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LIGO Fails To Detect Gravity Waves
planckscale writes "Last weekend, LIGO (the Laser Interferometer Gravitational-Wave Observatory) did not detect gravitational radiation in association with a gamma ray burst (GRB). The non-detection was actually a valuable contribution, as it helped to distinguish between competing models for what powers GRBs. The detector is due to be upgraded this year for even more accurate measurements. The interferometer is constructed in such a way that it can detect a change in the lengths of the two arms relative to each other of less than a thousandth the diameter of an atomic nucleus."
I'll spell it out for you. This is not a failure of gravitational wave detection technology.
What you apparently do not understand is that this device can detect gravitational waves. However, it did not detect gravitational waves that correlated with a gamma wave burst originating in Andromeda. Normally such bursts arise from well known phenomena, such as a collision of black holes. But in this case, the collision could not have been from one of these well known phenomena.
What the article suffers from is bad writing. It should have been put in the positive--something like "the gamma-ray burst originated from a novel mechanism". Now, because astrophysicists can not account for the burst, they must go back and (1) study other similar phenomena and/or (2) revise astrophysical theory to explain the heretofore inexplicable gamma ray burst. Why is this burst inexplicable at this point? Because they did not detect gravitational waves that correlated with the burst.
Just callin' it like I see it.
This is a press release, for general public who need illustrations they can somehow comprehend. Those of us who understand more know where to look for more accurate data, which would be both useless and confusing for the broader audience.
The creatures outside looked from Alt-Right to Antifa; but already it was impossible to say which was which.
The article says proton, not atom, so hydrogen I guess.
-Ariel
1) General Relativity as formulated by Einstein (and a lot of other similar derivates - are there many?) would be in serious doubt. An exam question I had was take GR and show gravity waves exist - you basically show how the wave equation falls out of the formulas and these things carry momentum out of a system.
2) You then need to explain stuff such as Mercury's orbit precession and observed effects of double Neutron stars slowing down - the FSM stirring his planetary meatball lunch slower?
The Singularity is closer than you think
Quant
It would be a serious blow to the picture in General Relativity of gravity warping space-time itself if we go for too long without detecting gravitational waves using length measurements as an interferometer does. This is especially true if we ever improve the accuracy of our measurements to the point where we can predict that we should observe gravitational waves but don't.
What we would replace it with that could explain all of the observations that GR predicts I don't personally know, but it's a good day in physics when a theory is proved wrong because it means that we've done our job.
licet differant, aequabitur
For all the people arguing over whether or not this is a failure of LIGO or not...it doesn't really say much at all. Initial LIGO (which is currently running) is more of a proof of concept sold as a viable project. But if you look at the expected rates of detection, the absolute high end for all binary sources is less than one event/year. The low end is between 4 events every 10000 years and 4 events every 100 years. The other source types are not any better.
This article basically says that because LIGO is known to not be sensitive enough to measure past a certain distance from Earth (which encompasses the Andromeda galaxy, in whose direction this burst occurred) and because no detection was seen, the burst was not caused from a source in the Andromeda Galaxy.
I suppose that after spending all this money its not a bad thing that LIGO can actually produce some useful results (though I doubt they were amazingly useful). Advanced LIGO should be able to do the job - but not for another 5-6 years. At that point, the minimum event rate is supposed to be around 1/year and we should finally get some sort of positive detection.
Personally I'm hoping Advanced LIGO does work, because otherwise all this money will have gone to waste and the field of gravitational wave astronomy will be even more damaged than it already is. The thing is, many people in astronomy who are not affiliated with LIGO are not excited by it. Maybe that interest will be rekindled when Adv LIGO actually works, since right now its more of an engineering problem than an astronomy or physics problem. More people are interested in LISA which (if it ever launches) should have many more interesting sources. Its amusing seeing LIGO people try to point out the flaws of LISA while trying to explain why LIGO doesn't work, but then maybe I'm biased since I am working on LISA (though I have worked on LIGO in the past).
Perhaps the LISA (NASA/ESA) project will have more luck (2015+).
We have seen binaries losing energy in a manner consisted with GW predictions, so there is a good chance the theory of GWs is correct.
> As the gravity wave compresses and then dilates space-time, the LIGO tube and the laser beam within it will compress and dilate in perfect synchrony.
This part isn't correct. The laser beam will be redshifted and change its wavelength, however it will still travel at the speed of light, c. Since the distance between the two ends is less, it will travel that distance in a shorter time.
Yes. This is no failure in the dectection technology. People at LIGO have estimated what they can detect and what they cannot. This puts an upper bound in the energy of the gravitational waves that were emitted by the GRB source. If it emitted more they would have detected them. This shows GRBs theories have a long way to go. We dont even know the total intrinsic amount of energy of a GRB source. If the source radiates in a polar pattern (like a lighthouse) we only see a small fraction of the GRB sources that exists (when the beam is directed toward the earth), in this case the intrinsic amount of energy is much smaller. If the GRB radiates like a star in all directions the intrinsic amount of energy is MUCH bigger. We can estimate the maximum size of the source bases in the timing of the event (if it has very fast variations it must be smaller because the information to coordinate this variation cannot propagate faster than light). But we dont know much more. This "failed" experiment is as important fot GRBs theory as the "failed" experiment in detecting the aether wind by Michelson and Morley was for the birth of Relativity. It shows we must review our theories. Airton da Fonseca Granero
Also note that the effect of the gravitational wave will depend on the gauge used. Basically there's too many free variables and based on the ones that you lock down, you get a different effect (but the same result).
(If I messed this up I apologize and feel free to correct me.)
Well, light is sort of separated. You won't feel the gravitational wave passing through you because the effect is so small and everything will be stretched along with you. However, light has no width and so when the distance between the mirrors increases, it goes out of phase with the cavity and turns up as a signal. Remember that light always travels at the same speed in vacuum, which is why this works.
There are others? I've never heard of any.
http://en.wikipedia.org/wiki/Alternatives_to_general_relativity
http://en.wikipedia.org/wiki/Parameterized_post-Newtonian_formalism
At least not as an overall theory.
I don't know what that means. How is one field theory more "overall" than another? People have attempted to apply GR to many more problems than any of the others, but that doesn't make GR any more complete than any of the others.
That would be the phonon.
Only his tendency toward a dazed stupor prevented him from screaming aloud.
Gravity is a quadrupole wave, vs a dipole wave (up and down) like electromagnetism. (It's related to why there is no antigravity, sorry!) Take your thumbtips and touch them together, and touch your index fingertips together, and make a vertical oval. Now distort it past circular to a horizontal oval. Now back to vertical. Repeat. That's what a quadrupole wave looks like as it's coming toward you. So if a gravitational wave hits the L-shaped LIGO face-on, one leg will shorten while the other leg will lengthen, back and forth.
The General Theory of Relativity says they should move at the speed of light. They are simply warps in space-time that are caused events involving really big masses (like black holes colliding). They are basically changes in gravitational forces as very massive object move. A classical example is two black holes rotate around each other and approach and collide. The gravitational forces vibrate up and down in magnitude as the objects move towards and away from us. That is the wave we're trying to detect.
In fact the length of the space between the mirrors (and any length whatsoever) is _defined_ as the time light spends traveling between the two. This is the definition of distance in GR. It works because the speed of light is constant for everyone everywhere (in GR); the same thing causes all the other funny effects of relativity, for instance the same object having different lengths for different observers.
So, by the same definition, a piece of space is lengthened or shortened _iff_ light spends a longer or shorter time traveling it. The speed of light never changes, but due to conservation laws its _frequency_ changes.
Very approximately, imagine the pulse of light starting at the far mirror. The EM wave makes (say) 100 oscillations in 100 seconds (totally out of scale with the real experiment, but that's not important). If the length between the mirrors is constant, the 100 wave peaks will hit the close mirror in 100 seconds. But if the distance between mirrors changes (eg, due to a gravity wave compressing space) _during_ the 100-pulse emission, the last peaks will have less space to travel than the first peaks. This means that the close mirror will be hit by 100 peaks in, say, 90 seconds. So the frequency of the wave went from 1 Hz to 10/9=1.1Hz. The waveform was deformed (compressed), but its speed was constant. (Note that the effect happens _only_ if the space changes shape _during_ the pulse. If it changes, say, between two 100-oscillation pulses spaced apart, you'll still get the travel time difference, but not the frequency shift. LIGO uses continuous lasers, though.)
The LIGO can't actually measure the change because it's much smaller than in this example. So it sends the lasers in perpendicular directions, and reflects them back. Because gravity waves stretch space differently in each direction (except if their direction happens to exactly bisect the angle between the arms), a passing gravity wave will force the two beams to go slightly out of phase. The difference between the two beams is (barely) detectable for big waves.
"I think I am a fallen star. I should wish on myself."
Nothing quite so ridiculous. See:
http://en.wikipedia.org/wiki/Gravitational_wave#Effects_of_a_passing_gravitational_wave
Nerd rage is the funniest rage.
Which doesn't actually answer my question at all... It explains how space time would affect the observer, but not the light. As I understand it, it wouldn't affect the light at all, which is different from what you are saying.
Based on the Wikipedia article about the LIGO, it looks like my understanding that light would appear to change speed is correct. LIGO works on the theory that the two light beams down the different legs are normally out of phase, so there is no resulting wave when they are interfere at recombination. If one of the legs changes length, the light travels down it AT THE SAME SPEED AS BEFORE, so comes back slightly out of phase from what it should be, so you get some resulting light after recombination.
As the light DOES travel at the same speed, despite the gravity wave making the leg shorter or longer, that would mean that to US the light would appear to be travelling faster or slower than normal (by a minute amount. LIGO effectively measures how much the speed of light appears to change (as long as it doesn't change by a whole wavelength per time taken to travel the 75x4km).
General relativity predicts that gravity waves exist. Quantum mechanics predicts that all energetic wavelike phenomena can be thought of as made up of particles. So putting the two together suggests that gravity waves can be thought of as being made up of particles, and a good name for these particles is 'graviton'. But there are big problems with combining general relativity and quantum mechanics and there isn't a very good theory of gravitons. Physicists are still fairly confident that however the combination of these two theories works out, it'll probably have gravitons in it, but a lot of physicists probably wouldn't stake their lives on that because we just don't (yet) have a good physical model.
> gravitons are not proven to exist.
Neither gravity waves nor gravitons have been detected.
> If a gravitational wave has energy (as well as momentum and angular momentum) then what kind of energy is contained in the wave?
Gravitational energy.
> where does this energy come from?
Whatever generated the gravity wave in the first place. For example this pulsar is in a binary star system. The theory of general relativity predicts that it should lose energy through gravity waves. It does in fact seem to lose energy at the same rate as predicted. So the energy of the gravity waves comes from the energy that the binary star system used to have due to its rotation.
Actually, there are a lot of subtleties in discussions about the energy of gravity waves as it's tricky to pin down exactly where the energy is. But what I've said above is a good approximate start.
Doesn't it make you feel good to know that our freedoms are protected by politicans, lawyers and journalists.
Those two gravity events are outside the light cone. Other observers can see those events differently. For example, another observer can see that the gravity increased at the "receiver" before it was "generated". If you can send information that way, then someone else can send information backwards in time.
Who ordered that?
....The General Theory of Relativity says they should move at the speed of light......
If gravity is confined to the speed of light, the Sun should have lost its planets long ago. For example, simple Newtonian math tells us that the Sun and Jupiter "KNOW" about each other right NOW or in a very short amount amount of time, not 43 minutes later. the Earth and the Sun "feel" each other's gravity instantaneously, not with an eight minute delay. The sun and the center of our galaxy communicate by gravity without a many light year time delay. Otherwise, the galaxy would fly apart.
Matter and electromagnetic energy have a speed limit, but gravity either doesn't have a limit at all, or it is incredibly high. Gravity equations do not contain any time values, only mass and distance.
All theory is gray
Unfortunately, too many physicists aren't very familiar with the theory of information.
If one can state the one basic principle in that theory it is that to send or store information you have to spend energy, increasing the entropy in the universe. However, thermodynamics is a macroscopic phenomenon, at quantum dimensions all phenomena are reversible. In quantum dimensions one could say that time is bidirectional. Coincidentally or not, it seems that in quantum dimensions there is no limitation in speed, information can be transmitted instantly. And, what is more, there are experimental results confirming this.
I confess I'm not too confident on those proofs that information cannot be transfered faster than light. Until someone creates a theory that conciliates quantum mechanics with general relativity, I'm willing to believe anything. Maybe irreversible time is just an illusion created by the thermodynamic effects in our macroscopic brains...
Newtonian gravitational equations. In that you are correct. But reality is Non-Newtonian. All Jupiter knows is what the Sun was doing 43 minutes ago. All we know is what the Sun was like 8 minutes ago. Relativity puts a speed limit not just on light but on any type of information transfer. Unless you want to throw out causality. Go read your Einstein again. Or even any introductory physics text on relativity. They explain it quite well. Or here, for anyone who likes small words, try this.
...It is integral to the equations that model electromagnetic phenomenon.....True. It is however NOT integral to equations that describe gravity! There is no time value in them. This is false. Time most definitely appears in the Einstein field equations. The linearized version of them is almost identical to Maxwell's equations.
You are thinking of Newtonian gravity, in which gravity is instantaneous. This is not true of relativistic gravity, in which gravity propagates at the speed of light. Therefore, if someone turned on a gravity generator or suddenly removed a gravity shield, the effects of that should be instantaneous, or at least very fast over large distances. This is false, as can be proven from the Einstein field equations. See here. Until Mr. Roemer first measured the speed of light, it too was thought to be instantaneous. Maybe someday the speed of gravity will be measured. It has been, albeit indirectly. The 1993 Nobel Prize in physics was awarded for this work. It equals the speed of light, plus or minus a few percent experimental error. The sun and the center of our galaxy are in gravitational "touch" with each other NOW, not how they were thousands of light years ago. This too is false. See the paper I cited here.
....But reality is Non-Newtonian.....
For the electromagnetic interaction, relativity has been experimentally shown, but gravity is still pretty much a mystery. We know that matter somehow gives rise to an acceleration we call gravity. There is no way to tell the difference between this acceleration due to gravity and the acceleration of matter by some means. There is no experiment you could do inside, if you were sealed into a closed rocket, to tell whether your cabin was being accelerated through space at 32ft/sec/sec or if that cabin was simply parked on the launch pad on earth.
We also know by experiment, that the gravitational interaction is some 36 orders of magnitude smaller than the electric interaction. Two neutron stars or black holes in close proximity, could have way more influence on each other and the radiation emitted, by way of the electric interactions between them, than by gravity.
Sure, the theory of relativity asserts that gravity is also subject to the speed of light, but that part of relativity has never been experimentally verified. Newtonian mechanics mandate that gravity ITSELF not be in any way be impacted by time. Only the ACCELERATION produced by gravity has a time value. Since the planets and galaxies don't move relative to each other at anywhere close to the speed of light, relativity doesn't enter into the picture here.
All theory is gray
Seriously, what history books have you been reading? You've got parts right. Gravity is indistinguishable from acceleration if you attempt to treat an accelerating body as an inertial reference frame. Yes, an imaginary force does appear if you do the coordinate substitution in this case (see any number of textbook, or, if you enjoy comedy this).
But to assert that the gravitational portions of relativity have never been shown experimentally is ignoring history. I point you towards the precession of Mercury's orbit and the first observation of gravitational lensing in 1919 (See the Story of Eddington and the Eclipse, laid out in many places and quite humorously here. And that's just the simple ones. It's called General Relativity.
Now, let's look at some of your errors here:
1)"Two neutron stars or black holes in close proximity, could have way more influence on each other and the radiation emitted, by way of the electric interactions between them, than by gravity."
Ok, true, given certain things. Even true about the radiation. Except for we're not talking about EM radiation in this context, we're talking about gravitational radiation, which shows no response to EM forces.
2)"Newtonian mechanics mandate that gravity ITSELF not be in any way be impacted by time. Only the ACCELERATION produced by gravity has a time value. Since the planets and galaxies don't move relative to each other at anywhere close to the speed of light, relativity doesn't enter into the picture here."
I'm not even sure where to begin here. Newton is wrong. Sorry to say it. I teach his laws every day in class, and he's wrong. He got damn close but his basic assumptions about the nature of reality are off. Space and time are not static. The force of gravity is only varies with time as the location and the mass creating the space-time curving effect that we see as gravity varies. So, if the object moves (like, oh, I don't know, a binary black hole system) the force we notice will vary. It takes time for the information that the force has changed to reach us. We don't notice instantly. That would mean information has traveled faster than light. Which is not possible.
Oh, and by the way, relativity does enter into this as General Relativity is a description of gravity.
Seriously, go read the first couple of sections ofThe Elegant Universe. And then go read a text book.