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View Full Version : Ep. 71: Gravitational Waves



Fraser
2008-Jan-15, 04:40 AM
When he put together his theories of relativity, Einstein made a series of predictions. Some were confirmed just a few years later, but scientists are still working to confirm others. And one of the most fascinating is the concept of gravitational waves. As massive objects move in space, they send out ripples across the Universe that actually distort the shape of matter. Experiments are in place and in the works to detect these gravitational waves as they sweep past the Earth.http://feeds.feedburner.com/~r/astronomycast/~4/216821373

More... (http://feeds.feedburner.com/~r/astronomycast/~3/216821373/)

jamesabrown
2008-Jan-17, 03:58 PM
Just a minor technical point, but early in this episode, you took a moment to announce something like, "And now a word from our sponsor." But your blurb for audible.com was missing.

AmroB
2008-Feb-05, 12:03 AM
Hi everyone,

Just got round to listen to this episode and I've got a question please. Does the universe expansion affect gravitational waves in anyway? e.g. do gravitational waves get red shifted?

Thanks,
Amro

FriedPhoton
2008-Feb-06, 02:37 AM
I could be mistaken but I believe I have heard Pamela state that gravitational effects are felt instantly in one show and travel at the speed of light in another. Am I confusing something or do I need a new battery for my neurons?

FriedPhoton
2008-Feb-06, 02:39 AM
do gravitational waves get red shifted?

Oh man... great question... now I have something else to hurt my brain with until someone answers it. I'm still wondering how we can have gravity waves when there is no gravity, only the warping of space... or at least that's what I thought Albert told me... his accent was a little thick.

Steve Limpus
2008-Feb-06, 09:34 PM
I could be mistaken but I believe I have heard Pamela state that gravitational effects are felt instantly in one show and travel at the speed of light in another. Am I confusing something or do I need a new battery for my neurons?

Light and gravity travel at the same speed as Einstein predicted.

Redshift? Don't know, but you're right, it's a great question.

Try this link, I haven't had time to study it, but it looks like it has what you're looking for.

http://en.wikipedia.org/wiki/Redshift

Cheers. :)

Steve Limpus
2008-Feb-06, 11:24 PM
Light and gravity travel at the same speed as Einstein predicted.

Redshift? Don't know, but you're right, it's a great question.

Try this link, I haven't had time to study it, but it looks like it has what you're looking for.

http://en.wikipedia.org/wiki/Redshift

Cheers. :)

I think this article is referring to light being redshifted by high gravity objects (among other things) rather than gravity itself being 'redshifted' by expansion, so - still a good question.

FriedPhoton
2008-Feb-06, 11:56 PM
Ok, so I followed the redshift link, read a bit there, went on to gravitational redshifting, ended up at frame-dragging which I recall Pamela mentioning in at least one show. And now I have another question...

I gather that frame-dragging is a very slight effect, but could you theoretically have a black hole so massive that it twisted space like a candy-cane, and if so, what would happen in that region of space. Would space be damaged (ok, dumb question, but I have to ask)? What would light traveling through a region like this look like?

Steve Limpus
2008-Feb-07, 12:06 AM
I'm not sure if this is relevant, but - is it the case that there are fundamental differences between gravity waves and light waves? For example, gravity waves are thought to propogate from black holes right? Light waves can't escape black holes. So maybe the comparison to red-shifted light is invalid in some way? I can't help but feeling though, that the wave-length of gravity waves must increase in expanding space. Then I try to consider gravitons, higgs fields, inverse square law, blah blah...

...meltdown.

Someone pass the burritos?

FriedPhoton
2008-Feb-07, 12:12 AM
You almost had me on that one. Light can be shifted due to gravity but it can also be shifted because of your speed relative to the light. I imagine the same could be said for gravity waves. And because of that, I'm going to go out on a limb and say, yes, gravity waves can be red-shifted.

Steve Limpus
2008-Feb-07, 12:22 AM
Ok, so I followed the redshift link, read a bit there, went on to gravitational redshifting, ended up at frame-dragging which I recall Pamela mentioning in at least one show. And now I have another question...

I gather that frame-dragging is a very slight effect, but could you theoretically have a black hole so massive that it twisted space like a candy-cane, and if so, what would happen in that region of space. Would space be damaged (ok, dumb question, but I have to ask)? What would light traveling through a region like this look like?

I think for this, look no further than supermassive black holes. Wikipedia would be a good place to start again:

http://en.wikipedia.org/wiki/Supermassive_black_hole

I haven't found anything online at all about redshifted gravity, except a few forums which are stumped too. You could try the other BAUT forums, some pretty clever dudes (and dudettes) hang out there. Post back here if you find something. Dunno if Amro found anything?

Steve Limpus
2008-Feb-07, 01:57 AM
Finally found something.

It's an arcticle comparing the GWB (Gravitational Wave Background) to the CMB (Cosmic Microwave Background) and is pretty cool in it's own right... and it says:


"...the GWB would be redshifted, like the CMB. But because of the GWB's earlier provenance, the reshifting would be even more dramatic: the energy (and frequency) of the waves would be downshifted by 24 orders of magnitude."

http://www.aip.org/pnu/2007/split/809-1.html

It's a pretty obscure reference, but for my money gravity is redshifted by expansion.

FriedPhoton
2008-Feb-07, 02:16 AM
Cool article. I was glad to see that real scientists agree with my crackpot theories for once.

Steve Limpus
2008-Feb-07, 02:34 AM
and this:


"..the GW spectrum will be redshifted. All energy redshifts in an expanding universe, and these waves redshift as photons of the CMB do.
Gravitational Waves from the Big Bang
Prof Juan Garcia-Bellido
http://scitizen.com/screens/blogPage/viewBlog/sw_viewBlog.php?idTheme=8&idContribution=354&PHPSESSID=df479587d2d27ba5c5553803696467cf

It would be a pretty dull world without us crackpots... my wife thinks its cute. Sometimes.

AmroB
2008-Feb-07, 08:19 PM
Many thanks for this, Steve. That explains it. I thought that the universe expansion should have some effect on GV!

Cheers

Steve Limpus
2008-Feb-07, 09:38 PM
No problem Amro - great question.

I'm pretty excited about the GWB... kinda like the CMB but a trillionth of a second after the Big Bang (or the Big Bang itself depending how you look at it)... wow!

They say Einstein wanted to 'know the mind of God'... I reckon this is getting pretty close.

LISA will be one to follow. Hope it works!

http://lisa.jpl.nasa.gov/#

Steve

NHR+
2008-Feb-08, 01:00 PM
I'm pretty excited about the GWB...



What an acronym you've got there! Reminds me all too much of the current POTUS (http://en.wikipedia.org/wiki/President_of_the_United_States) and his "nucular" weapons... oh, how I HATE that pronunciation...

Oh, sorry 'bout the OffTopic. :silenced:

Lake
2008-Apr-03, 03:02 PM
In the podcast, it's mentioned that two pulsars that gravitate around each other and also approach each other. It is said that they are getting closer and closer because the system is loosing energy and that energy is used to create gravitational waves.

Now this bring me back to basic physics which talks about all the fun and provocative ways to lose energy: friction, air resistance, and so on.

Apparently, to build a perpetual motion system you need something like frictionless material in a void. But even if you had that, the system still wouldn't work because it's losing energy in the form of gravity ways.

Does that make sense ?

Vanamonde
2008-Apr-05, 07:39 AM
We can speculate all we want but is it not true that right now, in March of 2008, not a single gravity wave has been measured yet?

Fraser
2008-Apr-08, 08:58 AM
Gravity waves have been seen in the interactions between massive objects orbiting one another, like pulsars. They haven't been detected here on Earth with LIGO or any other detector.

Vanamonde
2008-Apr-09, 11:40 AM
Whoa! Pulsars surf on gravity waves! That is too cool! Dick Dale and Vangelis could get together to make the music for that!

I know you're busy but if you could find time to post a reference, I would love to read it.

P.S. Astronomy Cast RAWKS! You and Pamela are doing the wonderful work.

undidly
2008-Apr-10, 01:42 AM
Vanamonde

>We can speculate all we want but is it not true that right now, in March of 2008, not a single gravity wave has been measured>

I live on earth.Here the ground and water goes up and down about every 12.5 hours.
I think it is because of very low frequency gravity waves from a nearby object.

(OK I know the earth is spinning and the moon is hardly moving but what if the earth was still and the moon orbiting quickly would it be right to say the tides are caused by gravity waves?.)

If the earth always showed the same face to the sun and the moon orbited as now then the tides frequency would be set by the moon only.Would the tides be caused by gravity waves from the moon?.

Jupiter causes tides on earth(very very very very small),by gravity waves or what?.

By analogy gravity is equivalent to DC electricity,gravity waves equivalent to AC.
Not a lot of difference between DC and AC.One is easily converted to the other.

Vanamonde
2008-Apr-11, 12:39 AM
I do not believe that tides are the same as gravitational waves.

Tides result from the mutual effect of two static gravity wells. Gravitational waves are produces in both the Earth-Moon and Earth-Sun system but they are so tiny, there is no hope of detecting them.

Gravitational waves result from a changes to a gravity well that causes a radiation of gravitons, something different - a rotation of large masses, "out of round". It creates ripples in the spacetime continuum in a unique and periodic way has yet to be detected on our Earth.

I found a reference to the inference of this in a pulsar pair - by Russell Alan Hulse and Joseph Hooton Taylor, Jr., of Princeton University go the 1993 Nobel Prize in Physics for this. See the wiki at http://en.wikipedia.org/wiki/Hulse-Taylor_binary

And the wiki articile on gravitational waves references this Astronomy Cast episode!

undidly
2008-Apr-12, 02:30 AM
Vanamonde

>Gravitational waves result from a changes to a gravity well that causes a radiation of gravitons, something different - a rotation of large masses, "out of round". It creates ripples in the spacetime continuum in a unique and periodic way has yet to be detected on our Earth.>

I know the earth moon system makes only very weak G waves.If the earth/moon could be detected from the distance of Pluto would you say G waves are being detected?.
Why not from closer?.
Why not from within the system?.
A weight on a spring goes up and down because of the moon.
A -----------------------------------------------a passing asteroid
A ----------------------------------------------- of gravity waves.

Forget gravitons.Nonsense to get government research grants.
Einstein said there is no "force of gravity".(just seems like it,long story)
For a lots of money I will search for centrifugalons and centripetalons.

Vanamonde
2008-Apr-12, 08:46 AM
I know the earth moon system makes only very weak G waves.

No one else knows this. No one has detected any. The best detector we have is so far called LIGO. See http://www.ligo.caltech.edu/

The reference I give earlier to the Wikipedia article is an indirect observation of the effect of G waves causing a binary pulsar system to slow down.

Check it out.

undidly
2008-Apr-13, 01:01 PM
Vanamonde

>Gravitational waves are produces in both the Earth-Moon and Earth-Sun system but they are so tiny, there is no hope of detecting them.>

Tiny but very near.
Again I ask,if we detected the moon orbiting the earth from far away by measuring the small changes in gravity as the moon changes position ,are we detecting gravity waves?.


>Gravitational waves result from a changes to a gravity well that causes a radiation of gravitons, something different - a rotation of large masses, "out of round". It creates ripples in the spacetime continuum in a unique and periodic way has yet to be detected>

And if the moon was touching the earth would the pair not be "a rotating large
mass ,out of round".It is quite near to earth.

Any tides caused somewhere else take energy from the system.
If far away tides do not take the energy then it just spreads out,it's gravity waves.

Vanamonde
2008-Apr-21, 10:53 AM
undidly, I do apologize, I am not Isaac Asmiov and I am in over my head and at the limit of my explanation skills, except to say, a change in the gravity you feel from the moon is not gravitational radiation. It is something different. No one has directly measured a gravitational wave, but of course, we have been feeling tides since we evolved.

The same would go for a dip in an orbit of a satellite when it pass a concentration of mass in the earth or moon. They are called mascons but again, this is not gravitational radiation

I have nothing to add and I am leaving this topic. Feel free to PM me if you have anything new or different to say or ask. I wonder someone else can explain it better.

dcl
2008-Apr-21, 09:47 PM
Vanamonde is correct in what he is saying about gravity waves.

Gravity waves are a phenomenon in fluid dynamics and are commonplace. Gravitational waves are a phenomenon predicted by Einstein's general theory of relativity and have not yet been detected although attempts were made to detect them back around 1970. They are disturbances in spacetime; gravity waves occur on interfaces between different fluids, for example, between water and air.

Gravity waves are produced whenever a fluid moves in a way that disturbs its surface. It takes extremely large masses compressed to extremely high density and in accelerated motion to produce gravitational waves of significant magnitude. They are expected to be detected, if ever, arising only from such extremely dense and massive objects as neutron stars in tight orbits about other objects. In principle, water tides on earth would produce such slight distortion of spacetime that their gravitational waves would be many orders of magnitude too weak to be detected. Ocean tides driven by the moon are gravity waves, not gravitational waves.

NilsJeppe
2008-Jun-02, 05:15 AM
Gravity waves have been seen in the interactions between massive objects orbiting one another, like pulsars. They haven't been detected here on Earth with LIGO or any other detector.

Hm. Can you be more specific? "Seen" but not "observed"? I assume you mean the behaviour of those objects changed in a way consistent with gravity waves?

NilsJeppe
2008-Jun-02, 05:30 AM
Okay, and I would like to add another question to this, or make it two related questions:

If space expands, why shouldn't this affect gravity waves? So, shouldn't gravity waves be "redshifted" for lack of a better term? After all, the space "in" the wave should also expand and thus increase its frequency.

Also, can't space expand faster than the speed of light? So, if spacetime can change its properties (shape) FTL in one way, why are gravity waves required to travel at the speed of light? Not that I disagree that they do, lacking any evidence to the contrary, but I'd be interested in the reason why we think that ST can change in one way faster than light, but not in another way?

(I am aware that the expansion is only at speeds higher than lightspeed if considered over vast distances, but iirc it was much faster initially?)

dcl
2008-Jun-05, 10:42 PM
The wavelength will be greater at points more distant from the source as you suggest, but this would result, as in Doppler effect, in lower, not higher, frequency at those points.

In talking of speed of expansion of space, you need to talk in terms of how fast one point in space is moving with respect to another point, just as in talking of speed of movement of one point in a rubber sheet with respect to another point in the same rubber sheet while the rubber sheet is being stretched uniformly in all directions. The distance between one point in space and another such point can certainly increase faster than the speed of light if the two points are far enough apart. Gravity waves move at the speed of light relative to where they are, not with respect to points arbitrarily far from where they are. The rate of expansion of space at one point relative to another is negligible if the points are close enough together. It works exactly the same for points in space as for points on a rubber sheet. The velocity addition formula in special relativity is not applicable to the expansion of space.

Incidentally, I wish I knew what FTL and ST mean. I wish forum people would stop using acronyms that some of us like me are not familiar with.

dhumphrey
2008-Jun-07, 10:02 PM
Never posted before but had to find an answer to this which I was left with after hearing the gravity wave show! During the show I think it was said that observations of two large objects had shown a decaying orbit indicative of the energy loss due to creation of gravity waves. It was also said that nothing stops gravity waves and they go on forever. Surely they gradually lose energy as they cause matter to grow and shrink and will thus eventually decay away?

dcl
2008-Jun-09, 06:37 PM
Never posted before but had to find an answer to this which I was left with after hearing the gravity wave show! During the show I think it was said that observations of two large objects had shown a decaying orbit indicative of the energy loss due to creation of gravity waves. It was also said that nothing stops gravity waves and they go on forever. Surely they gradually lose energy as they cause matter to grow and shrink and will thus eventually decay away?

Gravity waves would necessarily lose energy as they cause matter to expamd and contract, so your surmise is correct. on the other hand, their interaction with matter being to slight, their loss of eneregy would also be slight.

dhumphrey
2008-Jun-09, 10:01 PM
Thanks for the clarification