If all motion stops in the universe, then can’t time also be said to have stopped? Is time without motion a meaningless and immeasurable concept? I’m missing something here and it is bothering me.
Also, I’m (obviously) not a physicist, so please no extra big words or math in the answers.
Motion is just a change of position over time. So time is a component of motion.
That said, if all motion stopped (say, trillions of trillions of years from now, as the universe is in heat death, and all particles slowed to a stop), I guess you would have no way of telling if time was passing. But nor could you be there to observe that, so moot point.
But by the same token, isn’t time also capable of being expressed as a change of position as a result of motion? Without motion, time cannot be observed. To observe motion is to observe time.
What I don’t understand is how time can be comprised of anything except motion, and if it cannot be, why time is accorded independant significance at all.
In a sense, and according to special relativity, they are. This is going to get complicated, but I’ll try to explain it as simply as possible. The key thing you have to realize is that everything, (planets, cheetas, your grandmother, an '85 Yugo sitting in a parking lot) moves at the speed of light.
That’s insane, monkey! It can’t possibly be true! Ah, but it is. It’s Special Relativity. Time is not separate from space, which is why it’s properly called “spacetime”.
The Universe has four dimensions - three spatial dimensions and one of time. You may have heard that time slows down when an object approaches the speed of light. I’m going to attempt to explain why.
Light has zero mass and all of its motion is expressed in the three spatial dimensions. It’s not moving through the dimension of time at all. That '85 Yugo sitting in a parking lot isn’t moving in the spatial dimensions. Instead, all of its motion occurs in the dimension of time. If you start that puppy up and get on the highway, then some of its motion is transfered to the spatial dimensions and it slows down in time.
Courtesy of Dr. Brian Greene, there’s a good analogy for this. Imagine two Yugos traveling North bound at fifty miles an hour toward a finish line 100 miles away. They are traveling in only one dimension: the N/S axis. Yugo 2 turns and starts traveling NE at fifty miles an hour, while Yugo 1 continues N at 50 mph. Yugo 2 is now traveling in two dimensions - the N/S axis and the E/W axis. Half of Yugo 2’s motion is still North bound, but now half of its motion is east bound. 25 mph along the North/South axis and 25 mph along the East/West axis. Moving East has caused Yugo 2 to slow down along the N/S axis, but it is still moving at a total of 50 mph. Yugo 1 wonders why he can cross the finish line so much sooner than Yugo 2 when they’re traveling at the same speed.
Now imagine that the N/S axis is time and the E/W axis is space. If you stay still in space (i.e. stationary on the E/W axis) you are zipping along in time at the speed of light. If you start moving in space, i.e. the E/W axis, your speed w/ regards to time slows down. You are still moving at the same total speed, but you are moving in more than one dimension.
Light travels purely along the E/W (space) axis. It doesn’t move at all along the N/W (time) axis. We are made of mass so we can’t do that. Some of our motion must occur in the N/W (time) axis (the vast majority of it, as it turns out). This is why light seems so quick. It’s only traveling in three dimensions, while we have to travel in four. But if you add up our total speed in those four dimensions, guess what? It totals c - the speed of light. That’s Special Relativity.
To dopers who may quibble with the above explanation: Be kind, I know it is a gross over simplification. I am only talking about special relativity, not general relativity, and sure as hell not string or m theory. Trying to keep it simple.
Good except when you go NE your N and E speed components are not 50% of the original velocity. You need to use trig; upshod is your speeds are 70.7% of your 50 mph or roughly 35.3 mph N and 35.3 mph E
Carry on.
:smack:
Good, except for “upshod,” which should be “upshot.”
Carry on.
Thanks - I was typing fast. 
So by stopping motion, I am not zeroing out time, I am speeding it up? And from the perspective of a photon, time is not moving at all. Did I get it right?
From “A Short Talk On The Universe”:
Then time should have slowed down for you.
Yes, you did. Good job.
There’s an addendum here, though. We are only discussing Special Relativity, which is imperfect. General Relativity was developed as a refinement and later we discovered quantum mechanics. Quantum mechanics doesn’t mesh with Relativity at all, and the current rage is to try to get the two to work together under the frame work of String Theory (that one doesn’t use four dimensions - it uses freakin’ NINE! and we won’t go into them here).
In short, even our best physicists can’t properly explain time. The biggest question: Why does time have a direction? The Yugo in my earlier analogy can move East, West, or North, but it can not move South. Physics is having a hard time explaining exactly why time can not flow backwards. It’s a dimension, right? You ought to be able to move in either direction, right? 'cept you can’t. Our current best guess is that it has something to do with the Big Bang and “Expansion Theory”, but the dirty little secret is… nobody knows why!
It’s time for Zeno’s Paradox to make an appearence. Zeno pointed out that if time is made up of all the infinite moments of time, and during any moment of time a moving object will move zero distance, then how can an object move? The modern answer, since the advent of Newtonian mechanics, is to postulate that moving objects possess a quality- momentum- that means that even in a single frozen instant of time, a moving object is qualitively different from a non-moving object.
I don’t know why, but the physics threads here always make me happy. You folks have a real knack for explaining some pretty abstract concepts in coherent ways. Thanks, monkey and Lumpy!
Yes, forgot to say: thanks! That was keeping me up the last couple nights.
(From a non-physicist)
What is missing in our paradigms–missing from both Relativity and Quantum Mechanic frameworks–is an understanding of what space itself is. What is the matrix that “contains” mass and energy? Does a particle “move” in space, or is a particle space, moving? Until we get a paradigm which includes that third component–space–all answers will be constrained by an incomplete Theory of Everything. Right now our theories only include mass and energy (different forms of the same “thing”) interacting within a completely non-understood thing: space. This is why space seems so Euclidean on a local level and so bizarre on a large-scale level (and of course, even more bizarre at a quantum level.) Physicists happily talk about space expanding, b/c in a mathematical model such a concept can exist. The layman asks, “What is it expanding into?” and the physicist tries to explain that that question makes no sense. But what is really the problem is that we have absolutely no idea what space is.
I think of Time as the result of a change in the distribution of mass and energy within the matrix of space. If there is no change in that distribution, Time “stops.” But without knowing what space is, we cannot really describe Motion and Time at a fundamental enough level to answer your question. In my completely uneducated opinion, a complete understanding will present a model in which the question you aask is nonsensical, because Time and Space (the actual “stuff” of space; not the behaviour of Time and Motion in Space–i.e. not just a mathematical modeling of “spacetime”) and Mass and Energy will all be expressions/phases/whatever-new-word-is-suitable of the same thing in the same way that mass and energy are the same thing.
In such a paradigm, without Motion, Time does not “stop” but instead, without Motion there is Nothing at all.