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Q & A: what is motion

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Most recent answer: 10/26/2007
What do we mean if we say "MOTION"?
What is motion?
- Jules (age 17)
These very simple-sounding questions can be the hardest ones to answer.
People used to think that there was a big thing- the Earth- which was not moving. Then you could say that something was ’in motion’ if its distances to various points on Earth were changing in time.
The problem is that there’s no real sense in which the Earth is not moving. In fact, Einstein argued successfully (following Galileo) that there are no physical effects at all which can tell us if something is moving or not. So we shouldn’t even bother to ask.
What we can ask is whether two objects are in relative motion. From your point of view, you could lay out grids of points which are at fixed distances from you, and also not spinning around. If some other object is going from one point to another on that grid, then it’s in motion relative to you.
It gets more complicated in General Relativity because you have to be clear about what it means for the grid to stay rigid, but probably we’ve already been more complicated than you wanted.

Mike W.

(published on 10/22/2007)

Follow-Up #1: relative fields

Mike, I am most grateful for all of your answers to past questions of mine, so please know it is with deep respect that I prod one of your answers (purely in the spirit of fun just to see where it goes). In the previous answer you said that Einstein argued successfully that there are no physical effects at all that can tell us if something is moving or not. I recognize velocity is indeed distinguished from acceleration in not having an accompanying force. However, I would suggest your statement is not the case if the "something" is a charged particle. If a charged particle is in motion then it creates a magnetic field, does it not? Does this not distinguish it from a charged particle at rest?
- Scott (age 38)
Denver, CO
Great question!

Yes, a moving charged particle has a magnetic field as well as an electric field. The magnetic field has physical effects, such as giving additional force on other moving charged particles.

Now let's look at our initial particle in a frame in which it stands still- its own frame. It makes no magnetic field in that frame. However, its electric field in that frame is also different from the one in the initial frame. So are the velocities of the neighboring particles, and so are the rates of clocks and the lengths of sticks. The result is that the same net laws of physics, including the combined effects of the magnetic and electric fields, are obeyed in both frames. So you can't say that one or the other is more 'right'.

By the way, in some texts (such as Purcell's) the principles of special relativity are used to show that if there is an electric field in one frame, then in other frames there must be some additional effects on moving particles- the very same effects which we say are caused by magnetic fields. So far from there being a contradiction, magnetism is a necessary consequence of special relativity.

Mike W.

(published on 10/26/2007)

Follow-up on this answer.