Astronomers close in on new way to detect gravitational waves | Hacker News Reader

Astronomers close in on new way to detect gravitational waves

nature.com

79 pointspseudolus18 comments

Another tricky thing about pulsar observations is that the integration times are very short.

Obviously. You want the thousands to millions of beats per second.

With short exposure time, not many photons to grab. You need a bigger aperture.

Aericibo used to be a nice telescope for pulsars. Sigh...

China has a huge dish, sort of a next-gen Aericibo, the 500-meter FAST https://en.m.wikipedia.org/wiki/Five-hundred-meter_Aperture_...

At a conference in 2000 or 2001, they had a brief presentation where they noted that they have multiple such geological features that could be used to create such telescopes.

Interferometric arrays have so far been easier to build, I think because they have proven to be capable of more types of observation than single fixed dish, and therefore a better investment.

The NRAO can do long baseline observations that include large single dishes, although Areicibo was never able to do so; they used the Green Bank Telescope. Which is a monster.

https://en.m.wikipedia.org/wiki/Green_Bank_Telescope

https://greenbankobservatory.org/science/telescopes/gbt/

A dedicated VLBI array of FAST-class dishes would be mighty nice, though, and could spend many hours to gravity wave observations.

Arecibo and FAST are giant immovables dishes (essentially a parabolic valley covered by some electrically conducting material). The receivers suspended above these dishes are somewhat moveable, but that is about it. That means they can only observe a small region of the sky exactly overhead (this region changes together with the rotation of the earth). VLBI requires multiple telescopes significantly separated from each other in physical space (think thousands of km) to track the same object on the sky during an observation. Thus VLBI is hard/impossible for giant immovable dishes like FAST :)

> VLBI requires multiple telescopes significantly separated from each other in physical space (think thousands of km) to track the same object on the sky during an observation. Thus VLBI is hard/impossible for giant immovable dishes like FAST. :)

Good point... But the VLBI people I know are "here, hold my beer..." engineers. And astronomers will use Vegemite for thermal paste, if it lowers their Tsys in a tight spot.

If FAST-class dishes are cheap enough, you could build an assortment of them, pointing in different (fixed) directions. The primary azimuth would still train close to zenith, but the hard part is that suspended receiver assembly, the moving bits, a relatively heavy platform (that eventually fell into Arecibo's dish, after decades of minimal budget and maintenance:-( ... Dang, the Canadians have proposed large dirigibles, tethered like weather balloons, for the primary focus assemblies.

I miss that crowd. My first drive was the NRAO VLBA correlator, 23 (!) years ago. I'm out of the loop now.

Can’t wait to see what Astrophysics and Cosmology will look like in 20 years. Just a decade ago we were bound to the radio spectrum and had 0 experience with gravitational waves…

Bound to the radio spectrum?! Most astronomers work with more energetic photons than the puny things you get get from radio sources. Talking about being bound to the entire electromagnetic waveband is more correct, but we've not been bound to any narrow subset of it since like the 1930's.

Also, arguably the first non photon astronomical detection was the detection of neutrinos from SN1987A by Kamiokande, Baksan and IMB. There's also cosmic rays that we've been detecting for ages, but they don't really point back toward anything so it's really hard to argue you're doing astronomy with them.

I'm pretty sure they meant electromagnetic spectrum. Gravitational waves moves us beyond that.

As does the neutrino astronomy SiempreViernes mentioned.

Yes, I didn't mean otherwise, I was only interpreting the GP.

Sure, electromagnetic waveband is exactly what I meant, just phrased it in more accessible terms. AP is my academical background. I didn't think I could contribute much there, so I quit even before LIGO was constructed. Now it looks interesting again :)

Yes very exciting, sounds like this method has achieved a snr ~ 1 revealing the the frequency distribution of the underlying signal. Like you I cant wait to see what happens in the coming decades.

Indeed. Another aspect that could cause quite a disruption is cheaper access to space. With Starship coming to life, launching heavy and large stuff for very little money should enable previously "impossible" space telescopes (be they electromagnetic, neutrino or gravitational wave based).

Nice to hear the pulsar timing arrays are getting somewhere!

In what I'm sure is not entirely purely coincidental, I've been waiting and looking for any news from the PTA efforts for at least 5 years, and then yesterday I found a pulsar timing meme account and now here's this update! Nice!

You should share the meme account

We want the goods

So is this still a form of Interferometertry where differences from multiple signals are combined or something else?

Think of pulsars, in this context, as faraway precise clocks that chirp out a regular sequence of pulses [1]. We don't get to pick where they are, or how far away they are, but they're scattered all over the sky.

A gravitational wave passing through our solar-system/region-of-space will cause a very well-prescribed variation in the properties of spacetime. It will cause chirps from pulsars from some parts of the sky to arrive a tiny bit early and chirps from other parts of the sky to arrive a little bit late. As the wave passes by, those "early" and "late" signals will oscillate, making a readily-discernable signal.

The tricky bit, or the elegant bit, is that the signals accessible through this technique are very slow, in the nanohertz [2]. So, the experiment must watch signals from pulsars quasi-continuously for a long time, long enough to see multiple cycles of the oscillation.

It was clear from the outset that the approach had great promise. It was also clear from the outset that the measurement would take decades. We're starting to come to the time where the results are beginning to become quite interesting. There are signals, but they haven't yet oscillated enough to separate oscillations from statistical variation.

The pulsar-timing arrays are a wonderful example of the kinds of things one can do by thinking (and funding) long-term.

[1] Yes, they're imperfect, but the imperfections can either be handled or their statistical variation correctly handled when estimating the measurement's uncertainty.

[2] If you just turned 31, you're about to turn one billion seconds old. Put it on the calendar!

It is so cool how the universe acts as a lab for research of the very tiny.

> of the very tiny

Only in the perspective that a ripple on the far side of the world from a devastating tsunami is technically small compared with the noise of the ocean at that far side!

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