# B Why remain stationary?

1. Oct 15, 2016

### Mohd Abdullah

Hey guys,

Assume that there is a lone object in existence. No stars, no planets, no atoms, no gravity. Just absolute nothing besides this lone object. What makes this lone object remains stationary? From what I understand, motion is possible if there are more than one object and if there is gravity acting upon several objects. Thoughts?

2. Oct 15, 2016

### phinds

Unaccelerated "motion" is a meaningless concept except in reference to something. If there is only one object in the universe, it is not meaningful to talk about whether or not it is unaccelerated motion. If it is accelerating it can detect that itself (this is a bit simplified)

3. Oct 15, 2016

### Mohd Abdullah

So if I understand you, a lone object assumption is impossible. Right?

Sorry but what do you mean by "If it is accelerating it can detect that itself"? Seems like an unfinished sentence.

4. Oct 15, 2016

### phinds

Yes, "motion" of a lone object is not a meaningful concept if you are talking about unaccelerated motion.

Unaccelerated motion is relative but accelerated motion is not. Example. You are sitting on a train, asleep. You wake up. You see a train right next to yours but going in the opposite direction very fast. You actually cannot tell whether it is moving relative to the ground and you are standing still relative to the ground or it is standing still relative to the ground and you are moving relative to the ground or if BOTH of you are moving relative to the ground.

Now your train comes into a station and slows down (DEcelerates). You don't need any reference point outside of your train to tell that you are decelerating because you can FEEL it.

5. Oct 15, 2016

### Staff: Mentor

No. But it does not make sense to assign an absolute speed value to it. This is also true in our universe with many objects. There is no such thing as "the speed of an object". Only "the speed of an object relative to an arbitrary point of view".

6. Oct 15, 2016

### PeroK

If you were in a region of space so far from anything else that you could detect no other objects: no source of light, nothing; then, what experiment could you carry out to determine how fast you are moving?

Note that if space had some substance, then you could measure how fast you were travelling with respect to the empty space that surrounds you. But, space has no substance and you cannot measure your speed relative to empty space. There is, therefore, no experiment you can do to give yourself an "absolute" velocity.

But, if you have some means of propulsion: something you could throw would accelerate you by conservation of momentum, then you could definitely say that you had accelerated. And if you can accelerate, then motion is certainly possible: it's relative to the thing you threw.

So, the question is not whether motion is possible, but whether you can assign a definite, absolute value to your speed at any time. And, this is what you cannot do. You cannot say: I was absolutely at rest before I threw the object and now I'm travelling at $5m/s$. All you can say is that you have accelerated (by $5m/s$) and are now moving relative to the thing you threw.

If you imagine that the thing you threw eventually disappears from view and is no longer detectable, then there is no expeiment that can distinguish between your original state and your final state. You know that you accelerated and changed your velocity from something to something else, but you cannot distinguish between the two. All you can say is that at the beginning and the end you were moving with constant velocity but you cannot assign a specific value to your velocity at any time. You know the difference between your initial and final velocities ($5m/s$ - and this has the same value in any inertial reference frame) but that's all you can say.

That's what's meant by "all motion is relative".

7. Oct 15, 2016

### FactChecker

As long as there is a defined space-time coordinate system, motion and acceleration wrt that coordinate system are defined. So the question can be posed in terms of a space-time coordinate system without specifying how the coordinate system was determined -- even without stating the existence of other specific objects.

8. Oct 15, 2016

### Mohd Abdullah

Ok there are many responses here. So, in conclusion, if an assumed lone object (be it a lone atom, etc.) is neither moving nor still then this kind of assumption is not possible. It seems, at least, need a multiplicity of objects in existence so the term "still" and "moving" have meanings.

Thoughts?

9. Oct 15, 2016

### phinds

You persist in applying the terms "moving" and "still" to a single object. That is not appropriate. They don't apply since they are relative terms and you have specifically stated that there is no reference object.

10. Oct 15, 2016

### Mohd Abdullah

I see. Sorry because I just want to see the conclusion.

So, that means single object assumption is impossible.

11. Oct 15, 2016

### FactChecker

The approach of specifying a reference object has some complications. If there are only two objects, there is complete symmetry and you can only say that distances are changing, not which one is moving or which one is accelerating. (This is an issue that comes up often in the Twins Paradox.)

12. Oct 15, 2016

### phinds

13. Oct 15, 2016

### phinds

Any object can always tell if it is accelerating, so I don't agree w/ that part of your statement but I do agree w/ the rest.

14. Oct 15, 2016

### FactChecker

Not mathematically except by reference to a space-time coordinate system. But that leaves the issue of why object A's coordinate system is better than object B's. Motion and acceleration wrt a coordinate system is well defined, but the question of why (other than magic and metaphysics) one coordinate system is better than another still should be addressed. Of course, one object feels acceleration and the other does not, but why? The mathematical / logical symmetry of a two object system can only be broken by referencing more objects.

EDIT:
Example:
Two objects, A and B.
With A as the reference object:
Time: 0, 1, 2
A position: 0, 0, 0
B position: 1, 2, 8

With B as the reference object:
Time: 0, 1, 2
A position: -1, -2, -8
B position: 0, 0, 0

Clearly there is relative motion and relative acceleration, but there is no mathematical way to say which is moving and accelerating. Maybe both are. The mathematical symmetry can only be broken by adding something external to this example. That would bring up the question of why that external influence would cause the physical result of one object feeling acceleration and the other not.

Last edited: Oct 15, 2016
15. Oct 15, 2016

### Ibix

Can an isolated object even accelerate? A rocket has to detach a chunk of its mass as exhaust in order to accelerate - then it's not isolated anymore. And arguably wasn't a single particle to start with.

16. Oct 15, 2016

### phinds

Yeah, I'm knowingly glossing over the obvious flaws in saying that a single object can accelerate AND that it can "tell" that it is accelerating

17. Oct 15, 2016

### Ibix

I'm kind of wondering if the "obvious flaws" are why it's a bit tricky to answer. Any macroscopic object can always be divided into two non-isolated objects. And a truly isolated single particle would need some thought from someone who knows more quantum than I do...

18. Oct 15, 2016

### FactChecker

It may not be possible physically, but within a time-space coordinate system there are accelerated and unaccelerated paths defined. I think that the physical issues are mute since the universe has many objects and the 1 object question is so hypothetical. We can imagine almost anything in a universe of only 1 object. If we are talking about concepts, I think that is more of a mathematical question and we would need to specify a space-time coordinate system.

19. Oct 15, 2016

### phinds

Just FYI it's "moot", not "mute". On the actual (substantial) issues I defer to your knowledge.

20. Oct 15, 2016

### FactChecker

Ha! Yes. I can't spell worth s*\$t. I have no answer. That leaves me moot.