Successful detection of gravity waves!

[freyar]

I was looking more into the amount of energy that is present in the universe as gravity waves, and found this:

http://www.tapir.caltech.edu/~teviet/Waves/index.html

In particular:

http://www.tapir.caltech.edu/~teviet/Waves/gwave_spectrum.html

Relic background: A stochastic signal from the Big Bang itself, this consists of quantum fluctuations in the initial explosion that have been amplified by the early expansion of the Universe. While the spectral shape of this source can be predicted, its overall strength is highly uncertain, but is constrained by the fact that gravitational wave perturbations are one of several components contributing to the observed temperature fluctuations in the cosmic microwave background. This limits the maximum strength of gravitational waves at cosmological length scales. Two curves are shown: one at the upper limit of the observational constraints, and another an order of magnitude weaker.

Those TAPIR pages at Caltech are really great. Anyway, I just wanted to mention that relic background. There's a relic background of neutrinos (not yet observed), gravitational waves (not yet observed), and light (observed very precisely all the time). The gravitational wave background may eventually be detected by looking at pulsars, which are neutron stars that rotate and pulse with a well-determined period that would be affected by gravitational waves. The other way the gravitational wave background could be detected is through its influence on the polarization of the relic background light. If you remember a big fuss (from the BICEP2 collaboration) about two years ago, that was because one experiment thought they had detected this effect. However, another experiment showed that they just measured dust. Anyway, that's something LIGO can't measure but would be another very interesting.

 

[freyar]

so the"waves are of a heaving sigh. I figured as such or we would be tossed about like a sip in a storm with no rudder or sails.

about my question I mad, I asked it backwards.

let me put it this way: I moved to Missouri when I was 12. There was a pond about 100 yards away we went fishing and swimming in. During the dog days of summer - no wind at this time - the pond surface was glassy smooth. Now there was an old dead tree in the near center of this pond and we would row out to it and fish there.

Now that I have given you a picture, Here is my point. I remember throwing rocks in the pond and watching the ripples. The tree, a steadfast object, cause a disturbance. from what I watched it was like using echo location as the ripples bounced off the pond's edge and the tree. Are the objects or particles that the gravity waves do not affect that would bounce off of to give their presence away alike echo location?

Sorry about the other question, I pretty much knew the answer to that.

Scott "the overly curious" DeWar
AKA in RL As David A Johannes

You'd need something pretty big to affect gravitational waves like the ones announced this week very much for two reasons. One is just that their wavelength is roughly 1 million to 100 million meters in wavelength (that's a range from a bit smaller than the radius of the earth up to almost the radius of the sun), and waves aren't typically affected much by objects smaller than their wavelength. Second, gravitational waves are only affected by gravity, and gravity is pretty weak. For example, the sun's gravity only deflects light by a tiny amount (only measurable for light that passes very close to the sun's surface). So, for a significant effect, you'd really need the gravitational wave to pass close to a very massive object, like a galaxy or cluster of galaxies. But then it could be "gravitationally lensed" just like light is. This is something we see with light: light from a distant galaxy passing by an intervening galaxy or cluster gets distorted a lot, to the point that there can be multiple images of the same thing.

 

[Scott DeWar]

. . . . .edit some good stuff . . . . . But then it could be "gravitationally lensed" just like light is. This is something we see with light: light from a distant galaxy passing by an intervening galaxy or cluster gets distorted a lot, to the point that there can be multiple images of the same thing.

I am aware of that phenomena, I have read some layman level articles on it regarding the Hubble scope. I also remember A Einstein trying to prove it once while in pre-communist Russia during an eclipse as it pertained to star light near the sun, If I recall correctly. So I see this as a gentle tidal surge rather then what is perceived as a ripple. Not unlike a low level explosion used in excavation on a small scale: Felt more then heard.

I see it like this:

you and a friend are on two ships that are 100 meters from each other. There is a laser beaming from your ship to your friend's ship. The sea is heaving lightly, 6 inches. but each wave lengthe is 1000 meters wide. Now both ships are moving nearly in tandem being so close, but as the ships get further apart, the laser is detecting a greater and greater difference. Once your friend has taken his ship 5 mile away, the heaves and sighs of the tide are more visible as you look and see you are no longer in tandem with each other. In fact, the difference is so great the laser can no longer keep on target.

But still, you do not feel the waves.

Like that?

 

[Umbran]

So, for a significant effect, you'd really need the gravitational wave to pass close to a very massive object, like a galaxy or cluster of galaxies.

Super-massive black hole, perhaps? Physically large and massive enough, but not diffuse like a cluster of galaxies.

 

[freyar]

I am aware of that phenomena, I have read some layman level articles on it regarding the Hubble scope. I also remember A Einstein trying to prove it once while in pre-communist Russia during an eclipse as it pertained to star light near the sun, If I recall correctly. So I see this as a gentle tidal surge rather then what is perceived as a ripple. Not unlike a low level explosion used in excavation on a small scale: Felt more then heard.

Yes, what made Einstein famous was the fact that measurements of the deflection of starlight during an eclipse matched his prediction on the nose (there's a bit of a story behind that, but that's really another post). That's what we call weak lensing, since it's just a slight deflection. Larger objects, like clusters of galaxies, can actually lens light strongly enough to create multiple images. I don't know if LIGO has considered what a strongly lensed signal would sound like, but that's probably easier to work out than the actual black hole merger calculations.

I see it like this:

you and a friend are on two ships that are 100 meters from each other. There is a laser beaming from your ship to your friend's ship. The sea is heaving lightly, 6 inches. but each wave lengthe is 1000 meters wide. Now both ships are moving nearly in tandem being so close, but as the ships get further apart, the laser is detecting a greater and greater difference. Once your friend has taken his ship 5 mile away, the heaves and sighs of the tide are more visible as you look and see you are no longer in tandem with each other. In fact, the difference is so great the laser can no longer keep on target.

But still, you do not feel the waves.

Like that?

This is a pretty good explanation of why the LIGO detectors can observe the wavelengths they can --- due to the size they are (and also how far apart they are).

We can make a similar analogy to explain what can affect the wave itself. If you are in a boat and have a kilometer-wavelength wave come at you, your boat will not do much to the wave. However, short wavelength waves, like 1 meter, bounce off the boat in all different directions.

 

[freyar]

Super-massive black hole, perhaps? Physically large and massive enough, but not diffuse like a cluster of galaxies.

Yes, there'd be serious lensing near one, but they're small enough that they'd be unlikely to lie along the line of sight to anything. Galaxy clusters make good strong lenses because (a) they have a large enough angular extent that there are objects behind them to be lensed and (b) they have lots of supermassive black holes and stars and gas and dark matter, which makes them very massive/very deep gravitational potential wells.

 

[Umbran]

Yes, there'd be serious lensing near one, but they're small enough that they'd be unlikely to lie along the line of sight to anything.

I was actually considering *reflection*, which is what he originally asked about (waves bouncing off the tree in the lake), not lensing.

 

[freyar]

I was actually considering *reflection*, which is what he originally asked about (waves bouncing off the tree in the lake), not lensing.

Ahh, ok, I was just thinking about disturbance. I'd expect lensing to include some reflection/scattering generally, but I'm not sure. A black hole may also work, though the black hole may just absorb the gravitational wave.

 

[freyar]

Coming back to the idea of matching the gravitational wave events to other observations: there are lots of papers today where other experimental collaborations looked for corresponding events. Most of them saw nothing, but the Fermi gamma ray telescope reports a dim, brief event that showed up 0.4 seconds after the gravitational waves. (They estimate about a 0.2% chance that it's just an instrumental artifact.) It's not exactly expected that a black hole merger would be accompanied by a gamma ray burst like this, but I could see that matter falling into the black holes at the time of the merger could light up a bit (the gravitational waves carry out 100x as much energy as the gamma rays, it looks like). So, it's hard to know if this is real, but it would at least partially solve the mystery of what gamma ray bursts are if this holds up.