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

[maaaaattttt]

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.

[spauldo]

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.

[mr_mitm]

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.

[kmm]

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.

[biorach]

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

"Brooklyn is not expanding"

[itgoon]

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

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

[ahazred8ta]

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.

[freedomben]

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

[3cats-in-a-coat]

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.

[3cats-in-a-coat]

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.

[NitpickLawyer]

> 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?

[vishnugupta]

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

[bawana]

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

[antiquark]

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]

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

[sockboy]

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.

[dboreham]

Obviously as "a disturbance in the force".

[jeanlucas]

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

[spauldo]

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.