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

physics.stackexchange.com

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

#31

Post by mr_mitm »

maaaaattttt wrote: post #23
spauldo wrote: post #14

Earlier quoted context omitted.

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.

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…

It's 3), I'm afraid. There is no shortest length unit (as far as we know), the Planck length is a constant, and its only significance is that at these length scales, we need a theory of quantum gravity to describe what's happening.

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

#32

Post by rantallion »

eggn00dles wrote: post #28
spauldo wrote: post #14

Earlier quoted context omitted.

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.

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…

> 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.

Gravity wells aren't pulling surrounding space toward their centre. They're only pulling other masses that occupy the surrounding space.

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

#33

Post by Kranar »

oersted wrote: post #20
mr_mitm wrote: post #10

Earlier quoted context omitted.

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 h…

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…

No expansion does not happen at every scale. Expansion is a phenomenon that applies only when you average out unbounded systems at very large scales. By unbounded I mean systems whose kinetic energy is greater in magnitude than its potential energy.

Even galaxies themselves don't expand over time.

One question would be what do you hope to gain from a potential analogy. If you want a very down to Earth, practical and somewhat physical understanding, then the simplest and best explanation is that there are systems of objects in our universe, at very very large scales, that have a group velocity that is greater than the escape velocity needed to attract it to any other system of objects, and those systems are observed to be accelerating away from each other. No analogy is needed for this, it's just a fact presented plain and simply.

There is no theory that predicts a cause for this, but the best theory of gravity, general relativity, is a very flexible and open-ended framework that allows one to plug all kinds of different and imagined scenarios into it and see the results, even if those scenarios have no actual physical interpretation. Some people did play around with imagined results like an expanding universe, a contracting universe, an infinitely large universe, a closed but unbounded universe etc etc... General relativity doesn't predict any of these universes but it does let you explore these possibilities.

When Edwin Hubble observed that galaxies are moving away from each other then this observation was made to fit into the existing theory of general relativity. In order to take the raw observation and fit it with general relativity which interprets gravity as a purely geometric phenomenon, it did so by reframing this behavior not exactly as an intrinsic motion belonging to these large scale systems but rather as if these systems are stationary but there is more and more space filling up the universe in-between these large scale objects which gives them their apparent motion.

This is a means of reconciling the geometric view of gravity, ie. space-time, with the actual observed data. The specific technical details of how this reconciliation is performed is as OP mentioned, the FLRW metric which is here but as you said is too technical for most people to appreciate:

https://en.wikipedia.org/wiki/Friedmann%E2%80%93Lema%C3%AEtr...

You might then ask, what does this solution predict happens to atoms, or our solar system? Perhaps it predicts a very small and imperceptible expansion because other forces dominate, but nevertheless it must predict something, right? This is a tempting position, but it's not quite right.

The key reason is that the FLRW metric, which explains Hubble's observation as the literal stretching of space, literally don't make sense and can't be solved for systems like planets, solar systems, or even galaxies because it can only be used if certain requirements/preconditions are fulfilled.

These requirements are present only on the absolute largest scales where the universe looks fairly even/balanced, there is no center of mass, there is no region of the universe that is more special than any other region. In our solar system the sun is a pretty special center of mass and the solar system is not evenly balanced, same thing goes for our galaxy, and hence none of the models currently studied to describe Hubble's observations work for both the extremely large scale universe as well as for other scales.

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

#35

Post by ahazred8ta »

maaaaattttt wrote: post #3

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…

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.

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

#36

Post by spauldo »

eggn00dles wrote: post #28
spauldo wrote: post #14

Earlier quoted context omitted.

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.

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…

You apparently didn't try walking around. Give it a shot - you'll find that the matter you are made of isn't pinned to the space it's in.

Gravity ensures that structures at the cluster level and below don't expand as the space they're in expands. The space they're in is expanding just like it is everywhere (assuming a cosmological constant) - gravity just holds them together. Which is what I mean when I say matter isn't pinned to space - it just slides through it.

Gravity is too weak to affect distant objects, so we see the effects of the universe's expansion when we look at them.

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

#37

Post by eggn00dles »

spauldo wrote: post #36
eggn00dles wrote: post #28

Earlier quoted context omitted.

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…

You apparently didn't try walking around. Give it a shot - you'll find that the matter you are made of isn't pinned to the space it's in. Gravity ensures that structures at the cluster level and below don't expand as the space they're in expands. The space they're in is expanding just like it is everywhere (assuming a cosmological constant) - gravity just holds them together. Which is what I mean when I say matter is…

i walked around and it involved force and energy.

i will clarify. in the absence of other forces, matter is indeed pinned to the space it is in.

you talk about gravity as if its something distinct from what is driving the expansion of the universe.

we have that model, its what newton proposed.

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

#38

Post by spauldo »

eggn00dles wrote: post #37
spauldo wrote: post #36

Earlier quoted context omitted.

You apparently didn't try walking around. Give it a shot - you'll find that the matter you are made of isn't pinned to the space it's in. Gravity ensures that structures at the cluster level and below don't expand as the space they're in expands. The space they're in is expanding just like it is everywhere (assuming a cosmological constant) - gravity just holds them together. Which is what I mean when I say matter is…

i walked around and it involved force and energy. i will clarify. in the absence of other forces, matter is indeed pinned to the space it is in. you talk about gravity as if its something distinct from what is driving the expansion of the universe. we have that model, its what newton proposed.

I believe all of the current theories being seriously considered hold that whatever is causing expansion isn't gravity, so yes I do talk as if gravity is distinct from what is driving the expansion of the universe.

Regarding being "pinned," that still fails to account for inertia. The idea that there's a specific piece of space that we're stuck to implies there's a rest state at which there is no motion independent of any observer. We know that's not the case.

My original point was that gravity and the other forces that hold us together are so much stronger than whatever is causing expansion that the expansion of space doesn't affect us at small scales. The space we're occupying is expanding. We're not dragged along with it. The Triangulum galaxy doesn't move away from us because gravity keeps the Local Group together. We do see the expansion of space between us and distant objects, but that's because there's no force strong enough to hold those distant objects to us. That's not because we're "pinned" to our location, but because the space between us is getting larger.

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

#39

Post by spauldo »

CGMthrowaway wrote: post #21
spauldo wrote: post #14

Earlier quoted context omitted.

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.

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

The magnet analogy works here, so let's give it a try. Put one magnet on a table and then put one under the table so that it's held up by the magnet above it. Then observe what happens. This will take a while, so maybe go get a 2^128 cups of coffee.

The space between the magnets is expanding, just like space everywhere. Assuming the table and magnets are immune to deterioration over time, you can come back after several billion years and the distance between the magnets will have stayed the same. Space expanded, sure, but the stuff occupying the space didn't.

The forces that hold an atom together are significantly stronger than what's holding the two magnets in the example above.

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

#40

Post by spauldo »

maaaaattttt wrote: post #23
spauldo wrote: post #14

Earlier quoted context omitted.

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.

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…

The Plank length thing is just going to add confusion here. Quantum stuff is weird. That's where analogies go to die.

But generally speaking, the answer is 2. That's assuming there's no forces between the two objects.

Space doesn't expand like the outside of a balloon or a rubber sheet - I hate those analogies because they give you the wrong idea.

I'm not aware of any major current theories that say space is quantized, or any theories that have a way of pinpointing a "piece" of space, so the following analogy is flawed. But it should at least point you in the right direction.

Draw a line and put eight dots on it. Draw arrows pointing to the fourth, fifth, and eighth dot. We'll call those dots A, B, and C respectively.

We're going to measure distance by dots. Dots A and B have a distance of 1. Dots A and C have a distance of 4.

Now for each dot, add a new dot before and after it. Measure the distances. Dots A and B now have a distance of 3. Dots A and C now have a distance of 12.

Repeat. Measure the distances. Repeat again. Measure the distances. You'll notice that the distances between the dots you've marked is increasing faster with each repetition, and that the distance between A and C is increasing faster than A and B. An object at any of those points would not be experiencing any force - nothing is pushing or pulling on them - but an observer at any of those points would observe the objects at the other points to be accelerating away from them.

That's sort like how space expands. Of course, space doesn't have "points" as far as we can tell, so there's all kinds of problems with the above analogy, but hopefully it helps.

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