Exciting news: Gravitational Waves detected!

We want to share our excitement about the first-ever direct detection of gravitational waves! The event happened right before the beginning of the first observing run of the advanced LIGO detectors, on 14 September 2015. The waves were generated as two black holes merged into a single black hole about 1.3 billion light years from Earth. In astronomy units this is 410 Mpc, approximately 10% of the way across the visible Universe!

Just as exciting: this is also the first-ever observation of binary black holes. In fact, since black holes are black, and emit no light or electromagnetic radiation, this is the only way we can see them.

Did Einstein@Home play any role in this? No, it didn’t. The signal in the instrument lasted only about 1/4 of a second. It’s not a continuous-wave signal like the type that Einstein@Home has been searching for. But since the observing run ended in mid January, we have been preparing the data to start a new low-frequency all-sky search for continuous gravitational waves. We are now starting to run this on Einstein@Home, so please sign up your computers and disable their sleep mode! In the next months we will extend the frequency range of the continuous waves all-sky searches, target interesting point sources and we are also gearing up to perform broader surveys for binary black hole mergers.

Bruce Allen
Director, Einstein@Home

Comments

Mumak
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RE: Using GPUs would be

Quote:
Using GPUs would be nice and faster, also if it takes alil longer prolong the report date may be a good idea... just an idea...

Yes, but not all calculations can be (effectively) ported to GPGPU.
I'm sure there will be a GPU version if it's feasible.

-----

Alexander W. Janssen
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RE: But since the observing

Quote:
But since the observing run ended in mid January, we have been preparing the data to start a new low-frequency all-sky search for continuous gravitational waves.

Are low-frequency continuous gravitational waves a sign for pending binary black hole mergers, contrary to the "chirp" encountered at the actual event?

Alex.

"I am tired of all this sort of thing called science here... We have spent
millions in that sort of thing for the last few years, and it is time it
should be stopped."
-- Simon Cameron, U.S. Senator, on the Smithsonian Institute, 1901.

AgentB
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RE: Apart from the actual

Quote:
Apart from the actual detection of the GW, it is exiting how well the data can already be interpreted.

It is, and so many different things such as in first post by Bruce first-ever observation of binary black holes its the first "heavy" black hole

Quote:
GW150914 demonstrates the existence of stellar-mass black holes more massive than ≃25 solar masses, and establishes that binary black holes can form in nature and merge within a Hubble time.

and

Quote:
our observations constrain the Compton wavelength of the graviton to be >10^13 km

edit: I don't even know what a Hubble time was until i looked it up, and it is ~14 billion years.

capnrob97
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If BeatleJuice goes supernova

If BeatleJuice goes supernova in our lifetime, would we be able to detect gravitational waves from that event?

AgentB
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RE: If BeatleJuice goes

Quote:
If BeatleJuice goes supernova in our lifetime, would we be able to detect gravitational waves from that event?

Advanced LIGO i seem to recall reading should be sensitive to detect a core collapse supernova anywhere in our galaxy, but there is a lot of unknowns i guess. We'll be able to detect Betelgeuse our eyes! but i guess LIGO will hear it a short time before we see it.

Hopefully we'll get one soon.

Gary Charpentier
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RE: RE: If BeatleJuice

Quote:
Quote:
If BeatleJuice goes supernova in our lifetime, would we be able to detect gravitational waves from that event?

Advanced LIGO i seem to recall reading should be sensitive to detect a core collapse supernova anywhere in our galaxy, but there is a lot of unknowns i guess. We'll be able to detect Betelgeuse our eyes! but i guess LIGO will hear it a short time before we see it.

Hopefully we'll get one soon.


Not too close or the high energy radiation will fry us.

Tiers Jean-Francois
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Fantastic new even if E(a)H

Fantastic new even if E(a)H didn't contribute !
Congratulations to LIGO teams for having made it possible. I did not expect to live old enough to see this.

JF

AgentB
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RE: RE: Advanced LIGO i

Quote:
Quote:

Advanced LIGO i seem to recall reading should be sensitive to detect a core collapse supernova anywhere in our galaxy, but there is a lot of unknowns i guess. We'll be able to detect Betelgeuse our eyes! but i guess LIGO will hear it a short time before we see it.

Hopefully we'll get one soon.


Not too close or the high energy radiation will fry us.

The estimated safe distance is 50-100 light years, and Betelgeuse is over 600 ly away, this wiki article has some interesting details.

There is of course the possibility of a two faint white dwarfs or neutron stars in a nearby close binary that we can't see... spiralling away, and E@H discovers them via gravity wave search! I can see the headlines now "E@H predict a mass extinction event"

capnrob97
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Regarding supernovae, found

Regarding supernovae, found this while googling

https://labcit.ligo.caltech.edu/~ajw/bursts/burstsim.html

The search for gravitational wave bursts with LIGO (from, eg, supernovae, GRB engines, binary black hole mergers and ringdowns, and other very energetic phenomena) is made complicated by the fact that the burst waveforms are not known, or at best, only crudely modeled. The burst search algorithms employed by LIGO aim to detect bursts with a broad range of waveforms; the main tools are to detect non-stationary deviations from typical detector noise, and requiring coincidence between detectors in time, amplitude, frequency band, and waveform.
We are guided by supernova GW burst simulations from Zwerger & Muller (see below), which suggest that such bursts would have durations ranging from a fraction of a ms to 10's or 100's of ms, and with power in the LIGO frequency band.

astro-marwil
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Hallo! Also my

Hallo!
Also my congratulations to this extraordinary results and event!

I´ve just some small questions to the results:
I red the paper from B.P. Abbott et. al.. On page 2, fig. 1 are shown the reconstructed signals from both detectors. Easily one can see the differences in the reconstructed waveforms between both detectors, especially during the time of merging of the two BHs. (I´d plot this from the published data.) According to Fig. 3a in the paper, the detectors are twisted by about 20° only.
Now my questions: Does this differences in amplitude result from the polarisation of the GW? Or is this only due to the increasing phaseshift with increasing frequency and varying amplitude between both detectors due to their long geometrical separation? In the list of estimated source parameters is nothing said about polarisation. Presumably it needs at minimum one more widely separated detector to dig out the polarisation of the wave + or x from all the data.

Kind regards and happy crunching
Martin

Bernd Machenschalk
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I love that New Yorker

I love that New Yorker article.

BM

BM

Gary Charpentier
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Time for a question: A hot

Time for a question:
A hot object radiates away energy with photons

Does a mass radiate away energy with gravitons? If so, what is left?

Yes, I know gravity is so weak this will take a long time.

capnrob97
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Off topic and old news I

Off topic and old news I guess for most here, but if a science geek like me, a good watch.

Watching it now on the big TV via Apple AirPlay, about the big science search for the Higgs.

Also have donated money for years to IAS in Princeton, who one of the guys in the movie is from.

Higgs and GW in my lifetime, wow.

https://www.youtube.com/watch?v=_OfkNqUyZjw

archae86
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RE: I love that New Yorker

Quote:

I love that New Yorker article.

BM


Fabulous. They obviously had someone assigned to article a fair number of weeks ago, and got lots of the players to talk to them.

I thought they handled the delicate problem of what to say about Joseph Weber well.

MAGIC Quantum Mechanic
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RE: I love that New Yorker

Quote:

I love that New Yorker article.

BM

NICE.....that is the best the New Yorker has ever done.

-NW of Hanford since 1958

tullio
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RE: Time for a question: A

Quote:

Time for a question:
A hot object radiates away energy with photons

Does a mass radiate away energy with gravitons? If so, what is left?

Yes, I know gravity is so weak this will take a long time.


The 2 black holes merging have radiated away 3 solar masses, because the mass of the final black hole is 3 solar masses less than the sum of the 2 masses.
Tullio

Gary Charpentier
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RE: RE: Time for a

Quote:
Quote:

Time for a question:
A hot object radiates away energy with photons

Does a mass radiate away energy with gravitons? If so, what is left?

Yes, I know gravity is so weak this will take a long time.


The 2 black holes merging have radiated away 3 solar masses, because the mass of the final black hole is 3 solar masses less than the sum of the 2 masses.
Tullio


Yes, but that is the merger of two black holes. Does the 1 kilo mass sitting on the shelf also radiate away?

PhiAlpha
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Congratulations, very

Congratulations, very exciting news! I can't wait to crunch units from the new run.

"Everything should be made as simple as possible, but not simpler." A. Einstein

Maximilian Mieth
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RE: RE: RE: Time for a

Quote:
Quote:
Quote:

Time for a question:
A hot object radiates away energy with photons

Does a mass radiate away energy with gravitons? If so, what is left?

Yes, I know gravity is so weak this will take a long time.


The 2 black holes merging have radiated away 3 solar masses, because the mass of the final black hole is 3 solar masses less than the sum of the 2 masses.
Tullio

Yes, but that is the merger of two black holes. Does the 1 kilo mass sitting on the shelf also radiate away?

As far as I understand, a part of the masses of the black holes is being transformed into energy during the merger and send out in the form of gravitational waves. So as long as your kilo on the shelf is not being thrown into a black hole or so, nothing will happen to it. As far as I remember from physics at school, relativistic effects only really occur under such extreme circumstances (actually they occur all the time and everywhere, but they are very, very small).

tullio
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AFAIK a mass sitting still

AFAIK a mass sitting still does not emit gravitational waves. It does only if it is accelerated. The same thing happens to charged particles, they emit radiation when accelerated like electrons in a synchrotron which emit radiation (synchrotron light).
Tullio

Mike Hewson
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Hmmmm ... well a crucial idea

Hmmmm ... well a crucial idea is that the gravitational field has energy and as such has a mass equivalent which in turn may gravitate* ! This is, as they say in the classics, non-linear behaviour. Which is why it is rather difficult to solve the Einstein GR equations in such high fields ( out here in the quiet boondocks it is close to linear ). The 29 + 36 = 65 != 62 are the effective masses as experienced by a distant observer. That is : if you had a test mass tooling around in the far distance from the various black holes ( 2 before and 1 after ) they would act on that test mass with those amounts. So was it mass equivalent or field energy that radiated away ? Same, same but different .... :-)

Cheers, Mike.

* and the field energy associated with that has a mass equivalent etc ...... yes it is recursive, but it converges to a finite amount ( at least for us mugs on the outside of the hole ).

I have made this letter longer than usual because I lack the time to make it shorter ...

... and my other CPU is a Ryzen 5950X :-) Blaise Pascal

tullio
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RE: AFAIK a mass sitting

Quote:
AFAIK a mass sitting still does not emit gravitational waves. It does only if it is accelerated. The same thing happens to charged particles, they emit radiation when accelerated like electrons in a synchrotron which emit radiation (synchrotron light).
Tullio


I've checked on a book by Peter G.Bergmann, "The riddle of gravitation" whose Italian edition I have edited. He says:
Gravitational waves are emitted by massive bodies subjected to an acceleration and travel at the speed of light.
Prof.Bergmann was a coworker of Einstein.
Tullio

Chris S
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Ummm I don't see gravity

Ummm I don't see gravity working like that. If I hang a 2kg weight from a piece of string it will possess potential energy because of its mass and it has weight because of the effects of gravity. If that string is cut it will fall to the earth under the "attraction" of what we call gravity. Water always finds the lowest point it can flow to, because it is in effect being attracted to the core of the earth by gravity, and only the physical construction of the earth stops it.

I don't think it matters what gravity consists of, whether it be waves, sine waves, straight or curved lines, or star trek tractor beams. It has been measured at the earths surface to have an average acceleration value of 32.1 f/s or 9.8 m/s. That is the value of the attractive force at that point. Gravity exists, why does it matter so much what it consists of??

Waiting for Godot & salvation :-)

Why do doctors have to practice?
You'd think they'd have got it right by now

tullio
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Gravity is the weakest of the

Gravity is the weakest of the four basic interactions but it is the more important at great distances.It manages planets, stars, and galaxies and also dark matter, as is evidenced by the deflection on light beams coming from distant objects if there is a massive object, also made by darkmatter, in the foreground. This is called the Einstein lens and has been demonstrated by observing several images of the same supernova arriving at different times depending on the light paths. The gravitational constant G, also called the Newton constant, has been measured in different ways and there is an international effort to measure it with greater accuracy. The Lisa Pathfinder spacecraft, now orbiting the L1 Lagrange point, might be used to measure it, according to ESA.
Tullio

Chris S
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Yes I accept all that, and it

Yes I accept all that, and it is known as you say that gravity can bend light.

Now lets look at centrifugal force for instance. In the cases of the fairground rides where people are glued to the wall of a rotating barrel, or the wall of death motorcycle, they can be thought of as the opposite of gravity. i.e it is a force which repels not attracts from the centre of rotation. Does centrifugal force travel in waves?

Look at two bar magnets where the like poles repel and unlike poles attract, does the magnetism force travel in waves?

They are all similar forces that have an effect on other objects.

Waiting for Godot & salvation :-)

Why do doctors have to practice?
You'd think they'd have got it right by now

tullio
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I think there is a difference

I think there is a difference between static fields and dynamic fields. A massive object has a static field and can bend space-time. This action was considered a force in Newton times, now in Einstein times one does not speak of forces. Naturally it is easier to think of forces than of a warped space-time and we all do it in practice, even if it no longer correct.
A dynamic field results from an accelerated mass. It is also pictured as a force, and this gives the familiar pictures of centrifugal forces. Far easier than speaking of gravitational waves emitted by the accelerated mass. A gravity force is the equivalent of a constant accelerated system, as in the gedanken experiment. If an elevator is accelerated upwards in a gravity free space, the person in the elevator thinks he is subjected to a gravity force. I am very interested in the Lisa Pathfinder experiment where two masses will be freed in a gravity free space at the Lagrange point. Presumably they will attract each other and this will give the opportunity to measure the G constant of gravitational force.
A steady electric current running through a wire produces a magnetic field but it is a static field. Only if the current is alternating, at the frequency of 50 or 60 Hz, it will produce an electromagnetic wave of that frequency. Once at the University I put my finger to the input of an oscilloscope set at 50 Hz and I saw a sine wave on its display. My body was acting as a collector of electromagnetic waves. The magnetic field of a common magnet is a static field.
Tullio

Tiers Jean-Francois
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Hello all. What an exciting

Hello all.
What an exciting topic !

I have just one question. I suppose I should remember what was told me when at the University, a long, very long time ago. But I forgot : you know the life of a student : girlfriends, guitar parties "killing" Cohen, Dylan and others and, sometimes, courses :=)

I know that nothing is still in our universe, from sub-atomic particles to galaxies, an even the limits of our universe.
But if we consider the attraction of two bodies, the classical physics do not mention acceleration. Just a question of mass and distance, but not motion.
Do GW can/must exist there ?

Black holes merging is an event, but only a temporalily event. I guess it gives an exhaust of GW we can fortunately observe, even if it is not easy. But the rest of the time, GW should exist.

So, be kind in telling me if accelaration is from the theory ABSOLUTELY necessary for the existence of GW, even if they are so weak that we have no chance to detect them ?

Cheers and thanks in advance for your infos.
JF

tullio
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I have cited a book by Peter

I have cited a book by Peter G. Bergmann. He says that GWs are produced by an accelerated mass. If a mass travels at constant speed,as in special relativity, it will not produce GW. A static mass neither.
Tullio

Tiers Jean-Francois
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Thanks a lot Tullio; Now,

Thanks a lot Tullio;
Now, things are more clear in my head.
JF

Tiers Jean-Francois
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Additional... Tullio,

Additional...

Tullio, thanks for having recalled me some basics !
I can hardly be foregiven to have forgot them.
I take an example you gave in a previous post that I should have read with more attention : statics never create waves (electromagnetics for instance).
Thanks again.
JF

Jim1348
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I am not sure it is quite so

I am not sure it is quite so simple. Einstein famously noted that to the isolated observer, an acceleration force on a mass was indistinguishable from the force of gravity (I will use "force" here, since I think it is quite appropriate). So if you conclude that an acceleration produces gravity waves, while the force of gravity doesn't, that must have to do with the reference frame of the observer. And so presumably the force of gravity produces gravity waves also, just in a different reference frame. Correct?

Maximilian Mieth
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RE: AFAIK a mass sitting

Quote:
AFAIK a mass sitting still does not emit gravitational waves. It does only if it is accelerated. The same thing happens to charged particles, they emit radiation when accelerated like electrons in a synchrotron which emit radiation (synchrotron light).
Tullio

Isn't everything in our universe constantly accalerated? The mass may seem to sit still in the shelf, but for sure it is accelarated relatively to any other frame of reference. Hence, it should emit gravitational waves.

edit: Didn't read the previous post. It think we try to make the same point.

tullio
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According to Galileo a body

According to Galileo a body traveling at constant speed will maintain this constant speed, hence it will not accelerate. To accelerate it, you must use a force but even the most powerful force will not accelerate it to the speed of light, which is a limit. The mass of the Earth will exert a force on a body which is proportional to the product of the Earth's mass with its mass but, if you exert a force on a body it will react to the force with the same intensity because the mass is also inertial, that is it resists to be accelerated. So bodies of different masses undergo the same acceleration in the Earth's field and fall at the same rate. This was proven by Galileo using the Pisa Leaning Tower and bodies of different mass.
The equivalence of inertial mass and gravitational mass is the basis of General Relativity.
Tullio

Maximilian Mieth
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RE: According to Galileo a

Quote:
According to Galileo a body traveling at constant speed will maintain this constant speed, hence it will not accelerate. To accelerate it, you must use a force but even the most powerful force will not accelerate it to the speed of light, which is a limit. The mass of the Earth will exert a force on a body which is proportional to the product of the Earth's mass with its mass but, if you exert a force on a body it will react to the force with the same intensity because the mass is also inertial, that is it resists to be accelerated. So bodies of different masses undergo the same acceleration in the Earth's field and fall at the same rate. This was proven by Galileo using the Pisa Leaning Tower and bodies of different mass.
The equivalence of inertial mass and gravitational mass is the basis of General Relativity.
Tullio


Well, the kilo (together with the shelf and the earth) is rotating around the sun. If I remember correctly this is a kind of acceleration because the vector of the speed is constantly changing. So the kilo (together with the shelf and the earth) should emit gravitational waves.

astro-marwil
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Hallo Maximilian! You´r

Hallo Maximilian!
You´r right. The earth on its way around the sun emits about 200W GW-energy. This ist taken from it´s kinetetic - moving - energy. So our earth is inspiralling onto the sun, but soooo slooowly, that the sun has died long before our earth will reach her.

Kind regards and happy crunching
Martin

rbpeake
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RE: Hallo

Quote:
Hallo Maximilian!
You´r right. The earth on its way around the sun emits about 200W GW-energy.
Martin


Amazing, because 200W is so minimal! No wonder gravity waves are hard to detect!

Gary Charpentier
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RE: AFAIK a mass sitting

Quote:
AFAIK a mass sitting still does not emit gravitational waves. It does only if it is accelerated. The same thing happens to charged particles, they emit radiation when accelerated like electrons in a synchrotron which emit radiation (synchrotron light).
Tullio


Ah, acceleration. So our mass sitting on a shelf, which we can't tell if it is moving as there is no absolute frame, does not radiate unless it changes velocity. (Of course one on a shelf on earth does radiate as it does accelerate to stay on the shelf.)

However as soon as we have two masses in the universe they will fall towards each other and thus accelerate. Correct? Would this radiated energy equal the gain in kinetic energy?

Mike Hewson
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It has to be a particular

It has to be a particular pattern of acceleration to emit waves : non-axisymmetric. This is code for (a) not a simple injection of extra mass (b) not a simple displacement. The technical word is quadripolar or higher mode. For example if a star expanded and contracted in radius only, but evenly all over it's surface, then no GW's arise. It is conservation of mass/energy that prevents a monopolar pattern, and conservation of momentum that prevents a bipolar pattern.

In the case of the Earth going around the Sun think of it like a very uneven barbell. The axis of rotation is close to the heavier end of the barbell and as the Earth progresses around the Sun the motion is 'non-axially symmetric'.

{ Now IF the axis of rotation was the line joining the centers of the Earth and Sun ( which it isn't ) then that would be 'axially symmetric' and hence produce no GW's }

The other example of special interest at E@H is 'mountains' on neutron stars. These low bumps - but containing alot of mass - sit out from the general spherical shape. So it is like an unevenly balanced spin cycle in the clothes washer, but we can't stop it to distribute the wet clothes more evenly around the bowl ! This will radiate GW's and we hope one day to detect that.

Cheers, Mike.

( edit ) In generality ( very very Back Of The Envelope here ! ) one ought consider a given system with respect to G/c^2 ie. Newton's gravitational constant divided by light speed squared. In MKS units that is 6.67 * 10^[-11]/(3 * 10^[8])^2 ~ 10^[-27]. So you need objects of order 10^[30] kg ( the Sun ) and/or velocities of the order of light speed to get a decent measurable effect as well as moving in the above pattern.

You will note that as c -> infinity ( Newton's instantaneous transmission of gravity ) then G/c^2 -> zero ie. no waves. If gravity was stronger ( increase G ) or the speed of light was less ( decrease c ) then we'd see these GW's more routinely without the need for the really fancy gadgets. On useful way of thinking of any field ( gravitational or other ) is for the transmission of effects, rather like the legendary cheque being 'in the mail' where for a short time at least neither sender nor receiver has actual custody of the money. With those fast moving black holes there was a lot of funds in transit .... :-)

I have made this letter longer than usual because I lack the time to make it shorter ...

... and my other CPU is a Ryzen 5950X :-) Blaise Pascal

Chris S
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Well Mike, a lot of that was

Well Mike, a lot of that was I'm afraid above my head, and I'm happy to admit it, but these Neutron star mountains is a new one on me, and sound quite interesting. It was quite clever to have detected the bumps in the first place.

This fictional kg mass sitting on a shelf possesses potential energy due to gravity. It stays on the shelf because the shelf imposes an equal upward force of 1kg. The shelf itself is probably screwed to a wall and is held there by the shear force on the screws being stronger than the downward thrust of the shelf and the kg mass. If the shelf were flimsy or the screws too small then it would collapse and the whole lot would fall to the floor under the attraction of the earth's Gravity. Whether the kg mass would emit its own GW on its journey to the flow I am not clear about.

Waiting for Godot & salvation :-)

Why do doctors have to practice?
You'd think they'd have got it right by now

Gary Roberts
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I remember reading a long

I remember reading a long time ago about the possibility of 'millimeter mountains' on neutron stars. We might perhaps know if they exist or not when we, here at E@H, detect what we are looking for - continuous GW. I imagine such 'mountains' would be a possible source. Without detecting the GW, how would you ever be able to 'see' them or even be sure they existed?

So, keep searching, even if you don't understand why :-).

Cheers,
Gary.

Mike Hewson
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@Chris : think => favourable

@Chris : think => favourable geometry + big + fast = GW emission.

@Gary : touche ! How do we know we have heard the sound of the unknown bird ? Various answers come to my mind, but I like best the one Bikeman pointed out several years ago : if it is real and in the distant sky then the motion of the Earth around it's own axis and travelling around the Sun will consistently modulate the received signal. That is the equivalent to cocking your head this way and that away while listening for bird calls when walking through the bush. This is why we yearn for un-interrupted data collection at the interferometers, plus 'long integration times' which is our role here.

Cheers, Mike.

I have made this letter longer than usual because I lack the time to make it shorter ...

... and my other CPU is a Ryzen 5950X :-) Blaise Pascal

Gary Charpentier
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Mike, I think you have mixed

Mike, I think you have mixed up there and given the answer for what we could detect and not the field itself.

Consider an electromagnetic field. We get radio from frequencies > 0. However a frequency of zero also changes the field, but only when it is established or ends. Mathematically it is a translation. So just like when you put AC into an antenna and transmit a signal, if you put DC into an antenna you also transmit a signal (permanent magnet). Of course if the remote observer doesn't happen to catch it when it starts, he won't know it is there as there is no absolute frame he can compare to. Kind of like us here on earth, we can only tell which way Earth's pole is, we don't know where the galactic pole is until we get our compass needle away from earth.

The same applies to the gravitational field. If you create a mass you have warped space time. Half of a square wave which propagates out at c. Unless the distant observer was watching at the time you created it he won't know because there is no absolute frame to refer to.

Thinking about this a bit more, a single mass moving at a constant velocity in the universe should warp space time, but there is no frame of reference to measure it against, except itself. To perform a measurement (real not thought) you have to introduce a second mass and as soon as you do the two masses will accelerate towards each other.

Perhaps the result here is that all frames of reference in the real world are accelerating frames of reference and there is no rest frame. Actually that seems to agree well with my understanding of relativity.

Now as G is such a dang small constant, yes it takes huge masses being offset to launch a wave that will have enough power when it gets here to earth to be above the noise floor. But that is engineering, not physics. In physics a virtual particle pair involved in Hawking radiation creates a gravitational wave.
posted before I saw your touche

Mike Hewson
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Gary. There are crucial

Gary. There are crucial nuances of gravity which distinguish it from other fields, and have prevented the 'unified field theory' that Einstein yearned for. So 'what we could detect' and 'the field itself' are one and the same question alas. It comes back to the equivalence of mass with energy as per SR. That in effect requires another derivative beyond acceleration which in turn locks the result into the magnitude of G/c^2. One can think of it in other terms but eventually one has to accept ( or suffer later contradiction ) that 'create a mass' or 'introduce a second mass' can't really be done. Quite unlike EM there is only one 'charge' - mass/energy - that is measured using two related physical units. In gravity we can't do the equivalent of splicing a neutron into an electron and a proton.

There are any number of reference frames to describe events in. None are truly special except in our thinking perhaps. That's the business end of GR : to provide transforms b/w said viewpoints. The results from this discovery are quoted in the frame you would expect ie. what we have sensed in the far field well away from the thing itself. We have inertial frames and non-inertial frames defined respectively by their adherence or not to the rule(s) of inertia : if you don't apply a force ( to an object within ) does it persist in it's state of motion ? But here a 'force' means a non-gravitational one as we are morphing/replacing gravity away from the force language to geometric concepts. Therein lies the rub.

Cheers, Mike.

I have made this letter longer than usual because I lack the time to make it shorter ...

... and my other CPU is a Ryzen 5950X :-) Blaise Pascal

astro-marwil
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Hallo! Yesterday Karsten

Hallo!
Yesterday Karsten Danzmann (director of AEI Hannover) answered in an interview for a major local newspaper (Der Tagesspiegel) on the question: “Why do you need so many detectors, isn´t just one sufficient?†“Each of them is omnidirectional, that means, he registers signals from all directions and can´t distinguish the polarization, the direction of oscillation. With two of them one can fairly reconstruct the direction of the source, with three that works better, but is unable to distinguish between up and down. Only with a minimum of 4 detectors one can gather the total signal in regard of direction and polarization.â€
By the way: From Google Maps data one can learn that in regard to Livingston Hanford is twisted by 81° clockwise, looking from above, and tilted by nearly 33° due to the curvature of the earth surface, taking the earth as a perfect ball.

Kind regards and happy crunching
Martin

Tiers Jean-Francois
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Hello Mike. I had no idea

Hello Mike.
I had no idea that only quadripolar (or higher) modes could generate GW.
BTW, are there systems in our universe that are not ?

Thanks and cheers

JF

Mike Hewson
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RE: Hello Mike. I had no

Quote:
Hello Mike.
I had no idea that only quadripolar (or higher) modes could generate GW.
BTW, are there systems in our universe that are not ?


Sure, but that depends upon your degree of approximation to symmetry. One could reasonably model the Earth as having uniform density, or at least has the only interesting variation dependent purely on the radial distance ( onion like ), or furthermore features only related to displacement from the axis of rotation. If so the Earth's rotation won't produce GW's. Now if you want to care about the uniqueness of Mt Everest say, then some miniscule radiation will occur ( best of luck measuring that ).

Maybe a gas giant planet would be more like it. Their lack of solidity ought preclude any lasting non-axisymmetry, but that depends on how sure you are of the interior.

If you are thinking of systems with some number of discrete bodies then you'd have to come up with something pretty strange. Not impossible, just unlikely to form naturally. Hmmmm. Hang on. Accretion discs form frequently enough, so how many of those are suitably arranged ? Again it comes back to how close to pure symmetry you want which in turn ( upper ) bounds any radiation ....

Try Larry Niven's Ringworld ? Or alternatively just play Halo. :-)

Cheers, Mike.

I have made this letter longer than usual because I lack the time to make it shorter ...

... and my other CPU is a Ryzen 5950X :-) Blaise Pascal

Gary Charpentier
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RE: that 'create a mass' or

Quote:
that 'create a mass' or 'introduce a second mass' can't really be done.

I believe Mr. Hawking and Mr. Heisenberg have a bone to pick with you.

Tiers Jean-Francois
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Thanx a lot Mike. My question

Thanx a lot Mike.
My question was quite unclear, sorry.
It concerned only the theoretical possibilities to have somewhere a non quadripolar (or more) system in our common universe (not talking of worm holes or multivers).
Take for example Jove. It has satellites which from time to time are in conjunction. Then, its shape is distorted (a little) because of the mass of its satellites and Jove is no longer perfectly spherical.
The Moon tides on earth must have the same effect and I guess that it is the same elsewhere.

I agree that the result in terms of GW is not measurable at the moment (and maybe for ever !) but I remain convinced that we live in a swimming pool of GW.
That we can catch them or not was not my question.
The proof of existence is a thing, the proof of non existence is an other thing.
Am I wrong ?

Maximilian Mieth
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RE: Thinking about this a

Quote:

Thinking about this a bit more, a single mass moving at a constant velocity in the universe should warp space time, but there is no frame of reference to measure it against, except itself. To perform a measurement (real not thought) you have to introduce a second mass and as soon as you do the two masses will accelerate towards each other.

Perhaps the result here is that all frames of reference in the real world are accelerating frames of reference and there is no rest frame. Actually that seems to agree well with my understanding of relativity.


That is also my understanding. I think I tried to say that when I posted this:

Quote:
Isn't everything in our universe constantly accalerated? The mass may seem to sit still in the shelf, but for sure it is accelarated relatively to any other frame of reference.


Probably I was a bit unclear. This topic is really challenging.

Thanks everybody for explaning so much about GW!

Mike Hewson
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RE: Take for example Jove.

Quote:
Take for example Jove. It has satellites which from time to time are in conjunction. Then, its shape is distorted (a little) because of the mass of its satellites and Jove is no longer perfectly spherical.
The Moon tides on earth must have the same effect and I guess that it is the same elsewhere.
I agree that the result in terms of GW is not measurable at the moment (and maybe for ever !) but I remain convinced that we live in a swimming pool of GW.


Ah. Yes. These waves of whatever magnitude abound. Most are irrelevant in that far, far larger magnitude affects dominate.

Quote:
That we can catch them or not was not my question.
The proof of existence is a thing, the proof of non existence is an other thing.
Am I wrong ?


All science is contingent. Even the best efforts and theories may suffer contradiction by reality tomorrow. Often that leads not to total refutation of a set of ideas but refinement or bounding of applicability. Not detecting GW's was a possible outcome and as such ( with sufficient confidence in technique/execution etc ) would have lead to a re-frame of theory. GW's were detected and so people have hailed the experiment as a success, but it would also have been a success if we were correctly informed that GW's don't occur. A good experiment doesn't validate precepts. A good experiment tells you what nature is, not what you'd like it to be. In generality this is a heavily misunderstood foundation of science, and much confusion flows downstream because.

{ The second most misunderstood aspect of science is that any theory has a realm of applicability defined by it's assumptions. Hence one ought not complain that semiconductor lattice wave functions fail to predict anything useful in stellar cores. }

Cheers, Mike.

I have made this letter longer than usual because I lack the time to make it shorter ...

... and my other CPU is a Ryzen 5950X :-) Blaise Pascal