# The cause of motion

1. Sep 12, 2006

### topito2

I have to participate in a debate about whether motion has cause or not. Do you know of any book or website I could check to get prepared for my debate?

2. Sep 12, 2006

### Staff: Mentor

What do you mean by cause? Do you mean in the sense of Newton's Laws? Or in some more philosophical sense?

3. Sep 12, 2006

### Danger

There are a lot of different types of motion, as well, so it might be helpful to narrow it down. You have the paths of subatomic particles, Brownian motion in liquids, vibrations in solids, 'regular' motion such as things falling or rolling, stellar and galactic movements, etc..

4. Sep 12, 2006

### -Job-

Motion is a very basic phenomenon. Possibly motion is at the source of human logic. For instance, addition, a basic operation, can be observed through motion. Example, a body is at position x, you move it by y (add y to x) and obtain the new position x+y.
There is some equivalence between motion and addition. And there is another smililarity between motion & addition in that they're hard to explain without making use of the very thing you want to explain.
You'll find it hard to explain motion such that your explanation does not make use of motion of some kind. Similarly, if you try to describe addition as some function f(x), it will be hard to avoid using addition itself in such an f(x) (or some operation that is itself reducible to addition).
What is deducible from this is that, possibly, if motion is what we use to explain phenomenona, then we can't explain motion itself, because explaining motion with motion is not a valid, or useful, explanation.

Last edited: Sep 12, 2006
5. Sep 12, 2006

### Danger

Well, you can simplify that by saying that addition would not exist without thought, which is dependent upon chemical reactions, and therefore atomic motion, within the brain.

6. Sep 13, 2006

### Astronuc

Staff Emeritus
Somewhere there has to be a force to get things going.

As berkeman suggested, Newton's laws is a good place to start.

7. Sep 13, 2006

### topito2

The type of motion I'm talking about is regular motion as things rolling or falling. Newton's Laws do not give a cause for motion. They explain how something gained the motion it has, but not the cause of motion itself.
Forces are not the cause of motion, but forces cause "a change" in motion. I mean, if something has a straight line motion with a velocity of 3 m/s and a second later it has a velocity of 5 m/s, Newton's Laws would say a force interacted with that someting, changing its motion status, but Newton's Law would not explain why that something had a straight line motion with a velocity of 3 m/s at the beggining.

8. Sep 13, 2006

### Staff: Mentor

Of course it does. Unless you want to go back to the Big Bang or something, and then yes, Newton's Laws would not apply for a while after the Big Bang.

9. Sep 13, 2006

### Danger

That's getting pretty tricky, because to itself, nothing has any motion. ie: It's at rest within its own reference frame and everything else is moving. If I go to the fridge for a wobbly-pop, for instance, I'm not actually moving. The movement of the planet is bringing my beer to me. That's quite handy, since I'm too lazy to do it myself.

10. Sep 13, 2006

### -Job-

Of course if my fridge is on a corner of the room opposite to your fridge and we both go for a beer at the same time we have to be careful lest we unintentionally split the planet in two.

11. Sep 13, 2006

### Danger

:rofl:
Yeah, that was a pretty silly example.

12. Sep 13, 2006

### WhyIsItSo

Ah, that explains why the return trip to your couch is such hard work; you are working against an entire planet...

...and all this time I've been blaming the beer *chuckle*

13. Sep 13, 2006

### cliowa

Make it simple: god, design.
Be honest: You don't know, and nobody does.
But don't you feel that somehow it's not a sensible thing to ask?

14. Sep 13, 2006

### masudr

Motion? easy.

Choose your object's position. Choose your position. Define some co-ordinates. Do a first-derivative of the co-ordinates of your object with respect to one of your co-ordinates. If it's not zero, then you've got motion.

15. Sep 13, 2006

### -Job-

So what you're saying then is that a non-zero derivative is what causes motion. Somewhere in the universe someone is doing a derivative and whenever it's not 0, something moves.

16. Sep 13, 2006

### masudr

Sorry, I meant to say, that's what motion IS. The reason I said that is explained below.

Now, many things can cause motion. For example, an odd co-ordinate system. And I can eradicate the notion of motion by transforming to a co-ordinate system in which the object appears to be stationary.

17. Sep 13, 2006

### Danger

What on Earth are you saying? Leave the theology out of it; this is a serious science forum. There is no question that isn't sensible to ask, because the asking is what engenders answers. If scientists had your attitude, everyone would still think that the Earth was flat and the universe revolved around it. Putting your head in the sand doesn't work around here. :grumpy:

edit: Just saw your newest post, Masudr. Well said. That is the basis of the reference frames that I mentioned earlier. Relativity at its best.

Last edited: Sep 13, 2006
18. Sep 14, 2006

### cliowa

Clearly in general I agree that there is no question you shouldn't ask. What I wanted to illustrate is that this question, profound as it may seem, is rather philosophical, not scientifical. Don't you see that the "cause argument" never ends? When you found "the" cause for the motion, you're gonna ask: What's its cause? You'll end up nowhere.
Physics does not give answers to any cause at all. We simply describe nature, HOW nature works, NOT WHY nature's working that way. But still, physics offers many explanations, which may seem as if they were causal, but in the end of course they're not. The great power of physics is uniting different descriptions, finding basic mechanisms which are common to some phenomena. You can then describe - and if you like explain - the rest on the basis of these basical mechanisms. Aside from those however, there is no explanation available, for the reasons I mentioned above.
So asking for the ultimate cause is a little tricky. Firstly, we don't even know whether there is an ultimate cause at all. Secondly, it is not part of the physics program to find it. But please don't think I wouldn't like to find a final cause, it's just that I would not be so overhasty as to postulate that nature is working in exactly the same way, thinking in terms of causes as we do.

Now, that is quite untrue. In the history of science one important thing has been the existence of people who know what kind of explanation they are looking for. Galileo observed and described, he did not explain. Kepler got his laws out of observations, it was a purely statistical thing (guided by some great intuition, of course). For Newton everything was a god-given puzzle he was to solve. So when he had made up his theory of universal gravitation, lots of people did not like it, for the obvious lack of an explanation. Nobody knew why on earth there was something like this spooky force at a distance. Newton knew very well of this and attributed it to the divine.

So you see, these were all descriptions of observations in a mathematical setting. These descriptions then led to the explanation of some phenomena, which could not be described equally well without the assumption of validity of the mathematical models.

That's not what it is. Do you know what I'm saying?

19. Sep 14, 2006

### Danger

Now that you've explained your position, I agree completely... with one critical exception. To me, the word 'cause' references how, not why. That leaves it to scientists, not philosophers. Had the OP said 'reason' or 'purpose' or something similar, I would have recommended that it be moved to the Philosophy section.

20. Sep 14, 2006

### topito2

The debate date is close and I'm getting as much info as I can. I am kind of nervous since my teacher is very tough when it comes to arguments. If you say something in her class you better be prepared to support what you say

Most of you has given me definitions of motion, which is not what I need, but perhaps the clue to the answer is in the definition of motion itself. According to Physics for Scientists And Engineers 6E by Serway & Jewett force is not the cause of motion but the cause of changes in motion. So Newton's Laws would explain why an object had a change in its motion status but would not explain what caused the motion it has in the very beggining.

After thinking about it, I've decided the argument I will use is that motion has a cause: the observer's system of reference. If an object has motion it is so because the system of reference of the observer has the observer to see the object moving.

21. Sep 14, 2006

### ZapperZ

Staff Emeritus
Maybe what you need to do is this.

Consider yourself to be sitting on one of the gas particles. Everyone has (at least I hope they have) a video (or even under a microscope) of Brownian motion. So if you are one of the Brownian particle, what do you see? You see EVERYTHING in motion with respect to you.

Now let's say you are the earth. You look out into the rest of the universe. What do you see? You see EVERYTHING is in motion relative to you.

What is the conclusion here based on such observations alone? That the NATURAL condition of our universe is that everything IS in motion with respect to some observer somewhere. In fact, it is the LACK of motion of something that is unusual. Something that happens to not be moving with respect to something else is a special case and in our universe, it doesn't occur very often. We just think it is common due to our limited perception of our world.

Why do these things move? There can be 2 different reasons (if not more). The first one is the "initial condition". For example, take a mass of something into space, and explode it into a gazillion pieces. Without any external forces acting on all the gazillion pieces, what do you think will happen at a later time? Each of the gazillion pieces will see other pieces all moving. It will HIGHLY UNUSUAL for one piece to be moving with exactly the IDENTICAL VELOCITY as another. The probability of that occuring is extremely small. So given the initial condition, it is EXPECTED that every single pieces are moving with respect to each other.

The second explanation is that at the microscopic scale, and at finite temperature, motion is the rule (we don't even need to go into QM and invoke the HUP or zero-point energy). Just accounting for thermal vibrations alone is sufficient to indicate that everything is in motion.

Zz.

22. Sep 16, 2006

### -Job-

If the universe was frozen for a moment, would we be able to distinguish the objects that had motion prior to the freezing from those that didn't? I don't mean from an abstract point of view, as in "this egg is floating in mid air, hence it must be falling" but rather, is there any concrete physical experiment that would indicate that an object had motion?
I think not because motion is a property of a 4D object. There's no 3D object with the property of motion, so there shouldn't be any indication, from looking at a 3D object, that it had motion.

Last edited: Sep 16, 2006
23. Sep 16, 2006

### Danger

In that last scenario, I don't think that it would be possible. You can't even measure the Doppler shift of 'frozen' photons. By that same token, you wouldn't be able to see anything because the photons would never get to your eyes.

24. Sep 16, 2006

### turbo

This guy's treatment of motion is quite interesting, and he poses lots of interesting questions.

http://motionmountain.dse.nl/welcome.html [Broken]

Last edited by a moderator: May 2, 2017
25. Sep 16, 2006