Gary McGath <
garym@mcgath.com> wrote:
Keith F. Lynch wrote:
There will be a livestreamed "nanograv" announcement today at 1 pm
EDT. Presumably it has something to do with very low frequency
gravitational waves.
Eugen Rochko just posted "Potsdam gravity potato" on Mastodon.
Clearly something weighty is happening, maybe even a Mission of
Gravity, but I have no idea what.
The project has been carefully tracking the timing of pulsars for the
past 15 years. Pulsars are neutron stars which rotate very rapidly
and very regularly. Once you compensate for small frequency shifts
due to Earth's rotation and Earths' revolution around the sun, for
the pulsar's proper motion, and for interstellar dispersion, they're
almost as precise as a cesium clock.
Seven years ago LIGO detected gravitational waves. LIGO consists of
two observatories, each of which consists of two tunnels at right
angles. The lengths of the tunnels are measured in real time by laser interferometers. When a gravitational wave passes by, it stretches
one of the tunnels and contracts the other, and this can be detected
by interferometry. The wave sources are typically collisions between
black holes, between neutron stars, or between one of each.
The Nanograv project replaces the 4-kilometer-long tunnels with the
thousands of light years between us and the pulsars. By measuring
slight variations in pulsar timing and compensating for various
sources of noise, they've detected very low frequency gravitational
waves, believed to be caused by very distant very immense (billions
of solar masses) black holes revolving around each other.
Conceptually it's fairly simple. The main difficulty is in
compensating for sources of noise, i.e. non-gravitational-wave causes
of timing variations. Earth's revolution is one such cause. It has
been well understood since Copernicus half a millennium ago. But not
well enough. To get clean data, the Nanograv project had to pin down
the center of gravity of our solar system to within an unprecedented
precision of 100 meters.
Dispersion is a harder problem. It consists of a delay in the radio
signal from the pulsar due to the hard vacuum between the pulsar
being not quite hard enough. The signal which took several thousand
years to get here is delayed by perhaps a millionth of a second.
Fortunately, those delays don't matter, only how they vary with time
do. And *that* varies with which radio frequency they're received on.
But timing variances that are due to gravitational waves are
independent of the radio frequency.
Also, different pulsars are in different directions, resulting in
completely different patterns of dispersion. But pulsars in the same
general direction should "see" the same gravitational waves.
Pulsars are broadband sources. The radio frequencies the pulsars are
received on varies from a few megahertz to a few gigahertz. (They're
also observable in visible light, x-rays, etc.) Whichever frequency
we listen to, each individual pulsar has a specific *audio* frequency.
For instance PSR J1748-2446ad rotates at 716.355563 Hz. (Yes, a "day"
on that neutron star just lasts a 716th of a second.) If you point
a radio telescope in its direction, and hook it to a speaker, you will
hear that F#5 musical tone regardless of what radio frequency you're
tuned to. The same if you instead use an optical telescope, and hook
a photocell to your speaker.
Those radio frequencies and rotational frequencies shouldn't be
confused with the frequencies of the gravitational waves themselves.
Nanograv can only detect gravitational waves with frequencies in the
nanohertz range, hence the name. That is, each cycle lasts several
years, and the wavelengths are several light years.
The frequencies of the gravitational waves detectable by LIGO are in
the kilohertz range. So there's no overlap.
LIGO consists of two observatories, one in Louisiana, one in
Washington state. Sometimes a third similar observatory, such as
VIRGO in Italy, is online. In that case the direction of the source
can be determined. With further data recording and analysis, it's
believed that it should be possible for Nanograv to determine the
directions of the sources it's detecting.
Another difference between LIGO and Nanograv is that the former tends
to detect sudden brief violent events, while the latter probably tends
to detect continuous waves.
The Nanograv sources are believed to be extremely distant, even in
comparison to the distances to the pulsars. The path to the pulsars
is the antenna for detecting the gravitational waves, and is as small
in comparison with the distances to their sources as a typical radio
antenna is small in comparison with the distance to the transmitter.
I hope that's clear. In any case, it's good to hear news that isn't
all doom and gloom.
--
Keith F. Lynch -
http://keithlynch.net/
Please see
http://keithlynch.net/email.html before emailing me.
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