* Posts by Garret Cotter

25 posts • joined 19 Mar 2009

Boffins 'write directly to memory' of living brains

Garret Cotter

@Stevie: This is important

Understanding dopaminergic neurone response is not just at the core of understanding learning and memory but is essential for the understanding and treatment of several severe mental illnesses.

Don't make a knee-jerk reaction to something you clearly do not even remotely understand.

Cosmic flashbulb effect caused by 'black-hole invaders'

Garret Cotter


Hugo - in principle yes and lots of work has been done on lensing of GRBs for several reasons, but it's not thought to be a useful mechanism for the long/short burst dichotomy.

In practice the differential path lengths from a typical single-galaxy lens (the sort of "Einstein Cross" setup that provides pretty HST pics) are of order hours to months. So a single short-duration GRB would appear as multiple short bursts separated by that timescale - I don't think such a GRB has ever been unambiguously observed, and I think that's consistent with the rarity of the very close source-lens alignment you would need.

Many a GRB, though, lies on a line-of-sight that skirts nearby a foreground galaxy, usually evidenced by absorption lines in the GRB optical spectrum. In such cases micro-lensing by individual stars in the foreground galaxy would be expected to occur, but if IIRC that can only give you fractions of a second of temporal smearing - not enough to make a 100-second long-duration burst from an intrinsically short one.

(And of course the near-alignment will also cause some magnification too - there's been some controversy here recently because of a claim that "GRBs are more frequently lensed than quasars" - but as always the devil is in the detail and there's still plenty of head-scratching going on!)

Ford says new Taurus 'is fitted with stealth fighter radar'

Garret Cotter

Jodrell Bank

In fact, fundamentally it *is* a radio interferometer. More "useless mediocrity." :)

Hubble back in form with stunning new images

Garret Cotter

So, so beautiful, but...

... to quote the wise AC on the Drayson story, isn't this just an example of our "mediocre, low-grade, dull-witted, grunt academic 'research' 'work' ... of value to no-one in industry." No?

Congratulations and thanks to everyone at NASA, in the aerospace industry, in the universities, and fundamentally the US, Canadian and European taxpayers who have helped get HST back into shape. With luck those who use HST will now be able to advance their useless mediocrity for a few more years. And when spectacular results show up I'm sure press releases will be sent to El Reg.

Brown apologises for 'appalling' treatment of Turing

Garret Cotter

Re: Cringeworthy

Thought I would get INB4 "It was my ancestors wot did it!". Oh well.

This happened very recently. Living relatives of mine may have been involved. We're not talking about gifting indigenous americans blankets full of smallpox - section 28 applied to almost all of us, and homophobia is still rampant in this country. The UK government is my representative as well as yours, and I am pleased that they have apologised for the behaviour of their very-near predecessors.

One has to be very careful about conflating issues here, but if the present UK government apologised for Bloody Sunday I for one would not sneer "You didn't do it!"

Stargazers spot oldest supernova yet

Garret Cotter

Sn nucleosynthesis

There's plenty of time - seconds - for the r-process to churn out heavy elements all he way up to

Uranium in a Type II Sn (the r stands for rapid!). If anything, the problem is that observed Sn rates ought to produce too *many* trans-ferrous elements. And the s-process is believed to be simmering away in AGB stars all the time.

NASA takes stick over feet and inches

Garret Cotter


The kilo is defined as the mass of the standard kilo at Sevres. It was made to be close to the mass of a litre of water at STP, but that's not the *definition* - it still relies on comparison with one particular lump of metal. Pedantic, but important to remember if one is trying to claim some sort of metaphysical superiority for S.I.

iPhone owners are superior beings, says survey

Garret Cotter

@ Craig 12

Well said Sir... as the Reverend Bayes told us, P(A|B) != P(B|A)

One fifth of humanity deprived of Milky Way

Garret Cotter

You don't need smog...

Simple Rayleigh scattering off nitrogen and oxygen molecules is fine. Frex, the light from Sydney is clearly visible on a dark night at the AAT, in the middle of country NSW 400km away. And, yes, the light going straight up from traditional streetlamps is more significant than that reflected from the ground - the ground is reasonably "black". Virtually all streetlamps in Hawaii are now down-pointers and it makes a difference.

@Stevie - what are you going on about re: Pluto. A small fraction of the world's astronomers spent an even smaller amount of time worrying about classification within the Solar System. You insinuate that most astronomers send their time arguing about angels on the head of a pin instead of doing "real" astronomy (which you apparently have some authority to define?). Classification is very labour-light and experience tells us that it usually helps delineate the underlying physics.

Zaphod Beeblebrox home sun 'shrinking', may have blown up

Garret Cotter


Feel free to choose an orthogonal pronunciation :) I know one theorist who works on it and called Alpha Ori for years without even being able to point at it in the sky. As Feynman said, there's a difference between knowing something, and knowing the name of something.

Consensus among the stellar astrophysics experts I've talked to (I'm an AGN person myself) is that its not likely to go bang for a while yet (10^4-10^5 years), it's still a bit too red on the standard evolutionary tracks. Probably this is just an envelope pulsation (details of which we don't understand!) Even if something weird were going on it really ought to start oscillating pretty erratically before the explosion.

As has been said it'll be a relatively modest Type II, the light won't hurt us, and the shock/cosmic rays will probably be pretty dilute by the time they get here much much later. Yes too light to be a GRB by current models (incidentally Ia's don't give GRB's, similary too light/not enough spin, they deflagrate away) and as I think someone linked, its not pole-on anyway. I'd put my money on Eta Carinae instead.

YMMV, this is science, all predictions are subject to uncertainties. If it does go pop tomorrow we'll have a nice light show!

Boffins develop interstellar alien ocean-spotting tool

Garret Cotter


I've read the paper and their result looks robust, but there's nothing really quantitative about how to apply it to exoplanets, there's basically just a throwaway sentence "we conclude that it should be possible to infer the existence of water oceans on exoplanets with time-resolved broadband observations taken by a large space-based coronagraphic telescope." Hmm.

OK then put in some numbers. Their imaging is done at ~0.5-1.0 microns, at which wavelengths the Earth is about 1E10 times fainter than the Sun, so at 10 parsecs the Earth would be about 30th magnitude. For this technique to work you need to image a few times a day to see the rotation - you can't just sit on the target for a long time to build up S/N.

So, ballpark, we want to get a 30th magnitude target at optical wavelengths in an hour. For comparison, the Hubble Deep Fields get to about ~29th magnitude at these wavelengths with 1E5 second exposures in each band, and a 2.5m mirror. So with S/N proportional to sqrt(time) that's about 27 mag in an hour with 2.5-m mirror, we want to go three mags deeper which is a factor of 15, which gives a ~10-m mirror (S/N proportional to D^2). That might be on the small side depending on how strict we are with our S/N requirement. Anyway - big, but not technologically impossible.

However this does assume that you can blot out the light from the 5th mag star 0.1 arcsec away!

One of the primary reasons that planet hunters are aiming for the IR rather than the optical is that the contrast between star and planet is "only" about 1E6 rather than 1E10. I'm not sure how easy it would be to divert their attention to short wavelengths...

Atlantis safely back to Earth

Garret Cotter

Thank you

From an HST user - congratulations and thanks to the crew of STS-125, everyone on the ground at NASA, in the aerospace industry and universities, and ultimately the taxpayers, who made this possible.

Swivel-shed v polar stilt-scope in Big Bang boffinry clash

Garret Cotter

@Pad & Biff

Cheers, folks, you're welcome - I guess it means I can put "Arseing around on t'internet" under Public Outreach Activities in my next grant application :)

Coincidentally, Pad, I went to a very good talk yesterday about LISA, the planned gravitational wave detector in space. The speaker showed a plot of the instrumental sensitivity which made the same point as your analogy with the bubbles popping in the pond. Primordial gravitational waves down the bottom at low amplitude, and a huge swarm of "louder" green dots drowning them out - the green dots mainly being white dwarf binaries within the Milky Way with a period of a few hours. Direct detection of primordial GWs may be a long, long way off...

Garret Cotter

Gravitational wave signatures

They're looking for a particular pattern in the polarization of the CMB (at very low brightness and small angular scales). The idea is that the dominant "wobbles in the jelly" are spherically symmetric - think of a very big jelly in empty space starting to fall in on itself under its own weight and then springing back. However gravitational waves do something weird - if you pass one through the jelly, it'll shrink first along one axis, and then along another axis. Now, light passing by a lump of plasma that's wobbling becomes slightly polarized, and if it's wobbling gravitational-wave style, the orientation of the planes of polarization make a characteristic pattern that you don't get from regular wobbling.

Counter-intuitively, the hot spots in the CMB correspond to the less dense patches - light coming out of the dense patches gets gravitationally redshifted.

@Adrian: There are many very good generic scientific reasons for having several experiments make the same measurement. Can you not think of any? And I'm sure the teams involved in these experiments who have spent many years developing them and will spend many more analysing the data would be mightily disabused to hear you call them "rush-jobs".

Herschel and Planck safely away

Garret Cotter


It comes down to money in the end - they're both going to L2 and an Ariane V launch costs about €200M so it's a substantial saving, bringing the total cost of both missions down from €1900M to €1700. Is it worth the risk? Well, Ariane V had a shaky start, I remember many people in the community being worried when ESA made the decision to launch both together, which is now maybe ten (or more?) years ago. But there's only been one failure in the last 30-something Ariane V launches so I think the risk/benefit looks reasonable from today's point of view.

The general attitude with these gigabuck science missions is to take the risk. When you've had a 20-year gestation period, and the satellite design gets frozen many years before launch, the technology will always be dated by launch time. Even if you had the funds to build a copy of the satellite, it'd be even further behind by the time it got launched, so better to look ahead and get working on the next generation. The only mission I can think of that was replaced by a near-copy was Cluster.

@David Wilson: Well said sir! That sort of common sense could go a long way in some space agencies :)

Garret Cotter

Shielding at L2

The Earth's angular diameter at L2 is 30 arcminutes, the Sun's is 31.5. So if you were sitting right at L2, you would gain some pretty valuable shielding.

However, L2 is a saddle point and satellites need to orbit it. In the case of Herschel, this is at a mean radius of about 800 000 km. Hence the Sun pops out from behind the Earth. But L2 does have a major advantage for shielding because the main heat sources - Earth as well as Sun - are always in the same part of the sky relative to the telescope. So you can just put a single heat shield on one side of your spacecraft and hide behind it. Look at e.g. the design of JWST - naked primary miror hiding behind a big parasol, basically...

Too much sunshine makes you commit suicide

Garret Cotter

Please look before you leap

Folks, this is a well-known part of the seasonality of mood disorders. Many people are seriously investigating how it relates to the molecular biology of circadian rhythms, with mounting evidence. I refer you to chapter 16 of Goodwin & Jamison (2nd ed) and references therein. Please don't denigrate the valuable work being done in this field on the basis of one media report. Any monkey can spout out "correlation doesn't imply causation" - look behind the headlines before trying to wave your willy.

NASA's Kepler dazzles with first pics in new-Earth hunt

Garret Cotter

@John Smith 23 April 12:48

Sorry for being off line have been tied up in teaching!

Yes within in individual system, the planets ought to be reasonable

coplanar, if they've formed out of an accretion disc as we expect (and

conservation of momentum is a strong prior!). But that only means

within a few degrees, once they've had a bit of post-formation

battering - I think back-of-an-envelope for example, if Kepler found

Earth, it would have a about a 50/50 chance of getting one transit of

Mars; Mercury and Venus would be out through inclination, Jupiter and

further would be out through mission length.

But between stars - the ecliptics will be pretty much at random. So

Kepler would still need 200 solar systems to detect one Earth.

I hadn't head about the Daedalus revisit - will keep my eyes open for

that, it's still the horse I'd put my money on. Starwisp though -

unsteerable mirror that has to be also a Fresnel lens from Earth?

Free-flying optical arrays in ~AU orbit would have much greater senstivity

& mapping power - local angular resolution at a wavelength where the

targets are dim and with an unsteerable antenna isn't FTW! Seriously - blackbody in the Rayleigh-Jeans regime goes as nu^2. Push me offline and I'll calculate precisely how close you'd need to be to Earth with an off-axis 30-m cm-wave antenna to detect it. There's several orders of magnitude on the back of my bus ticket in favor of going for a local optical/IR search!

Garret Cotter

@ John Smith

Well, the chance of detecting the Earth by occultation against the Sun from a random line-of-sight is about 0.5%. (You'd be looking for a 0.001% dip in brightness that lasted a few hours once a year). So, given that there are only 11 stars with 10 light years of the Sun, it's not a huge surprise that we haven't detected any Earth-like planets yet - they may well be there, but with their orbits too far from edge-on. But my understanding is it won't be more that a few years before we start getting detections or good upper limits from direct imaging/interferometry.

Interestingly just yesterday the HARPS team (radial velocity) announced they had found a 2 Earth-mass planet - but in a 3-day orbit. They've been studying that particular system for a long time a have been beating down the signal/noise ratio, but the technique still fundamentally needs the planet to be throwing the star around at a fair speed.

As is often lamented we *could* build an Orion-type 0.05c ship now... more realistic I think would be a Deadalus-type 0.1c ship, in ~100 years maybe. As you say, knowing there was a terrestrial planet at the target star would be a huge motivation (that was indeed the basis for the Daedalus mission profile to Barnard's Star if I remember correctly) but I wonder if even on 100-year time scales we'd be rich and motivated enough to do it...

Garret Cotter

@Impressive but disappointing

Yes the local stars have all been tried - with existing technology. Radial Doppler can only detect tight-orbit Jupiters, and nulling interferometers (e.g. the Large Binocular Telescope) and dedicated planet-finding coronographs are in their infancy. So watch this space...

Kepler - and Corot - can find Terrestrials, but by occultation, so you need the planet's orbit to be just-so aligned with the line of sight, and exquisite photometry - hence the need for a wide-field camera in space. The optical arrays that you mention are made of ~1-m telescopes and don't have the sensitivity to detect planets - they're mainly used to study the atmospheres of red giant stars.

But on the motivation side... nobody's doing this to find a home for us when the Sun dies. The fraction of stars with planetary systems is a fascinating science question in its own merit. Cue arguments about the decline and fall of Rome, etc...

Garret Cotter

@Astronomer, meet layman

Your hand is a couple of feet away from you face, so you see it as it was a couple of nanoseconds ago. How do you know it still exists?

Sorry that's a bit trite - but we can only ever see our past light-cone. In astronomy, you see things as they were and you work in that time frame. They may be different "now", with a God's eye view of the universe that sees everything simultaneously, but you work with their evolution as it is seen under the well-supported assumption that the laws of physics are invariant in time and space.

FWIW, 13,000 years is a small timescale compared with stellar evolution, so they probably actually still do look the same today. Maybe if there was a very, very massive star in there it might have gone POP since and be sending its detritus towards us....

Garret Cotter

@the sun will supernova

The Sun won't supernova... it'll go red giant in about 5 billion years and will shortly thereafter shrink to a white dwarf. It won't go bang unless someone then sneaks up about another solar mass of material and dumps it in an outrageously explosive act of cosmic fly-tipping...

Boffins build super-accurate atomic clock

Garret Cotter


Fermions are particles which obey Fermi-Dirac statistics - that is all. Composite particles such as atoms (and indeed protons and neutrons) can be bosons or fermions. E.g. He3 is a fermion and He4 is a boson.

Astroboffins probe mysterious 'blazar'

Garret Cotter


Just means "not active at the moment", by analogy with volcanoes I guess. The black hole is as you say absolutely still there but not scoffing gas/dust/etc, so no luminous accretion disc or jets. But the presence of a black hole can be inferred e.g. from the velocities of stars very close to the centre of a galaxy - if the speeds are high enough and the orbits are small enough the best bet is a black hole. If you were able to funnel enough stuff down into the core of the galaxy (perhaps in a merger as mentioned above) then presumably it might light up.

Garret Cotter


Not a silly question at all. Most, maybe even all, massive quiescent galaxies nowadays have a dormant supermassive black hole. The Milky Way has one, though it's only about three million solar masses which means we were probably only a fairly wimpy quasar in the past.


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