As clearly stated in the article, this was not a supernova (SN), but rather a gamma ray burst (GRB). The network of satellites referred to all trigger on high energy gamma rays, and look for the afterglow of the event which caused the trigger. The time scale of GRBs is typically on the order of seconds. Core collapse SNe, by comparison, are optically visible and have a brightening and fading timescale on the order of days or tens of days. Much more is understood of core collapse SNe than the progenitors of GRBs. One of the leading models for short GRB progenitors is the binary inspiral of two massive, compact objects, at least one of them being a neutron star. Obviously we can't resolve the region that the GRB came from, but from the above model, it's inferred that there is a region dense in stars out there, rather than just one isolated star. The second piece of evidence is that the afterglow was actually visible: this afterglow is most likely from shock waves in the interstellar gas, having associated high densities and temperatures, glowing in the optical or xray. If there was no gas by the progenitor of the GRB, there would not have been an afterglow (or the model is wrong). The slashdot title was therefore wrong in two ways: this was not a SN event, and it was not in empty space - it was just not in a host galaxy.
It distresses me a little to see a post modded so highly just because it throws together the right words; but I suppose that says something about me as well, given my choice of forum.
Anyway, since I nominally study gravitation, I feel like I should clarify some things in a reply.
Firstly, I'm going to guess that frame dragging is verified at no better resolution than the curvature of space/time, but that as far as they can tell, it exists and meets the values expected by Einstein.
Frame dragging is the name of one particular way in which spacetime curves. It is curvature. To say something about frame dragging or curvature is to say something about the other. I don't know if the parent statement makes sense or not.
The group has not released their frame dragging measurements yet, just the geodetic precession measurements (the precision of which will likely go up as they isolate more systematics in their data as they move toward making a statement about frame dragging).
Frame dragging is about 100 times harder to measure than geodetic precession, for the mass and spin of the Earth.
Secondly, I'm also going to guess that QM experts will start to get a little nervous. The properties any future QM model of gravity must have contradict the GR model. They cannot both be right. The more "right" the GR model, the more problematic a QM model. This doesn't mean a QM model does not exist, only that it is most undesirable (from a QM perspective) for the GR model to make highly precise and accurate predictions.
GR is arguably the most successful physical theory to date (I would say that electrodynamics rivals it since it has been formulated classically in curved spacetime and also has been quantized successfully in flat spacetime, but that is another discussion).
Newton was not "right", but note that GR simplifies to Newtonian mechanics in the weak field and non-relativistic limit. Any theory which supersedes a highly successful physical theory must reproduce said theory in the proper limits. A quantum theory of gravity must reproduce GR in the macroscopic limit, just as quantum mechanics has a correspondence principle which allows it to reproduce classical wave and particle phenomena in the appropriate limit.
I don't think any physicist is nervous about these results - everybody expects GPB to verify the predicted frame dragging. Deviations from the values predicted would excite fans of MoND, SVT theories, and other alternative theories of gravity.
Thirdly, frame-dragging occurs at a non-zero distance from an object.
Frame dragging curves spacetime globally, but falls off to asymptotic flatness. The parent statement probably makes sense.
This doesn't matter, for the purpose of these observations, as they're nowhere near accurate to measure the relativistic effects that apply to the information passed that creates the effects in the first place. Nonetheless, such an affect must exist, or you'd end up with infinitely fast rates of change of state, which is expressly forbidden in GR.
The NSF and NASA don't spend this much money to throw an instrument into space unless they think it will actually measure what it's supposed to. The gyros are the most spherical macroscopic manmade objects, which used superconducting quantum interference devices (SQUIDs) to precisely measure their precession, blah blah blah, read about it on their web site. I sure hope they're accurate enough to measure those relativistic effects, because that's exactly what they've been designed to do.
I don't know what information you are talking about. The Einstein Field Equations are local, so there is an inherent limit on the speed at which 'information' (curvature) propogates through spacetime.
It's a gross simplification and it's not an "obvious" conclusion to reach by any means, but if the curvature (and restoration) of space/time has nothing analogous to Hooke's Constant, then after a gravitationally massive object has move
What is meant is the following: One of the exact solutions to the Einstein field equations is a decent assumption for the Earth's (or anything approximately spherical which is not moving relativistically) gravitational field. The curvature of space-time is greater the closer to the center of the massive body. A light ray travelling some distance from the massive body will be deflected from a "straight line" (which is hard to define in curved space). If you are taking the view that you start rotating the rest of the universe around us, then it is equivalent to having your coordinate system spin around the massive body (well, there is nothing besides the massive body in the universe I am imagining). Physically, light will follow the same path as it did before, since all you have done is redefine the coordinate system, which does not change physics! Now instead, consider spinning the Earth, instead of the coordinate system. The matter making up the earth now has more energy-momentum (the magnitude of which is a physical quantity which can be measured independent of reference frame, if your frame is freely-falling). Energy-momentum is what causes space-time to curve, so a light ray travelling the same distance from the earth will be deflected by a larger amount, since space will be more curved.
Actually, the frequency of photons will change due to the changing gravitational potential to satisfy conservation - this is the gravitational wave doing some work on the light. But this change is also the same change which is being measured by the device.
In essence, you can think of it as doing that. Think about how you detect frequency shifts: when you tune a guitar, you listen for a frequency difference because one string is accruing phase faster than the other one. When they combine, you hear beats. One you look at the output of a Michelson interferometer, there are differences in intensity because one arm may have accrued more phase than the other one. You can think about it in the time domain, measuring length of the arms, or in the frequency domain, because the arms (which are resonant cavities) have had their resonant frequencies shifted.
This is a reply to this post and some of its' ancestors. Gravitational waves are predicted to weakly interact with everything which is matter-energy. For that matter, gravity interacts with itself (which is why GR predicts black holes and other such singularities). However, in the weak-field regime (that is, space-time is flat except for a deviation which is orders of magnitude less, meaning we can take the leading term in the expansion, so the theory is linear), gravitational waves just pass through everything. Since they pass through things, their energy falls off like the square of distance from the source. In the strong-field interacting picture, they certainly should exhibit non-linear exotic behaviour, but those are precisely the parts of GR we are trying to probe with LIGO. The exchange between matter-energy and curvature (gravitational waves) that you are thinking about is from the latter to the former, but just think about the former being turned into the latter - that is the prediction of the source of gravitational waves. However, it works both ways. On the levels at which LIGO hopes to detect gravitational waves, we will see about 10^53 gravitons. I am quoting this figure without understanding where it comes from, since we certainly don't have a quantum theory of gravity. But gravity is predicted to be quantum in nature as well, but we won't see the quantization from where we stand.
One of the ancestors addressed the issue of measuring while your meter stick is being squashed and expanded, and another about the local speed of light. These issues are related. One of the postulates (argue argue whatever) of GR is that the speed of light is constant in every frame, and it has the same constant value compared between all frames. Light is the perfect meter stick (or clock) for making measurements with. I had the same thought about measuring the arm lengths as you did for a while until I started taking GR. Here's how the thought goes: "If space is being stretched, and a meter stick is sitting in front of my face, I will always see the meter stick as being one meter long." Here's what GR predicts: the proper distance between free test masses sitting in space as a gravitational wave passes by will exhibit the increase in the X, decrease in the Y and vice versa oscillation pattern. To measure this, you need to use something free, like the mirrors at the ends of the beam tubes (they are really only free in one dimension). To measure distance, you can't use a meter stick, because it is not an ideal measuring device which you need to measure space with in GR. The ideal device is light. To think about it without resorting to a meter stick increasing and decreasing, think about the light travel time. Since light has a constant speed in all frames, if the proper distance is what is really increasing (disregard what happens to the meter stick, since it is made up of fallible matter and might stretch along with space, but light won't), then it will take longer to go down one arm and shorter down the other. Therefore one arm will add phase relative to the other, which will no longer perfectly interfere at the end.
Each of the mirrors have magnets glued to them in a quadrupole configuration (so they are not affected by stray magnetic fields) which are used to move the mirrors around. The mirrors are moved around for a few reasons: 1) The detector works in a closed loop feedback system, so they keep the mirrors they same distance (mod the wavelength) from the source (and the error signal which is used to feed back on them is the actual "gravitational wave" signal. 2) Calibration: during data collection, the mirrors are periodically driven with a known magnetic field by the same set of magnets. Because we know the mechanics of moving a mass with the force of a magnet, we know how far it went. And when we look at the error signal to keep the device in lock while it is being forced, we know the ratio between distance the mirrors travel and digital signal coming out of the detector. So after the data are collected and ready to be analyzed, a lot of people spend lots of time preparing and reducing the data, which includes coming up with a calibration from the forcing.
This is in reply to this post and a number of others on the same topic. Major sources of noise: seismic, acoustic, photon shot noise, thermal noise. 1) Acoustic noise: the entire beam tube system is in vacuum, so the only mechanical vibrations can be coupled in through the mirror supports, which are suspended on thin wire. The pendulum created by the hanging mirror essentially creates a mechanical low-pass filter which reduces the effects of noise above about 10 Hz. The gravitational wave projects (on Earth, not talking about LISA here) are mostly interested in frequencies around a few hundred to a few thousand Hz. 2) Seismic: this can cause pretty large displacement. Each of the mirrors (on its' hanging suspension) is sitting on a system of masses and springs (three levels) which creates a third order lowpass filter which further reduces noise. 3) Photon shot noise: this rises with frequency; essentially, photons are uncorrelated random events which create a Poisson noise distribution. In a Poisson distribution, the standard deviation of count rate is equal to the square root of the count rate, so the variance is decreased by decreasing count rate at the detector. This is why the interferometric detectors operate "in null," meaning they keep the mirrors at a differential path length which is equal plus or minus integer multiples of wavelengths. This way, the output at the point where they interfere is kept dark. The idea is that it's easier to detect a difference between 0 and 1 than between 100 and 101. (There is a ton of feedback to keep the whole system in null. Read up on Pound-Drever locking to understand it.) 4) Thermal noise: the surface of the mirror is made of atoms which jiggle in random Brownian motion. This is unavoidable unless the mirror is cooled sufficiently, which is difficult to do because of how well isolate the mirrors are. However, the Brownian motion can be averaged out over a large area by making the laser's spot size large.
So they've thought about it a little bit. And they are also measuring other non-detector channels like seismic activity and acoustics near the detector and wind speed and... so they can correlate with those sources.
The NSF doesn't go around giving millions to any old project:)
Here's the deal with local sources: their masses are tiny compared to astronomical sources! But here's a more local source that we have detected: the moon. The moon causes tidal deformations in the Earth's crust, which LIGO (disclosure: I am involved with the LIGO project) and the other large scale interferometers (GEO, VIRGO, TAMA) have to subtract out in order to see anything besides the moon. Essentially, to make gravitational waves large, the conditions which need to be satisfied are 1) large amounts of matter 2) moving quickly. Things which satisfy this are: supernovae core collapses which are sufficiently non-axisymmetric, compact (eg. black hole or neutron star) binary system decay, and maybe some events we don't yet know of.
Other posters have already stated this: information can not travel faster than the speed of light. The fact that phase velocity can be faster than c, as this article points out (which has been known for a long time! read about anomolous dispersion. we've known about that for a long time now.) can _not_ improve telecom by speeding up information transfer. Advanced techniques (better fiber optics, optical routers, etc) which still abide by the c speed limit are the only way to reduce your ping time. Anyway, the current bottleneck is not the fiber part of telecom. The optical-electronic interface and the electronic switching is the real culprit. Once optical switching and routing is prevalent, then more technology spent on optics will really pay off.
Sure, that phenomenon being general relativity. A gravitational wave would be a time varying ("AC" to an electrical engineer) curvature of spacetime. Frame dragging would be a constant ("DC") curvature due to being close to a massive rotating body.
Is it difficult to believe that we see distant objects? The light waves reach us after travelling through spacetime; we don't have to be touching a distant star to see it. Similarly, we feel the pull of gravity of every other object in the universe (and not only massive objects, such as matter. It is the curvature or energy density of spacetime which couples to the gravitational force. Weakly Interacting Massive Particles should contribute to the pull of gravity we feel, as should the energy density of a vacuum itself!) However, neither light nor gravity travel at infinite velocities. Both are predicted to propogate at the same velocity, c. Imagine a distant (or even close) system of two stars orbiting each other. In one orientation, when they are the same distance from you, you would feel a certain gravitational attraction to them. When they are in line with you, you will feel different attraction to them. It is this fluctuation in gravitational attraction (which in turn curves spacetime) which you would feel.
Actually, the angular resolution is worse than that. The antenna pattern of the LIGO and VIRGO projects is close to 90 degrees of sky. However, work is under way at Caltech to use multiple detectors (like LIGO Hanford and Livingston) in a fashion similar to how radio astronomy uses multiple dishes to form a more sensitive, finer resolution antenna (this is based on interferometery). The stochastic gravity wave background, which is a prediction of inflation, is predicted to be at power levels which are currently below the noise level of the detectors. Advanced LIGO and LISA may have noise levels low enough to verify or disprove this prediction.
1) Any theory in contention with either of these would probably only be an alternative to one or the other, considering that GR and QM make predictions on completely different scales and are generally not unified. 2) GR does not make any prediction such as "in the far field, gravity waves should look like the result of dipole excitations" or quadrupole. In the far field, in a linearized patch of spacetime (a small patch of it, in which special relativity can be applied) gravity waves should obey a linear wave equation. Therefore, both binary inspiral sources such as coalescing black holes or other massive objects and any other gravity wave source are superimposable and can therefore be predicted by GR. 3) Be careful with the usage of the words dipole and quadrupole: a binary inspiral system is not actually a dipole, since both bodies have the same "sign" - the gravitational force only cares about energy density. Two massive bodies both have positive energy density.
Sorry, gravity probe B (the recent satellite) is not trying to confirm the existence of gravity waves. GPB is looking for "frame dragging," another predicted effect of general relativity. Gyroscopes in GPB should precess, despite the fact that they are over the poles of the earth and (to first order, excluding motion about the sun and the motion of our solar system itself) not in a rotating frame. Even though the gyroscopes won't be in a rotating frame, their spacetime metric will be 'dragged' by the rotating massive earth, causing a precession of some parts of arcseconds (check the web page for more).
OK, there are a lot of posts here. I will try to relate to many of them. First of all, anybody who is complaining about ADC/DAC technology must have extremely good equipment in the rest of the audio chain, because these precision instruments are not the major cause of distortion or coloration in reproduction. The largest culprit is at the very end: the speaker. It is quite difficult to make a single driver cover the entire audio range with flat frequency response and equal phase response (equal phase response is necessary to preserve timbre, as it is the relative amplitudes and phases of harmonics which give instruments their timbres). Most speakers sold today have crossover networks to try to bridge the response between multiple drivers, and these crossovers are very difficult to design without distorting the audio. Now let's assume you have perfect speakers; now you might be able to hear the quantization effects of the DAC, but only if it is low resolution and there is no output filter, and you have ears that can hear up to the sampling frequency of the DAC, which you do not. Your dog might, but I'm not sure. Your ears can't hear far past 20kHz, definitely not past 30kHz. 44.1kHz (CD audio) gives a nyquist limit of 22kHz on reproduced frequencies (nyquist sampling theorem). Now, with your perfect speakers and your ADC/DAC which are faster and higher resolution than audible, you might place a DSP between the ADC and the DAC. There is one thing DSPs are very good at doing: multiplying and accumulating (a MAC operation). It is trivial to implement a linear time-invariant (LTI) filter on a DSP. This is how you do digital equalization. Many functions you might want to do with audio can be represented as an LTI filter. You will be able to implement all of these on a DSP. It will sound right with your perfect speakers and perfect ADC/DAC. However, a vaccuum tube can not be modeled as an LTI filter. That's because tubes are not linear (that's the first letter in LTI!). A linear amplifier would not introduce harmonics into a signal which are not there. The reason tube amps sound "warmer" than transistor amps are that they tend to introduce even ordered harmonics. Compare this to transistors, which tend to introduce odd ordered harmonics. Trying to use a DSP to model a non-linear system will be nowhere near as trivial as modeling an LTI. Instead of O(n) MAC operations per sample, where n is the order of the LTI filter, you will have to have O(m^3*n) where m is the order of the polynomial you are using to model the non-linear system, and still it will only be a polynomial model of a non-linear system.
But none if it really matters that much, because speakers still suck:)
This goes in the spirit of not having hard limits, which is generally a good programming philosophy. Generally, there are only 3 cases: 0 items, meaning a restriction, 1 item, like the normal clipboard, or an infinite (until memory runs out) number of items. This is a good philosophy to work by, unless you have a standard (which you assume won't get broken). People making fixed size buffers are what get people into buffer-overflow attack problems.
Formula for getting a story on/. follows: 1. Link to an actual story. 2. Throw in lateset buzzwords (grid computing, XML, your choice!) 3. Don't sound completely illiterate (use four syllable words like ubiquitous).
Not all auto-focus cameras suffer from the feature bloat problem. Some which I have seen (Canons, Olympus, some other) have a dial on the back, a dial on the top, a dial on the left, and several pushbuttons... it's not something you can just pick up and already know how to use. However, Pentax's ZX-50 which I have has the features of you want from an autofocus camera, but very simple to use as a manual camera. Switching between AF and MF is just a 2-position sliding switch. You can use the aperture ring on your pentax lens, or you can electronically set the aperture automatically or manually with a dial. And the mode dial doesn't have 20 different modes like bloated cameras - just 5 - full auto, aperture priority, shutter priority, full manual, and ISO setting. I really don't like feature bloat; if there are features I'm going to use, though, and are as easy to use as on a manual, I'll use them.
The C64 was the first computer I ever used, and I am grateful for it. When you learned to use a C64, it was a simple machine and you could understand how to program it (I had no programming experience, but I picked up BASIC in no time). I think nowadays people are handed a complex system which is understood by those who designed it and have seen it evolve, but is not intuitive to newbies. Of course computers are hard to use if you don't know why anything does what it does! With a simple system like the C64, if you learn to program at the same time as just use the computer, computers make sense.
Harnessing energy release is what all generators are about. It's not the release of energy that is difficult, but the efficient release and harness. Coal/oil/gas generators all generally heat water, turning it into steam, spinning a turbine to produce mechanical energy which is converted to electricity through induction. Fission also releases massive amounts of heat energy which is absorbed by water and turns a turbine. The majority of energy in these fusion reactions (Inertial confinement fusion (laser driven), magnetic confinement fusion (in a tokamak), electrically pulsed like in this article) leaves the system in the kinetic energy of the resulting particles. For example, Deuterium and Tritium are often fused yielding normal Helium and a neutron. Both are moving very fast after the fusion. This velocity is where most of the energy of fusion is. You can capture this again by letting the fast particles transfer their energy to a big resorvoir which would heat up from this energy transfer and again heat water to steam to turn a turbine. With matter-antimatter collisions, the gamma rays would have to be absorbed by some matter, which energizes the matter, either thermally or electrically (that's how solar cells work - by liberating electrons by light interaction) or some other means I can't think of. But you have to find the antimatter first:)
Unfortunately, the majority of the energy created in the system (which I think could plausibly break even or even function as a reactor, but if it were constructed to the highest precision, perfect sphericity, which we can not really obtain) is not in what particle is created, but the speed that particle is given due to the reaction. That's right, most of the energy from mass-energy conservation equation (E=M*C^2) is in the kinetic energy of the particles which have reacted. So using their electrical properties to evolve electrical energy is ignoring the vast majority of the energy. Most generators (as far as I know) would convert this kinetic energy into thermal energy by using the velocity of the particles to heat some sort of water resorvoir, which would generate steam and drive a turbine like any old coal generator, except without the fire and coal and soot and yuck.
Slashdot Mirror
As clearly stated in the article, this was not a supernova (SN), but rather a gamma ray burst (GRB). The network of satellites referred to all trigger on high energy gamma rays, and look for the afterglow of the event which caused the trigger. The time scale of GRBs is typically on the order of seconds. Core collapse SNe, by comparison, are optically visible and have a brightening and fading timescale on the order of days or tens of days.
Much more is understood of core collapse SNe than the progenitors of GRBs. One of the leading models for short GRB progenitors is the binary inspiral of two massive, compact objects, at least one of them being a neutron star. Obviously we can't resolve the region that the GRB came from, but from the above model, it's inferred that there is a region dense in stars out there, rather than just one isolated star. The second piece of evidence is that the afterglow was actually visible: this afterglow is most likely from shock waves in the interstellar gas, having associated high densities and temperatures, glowing in the optical or xray. If there was no gas by the progenitor of the GRB, there would not have been an afterglow (or the model is wrong).
The slashdot title was therefore wrong in two ways: this was not a SN event, and it was not in empty space - it was just not in a host galaxy.
Firstly, I'm going to guess that frame dragging is verified at no better resolution than the curvature of space/time, but that as far as they can tell, it exists and meets the values expected by Einstein.
Frame dragging is the name of one particular way in which spacetime curves. It is curvature. To say something about frame dragging or curvature is to say something about the other. I don't know if the parent statement makes sense or not. The group has not released their frame dragging measurements yet, just the geodetic precession measurements (the precision of which will likely go up as they isolate more systematics in their data as they move toward making a statement about frame dragging). Frame dragging is about 100 times harder to measure than geodetic precession, for the mass and spin of the Earth.
Secondly, I'm also going to guess that QM experts will start to get a little nervous. The properties any future QM model of gravity must have contradict the GR model. They cannot both be right. The more "right" the GR model, the more problematic a QM model. This doesn't mean a QM model does not exist, only that it is most undesirable (from a QM perspective) for the GR model to make highly precise and accurate predictions.
GR is arguably the most successful physical theory to date (I would say that electrodynamics rivals it since it has been formulated classically in curved spacetime and also has been quantized successfully in flat spacetime, but that is another discussion). Newton was not "right", but note that GR simplifies to Newtonian mechanics in the weak field and non-relativistic limit. Any theory which supersedes a highly successful physical theory must reproduce said theory in the proper limits. A quantum theory of gravity must reproduce GR in the macroscopic limit, just as quantum mechanics has a correspondence principle which allows it to reproduce classical wave and particle phenomena in the appropriate limit. I don't think any physicist is nervous about these results - everybody expects GPB to verify the predicted frame dragging. Deviations from the values predicted would excite fans of MoND, SVT theories, and other alternative theories of gravity.
Thirdly, frame-dragging occurs at a non-zero distance from an object.
Frame dragging curves spacetime globally, but falls off to asymptotic flatness. The parent statement probably makes sense.
This doesn't matter, for the purpose of these observations, as they're nowhere near accurate to measure the relativistic effects that apply to the information passed that creates the effects in the first place. Nonetheless, such an affect must exist, or you'd end up with infinitely fast rates of change of state, which is expressly forbidden in GR.
The NSF and NASA don't spend this much money to throw an instrument into space unless they think it will actually measure what it's supposed to. The gyros are the most spherical macroscopic manmade objects, which used superconducting quantum interference devices (SQUIDs) to precisely measure their precession, blah blah blah, read about it on their web site. I sure hope they're accurate enough to measure those relativistic effects, because that's exactly what they've been designed to do. I don't know what information you are talking about. The Einstein Field Equations are local, so there is an inherent limit on the speed at which 'information' (curvature) propogates through spacetime.
It's a gross simplification and it's not an "obvious" conclusion to reach by any means, but if the curvature (and restoration) of space/time has nothing analogous to Hooke's Constant, then after a gravitationally massive object has move
What is meant is the following:
One of the exact solutions to the Einstein field equations is a decent assumption for the Earth's (or anything approximately spherical which is not moving relativistically) gravitational field. The curvature of space-time is greater the closer to the center of the massive body. A light ray travelling some distance from the massive body will be deflected from a "straight line" (which is hard to define in curved space).
If you are taking the view that you start rotating the rest of the universe around us, then it is equivalent to having your coordinate system spin around the massive body (well, there is nothing besides the massive body in the universe I am imagining). Physically, light will follow the same path as it did before, since all you have done is redefine the coordinate system, which does not change physics!
Now instead, consider spinning the Earth, instead of the coordinate system. The matter making up the earth now has more energy-momentum (the magnitude of which is a physical quantity which can be measured independent of reference frame, if your frame is freely-falling). Energy-momentum is what causes space-time to curve, so a light ray travelling the same distance from the earth will be deflected by a larger amount, since space will be more curved.
Actually, the frequency of photons will change due to the changing gravitational potential to satisfy conservation - this is the gravitational wave doing some work on the light. But this change is also the same change which is being measured by the device.
In essence, you can think of it as doing that. Think about how you detect frequency shifts: when you tune a guitar, you listen for a frequency difference because one string is accruing phase faster than the other one. When they combine, you hear beats.
One you look at the output of a Michelson interferometer, there are differences in intensity because one arm may have accrued more phase than the other one. You can think about it in the time domain, measuring length of the arms, or in the frequency domain, because the arms (which are resonant cavities) have had their resonant frequencies shifted.
This is a reply to this post and some of its' ancestors.
Gravitational waves are predicted to weakly interact with everything which is matter-energy. For that matter, gravity interacts with itself (which is why GR predicts black holes and other such singularities). However, in the weak-field regime (that is, space-time is flat except for a deviation which is orders of magnitude less, meaning we can take the leading term in the expansion, so the theory is linear), gravitational waves just pass through everything. Since they pass through things, their energy falls off like the square of distance from the source. In the strong-field interacting picture, they certainly should exhibit non-linear exotic behaviour, but those are precisely the parts of GR we are trying to probe with LIGO.
The exchange between matter-energy and curvature (gravitational waves) that you are thinking about is from the latter to the former, but just think about the former being turned into the latter - that is the prediction of the source of gravitational waves. However, it works both ways.
On the levels at which LIGO hopes to detect gravitational waves, we will see about 10^53 gravitons. I am quoting this figure without understanding where it comes from, since we certainly don't have a quantum theory of gravity. But gravity is predicted to be quantum in nature as well, but we won't see the quantization from where we stand.
One of the ancestors addressed the issue of measuring while your meter stick is being squashed and expanded, and another about the local speed of light. These issues are related. One of the postulates (argue argue whatever) of GR is that the speed of light is constant in every frame, and it has the same constant value compared between all frames. Light is the perfect meter stick (or clock) for making measurements with.
I had the same thought about measuring the arm lengths as you did for a while until I started taking GR. Here's how the thought goes: "If space is being stretched, and a meter stick is sitting in front of my face, I will always see the meter stick as being one meter long." Here's what GR predicts: the proper distance between free test masses sitting in space as a gravitational wave passes by will exhibit the increase in the X, decrease in the Y and vice versa oscillation pattern. To measure this, you need to use something free, like the mirrors at the ends of the beam tubes (they are really only free in one dimension). To measure distance, you can't use a meter stick, because it is not an ideal measuring device which you need to measure space with in GR. The ideal device is light. To think about it without resorting to a meter stick increasing and decreasing, think about the light travel time. Since light has a constant speed in all frames, if the proper distance is what is really increasing (disregard what happens to the meter stick, since it is made up of fallible matter and might stretch along with space, but light won't), then it will take longer to go down one arm and shorter down the other. Therefore one arm will add phase relative to the other, which will no longer perfectly interfere at the end.
Each of the mirrors have magnets glued to them in a quadrupole configuration (so they are not affected by stray magnetic fields) which are used to move the mirrors around. The mirrors are moved around for a few reasons:
1) The detector works in a closed loop feedback system, so they keep the mirrors they same distance (mod the wavelength) from the source (and the error signal which is used to feed back on them is the actual "gravitational wave" signal.
2) Calibration: during data collection, the mirrors are periodically driven with a known magnetic field by the same set of magnets. Because we know the mechanics of moving a mass with the force of a magnet, we know how far it went. And when we look at the error signal to keep the device in lock while it is being forced, we know the ratio between distance the mirrors travel and digital signal coming out of the detector. So after the data are collected and ready to be analyzed, a lot of people spend lots of time preparing and reducing the data, which includes coming up with a calibration from the forcing.
This is in reply to this post and a number of others on the same topic.
... so they can correlate with those sources.
:)
Major sources of noise: seismic, acoustic, photon shot noise, thermal noise.
1) Acoustic noise: the entire beam tube system is in vacuum, so the only mechanical vibrations can be coupled in through the mirror supports, which are suspended on thin wire. The pendulum created by the hanging mirror essentially creates a mechanical low-pass filter which reduces the effects of noise above about 10 Hz. The gravitational wave projects (on Earth, not talking about LISA here) are mostly interested in frequencies around a few hundred to a few thousand Hz.
2) Seismic: this can cause pretty large displacement. Each of the mirrors (on its' hanging suspension) is sitting on a system of masses and springs (three levels) which creates a third order lowpass filter which further reduces noise.
3) Photon shot noise: this rises with frequency; essentially, photons are uncorrelated random events which create a Poisson noise distribution. In a Poisson distribution, the standard deviation of count rate is equal to the square root of the count rate, so the variance is decreased by decreasing count rate at the detector. This is why the interferometric detectors operate "in null," meaning they keep the mirrors at a differential path length which is equal plus or minus integer multiples of wavelengths. This way, the output at the point where they interfere is kept dark. The idea is that it's easier to detect a difference between 0 and 1 than between 100 and 101. (There is a ton of feedback to keep the whole system in null. Read up on Pound-Drever locking to understand it.)
4) Thermal noise: the surface of the mirror is made of atoms which jiggle in random Brownian motion. This is unavoidable unless the mirror is cooled sufficiently, which is difficult to do because of how well isolate the mirrors are. However, the Brownian motion can be averaged out over a large area by making the laser's spot size large.
So they've thought about it a little bit. And they are also measuring other non-detector channels like seismic activity and acoustics near the detector and wind speed and
The NSF doesn't go around giving millions to any old project
The article is writing about GEO600, whose two arms are 600m which is about 2000ft.
Here's the deal with local sources: their masses are tiny compared to astronomical sources!
But here's a more local source that we have detected: the moon. The moon causes tidal deformations in the Earth's crust, which LIGO (disclosure: I am involved with the LIGO project) and the other large scale interferometers (GEO, VIRGO, TAMA) have to subtract out in order to see anything besides the moon.
Essentially, to make gravitational waves large, the conditions which need to be satisfied are 1) large amounts of matter 2) moving quickly. Things which satisfy this are: supernovae core collapses which are sufficiently non-axisymmetric, compact (eg. black hole or neutron star) binary system decay, and maybe some events we don't yet know of.
Other posters have already stated this:
information can not travel faster than the speed of light.
The fact that phase velocity can be faster than c, as this article points out (which has been known for a long time! read about anomolous dispersion. we've known about that for a long time now.) can _not_ improve telecom by speeding up information transfer. Advanced techniques (better fiber optics, optical routers, etc) which still abide by the c speed limit are the only way to reduce your ping time.
Anyway, the current bottleneck is not the fiber part of telecom. The optical-electronic interface and the electronic switching is the real culprit. Once optical switching and routing is prevalent, then more technology spent on optics will really pay off.
Sure, that phenomenon being general relativity.
A gravitational wave would be a time varying ("AC" to an electrical engineer) curvature of spacetime. Frame dragging would be a constant ("DC") curvature due to being close to a massive rotating body.
Is it difficult to believe that we see distant objects? The light waves reach us after travelling through spacetime; we don't have to be touching a distant star to see it. Similarly, we feel the pull of gravity of every other object in the universe (and not only massive objects, such as matter. It is the curvature or energy density of spacetime which couples to the gravitational force. Weakly Interacting Massive Particles should contribute to the pull of gravity we feel, as should the energy density of a vacuum itself!)
However, neither light nor gravity travel at infinite velocities. Both are predicted to propogate at the same velocity, c.
Imagine a distant (or even close) system of two stars orbiting each other. In one orientation, when they are the same distance from you, you would feel a certain gravitational attraction to them. When they are in line with you, you will feel different attraction to them. It is this fluctuation in gravitational attraction (which in turn curves spacetime) which you would feel.
Actually, the angular resolution is worse than that. The antenna pattern of the LIGO and VIRGO projects is close to 90 degrees of sky. However, work is under way at Caltech to use multiple detectors (like LIGO Hanford and Livingston) in a fashion similar to how radio astronomy uses multiple dishes to form a more sensitive, finer resolution antenna (this is based on interferometery).
The stochastic gravity wave background, which is a prediction of inflation, is predicted to be at power levels which are currently below the noise level of the detectors. Advanced LIGO and LISA may have noise levels low enough to verify or disprove this prediction.
1) Any theory in contention with either of these would probably only be an alternative to one or the other, considering that GR and QM make predictions on completely different scales and are generally not unified.
2) GR does not make any prediction such as "in the far field, gravity waves should look like the result of dipole excitations" or quadrupole. In the far field, in a linearized patch of spacetime (a small patch of it, in which special relativity can be applied) gravity waves should obey a linear wave equation. Therefore, both binary inspiral sources such as coalescing black holes or other massive objects and any other gravity wave source are superimposable and can therefore be predicted by GR.
3) Be careful with the usage of the words dipole and quadrupole: a binary inspiral system is not actually a dipole, since both bodies have the same "sign" - the gravitational force only cares about energy density. Two massive bodies both have positive energy density.
Sorry, gravity probe B (the recent satellite) is not trying to confirm the existence of gravity waves. GPB is looking for "frame dragging," another predicted effect of general relativity. Gyroscopes in GPB should precess, despite the fact that they are over the poles of the earth and (to first order, excluding motion about the sun and the motion of our solar system itself) not in a rotating frame. Even though the gyroscopes won't be in a rotating frame, their spacetime metric will be 'dragged' by the rotating massive earth, causing a precession of some parts of arcseconds (check the web page for more).
OK, there are a lot of posts here. I will try to relate to many of them.
:)
First of all, anybody who is complaining about ADC/DAC technology must have extremely good equipment in the rest of the audio chain, because these precision instruments are not the major cause of distortion or coloration in reproduction. The largest culprit is at the very end: the speaker. It is quite difficult to make a single driver cover the entire audio range with flat frequency response and equal phase response (equal phase response is necessary to preserve timbre, as it is the relative amplitudes and phases of harmonics which give instruments their timbres). Most speakers sold today have crossover networks to try to bridge the response between multiple drivers, and these crossovers are very difficult to design without distorting the audio.
Now let's assume you have perfect speakers; now you might be able to hear the quantization effects of the DAC, but only if it is low resolution and there is no output filter, and you have ears that can hear up to the sampling frequency of the DAC, which you do not. Your dog might, but I'm not sure. Your ears can't hear far past 20kHz, definitely not past 30kHz. 44.1kHz (CD audio) gives a nyquist limit of 22kHz on reproduced frequencies (nyquist sampling theorem).
Now, with your perfect speakers and your ADC/DAC which are faster and higher resolution than audible, you might place a DSP between the ADC and the DAC. There is one thing DSPs are very good at doing: multiplying and accumulating (a MAC operation). It is trivial to implement a linear time-invariant (LTI) filter on a DSP. This is how you do digital equalization. Many functions you might want to do with audio can be represented as an LTI filter. You will be able to implement all of these on a DSP. It will sound right with your perfect speakers and perfect ADC/DAC.
However, a vaccuum tube can not be modeled as an LTI filter. That's because tubes are not linear (that's the first letter in LTI!). A linear amplifier would not introduce harmonics into a signal which are not there. The reason tube amps sound "warmer" than transistor amps are that they tend to introduce even ordered harmonics. Compare this to transistors, which tend to introduce odd ordered harmonics. Trying to use a DSP to model a non-linear system will be nowhere near as trivial as modeling an LTI. Instead of O(n) MAC operations per sample, where n is the order of the LTI filter, you will have to have O(m^3*n) where m is the order of the polynomial you are using to model the non-linear system, and still it will only be a polynomial model of a non-linear system.
But none if it really matters that much, because speakers still suck
This goes in the spirit of not having hard limits, which is generally a good programming philosophy.
Generally, there are only 3 cases: 0 items, meaning a restriction, 1 item, like the normal clipboard, or an infinite (until memory runs out) number of items. This is a good philosophy to work by, unless you have a standard (which you assume won't get broken). People making fixed size buffers are what get people into buffer-overflow attack problems.
Formula for getting a story on /. follows:
:)
1. Link to an actual story.
2. Throw in lateset buzzwords (grid computing, XML, your choice!)
3. Don't sound completely illiterate (use four syllable words like ubiquitous).
Rinse, Repeat, Lather!
Not all auto-focus cameras suffer from the feature bloat problem. Some which I have seen (Canons, Olympus, some other) have a dial on the back, a dial on the top, a dial on the left, and several pushbuttons ... it's not something you can just pick up and already know how to use.
However, Pentax's ZX-50 which I have has the features of you want from an autofocus camera, but very simple to use as a manual camera. Switching between AF and MF is just a 2-position sliding switch. You can use the aperture ring on your pentax lens, or you can electronically set the aperture automatically or manually with a dial. And the mode dial doesn't have 20 different modes like bloated cameras - just 5 - full auto, aperture priority, shutter priority, full manual, and ISO setting.
I really don't like feature bloat; if there are features I'm going to use, though, and are as easy to use as on a manual, I'll use them.
When will I finally have a good user interface to play music under linux? iTunes definitely has the best user interface I've seen in any player.
The C64 was the first computer I ever used, and I am grateful for it. When you learned to use a C64, it was a simple machine and you could understand how to program it (I had no programming experience, but I picked up BASIC in no time).
I think nowadays people are handed a complex system which is understood by those who designed it and have seen it evolve, but is not intuitive to newbies. Of course computers are hard to use if you don't know why anything does what it does!
With a simple system like the C64, if you learn to program at the same time as just use the computer, computers make sense.
If you remember the /. story on buddyzoo, the guy who did that is half responsible for this. And he lives upstairs from me :)
Harnessing energy release is what all generators are about. It's not the release of energy that is difficult, but the efficient release and harness. :)
Coal/oil/gas generators all generally heat water, turning it into steam, spinning a turbine to produce mechanical energy which is converted to electricity through induction.
Fission also releases massive amounts of heat energy which is absorbed by water and turns a turbine.
The majority of energy in these fusion reactions (Inertial confinement fusion (laser driven), magnetic confinement fusion (in a tokamak), electrically pulsed like in this article) leaves the system in the kinetic energy of the resulting particles. For example, Deuterium and Tritium are often fused yielding normal Helium and a neutron. Both are moving very fast after the fusion. This velocity is where most of the energy of fusion is. You can capture this again by letting the fast particles transfer their energy to a big resorvoir which would heat up from this energy transfer and again heat water to steam to turn a turbine.
With matter-antimatter collisions, the gamma rays would have to be absorbed by some matter, which energizes the matter, either thermally or electrically (that's how solar cells work - by liberating electrons by light interaction) or some other means I can't think of.
But you have to find the antimatter first
Unfortunately, the majority of the energy created in the system (which I think could plausibly break even or even function as a reactor, but if it were constructed to the highest precision, perfect sphericity, which we can not really obtain) is not in what particle is created, but the speed that particle is given due to the reaction. That's right, most of the energy from mass-energy conservation equation (E=M*C^2) is in the kinetic energy of the particles which have reacted. So using their electrical properties to evolve electrical energy is ignoring the vast majority of the energy.
Most generators (as far as I know) would convert this kinetic energy into thermal energy by using the velocity of the particles to heat some sort of water resorvoir, which would generate steam and drive a turbine like any old coal generator, except without the fire and coal and soot and yuck.