# Sit still!November 17, 2008 2:40 PM   Subscribe

Can any object stay in one place in outer space?

Allowing for the expansion of the universe, etc. It seems that nothing could ever, even possibly, stay in the same place because the same place is never in the same place.

Are black holes affected by the expansion of space? Maybe they stay in the same spot and just chew up the expanding space?

Obviously, I know very little on this topic.

Any thoughts?
posted by zerobyproxy to Science & Nature (22 answers total)

Define place.
posted by weapons-grade pandemonium at 2:48 PM on November 17, 2008 [2 favorites]

I think it would depend on how you determine "one place". If that "one place" is where that object is, then yes, it's staying there-- but everything else is going to be moving around it.

[IANAAP, IANYAP.]
posted by dunkadunc at 2:49 PM on November 17, 2008

It's all relative?
posted by Echidna882003 at 2:50 PM on November 17, 2008

Same place relative to what? There are lots of ways to make something stay in the same place relative to some frame of reference. About frames of reference. Since there's no preferred frame of reference, there's no universal definition of "not moving", true, but there are plenty of other frames of reference in which stuff sits still. For example, for every object in the universe O, there's a frame of reference relative to O such that O is immobile (i.e. has zero velocity and zero velocity change). So really, everything is sitting still.
posted by GuyZero at 2:50 PM on November 17, 2008

First, you need to define "place" - specifically, what is your reference point? When I say, "My cup of coffee is staying in one place," I mean that it is staying in one place relative to myself and to the earth, but me and the earth are hurling through space at an incredible rate.

The fundamental principle of special relativity is that there is no preferred point or reference plane that is "more correct" than any other intertial point or reference plane. So my coffee cup could be "staying in one place" with reference to the surface of the earth, but would be moving very fast in reference to the sun. It could be still in reference to the sun, but moving in reference to another star.

posted by muddgirl at 2:52 PM on November 17, 2008

If you're talking about the expansion of the universe...

Whatever place you're talking about, the universe is expanding from that point, everything radiating out from it. Wherever you are in the universe, it will appear as though space is moving away from that point in all directions, so if you're defining 'place' as it relates to other 'places' in the universe, whatever space you're in is still while all other space is moving.

Place can only be defined as it relates to other places. Place is intimately related to perception and perspective and point of view.
posted by incessant at 2:56 PM on November 17, 2008

OK, let me see if I can clarify. Stationary. So other things move, space expands but the object stays where it was (at the beginning of this example). Let's consider a big rock, in space, at this particular moment. Could it stay at the place and time that it is in right now while other things move? What would allow this to happen?

Lots of interesting responses so far. So it seems that there is a duality to the answer. To my mind though, it seems that it is impossible for any object not to be moving (in the larger sense).
posted by zerobyproxy at 3:24 PM on November 17, 2008

You have to define that it's stationary in respect to something else. Do you perhaps mean that it's stationary with respect to the center of the universe?
posted by zsazsa at 3:28 PM on November 17, 2008

For another aspect of staying in place in space - there are Lagrangian points...
posted by birdsquared at 3:30 PM on November 17, 2008

OK, let me see if I can clarify. Stationary. So other things move, space expands but the object stays where it was (at the beginning of this example). Let's consider a big rock, in space, at this particular moment. Could it stay at the place and time that it is in right now while other things move? What would allow this to happen?

Sure, but I think what other people are saying is that everything can be thought of that way. Say I throw a baseball, and it moves away from me at 80mph (as if). There's really no reason to see it from the ball's point of view, where it's staying still and everything in its immediate vicinity is moving past it at 80 mph.

Same thing with an object floating in space. The object's location can only be determined relative to other objects around it, which are pretty much all in some sort of motion. Your whole situation is basically going to boil down to what you end up defining as "stationary", and how you could ever determine that something is not, in fact, stationary. I'm thinking through it right now, but I think the only way to define something as stationary is to compare it to something else -- either another planet, a star, or the observer. If you and your rock were both hurtling through space at some velocity, but maintained a constant distance from each other, how could you possibly determine that you weren't stationary while other things were moving?

Do you perhaps mean that it's stationary with respect to the center of the universe?
Is there a center of the universe? How would you know where it was? The answer is not "the point where everything is expanding from," because the universe is expanding from every point within it.
posted by LionIndex at 3:50 PM on November 17, 2008

Let's consider a big rock, in space, at this particular moment. Could it stay at the place and time that it is in right now while other things move? What would allow this to happen?

There is no preferred frame of reference. Everything must be measured relative to something else. Ergo, measured relative to the rock, the rock is always stationary. If you're standing on the rock and you see something moving then relative to that the rock is moving.

This is why it's called "The Theory of Relativity". Everything is relative.

There's in no black-on-black hidden coordinate system that demarcates the universe like a gigantic piece of 3D graph paper. Similarly, there is no "center" of the universe. An observer measures the entire universe relative to itself. That's all there is.

If you agree that the universe is expanding, it is always expanding away from you in all directions, regardless of where you are. And, inversely, it is not expanding at all where you are. Therefore the expansion of the universe doesn't really play into it. But whether the universe is expanding or not is a separate issue from that of relative frames of reference.
posted by GuyZero at 3:52 PM on November 17, 2008

Ah, I see what you're saying, zerobyproxy.

Imagine that you are sitting at the table, and you've got a big jar of Silly Putty that you've spread out so that it's flat. That's a reasonable 2-D approximation of the universe. You can mark a bunch of points on the silly putty and watch the distance between them expand as you stretch out the putty in every direction, just like the universe does.

However, in our thought experiment, we can stick a straight pin through the putty and into our table. Then, when we stretch out the putty, the pin "stays in place" while everything else moves, right?

The problem is that, when it comes to the universe, there is no dining room table that a rock could be attached to. Everything we can observe is suspended inside "the putty", so we've got no reference to say, "Oh, that rock is stuck to our metaphorical table, so it's not moving while everything else is". We are, in essense, tiny little molecules inside the putty itself - we can see our neighbors moving farther away, but there's no way we can tell that some pin is "stationary", or even whether the table we're sitting on exists.
posted by muddgirl at 3:53 PM on November 17, 2008 [2 favorites]

To hit another point in the question, yes, black holes are "affected" by the expansion of space. There are plenty of black holes in the centers of other galaxies, and we know that those galaxies and their black holes are receding from us with the Hubble flow. In general black holes behave no differently than any other massive object when it comes to the expansion of space.

[IAAAStudent]
posted by kiltedtaco at 4:32 PM on November 17, 2008

I'm really just expanding on things other people have said here.

The problem is that the definitions of "in the same place" that work on human scales don't work so well when considering things on astronomical scales. My keys stay in the same place most of the time - my jacket pocket, and London's has been in the same place for the last thousand years or so (although it's grown just a bit since them). But what I mean by 'in the same place' is different for both of them. My keys can travel about quite a lot, but if they're in my jacket they're, in an intuitive sense, in the same place as usual. If I want to find them, I know exactly where to look. The same is true of London, though not in the same way. If I want to go to London, then it's easy to locate, although it twirls around the Earth's axis and circles the sun with the rest of the planet.

We might decide that "in the same place" means that we can give the same directions to locate it in a given context. That could be refined further, I'm sure, but it's a reasonable first stab at a definition. As others have mentioned, one of the consequences of relativity is that there's no absolute definition of "in the same place" - there's no top context. One of the problems with your definition of "in the same place" is that it assumes there's a top context, an absolute that we can measure from. Since most things we encounter have a fairly obvious context (the keys relative to the various places I could put them, London relative to the rest of the world), it's counter-intuitive that the universe isn't like this.

So yes, some things in space are in the same place, but it depends on what you mean by in the same place. That varies depending on what you want to know. The Earth orbits the Sun between Venus and Mars, which is a reasonable definition of its place. But if you're sending a rocket to either planet, you need to know where in the orbit it is, so in that context it's not in the same place.

(Not to confuse things too much, but it's maybe worth noting that place with regard to my keys doesn't have any particular physical reference at all, rather it's a choice out of a number of places I could generally put them in - my jacket, my backpack, the table, etc.. Place doesn't even necessarily refer to a location in a measurable space. It can even refer to imaginary spaces. I could tell you exactly where the city of Ankh-Morpork is, for example. Lots more people could tell you where Mordor, Gondor and The Prancing Pony are.)
posted by xchmp at 4:44 PM on November 17, 2008

No.
posted by Benjy at 5:06 PM on November 17, 2008

OK. So, is it possible for an object to be stationary (relative to all other objects) if it could expand on all axis as everything expands away from that moment in space/time?

This stuff is really interesting to me. Thanks for all of the input.
posted by zerobyproxy at 7:53 PM on November 17, 2008

is it possible for an object to be stationary (relative to all other objects) if it could expand on all axis as everything expands away from that moment in space/time?

It would seem that it would never be possible for an object to be stationary relative to all other objects, if so much as any one object were moving relative to any other object.
posted by kidbritish at 8:00 PM on November 17, 2008

Since no reference frame is preferable to any other, you can pick any object you want in the universe, arbitrarily define it to be "stationary," and then say that everything else is moving relative to it. As the universe expands, everything else would seem to be moving away from your arbitrarily-chosen "stationary" object (although at varying rates).

I'm not sure I completely understand your question "is it possible for an object to be stationary (relative to all other objects) if it could expand on all axis as everything expands away from that moment in space/time?"

What I got from that was basically: if you have some solid object (i.e. not a point) could you make the distance between its surface and any object in the universe remain constant as the universe expands, by expanding the object?

The answer, I think, is 'no.'

To see why, consider the one-dimensional case: imagine a piece of putty with marks at 0, 1, 5, and 10. Then we stretch the putty to twice it's starting length. The point at zero is still at zero, but the point formerly at 1 is now at 2, the point at 5 is now at 10, and the point now at 10 is now at 20. Not only have they moved further away from the origin, but the distances between them have increased. Because of this, you cannot simply slide your point at 0 around and find a place where the original distance relationship exists. If you try to move your origin point (this would be the surface of your hypothetical object in space) so that it remains a constant distance away from the point originally at 10, and later at 20, you would end up actually overtaking the nearer points, because they are not receding as quickly.
posted by Kadin2048 at 9:54 PM on November 17, 2008

A clarification:
What I got from that was basically: if you have some solid object (i.e. not a point) could you make the distance between its surface and any object all objects in the universe remain constant as the universe expands, by expanding the object?

The answer, I think, is 'no.'
You can maintain a constant distance between the surface of your solid and any other object (or one object in each direction that you expand the surface towards), but just not all objects in that direction.

Pardon my sloppiness; didn't catch it until after preview.
posted by Kadin2048 at 9:59 PM on November 17, 2008

Realistically the only stationary object that could and does possibly exist is the bubble of spacetime expanding at lightspeed from the original point of the big bang for the past 15 billion or so years.
posted by blue_beetle at 11:27 PM on November 17, 2008

blue_beetle: see everything above. No original point, no bubble of spacetime expanding at lightspeed.

It is meaningful to be at rest with respect to the larger universe and with respect to the cosmic microwave background though. We happen not to be, moving at some hundreds of kilometres per second thataway. But you could easily arrange for something to be at rest. It wouldn't be terribly interesting though.

Such an object would have everything expanding evenly around it and wouldn't be moving through the larger structure of the universe basically. It's not so much that there's something else it's staying still with respect to, it's just that it's no longer seeing slightly different recession with respect to a distant object in front of it and a distant object behind it.
posted by edd at 3:43 AM on November 18, 2008

« Older How to create an interest-based survey that groups...   |   Who pays for the groom's cake? Newer »