Holometer is to eliminate all other sources of movement until they are left with a fluctuation they cannot explain. According to Fermilab’s Chris Stoughton, a scientist on the Holometer experiment, the process of taking data was one of constantly adjusting the machine to remove more noise.

“You would run the machine for a while, take data, and then try to get rid of all the fluctuation you could see before running it again,” he says. “The origin of the phenomenon we’re looking for is a billion billion times smaller than a proton, and the Holometer is extremely sensitive, so it picks up a lot of outside sources, such as wind and traffic.”  

If the Holometer were to see holographic noise that researchers could not eliminate, it might be detecting noise that is intrinsic to space-time, which may mean that information in our universe could actually be encoded in tiny packets in two dimensions.

The fact that the Holometer ruled out his theory to a high level of significance proves that it can probe time and space at previously unimagined scales, Hogan says. It also proves that if this quantum jitter exists, it is either much smaller than the Holometer can detect, or is moving in directions the current instrument is not configured to observe.

So what’s next? Hogan says the Holometer team will continue to take and analyze data, and will publish more general and more sensitive studies of holographic noise. The collaboration already released a result related to the study of gravitational waves.

And Hogan is already putting forth a new model of holographic structure that would require similar instruments of the same sensitivity, but different configurations sensitive to the rotation of space. The Holometer, he says, will serve as a template for an entirely new field of experimental science.

“It’s new technology, and the Holometer is just the first example of a new way of studying exotic correlations,” Hogan says. “It is just the first glimpse through a newly invented microscope.”

The Holometer experiment is supported by funding from the DOE Office of Science. The Holometer collaboration includes scientists from Fermilab, the University of Chicago, the Massachusetts Institute of Technology and the University of Michigan.

Fermilab

What are WIMPs, and what makes them such popular dark matter candidates?

Invisible dark matter accounts for 85 percent of all matter in the universe, affecting the motion of galaxies, bending the path of light and influencing the structure of the entire cosmos. Yet we don’t know much for certain about its nature.

Most dark matter experiments are searching for a type of particles called WIMPs, or weakly interacting massive particles.

“Weakly interacting” means that WIMPs barely ever “talk” to regular matter. They don’t often bump into other matter and also don’t emit light—properties that could explain why researchers haven’t been able to detect them yet.

Created in the early universe, they would be heavy (“massive”) and slow-moving enough to gravitationally clump together and form structures observed in today’s universe.

Scientists predict that dark matter is made of particles. But that assumption is based on what they know about the nature of regular matter, which makes up only about 4 percent of the universe.

WIMPs advanced in popularity in the late 1970s and early 1980s when scientists realized that particles that naturally pop out in models of Supersymmetry could potentially explain the seemingly unrelated cosmic mystery of dark matter.

Supersymmetry, developed to fill gaps in our understanding of known particles and forces, postulates that each fundamental particle has a yet-to-be-discovered superpartner. It turns out that the lightest one of the bunch has properties that make it a top contender for dark matter.

“The lightest supersymmetric WIMP is stable and is not allowed to decay into other particles,” says theoretical physicist Tim Tait of the University of California, Irvine. “Once created in the big bang, many of these WIMPs would therefore still be around today and could have gone unnoticed because they rarely produce a detectable signal.”

When researchers use the properties of the lightest supersymmetric particle to calculate how many of them would still be around today, they end up with a number that matches closely the amount of dark matter experimentally observed—a link referred to as the “WIMP miracle.” Many researchers believe it could be more than coincidence.

“But WIMPs are also popular because we know how to look for them,” says dark matter hunter Thomas Shutt of Stanford University and SLAC National Accelerator Laboratory. “After years of developments, we finally know how to build detectors that have a chance of catching a glimpse of them.”

Shutt is co-founder of the LUX experiment and one of the key figures in the development of the next-generation LUX-ZEPLIN experiment. He is one member of the group of scientists trying to detect WIMPs as they traverse large, underground detectors.

Other scientists hope to create them in powerful particle collisions at CERN’s Large Hadron Collider. “Most supersymmetric theories estimate the mass of the lightest WIMP to be somewhere above 100 gigaelectronvolts, which is well within LHC’s energy regime,” Tait says. “I myself and others are very excited about the recent LHC restart. There is a lot of hope to create dark matter in the lab.”

A third way of searching for WIMPs is to look for revealing signals reaching Earth from space. Although individual WIMPs are stable, they decay into other particles when two of them collide and annihilate each other. This process should leave behind detectable amounts of radiation.

Researchers therefore point their instruments at astronomical objects rich in dark matter such as dwarf satellite galaxies orbiting our Milky Way or the center of the Milky Way itself.

“Dark matter interacts with regular matter through gravitation, impacting structure formation in the universe,” says Risa Wechsler, a researcher at Stanford and SLAC. “If dark matter is made of WIMPs, our predictions of the distribution of dark matter based on this assumption must also match our observations.”

Wechsler and others calculate, for example, how many dwarf galaxies our Milky Way should have and participate in research efforts under way to determine if everything predicted can also be found experimentally.

So how would researchers know for sure that dark matter is made of WIMPs? “We would need to see conclusive evidence for WIMPs in more than one experiment, ideally using all three ways of detection,” Wechsler says.

In the light of today’s mature detection methods, dark matter hunters should be able to find WIMPs in the next five to 10 years, Shutt, Tait and Wechsler say. Time will tell if scientists have the right idea about the nature of dark matter.

http://www.nanotechnologyworld.org/#!What-are-WIMPs-and-what-makes-them-such-popular-dark-matter-candidates/c89r/55b797610cf228fd5ebad145

What’s on the surface of a black hole?

Are black holes the ruthless killers we’ve made them out to be? Samir Mathur says no.

According to the professor of physics at The Ohio State University, the recently proposed idea that black holes have “firewalls” that destroy all they touch has a loophole.

In a paper posted online to the arXiv preprint server, Mathur takes issue with the firewall theory, and proves mathematically that black holes are not necessarily arbiters of doom.

In fact, he says the world could be captured by a black hole, and we wouldn’t even notice.

More than a decade ago, Mathur used the principles of string theory to show that black holes are actually tangled-up balls of cosmic strings. His “fuzzball theory” helped resolve certain contradictions in how physicists think of black holes.

But when a group of researchers recently tried to build on Mathur’s theory, they concluded that the surface of the fuzzball was actually a firewall.

According to the firewall theory, the surface of the fuzzball is deadly. In fact, the idea is called the firewall theory because it suggests that a very literal fiery death awaits anything that touches it.

Mathur and his team have been expanding on their fuzzball theory, too, and they’ve come to a completely different conclusion. They see black holes not as killers, but rather as benign copy machines of a sort.

They believe that when material touches the surface of a black hole, it becomes a hologram, a near-perfect copy of itself that continues to exist just as before.

“Near-perfect” is the point of contention. There is a hypothesis in physics called complementarity, which was first proposed by Stanford University physicist Leonard Susskind in 1993. Complementarity requires that any such hologram created by a black hole be a perfect copy of the original.

Mathematically, physicists on both sides of this new fuzzball-firewall debate have concluded that strict complementarity is not possible; that is to say, a perfect hologram can’t form on the surface of a black hole.

Mathur and his colleagues are comfortable with the idea, because they have since developed a modified model of complementarity, in which they assume that an imperfect hologram forms. That work was done with former Ohio State postdoctoral researcher David Turton, who is now at the Institute of Theoretical Physics at theCEA-Saclay research center in France.

Proponents of the firewall theory take an all-or-nothing approach to complementarity. Without perfection, they say, there can only be fiery death.

With his latest paper, Mathur counters that he and his colleagues have now proven mathematically that modified complementarity is possible.

It’s not that the firewall proponents made some kind of math error, he added. The two sides based their calculations on different assumptions, so they got different answers. One group rejects the idea of imperfection in this particular case, and the other does not.

Imperfection is a common topic in cosmology. Physicist Stephen Hawking has famously said that the universe was imperfect from the very first moments of its existence. Without an imperfect scattering of the material created in the Big Bang, gravity would not have been able to draw together the atoms that make up galaxies, stars, the planets—and us.

This new dispute about firewalls and fuzzballs hinges on whether physicists can accept that black holes are imperfect, just like the rest of the universe.

“There’s no such thing as a perfect black hole, because every black hole is different,” Mathur explained.

His comment refers to the resolution of the “ information paradox,” a long-running physics debate in which Hawking eventually conceded that the material that falls into a black hole isn’t destroyed, but rather becomes part of the black hole.

The black hole is permanently changed by the new addition. It’s as if, metaphorically speaking, a new gene sequence has been spliced into its DNA. That means every black hole is a unique product of the material that happens to come across it.

The information paradox was resolved in part due to Mathur’s development of the fuzzball theory in 2003. The idea, which he published in the journal Nuclear Physics B in 2004, was solidified through the work of other scientists including Oleg Lunin of University at Albany, Stefano Giusto of theUniversity of Padova, Iosif Bena of CEA-Saclay, and Nick Warner of the University of Southern California. Mathur’s co-authors included then-students Borun Chowdhury (now a postdoctoral researcher at Arizona State University), and Steven Avery (now a postdoctoral researcher at Brown University).

Their model was radical at the time, since it suggested that black holes had a defined—albeit “fuzzy”—surface. That means material doesn’t actually fall into black holes so much as it falls onto them.

The implications of the fuzzball-firewall issue are profound. One of the tenets of string theory is that our three-dimensional existence—four-dimensional if you count time—might actually be a hologram on a surface that exists in many more dimensions.

“If the surface of a black hole is a firewall, then the idea of the universe as a hologram has to be wrong,” Mathur said.

The very nature of the universe is at stake, but don’t expect rival physicists to come to blows about it.

“It’s not that kind of disagreement,” Mathur laughed. “It’s a simple question, really. Do you accept the idea of imperfection, or do you not?”

Source: http://www.nanotechnologyworld.org/#!What’s-on-the-surface-of-a-black-hole/c89r/5582da250cf23681a55f1ccb