How would a passing gravitational wave look or feel? (2017)

https://physics.stackexchange.com/questions/338912/how-would-a-passing-gravitational-wave-look-or-feel

by ynoxinul • 1 year ago

90 37 1 year ago

physics.stackexchange.com

sockboy• 1 year ago

It's fascinating to think about how such tiny ripples in spacetime could be detected and what it might feel like if we could perceive them directly. The scale and subtlety are just mind-blowing.

maaaaattttt2• 1 year ago

On a somewhat related note I'm wondering what the expansion of the universe means for our bodies and matter in general? I think, like the accepted answer suggests, the forces on the atomic level make it so that larger structures get back to a certain equilibrium even if constantly streched equaly in all directions. But I have a hard time imagining what the universe expanding really means on a human/solar system scale. I know of the inflatable balloon analogy, but to me, matter is not on the ballon, rather it is the rubber the ballon is made of.

I have never seen this really explained in details to the general public which I belong to. Maybe that's a sign I'm completely misunderstanding the subject though.

freedomben• 1 year ago

Personally I like the raisins in a baking muffin analogy better as it's 3 dimensional

randallsquared• 1 year ago

The inflatable balloon analogy has the advantage that, two-dimensionally, there's no "edge" beyond which you could travel.

3cats-in-a-coat4• 1 year ago

The whole "expanding Universe" model is repeatedly dealt blows in the past few years, so I'd accept it as a plausible, but not sufficiently demonstrated hypothesis and not waste time bothering about the impact on our bodies.

But if you will, think about it like that. All life adapts to its environment. All life. All the time. Everywhere. And the expansion is not that fast that a single generation of anything from a one day fly to a centenarian turtle, or a millennia old tree has to bother about it. It's invisible at our timescale.

I've always thought why animals were so huge at the beginning. Not just dinosaurs, but insects the size of a large dog. Maybe it's the oxygen rich environment. Maybe it's evolutionary processes shrinking size in time to optimize energy needs and improve survival. Maybe gravity somehow changed in time, or it was the expansion of the Universe or a myriad of sci-fi reasons we can come up with. But life adapted and moved on.

So that's what expansion means for us. One of millions of variables we constantly adapt to. If it's slow enough, no problem. But if it's hitting us fast like the accelerating climate change or technological progress, that... we may have problems with. That's when you see unrest, violence, crime. Wars. Famine. Suffering.

The Universe is not our problem. We... are our own problem.

TeMPOraL• 1 year ago

Almost agreed, but I feel compelled to correct one thing, because it's a common and annoying misconception "nature lovers" harbor:

One of millions of variables we constantly adapt to. If it's slow enough, no problem. But if it's hitting us fast(...), that... we may have problems with. That's when you see unrest, violence, crime. Wars. Famine. Suffering.

No. Famine, starvation, disease, suffering, mass deaths - and even wars - are exactly the process through which life adapts to slow changes. That's what it means for ecosystems to thrive, for nature to be in balance - that balance is held dynamically, by constant cycles of excessive slaughter followed by mass starvation.

We brought a lot of new problems on the table, both for ourselves and all other life, the latter of which can't even keep pace. But senseless suffering and comically painful death - that one came from nature, and we're actually successfully reducing it.

mr_mitm2• 1 year ago

Expansion happens only in the Lemaitre-Friedmann-Robertson-Walker walker metric, which is a solution to the Einstein equations in a homogeneous universe. That's a fine approximation to our universe at the largest scales, but not on the scale of a solar system. Spacetime locally around the earth looks much more like a Schwarzschild solution. So we're not experiencing expansion.

If you are asking hypothetically, if a human body were floating in the intergalactic medium, then yes, the accepted answer you quoted would apply.

oersted• 1 year ago

A translation would be appreciated, OP was asking for an explanation for the general public. Even highly technical people will struggle to understand this if you use such insider jargon without clarification.

Surely expansion is happening at every scale but locally other factors dominate right? To what degree? Is it mainly gravity? Electromagnetic attraction between atoms and/or molecules?

The naive mental model that I have is of two balls tied with a rubber band, each on a treadmill going in opposite directions. Since the rubber band attraction dominates, they slip on the surface of the treadmill and their distance is barely affected even if the surface underneath “expands” outwards quite quickly. Is this a reasonable analogy or is it too simplistic?

Of course the rubber band force is proportional to the distance, while the attraction forces we are talking about are inversely proportional. And I have no idea if the expansion of the universe can be reasonably modeled as an outwards “drag force” on matter. How “sticky” is matter with respect to space?

itgoon• 1 year ago

https://www.youtube.com/watch?v=gEyXTQ9do-c

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

spauldo6• 1 year ago

Matter isn't pinned to the space it's in (source: try walking around). As space expands, the other forces which are orders of magnitude stronger than the expansion of space slide matter along so that distances don't change. You can only detect the expansion of space by measuring the distance between things that are so spread apart that the other forces between them are essentially zero.

CGMthrowaway• 1 year ago

That's an interesting point. The expansion of space still impacts matter on an atomic level though. The space between atom core and electrons influences its bonding abilities and other properties

Analogy, take two attracted magnets, or two opposing electrodes, and expand the space between them. Things change

maaaaattttt2• 1 year ago

That's what I understood from the explanation on stackexchange. But given what you said, if we take the plank length as the shortest length unit, and we consider two theoretical "objects" placed at one plank length away from each other. Does the universe expanding for these two objects mean: 1. the plank length is becomming bigger, 2. more plank lengths are added in between the two objects, 3. Something else and I'm completely off

eggn00dles3• 1 year ago

Matter is very much pinned to the space its in. If the space between two galaxies expands, the distance between the two galaxies grows. If matter wasnt pinned the distance between the two galaxies would remain the same despite the expansion.

Things dont fall to the ground because the earth pulls on them. Earth is pulling in the space around it, and those things come with it. See the river model of general relativity for a more thorough explanation.

biorach• 1 year ago

https://m.youtube.com/watch?v=5U1-OmAICpU

"Brooklyn is not expanding"

kmm• 1 year ago

Without taking into account dark energy or a cosmological constant (so on scales smaller than a few hundreds of millions of lightyears), in the usual cosmological model you can see the expansion of the universe simply as a remnant of the initial kick all matter got from the Big Bang. There is no active pushing anymore, it's just matter moving apart, constantly slowing down due to the mutual gravitational attraction.

So for our bodies, planets, solar systems, even galaxies and clusters, because these are bound (either electromagnetically or gravitationally), the influence of the expansion of the universe on them is not just negligible, it's non-existent.

It's a little different when wo do include dark energy and other mechanisms more complicated than a simple matter or light content. For your intuition, you can think of this as a constant omnipresent negative pressure. We have no idea how it works on scales smaller than those of the observable universe, but if we imagine it works the same on every scale, then it's an extremely tiny force constantly pulling your body apart.

ahazred8ta• 1 year ago

As near as we can tell, space is expanding by about 10 meters per year per astronomical unit. It's 7% per billion years. In practice, it would show up as a very small fudge factor in the tenth decimal place of the gravitational constant.

3cats-in-a-coat4• 1 year ago

Gravitational waves move at the speed of light, doubtful you can "look" at it. If it's that strong it'll just seem like a shake. Like an earthquake. Except it's the universe that's quaking.

vishnugupta• 1 year ago

If everything is quacking then would anyone “feel” it?

3cats-in-a-coat4• 1 year ago

That's a very good question.

If light speed was infinite. If sound speed was infinite. Or even if water waves were infinite, we'd never feel or see anything waving.

We feel a wave by seeing the differential effect of it propagating. Which requires speed with a detectable progression. Compression, decompression. Excitation, de-excitation.

So when everything is quaking, we detect it by neighboring regions not being in sync as to the direction, amplitude, phase etc. of where they're quaking at any given moment.

Just like with normal earthquakes. The ground is shaking. If everything atop shook in perfect sync, you'd miss the earthquake. But otherwise, you feel it, you're destabilized. Not in sync with the ground.

For gravitational waves, it's much harder for us to feel it without special, very sensitive equipment. Normally. But if it was strong enough, we would.

NitpickLawyer• 1 year ago

Gravitational waves move at the speed of light

Do we know if there are mediums (sp? media?) where gravitational waves move slower than the speed of light? Like light does in glass?

3cats-in-a-coat4• 1 year ago

That's an interesting question and it's quite possible the medium matters about gravitational transmission speed. I.e. the question is how is gravity transmitted. We'd need to know a lot more about the structure of subatomic matter & interactions to answer this one. As it stands, we have no theory of gravity at all at the quantum level.

I'm firmly of the opinion there's no "spooky action at a distance". All waves propagate gradually (if rapidly) through a medium, even if we decide to call that medium "vacuum" and define it as empty. Or more modernly, a "field". Waves don't propagate in nothing by definition.

antiquark• 1 year ago

So (in theory) you could hear the chirp of merging black holes, if they were close enough.

In fact, everyone on the planet would hear the same chirp. Someone should comb the historical records (or even, mythologies) for a birdless chirp heard by many people.

yetihehe• 1 year ago

I don't think life on earth would survive aftermath if such chirp was close enough to be heard.

dboreham• 1 year ago

Obviously as "a disturbance in the force".

jeanlucas• 1 year ago

This would be an awesome xkcd video, they're killing it on YouTube

spauldo6• 1 year ago

Probably better as a MinutePhysics video. It's not a real xkcd What-If if the Earth doesn't get destroyed.

I'm really hoping Randall makes some new material soon - I enjoy the videos but I've read all the What-Ifs on his site and own both the books.

bawana• 1 year ago

Interestingly, from a statistical mechanical viewpoint , entropy decreases as space expands.

How would a passing gravitational wave look or feel? (2017)

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