Galaxy Wide Telescope

My favourite bit from today’s Astronet sessions … The Square Kilometre Array (SKA) guys say they will find 30,000 pulsars , and 1000 of these will be millisecond pulsars. So we will have a network of these beasts spread through the Galaxy, and will know their frequencies and phases very accurately. Then … here comes the cool bit …. as gravitational waves pass through the Galaxy, and temporarily alter the path lengths to different parts of this network, we will see phase shifts. So we will be using the whole Galaxy as a gravitational wave detector. Its as sensitive as LISA but sensitive to higher frequencies.

How cool is that.

12 Responses to Galaxy Wide Telescope

  1. Terry Johnson says:

    I found this post and read through the first page of your posts. I’m very glad I found you. I’ll be subscribing now. :o)

    I’m also linking to this post from my blog, which sadly doesn’t include a cool photo of the desk of a Nobel laureate.


  2. Stephen says:

    Best news item since the detection of xrays from a supernova.

  3. MikeW says:

    Certainly cool idea, but I don’t think it actually uses the whole Galaxy in the way you imply ….

    must be detecting local effects on pulsar timing surely?

  4. andyxl says:

    Quite right Mike. John Peacock pointed this out to me too – any particular event takes 26,000 years to cross the Galaxy … so you only see a local effect as you say. However, the whole network will show a jitter due to the background of cosmic grav waves. Measuring the amplitude and spectrum of that jitter should be a test of inflation. Of course, as my other chum Andy T pointed out to me, there may be other such sources of pulsar noise. Damn. One should never grasp something too firmly or it crumbles.

  5. Tony says:

    I seem to recall that adding noise to a system can make some undetectable information detectable. Wonder if it is possible to work backwards from knowing that you are detecting something that ought to be undetectable to working out the level of noise and thereby, in the above example, figuring out the level of gravitational waves?

  6. andyxl says:

    Tony : (a) wuh ? (b) its easier than you make it sound. Expanding on this a tad … (a) As far as I know, adding noise always loses information. If you are right, find me the reference. (b) You hypothesise a given level of grav waves; that predicts the pulsar noise; then you see if the measured noise is more or less than this… Add flour and eggs, scale as appropriate. I am hoping Andy T might tell us what the other possible sources of pulsar noise are, but I am not sure if he’s a reader …

  7. Tony says:

    Reason it came to mind was article in last week’s New Scientist ( but I’ve seen the same thing several times before in NS articles: One of those sums it up as ‘Stochastic resonance is the name given to the enhanced sensitivity of a device to a weak signal that occurs when random noise is added to the mix’. But maybe that is where I’m misinterpreting: the system above would be adding noise to the signal, not to the receiving instrument. Then again, at it says ‘stochastic resonance is a mechanism by which a system embedded in a noisy environment acquires an enhanced sensitivity towards small external time-dependent forcings’ so maybe external noise does work to increase sensitivity.

    It would be good to know if I have misunderstood the whole concept. I’ll await your analysis.

  8. Tony says:

    Only just realised I would have been better simply googling for ‘gravitational wave stochastic resonance’ than looking at what had been written in NS: returns ‘about 201,000’ hits.

    I didn’t bother looking far since I can’t interpret the results but near the top of the list is, ‘A Macroscopic Gravity Wave Effect’. From the abstract: ‘Part of the scheme is a new, purely numerical detection technique that can also be used in the data processing of other projects of periodic gravity wave detection.’ That technique is ‘in a manner that is akin to stochastic resonance’ (p2).

    No idea if this is relevant to the idea in Andy’s post. I’ll leave that to the experts.

  9. andyxl says:

    Oh noes, brane hurtz !

  10. Tony says:

    Sorry – too much for a weekend – I’ll go back to sleep 🙂

  11. Simon says:

    What this is measuring is the Earth, bobbing about on the gravitational waves, so indeed the Galaxy is not the detector (the pulsar timing residual is the integral of the fractional doppler shift over time). The pulsars are used as timing references and actually you would only need one pulsar if it a) was not bobbing about too and b) was intrinsically a perfect clock. Having the pulsar array gives a slightly less noisy reference frame. The important thing is that gravitational waves have no overall effect on the radio propagation through the Galaxy, just on the end points (the rest cancels out). This works for non-stochastic waves too, see for a lovely example.

    The timescales for these measurements are days to 10s of years, so sensitivity is to frequencies that are actually much *lower* than LISA (which bottoms out at about 0.1 millihertz)

  12. andyxl says:

    Simon – many thanks for this very useful clarification. Makes much more sense. A good example of the dangers of blogging – shooting from the hip with a spontaneous reaction can lead to some silly errors. My apologies to all. Its still very cool ….

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