Nature News Article 'Pulsar Watchers Race for Gravity Waves
14 Jan 2010 13:08:40 UTC
Topic 194726
(moderation:
Its old news but it quotes Dr. Allen..
Pulsar Watchers Race for Gravity Waves
Edit: This article is free.. But I don't know whether you need an account..
There are some who can live without wild things and some who cannot. - Aldo Leopold
Nature News Article 'Pulsar Watchers Race for Gravity Waves
)
I have an account on Nature magazine, without paying anything. Some articles I can read and some not, in a rather random way. But I could read this one without even logging in.
Tullio
A comment on this article was clearly only advertisement for bags, shoes, etc. I complained and am waiting for an answer from Nature magazine.
RE: RE: Its old news but
)
You can get access to all articles in online nature news for 10.00 a month. I don't because I find that expensive. I do have a Google widget on my home page for Nature News. Like you I can access some articles and just a taste of others
There are some who can live without wild things and some who cannot. - Aldo Leopold
RE: Its old news but it
)
The latest (January 2010) Physics Today also has an article in the back about using pulsar timing to detect gravitational waves. Unfortunately the web access requires a subscription AFAIK so I didn't bother making a link.
"Better is the enemy of the good." - Voltaire (should be memorized by every requirements lead)
RE: ... A comment on this
)
I reported it too, but the link back to the article leads right back to the 'Report a comment' page – maybe they're not receiving the submittal?
Good article – it gives the best overview compared to other articles I've read on the subject recently.
Had to look for the source – I think this is it, an international collaboration: Parkes Pulsar Timing Array project
I'm guessing it's an equally challenging way to detect GWs, e.g., similar pattern-matching algorithms, contending with noise from electromagnetic interference, etc.
Interesting aside that pulsar timing has been used to validate the predictions of Einstein's general theory of relativity in the presence of very strong gravitational fields to within 0.05% - see 'General relativity survives grueling pulsar test'. Pretty good upgrade of the Hulse-Taylor lab :)
I registered with my account
)
I registered with my account name and left a comment on a comment but also mentioned what we are trying to do.
Tullio
Speaking of labs and pulsars,
)
Speaking of labs and pulsars, and of the challenges unique to the pulsar technique for detecting GWs, it's possible to observe the arrival of a pulsar's pulse that appears to have traveled faster than light. The cause is anomalous dispersion, an effect studied previously only in earthbound labs. It's now been observed (at Arecibo) for the first time occurring in the interstellar medium, between here and the radio pulsar PSR 1937 +21 – see this arxiv preprint. So the pulsar trick for detecting GWs is a little trickier than just measuring the arrival time of each pulsar's pulse. :)
It's stated that there's no violation of special relativity when it comes to a pulse's group velocity being greater than the speed of light, yet in this case, doesn't information about neutral hydrogen arrive ahead of schedule?
RE: It's stated that
)
From what I remember of my studies of many years ago it is the phase velocity that can be greater than c without violating relativity. Group velocity is connected to a transfer of energy-momentum so it could not be greater than c without violating relativity. But maybe physics has progressed...
Tullio
RE: RE: It's stated that
)
That's right. We only ever measure the group velocity ( speed of travel of a superposition of states ), the phase velocity is the presumed speed of non-measured components. There are many media for which the phase velocity depends upon frequency ( that's what dispersion means ) so if we have a pulse/group narrowing then we assume some components have 'caught up' - and to do so you must assign a phase velocity exceeding c. But relativity is still safe .... as those are never separately measured.
I reckon the key to understanding this is how a pulse profile is formed from the sum of unequal components of differing frequencies and the cyclic nature of phases. I think the authors seem to be dividing the situation into two parts - (a) the propagation of the pulsar photons as they arrive, enter and leave the cloud and (b) the propagation of disturbance of the electrons due to the passage of those incident photons. At the far side of the cloud ( ie. away from the source, where the photons are exiting ) the 'electric field' will be contributed to by both. But 'electric field strength' is another mechanism for deducing the probability of photon detection - so the authors' treatment is tantamount to distinguishing between actual and virtual photons. But as both types summate to give a detectable response, then it's a moot distinction.
Anyhow it's a pre-print. So it may take a clean up of the wording to catch the right descriptive paradigm, as currently there's a free mix of particle/wave language.
And it's worth mentioning that just because something is overall neutrally electrically charged doesn't mean it will not have an electromagnetic effect - take the neutron with it's magnetic moment for instance. You can still get an induced dipole in neutral hydrogen ....
Cheers, Mike.
( edit ) However you interpret the causality aspects, they are using the timing delays to map temperatures, densities and motions of the clouds ( along the line of sight from pulsar to receiver ).
I have made this letter longer than usual because I lack the time to make it shorter ...
... and my other CPU is a Ryzen 5950X :-) Blaise Pascal
Thanks, gents. I have a
)
Thanks, gents. I have a clearer understanding of it than before, but still feel the need for some review. If the index of refraction (n) of a material is defined as the ratio of the velocity of light in vacuum (c) to the velocity of light in the material (v), then you have n = c / v. Since the speed of light is always less in some material than it is in vacuum, v will always be a little less than c, which means that n will always be a little greater than 1 (n = 1 only when v = c).
This would be valid for some specific frequency and, after a little algebra, you could then say that the phase velocity is given by the speed of light in vacuum divided by the refractive index, v_phase = c / n.
But if you have a pulse consisting of many different frequency components, traveling as a 'group' in an envelope that gives the pulse its overall amplitude, and if things are further complicated with a material where the refractive index changes as the frequency changes (the refractive index being a function of frequency), then the velocity of the group (or envelope, or pulse of different frequency components) will not be as simple as v = c / n. Instead of just the 'n' in the denominator of the equation there will also have to be terms that take into account the rate of change of the index of refraction per rate of change in wavelength at each specific wavelength. That being the difference between group velocity and the simpler case for the phase velocity of a single frequency (with regard to just 'n' in the denominator), the equation for group velocity is v_group = c / (n - λ[dn/dλ]).
The denominator is where the action is because as long as it's greater than 1 then the velocity term will be less than the speed of light. If the quantity in the denominator should ever equal 1 then the velocity will be exactly the speed of light, and jumpin' jimeny, if the denominator should ever be less than 1 then the velocity would be greater than the speed of light!
Now for the effect of dispersion and what happens in the denominator. Recall that for all naturally occurring optical media that the speed of light will be slower than in vacuum, and so 'n' will always be a little greater than 1. So to start with in the denominator you have something a little bigger than 1 and you're subtracting something from it – the wavelength times the rate of change of index of refraction over the rate of change of the wavelength. For materials the cause normal dispersion, the index of refraction increases as the wavelength decreases and so the term dn/dλ is less than zero (it's negative). Subtracting a negative number is the same as adding the positive value of the number, in this case adding it to a value already a little bigger than 1, so the group velocity will actually be less than the phase velocity.
When it comes to anomalous dispersion, the index of refraction increases as the wavelength increases – the dn/dλ term is positive! A change of signs, so now you're actually subtracting something from a value that's only a little bit greater than 1 to start with. As it happens there are materials that result in a denominator less than 1 and hence a group velocity greater than the speed of light. For the pulsars it's neutral hydrogen (having a resonance at 1420.4 MHz) causing the anomalous dispersion, and free electrons causing normal dispersion. Either way an ingenious probe of the interstellar medium :)
I don't think it's so much that one phase catches up to the others, but rather the others slow down more so than the ones that appear to catch up? At any rate, when the group velocity is greater than the phase velocity then the group velocity is no longer the speed at which information or energy is actually propagating in each phase. This page, Phase, Group, and Signal Velocity, has a good illustration showing how group velocity can actually be made arbitrarily great.
I was wondering how many
)
I was wondering how many known pulsars are there, and how many of those are extragalactic – currently about 750 known pulsars, all but a few are in our galaxy – when the Square Kilometer Array comes online they expect to discover, in just a few hours time, about 1000 millisecond pulsars in a single globular cluster that's orbiting our galaxy (47 Tucanae, second brightest globular cluster), and there are over 200 of these types of galaxies in orbit around the Milky Way, wow! There's a good overview of pulsars and benefits of the SKA on this page from the ASA.
For an idea of the difficulties involved when it comes to determination of distances to pulsars, see this article from SAO/NASA ADS. Dispersion is certainly taken into account ... but what if there's some dispersive material that red-shifts hydrogen α-lines (and other transitions), instead of recessional velocity? Like a more or less uniform distribution of “transparent†dark matter? Seems easier to grasp something like that compared to spacetime itself expanding – what are some reasons it has to be recessional velocity causing the redshift, instead of dispersion, or some combination of the two?