A truly motionless object. Possible?

[DasUnterMensch]

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?

[Q.E.D.]

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.

[CalMeacham]

1.) Motionless relative to what?

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

[Q.E.D.]

Originally posted by CalMeacham
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.

[DasUnterMensch]

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?

[Earthworm Jim]

Originally posted by CalMeacham
1.) Motionless relative to what?

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?

[Q.E.D.]

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.

[DasUnterMensch]

God astrophysics is just sooo intuitive

[other-wise]

Q.E.D. said: 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.

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.)

[cybersnark]

Originally posted by DasUnterMensch
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?

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.

[Q.E.D.]

Originally posted by other-wise
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.)
"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.

[CalMeacham]

quote:
--------------------------------------------------------------------------------
Originally posted by CalMeacham
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.

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.

[Squink]

Closely related thread:
What speed am I ACTUALLY moving through space? (http://boards.straightdope.com/sdmb/showthread.php?s=&threadid=167922&highlight=microwave)

[MC Master of Ceremonies]

And then of course you have zero-point energy........................

[FranticMad]

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.

[other-wise]

Q.E.D. said: "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.


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?

[ForumBot]

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

That would be my only guess.

:smack: Stupid apostrophe.

[MC Master of Ceremonies]

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

[CalMeacham]

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.


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.

[CookingWithGas]

Originally posted by FranticMad
Yes it is possible to be motionless in the most abstract sense, but how would you know? 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.

[DasUnterMensch]

So Heisenberg';s Uncertainty applies to the macro as well?
I'm totally confused now.

[MC Master of Ceremonies]

Of course hesiebergs uncertainity principle applies to macroscopic objects too, it's just that the degree of uncertainty will be microscopic (if you want to look at idealized situations).

Another important thing about the hesienberg uncertainty principle is that it does not just limit the knowledge of the observer, but the 'knowledge' (or behaviour if you prefer) of individual particles.

[eburacum45]

If you took the background microwave radiationas a frame of reference, the Milky way galaxy would be rushing past at 600km per second, which sounds like a lot;
but is a tiny fraction of the speed of light- you would hardly be able to detect any motion once the solar system had scooted away into the distance.
Then you would hit another problem... you would be falling toward the centre of the galaxy..., and also toward any other object you might pass near... if you have mass you would not be able to remain stationary relative to the background radiation without accelerating against gravity.
anyway, all the particles within such a 'stationary ' object would be moving relative to each other unless it were at zero kelvin.

[eburacum45]

And as MC says, ZPE fluctuations perturb even at 0 kelvin.

[Phage]

HIJACK!

What is up with there not being a universal frame of reference? If nothing can move faster than the speed of light (in a vacuum), not even light, then what frame of reference is light using? If you try to shine a light forward in a craft that is moving close to the speed of light, the light would move forward at a speed which from the craft’s standpoint would be comparatively slow. So, isn’t light using a frame of reference that never changes?

[MC Master of Ceremonies]

No, because time is relative too, at whatever speed you are travelling at the speed of light is always the same. So it cannot be used as a reference point.

[dylan_73]

Originally posted by Phage
If you try to shine a light forward in a craft that is moving close to the speed of light, the light would move forward at a speed which from the craft’s standpoint would be comparatively slow.No, from the craft's viewpoint the light would move away at the speed of light.

[scr4]

Originally posted by CalMeacham
So the answer is "no", you can't have an object perfectly motionless in space, because that implies perfect knowledge of position and momentum.
I think "perfectly motionless" only implies a perfect knowledge of momentum. You don't need to know the location.

Although come to think of it, you might have to set the uncertainty in position to be the size of the visible universe, or something like that. But then we get into the unknown territory where quantum mechanics and relativity collide.

By the way, I think Phage has a point. The cosmic background radiation is uniform in only one rest frame, so that is in effect the rest frame of the universe.

[RM Mentock]

Originally posted by other-wise
(I'd really like feedback on this... it caused a minor philosophical brawl among my friends just last night.)
Brain bout!!

Love to see 'em eggheads rolling around in a morass.

[scr4]

Hm, I guess I misread Phage's post. Light itself is not define a reference frame, because the speed appears constant regardless of how fast you are moving.

[Phage]

So, if the speed of light appears constant regardless of how fast you are moving, then the ship with the light would be able to flip them on and illuminate objects ahead of them with the expected speed (e.g.: something 3 light minutes away would be seen as getting brighter about 6 minutes later). How would the object illuminated perceive this approaching beam of light? If they saw the craft, it would be for only an instant as any light that hit it and returned would be only moments ahead of the ship itself. However, they would also be struck with the headlights of the ship well ahead of the ship's passing. Wouldn't that indicate the light from the ship's headlights was exceeding the speed of light?

[Revtim]

Before I thought about the universe expanding, I was about to write that if *everything* in the universe was motionless then there would only be one frame of reference, and hence "true" motionlessness achieved. But I guess expansion rules out even that theoretical possibility.

[eburacum45]

However, they would also be struck with the headlights of the ship well ahead of the ship's passing. Wouldn't that indicate the light from the ship's headlights was exceeding the speed of light?
----------------------
No, the light from the headlights and the image of the ship would arrive simultaneously, at the speed of light.
(thats why it is called the speed of light.)

[SCSimmons]

Originally posted by eburacum45
However, they would also be struck with the headlights of the ship well ahead of the ship's passing. Wouldn't that indicate the light from the ship's headlights was exceeding the speed of light?
----------------------
No, the light from the headlights and the image of the ship would arrive simultaneously, at the speed of light.
(thats why it is called the speed of light.)

I think that Phage's puzzlement is that this seems inconsistent with the fact that, from the point of view of the ship, their headlights do strike the object well ahead of the ship's passing. How can the two events seem virtually simultaneous to the observers at the object but not to the observers on the ship?

Let's run it through from each point of view. For the ship, the object is moving toward them at, say, .99 c. Their headlights strike it as it passes three light-minutes away, then the reflected light reaches the ship three minutes later, followed ~1.8 seconds later by the object. (Evasive maneuvers, Mr. Sulu!) The light from the headlights that was first reflected from the object must therefore have been emitted while the ship was six light-minutes away (plus a few light-seconds; we can ignore that for the moment). Now, what does this look like from the point of view of the observer on the object? Well, the light from the headlights first struck the object when the ship was three light-minutes away, right? So, the ship moving at .99 c won't arrive for more than three minutes after the glow of its headlights turn up, right? Uh, no, that can't be right, because they collide only 1.8 seconds later, barring some impressive reaction times on Mr. Sulu's part. So, obviously the ship only emitted that light about 1.9 seconds ago, at a distance(therefore) of only 1.9 light-seconds. But that can't be right either! WTF?!

You can find the real answer in any good textbook on special relativity. Suffice to say that the two sets of observers will report different time intervals and space separations between the three events (light emitted, light reflected, light returns to emitter), and both sets of observers will be right in spite of their disagreement ...

[Phage]

eburacum45, that is my point! Your reply is in direct contrast to what dylan_73 said: "No, from the craft's viewpoint the light would move away at the speed of light." Same thing with scr4's second post.
The way I see it, the light would not move away at the speed of light, it would move away at whatever speed the craft is below the speed of light. Were the light to appear to the craft to move forward at the speed of light, observers would see a craft emitting a beam moving almost twice the speed of light as far as they could tell.

[Phage]

So basically what SCSimmons is saying is that it works for each person, but in a different way... and somehow reality is ok with that. That is really f*cked up, but it is the best explanation I have been given yet.

[Derleth]

Phage, you're hitting your head on the concept of frames of reference. Each frame of reference (be it inside the craft with the headlights or on the object that craft will hit) is perfectly correct, even if the only thing they agree on is the velocity of light. But they are guaranteed to agree on that much (the speed of light).

Common sense has no place in relativity.

[JasonFin]

Phage, it is an axiom of special relativity that light moves at a constant speed, c, in every inertial (non-accelerating) frame of reference. If a spaceship is moving at .99c in my frame of reference, and shines a light forward, then the light will move at c in my frame of reference (pulling ahead of the spaceship at .01c) and it will also move at c in the spaceship's frame of reference (pulling ahead of the spaceship at c). This works out because each frame of reference will see every other frame of reference as having lengths which are contracted in the direction of motion and clocks which run slowly (both by a factor of gamma=1/Sqrt[1 - v^2/c^2]), and they will also measure different pairs of events as occuring simultaneously. In each frame, this suffices to precisely explain all disagreements.

This may seem unintuitive, but the predictions of special relativity have been confirmed to an extraordinary degree of precision. It is the basis of all modern physical theories.

I see on preview that Darleth beat me to it, but such is life.

[Chronos]

If I am in any sort of reference frame whatever (a planet, a moving starship, even almost falling into a black hole) and you're also in any frame of reference whatever, and I shine a flashlight at you, I'll measure the exact same speed for that light that you will, and that speed will be the same as the speed anyone else would ever measure for any light. Yes, this is counterintuitive, but it's also absolutely true. The light will, however, probably appear a different color to you.

And the "quantum foam", if it exists, almost certainly takes such a form that it can't be used to define a reference frame, either.

[Kivick]

A bit of oversimplification and obvious but:

It is possible to do this but from our perspective it would appear to be moving..

[filmore]

I've often wondered how an atom knows it has stopped in a Bose-Einstein Condensation (http://www.colorado.edu/physics/2000/bec/) experiment. Basically an atoms are hit with a laser beams until they "stop" and then they all condense to the same place. But the atoms are only stopped relative to the tabletop of the experiment, so how do they know to condense?

[MC Master of Ceremonies]

Well filmore, in that experiment, they are not 'stopped', as I said before it is impossible to stop atoms from moving even at 0 K, they are brought down to a very low temp.

[MC Master of Ceremonies]

Oh I see now, do you mean photons? This is a completely different ballgame, nothing to do with relativity, everything to do with quantum physics (it's more like the information of the photon is frozen, than the photon actually stops)

[filmore]

Originally posted by MC Master of Cermonies
Well filmore, in that experiment, they are not 'stopped', as I said before it is impossible to stop atoms from moving even at 0 K, they are brought down to a very low temp.

But they are only at 0 K relative to the tabletop. To another observer, those atoms could appear to be moving very fast and would have a much higher temperature to that other observer. Would the other observer see the BEC as well?

I think I might be misunderstanding something here so feel free to correct me.

[Apex Rogers]

Originally posted by filmore
But they are only at 0 K relative to the tabletop.

0 K refers to absolute zero, which is not measured relative to anything. 0 K is always 0 K.