Yesterday there was brief moment when I thought that they had announced the first detection of a gravitational wave by LIGO. Needless to say, this turned out to not be the case. If it were then you would have heard about it - most likely from a newsreader doing their very best.
The paper that caused this trouble describes how LIGO has been used to place a low-limit on some properties of the Crab Nebula pulsar (you can read it here if you like). The way this paper was announced at the AAS meeting in St. Louis made it sound like they had a detection. But they didn’t and don’t (yet). Interestingly, I wasn’t all that excited though. For a few moments I was quite convinced they had finally made a detection and although it would have been historic I could only think of one thing: so what now?
The scientific community has been thinking about gravity waves for a very long time. They are a part of general relativity - although they were conceived of before that. To the non-astronomer, they basically answer the question, ‘what would happen if the Sun suddenly vanished?’. Would the Earth instantly fall out of orbit or wold it take time? Would we see it happened before or after we felt it happen?
The answer is that gravity is ‘transmitted’ via waves and that it travels at the speed of light - so we’d see and feel it at the same time. Gravity is thought to propagate through spacetime, much as light propogates through the electromagnetic field. The ripples it creates in spacetime are very tiny though and so they are extremely hard to detect.
So if you could ’see’ gravitationally energetic events, then what would you do? Well it would allow you to perform a new kind of astronomy. It would open a new spectrum of analysis and viewing on the universe. This new spectrum would range from highly energetic events (e.g. coalescing black holes) to lower energy events (e.g. accretion disks). Mainly it it interesting in the way it would let us look into the physics of black holes.
Huge amounts of money have been ploughed into gravity wave physics. Interestingly the gravity waves groups around the UK always seem to have a lot of money! They use it to meet up and discuss theoretical results. They create lavish PR campaigns and recruit PhD students. They take data with LIGO and its contemporaries. What they do not do, and have not yet ever done, is detect a single gravity wave!
I realise this is political, but it always irks me slightly. Gravity waves have lots of money but no results. ESA is potentially going to bump another project in favour of contributing to LISA, a gravitational wave detector in space.
So I am asking for anyone that knows something about gravity waves to give me some reasons to like the idea of studying them again. I used to when I was a kid. They are very cool, they are high-tech, but they are - for now - undetected and very, very expensive.
So any general relativity enthusiasts/experts out there who can offer me something to work with? Anyone?
June 4th, 2008 at 7:54 am
When I did science I used to work on 3D models of colliding black holes, so I have some expertise in the area. I think the confirmation of direct detecting a gravitational wave would be very exciting. As you know, we know that they are there. And we have the Einstein equations to predict gravitational signatures. But the truth is that the Einstein equations are extremely difficult to solve, and we have, so far, had no experimental data to compare our models to. We have no idea if the assumptions we put into our models are correct. In, for example, large scale structure cosmology or in stellar modeling a theoretician can compare his/her result to experimental data. If the model looks like reality, we have learning something. It is the fundamental process of scientific discovery, and General Relativists do not have it (yet). Imagine the insights we can gain from getting experimental results to compare to. Already LIGO has given results without detecting gravitational waves, because the fact that we cannot detect the waves says something about the properties of the universe.
As an astronomer you want yo know how the universe works, right? And the detection and study of gravitational waves will give of insight to figure it out. It will let us look at the universe with new eyes. It’s not even comparable say, to seeing the universe with X-ray or Gamma ray telescopes, because although we have learned much by having those telescopes, they still only study the electromagnetic spectrum. Imagine what we might see when we study the gravitational spectrum.
The scientific rewards of detecting gravitational waves are huge, which in my opinion justifies the money spent.
June 4th, 2008 at 9:32 pm
What we ought to remember is that we already know gravitational waves
exist — we have seen their effects indirectly in the way other pulsars
and neutron stars interact and how their orbits evolve. As things stand,
only gravitational waves explain these phenomena to the high precision
that the observations demand. What we are yet to do though is to
detect the waves directly, with a gravitational telescope, but our
confidence in their existence, combined with the highly sensitive
detectors that now are in operation, allows astronomers to make statements
about sources when we do *not* see the waves. This is the case here: at
some level gravitational radiation plays a part in the spin-down of the Crab,
but we have shown that it’s a small part.
The field of gravitational wave astronomy is growing fast, and already
generating lots of interaction between the worldwide gravitational wave
community (including the LSC who have authored the new Crab paper you’re referring to) and “electromagnetic” astronomers who are not themselves
directly involved in the design and operation of the various ground-based interferometric detectors. However they recognise the enormous potential of
these detectors for addressing major questions across a number of areas of astrophysics - from stellar evolution to cosmology. So the potential
applications of gravitational wave research are definitely *not* just in relativity (although there are undoubtedly fascinating questions to be
addressed there too).
ttfnRob, in your own field, for example, Sandick et al (2006) astro-ph/0603544
estimate the GW signatures of the first stars, and show that future ground-
and space-based interferometric GW detectors may put constraints on the SFR
from Population III stars at redshifts where it would be extremely challenging
to make E-M observations.
If you’d like to find out more about the rich potential of gravitational wave
astronomy, why not have a chat to Sathya in your Dept., or take a look at the
A&G article I wrote with Graham Woan (one of the main people behind the Crab result) on “Gravitational Astrophysics”
Astronomy and Geophysics, 2007, 48, 1.10-1.17)
June 4th, 2008 at 10:17 pm
Martin, thank you for such a thoughtful reply. I knew someone out there would be able to get me fired up about gravity waves again! Pop III stars, eh? I will chat to Sathya about that, it sounds very interesting. Its too easy to get swept up in politics as a PhD student. That’s why its great running this blog.
June 5th, 2008 at 5:30 am
No worries! These are troubling times for UK astronomy, and it’s easy for me
too to get riled about the politics and forget why I wanted to get into this
subject in the first place: to answer the ‘big questions’.
I’m currently at a workshop in Aspen on gravitational wave astronomy: there’s
really going to be some amazing problems to work on that best combine E-M and
GW data, if we can just convince the politicians to let us build the telescopes!
Cheers
Martin
June 5th, 2008 at 9:34 am
I’m not sure what a gravity wave telescope would look like. Could LISA resolve an image?
October 31st, 2008 at 4:27 am
From MY understanding of gravity, it doesn’t require anything even resembling waves– it merely involves “irregularities” in the space-time matrix. An object in orbit follows the same law of inertia that an object is subject to when it travels in a “straight” line. These lines are merely “distorted”. If such an object could “think”, an orbiting object would “imagine” it was following the straightest possible path.