# Do gravitational waves have different frequencies?

1. May 9, 2010

### Mu naught

I'm wondering if gravitational waves can have a wide range of frequencies and wavelengths much like EM waves, or if they are more uniform and only vary in amplitude.

Also, should gravitational waves experience a Doppler shift?

2. May 9, 2010

### Staff: Mentor

They would vary depending on the orbital period of the source.

3. May 9, 2010

### Mu naught

ah, but we can measure the orbital period of things quite accurately can we not? I ask because, if we can someday detect gravitational waves, it seems to me they should be a very accurate way to measure the distance to objects which we only have rough ideas about now.

4. May 9, 2010

How so?

5. May 9, 2010

### Mu naught

Because if we know the intrinsic frequency of the G waves a body is emitting by being able to visually measure it's period and determine it's mass, then by measuring Doppler shift we'll know exactly how far away it is. Unlike EM waves where we have no way of knowing what frequencies a body emits intrinsically.

6. May 9, 2010

### Staff: Mentor

The Doppler shift would be the same for the visual measurement and the gravitational measurement, so it would not provide any additional information. Also, we do know characteristic spectral frequencies that are emitted for EM waves, so we can already determine Doppler shifts.

7. May 9, 2010

It is true that gravitational waves from inspiral compact binaries would provide a standard candle for accurate distance measurements. I don't think this is quite true for general sources of gravitational waves.

8. May 10, 2010

### hamster143

Consider this: if the binary system recedes from us at velocity that gives it measurable z>0, then its successive observations would be shifted by the same amount as the EM Doppler shift. So we can't really measure its "true" frequency. We'll measure some number that will happen to be equal to the frequency of the G wave, rendering the mechanism useless for the purposes of measuring Doppler shifts. (Well, maybe those frequencies won't be 100% exactly equal, I think you'd need to do some GR calculations to confirm that ... but I'm pretty sure that first-order effects will cancel out.)

In this aspect, EM is much nicer, because there are specific patterns corresponding to spectrum lines of known elements. The problem is usually with determining the natural brightness of the object.

And, most important of all, we are nowhere near close to measuring G waves of objects at z>>0. For now, we're talking about launching some space probe by 2020 that _might_ be sensitive enough to detect G waves of binary star collisions and black hole mergers within our own galaxy. As of today, I'd say we're about 5 orders of magnitude short in terms of distance, therefore 10 orders of magnitude short in terms of sensitivity.

Last edited: May 10, 2010
9. May 10, 2010

### nicksauce

Completely false. LIGO could today detect black hole mergers of 10 solar mass black holes out to a distance of 3Mpc, and a merger of 50 solar mass black holes out to a distance of 120 Mpc. (Ref: http://arxiv.org/abs/0711.3041). So we can see well out of our galaxy.

10. May 10, 2010

11. May 10, 2010

### nicksauce

Redshift 2 or 3 sounds a bit high to me... I wonder if, being from 1994, that paper is outdated.

12. May 10, 2010

It's possible but I don't have any more modern numbers. The noise curves for Advanced LIGO predicted in that paper are very similar to the ones predicted today by LIGO.

13. May 10, 2010

### hamster143

I stand corrected.

Yes, it seems that we can observe black hole mergers in our Local Group with LIGO already ... unfortunately, since LIGO has been running for close to 10 years and so far it has found squat, we can conclude that either G waves do not exist or black hole mergers are exceedingly rare.

What I should have said is that we're hoping to launch a probe that might be sensitive enough for gravitational waves originating from some of the "regular" orbiting binary systems in our galaxy.

14. May 10, 2010