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The Earth As a Gravitational Wave Detector
b30w0lf writes "Gravitational wave detection — i.e. the detection of propagating ripples in spacetime — is a hot subject these days, with ground-based interferometer experiments like LIGO active, and hopes for a space interferometer like LISA. However, physicist Freeman Dyson proposed back in 1969 that the earth itself could be used as a gravitational wave detector. The idea is behind the approach is that gravitational waves impact the earth's crust, causing potentially detectable seismic waves. Using Dyson's approach, Physicists at Harvard and NINP, Florence were able to put an upper limit on the intensity of gravitational background radiation based on a year of observational seismic data (abstract, full pre-print). The upper limit they found improved currently laboratory upper limits by 9 orders of magnitude."
The crucial thing is that they improved the limits in the narrow frequency band where the Earth is a resonant detector :
This is very cool, but note that it is at a frequency where there are not a lot of expected sources (stellar-mass binary black hole coalescence is up in the kHz range).
The announcement on Monday about inflationary gravitational waves is likely to get a good deal more scientific attention.
Upper Limit on a Stochastic Background of Gravitational Waves from Seismic Measurements in the Range 0.05–1 Hz
Michael Coughlin and Jan Harms
Phys. Rev. Lett. 112, 101102 (2014)
Published March 13, 2014
Since the moon is much more stable than the Earth, would it be a better detector? Have seismic readings been taken on the Moon?
love is just extroverted narcissism
Thay're not saying that gravity waves are creating the signals. What they are saying is that if gravity waves are creating any signals at all, the size of the signals being measured limits the possible size of the gravity waves to smaller than a certain size. It is putting an upper bound on the possible size of gravity waves. This is important, because the previously determined upper bound on gravity wave size was 9 orders of magnitude bigger than it is now that we have these experimental results.
I wish I had a good sig, but all the good ones are copyrighted
You know, "orders of magnitude" are not that thing when you ask for extra large dishes in a restaurant.
Ezekiel 23:20
but what about other things that could generate similar signals
What "other things"? Electromagnetism is the only thing that has comparable speed to gravity waves, and unlike gravity waves, it only penetrates Earth-sized solid matter with exponential falloff at short distances. Forget possible seismic causes for the kind of measurements they must be looking for. Just recalling my high school physics 101, you know...
Also, from TFA:
Gravitational waves are the last untested prediction of Albert Einstein's General Theory of Relativity.
I'd argue that this is debatable, seeing that we've already measured the decay of at least one binary pulsar that wonderfully corresponds to the predicted gravity-wave-mediated energy loss.
Ezekiel 23:20
Neither.
What they did is say is basically "We now have a detector 10^9 times more sensitive, which is capable of detecting gravitational waves up to 10^9 times smaller than previous detectors, if there are waves. We didn't see any waves with this detector. Therefore if they exist, they are smaller than what our new detector can detect".
In other words, if there are gravitational waves, they are smaller magnitude than they are able to detect with the new detection system. This doesn't rule them out, it just blacks out a potential energy/amplitude range in which they might have existed before nothing was seen in that search band.
They've more or less reduced the probability set, and pissed in a number of esoteric theories cheerios, but not done a lot else to prove or disprove gravity waves.
It's the difference between having to look for a lost item in an entire warehouse, or having to look for it in a crackerjack bix sized area of the warehouse - albeit it'll take a lot more expensive and redesigned equipment to even look in part of the crackerjack box.
Frankly, if we threw 4 ten ton spheres into relatively deep space (e.g. solar orbit), arranged them at the vertices of a tetrahedron, and then used laser interferometry between the spheres, and then threw another ten ton sphere across the solar system at a non-trivial speed, and through the tetrahedron, not intersecting a face or the body center, we could pretty much say once and for all if there were gravity waves or not, based on delay (or non-delay) of the effect of the moving sphere being "not there, then suddenly there, then suddenly not there", at least to about 1/2 the wavelength used by the interferometers (hence the need for a "non-trivial speed" for what is, in effect, a gravitational probe inserted into the system, to do the experiment).
Doing the more or less definitive experiment would be expensive (as in "on the order of the cost of the LHC").
They've sunk over a billion into the Hanford and Livingston observatories. The LIGO observatories from 2002 to 2010 were only operational for a very small fraction of the time, plagued by equipment problems, never acheived the design sensitivity, and NEVER detected anything useful. Most of their data was contaminated by local noise, including the highway a few miles away. They blindly collected terabytes of raw data that has never been fully analyzed and they have minimal local data analysis capability.
Now NSF is pouring even more money into it in the hopes they can improve the sensitivity and actually detect something? At best they might record a perturbance that is correlated between multiple sites (they also partner with an Australian site I believe), of which the value of that data is still debatable.
I wish the NSF would pull the plug on this waste of resources and invest in something more useful like cleaner nuclear power.
Here is the null result. I presume there are new analytical techniques every few years.
You can't increase and decrease mass - so no monopole gravity waves.
You can't move the center of mass (conservation of momentum) so no vector gravity waves.
You can change the distribution (imagine two masses moving closer and further apart), and this generate tensor (spin 2) gravity waves.
The coupling is VERY small - so the energy radiated is tiny unless you are dealing with near black-hole conditions.
December 27, 2004 at 21:30:26 UT, a burst of gamma rays from SGR 1806-20 passed through the Solar System. The burst was so powerful that it had effects on Earth's atmosphere, at a range of about 50,000 light years.
At 00:58:53 UTC on Sunday, 26 December 2004, an undersea megathrust earthquake occurred in the Indian Ocean which caused a tsunami which killed 250,000 people.
Where does a gravity wave theoretically come from? All I can imagine is that they would come from a mass increasing or decreasing in magnitude, and I don't know of any way that happens.
The Guardian article refers to a detector which might have made an indirect detection of gravitational waves.
If two massive bodies such as neutron stars or black holes collide, the energy they lose in the form of gravitational energy is propagated away in waves. These waves are ripples in spacetime, and they are quadropolar in nature. This means that they stretch spacetime in one direction while squeezing it in the other.
Gravitational waves form part of the predictions of Einstein's Theory of General Relativity. They are the last piece of the theory yet to receive a direct detection. A notable indirect detection of gravitational waves is the measurements of the orbital decay of the PSR B1913+16 binary pulsar, for which Hulse and Taylor received the Nobel Prize in 1993.
For the purposes of direct detection of these waves, on Earth we've set up a network of laser interferometers (the major players are Advanced LIGO, Advanced Virgo, GEO-HF and KAGRA, though all but GEO are currently in the process of being commissioned). If we arrange our detectors on Earth at right angles, we become optimally sensitive to the majority of gravitational wave sources. If the masses of the bodies involved in the collision are big enough, the ripples in spacetime will be strong enough to change the time in which it takes light to travel along each arm of the interferometer - in one arm the light will take longer time to travel, and in the other it will take shorter time. If we recombine the light in each arm, we can sense via the interference pattern of the light whether a gravitational wave has passed through the detector. In practice there are loads of other signal sources present in the interferometer, and quantifying and eliminating these sources of noise are the major tasks facing these detectors. With these noise sources accounted for, the first direct detection of gravitational waves might be made in the next few years.
The LISA project referred to in the main article is dead since NASA pulled out funding. The project lives on in the form of the ELISA project, funded by European organisations. This has tentative approval for launch in the next 20 years. This mission is not intended to directly detect the 'first' gravitational wave, but rather to detect them in abundence. Indeed, the problem with this type of detector is dealing with the huge number of potential detections. ELISA is also designed to detect waves in a completely different spectrum from the ground detectors, and from different astronomical sources. By the time ELISA launches it is likely that the network of detectors as part of the LIGO Scientific Community will have made the first detection, here on Earth.