Mike asked:

This may be a question for a Physics professor, but it does have some philosophical implications too!

Can physical things actually "move" without an external reference to relate its motion to? Let's
imagine a universe where there are no other physical bodies (planets, stars, pogo sticks, Big Macs,
etc) besides our own body. We have attached to our backside a rocket pack. We throttle-up the
rocket and immediately feel the intense acceleration. After a good long burn we disable the rocket.
Are we moving? If you say, "Of course you are moving! You felt the acceleration, didn't you?" Then
prove it! Movement is defined as a continuous change in position RELATIVE to another physical
object. So, without the existence of any other physical objects to relate to, is it possible to move?

============

Consider this situation: suppose you hold a bucket full of water in your hand, just letting it hang. Now,
suppose the room, even the earth, rotates around that bucket. Will anything happen to the water in
the bucket? No. Now suppose you rotate the bucket (not revolve, rotatearound a central axis) and
keep the room and the earth still. What happens? The water climbs the sides of the bucket, right?
This seems to be an example where absolute motion is not reciprocal; where you can tellwhether it's
you or the universe which is moving, doesn't it. I'm still not sure how this is resolved... one proposal
I've seen is that this kind of spin, if enoughof the universe were moving around the bucket, would
cause the water in the bucket to rise. Of course, another thing about this example is that the water in
the bucket is accelerating, not moving uniformly (i.e., its motion vector is continually changing in
direction, although not in length), and we're out of the realm of special relativity, which is what your
question seems to assume, but actually is outside of, and into the realm of generalrelativity, where
gravity as acceleration, time dilation, and various other nonlinear effects start to take place. To relate
it to your example, in a continuously acceleratingrocket, you wouldbe able to tell that you were
moving, because a) you'd experience weight from the acceleration, and b) when you got out of your
rocket, after you returned, you'd look around and see that while 5 hours, say, had passed for you, 5
days, perhaps, would have passed for the rest of the universe while you accelerated, turned around,
and decelerated back to your original starting point. This is the classical "time dilation" effect of
general relativity, and there are other effects, e.g., a change in shape and increased mass. Now, the
restof the universe could nothave been moving, because the time dilation effect is asymmetric, as
you can see. If you had sat still and everyone else shot away from you into space, then returned, then
everyone elsewould have had the dilation, while you would not. Things are symmetrical so long as
acceleration is zero.

The question is, whydoes this happen? Is there an effect of the total mass of the universe on all its
components (so that if the wholeuniverse rotated around the bucket, the water wouldrise)? Is there a
"fabric" of space (despite Michelson & Morley) which is being "pushed against"? Is the speed of light
limitation causing the particles in your body, etc., to require more and more energy to accelerate as
their speed increases (i.e., the energy has to "catch up" with them)? Well... none of these are
employed today. The explanation of these effects is a geometrical one, involving the curvature of
space and time combined: space-time curvature, and the math is rather complex.

Here are some websites that might help:

http://en.wikipedia.org/wiki/General_relativity

http://math.ucr.edu/home/baez/RelWWW/wrong.html

http://www.astro.ucla.edu/~wright/relatvty.htm

Have fun!

Steven Ravett Brown

That's a very good observation, Mike! And at least in philosophy your question belongs among the
oldest to have been discussed. I call it the "Paradox of Motion", for the same reason you do; and
among the very first philosophers there were two, Heraclitus and Parmenides, who acquired
"immortal fame" by proposing exactly opposite solutions. Very roughly, the first proposed that motion
is the fundamental state of the universe anyway and that nothing ever sits still. The other said the
whole thing is a sophisticated illusion, that we live in a static universe (single block) and movement is
a play of light and shadows on our senses. Well, there's a little more to it than I've given here, but in
any case theirs was not the last word ever spoken on the issue. It may not be beside the mark to
mention in passing that in science the question tends, unfortunately, often to get shoved under the
carpet.

In addition, a much deeper issue is, as it were, attached to it. As long as we speak about matter only,
it could be argued that motion or no motion amounts pretty much to the same thing. If no conscious
intelligence is there to register the motion, by what logic might we wish to say yea or nay? But there
does exist a conscious intelligence; and now the deep issue I alluded to is this.

Imagine you are in a pitch black room which is also asensory in other respects. Suddenly you see two
billiard balls, one white, the other red, stationery in the middle of nothing. (Assume their light source
to be internal so that you still cannot see anything else.) Now you would, as a matter of course,
assume that these balls are situated on a billiard table. So be it, even though for you it is mere
conjecture. But all of a sudden, the white ball rolls rapidly towards the red and collides with it. In the
result the white ball is deflected from its course and the red one takes off and disappear a fraction of
a second later lets say into a pot. A question arises from this scenario which offers testimony either to
an ingrained prejudice or lets you in on a fundamental truth. You choose.

It is this: if you were asked, what moved the white ball, you would without a seconds deliberation urge
some cause: whether a third unseen (black) billiard ball, or an unperceived tilting in the table, or a
sudden influx of directed air pressure etc. With any of these causes, however, you would immediately
understand, without being prompted, that they in turn demand similar causal explanations; that tilting
the table (for example) might be due to a crack in the floor, which occurred because of an
earthquake, etc etc. This way you could, if that was your frame of mind, continue through to the Big
Bang. Once there, of course, you're not allowed to ask for further causes: for this is the great deus ex
machina that answers all questions.

(Mind you: the white ball hitting the red in such a way that it rolls straight into a pot is a bit much of a
coincidence too!).

If, on the other hand, you were informed that, also unseen by you, a billiard player happened to be
present and used an ordinary queue to push the white ball, then you would know the cause of motion
of the white ball. But you would not enquire after, indeed hardly feel a need to do so, the cause of the
queues motion. The billiard players motion does not require a causal explanation.

So there is an answer of a sort in all this thought experimenting. The motion of matter demands an
explanation, and except in the trivial sense of supposing "impetus" or "momentum" to be conveyed
from one object to another, we have none. All the standard explanations like gravitation, heat,
electrical potential etc. simply beg the question or push it up from one explanatory level to the next,
where the same question awaits you again. By contrast, agency requires no explanation beyond
assuming a "free will", and this applies to bacilli as much as to us. In consequence some
philosophers, aware of this fundamental discrepancy in causal chaining, have proposed either that
the same causes operate hidden from us in organic matter as well, while others maintain that agency
precedes matter, that without it, we could not explain the simplest facts of the universe.

If you're interested in pursuing this (but be warned it is a difficult as well as contentious subject
matter), you might sample Schopenhauer (The World as Will and Imagination) or Leibniz (New
System) together with some secondary literature (check Janaway or Magee on Schopenhauer, and
the Oxford annotated edition of the New System), for versions of "agency first" idea; or you might
delve into Barbour's The End of Time. And of course, let no-one discourage you from reading
Heraclitus and Parmenides, who started this ball rolling (lots of cheap editions, e.g. The Origins of
Scientific Thoughtby G. de Santillana in Mentor, or Presocratic Philosophersby Jonathan Barnes in
Penguin).

Jürgen Lawrenz

Newton's law: to each action there is an equal and opposite reaction. Here you are moving relative to
the contents of your your rocket exhaust (which is a physical object besides your own body). In
practice, I think you will find it impossible to construct a thought experiment in which the traveller is
both (a) accelerating and (b) the only object in the universe.

David Robjant

37