Exploring Black Holes

Rod
Rod
Joined: 3 Jan 06
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Topic 193741

Greetings:

For anybody who has iTunes, ITunesU has a series of movie lectures through MIT open courseware on:

Exploring Black Holes: General Relativity & Astrophysics Spring 2003

They look into Einstein's field equations. I have not gone through all the lectures but any layman can get something out of them. It like sitting in the classroom ... and they are free..:-)

I find ITunesU a marvelous resource...

There are some who can live without wild things and some who cannot. - Aldo Leopold

Fred J. Verster
Fred J. Verster
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Exploring Black Holes

Thumbs up for Einstein's gravitational prediction and law.
An artical about this.

Taking advantage of a unique cosmic coincidence, astronomers have measured an effect predicted by Albert Einstein's theory of general relativity in the extremely strong gravity of a pair of superdense neutron stars. The new data indicate that the famed physicist's 93-year-old theory has passed yet another test.

The scientists used the National Science Foundation's Robert C. Byrd Green Bank Telescope (GBT) to make a 4-year study of a double-star system unlike any other known in the universe. The system is a pair of neutron stars, both of which are seen as pulsars that emit lighthouse-like beams of radio waves.

"Of about 1,700 known pulsars, this is the only case where two pulsars are in orbit around each other," says Rene Breton, a graduate student at McGill University in Montreal, Canada. In addition, the stars' orbital plane is aligned nearly perfectly with their line of sight to Earth, so that one passes behind a doughnut-shaped region of ionized gas surrounding the other, eclipsing the signal from the pulsar in back.

"Those eclipses are the key to making a measurement that could never be done before," says Breton.

Einstein's 1915 theory of predicted that, in a close system of two very massive objects, such as neutron stars, one object's gravitational tug, along with an effect of its spinning around its axis, should cause the spin axis of the other to wobble, or precess. Studies of other pulsars in binary systems had indicated that such wobbling occurred, but could not produce precise measurements of the amount of wobbling.

"Measuring the amount of wobbling is what tests the details of Einstein's theory and gives a benchmark that any alternative gravitational theories must meet," says Scott Ransom of the National Radio Astronomy Observatory.

The eclipses allowed the astronomers to pin down the geometry of the double-pulsar system and track changes in the orientation of the spin axis of one of them. As one pulsar's spin axis slowly moved, the pattern of signal blockages as the other passed behind it also changed. The signal from the pulsar in back is absorbed by the ionized gas in the other's magnetosphere.

Pulsars, first discovered in 1967, are the "corpses" of massive stars that have exploded as supernovae. What is left after the explosion is a superdense neutron star that packs more than the mass of our Sun into the size of an average city. Beams of radio waves stream outward from the poles of the star's intense magnetic field and sweep around as the star rotates, as often as hundreds of times a second.

The pair of pulsars studied with the GBT is about 1,700 light-years from Earth. The average distance between the two is only about twice the distance from the Earth to the Moon. The two orbit each other in just under 2.5 hours.

"A system like this, with two very massive objects very close to each other, is precisely the kind of extreme 'cosmic laboratory' needed to test Einstein's prediction," says Victoria Kaspi, leader of McGill University's Pulsar Group.

Theories of gravity don't differ significantly in "ordinary" regions of space such as our own solar system. In regions of extremely strong gravity fields, such as near a pair of close, massive objects, however, differences are expected to show up. In the binary-pulsar study, general relativity passed the test provided by such an extreme environment, the scientists said.

"It's not quite right to say that we have now 'proven' general relativity," Breton says. "However, so far, Einstein's theory has passed all the tests that have been conducted, including ours."

tullio
tullio
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RE: Thumbs up for

Message 82584 in response to message 82583

Quote:

Thumbs up for Einstein's gravitational prediction and law.
An artical about this.

"It's not quite right to say that we have now 'proven' general relativity," Breton says. "However, so far, Einstein's theory has passed all the tests that have been conducted, including ours."


I think, after reading the McGill University news, that this observation has measured the geodetic precession and not the frame dragging as I wrote elsewhere. The geodetic precession has been measured with great precision (about 1.5% uncertainty) by the Gravity Probe B satellite. What was lacking was a measurement of the frame dragging, a much weaker effect, but it seems that NASA, for financial reasons, is not going to complete the analysis of data. Maybe a BOINC project (Gravity@home?) could do it. If you go to the Science magazine site (www.science.org) and go to the July 4 issue you'll not be able to read the full article, only the abstract unless you have a subscription. But you can watch two Quicktime movies. Here is a link to the second.
double pulsar
Tullio

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