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Author Topic: Why is Gravitational wave detection important?  (Read 7612 times)

Offline Ethos_

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Re: Gravitational waves detected?
« Reply #25 on: 12/02/2016 15:14:28 »
How far does this result tip the balance in favour of the existence of the graviton?
While this experiment supports gravity's ability to squeeze and stretch space/time, whether that space/time disturbance is caused by the theorized force carrier "graviton" is a question that I don't believe has been answered yet. Nevertheless, your question is a very important one Jeff, and of great interest to us all.
« Last Edit: 12/02/2016 15:17:08 by Ethos_ »
 

Offline jeffreyH

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Re: Gravitational waves detected?
« Reply #26 on: 12/02/2016 16:51:22 »
Evan at one point there was a discussion about gravitons outside the event horizon of a black hole. Can you remember which thread that was in?

I have found that the search tool within TNS forum is not always great, but that searching google using some of the terms you're looking for as well as the names of likely posters will often lead to the right place.

I found these using that method:
http://www.thenakedscientists.com/forum/index.php?topic=47582.0
http://www.thenakedscientists.com/forum/index.php?topic=20014.0

I had a similar idea to what pmbphy (Pete) said about virtual particle pairs. In this case gravitons. So that one escaped but the other didn't. I didn't like that idea at all.
 

Online evan_au

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Re: Gravitational waves detected?
« Reply #27 on: 12/02/2016 19:04:11 »
The published paper is here: "Observation of Gravitational Waves from a Binary Black Hole Merger"
https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.116.061102
It is fairly readable, if you have done first-year physics at university level.

Quote from: Observation
the events have a combined signal-to-noise ratio (SNR) of 24
I interpret this to say that the signal is 24 times the background noise.

In telecommunications, we would normally specify a SNR in dB, ie 24dB means that the signal is 102.4 times the noise, or 251 time the noise. That is not the case here.

However, the statistics they use suggest that a false alarm like this would occur less than once in 200,000 years (or 8,000 years, depending on which assumptions you make). 

Quote from: evan_au
The gravitational wave signal is more intense in the last couple of days of a merge process that can take millions of years
In fact, the signal is only visible above the noise for the last 100ms, while the two black holes do their last 8 orbits.

Quote from: evan_au
you need to be "lucky" in the timing
Quote from: Observation
Only the (two) LIGO detectors were observing at the time of GW150914. The Virgo detector was being upgraded, and GEO 600, though not sufficiently sensitive to detect this event, was operating but not in observational mode.
In terms of being "lucky", only two of the world's gravitational wave observatories were operating at the time.
LIGO was shut down for an upgrade in October 2010, and only reopened in September 2015.
This event was detected on 14th September 2015.  http://en.wikipedia.org/wiki/LIGO

Quick analysis algorithms highlighted a possible event within 3 minutes of the event arriving at the Earth.

Given that this first detection happened with a couple of weeks of reopening, I wonder how many more they have discovered since then?

Quote from: evan_au
fairly "close" to Earth (in astronomical terms)
The distance surprised me: 400 Megaparsecs; redshift z=0.09.

So this is a distant object, outside our local group of galaxies.
Part of this sensitivity may be due to the surprisingly large mass of these black holes: about 30 solar masses each. Experimenters were expecting smaller black holes to be much more common.

They estimate that 3 times the mass of the Sun was turned into gravitational waves during the merger.

Quote from: jeffreyH
How far does this result tip the balance in favour of the existence of the graviton?
If gravitons exist (as many physicists believe), this experiment will not detect them; it is looking for coherent graviton waves.
Just like someone seeing coherent light waves from a laser pointer will not detect individual photons with their eyes.

However, this measurement was able to place some constraints on the characteristics of the graviton: 
  • Wavelength > 1013km (ie very big)
  • Mass < 10-22 eV/c2 (ie very low mass; perhaps massless?) 
 

Offline syhprum

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Re: Gravitational waves detected?
« Reply #28 on: 12/02/2016 22:44:24 »
I understand that that the source detected by the LIGO experiment radiated gravitational waves with a power of 3.8*10^49 watts for a few milliseconds, and was received from 1.2 billion light years away.
Are there any local sources nearer, the Earth/Moon system must be generating some and the effect on us are quite apparent.
How about a pair of tethered 1000Kg satellites rotating at 10hz 1000km away would they generate enough power to be detectable .
I understand planets like Jupiter generate a few tens of Watts but they are a long way away.

PS
I read the paper quoted and lacking even an O level had no problem understanding it .
« Last Edit: 12/02/2016 23:00:17 by syhprum »
 

Offline jeffreyH

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Re: Gravitational waves detected?
« Reply #29 on: 12/02/2016 23:01:30 »
Quote from: jeffreyH
How far does this result tip the balance in favour of the existence of the graviton?
If gravitons exist (as many physicists believe), this experiment will not detect them; it is looking for coherent graviton waves.
Just like someone seeing coherent light waves from a laser pointer will not detect individual photons with their eyes.

However, this measurement was able to place some constraints on the characteristics of the graviton: 
  • Wavelength > 1013km (ie very big)
  • Mass < 10-22 eV/c2 (ie very low mass; perhaps massless?) 

I would expect the energy to be related to wavelength in much the same way as it is with the photon. The graviton should be massless. The much longer wavelengths are then more consistent with the weakness of the gravitational field.
 

Online evan_au

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Re: Gravitational waves detected?
« Reply #30 on: 13/02/2016 02:26:26 »
The power radiated in Gravitational waves for two orbiting bodies is:

P=dE/dt=-(32/5)*(G4/c5)*(m1m2)2*(m1+m2)/r5

A few examples:

System               Mass1 (kg)   Mass2 (kg)   Radius (m)   Power (W)
Earth orbiting Sun:      5.97E+24   1.98E+30   1.5E+11      193
Moon orbiting Earth:      5.97E+24   7.34E+22   385e7      7.19E-06
Pulsar B1913+16:         2.78E+30   2.78E+30   2e9      5.47E+23
1 ton satellites at 10m:   1.00E+03   1.00E+03   10      1.05E-72



Quote from: syhprum
Are there any local sources nearer (than the one detected by LIGO)?
There are binary pulsars that have been detected in our galaxy (eg PSR B1913+16); by measuring their orbital period, these are known to be emitting gravity waves.

These neutron stars have 40% more mass than the Sun, but these are separated by greater than the radius of the Sun. They are emitting huge amounts of gravitational waves (1023 Watts), but this is microscopic compared to their kinetic energy - their orbital period is only changing by microseconds per year. They will not be radiating enough gravitational waves to be detectable on Earth until they are about to collide.

Quote
the Earth/Moon system must be generating some
From the table above, it's about a microWatt. Negligible impact on the Moon's considerable kinetic energy.
Quote
and the effect on us are quite apparent.
The Moon produces tides, and these tides slow down the Moon's orbit & Earth's rotation via friction. But these friction effects are huge, and the gravitational radiation is negligible.

Quote
How about a pair of tethered 1000Kg satellites rotating at 10hz 1000km away would they generate enough power to be detectable?
Gravitational radiation occurs when masses are accelerated in their mutual gravitational fields.

If they are tethered, they accelerated by the tension in the tether. I'm not sure if they would radiate gravitational waves (help, someone?).

If the 1 ton masses were in orbit around each other at a radius of 1 meter, the power radiated would be 10-67W, which would have negligible impact on their kinetic energy.
« Last Edit: 13/02/2016 02:28:51 by evan_au »
 

Offline syhprum

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Re: Gravitational waves detected?
« Reply #31 on: 13/02/2016 10:20:44 »
Applying the inverse square law I believe the power received from the tethered satellites at 1000 km distant is about 10^-23 times that from the black holes hence not receivable.
Many thanks for these calculations.

PS I wonder what the effect of this massive burst of radiation would be to nearby objects to this collision ? 
PPS
Viewed as a communication receiving device what is the bandwidth of LIGO I understand the signal was about 10Hz 
« Last Edit: 13/02/2016 10:27:06 by syhprum »
 

Online evan_au

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Re: Gravitational waves detected?
« Reply #32 on: 13/02/2016 12:08:55 »
Quote from: syhprum
Viewed as a communication receiving device what is the bandwidth of LIGO?
In the paper, Figure 3 in Section III shows the optimum sensitivity around 100-300Hz.
However, the useful sensitivity is about 20Hz to 2kHz.

Figure 1 in Section II of the paper shows the frequency of the chirp sweeping upwards from 50Hz to 400Hz over a period of about 7ms.

See: https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.116.061102
(Added link to paper)
« Last Edit: 13/02/2016 21:33:20 by evan_au »
 

Offline alancalverd

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Re: Gravitational waves detected?
« Reply #33 on: 13/02/2016 12:33:45 »
The power radiated in Gravitational waves for two orbiting bodies is:

P=dE/dt=-(32/5)*(G4/c5)*(m1m2)2*(m1+m2)/r5

A few examples:

System               Mass1 (kg)   Mass2 (kg)   Radius (m)   Power (W)
Earth orbiting Sun:      5.97E+24   1.98E+30   1.5E+11      193


Now that's 20 times the power of the transmitter on the Mars Rover, so should be detectable on Mars! Time to launch an experimental physicist on a one-way mission. Do I need a visa?
 

Offline syhprum

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Re: Gravitational waves detected?
« Reply #34 on: 13/02/2016 19:58:54 »
You are going to find it rather exhausting constructing the LIGO type receiver by yourself but the best of luck
Please quote the URL of the article I seem to have missed it.
« Last Edit: 13/02/2016 21:19:26 by syhprum »
 

Offline chiralSPO

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Re: Gravitational waves detected?
« Reply #35 on: 13/02/2016 20:50:30 »
The power radiated in Gravitational waves for two orbiting bodies is:

P=dE/dt=-(32/5)*(G4/c5)*(m1m2)2*(m1+m2)/r5

A few examples:

System               Mass1 (kg)   Mass2 (kg)   Radius (m)   Power (W)
Earth orbiting Sun:      5.97E+24   1.98E+30   1.5E+11      193


Now that's 20 times the power of the transmitter on the Mars Rover, so should be detectable on Mars! Time to launch an experimental physicist on a one-way mission. Do I need a visa?

Won't it have a period of 1 year (frequency of ~3e8 Hz)? Unless I'm mistaken, that would be hard to detect even if the power were through the roof!
 

Offline Donovan Rama

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Re: Gravitational waves detected?
« Reply #36 on: 13/02/2016 21:21:46 »
What I am about to say is just my thought and I could be wrong. BUT I recently read that space can be warped with the more objects in it thus creating gravitational waves IF that is true and considering earth is close to the sun than most other planets and starts and also considering that Stars eventually do explode or disappear or evaporate what ever fancies your vocab then when a few of those stars "explode" that would mean that there is less gravitational wave pull on earth and eventually would earth not to soon get closer and close to the sun ? And if that is true then would that mean that the theory of global warming is incorrect and In fact it is because of gravitational wave pull on earth that takes it closer to the sun that is the cause of increasing heat instead?  Just a thought or a theory but it is something that's tickles my brain
 

Offline chiralSPO

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Re: Gravitational waves detected?
« Reply #37 on: 13/02/2016 21:59:06 »
Welcome to the forum, Donovan!

An interesting idea, but I'm pretty sure it is not correct.

It's true that all objects with mass distort spacetime, and as they move will create some gravity waves. But I don't think that stars exploding will have any significant effect on our orbit (even the closest star outside our solar system is more than 4000000000000 km away). Also, I am unaware of any evidence that our orbit around the sun has changed substantially in the last 100 million years, and certainly not in the last century...
 

Online evan_au

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Re: Gravitational waves detected?
« Reply #38 on: 13/02/2016 22:20:04 »
Quote from: alancalverd
(200W of gravitational wave power from the Earth orbiting the Sun is) 20 times the power of the transmitter on the Mars Rover, so should be detectable on Mars!

The Mars Rover (and its orbiter) have fairly directional antennas, concentrating the radio power in the direction of Earth, increasing the signal by a factor of thousands.
The NASA Deep Space Network have some massive dishes (I've seen the 70m dish at Tidbinbilla) that collect signals from Mars, to the exclusion of all other directions, increasing the signal by factors of millions.

However, transmission of gravitational waves occurs in all directions, and gravitational wave detectors also receive signals from all directions. Directional microwave antennas improve the sensitivity by a factor of billions compared to omnidirectional antennas.

There is also the slight problem that our biggest gravitational wave antennas today are 4km long, and have the sensitivity to detect length variations smaller than a proton. To detect oscillations from Earth's orbit, you would like an antenna with far greater sensitivity, but with a baseline that stretches from Earth well into the Oort Cloud.

eLISA is a technology demonstrator for a space-based gravitational wave detector that was launched in December 2015. The proposed LISA telescope would provide a much longer baseline than one we could construct on Earth.

I think if you want to detect the effects of Earth orbiting the Sun, you should stick to a sextant, a lump of magnetite, and a water clock.
 

Offline syhprum

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Re: Gravitational waves detected?
« Reply #39 on: 14/02/2016 15:17:50 »
It is nice to have a correspondent here that understands radio communication technology
 

Offline syhprum

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Re: Gravitational waves detected?
« Reply #40 on: 15/02/2016 11:09:58 »
Is there a polarization component to gravitational waves ?
The LIGO installations consist of two horizontal tubes at right angles would a third vertical tube albeit expensive to construct provide useful information ?.   
 

Online evan_au

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Re: Gravitational waves detected?
« Reply #41 on: 15/02/2016 21:21:17 »
Quote from: syhprum
Is there a polarization component to gravitational waves ?
Yes. That is why researchers want at least 4 gravitational wave observatories around the world: 2 in USA, with 1 in Japan and 1 in India being built.

If the polarization is "wrong", one observatory may not be able to pick up the signal at all (or it might be so low that it is buried in noise).

However, the other 3 observatories, with different orientations on the surface of the Earth should (in principle) be able to pick up the signal (if it is strong enough).

Astronomers want the same signal to be picked up by 3 observatories, as this allows them to triangulate the position of the source in the sky - at least for a neutron star merger, that should produce a signal that can be picked up by optical, X-Ray and radio telescopes.

With the first signal back in September 2015, it was detected at just 2 observatories; the signal could have come from anywhere in a thin band that circles half way around the sky. That doesn't tell you where to point your telescopes (not that you would expect to see much from a merger of black holes).

Quote
The LIGO installations consist of two horizontal tubes at right angles would a third vertical tube albeit expensive to construct provide useful information?

Yes, but it's easier to build another two 4km vacuum tubes flat on the ground in another part of the world, than to build a 4km vacuum tube up in the air, or down into the ground.

Geographical dispersion also provides some protection from sources of interference like earthquakes, cosmic rays, power glitches, hardware failures, software bugs, etc.
 

Offline syhprum

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Re: Gravitational waves detected?
« Reply #42 on: 15/02/2016 21:47:03 »
Yes with tightly time controlled communication now available another installation 90 around the world is a better solution than a vertical tube.
As you say if a source could be observed by both electromagnetic radiation as well as gravitational waves by the variation in timing it would give some clues as to the mass of the Graviton (if it has any!) or confirmation of the mass of Neutrinos if they are also observed at the same time
 

Offline jeffreyH

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Re: Why is Gravitational wave detection important?
« Reply #43 on: 16/02/2016 14:59:17 »
If life had evolved in a zero gravity environment then lots of things would be different. How would Lavoisier have determined the conservation of mass without gravity? How would mass even be determined. Any experiments that can attempt to pin down the speed of gravity will open many new avenues in physics. Will string theory survive?
 

Offline dhjdhj

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Re: Why is Gravitational wave detection important?
« Reply #44 on: 16/02/2016 19:50:38 »
I also wondered about the speed of the wave and whether if new matter was created which included a graviton( if it exists ) its field would presumably extend to infinity, but would it do that instantly? or would it have a finite speed related to C?
 

Online evan_au

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Re: Why is Gravitational wave detection important?
« Reply #45 on: 16/02/2016 20:23:20 »
Quote from: dhjdhj
I also wondered about the speed of the (gravitational) wave
This is covered in another thread: http://www.thenakedscientists.com/forum/index.php?topic=65812.0
 

Online evan_au

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Re: Why is Gravitational wave detection important?
« Reply #46 on: 21/02/2016 06:06:38 »
Quote
The LIGO installations consist of two horizontal tubes at right angles would a third vertical tube albeit expensive to construct provide useful information?

A European proposal suggests:
Quote from: Scientific American
three 10-kilometer arms arranged in the shape of an equilateral triangle rather than an L. That configuration would help it pinpoint the sources of gravitational waves on the sky

Looking at it geometrically:
- three tubes at right-angles would represent a corner of a 3D cube, with sensitivity in all three planes of the cube surface.
- three tubes in an equilateral  triangle represents a cut-off corner of a 3D cube. They imply that this gives enough information to determine the direction of the source from a single detector?

eLISA (in space) would also be a triangle, but with sides of a million km.

Unfortunately, neither detector is currently funded.
See: http://www.scientificamerican.com/article/the-future-of-gravitational-wave-astronomy/ 

 

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Re: Why is Gravitational wave detection important?
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