PAnews.com, Port Arthur, Texas

Community News Network

January 7, 2014

Stellar trio could put Einstein's theory of gravity to the test

In a cosmic coup, astronomers have found a celestial beacon known as a pulsar in orbit with not one, but two other stars. The first-of-its-kind trio could soon be used to put Einstein's theory of gravity, or general relativity, to an unprecedented test.

"It's a wonderful laboratory that nature has given us," says Paulo Freire, a radio astronomer at the Max Planck Institute for Radio Astronomy in Bonn, Germany, who was not involved in the work. "It's almost made to order."

A pulsar consists of a neutron star, the leftover core of a massive star that has blown up in a supernova explosion. The core's own gravity squeezes it so intensely that the atomic nuclei meld into a single sphere of neutrons. The spinning neutron star also shines out a beam of radio waves that sweeps the sky just as the light beam from a lighthouse sweeps the sea. In fact, pulsars flash so regularly that they make natural timepieces whose ticking can be as steady as that of an atomic clock.

The incredible regularity makes it possible to determine whether the pulsar is in orbit with another object, as roughly 80 percent of the more than 300 fast-spinning "millisecond" pulsars are known to be. As the pulsar and its companion orbit each other, the distance between the pulsar and Earth varies slightly, so that it takes more or less time for the pulses of radio waves to reach Earth. As a result, the frequency of pulsing speeds up and slows down in a telltale cycle.

But such a simple scenario couldn't explain the peculiar warbles in the frequency of pulsar PSR J0337+1715, which Scott Ransom, an astronomer at the National Radio Astronomy Observatory in Charlottesville, Va., and colleagues discovered in 2007 with the Robert C. Byrd Green Bank Telescope in West Virginia. Training other radio telescopes on the object, Ransom and colleagues kept it under near-constant surveillance for a year and a half. Eventually, Anne Archibald, a graduate student at McGill University in Montreal, figured out exactly what's going on.

The pulsar, which has 1.4 times the sun's mass and spins 366 times a second, is in a tight orbit lasting 1.6 days with a white dwarf star only 20 percent as massive as the sun. A second white dwarf that weighs 41 percent as much as the sun orbits the inner pair every 327 days, as Ransom and colleagues reported online Sunday in Nature. "We think that there are not more than 100 of these [trios] in our galaxy," Ransom says. "They really are one-in-a-billion objects."

The distinctive new system opens the way for testing a concept behind general relativity known as the equivalence principle, which relates two different conceptions of mass. An object's inertial mass quantifies how it resists pushing or pulling: It's easier to start a stroller rolling than a car because the stroller has less inertial mass. A thing's gravitational mass determines how much a gravitational field pulls on it: A barbell is heavier than a feather because it has more gravitational mass.

The simplest version of the equivalence principle says inertial mass and gravitational mass are equal. It explains why ordinary objects like baseballs and bricks fall to Earth at the same rate regardless of their mass - as legend claims Galileo showed by dropping heavier and lighter balls from the Leaning Tower of Pisa.

The strong equivalence principle takes things an important step further. According to Einstein's famous equation, E = mc2, energy equals mass. So an object or system's mass can be generated by the energy in the gravitational fields within the system itself. The strong equivalence principle states that even if one includes mass generated through such "self-gravitation," gravitational and inertial mass are still equal. The principle holds in Einstein's theory of general relativity but typically does not hold in alternative theories, says Thibault Damour, a theoretical physicist at the Institute for Advanced Scientific Studies in Bures-sur-Yvette, France. So poking a pin in the principle would prove that general relativity is not the final word on gravity.

Researchers have already tried to test the strong equivalence principle. Since the 1970s, some have compared how the moon and Earth orbit in the gravitational field of the sun. More recently, others have analyzed the motion of pulsar-white dwarf pairs in the gravitational field of the galaxy. But those studies have been limited, Damour says. Earth's self-gravitation accounts for just a billionth of its mass. In pulsar studies, the galaxy's gravity is very weak. So the strong equivalence principle has been tested only to a precision of parts per thousand.

The new pulsar system opens the way to a much more stringent test by combining the strengths of the two previous methods. The self-gravitation of the pulsar accounts for roughly 10 percent of its mass, in contrast to less than 0.001 percent for the inner white dwarf. At the same time, both move in the gravitational field of the outer white dwarf, which is much stronger than the field of the galaxy. By tracking the system's evolution, Ransom and colleagues should be able to tell whether either the inner white dwarf or the pulsar falls faster toward the outer white dwarf and test strong equivalence about 100 times as precisely as before, Damour says.

So will strong equivalence principle be found wanting? "I would rather expect to get a better limit" on possible violations, Damour says. "But I'm open-minded. It would be great to get a violation." Freire says a violation would be "a complete revolution."

Researchers may not have to wait long, Ransom says. His team should be able to test the principle within a year.

 

1
Text Only
Community News Network
  • Celebrity quack moms are a terrible influence on everyday parents

    On April 15, the actress Alicia Silverstone released a book called "The Kind Mama: A Simple Guide to Supercharged Fertility, a Radiant Pregnancy, a Sweeter Birth, and a Healthier, More Beautiful Beginning." It's chock-full of attachment parenting lessons and dangerous misinformation.

    April 24, 2014

  • Affirmative action ruling challenges colleges seeking diversity

    The U.S. Supreme Court's support of Michigan's ban on race-based affirmative action in university admissions may spur colleges to find new ways to achieve diversity without using racial preferences.

    April 23, 2014

  • A 'wearable robot' helps her walk again

    Science is about facts, numbers, laws and formulas. To be really good at it, you need to spend a lot of time in school. But science is also about something more: dreaming big and helping people.

    April 23, 2014

  • Cuba is running out of condoms

    The newest item on Cuba's list of dwindling commodities is condoms, which are now reportedly in short supply. In response, the Cuban government has approved the sale of expired condoms.

    April 23, 2014

  • The waffle taco's biggest enemy isn't McDonald's. It's consumer habits.

    Gesturing to Taco Bell, Thompson said McDonald's had "not seen an impact relative to the most recent competitor that entered the [breakfast] space," and that new competition would only make McDonald's pursue breakfast more aggressively.

    April 23, 2014

  • Screen Shot 2014-04-22 at 4.42.47 PM.png VIDEO: Leopard attacks crowd in India

    A leopard caused panic in the city of Chandrapur when it sprung from the roof of a house and charged at rescue workers.

    April 22, 2014 1 Photo

  • The top 12 government programs ever

    Which federal programs and policies succeed in being cost-effective and targeting those who need them most? These two tests are obvious: After all, why would we spend taxpayers' money on a program that isn't worth what it costs or helps those who do not need help?

    April 22, 2014

  • In cuffs... 'Warlock' in West Virginia accused of sexual assault

    Police in West Virginia say a man claiming to be a “warlock” used promises of magical spells to lure children into committing sexual acts with him.

    April 22, 2014 1 Photo

  • Cats outsmart the researchers

    I knew a lot had been written about dogs, and I assumed there must be at least a handful of studies on cats. But after weeks of scouring the scientific world for someone - anyone - who studied how cats think, all I was left with was this statement, laughed over the phone to me by one of the world's top animal cognition experts, a Hungarian scientist named Ádám Miklósi.

    April 22, 2014

  • McCain 1 House Republicans are more active on Twitter than Democrats

    Your representative in the House is almost certainly on Twitter. Your senator definitely is. But how are they using the social network? Are Democrats more active than Republicans, or vice versa? Who has the most followers on the Hill?

    April 22, 2014 1 Photo

Video
Facebook
Sports Tweets
Photos