String Theory Finally Does Something Useful

String theory has finally made a prediction that can be tested with experiments — but in a completely unexpected realm of physics.

The theory has long been touted as the best hope for a unified “theory of everything,” bringing together the physics of the vanishingly small and the mindbendingly large. But it has also been criticized and even ridiculed for failing to make any predictions that could be checked experimentally. It’s not just that we don’t have big enough particle accelerators or powerful enough computers; string theory’s most vocal critics charge that no experiment could even be imagined that would prove it right or wrong, making the whole theory effectively useless.

Now, physicists at Imperial College London and Stanford University have found a way to make string theory useful, not for a theory of everything, but for quantum entanglement.

“We can use string theory to solve problems in a different area of physics,” said theoretical physicist Michael Duff of Imperial College London. “In that context it’s actually useful: We can make statements which you could in principle check by experiment.” Duff and his colleagues describe their findings in a paper in Physical Review Letters September 2.

String theory suggests that matter can be broken down beyond electrons and quarks into tiny loops of vibrating strings. Those strings move and vibrate at different frequencies, giving particles distinctive properties like mass and charge. This strange idea could unite all the fundamental forces, explain the origins of fundamental particles and connect Einstein’s general relativity to quantum mechanics. But to do so, the theory requires six extra dimensions of space and time curled up inside the four that we’re used to.


To understand how these extra dimensions could hide from view, imagine a tightrope walker on a wire between two high buildings. To the tightrope walker, the wire is a one-dimensional line. But to a colony of ants crawling around the wire, the rope has a second dimension: its thickness. In the same way that the tightrope walker sees one dimension where the ants see two, we could see just three dimensions of space while strings see nine or ten.

Unfortunately, there’s no way to know if this picture is real. But although string theorists can’t test the big idea, they can use this vision of the world to describe natural phenomena like black holes.

Four years ago, while listening to a talk at a conference in Tasmania, Duff realized the mathematical description string theorists use for black holes was identical to the mathematical description of certain quantum systems, called quantum bits or qubits.

Qubits form the backbone of quantum information theory, which could lead to things like ultrafast computers and absolutely secure communication. Two or more qubits can sometimes be intimately connected in a quantum state called entanglement. When two qubits are entangled, changing one’s state influences the state of the other, even when they’re physically far apart.

“As I listened to his talk, I realized the kind of math he was using to describe qubit entanglement was very similar to mathematics I had been using some years before to describe black holes in string theory,” Duff said. When he looked into it, the mathematical formulation of three entangled qubits turned out to be exactly the same as the description of a certain class of black holes.

In the new study, Duff and his colleagues push the similarity one step further. They used the mathematics of stringy black holes to compute a new way to describe four entangled qubits, an open question in quantum information theory.

“We made statements that weren’t previously known using string theory techniques,” Duff said. “Whether the result is some fundamental principle or some quirk of mathematics, we don’t know, but it is useful for making statements about quantum entanglement.”

What’s more, these statements are precise and experimentally provable, unlike previous suggestions for ways to test string theory, Duff says.

“So in a way, there’s bad news and good news in our paper,” he said. “The bad news is, we’re not describing the theory of everything. The good news is, we’re making a very exact statement which is either right or wrong. There’s no in between.”

Duff emphasized that this is only a test of string theory as it relates to quantum entanglement, not as a description of the fundamental physics of the universe. The battle over string theory as a theory of everything rages on.

“Already I can imagine enemies sharpening their knives,” Duff said.

And they are. A chorus of supporters and critics, including Nobel laureate and string theory skeptic Sheldon Glashow and string theorists John Schwarz of Caltech, James Gates of the University of Maryland, and Juan Maldacena and Edward Witten of the Institute for Advanced Study in Princeton agree that Duff’s argument is “not a way to test string theory” and has nothing to do with a theory of everything.

Mathematician Peter Woit of Columbia University, author of the blog Not Even Wrong, thinks even claiming that the new paper is a test of quantum entanglement is going too far.

“Honestly, I think this is completely outrageous,” he said. Even if the math is the same, he says, testing the quantum entangled system would only tell you how well you understand the math.

“The fact that the same mathematical structure appears in a quantum mechanical problem and some model of black holes isn’t even slightly surprising,” he said. “It doesn’t mean that one is a test of the other.”

Witten takes a more optimistic view of the theory’s chances, pointing out that the mathematics of string theory have turned out to be coincidentally useful in other areas of physics before.

“In general, this kind of work shows that string theory is useful, and in fact by now it has been useful in many different ways,” Witten said in an email to Wired.com.

“One might surmise that a physics theory that has proved to be useful in so many different areas of physics and math is probably on the right track,” he added. “But that is another question.”


Fuente: Wired Science

Dark Matter May Be Building Up Inside the Sun

The sun could be a net for dark matter, a new study suggests. If dark matter happens to take a certain specific form, it could build up in our nearest star and alter how heat moves inside it in a way that would be observable from Earth.

Dark matter is the mysterious stuff that makes up about 83 percent of the matter in the universe, but doesn’t interact with electromagnetic forces. Although the universe contains five times as much dark matter as normal matter, dark matter is completely invisible both to human eyes and every kind of telescope ever devised. Physicists only know it’s there because of its gravitational effect on normal matter. Dark matter keeps galaxies spinning quickly without flying apart and is responsible for much of the large-scale structure in the universe.

Current dark matter detectors are looking for WIMPs, or weakly interacting massive particles, that connect only with the weak nuclear force and gravity. Based on the most widely accepted theories, most experiments are tuned to look for a particle that is about 100 times more massive than a proton. The chief suspect is also its own antiparticle: Whenever a WIMP meets another WIMP, they annihilate each other.

“This is something that has always worried me,” said astroparticle physicist Subir Sarkar of the University of Oxford. If equal amounts of matter and antimatter were created in the big bang, the particles should have completely wiped each other out by now. “Obviously that did not happen, we are here to prove it,” he said. “So something created an asymmetry of matter over antimatter,” letting a little bit of matter survive after all the antimatter was gone.

Whatever made regular matter beat out regular antimatter could have worked on dark matter as well, Sarkar suggests. If dark matter evolved similarly to regular matter, it would have to be much lighter than current experiments expect, only about 5 times the mass of a proton. That’s a suggestive number, Sarkar says.

“If it were five times heavier, it would get five times the abundance. That’s what dark matter is,” he said. “That’s the simplest explanation for dark matter in my view.”

The trouble is, these light particles are much more difficult to detect with current experiments. In a paper in the July 2 Physical Review Letters, Sarkar and Oxford colleague Mads Frandsen suggest another way to find light dark matter: Look to the sun.

Because lightweight dark matter particles wouldn’t vaporize each other when they meet, the sun should collect the particles the way snowballs collect more snow.

“The sun has been whizzing around the galaxy for 5 billion years, sweeping up all the dark matter as it goes,” Sarkar said.

The buildup of dark matter could solve a pressing problem in solar physics, called the solar composition problem. Sensitive observations of waves on the sun’s surface have revealed that the sun has a much easier time transporting heat from its interior to its surface than standard models predict it should.

Dark matter particles that interact only with each other could make up the difference. Photons and particles of regular matter bounce off each other on their way from the sun’s interior to its surface, so light and heat can take billions of years to escape. But because dark matter particles ignore all the regular matter inside the sun, they have less stuff in their way and can transport heat more efficiently.

“When we do the calculation, to our amazement, it turns out this is true,” Sarkar said. “They can transport enough heat to solve the solar composition problem.”

Next, Sarkar and Frandsen calculated how being full of dark matter would affect the number of neutrinos the sun gives off. They found that the neutrino flux would change by a few percent. That’s not much, Sarkar said, but it’s just enough to be detected by two different neutrino experiments — one in Italy called Borexino and one in Canada called SNO+ — that are soon to get under way.

“It’s a speculative idea, but it’s testable,” Sarkar said. “And the tools to test it are coming on line pretty fast. We don’t have to wait 20 years.”

The idea of lightweight dark matter influencing the sun is “not too much of a stretch, in my opinion,” said physicist Dan Hooper of Fermilab in Illinois. “I look at their numbers, and they’re very plausible to me.”

Some puzzling results from dark matter detectors hint that these lightweight particles could have already been detected. Earlier this year, a germanium hockey puck in a mine in Minnesota called the Coherent Germanium Neutrino Technology (CoGeNT) detected a signal from a particle about 7 times the mass of the proton, though they’re not sure yet whether it’s dark matter. Another detector in Italy called DAMA has reported similar results.

“There’s an increasingly compelling body of evidence accumulating” that dark matter is just a few times as massive as a proton, Hooper said. “The jury is still out, but if this is really what’s going on, we should be able to know it with some confidence in the next year or so.”

Update: Regular matter makes up 5 percent of the energy density of the universe, and dark matter makes up 25 percent (five times more than regular matter). The remaining 70 percent is dark energy.


Fuente: Wired Science

Sun’s Dust Ring Could Help Find Exo-Earths

Earth-like exoplanets could announce their presence through trailing clumps of dust — and new observations of the Earth’s own dust cloud could provide a way to find them. Over the course of five years, the Spitzer Space Telescope drifted through a diffuse but extensive ring of dust particles that orbit the sun in lockstep with the Earth, showing astronomers for the first time what the dusty signature of an exo-Earth might look like.

“For the first time we can measure the structure of that cloud along the Earth’s orbit, using this moving space probe that travels through the cloud,” said astronomer William T. Reach of the Universities Space Research Association, the author of a paper to appear in the journal Icarus. “We can use that as a key, as a template, to understand the dust around other stars.”

The observations showed that a ring of dust from comet tails and broken asteroids follows the Earth in its orbit, something astronomers had already suspected. The dust particles are about 0.02 millimeters in diameter or larger. An extra-thick cloud of these particles about 7 million miles wide trails behind the Earth at about 80 times the distance from the Earth to the moon. Spitzer, which follows the Earth in orbit around the sun, sent images from directly inside this cloud from its launch in 2003 until its coolant ran out in 2009.

Astronomers’ first whiff of this trailing dust clump came in 1984, when the IRAS spacecraft showed that the sky is brighter in infrared wavelengths when looking backward along the Earth’s orbit than when looking forward. Because dust glows in the infrared, the lightened sky was a clear sign that more dust follows the planet than leads it.

“We couldn’t figure out for the life of us what the hell was going on,” said astronomer Mark Sykes, now the director of the Planetary Science Institute in Arizona, who worked on the IRAS project. No good explanations emerged until the early ’90s, when astronomer Sumita Jayaraman, also now at the Planetary Science Institute, realized that individual dust particles could get temporarily trapped in a special gravitational relationship called a resonant orbit with Earth.

Most of the dust in the plane of the solar system, called the zodiacal cloud, will eventually spiral into the sun. But particles of the right size, tens of micrometers across, can feel a little gravitational push as they float by the Earth. That push counteracts the sun’s pull just enough to hold the dust particles in a loose halo around the sun. The subtle interactions of the Earth and the dust grains’ movements lead to the backward-facing clump.

Mathematical models of the dust ring gave astronomers an idea of the clump’s extent, but the Spitzer observations were the first chance to test them.

“This work is great because it provides us a novel way of probing the structure of this cloud, which could then feed back into these detailed dynamical models of the dust,” Sykes said.

The observations can feed models of what dust rings associated with extrasolar planets might look like. Of the few extrasolar planets to have their pictures taken by direct imaging, at least two hinted at their presence by warping the disk of dust and gas around their star. Earth-like planets that are too small or dim to find through usual methods may have a subtle but detectable influence on their dust disks.

“It’s a way that we can recognize planets around other stars that we can’t necessarily see,” said NASA exoplanet scientist Marc Kuchner. “This result make it much easier to compare solar system dust clouds with ones we see in the disks.”

But the dust can be misleading too, Kuchner warns. “They can be bad news if you’re trying to directly image a planet, because they can masquerade as planets themselves,” he said. “It’s both the signal and the noise.”


Fuente: Wired Science

Stephen Hawking afirma que los viajes en el tiempo son posibles, pero sólo al futuro

Si primero fue el contacto con los extraterrestres, Stephen Hawking sigue por la vía de la ciencia ficción y le da por hablar sobre los viajes en el tiempo. En esta ocasión, el científico estadounidense británico afirma que este tipo de viajes sí serían posibles, pero únicamente hacia el futuro.

Hawking menciona que, una vez que se construyan naves espaciales que sean capaces de desplazarse al 98% de la velocidad de la luz, un día transcurrido abordo equivaldría a un año en la Tierra. De acuerdo a los cálculos del físico, se tardarían cerca de seis años para alcanzar esta velocidad. Aunque la afirmación suena un poco descabellada, Brian Cox, físico de partículas de la Universidad de Manchester, apoya la teoría basado en algunas observaciones hechas en el Gran Colisionador de Hadrones: “Cuando aceleramos partículas diminutas al 99.99% de la velocidad de la luz en el LHC de Ginebra, el tiempo transcurrido para ellas es un sietemilésima más lento del que medimos con nuestros relojes”.

¿Tiene esta teoría alguna aplicación práctica? Para Hawking, podría suponer la salvación de la Humanidad, ya que los viajeros en el tiempo podrían llegar una Tierra post-apocalíptica a repoblar (claro, si consiguen resolver el problema de cómo aterrizar y todo lo demás). Claro, al buen Stephen ya no le importa mucho lo que hablen de él, por lo que esperamos que este tipo de declaraciones suyas sean más recurrentes. En fin, no sé ustedes, pero yo ya puedo respirar más tranquilo ahora que la paradoja del viaje del tiempo ha quedado descartada.

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