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Comment by mrkramer | Hacker News Reader

>If spacetime is the medium of the waves, why are these waves hitting anything, and what?

"Gravitational waves are disturbances in the curvature of spacetime, generated by accelerated masses, that propagate as waves outward from their source at the speed of light." [0]

They are hitting everything what is on their way just like sea waves are.

[0] https://en.wikipedia.org/wiki/Gravitational_wave

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It depends what “everything” is though. We exist in spacetime, so we are as much the medium of the wave as everything else is. What are the ‘rocks’ in the analogy? What’s ‘wearing’ like the rocks? We don’t know enough about the spacetime or what’s out of spacetime to say.

I'm not a physicist but it doesn't seem hard to understand: "Water waves, sound waves, and electromagnetic waves are able to carry energy, momentum, and angular momentum and by doing so they carry those away from the source. Gravitational waves perform the same function[0]."

So as water wave is hitting rock or any other object(mass/energy) on its way gravitational waves should hit also everything and anything on their way until they lose energy and collapse.

"Gravitational waves are constantly passing Earth; however, even the strongest have a minuscule effect and their sources are generally at a great distance. For example, the waves given off by the cataclysmic final merger of GW150914 reached Earth after travelling over a billion light-years, as a ripple in spacetime that changed the length of a 4 km LIGO* arm by a thousandth of the width of a proton, proportionally equivalent to changing the distance to the nearest star outside the Solar System by one hair's width[0]."

Gravitational waves are all around us but they have minimal impact whatsoever unless I suppose some large event(source) propagates strong gravitational waves near us.

*Laser Interferometer Gravitational-Wave Observatory

[0] https://en.wikipedia.org/wiki/Gravitational_wave

>So as water wave is hitting rock or any other object(mass/energy) on its way gravitational waves should hit also everything and anything on their way until they lose energy and collapse.

Gravitational waves are vibrations in spacetime. They interact very weakly with matter. They travel at more or less the speed of light without 'hitting' any rocks. The question is what are the 'rocks' - since gravitational waves don't really interact with matter significantly, and we don't really understand the makeup or source of spacetime.

That was my original question to you, and the answer isn't obvious, because we don't actually know.

>Gravitational waves are vibrations in spacetime. They interact very weakly with matter. They travel at more or less the speed of light without 'hitting' any rocks. The question is what are the 'rocks' - since gravitational waves don't really interact with matter significantly, and we don't really understand the makeup or source of spacetime.

They can have big impact: "The waves can also carry off linear momentum, a possibility that has some interesting implications for astrophysics. After two supermassive black holes coalesce, emission of linear momentum can produce a "kick" with amplitude as large as 4000 km/s. This is fast enough to eject the coalesced black hole completely from its host galaxy. Even if the kick is too small to eject the black hole completely, it can remove it temporarily from the nucleus of the galaxy, after which it will oscillate about the center, eventually coming to rest. A kicked black hole can also carry a star cluster with it, forming a hyper-compact stellar system. Or it may carry gas, allowing the recoiling black hole to appear temporarily as a "naked quasar"[1].

And idk what do you mean by "source of spacetime" you mean quantum origin[2]? https://knowablemagazine.org/article/physical-world/2019/qua...

[1] https://en.wikipedia.org/wiki/Gravitational_wave

[2] https://knowablemagazine.org/article/physical-world/2019/qua...

"I'm not a physicist but"... You should be. That was a nice postulation.

When we have a sound-wave it means air-pressure changes along the peaks and troughs of the wave.

When we have a gravitational wave what measurable quantity changes?

Gravitational waves change the definition of length and time. If you have some baseline pressure in a room, a pressure wave causes a small region to be briefly higher or lower pressure. If you have a meterstick and a stopwatch, a gravitational wave causes the meter to grow bigger or smaller and the stopwatch to run fast or slow. It's as if you are briefly closer to some immense object like a black hole which causes time dilation - indeed that's exactly what it is. No matter how ridiculously far away they are, as two celestial objects orbit each other they are continually moving closer and then further away from you, and while gravity drops off with distance it never goes to zero. You are feeling the gravitational pull of that celestial object on the other side of the universe getting infinitesimally stronger and weaker as the distance between you and it shrinks and grows, or more accurately as the distance between you and where it was all those millions of years ago when the waves were first emitted shrinks and grows.

So you're saying that measuring gravitational waves means we measure how much a given distant mass pulls us towards it and the value of that pull-force goes up and down like a wave?

Can we measure the "pull" of a specific distant mass?

Or do we measure something else like the change in distance between two points?

Well the pull of gravity is really just the warping of spacetime. A gravitational wave communicates the change in the gravitational field caused by an accelerating mass. We measure this by shooting lasers along two paths and comparing how long it takes light to traverse the paths, the warping of spacetime causes this value to change. You could use this value to directly calculate how much the pull of the distant object upon us varied. But we can only see the variation, not the absolute pull, of these distant objects; and even then only for extreme cases like black hole and neutron star pairs moments before they collide.

Good explanation thanks.

// we can only see the variation

Can we also see the direction from which the wave is coming from ?

Ask a physicist but as far as I understood gravitational waves are dents in spacetime[0] and as they travel their energy, momentum, and angular momentum change over time because of interaction and collision with other "stuff" in space. Idk how you can measure any of that.

[0]https://www.esa.int/Science_Exploration/Space_Science/Gravit...

Is there a simple way to explain what a "dent in spacetime" means? How can it be measured.

I don't doubt that science and you all are right, I'm only asking questions in the hope that there is a simple explanation that would make a light-bulb go off in my head.

Correct, it's all just spacetime. This is what Einstein was after in the last years of his life - a unified geometric theory, where mass was not separate from the sheet, but rather, part of it - a deformation of it.

[1] https://www.arxiv.org/pdf/0706.0190v2

Water waves can crash into eachother. There's no reason that spacetime waves cannot either. You can get some hint of this if you look at the oscillations that comprise the Higgs field.

The negative curvature of gravitation should be cancelled out by the positive curvature of matter and energy.

https://en.wikipedia.org/wiki/Zero-energy_universe

We, and everything else, are stuff floating in the sea of spacetime. Just as a ship would bob up and down as an ocean wave passed along but is not itself part of the wave, so too are we affected by gravitational waves as the pass through spacetime. The difference is instead of up and down motion, the distance between two points in space and time expands and contracts.

>We, and everything else, are stuff floating in the sea of spacetime. Just as a ship would bob up and down as an ocean wave passed along but is not itself part of the wave, so too are we affected by gravitational waves as the pass through spacetime.

Exactly it's called Frame-dragging[0]: "Frame-dragging is an effect on spacetime, predicted by Albert Einstein's general theory of relativity, that is due to non-static stationary distributions of mass–energy. A stationary field is one that is in a steady state, but the masses causing that field may be non-static -rotating, for instance."

[0] https://en.wikipedia.org/wiki/Frame-dragging

I'll make a guess that matter is like bubbles in spacetime. For some reason the bubbles remain stable and due to cleverness of equations governing spacetime, it allows to form stable complex structures around those bubbles. This is even a verifyable hypothesis: check what happens with GR equations in topologies with tiny spherical holes.

I guess the question is what is spacetime made of, which is the big problem in physics right now. Whether it's loop quantum gravity, string theory, or one of the many other theories that exist, the question of what is spacetime is what people are trying to solve to figure out quantum gravity.

What is spacetime made of? Nobody knows yet.

Bubbles + thermal noise. Noise causes vibration, vibration makes bubbles stable. Vibration also causes bubbles to stick together due to Casimir effect.

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