# What the heck is motion anyway?

1. May 13, 2004

### Namloh2000

what the heck is motion anyway??

How can things even move?

2. May 13, 2004

### arildno

The german philosopher Hegel liked Zeno's paradoxes a lot; perhaps you'll find your answer in "Wissenschaft der Logik"

3. May 15, 2004

### Alkatran

How could things not move? If things couldn't move would you have a static universe, or no universe at all because everything was jammed into the same, unmoving spot? What if everything is moving at speed Y, except Object X, is X moving, or everything else? If it was X, if X is going c right and Y is going c left, what's the distance between them after 5 seconds?

You couldn't ask the question on how things move if they didn't, so they do.

4. May 18, 2004

### Namloh2000

It was after reading all of Zeno's paradoxes that I decided to pose the question. I am also familiar with Hegel.

5. May 18, 2004

### MiGUi

Let be an object in a n-dimensional space. If we have a frame system and it base of n independent coordinates {$$x_1, x_2, ..., x_n$$} to refer its position, we say that the object is moving or is variating its j-esim coordinate if
$$\frac{dx_j}{dt} \ne 0$$

Math is cool :rofl:

6. May 21, 2004

### Stevo

You assume that your mathematics describes something physically real - you are assuming that real things move, which is precisely what we want to prove (or disprove).

Of course, we may define motion purely mathematically, but that doesn't answer the physical question of how things move.

7. May 21, 2004

### Stevo

What Zeno was criticising was the view that space is a continuum. In mathematical terms, he was criticising the view that space is complete.

Zeno basically assumes two things: An infinite summation of objects of finite size will not converge; Any summation of objects of zero size will give zero.

What he purports to show is that if space is constituted by an infinite number of points (which allows for infinite divisibility), which have zero size, then you can't add them up to get a finite distance.

Of course, we know now why he's wrong. You can't deduce the length of a line by adding up the number of points on it. The number of points on any line is infinite, regardless of the length of the line.

8. May 29, 2004

### loseyourname

Staff Emeritus
Zeno's issue was that an infinite sum cannot be finite, that is, an infinite number of points cannot be summed up to equal a finite line. This is his first paradox, where he divides a line in half, then the half in half, and so on infinitely. If quantum theory is extrapolated to say that all quantities are made up of discrete units, then you can't even say that a line is made up of points. Points would have no real existence. Objects with no spatial extent cannot constitute an object that has spatial extent. Looking at it this way, you reach a point at which you no longer can divide the distance in half; there is a minimum distance. I'm not sure if this is the planck length in M theory or in quantum theory, of if it is some other distance (I'm not that well-versed on physics). Does anyone here know?

Of course, I'm not considering Zeno's other three paradoxes of motion, but this should deal with the first.

9. Jun 3, 2004

### wuliheron

According to the best theory today, motion/change and the lack of it are relative. If I could fly fast enough, it would appear that I am not moving, the earth is simply rotating under me. Have I actually slowed down? There is no absolute standard for determining what is and is not moving. All we can note is that relative to something else we perceive motion or the lack of it.

As for Zeno, he believed the universe is indivisible, indestructable, immortal, and unchanging. All motion/change he insisted was illusory and God's creation is the imbodiment of timeless perfection.

10. Jun 4, 2004

### p-brane

What is the reading on your radar gun when you measure the speed of a car going 90 miles an hour from your position on board a car doing 60 miles an hour? (Both cars are heading in the same direction)

11. Jun 4, 2004

Staff Emeritus
At those speeds, the deviation between the correct relativistic addition formula and the approximate linear one are too small to measure without special equipment, so the linear answer 30 mph is OK. Now try it with the speeder going .9c relative to earth and the radar going .6c.

12. Jun 5, 2004

### mikesvenson

quite a difference there

13. Jun 5, 2004

### p-brane

This is a good question. Do I calculate the relativistic time warp/differences that come with the proximity to doing 1.0c?

If fact, when we say that c equals 186,000 miles per second we can't be sure seconds are "seconds" at c since time is severely distorted at the speed of light. Damn physicists have to screw everything up!

14. Jun 14, 2004

### mikesvenson

Is it true that every object down to the smallest is completely at rest in relation to itself, and it only is in motion in relation to some other perspective? Wouldnt that mean that every object in existance is in motion at every speed in every direction up to c simultaneously when in relation to every other existing object?

15. Jun 14, 2004

Staff Emeritus
Yes. If I am unaccelerated then relative to myself I am at rest. Then as you say given any velocity vector whatsoever, of speed less than c, there can be an observer traveling inertially on the reciprocal vector and they will see me as possessing that velocity.

This works for everything massive, down to the smallest. Massless objects like photons don't have rest frames, so the reasoning doesn't work for them.

16. Jun 15, 2004

### mikesvenson

if any object is at all relative velocities up to c when in relation to all other objects, like you confirmed, then would that mean since mass increases with velocity, that every object has every amount of mass in relation to every other object?

17. Jun 15, 2004

Staff Emeritus
Well, I don't use the formalism where mass increases. I use the invariant mass formalism and put the increase into the energy. So that your statement would become an object has every possible energy with respect to other intertial frames. Yes.

18. Jun 15, 2004

### p-brane

All of this may explain the old saying "all things being equal".

I'm enjoying the expansive chat you all'r' havin'. Thank you.

"Massless objects like photons don't have rest frames, so the reasoning doesn't work for them."

Photons would not be photons if they were at rest. Yes?

19. Jun 16, 2004

### mikesvenson

Does that imply that photons exist outside of the dimension of time?, since time is the measured duration for a constantly changing position of massive objects?
Does that mean "at rest" = no motion = no time? Wouldnt that mean the photon would indeed exist as a photon, but frozen in its inertial frame? Isnt this the trueth behind the existance of instances in constant change and reformation(motion, or time)?

20. Jun 16, 2004