News: Intergalactic Race Ends in a Virtual Tie.
- From: Ye Old One <usenet@xxxxxxxxx>
- Date: Wed, 28 Oct 2009 22:52:44 GMT
Intergalactic Race Ends in a Virtual Tie
By DENNIS OVERBYE
http://www.nytimes.com/2009/10/29/science/space/29light.html?partner=rss&emc=rss
Astronomers announced Wednesday that a race halfway across the
universe had ended in a virtual tie. And so the champion is still
Albert Einstein ? for now.
The race was between gamma rays of differing energies and wavelengths
spit in a burst from an exploding star when the universe was half its
present age. After a journey of 7.3 billion light-years, they arrived
within nine-tenths of a second of each other in a detector on NASA?s
Fermi Gamma-Ray Space Telescope, at 8:22 p.m., Eastern time, last May
9.
Astronomers said the gamma-ray race was one of the most stringent
tests yet of a bedrock principle of modern physics: Einstein?s
proclamation in his 1905 theory of relativity that the speed of light
is constant and independent of its color, energy, direction or how you
yourself are moving.
?I take it as a confirmation that Einstein is still right,? Peter
Michelson of Stanford, principle investigator for Fermi?s Large Area
Telescope and one of 206 authors of a paper published online in the
journal Nature, said in an interview.
So far, there is no evidence that the energy or wavelength of light
affects its speed through space.
That is important because of what it could say about the structure of
spacetime. Some theorists have suggested that space on very small
scales has a granular structure that would speed some light waves
faster than others ? in short, that relativity could break down on the
smallest scales.
Dr. Michelson and others emphasize that while the new Fermi results do
not yet eliminate the prospect, further observations with more gamma
ray bursts could eventually verify or refute the hypothesis. That
would have a major effect on efforts by physicists to unify the
Einsteinian gravity that governs outer space with the weird quantum
laws that govern the inner space of the atom.
Mario Livio, an astronomer at the Space Telescope Science Institute in
Baltimore, called the Fermi results an interesting effect but not
revolutionary by any stretch, saying, ?The beauty of the experiment is
not as much in what it achieves as in the fact that you can use
astronomical observations to place some interesting limits on very
fundamental physics.?
Quantum theory, as Einstein discovered to his chagrin, reduces life on
subatomic scales to a game of chance in which elementary particles can
be here or there but not in between. One consequence is that spacetime
itself should become discontinuous and chaotic when viewed at very
close distances, the way an ocean can look smooth from an airplane but
appears choppy and foamy up close.
This, the story goes, could have an effect on the propagation of
light, or photons as they are called in quantum-speak, slowing light
with short wavelengths relative to light with longer wavelengths. The
higher the energy of a photon, the shorter is its wavelength. One way
to think about it is to envision the photons as boats on this choppy
sea. The small ones, like tugboats, have to climb up and down the
waves to get anywhere, while the bigger ones can slice through the
waves and bumps like ocean liners, and thus go a little faster.
Until now such quantum gravity theories have been untestable.
Ordinarily you would have to see details as small as 10-33 centimeters
? the so-called Planck length, which is vastly smaller than an atom ?
to test these theories in order to discern the bumpiness of space.
Getting that kind of information is far beyond the wildest
imaginations of the builders of even the most modern particle
accelerators, which has left quantum gravity theorists with little
empirical guidance.
?What?s really lacking,? Dr. Michelson explained, ?is a laboratory
experiment that tells us anything. So we have to use cosmology: we use
universe as the lab.?
Fermi, which was launched into orbit around the Earth in 2008, is a
particle physics experiment in space. Its main instrument, the Large
Area Telescope, records both gamma rays, which are a form of energetic
light, and high-energy charged particles like electrons or protons
raining through space.
The explosions known as gamma-ray bursts, which are thought to signal
the implosions of stars into black holes, are ideal for gamma-ray
racing. They are far away, and their photons have a wide range of
wavelengths or energies. By timing the arrivals of photons of
different energies, astronomers can determine if the more energetic
ones are arriving a little later than the less energetic ones.
The photons from GRB 090510, detected on May 9, ranged from 31 billion
electron volts ? the energy unit of choice in physics ? to 10,000
electron volts, a factor of more than a million, in seven brief bursts
over about a two seconds.
The spread in travel time of 0.9 second, if attributed to quantum
effects rather than the dynamics of the explosion itself, suggested
that any quantum effects in which the slowing of light is proportional
to its energy do not show up until you get down to sizes about
eight-tenths of the Planck length, according to the Nature paper,
whose lead author was Sylvain Guiriec of the University of Alabama.
Dr. Livio emphasized, however, that this was only one of many classes
of models. ?It would be amazing that in effect we don?t need a quantum
theory of gravity,? he said. ?This only tells us where there are the
dead ends.?
Indeed, other physicists said that even this model would not be ruled
out until the size limit had been set much below the Planck size.
The good news, astronomers said, is that more data expected from Fermi
could decide the question. As Lee Smolin, a quantum gravity theorist
from the Perimeter Institute for Theoretical Physics in Waterloo,
Ontario, said, ?So a genuine experimental test of a hypothesized
quantum gravity effect is in progress.?
In the meantime, the last word belongs to Einstein, as Robert Kirshner
of the Harvard-Smithsonian Center for Astrophysics wrote in an e-mail
message paraphrasing a New York Times headline from 1919 about
observations that confirmed Einstein?s general relativity. ?But the
Nature story is, ?Einstein found right again. Heavens not askew!
Savants not agog!? ? Dr. Kirshner wrote.
--
Bob.
.
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