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tommac
2009-May-06, 04:14 AM
Are gravitational "waves" really waves? Like would a gravitational wave redshift when it was moving away from you?

Jeff Root
2009-May-06, 05:41 AM
It would "redshift" or "blueshift" the same way light does.

-- Jeff, in Minneapolis

alainprice
2009-May-06, 04:10 PM
It would either be a longitudinal or transverse wave. Same was a sound or water wave.

I would love to know why you are so obsessed with 'shifts'. It's a change in relative energy, bro. You can doppler shift, gravitational shift, etc... It's just a plain old shift and really doesn't mean much about the energy content of what you are considering.

tommac
2009-May-06, 06:01 PM
It would either be a longitudinal or transverse wave. Same was a sound or water wave.

I would love to know why you are so obsessed with 'shifts'. It's a change in relative energy, bro. You can doppler shift, gravitational shift, etc... It's just a plain old shift and really doesn't mean much about the energy content of what you are considering.

Shifts are totally rad! :cool:

Amber Robot
2009-May-06, 06:02 PM
Are gravitational "waves" really waves? Like would a gravitational wave redshift when it was moving away from you?

You mean would a gravitational wave coming towards you be redshifted from a source that is moving away, relative to the rest frame of the source. I think it would be difficult to observe waves that are moving away from you.

tommac
2009-May-06, 06:02 PM
It would "redshift" or "blueshift" the same way light does.

-- Jeff, in Minneapolis

What would that mean? Would the gravitational wave have a wave length, how could we measure that? Is it a pure wave? or some sort of hybrid like light?

robross
2009-May-06, 06:03 PM
It would either be a longitudinal or transverse wave. Same was a sound or water wave.

I would love to know why you are so obsessed with 'shifts'. It's a change in relative energy, bro. You can doppler shift, gravitational shift, etc... It's just a plain old shift and really doesn't mean much about the energy content of what you are considering.

This is a follow-up question to your point. If we just observe a photon/light wave at a certain frequency, but do not know its source (i.e., where it came from), is there any way to determine if its frequency has been affected by a doppler shift?

trinitree88
2009-May-06, 06:12 PM
In the laser interferometers LIGO and LISA...first the approaching wave causes a distortion in one plane, then in another at right angles to the first plane. So, picture you standing at plus x in a big cartesian cube.... gravitational wave approaching, from minus x and for convenience, it is symmetric to the x axis. You'd see the y-axis stretch, then shrink, then return to normal...followed by the z-axis doing the same, as the wave passes by.
If your three mutually perpendicular axes happen to be carrying light beams that are monochromatic, you see the wave interference patterns shift if you've set up Michelson's interfereometers. Long arms give better sensitivity but are inherently more noisy, too.
They are seeing chronic noise at ~ 1500 khz from what they think is the Earth...(it is also the threshold of the Canaries of the Arctic...right whales....which would be odd, "seeing" whales with an interferometer:shifty::lol:)



see:http://oceanlink.island.net/oinfo/acoustics/accomplish.html

01101001
2009-May-06, 06:32 PM
[...] is there any way to determine if its frequency has been affected by a doppler shift?

A lone photon, with no companions to look at? No. Its original frequency is not recorded in it. The only frequency it can reveal is its current one.

alainprice
2009-May-06, 06:35 PM
This is a follow-up question to your point. If we just observe a photon/light wave at a certain frequency, but do not know its source (i.e., where it came from), is there any way to determine if its frequency has been affected by a doppler shift?

Not that I know of. At least not for a single photon.

Once you see a collection of photons, now you have an intensity curve for different frequencies and we can curve fit to guess the source. After the source is estimated, then the red/blue shift can be calculated.

tommac
2009-May-06, 06:47 PM
Usually standard candles such as supernovae are used, right?


Not that I know of. At least not for a single photon.

Once you see a collection of photons, now you have an intensity curve for different frequencies and we can curve fit to guess the source. After the source is estimated, then the red/blue shift can be calculated.

Jeff Root
2009-May-06, 07:02 PM
It would "redshift" or "blueshift" the same way light does.
Would the gravitational wave have a wave length, how could we
measure that?
Gravitational waves must have wavelengths. I believe the theory
of black hole collisions (and the like) predicts that there will be a
predominant wavelength for each collision, which depends on the
masses, energies, and speeds involved in the collision. If so, then
that wavelength should make itself visible in the LIGO detector as
a particular frequency in the detector's motions. The frequency-
wavelength relationship should be the same for gravity as it is for
light, since they should have the same speed.

However, I doubt that the source of any gravitational wave would
be known, and the original wavelength or frequency would not be
known, so the amount of Doppler shift would not be determinable.



Is it a pure wave? or some sort of hybrid like light?
You are obviously referring to the fact that light can be detected
either as waves or as particles. Light consists of photons. The
behavior of photons is described in terms of waves.

Gravitational waves should be the same. Particle theory predicts
that the force of gravity is carried by gravitons. The behavior of
gravitons is described in terms of waves. A difference between
photons and gravitons is that some photons are energetic enough
to detect individually, while no gravitons have enough energy to
detect individually. So the existence or nonexistence of gravitons
will probably never be demonstrated.

-- Jeff, in Minneapolis

Amber Robot
2009-May-06, 07:56 PM
Usually standard candles such as supernovae are used, right?

Depends on what you mean by 'usually'. It depends on what one is trying to get a redshift for. I would say that most redshifts to distant objects are not determined by supernovae.

tommac
2009-May-06, 10:23 PM
Do gravitational waves get weaker when they redshift? In a similar fashion to light waves?

korjik
2009-May-07, 04:34 AM
Usually standard candles such as supernovae are used, right?

Most commonly used ways to measure redshift would be to use a spectral line from absorption. I think the h and k calcium lines are commonly used, probably because they are common lines in stars.

tommac
2009-May-17, 09:30 PM
would a gravitational wave weaken ( red shift ) as it is moving away from you?

WayneFrancis
2009-May-18, 02:16 AM
This is a follow-up question to your point. If we just observe a photon/light wave at a certain frequency, but do not know its source (i.e., where it came from), is there any way to determine if its frequency has been affected by a doppler shift?

for a single photon there is no way to determine what frequency it was emitted at. It is only with large amounts of photons can we use spectral analysis to determine the what the emitting source was and what its red/blue shift is. The more photons collected the more accurate the result.

WayneFrancis
2009-May-18, 02:27 AM
would a gravitational wave weaken ( red shift ) as it is moving away from you?

Ummm yes. If a body is moving away from you then the gravity from said body will weaken.

Now if you are also asking how the passing of virtual photons, or gravitons, react with relativistic motion then I would say you would see the same effect but I would imagine that this effect is very small and would get drowned out even more by the kinetic energy's, of the object in relative motion, increase in gravitational potential.

IE the decrease in gravitons you receive from an object traveling away from you at high speeds will be dwarfed by the increase in mass of said object and even more by the simple inverse square law.

cbacba
2009-May-18, 01:01 PM
Do gravitational waves get weaker when they redshift? In a similar fashion to light waves?


the answer is in your question. Waves are waves. They are a pattern moving through space and conveying energy. Most wave properties are shared between sound and light waves despite being tremendously different.

tommac
2009-May-18, 09:21 PM
the answer is in your question. Waves are waves. They are a pattern moving through space and conveying energy. Most wave properties are shared between sound and light waves despite being tremendously different.

Yes I agree ... however a gravitational wave seems strange to me ...
like what would be the wavelength of a gravitational wave?

As far as blue shift and red shift goes ... it would seem to mean that if a gravitational object was moving towards you that you would feel more gravitaional effect than would be expected and if it was moving away from you , you would feel less than expected.

If this is true ... as you approached a black hole you would feel MORE gravity than expected. Depending on how fast you were approaching the black hole the greater the gravity that you would feel upon approach.

Does that sound right?

grant hutchison
2009-May-18, 10:49 PM
A gravitational wave is a transverse wave: it applies compression/stretch at right angles to its direction of propagation. When one passes it will buffet you, but it won't alter your acceleration towards the source of the wave.
Travel towards a source of gravity waves, and their alternating stretch and squeeze will occur in a faster cycle; travel away, and the cycle will come slower. That's your blue- and red-shift.

Grant Hutchison

WayneFrancis
2009-May-18, 11:45 PM
Grant, is this the same stretching and squeezing that I've heard would be another big effect of falling into a black hole that isn't talked about because it is "to confusing"?

I can't remember where I heard the astronomer talk about it but if I remember correctly he said this stretching and squeezing would probably get you before the spaghettification. I could be mistaken though.

grant hutchison
2009-May-19, 12:08 AM
Grant, is this the same stretching and squeezing that I've heard would be another big effect of falling into a black hole that isn't talked about because it is "to confusing"?

I can't remember where I heard the astronomer talk about it but if I remember correctly he said this stretching and squeezing would probably get you before the spaghettification. I could be mistaken though.I presume, since you mention "spaghettification" as a separate issue, that you're talking about something different from the conventional tidal stretch and squeeze?

Maybe what you're recalling relates to the inner, Cauchy horizon of a rotating black hole. As well as being a surface of infinite blue shift, it's predicted to concentrate gravitational radiation emitted during the collapse of the parent star, forming a sheet of metric perturbations. In a supermassive rotating black hole, these metric perturbations could disintegrate you at the Cauchy horizon, before the tidal forces became too strong.
But there seems to be some doubt about whether the Cauchy horizon can form in a realistic collapse, and about what it would actually do to objects passing through it. All I know about this I learned from Chapter 6 of Michael Lockwood's excellent The Labyrinth of Time: Introducing the Universe.

Grant Hutchison

publius
2009-May-19, 12:35 AM
You know, the simple non-rotating, charged black hole solution has two horizons, with the inner being a Cauchy horizon, as well.

Whether they can exist or not is a matter of some debate. There's these various "energy conditions" (further "common sense" restrictions on the stress energy beyond the regular constraints of GR) that are thought to prevent them from forming in any realistic real world dynamic collapse.

However, if these energy conditions are violated (the Casmir effect apparently violates one), then all bets are off. Apparently, appropriate violations of the energy conditions can prevent the "infinite blueshift" effect on approaching one. But this all well beyond my understanding.

And BTW, if gravitational waves are strong enough to rip you apart, that is the local tidal effect of the wave itself. :) I realize we're thinking of that as something apart from the "background tide", added to it, but from a free faller perspective, tides are tides. :)

-Richard

mugaliens
2009-May-19, 06:37 AM
And BTW, if gravitational waves are strong enough to rip you apart, that is the local tidal effect of the wave itself. :)

I've always found that to be an enjoyable special effect in the various Star Trek films!

agingjb
2009-May-19, 08:15 AM
I not sure I understand what the Doppler effect on gravitational waves would entail. How would the frequency be measured? Do gravitational waves have anything corresponding to a spectral lines?

A probably ill-informed speculation: suppose we could detect gravitational waves from, say, a pair of neutron stars, and determine their frequency, and observe the light or Xrays from the stars, to find their orbital period and their velocity of approach or recession.

Would we expect the observed frequency of the gravitational waves to be, in theory, different from the observed frequency of the orbit?

grant hutchison
2009-May-19, 09:33 AM
And BTW, if gravitational waves are strong enough to rip you apart, that is the local tidal effect of the wave itself. :) I realize we're thinking of that as something apart from the "background tide", added to it, but from a free faller perspective, tides are tides. :)Good point, thanks. I was thinking of tides as arising from the background average gravitational field, and gravitational waves as something superimposed on that. But if you've got a gravity gradient, you've got a tide.

Grant Hutchison