# B Can gravitational waves interfere with each other?

K

#### KBon

Related Astronomy and Astrophysics News on Phys.org

#### mfb

Mentor
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

Mentor
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.

K

#### 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

Mentor
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

Mentor
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

Mentor
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

Gold Member
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

Mentor
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

Gold Member
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

Mentor
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

Gold Member
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

Mentor
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

Gold Member
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

Mentor
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

Gold Member
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.

"Can gravitational waves interfere with each other?"

### Physics Forums Values

We Value Quality
• Topics based on mainstream science
• Proper English grammar and spelling
We Value Civility
• Positive and compassionate attitudes
• Patience while debating
We Value Productivity
• Disciplined to remain on-topic
• Recognition of own weaknesses
• Solo and co-op problem solving