quantum gravity

Axion monodromy inflation Let us assume that the BICEP2 result is confirmed as cosmological, and indicates primordial gravitational waves generated during inflation. Within the context of inflationary theory, this groundbreaking discovery has important implications for quantum gravity, for which string theory is our leading candidate. String theory contains a rather simple mathematical structure – monodromy […]

Axion monodromy inflation

Let us assume that the BICEP2 result is confirmed as cosmological, and indicates primordial gravitational waves generated during inflation. Within the context of inflationary theory, this groundbreaking discovery has important implications for quantum gravity, for which string theory is our leading candidate.

String theory contains a rather simple mathematical structure – monodromy – which naturally generates a significant tensor signal. In this guest post, I’ll describe that mechanism, and discuss its range of applicability as we currently understand it. (String theory also contains multiple axion fields, which in itself gives an interesting realization of assisted inflation, N-flation, covered nicely in an earlier blog post. It was later realized that along each such direction the monodromy effect operates; in general, one may consider a combination of these two mechanisms.)

Stable Orbits

Published on May 3, 2012 If gravity is so attractive, why doesn’t the earth just crash into the sun? Or the moon into the earth? The answer: Stable Orbits hyperbolic funnel video: http://bit.ly/r5xhng minutephysics is now on Google+ – http://bit.ly/qzEwc6 And facebook – http://facebook.com/minutephysics And twitter – @minutephysics Minute Physics provides an energetic and entertaining […]

Published on May 3, 2012

If gravity is so attractive, why doesn’t the earth just crash into the sun? Or the moon into the earth?

The answer: Stable Orbits

hyperbolic funnel video: http://bit.ly/r5xhng

minutephysics is now on Google+ – http://bit.ly/qzEwc6
And facebook – http://facebook.com/minutephysics
And twitter – @minutephysics

Minute Physics provides an energetic and entertaining view of old and new problems in physics — all in a minute!

Gravity

Check out this eye-popping behind-the-scenes making of Alfonso Cuarón’s space spectacle Gravity. It’s almost unbelievable how all this CG and motion capture transformed into that nail-bitingly realistic film. WARNING there are some major spoilers for the ending.

Check out this eye-popping behind-the-scenes making of Alfonso Cuarón’s space spectacle Gravity. It’s almost unbelievable how all this CG and motion capture transformed into that nail-bitingly realistic film. WARNING there are some major spoilers for the ending.

Edward Witten

Published on May 9, 2013
2010 Newton Medal winner, Edward Witten speaks to an audience at the Institute of Physics in London. He helped progress our understanding of the fundamental theories in physics. He touches on gravity, quantum mechanics and st…

Published on May 9, 2013

2010 Newton Medal winner, Edward Witten speaks to an audience at the Institute of Physics in London. He helped progress our understanding of the fundamental theories in physics. He touches on gravity, quantum mechanics and string theory.

relativity

Elwood H. Smith In the spring physicists had to withdraw a sensational report that the subatomic particles known as neutrinos were going faster than light, Einstein’s cosmic speed limit; they discovered they had plugged in a cable wrong. Now scientists … Continue reading

Elwood H. Smith

In the spring physicists had to withdraw a sensational report that the subatomic particles known as neutrinos were going faster than light, Einstein’s cosmic speed limit; they discovered they had plugged in a cable wrong.

Now scientists from NASA’s Jet Propulsion Laboratory have reported that they have explained one of the great mysteries of the space age, one that loomed for 30 years as a threat to the credibility of Einsteinian gravity.

The story starts with the Pioneer 10 and 11 space probes, which went past Jupiter and Saturn in the late 1970s and now are on their way out of the solar system. In the 1980s it became apparent that a mysterious force was slowing them down a little more than should have been expected from gravity of the Sun and planets..

Was there an unknown planet or asteroid out there tugging on the spacecraft? Was it drag from interplanetary gas or dust? Something weird about the spacecraft? Or was something wrong in our calculation of gravity out there in the dark?

That last explanation would have been big news indeed. Much of what we know about the universe — for example, the existence of dark matter, which seems to swaddle and shape the galaxies, and of dark energy, which seems to be speeding up the expansion of the universe — comes from presuming that Einstein’s General Theory of Relativity, which describes gravity as the warping of space-time geometry — is correct over cosmic distances.

General relativity has passed every test on Earth. Without correcting for it, GPS systems would not work. But some theorists have suggested that if gravity behaved differently over large distances from what Einstein thought, it would relieve astronomers of the embarrassing need to posit that 96 percent of the universe consists of various kinds of unknown dark stuff. A similar, but larger, kind of deviation from Einsteinian theory could explain the Pioneer anomaly, as it is called.

Pioneers 10 and 11 were launched in 1972 and 1973, respectively, and are now both about 10 billion miles out. They were last heard from in 2003, when the radio signal from Pioneer 10 got too weak to be detected. They were the first spacecraft to go past Jupiter and Saturn (though their biggest impact on pop culture until then might have been a controversy over the nude human figures on a plaque designed for the benefit of any distant aliens who might find them).

In 1998, however, when John D. Anderson of the Jet Propulsion Laboratory and his colleagues discovered that the spacecraft were running a little late on their timetable to eternity, it seemed as if general relativity might be up for grabs — allowing the news media to ask their favorite science question: Was Einstein wrong? There was talk of a special deep space probe whose only mission would be to track its own movements.

The effect was slight — slowing the spacecraft by about 300 miles a year — but the crack interplanetary navigators at J.P.L., who can slip a probe through Saturn’s rings or buzz the moons of Jupiter, take great pride in their knowledge of the forces and foibles of the solar system.

Slava G. Turyshev, a Russian physicist and gravitational expert working at the laboratory, heard the challenge and took it on, feeling “a sense of responsibility to get to the bottom of it.” And so he set out to reconstruct the history of the Pioneer voyages.

You might think that would be easy. But the Pioneers spanned the history of the space age and also of the computer age, occasioning a major effort in what Dr. Turyshev calls “space archaeology.”

He and his colleagues had to scour NASA labs for old punch cards and magnetic tapes and for vintage devices that could read the data stored on them — then reformat all that data to a single modern standard.

Among other things, that meant ascertaining the positions of every antenna in NASA’s Deep Space Network to an accuracy of one centimeter over all that time.

It took much longer than Dr. Turyshev had imagined, and he had to depend on money from the Planetary Society as well as from NASA to keep the project going, all the time aware that nobody was ever likely to retrace his footsteps. So he had better get it right.

“The more we learned, the less optimistic we became about new physics,” he said. It became apparent that the fault with the Pioneers’ travels turned out to lie not in the stars or the shape of space-time but in the spacecraft themselves.

As designed, they radiated more heat in one direction from the circuits and generators that produced their electricity. And that imbalance, Dr. Turyshev and his colleagues concluded in a recent paper in Physical Review Letters, was all that was needed to explain the Pioneers’ behavior.

Gravity did not need to be fixed. Einstein was right again.

In fact he was doubly right, as it turns out.

The idea that light, of which heat radiation is one form, can carry momentum and thus a propulsive force is implicit in the basic equations of electromagnetism. A comet’s tail, blown by sunlight, is one example. But it acquired new visceral meaning when Einstein, beginning in 1905, showed that light can be thought of as little particles — now called photons — that carry energy and momentum. As with bullets, if you shoot them in one direction, there will be a recoil in the other.

If you drive at night, Dr. Turyshev explained, “your high beams would emit photons and it would push you back.”

So the Pioneers’ problem is that they have been driving the solar system with their high beams on.

Dr. Turyshev said he had set out in search of new physics. When I asked him if he was disappointed at the final results, he laughed and replied: “Of course. People were ready for something big.”

Still, he said, the Einsteinian explanation “also helps.” The findings should help in designing new spacecraft for sensitive missions like measuring gravitational waves.

“It’s a win-win,” he said.

You may dream of freaky new physics, but sometimes freaky old physics is all you need.