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hissingnoise
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Gravitational waves: breakthrough discovery after two centuries of expectation
This is Earthshaking news . . . figuratively and literally?
https://www.theguardian.com/science/2016/feb/11/gravitationa...
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crystal grower
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Thanks for info, that's really amazing.
But I think the most cool thing about it is that A. Einstein has known it a century ago.
(Just curiosity: Has Einstein ever got wrong?)
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Eddygp
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Yes, he HAS!!!
there may be bugs in gfind
[ˌɛdidʒiˈpiː] IPA pronunciation for my Username |
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Marvin
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He included a constant to account for the universe being stationary. That constant is now being used to account for the universe accelerating. He
also refused to accept quantum theory. So not a perfect record.
I was really really hoping gravity waves would turn out to be false.
The maths behind GR is mind melting.
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annaandherdad
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These were relatively small black holes that combined, about 29 and 36 solar masses for the pair, plus or minus about 4 on each figure. It means
both black holes probably formed from the collapse of massive stars, perhaps in a binary star system, some time in the past. The event was a billion
light years away. It means that black hole coalescence is now observable via gravity waves within a sphere centered on the earth of about that
radius. The wave forms in Louisiana and Washington state differed in arrival time by 7 milliseconds, due to the 2000 km separation between the
detectors. This means that the waves came from the southern hemisphere. In the future if more than two detectors are active when a signal is
received, it will be possible to give more accurate data on the direction from which the signal came. The wave form is chirped, that is, the
frequency and amplitude increase at first, as the two black holes spiral in toward one another, and their orbital frequency increases. The
frequency of the gravitational wave is twice the orbital frequency. After the merger there is a ring-down as the two black holes radiate away their
quadrupole and higher moments and settle into a new and larger, stable rotating black hole. The system radiated a total gravitational energy of 3
solar masses, with a peak power of 200 solar masses (times c^2) per second. Various quantitative arguments exclude the possibility that the two
objects were a pair of stars (neutron or otherwise), or a black hole and a star; only two black holes fits the data.
Rather than correct the above, let me point out that the two detectors are separated by more than 2000km. The 2000km figure is the light travel
distance at 7ms. This is the distance between the detectors, times the cosine of the angle between the line joining the detectors, and the direction
of the incoming gravitational waves. This determines the direction of the incoming waves to lie on a cone of known opening angle, centered on the
line between the detectors. It is known that the cone points to the south because the signal was detected first in Louisiana. If three or more
detectors were to receive the signal, you would have overlapping cones that would select out a unique direction.
[Edited on 11-2-2016 by annaandherdad]
Any other SF Bay chemists?
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careysub
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Quote: Originally posted by Marvin  | He included a constant to account for the universe being stationary. That constant is now being used to account for the universe accelerating. He
also refused to accept quantum theory. So not a perfect record.
I was really really hoping gravity waves would turn out to be false.
The maths behind GR is mind melting. |
The Cosmological Constant is actually a component of the formal solution of GR, not an arbitrary addition - but its value is not set by the theory.
Einstein debated whether it was 'real' and changed his mind more than once. So his original formulation, with all of its details (including the CC)
appears to be correct in every aspect.
Yep, he could never adapt to quantum theory - even though he helped create it.
I am happy that GR is proving to be correct (just like I am happy that quantum theory as originally formulated also appears correct).
But we knew that gravitational waves were real already, the energy loss of black hole binaries already measured showed they were radiating
gravitational waves, exactly as GR predicts. The question was whether we could detect them.
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XeonTheMGPony
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https://www.youtube.com/watch?v=4UY5A3NJjls < Rated R for language
We're geting there, now we need to hurry up with warp drive!
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careysub
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One thing about gravitational waves I rarely see brought up is that gravitational waves can be extremely "bright", that is, have a lot of energy in
them; yet they are extremely difficult to detect. It is because of the ratio of field strength between gravity and electromagnetism: 10^-43!
Roughly speaking, a gigawatt of incident gravitational energy incident on a detector (or you) would generate a 10^-34 watt signal! This is why it was
doubted that they could ever be detected. Thank gods for coalescing black holes! Those suckers are the strongest source of radiation emissions in the
Universe.
How intense? About a billion times brighter (in the gravity spectrum) than the brightest quasar in the Universe (the blazar 3C 454.3) is in the
electromagnetic spectrum!
Its probably a good thing we cannot easily interact with their emissions, but if they weren't so incredibly intense we would never pick up any
gravitational waves.
3C 454.3 reached its all time recorded peak brightness of 13.4 magnitude (7 magnitudes dimmer than what the human eye can see) in June 2014. If we
could see a major black hole coalescence at the same distance (7.7 billion light years, over half the way back to the beginning of the Universe), it
would flash at about -10 magnitude (approaching the brightness of the full moon, which is -13 magnitude).
Check out this paper:
http://arxiv.org/pdf/1602.02872v1.pdf
[Edited on 12-2-2016 by careysub]
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phlogiston
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Your comparison helps a lot to gain some feeling for the effects relative to electromagnetic signals, thanks. But while full moon equivalent is indeed
astounding given the distance, it is not particularly bright in an absolute sense. A tiny LED flashlight at armslength easily outshines the entire
moon. This should be true for locally generated gravitational waves too.
Given the extreme distances between cosmic events and earth and the resulting 'dilution' of the signal with distance, I am still a little puzzled that
local signals of low absolute intensity are not far more easily detected.
I am wandering if perhaps the frequency comes into play here. Given their size, I suspect LIGO detecters are optimal for detecting waves with
kilometer wavelengths, wheres local events from small objects accelerating will generate much higher frequency waves. But LIGO has to be large because
the relative change in the length of the arms of the detector is still incredibly tiny. If the arms were smaller, the change in size would probably be
undetectable.
[Edited on 12-2-2016 by phlogiston]
-----
"If a rocket goes up, who cares where it comes down, that's not my concern said Wernher von Braun" - Tom Lehrer |
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Fulmen
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| Quote: Originally posted by careysub | | So his original formulation, with all of its details (including the CC) appears to be correct in every aspect |
As I understand it the original CC was introduced to fit the current accepted static universe as a rather ad-hoc addition. That would mean that he was
right, but for the wrong reason (which is pretty close to being wrong).
| Quote: | | But we knew that gravitational waves were real already, the energy loss of black hole binaries already measured showed they were radiating
gravitational waves, exactly as GR predicts |
There is a huge difference between measuring energy being lost and actually measuring the energy in the predicted form. Prior to this we didn't
know GW were real, although we were pretty confident.
We're not banging rocks together here. We know how to put a man back together.
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careysub
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| Quote: Originally posted by Fulmen | | Quote: Originally posted by careysub | | So his original formulation, with all of its details (including the CC) appears to be correct in every aspect |
As I understand it the original CC was introduced to fit the current accepted static universe as a rather ad-hoc addition. That would mean that he was
right, but for the wrong reason (which is pretty close to being wrong).
| Quote: | | But we knew that gravitational waves were real already, the energy loss of black hole binaries already measured showed they were radiating
gravitational waves, exactly as GR predicts |
There is a huge difference between measuring energy being lost and actually measuring the energy in the predicted form. Prior to this we didn't
know GW were real, although we were pretty confident.
|
The Cosmological Constant is a natural part of the most general form of the GR equations.
A very good analogy can be drawn to the constant that appears when performing an integration. The most general solution of an integral equation
contains a constant, which is arbitrary. Usually it gets ignored in practice, but it really is part of the solution (and is a reflection of the fact
that when you do differentiation constants disappear).
If you work an integration problem on a test or in homework, and this constant is not included you probably lose points.
Yes there is a big difference between knowing that GWs exist and actually detecting them (the first already generated a Nobel and the second will
generate another). But we did know they existed before direct detection.
An analogy with the neutrino can be drawn. We knew neutrinos existed before we detected them since we could detect the energy loss when they carried
energy away. But with GWs the case was much stronger since the observed loss exactly matched the predictions of the existing theory (unlike
neutrinos).
[Edited on 12-2-2016 by careysub]
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careysub
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| Quote: Originally posted by phlogiston | Your comparison helps a lot to gain some feeling for the effects relative to electromagnetic signals, thanks. But while full moon equivalent is indeed
astounding given the distance, it is not particularly bright in an absolute sense. A tiny LED flashlight at armslength easily outshines the entire
moon. This should be true for locally generated gravitational waves too.
Given the extreme distances between cosmic events and earth and the resulting 'dilution' of the signal with distance, I am still a little puzzled that
local signals of low absolute intensity are not far more easily detected.
I am wandering if perhaps the frequency comes into play here. Given their size, I suspect LIGO detecters are optimal for detecting waves with
kilometer wavelengths, wheres local events from small objects accelerating will generate much higher frequency waves. But LIGO has to be large because
the relative change in the length of the arms of the detector is still incredibly tiny. If the arms were smaller, the change in size would probably be
undetectable.
[Edited on 12-2-2016 by phlogiston] |
The frequency issue is part of it. Another part is how often these events occur.
If by "local" you mean the Local Group of galaxies for example (Milky Way, Andromeda, Triangulum, plus dwarf galaxies) then the volume of the whole
observable Universe is 100 billion times bigger. We are far more likely to see very distant events.
Note that the example I gave for GW brightness was assuming it was an extremely distant one. The one we detected is much closer.
I just calculated the actual brightness of this particular event, which was at 1.2 billion light years. The gravitational wave radiation field was 0.1
watts/per square meter! Sunlight at Earth's orbit (i.e. without any absorption) is 1400 watts/per square meter.
0.1 watt per square meter is about the illumination intensity of an iPhone LED light at one meter.
If the event were 10 million light years away the energy would have equaled solar illumination. If it has occurred in the Andromeda galaxy it would
have been 10 times brighter than the Sun. If it had occurred in the center of our galaxy it would have been 100,000 times brighter than the Sun.
Also I should point out the blazar I used as an example is so extremely bright (but still astronomically dimmer than the GW burst) because it has an
emission jet pointed directly at us. It is not omnidirectional, unlike the GW event, making the brightness of the black hole merger even more
incredible.
[Edited on 12-2-2016 by careysub]
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Fulmen
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Careysub: The point was that he introduced a value specifically to achieve a static universe, rather than predicting a dynamic one. As I understand it
Einstein abandoned the idea in 1929 after Hubble's discovery, calling it the biggest blunder of his life. If he's willing to call it a blunder I have
no problem taking his word for it ;-)
We're not banging rocks together here. We know how to put a man back together.
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careysub
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| Quote: Originally posted by Fulmen | | Careysub: The point was that he introduced a value specifically to achieve a static universe, rather than predicting a dynamic one. As I understand it
Einstein abandoned the idea in 1929 after Hubble's discovery, calling it the biggest blunder of his life. If he's willing to call it a blunder I have
no problem taking his word for it ;-) |
My point is that there is a subtlety about what the "error" actually was.
The most general form of GR includes a CC naturally, as I said it is not ad hoc (although any particular value is).
Einstein set a value for this to make a static universe, which he believed in (and was mistaken).
With the discovery of the Hubble Law he dropped the CC, believing that including it was in error, since the Universe was not static as he believed.
But he was in error in dropping the CC, since it really is part of the true solution of GR, just not in the way he imagined (no one expected cosmic
acceleration, AFAIK). The Universe is stranger than even Einstein imagined.
You might say he was in error twice - once for believing in a static Universe, and once in dropping the CC.
His original form of the equations was the correct one however.
[Edited on 12-2-2016 by careysub]
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hissingnoise
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How the ancient merging of massive bodies produced this "tsunami" in the space-time continuum is a question dogging me since the detection was
announced.
Was it caused by the act of two massive bodies colliding or by the loss of mass in the huge release of energy, or was it a combination of both those
things?
Fuck!
http://thinkforyourself.ie/2010/02/21/wha%E2%80%99-is-the-st...
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careysub
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| Quote: Originally posted by hissingnoise | How the ancient merging of massive bodies produced this "tsunami" in the space-time continuum is a question dogging me since the detection was
announced.
Was it caused by the act of two massive bodies colliding or by the loss of mass in the huge release of energy, or was it a combination of both those
things?
|
It is the natural result of two extremely massive, maximally dense bodies in the process of merging which converts ~10% of their mass into
gravitational wave energy. The merger involved them spinning around each other at a large fraction of the speed of light.
Similar to the way oscillating charged particle naturally emit energy as electromagnetic waves.
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Reference Information
Observation of Gravitational Waves from a Binary Black Hole Merger
B. P. Abbott et al.
PHYSICAL REVIEW LETTERS
12 FEBRUARY 2016
DOI: 10.1103/PhysRevLett.116.061102
Abstract
On September 14, 2015 at 09:50:45 UTC the two detectors of the Laser Interferometer Gravitational-Wave Observatory simultaneously observed a transient
gravitational-wave signal. The signal sweeps upwards in frequency from 35 to 250 Hz with a peak gravitational-wave strain of 1.0 × 10−21. It
matches the waveform predicted by general relativity for the inspiral and merger of a pair of black holes and the ringdown of the resulting single
black hole. The signal was observed with a matched-filter signal-to-noise ratio of 24 and a false alarm rate estimated to be less than 1 event per 203
000 years, equivalent to a significance greater than5.1σ.
Thesourceliesataluminositydistanceof410þ160 Mpccorrespondingtoaredshiftz1⁄40.09þ0.03. −180 −0.04
In the source frame, the initial black hole masses are 36þ5M and 29þ4M , and the final black hole mass is −4⊙ −4⊙ 62þ4M , with 3.0þ0.5M c2
radiated in gravitational waves. All uncertainties define 90% credible intervals. −4 ⊙ −0.5 ⊙
These observations demonstrate the existence of binary stellar-mass black hole systems. This is the first direct detection of gravitational waves and
the first observation of a binary black hole merger.
Attachment: Observation of Gravitational Waves from a Binary Black Hole Merger -B. P. Abbott.pdf (914kB)
This file has been downloaded 513 times
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hissingnoise
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“It’s such a huge relief to get to share this with the world,” Lisa Barsotti, a principle research scientist at LIGO said.
“I felt like time between September and February was just being stretched out.”
[Edited on 12-2-2016 by hissingnoise]
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Fulmen
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I agree that it is subtle. But there is still good reason to call it a blunder. The original GR predicted a non-static universe, while the current
assumption was the opposite. But this wasn't supported by any evidence, just the lack of it. And while a CC would fix that it must have been a
precarious and delicate balancing act.
Considering the incredible "act of faith" it took Einstein to follow logic past common sense makes it that much worse.
That later findings has reintroduced the CC doesn't really vindicate his earliest use of it, being right for the wrong reasons isn't far from being
wrong.
At least that's how I understand it. Not that I hold it against him, it just underlines just how brief his time was. Few has ever made a similar
contribution to science, but the time period this was limited to is just as remarkable. Within a decade he had completely revolutionized science, then
nothing. I don't know if there was more for him to do, it takes time before enough observations accumulate to produce new discoveries of this
magnitude. But his failure to accept quantum mechanics pretty much eliminated the possibility of him producing more of real significance.
Perhaps the future will vindicate him on that point as well, perhaps we will in time "solve" QM into something more predictable. But does that make
him right? Considering the available evidence, did he really have any reason to doubt QM?
We're not banging rocks together here. We know how to put a man back together.
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wg48
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| Quote: Originally posted by careysub | One thing about gravitational waves I rarely see brought up is that gravitational waves can be extremely "bright", that is, have a lot of energy in
them; yet they are extremely difficult to detect. It is because of the ratio of field strength between gravity and electromagnetism: 10^-43!
Roughly speaking, a gigawatt of incident gravitational energy incident on a detector (or you) would generate a 10^-34 watt signal! This is why it was
doubted that they could ever be detected. Thank gods for coalescing black holes! Those suckers are the strongest source of radiation emissions in the
Universe.
snip
[Edited on 12-2-2016 by careysub] |
That,s not a fair calculation. The 10^43 ratio is the ratio between the gravity force between subatomic particles and the electromagnetic force
between them. For example electrons or protons. So unless the detector is a single hydrogen atom.. Unfortunately a Weber type detector (cryo bar) of
10^43 protons or neutrons is a very very large bar . One Weber detector used 1000kg mass which is only about 10^27 neutrons and protons so its still
not easy.
But don't forget that the gravitation constant was determined by measuring the gravity force between lead balls way back in the 1700s.
Perhaps a big part of the detection problem is that the waves effect all masses the same. So you can not use a simple torsion balance with a lead
ball.
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careysub
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| Quote: Originally posted by wg48 | | Quote: Originally posted by careysub | One thing about gravitational waves I rarely see brought up is that gravitational waves can be extremely "bright", that is, have a lot of energy in
them; yet they are extremely difficult to detect. It is because of the ratio of field strength between gravity and electromagnetism: 10^-43!
Roughly speaking, a gigawatt of incident gravitational energy incident on a detector (or you) would generate a 10^-34 watt signal! This is why it was
doubted that they could ever be detected. Thank gods for coalescing black holes! Those suckers are the strongest source of radiation emissions in the
Universe.
snip
[Edited on 12-2-2016 by careysub] |
That,s not a fair calculation. The 10^43 ratio is the ratio between the gravity force between subatomic particles and the electromagnetic force
between them. For example electrons or protons. So unless the detector is a single hydrogen atom.. Unfortunately a Weber type detector (cryo bar) of
10^43 protons or neutrons is a very very large bar . One Weber detector used 1000kg mass which is only about 10^27 neutrons and protons so its still
not easy.
But don't forget that the gravitation constant was determined by measuring the gravity force between lead balls way back in the 1700s.
Perhaps a big part of the detection problem is that the waves effect all masses the same. So you can not use a simple torsion balance with a lead
ball.
|
I said "roughly"... to give an indication of how difficult the detection is.
In fact gravitational wave detectors do not use the coupling between gravity and electromagnetism at all, but instead detect the distortion of space
directly. This was true of Weber detector also. The space distortion caused by the major detected event was on the order of 10^-21.
If the coupling were high (in the vicinity of 1) you could hope to capture all, or most of the energy in the wave, which electromagnetic detectors
commonly approximate.
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wg48
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| Quote: Originally posted by careysub |
In fact gravitational wave detectors do not use the coupling between gravity and electromagnetism at all, but instead detect the distortion of space
directly. This was true of Weber detector also. The space distortion caused by the major detected event was on the order of 10^-21.
snip . |
Can you explain what the 10^-21 space distortion means.
ie what can be measured or compared to detect the distortion?
My limited understanding of the Webber detector is it compares the force on side of the mass (loosely speaking) if they are not the same the resultant
force distorts the mass causing it to ring which is then detected.
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j_sum1
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A couple of curious questions. Although these might be betyer suited to Randal Munro of whatif fame...
How close to the converging black holes could you get without the massive energy expulsion doing indescribably destructive things to your body?
And how close woukd you need to get in order to observe it with your normal senses?
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Marvin
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I think it is a fair comparison, if electromagnetism was weaker we'd be using kilograms of ions to detect it. We'd also have to be stuck together by
a totally different physics, of course.
Another issue accessible to chemists here is that gravity wave coupling is quadrupolar.
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careysub
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BTW, the usual ratio quoted for the strength of fundamental forces is
Strong Force = 1
Electromagnetism = 1/137
Weak = 10^-6
Gravity = 6*10^-39
Giving a direct force strength ratio of 8*10^-37.
My reason for using 10^-43 is this page at Caltech discussing an oscillating charge mass:
http://ned.ipac.caltech.edu/level5/ESSAYS/Boughn/boughn.html
a system, that emits 10^43 times more energy in electromagnetic waves compared to gravity waves. I was postulating that you used such a system as a
gravity energy collector, seeking to collect the excitation of gravity into electromagnetism.
I was thinking just this morning about how large a space distortion effect it might take to be dangerous. Odd thing to imagine, very Star Trekkie. One
imagines that since this detection involved a distortion of 10^-21 that an event 10^10 times closer (inverse square law) would produce one of 10%
which ought to be impressive. That would 0.2 light years - or pretty close for such a (clearly infrequent) event.
The Weber Bar detector relied on space dimensional distortion just like LIGO. The passing of a wave would cause the cylinder to change in length by
10^-16 which would cause it vibrate at 1660 Hz.
[Edited on 13-2-2016 by careysub]
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