A very strange universe indeed!

[Washington]

The following was taken from the pages of Thinking Physics is Gedanken Physics 2nd ed. by Lewis C. Epstein. Insight Press, 1979. An excellent book on basic physics.

What are your thoughts on this strange nature of our universe?

 

[ArnautDaniel]

A more quantum mechanical approach would allow the object to take every path simultaneously between two points.

We'll assume we know the starting and ending points to simplify things a smidge.

Anyway, while the object is taking every path possible, the paths are sort of summed up to generate what the object actually does.

In appropriate limits the classical "straight-line" paths can be derived from this.

Now this all moves the issue back a bit, because while we are allowing the object to take every possible path we have specify just how we are adding the paths together...and so, why this way of adding and not another?

 

[Allegory]

I don't understand the question.

 

[Paconious]

A more quantum mechanical approach would allow the object to take every path simultaneously between two points.

Something that is truly bizarre! How could the laws of physics break down when dealing with quatum objects? Let me attempt to illustrate this further with a little wave-particle video as my amateur knowledge of physics wont do.

http://www.youtube.com/watch?v=DfPeprQ7oGc

How can matter be at different locations? and how can perception change the whole thing? our universe is indeed very strange.

 

[Washington]

I don't understand the question.

The nature of linear motion is that it is only detectable when there is something by which its possessor (a moving body) can be measured against: another object. If you were the only object in all of space there would be no way to tell if you were moving or not. But, place a rock somewhere else in the universe and you immediately have a reference point to tell you if you were moving or not (assuming the rock was not also moving in the same direction and at the same speed). So linear motion is relative. It only exists relative to other objects.

But the nature or rotational motion is that even without another object in space, it can be detected. This is even apparent in our day to day life. Consider: you're riding down a smooth road at 35 miles per hour. If you close your eyes you would not detect any more forward motion than if you were sitting at home in a chair watching TV. Try hanging a small weight from a string while in a moving car and at home. In both cases the weight will hang straight down. Now go around a curve at that speed. Even with your eyes closed will immediately be able to tell you are moving. And, the small weight will no longer be hanging straight down. And this rotational movement would be detectable if you underwent it as the sole object in the universe. So rotational motion is not relative to anything else, but is an absolute motion unto itself. The question posed by the book is, Why is this so?

 

[Chalnoth]

Why the difference? Well, angular motion isn't a fundamental quantity in Newtonian physics. Its properties are entirely derived from linear motion. That is, most basically, the essence of it. After all, to have angular motion requires a center of motion, which is an arbitrary quantity for a freely-moving body (i.e. one not bound into, say, a solar system or other gravitational potential).

 

[Allegory]

The nature of linear motion is that it is only detectable when there is something by which its possessor (a moving body) can be measured against: another object. If you were the only object in all of space there would be no way to tell if you were moving or not. But, place a rock somewhere else in the universe and you immediately have a reference point to tell you if you were moving or not (assuming the rock was not also moving in the same direction and at the same speed). So linear motion is relative. It only exists relative to other objects.

But the nature or rotational motion is that even without another object in space, it can be detected. This is even apparent in our day to day life. Consider: you're riding down a smooth road at 35 miles per hour. If you close your eyes you would not detect any more forward motion than if you were sitting at home in a chair watching TV. Try hanging a small weight from a string while in a moving car and at home. In both cases the weight will hang straight down. Now go around a curve at that speed. Even with your eyes closed will immediately be able to tell you are moving. And, the small weight will no longer be hanging straight down. And this rotational movement would be detectable if you underwent it as the sole object in the universe. So rotational motion is not relative to anything else, but is an absolute motion unto itself. The question posed by the book is, Why is this so?

Sorry, I should have been clearer. I understand inertia. My statement was more of a "so what?" I don't understand why you suppose this is so out of the ordinary. After all when you're rotating you have different parts of your reference frame accelerating constantly; when you're moving in a straight line at a constant speed that doesn't happen. So why is it surprising, then, that you should be able to detect acceleration vs. no change in velocity?

 

[Washington]

Chalnoth,

Thanks for your answer; however, I don't believe this goes to the heart of the author's questions. BTW, just as a matter of information, the author, Lewis Epstein, is . . .

" a popular physics lecturer and author. He has worked on the Saturn Rocket Project with the Chrysler Corporation and helped develop the original large space telescope intended to be orbited by a Saturn rocket. He was a science writer for the New Orleans Express and is currently a physics teacher/lecturer at San Francisco City College."
​

so I believe his questions are reasonable and not the simple musings of the uninformed. He has also written an excellent introductory book on relativity.

Sorry, I should have been clearer. I understand inertia.

I don't believe the issue is a matter of inertia.

My statement was more of a "so what?" I don't understand why you suppose this is so out of the ordinary.

I don't suppose it is. It's quite ordinary. As the book puts it, the question is one of, why is one type of motion relative and the other absolute? Why are not both either relative or absolute? In other words---as I interpret the author's puzzlement---in respect to the two types of motion, why should nature be so constructed so as to imbue different forms of motion (relative and absolute) to its two variations?

 

[Allegory]

So you're asking why we can tell when we're accelerating, but not when we're not accelerating?

This question doesn't really make a lot of sense. It's like asking why we can see light instead of dark. In one case, something is happening, in the other there's nothing happening. I don't really know how to explain it any better than that. I'm sure the guy who wrote that book is very clever, but it seems like he got caught up on some detail and had some verbal diarrhea.

 

[Washington]

So you're asking why we can tell when we're accelerating, but not when we're not accelerating?

No. The questions have already been well stated in the OP. However, as Epstein said, they are deep questions, which may be why you have failed to grasp them. No harm no foul, Allegory.

 

[Chalnoth]

Chalnoth,

Thanks for your answer; however, I don't believe this goes to the heart of the author's questions. BTW, just as a matter of information, the author, Lewis Epstein, is . . .

" a popular physics lecturer and author. He has worked on the Saturn Rocket Project with the Chrysler Corporation and helped develop the original large space telescope intended to be orbited by a Saturn rocket. He was a science writer for the New Orleans Express and is currently a physics teacher/lecturer at San Francisco City College."
​

so I believe his questions are reasonable and not the simple musings of the uninformed. He has also written an excellent introductory book on relativity.

Well, as far as education goes, it seems I'm more qualified than he is (Ph.D. in physics, after all), though I'm certainly not yet as experienced.

It seems to me that the issue becomes very clear if you just think about what happens to one of the constituent particles. For linear motion, the constituent particles each have the exact same motion as the object as a whole. But for a rotating body, the individual particles are moving in circles, an action which requires a constant force towards the center, making the rotating body under constant (detectable) stress.

 

[sinan90]

Circular motion has acceleration since you are changing direction and acceleration is a vector quantity. Since feeling the acceleration is the same as feeling the affect of gravity pulling you to the earth you can feel the motion.

Liner motion involves no acceleration, and since there is no ultimate reference frame the object in motion would be perfectly right in saying everything else is moving past it.

 

[Allegory]

Unfortunately Washington is going to come in here and accuse you both of being unable to grasp the deepness of this question, because both of you have given essentially the same answer I gave.

I think what the OP is trying to get at is asking why motion happens to work in such a way that linear motion is relative and acceleration is absolute. I think the heart of the answer is because in acceleration there is a change in your velocity vector, whereas with linear motion you experience no change with that regard.

So why, in this universe, can we measure change but not no change? Chanloth's answer looks like a good one, but I'm not nearly as qualified to talk about physics as he is, or qualified at all for that matter haha..(I'm in cognitive science ).

Is there a point when a question goes from being "good" to being "absurd"?

 

[ArnautDaniel]

Well, as far as education goes, it seems I'm more qualified than he is (Ph.D. in physics, after all), though I'm certainly not yet as experienced.

It seems to me that the issue becomes very clear if you just think about what happens to one of the constituent particles. For linear motion, the constituent particles each have the exact same motion as the object as a whole. But for a rotating body, the individual particles are moving in circles, an action which requires a constant force towards the center, making the rotating body under constant (detectable) stress.

Note though that the whole issue of rotation causing a stress arises precisely because linear motion is somehow "more natural".

The stress exists because the portions of the body "want" to travel in a straight line, and a force is needed to keep them travelling in such a line.

The point of the OP is to wonder why travel in a straight line is so natural in our universe, so saying (in effect), "well given that things want to travel in a straight line, rotational motion requires forces and stress and so is so much less natural than travel in a straight line, therefore things want to travel in a straight line" (as you have basically done) doesn't really answer anything.

In fact all the notions touched on in this thread from Newtonian (and Einsteinian) physics can be packaged up in the (poorly named) "Principle of Least Action" (actually stationary action).

So, why do objects "want" to travel the path of stationary action?

Well we can simplify things a bit and assume that there is going to be some functional we can use, that will give us via a variational principle the observed motion in the universe.

Let's assume this.

Then the question becomes:

Why does the functional we use (namely the action) have the form it has and not another (the other, presumably, giving us other paths)?

The answer is going to go back to whether the universe has preferred directions.

The beauty of the "straight lines" is that they allow the universe to have no preferred directions. Other types of motion will require some directions of space to have different effects on motion than others.

If we assume all directions are as good as any other, we get straight line motion.

In more physicsy terms, it gets back to symmetries.

 

[Washington]

Well, as far as education goes, it seems I'm more qualified than he is (Ph.D. in physics, after all), though I'm certainly not yet as experienced.

He doesn't have a Ph.D?

It seems to me that the issue becomes very clear if you just think about what happens to one of the constituent particles. For linear motion, the constituent particles each have the exact same motion as the object as a whole. But for a rotating body, the individual particles are moving in circles, an action which requires a constant force towards the center, making the rotating body under constant (detectable) stress.

So you believe it's the nature of centripetal force that makes rotational motion absolute?

Circular motion has acceleration since you are changing direction and acceleration is a vector quantity. Since feeling the acceleration is the same as feeling the affect of gravity pulling you to the earth you can feel the motion.
Liner motion involves no acceleration, and since there is no ultimate reference frame the object in motion would be perfectly right in saying everything else is moving past it.

I agree. Acceleration and its effects can be produced by two means: change in speed and/or change in direction. And I assume what you are saying here is that rotational motion is absolute because it necessarily incorporates a change in direction.

Unfortunately Washington is going to come in here and accuse you both of being unable to grasp the deepness of this question, because both of you have given essentially the same answer I gave.

What answer? The one you gave in #7? This answer, with its "reference frame " is a far shout from what the other two have said. So, no, I'm not going to do it again. And as for your inability to grasp the questions, YOU admitted this very thing in post # 3: "I don't understand the question." My remark was a matter of conjecture as to why this was.

I think what the OP is trying to get at is asking why motion happens to work in such a way that linear motion is relative and acceleration is absolute.

I think the heart of the answer is because in acceleration there is a change in your velocity vector, whereas with linear motion you experience no change with that regard.

That explains the mechanics, but I think Epstein is asking why is the necessarily so.

At this point, having been pressed into revisiting long-forgotten physics,---thanks to the responses of everyone here, which does include yours, Allegory,--- I'm beginning see Epstein's questions as not so much an issue of physics, but, if anything, one of philosophy, and then not a very meaningful question at that.
Now, with that said, I really appreciate ArnautDaniel's post, which does visit the more philosophical side of the question.

 

[Chalnoth]

Note though that the whole issue of rotation causing a stress arises precisely because linear motion is somehow "more natural".

The stress exists because the portions of the body "want" to travel in a straight line, and a force is needed to keep them travelling in such a line.

The point of the OP is to wonder why travel in a straight line is so natural in our universe, so saying (in effect), "well given that things want to travel in a straight line, rotational motion requires forces and stress and so is so much less natural than travel in a straight line, therefore things want to travel in a straight line" (as you have basically done) doesn't really answer anything.

In fact all the notions touched on in this thread from Newtonian (and Einsteinian) physics can be packaged up in the (poorly named) "Principle of Least Action" (actually stationary action).

So, why do objects "want" to travel the path of stationary action?

Well we can simplify things a bit and assume that there is going to be some functional we can use, that will give us via a variational principle the observed motion in the universe.

Let's assume this.

Then the question becomes:

Why does the functional we use (namely the action) have the form it has and not another (the other, presumably, giving us other paths)?

The answer is going to go back to whether the universe has preferred directions.

The beauty of the "straight lines" is that they allow the universe to have no preferred directions. Other types of motion will require some directions of space to have different effects on motion than others.

If we assume all directions are as good as any other, we get straight line motion.

In more physicsy terms, it gets back to symmetries.

Hmm, thought I mentioned this, but I guess I left it out of my first post. Sorry about that.

Yes, ultimately it has to do with symmetries. I'll see if I can't explain it in another way:

The reason that objects like to move in straight lines is that there exists some translational symmetry: when it is true that movement from place to place in some region changes nothing, then this means that linear momentum will be conserved. If linear momentum is conserved, then one has to have some sort of interaction to change the direction or speed of anything's motion: you have to have some sort of force.

The next question is, what about angular momentum? If linear momentum being conserved leads to linear motion being natural, won't conservation of angular momentum lead to circular motion being natural? In a word, no. Not in the least: movement in a straight line also conserves angular momentum, and what's more it conserves angular momentum about any point you might wish to measure it: remember that angular momentum requires one to define a point around which things rotate.

So, one might try to think of a situation where circular motion is more "natural" by proposing a system that is rotationally invariant, but not translationally: it's the same if rotated about some point, but different places are different. Our own solar system is a perfect example of this: rotate about the Sun and everything looks much the same (ignoring the planets and other objects for the time being). But move around from place to place, and things look quite different: the Sun will be either in a different direction or at a different distance from you. The first thing to notice here is that this is not nearly as natural as the system that conserved both linear and angular momentum, the system that led to straight line motion.

And yet, even here circular motion is by no means "natural" at all: orbits around the Sun can be any conic section you desire. You can have parabolic orbits, elliptical orbits, hyperbolic orbits. Circular orbits are an extreme special case. So there really is no way to get a system where circular motion is the "natural" thing.

 

[sinan90]

I think the answer of why accelerated motion is detectable while uniform, linear motion isn't is just simply the unsatisfying answer of that's just the way it is. I think the problem of this type of idea is people always want to, and quite rightly so, know why it happens, but sometimes I feel that there is no real explanation possible, which in my opinion is deeply unsatisfying.

 

[ArnautDaniel]

And yet, even here circular motion is by no means "natural" at all: orbits around the Sun can be any conic section you desire. You can have parabolic orbits, elliptical orbits, hyperbolic orbits. Circular orbits are an extreme special case. So there really is no way to get a system where circular motion is the "natural" thing.

Sure there is:

Place a charged particle in magnetic field which is constant throughout the space, and confine its motion to a plane to which the field is normal.

Give it a starting velocity and its motion is naturally "circular".

But of course we've introduced a special direction (the direction in which the magnetic field is pointing), and thus made a situation which isn't the same in all directions.

Again, you are assuming properties of space that make linear motion "natural", and then saying linear motion is "natural" without justifying those properties.

The OP could be recast as "Why should we assume space has the properties P which make linear motion natural as opposed to other properties?"

 

[Chalnoth]

Sure there is:

Place a charged particle in magnetic field which is constant throughout the space, and confine its motion to a plane to which the field is normal.

Give it a starting velocity and its motion is naturally "circular".

Actually, no. This is only the case if its velocity parallel to the field is identically zero. The natural motion is a corkscrew, of which a circle is an extreme special case (yet again).

 

[ArnautDaniel]

Actually, no. This is only the case if its velocity parallel to the field is identically zero. The natural motion is a corkscrew, of which a circle is an extreme special case (yet again).

The phrase in my post:

"confine its motion to a plane to which the field is normal"

requires that the velocity parallel to the field be zero.

...

You are an experimentalist, aren't you?