# A truly motionless object. Possible?

Just remember that your standing on a planet that’s evolving,
revolving at nine hundred miles an hour.
It’s orbiting at nineteen miles a second, so its reckoned,
a sun that is the source of all our power.
The sun and you and me and all the stars that we can see
are moving at a million miles a day.
In an outer spiral arm, at forty thousand miles an hour,
of a galaxy we call the milky way.

-Monty Python
http://www.ppsa.com/magazine/Galaxy.html

I was whistling this tune the other day and started wondering if a object could be made to stand motionless in space from a universal stand point for a measurable amount of time, or even if a state of true motionless could exist.

Well can it?

Since there isn’t any universal frame of reference, then no. It is possible for an object to be completely motionless with respect to some other arbitrary object. Empty space itself has no features that allow one to assign a universal reference point, and even if there were, space is expanding, so that wouldn’t work out either. Motion (or motionlessness) is relative, not absolute.

1.) Motionless relative to what?

2.) If you know position and momemtum simultaneously, you’re violating Heisenberg’s Uncertainty Principle. So, no.

It’s perfectly possible to know the position and momentum of a macroscopic object, like a baseball to an arbitrary degree of precision. It’s only at the paticle level where such things become uncertain.

If space is expanding, it has to be expanding from a single point in space. Could that point be the reference? Or am I totally missing something?

I’ve heard it said that at Planck scales space itself “becomes frothy”. Now, I have no idea what this is supposed to mean, but to me ‘froth’ implies that there is some quality of space itself that could potentially be used as the reference.

Is that not true? Or is this where the “Relativity / Quantum Mechanics” disparity exists? Or (perhaps most likely) did what I say have no meaning at all?

Not quite. Try this experiment: Get a ballon and partially inflate it. Use a magic marker to make dots all over its surface. Now blow it up some more and observe what happens. Every dot will move away from every other dot, and although you can arbitrarily choose one dot against which to measure the others, as soon as you change your point of reference to a different dot, the original point will seem to be moving as well. As I said, all relative.

God astrophysics is just sooo intuitive

Q.E.D. said:

What’s the difference between “an arbitrary degree of precision” and “uncertain”?

(I’d really like feedback on this… it caused a minor philosophical brawl among my friends just last night.)

Peachy, let’s try it! Go ahead and find that reference point from which the whole universe is expanding. Put your finger on it. If you can do that, I can put together a device that isn’t moving relative to it and your question will be resolved!

Of course, it may take me a few years and cost a few billion dollars. Got your checkbook?

Kidding aside, if you define a point of reference, I’m sure someone can build something that is motionless with respect to it.

“To any arbitrary degree of precision” means you can make a measurement to the limit of accuracy of any equipment you have. "Uncertain"means at some point, more sensitive equipment will not give you any more precise results.

In other words, it’s not possible.

Even for a macroscopic object, Heisenberg’s holds. You can’t know both position and momentum to arbitrary precision.

What speed am I ACTUALLY moving through space?

And then of course you have zero-point energy…

Yes it is possible to be motionless in the most abstract sense, but how would you know?

Heisenberg only talked about the impossibility of measuring an object accurately. As long as you don’t measure it, then it remains undisturbed. So this abstract object could be motionless as long as you ignore it.

And since everything is moving (as far as I know, anyway), there is no way to prove that your object is, in fact, motionless. However, my uncle Edward comes about as close to absolute motionlessness as humanly possible.

Q.E.D. said:

Yes, but if both the measurements and consequent “precise results” are arbitrary in the first place, isn’t it a bit redundant to describe any claims of accuracy as “uncertain”? Are we just claiming “accurate within this arbitrary context”? And if so, why are measurements at the particle level non-arbitrary?

Take it into space, and lower its temperature to absolute zero?

That would be my only guess.

:smack: Stupid apostrophe.

ForumBot, I mentioned zero-point energy earlier, even at absolute zero a particle still oscillates.

You fellas need to look into Heisenberg’;s Uncertainty Theorem more closely. Although it’s often given in terms of a measurement problem, the real import of it is that (provided you “buy” modern quantum mechanics) a state in which you can even have a precisely defined position and a precisely defined momentum simultaneously cannot exist, regardless of whether a person (or a machine, or anything) is actually measuring it.

So the answer is “no”, you can’t have an object perfectly motionless in space, because that implies perfect knowledge of position and momentum.

Posts prior to the quoted one have hit on this point but it may bear repeating.

What do you mean by “the most abstract” sense? The problem here, notwithstanding the Uncertainty Principle, is that motionless is undefined except with respect to another object.

If you are in an airplane, you think yourself motionless as you look at the interior of the craft, and with the windows shut there is no way to determine whether the plane is flying at 500mph or sitting on the ground. Physics in all intertial frames (i.e., not accelerating) is the same so even the concept of motionlessness is nonsensical except with regard to some other object.