Duration of recently measured gravitational waves

[fog37]

Hello,

I have a basic question about the temporal duration of these newly measured gravitational waves.

How long is the duration of these gravitational waves?
Two black holes collided and created these gravitational waves a while ago. How long can we "listen" to these waves? When did they start and when will they end? All waves have a beginning and an end.

I guess I would have a similar question about the background radiation (which is an electromagnetic wave instead). It does not seem to be just a short, finite duration pulse of radiation. All waves have a beginning and an end but both gravitational waves and background radiation seem to be a very lasting wavetrain...

thanks,
Fog37

 

[Orodruin]

The amplitude and frequency of the gravitational wave increase as the black holes spiral towards each other. The part of the wave which is actually detectable above the background is very short, around 0.1 s.

 

[fog37]

Thanks.

I can see how the amplitude and frequency would increase. But have the two black holes actually collided? Or are they still approaching each other? How long does all this process take?

I guess I am wondering if these gravitational waves have an actual beginning and end and which part of the wave we are actually measuring. I just imagine a wave packet of a certain duration...

 

[Vanadium 50]

They collided. See Figure 2 of the paper. PRL 116, 061102 (2016).

 

[Aaronvan]

What is the directional resolution of LIGO? Enough to observe an optical or radio counterpart to the BH merger?

 

[exponent137]

Now, it is expected that signals of gravitational waves will happens a few times per year.

But, LIGO was built a long time ago. What are reason that signals were not happen before sep 2015? When LIGO became operative?

 

[Aaronvan]

Now, it is expected that signals of gravitational waves will happens a few times per year.

But, LIGO was built a long time ago. What are reason that signals were not happen before sep 2015? When LIGO became operative?

I think it took a long time to calibrate and tweak LIGO. Also, it underwent a $600 million refurbishment two years ago. The original configuration wasn't sensitive enough.

 

[Vanadium 50]

What is the directional resolution of LIGO? Enough to observe an optical or radio counterpart to the BH merger?

This burst is localized to about 1.5% of the sky. That's a big area. Put another way, the volume is about 200 trillion cubic light-years. That's not small.

 

[sandy stone]

The amplitude and frequency of the gravitational wave increase as the black holes spiral towards each other.

I assume this happens (spiral towards each other) because of the energy lost due to gravitational radiation?

 

[jtbell]

I assume this happens (spiral towards each other) because of the energy lost due to gravitational radiation?

Yes.

 

[exponent137]

I think it took a long time to calibrate and tweak LIGO. Also, it underwent a $600 million refurbishment two years ago. The original configuration wasn't sensitive enough.

1. Preciseness now is ##10^{-21}## according to lenght. What preciseness was before refurbishment?
2. Can LIGO and other similar devices give any information about quantum gravity?
3. Does LIGO gives any information about earthquakes?

 

[fog37]

So, before measuring these gravitational waves, we only knew that gravitational waves were just a possibility ( a mathematical idea predicted by Einstein), the same way string theory is an interesting and promising theory but not scientific yet since it still lacks scientific evidence?

 

[Orodruin]

Before the measurement we only had indirect evidence such as binary systems losing energy at the right rate. We also had predictions that certain types of events, such as the observed merger, would be able to produce measurable signals.

 

[jtbell]

So, before measuring these gravitational waves, we only knew that gravitational waves were just a possibility ( a mathematical idea predicted by Einstein), the same way string theory is an interesting and promising theory but not scientific yet since it still lacks scientific evidence?

Gravitational waves are a specific prediction of a well-developed theory (general relativity) that has been tested extensively and successfully in many other ways.

As I understand it, string theory has not yet made any such testable predictions. In fact, there isn't even a single string theory, as I understand it, but instead a number of competing variations.

 

[fog37]

Great. Thanks a lot.

I am now wondering about the background radiation...is this radiation still around and can we "hear" it with antennas? How can it be still around? It should be an infinitely long wavefield...

 

[PeterDonis]

I am now wondering about the background radiation...is this radiation still around and can we "hear" it with antennas? How can it be still around? It should be an infinitely long wavefield...

Do you mean the CMBR? That does not have infinitely long wavelength. Its current temperature is about 2.7 degrees K, which corresponds to a wavelength of about 5 millimeters, easily detectable with antennas. In fact, that is how the CMBR was first detected; it showed up as background noise in a radio antenna.

 

[Ibix]

I am now wondering about the background radiation...is this radiation still around and can we "hear" it with antennas? How can it be still around? It should be an infinitely long wavefield...

The source of the gravitational waves LIGO detected was an event - a point in space and time. The waves from it have passed us and gone. The source of the cosmic background (and any associated gravitational waves) is everywhere in the universe 13.7bn years agi. We are currently seeing energy emitted 13.7bn years ago - next year we will be seeing energy emitted at the same time but from a bit further away. That's why we will always be able to see the background, but gravitational waves from black hole collisions comes and goes.

 

[fog37]

thanks,

As far as the CMBR, I see how it is still around and detectable by an antenna anywhere on earth. The wavelength is 5 mm. These wavefield is therefore omnidirectional.
But why does it not cease to exist? every wavefield has a beginning and an end? What sustains the CMBR? The temperature of space is not 0 K but close to a couple of degree Kelvin...

 

[gianeshwar]

The source of the gravitational waves LIGO detected was an event - a point in space and time. The waves from it have passed us and gone. The source of the cosmic background (and any associated gravitational waves) is everywhere in the universe 13.7bn years agi. We are currently seeing energy emitted 13.7bn years ago - next year we will be seeing energy emitted at the same time but from a bit further away. That's why we will always be able to see the background, but gravitational waves from black hole collisions comes and goes.

Please explain it in more detail!

 

[gianeshwar]

Please explain it in more detail!

Why for black holes it comes and goes?
If many such events of bkack hole collisions keep happening, it means gravitational waves keep crisscrossing us most of time and every few seconds we keep catching gravitational wave signals.
As I understand about the CMBR, it had started at the Bigbang event and eversince the radiation is being detected throuout universe.
But why this does not passby and finish like that of blck hole collision.

 

[Orodruin]

Why for black holes it comes and goes?
If many such events of bkack hole collisions keep happening, it means gravitational waves keep crisscrossing us most of time and every few seconds we keep catching gravitational wave signals.
As I understand about the CMBR, it had started at the Bigbang event and eversince the radiation is being detected throuout universe.
But why this does not passby and finish like that of blck hole collision.

Black hole mergers are isolated events ooccurring at different times and places. The CMB was released everywhere and not by isolated mergers.

 

[Ibix]

To expand slightly on Orodruin's answer - think of ripples spreading out on a pond.

The black hole merger is a single event, like a rock dropped into the pond. You get an expanding ring of ripples; once they've washed over you the pond is still again and you can't tell that the ripples passed.

The cosmic microwave background, however, comes from the time when the universe was extremely hot and dense - so hot and dense that light could not pass through it. When the whole thing cooled enough that light could travel without being absorbed, electromagnetic radiation spread out from everywhere. Instead of someone throwing a rock into a pond, think of someone throwing gravel uniformly across the surface of the pond. Again, once a ripple has passed you it's gone - but this time, there's another ripple right behind it, and another and another and another... And that is true whichever way you look.

I'm not sure that last is actually a valid physical model, and it certainly leaves out a lot of cosmological details (the universe is expanding, so the really hot glow is red-shifted into a 3K microwave glow, for example). But hopefully it gives you an idea why we continue to see the microwave background but only get one chance to see a black hole merger.

 

[gianeshwar]

Thank you Orodruin and Ibix !