I do billion-point 1-D FFTs in order to search for very weak pulsars in radio data. And since I need to know the exact phase of each Fourier amplitude (in order to sum the harmonics together for optimal sensitivity to "sharp" pulsar profiles) I need at least double-precision-level (around 15 digits) of pi in order to keep floating-point errors in the trig computations from killing me.
Sorry, but that is wrong. You are correct in that there is no "absolute position or absolute motion" as you put it, but who or what gets accelerated certainly does make a difference. Acceleration is how the twin paradox, for instance, is resolved (see here: http://www.weburbia.demon.co.uk/physics/twin_gr.ht ml The rest of the relativity FAQ is very good as well).
by Richard Rhodes is one of (if not the ) best book that I have ever read. It has something for everyone, but it is especially suitable for geeks as it details the development of quantum mechanics and atomic physics, before diving into one of the most amazing engineering projects ever undertaken by mankind.
In order to do this experiment they had to build what are, more likely than not, the two most perfectly round objects in the entire universe...
Actually, they are only the most spherical things in our little region of the galaxy. Neutron stars (of which the closest known is a couple hundred lt-yrs away) are even more spherical.
Toronto, Waterloo and McGill are all great choices for physics. Add to that list (at least) UBC and Queens and you have the best in Canada. I don't think that you'd go wrong with any of them. Given that, pick the place that you think you'd be happiest at (i.e. location, climate, culture, etc) and go for it.
Scott
PS: I actually just left McGill (I was there a a post-doc for 3 yrs) in August and am now a staff astronomer at NRAO in Virginia. I guess I'd better update my/. info...
"Seeing" is the wobbling back and forth of portions of an image caused by the turbulence of the atmosphere. The many "seeing cells" above a telescope act as lots of little lenses and distort an image taken from the ground. In general, the best sites in the world can sometimes allow "seeing"-limited observing down to around 0.2-0.4" (that is the best resolution possible -- which is much less than would be possible with a large telescope in space). However, adaptive optics (or interferometry) can sometimes beat this atmospheric limitation.
That is one of the most jargon and/or marketing-speak filled story descriptions that I have ever read on/. I have absolutely no desire to waste my time looking up those acronyms in order to see if I _might_ want to RTFA.
1. The Nyquist Theorem states the maximum possible encoded frequency in a digitized waveform. It says nothing about how the waveform may or may not suffer aliasing as the frequency approaches half the sample rate. I.e. a rate of 44.1khz is necessary (but may not be sufficient) to encode a 22.05khz tone. I'm not sure this was clear in your reply.
Sorry, this is simply not true. The Nyquist theorem states that you can completely reproduce a band-limited signal if you evenly sample at twice the bandwidth of the signal. If the signal is not band-limited, the content at frequencies above the half-Nyquist rate will be aliased back into the lower frequency spectrum.
A tone, by definition, is a sinusoid and has no higher harmonic content. Therefore, by your example, a 22.05kHz tone (i.e. a sinusoid) can be reproduced with no aliasing by a 44.1kHz sampling rate. However, when your signal is very near the half-Nyquist rate, the phase of the signal becomes important. In practical terms, that is why you usually slightly over-sample your band-limited signal if phase information (i.e. exact reproduction of the signal) is important. For CD audio, the goal was to accurately reproduce 20kHz signals, therefore, the slight oversampling to 44.1kHz (Note: low-pass filter responses also contributed to the need to slightly oversample).
Bottom line, the aliasing in your example comes about because you are not talking about a band-limited signal since you have a non-sinusoidal waveform with its fundamental at 22.05kHz but higher harmonic content at integer multiples of 22.05kHz.
What's really too bad is that most of your arguments are completely incorrect. First off, Hubble is a technological marvel -- it's current "best" detector, the ACS, is one of the most sophisticated instruments in the world. It is state-of-the-art. And the primary mirror is still outstanding (perfectly ground to the wrong, but _known_ shape).
Second, the new JWST will only work in the near infra-red. That is fantastic for cosmology, star formation and certain other sciences, but will not help with the optical and near-UV science that HST can provide.
And finally, while adaptive optics at most new ground based telescopes are doing great things, there are still _severe_ limitations to their use: only small fields of view are available and bright stars need to be nearby in the sky (this greatly limits the fraction of the sky that can be viewed by these systems). Note: yes, sodium laser-based AO systems can fix some of these problems, but the lasers are currently highly problematic and the systems have very low observing efficiency (i.e. useful scientific data per unit of telescope time).
So bottom line is that HST will be sorely missed by astronomers/astrophysicists. And yes, as a professional astronomer, I will be one of those missing it (even though most of my work is in the radio).
I'm heavily involved in a 5-6 year project to use the Arecibo telescope to search for new pulsars. The project uses a new 7-beam receiver system, each of which takes data from up to 1024 nearby frequency channels. The data is 16-bit sampled over 15000 times per second from each frequency channel. We need the time and frequency resolution to find exotic millisecond pulsars.
Over the couse of the survey we expect to take about 1 PB of data. We're still trying to figure out exactly how we will process and store it all.
High precision timing of millisecond pulsars (which accounts for every single rotation of a pulsar over the course of several years) can make observations with astrometric (i.e. positional) errors of several micro arcseconds.
An excellent example was published in Nature in 2001. Here is a preprint. The work describes the timing of the nearby (~450 lt-yrs) millisecond pulsar J0437-4715. The proper motion (movement across the sky) and parallax (apparent motion on the sky due to the earth's orbit) of the pulsar were measured to extreme precision, and a new test of General Relativity was also given.
Actually, that is not satisfactory... Your definition was valid in the 60's and 70's but not today.
Now we know that they are distant galaxies that have active nuclei. The nuclei are powered by supermassive (10^6-10^8 solar masses) black holes. What we are seeing is the point-like emission from near these black holes (i.e. the jets and/or an accretion disk). The radiation is often visible in radio, optical, and X-ray bands.
Actually, this is not the case. Hewish received the Nobel prize _precisely_ because he and Jocelyn Bell discovered pulsars -- one of the greatest discovies in 20th century astronomy.
And while Jocelyn has always been very gracious (and modest) when discissing this topic, the majority of astronomers believe that an injustice was done by the Nobel committee. It was precisely because of this injustice, in fact, that when The Nobel was awarded to Joe Taylor for work on the Binary Pulsar (for showing that the orbital period of the system was decaying as predicted due to gravitational radiation), that they also awarded it to Russell Hulse. Russell had effectively left astronomy by then and had done very little of the follow-up work which showed the workings of GR. Yet it was he that had discovered the system and determined many of its properties.
Note: I am a pulsar astronomer and have first-hand knowledge of this.
Not really. With the rise of adaptive optics, ground-based telescopes are increasingly able to achieve diffration-limited or near-diffraction-limited resolution in the optical and (in particular) the near-IR (which is of crucial importance for cosmology -- the current "Hot" area of astronomy).
Once you hit that physics-limited level of resolution (which has been the true advantage of HST), the gains come from light-gathering ability. This is where ground-based telescopes clean up. The $$/area is much lower (i.e. better) for ground-based telescopes. And the upkeep costs are much smaller as well. Space is expensive.
When you can have a telescope with near-diffraction limited resolution and 10-1000 times the light gathering ability of a space-based telescope of the same cost, astronomer's will choose that guy any day.
Actually, GPS satellites are in high-Earth orbits, but are lower (closer) than satellites in geosync orbits. Their orbital period is about 12 hours and is therefore not even close to the rotational period of the Earth.
The Earth's rotation is referenced to quasars at cosmological distances from us. Since they are so far away, they are for all intents and purposes located at fixed positions on the sky (unlike many nearby stars which show parallax and proper motion over the course of a year or more). The postions are measured using radio VLBI (Very Long Baseline Interferometry) that can provide astrometric positions on the sky to better than a milli-arcsecond.
For more info, browse here: http://hpiers.obspm.fr
I posted this a few years ago for a related story, but it seems worth repeating now:
So, I met him in an pseudo-interview with about 6 other students. I asked him if it ever bothered him to be the "Father of the H-Bomb" since his "baby" could be used for such evil and/or immoral purposes.
I thought he was going to jump out of his chair at me.
He got very upset and angrily announced that a scientist's only responsibility is to science. The possible uses of a discovery should not even be considered by the researchers -- that is someone elses business. And because of this, he did not feel even the slightest bit of remorse for his work on the bomb.
And then he upbraided _me_ (since I was on my way to grad school to become a scientist at the time) for thinking that a scientist _should_ worry about the moral implications of his/her work.
Needless to say, I didn't ask any more questions.;)
Re:Doppler Drift Rate "chirping" seems way redunda
on
SETI@Home Publishes Skymap
·
· Score: 2, Informative
Sorry, but cepstral techniques don't do what the SETI people need them to do. The de-chirping needs to happen coherently (i.e. without any loss of the phase information from the original data and signals that it might contain). The reason for this is that the signal-to-noise of a detected periodic signal is much less if you use an incoherent technique like the cepstrum rather than a coherent one. And since they are looking for very weak signals, they need every bit of S/N that they can get.
OTOH, I have developed a cepstral-like technique to detect binary pulsars in data almost identical to the SETI@home data. You can read about it here or here if you are interested.
The planet almost certainly formed before the pulsar went supernova.
I'm not quite sure why you would say this. First of all, most of the original neutron stars in globular clusters all would have formed within the first 100 million years or so of the clusters history. But, since the cores of clusters are so dense, NSs could have been created later by a more exotic process, such as accretion induced collapse of a white dwarf or during some other common envelope phase of a close stellar interaction.
Stellar collisions are (relatively) common in clusters and a common envelope phase is a typical result. Note that the common envelope phase could likely create a metal-rich planetary disk where a planet could form as well. If this is the case then the planet would be significantly younger than the pulsar.
Note: I am a post-doc working on millisecond pulsars in globular clusters.
No, not really. People who do serious number
crunching would love to be able to do it on a
couple cheap(ish) servers located in the room
next door or under their desks rather than
fighting with massive amounts of data transfer
over the 'net to some supercomputer center,
long job queues, inflexible work environments, etc.
Working on Alphas was great. Give me an Athlon64 today, please.
Uhhh...Science and Nature are the two most
prestigous and influential scientific journals
in the world.
And as for what to believe, while I agree with
your numbers, the point of this whole thing is
that this new measurement is yet another new and
independent way of measuring the age of the universe.
Since the error bars of virtually all the methods
are overlapping, that gives us confidence that the
numbers we are getting from all the various methods are correct.
That is why this is important.
I dual boot Linux (Debian) and OS X on an iBook...
on
Is Mac OS X Slow?
·
· Score: 2, Informative
and I stay in Linux 95% of the time because KDE2 is much faster in general than OS X (10.1.5).
Since my iBook2 (600MHz) can't handle the new Quartz rendering in Jaguar, I'm left with a functional - but still slowish - interface under OS X.
In general, though, I get the best of both worlds by running Mac-on-linux, which runs OS X beautifully (all except sound....) with a simple Ctrl-Alt-F8...
Slashdot Mirror
More or less. See the paper "The Effects of Moore's Law and Slacking on Large Computations".
It's quite entertaining....
http://xxx.lanl.gov/abs/astro-ph/?9912202
I do billion-point 1-D FFTs in order to search for very weak pulsars in radio data. And since I need to know the exact phase of each Fourier amplitude (in order to sum the harmonics together for optimal sensitivity to "sharp" pulsar profiles) I need at least double-precision-level (around 15 digits) of pi in order to keep floating-point errors in the trig computations from killing me.
Sorry, but that is wrong. You are correct in that there is no "absolute position or absolute motion" as you put it, but who or what gets accelerated certainly does make a difference. Acceleration is how the twin paradox, for instance, is resolved (see here: http://www.weburbia.demon.co.uk/physics/twin_gr.ht ml The rest of the relativity FAQ is very good as well).
IAAA.
The Galaxy Evolution Explorer (GALEX) http://www.galex.caltech.edu/ is cranking out great science (and spectacular pictures of galaxies...)
Yes, IAAA.
by Richard Rhodes is one of (if not the ) best book that I have ever read. It has something for everyone, but it is especially suitable for geeks as it details the development of quantum mechanics and atomic physics, before diving into one of the most amazing engineering projects ever undertaken by mankind.
And it is written incredibly well also.
Highly recommended.
Hey, don't blame me!
I didn't write the press release, just the Nature paper that they were trying to explain to Joe Blow.
Note: I moved from McGill to NRAO in August.
Actually, they are only the most spherical things in our little region of the galaxy. Neutron stars (of which the closest known is a couple hundred lt-yrs away) are even more spherical.
And yes, IAAA (I am an astronomer).
Toronto, Waterloo and McGill are all great choices for physics. Add to that list (at least) UBC and Queens and you have the best in Canada. I don't think that you'd go wrong with any of them. Given that, pick the place that you think you'd be happiest at (i.e. location, climate, culture, etc) and go for it.
/. info...
Scott
PS: I actually just left McGill (I was there a a post-doc for 3 yrs) in August and am now a staff astronomer at NRAO in Virginia. I guess I'd better update my
"Seeing" is the wobbling back and forth of portions of an image caused by the turbulence of the atmosphere. The many "seeing cells" above a telescope act as lots of little lenses and distort an image taken from the ground. In general, the best sites in the world can sometimes allow "seeing"-limited observing down to around 0.2-0.4" (that is the best resolution possible -- which is much less than would be possible with a large telescope in space). However, adaptive optics (or interferometry) can sometimes beat this atmospheric limitation.
And yes, IAAA (I am an astronomer).
That is one of the most jargon and/or marketing-speak filled story descriptions that I have ever read on /. I have absolutely no desire to waste my time looking up those acronyms in order to see if I _might_ want to RTFA.
Thanks for the great submission.
1. The Nyquist Theorem states the maximum possible encoded frequency in a digitized waveform. It says nothing about how the waveform may or may not suffer aliasing as the frequency approaches half the sample rate. I.e. a rate of 44.1khz is necessary (but may not be sufficient) to encode a 22.05khz tone. I'm not sure this was clear in your reply.
Sorry, this is simply not true. The Nyquist theorem states that you can completely reproduce a band-limited signal if you evenly sample at twice the bandwidth of the signal. If the signal is not band-limited, the content at frequencies above the half-Nyquist rate will be aliased back into the lower frequency spectrum.
A tone, by definition, is a sinusoid and has no higher harmonic content. Therefore, by your example, a 22.05kHz tone (i.e. a sinusoid) can be reproduced with no aliasing by a 44.1kHz sampling rate. However, when your signal is very near the half-Nyquist rate, the phase of the signal becomes important. In practical terms, that is why you usually slightly over-sample your band-limited signal if phase information (i.e. exact reproduction of the signal) is important. For CD audio, the goal was to accurately reproduce 20kHz signals, therefore, the slight oversampling to 44.1kHz (Note: low-pass filter responses also contributed to the need to slightly oversample).
Bottom line, the aliasing in your example comes about because you are not talking about a band-limited signal since you have a non-sinusoidal waveform with its fundamental at 22.05kHz but higher harmonic content at integer multiples of 22.05kHz.
What's really too bad is that most of your arguments are completely incorrect. First off, Hubble is a technological marvel -- it's current "best" detector, the ACS, is one of the most sophisticated instruments in the world. It is state-of-the-art. And the primary mirror is still outstanding (perfectly ground to the wrong, but _known_ shape).
Second, the new JWST will only work in the near infra-red. That is fantastic for cosmology, star formation and certain other sciences, but will not help with the optical and near-UV science that HST can provide.
And finally, while adaptive optics at most new ground based telescopes are doing great things, there are still _severe_ limitations to their use: only small fields of view are available and bright stars need to be nearby in the sky (this greatly limits the fraction of the sky that can be viewed by these systems). Note: yes, sodium laser-based AO systems can fix some of these problems, but the lasers are currently highly problematic and the systems have very low observing efficiency (i.e. useful scientific data per unit of telescope time).
So bottom line is that HST will be sorely missed by astronomers/astrophysicists. And yes, as a professional astronomer, I will be one of those missing it (even though most of my work is in the radio).
I'm heavily involved in a 5-6 year project to use the Arecibo telescope to search for new pulsars. The project uses a new 7-beam receiver system, each of which takes data from up to 1024 nearby frequency channels. The data is 16-bit sampled over 15000 times per second from each frequency channel. We need the time and frequency resolution to find exotic millisecond pulsars.
Over the couse of the survey we expect to take about 1 PB of data. We're still trying to figure out exactly how we will process and store it all.
For more info, you can poke around here.
High precision timing of millisecond pulsars (which accounts for every single rotation of a pulsar over the course of several years) can make observations with astrometric (i.e. positional) errors of several micro arcseconds.
An excellent example was published in Nature in 2001. Here is a preprint. The work describes the timing of the nearby (~450 lt-yrs) millisecond pulsar J0437-4715. The proper motion (movement across the sky) and parallax (apparent motion on the sky due to the earth's orbit) of the pulsar were measured to extreme precision, and a new test of General Relativity was also given.
PS: IAAPA (I am a pulsar astronomer)
Actually, that is not satisfactory... Your definition was valid in the 60's and 70's but not today.
Now we know that they are distant galaxies that have active nuclei. The nuclei are powered by supermassive (10^6-10^8 solar masses) black holes. What we are seeing is the point-like emission from near these black holes (i.e. the jets and/or an accretion disk). The radiation is often visible in radio, optical, and X-ray bands.
PS: IAAA (I am an astronomer)
Actually, this is not the case. Hewish received the Nobel prize _precisely_ because he and Jocelyn Bell discovered pulsars -- one of the greatest discovies in 20th century astronomy.
And while Jocelyn has always been very gracious (and modest) when discissing this topic, the majority of astronomers believe that an injustice was done by the Nobel committee. It was precisely because of this injustice, in fact, that when The Nobel was awarded to Joe Taylor for work on the Binary Pulsar (for showing that the orbital period of the system was decaying as predicted due to gravitational radiation), that they also awarded it to Russell Hulse. Russell had effectively left astronomy by then and had done very little of the follow-up work which showed the workings of GR. Yet it was he that had discovered the system and determined many of its properties.
Note: I am a pulsar astronomer and have first-hand knowledge of this.
Not really. With the rise of adaptive optics, ground-based telescopes are increasingly able to achieve diffration-limited or near-diffraction-limited resolution in the optical and (in particular) the near-IR (which is of crucial importance for cosmology -- the current "Hot" area of astronomy).
Once you hit that physics-limited level of resolution (which has been the true advantage of HST), the gains come from light-gathering ability. This is where ground-based telescopes clean up. The $$/area is much lower (i.e. better) for ground-based telescopes. And the upkeep costs are much smaller as well. Space is expensive.
When you can have a telescope with near-diffraction limited resolution and 10-1000 times the light gathering ability of a space-based telescope of the same cost, astronomer's will choose that guy any day.
Note: IAAA (I am an astronomer)
Actually, GPS satellites are in high-Earth orbits, but are lower (closer) than satellites in geosync orbits. Their orbital period is about 12 hours and is therefore not even close to the rotational period of the Earth.
The Earth's rotation is referenced to quasars at cosmological distances from us. Since they are so far away, they are for all intents and purposes located at fixed positions on the sky (unlike many nearby stars which show parallax and proper motion over the course of a year or more). The postions are measured using radio VLBI (Very Long Baseline Interferometry) that can provide astrometric positions on the sky to better than a milli-arcsecond.
For more info, browse here: http://hpiers.obspm.fr
Note: IAAA (I am an astronomer)
I posted this a few years ago for a related story, but it seems worth repeating now:
;)
So, I met him in an pseudo-interview with about 6 other students. I asked him if it ever bothered him to be the "Father of the H-Bomb" since his "baby" could be used for such evil and/or immoral purposes.
I thought he was going to jump out of his chair at me.
He got very upset and angrily announced that a scientist's only responsibility is to science. The possible uses of a discovery should not even be considered by the researchers -- that is someone elses business. And because of this, he did not feel even the slightest bit of remorse for his work on the bomb.
And then he upbraided _me_ (since I was on my way to grad school to become a scientist at the time) for thinking that a scientist _should_ worry about the moral implications of his/her work.
Needless to say, I didn't ask any more questions.
Sorry, but cepstral techniques don't do what the SETI people need them to do. The de-chirping needs to happen coherently (i.e. without any loss of the phase information from the original data and signals that it might contain). The reason for this is that the signal-to-noise of a detected periodic signal is much less if you use an incoherent technique like the cepstrum rather than a coherent one. And since they are looking for very weak signals, they need every bit of S/N that they can get.
OTOH, I have developed a cepstral-like technique to detect binary pulsars in data almost identical to the SETI@home data. You can read about it here or here if you are interested.
I'm not quite sure why you would say this. First of all, most of the original neutron stars in globular clusters all would have formed within the first 100 million years or so of the clusters history. But, since the cores of clusters are so dense, NSs could have been created later by a more exotic process, such as accretion induced collapse of a white dwarf or during some other common envelope phase of a close stellar interaction.
Stellar collisions are (relatively) common in clusters and a common envelope phase is a typical result. Note that the common envelope phase could likely create a metal-rich planetary disk where a planet could form as well. If this is the case then the planet would be significantly younger than the pulsar.
Note: I am a post-doc working on millisecond pulsars in globular clusters.
No, not really. People who do serious number crunching would love to be able to do it on a couple cheap(ish) servers located in the room next door or under their desks rather than fighting with massive amounts of data transfer over the 'net to some supercomputer center, long job queues, inflexible work environments, etc.
Working on Alphas was great. Give me an Athlon64 today, please.
ScottAnd as for what to believe, while I agree with your numbers, the point of this whole thing is that this new measurement is yet another new and independent way of measuring the age of the universe. Since the error bars of virtually all the methods are overlapping, that gives us confidence that the numbers we are getting from all the various methods are correct.
That is why this is important.
Since my iBook2 (600MHz) can't handle the new Quartz rendering in Jaguar, I'm left with a functional - but still slowish - interface under OS X.
In general, though, I get the best of both worlds by running Mac-on-linux, which runs OS X beautifully (all except sound....) with a simple Ctrl-Alt-F8...
Scott