B Can gravitational waves interfere with each other?

[KBon]

Just a thought I had...

 

[mfb]

Theoretically: Yes. They have energy, so the have an effect on each other. In practice: Completely negligible. Tens of orders of magnitude too weak to be relevant.

 

[snorkack]

Energy is irrelevant. Light gives the same interference pattern no matter how dim it is.
What matters is whether we can detect it.
For the common source of gravity waves - inspiralling black holes - are there any directions of space into which gravity waves are not emitted, for reasons of symmetry?

 

[mfb]

Ah, I interpreted the question as interaction. The regular wave interference is still there at any intensity, of course.

For the common source of gravity waves - inspiralling black holes - are there any directions of space into which gravity waves are not emitted, for reasons of symmetry?

Parallel to the axis of orbital angular momentum the intensity is tiny.

 

[KBon]

Ah, I interpreted the question as interaction. The regular wave interference is still there at any intensity, of course.Parallel to the axis of orbital angular momentum the intensity is tiny.

What would it look/feel like? Would there be gravity changes / oscilations?

 

[mfb]

Unless you are extremely close to black holes in the merging process, you don't feel anything. The events detected by LIGO changed the lengths of the arms by 1 part in 10^-21. If you are extremely close, it might feel like you are pushed/pulled a bit, but "from within".

 

[snorkack]

How much is a man stretched by 1 decibel sound at 250 Hz?

 

[mfb]

1 decibel is an odd value to choose, but whatever. It corresponds to about 20 µPa. No idea about Young's modulus for a human as a whole. Pick your favorite number. With the 14 GPa for the human bone listed there, we get 10^-12 length changes. Most parts of the human are significantly softer and will deform more.

 

[snorkack]

1 decibel is an odd value to choose, but whatever.

Well, that´s about the hearing threshold for healthy human ear. Wasn´t sure whether it is 0 db or 1 db that is quietest sound which can be heard.

we get 10^-12 length changes. Most parts of the human are significantly softer and will deform more.

The 10^-21 stretch of gravity waves sounds like a small number, but the 10^-12 stretch of 1 db sound also sounds like a small number.
Certainly 1 db sound at 250 Hz does not feel like "being pushed/pulled" - it is felt by ears alone.
So could sufficiently strong gravitational waves be perceived directly by naked ear as a quiet sound?

 

[mfb]

10^-12 is the stretch in bones, everything else will be stretched more, especially the eardrum.

So could sufficiently strong gravitational waves be perceived directly by naked ear as a quiet sound?

Maybe.

 

[Blibbler]

Good question and the interesting thing about gravitational waves is that they are a distortion of their medium so not only do they interfere by addition and subtraction but also by multiplication and exponentiation - amplitude and frequency modulation: two interacting gravitational waves will modulate each other's phases and their rates of change of phases.

 

[snorkack]

How do gravitational waves interact with (weak, test) electromagnetic waves?

 

[stefan r]

10^-12 is the stretch in bones, everything else will be stretched more, especially the eardrum.

I do not believe it works that way. If you stretch body parts using a force then the rigidity matters. The gravity wave is stretching space time. You would not feel anything. Soft tissues change in exactly the same way that hard tissues change.

 

[mfb]

The gravity wave is stretching space time. You would not feel anything.

The body parts would expand/shrink to their original size, assuming the frequency is low enough to make that possible. Different body parts would follow at different speed.

 

[stefan r]

The body parts would expand/shrink to their original size, assuming the frequency is low enough to make that possible. Different body parts would follow at different speed.

If you ride the vomit comet you perceive micro-gravity through one part of the cycle. At high altitude acceleration of Earth's gravity is lower than at low altitude. The distortion of time-space changes but you feel the same micro-gravity. The bicep, humorous bone, and a measuring tape wrapped around the arm are all effected by dilation in the same way. [Tidal forces are different. Might make this example bad]

My understanding is that the laser light in the LIGO apparatus is the same frequency in both arms and at any point in the apparatus. The photons are arriving at the detector at different times. The arrival time depends on which leg they traveled and the amplitude of the gravity wave passing through the leg. The wavelength of the light is the same if measured at any point. So interactions between the light and matter will be the same. Particle-particle interactions are also the same anywhere in the LIGO apparatus. The observation of when two distant events occurred is effected by the gravity wave. The events themselves are not effected.

 

[mfb]

LIGO is so long that the mirrors are effectively floating in space relative to each other. Your human body is not (at the frequencies LIGO is interested in).

 

[stefan r]

LIGO is so long that the mirrors are effectively floating in space relative to each other. Your human body is not (at the frequencies LIGO is interested in).

https://en.wikipedia.org/wiki/Gravitational_wave

This occurs despite such free objects never being subjected to an unbalanced force. The magnitude of this effect decreases proportional to the inverse distance from the source.

The area enclosed by the test particles does not change and there is no motion along the direction of propagation.[citation needed]

I am not an expert on this topic. Gravity waves passing through the galaxy would dissipate if energy was doing work on the molecules in objects.

A diagram of LIGO looks like a scaled up Michelson-Morley detector. Did I miss something.

 

[snorkack]

Does gravitational wave cause the shape of Earth to change, the way the change of direction to Moon does?

 

[mfb]

Gravity waves passing through the galaxy would dissipate if energy was doing work on the molecules in objects.

Gravitational waves - gravity waves are something different.
Gravitational waves are doing an incredibly tiny amount of work on molecules.
What you cited is the LIGO situation - independent objects.
A human body doesn't consist of independent objects. If the distances within a solid object decrease in one direction and increase in another, it induces stress in the object.

@snorkack: That is a different type of change.

 

[stefan r]

If the distances within a solid object decrease in one direction and increase in another, it induces stress in the object.

If a 1 meter iron bar is clamped and tightened to 0.99 meters it is under stress. If the length of the space in the clamp and length of the iron bar are equal before, during, and after an event then there was no stress.

 

[mfb]

Forget the clamp, the iron bar itself has the necessary force.

If the iron bar has absolutely no internal forces, the distance between its ends will decrease and increase periodically according to the GW. Imagine a 30 nHz gravitational wave at h=0.01, so we have one massive oscillation per year. Do you expect the iron bar to be 0.99 meters long for some month, and 1.01 meters for some other month?
That is not what would happen. The iron bar would keep its length to an extremely good approximation, which means the ends will not be in an inertial frame - they will feel an acceleration, outwards while the space is contracting and inwards while space is expanding along the length of the bar.

 

[snorkack]

What is the shape into which a gravitational wave will distort a sphere, such as a planet?

 

[stefan r]

What is the shape into which a gravitational wave will distort a sphere, such as a planet?

An ellipsoid is a shape. Planets are not usually spheres.

 

[snorkack]

And tides also distort spheres into ellipsoids. Correct?

 

[litup]

Well, that´s about the hearing threshold for healthy human ear. Wasn´t sure whether it is 0 db or 1 db that is quietest sound which can be heard.

The 10^-21 stretch of gravity waves sounds like a small number, but the 10^-12 stretch of 1 db sound also sounds like a small number.
Certainly 1 db sound at 250 Hz does not feel like "being pushed/pulled" - it is felt by ears alone.
So could sufficiently strong gravitational waves be perceived directly by naked ear as a quiet sound?

The difference between 10^-12 and 10-21 is a billion to one, so assuming things being equal, the gravity wave would have to be at least one billion times stronger to be heard. I think if you actually were close enough to say a binary black hole to hear it you would be in deep doo doo.

 

[stefan r]

And tides also distort spheres into ellipsoids. Correct?

Slightly ovoid. But I think it is fine to think of both tides and rotation as creating an oblate spheroid. A spheroid is a special case of ellipsoid where 2 axis are equal.

My impression is that the gravity wave does not create an oblate spheroid. Starting with a perfect sphere 2 of the axes change in opposite ways. So 3 equal diameters become 3 non equal lengths.

Might be worth pointing out that tides are different from wind wave height. Something well above high water could get wet. Tidal waves and storm surges are also different.

 

[stefan r]

The difference between 10^-12 and 10-21 is a billion to one, so assuming things being equal, the gravity wave would have to be at least one billion times stronger to be heard. I think if you actually were close enough to say a binary black hole to hear it you would be in deep doo doo.

Anyone close to a black hole is in deep. The wave would be minimal concern.

An oil tanker (or kayak) on the open ocean can get hit by a tsunami without the crew noticing. Same tsunami can destroy coastal civilization hundreds of kilometers away. Boats do not sink even if the swell is higher than the deck.

Your ear drum and all of the fluids that it contacts are inside the same wave. There is nothing to hear.

 

[snorkack]

The difference between 10^-12 and 10-21 is a billion to one, so assuming things being equal, the gravity wave would have to be at least one billion times stronger to be heard. I think if you actually were close enough to say a binary black hole to hear it you would be in deep doo doo.

You are not in deep doo doo when you hear 0 dB sound. You do have some discomfort and hearing problems when you hear 120 dB sound, but it´s still not immediately fatal.
The source of 10^-21 amplitude gravitational wave was estimated to be 10⁹ lightyears away. So a gravitational wave might have an amplitude of 10^-12 when the source is a lightyear away.
If a 60 solar mass black hole were to fly by Solar System at a distance of 1 lightyear, at a relative speed of 200 km/s, what effects are to be expected on Solar System?

 

[mfb]

If there is no matter falling in, we would just note the gravitational lensing effect on starlight passing close to it, most notably in Gaia data.

 

[stefan r]

If a 60 solar mass black hole were to fly by Solar System at a distance of 1 lightyear, at a relative speed of 200 km/s, what effects are to be expected on Solar System?

Can we change that to "minimum distance". 300 km/s is easier if you calculate in your head, 0.1% light speed. 1000 years to go from 1.414 light years to 1 light year.
Not much recognizable change to Earth orbit because the pass is a few thousand years. The path of the sun-earth barycenter shifts toward the black hole.
If the black hole was 64 solar masses then an Oort cloud object at 1/8th light year would feel equal gravity, the orbit changes a lot. A comet with an orbit not parallel to the black hole would shift orbital planes. You would have to calculate the effect for each orbit and the timing. Some of the comets will fly through the inner solar system. Some get captured by Jupiter and become regular comets. Kuiper belt objects shift a little. Some Kuiper objects could interact with each other but it is also possible that objects would have interacted but now they do not.
The heliopause and termination shock would move further from the sun in the direction of the black hole.

 

[snorkack]

If there is no matter falling in, we would just note the gravitational lensing effect on starlight passing close to it, most notably in Gaia data.

The heliopause and termination shock would move further from the sun in the direction of the black hole.

Why in that direction?
There would be some matter falling in. The sparse interstellar gas of Local Bubble.
But if you have a black hole moving at high relative speed through sparse interstellar gas, what kind of disturbance is created? The gas accelerates on approach to the hole - then falls past event horizon with most of its kinetic energy.