The 4th dimension?

Question

Why is time the 4th dimension of space? Why not something else? What led physicists to conclude time is the 4th dimension?

What is it we can move on in one direction in time, ie, forward? If we can move freely in the 3 dimension, why is it we can move only forward in time?

Answer 1

You are very insightful, and COMPLETELY over my head on this one.

Answer 2

We don't have to get all technical to figure this out. One of the reasons I disliked history in high school was all of those dates I had to memorize. People and dates have to have have a time reference. Time and space go hand in hand, they can't be separated, space contains all four detentions, when Henry the eight told the Pope to get lost the universe was in a finite space, the planets and everything else occupied a specific place in this space, at a specific time. Granted, clocks and calendars have nothing to do with time in the scientific context of time, but in case the analogy is okay. Time does not move, events occur and time is the baloney in the sandwich. Time can be altered by velocity as Einstein so majestically predicted. If you could move forward in time you would travel into the future. It is always, NOW and it never be anything but, Now.

Answer 3

Time is not "the" fourth dimension, just as length is not "the" first dimension. It is simply one of the four space-time dimensions of our universe.

An excellent book that can explain what "the" fourth spatial dimension is, and can also answer your question about why time only seems to go "forward" is 'The Fourth Dimension', by Rudy Rucker. ISBN 0-395-39388-4.

Answer 4

There were quite a few things that led physicists to conclude that time should be unified with space to make a more general framework. Look up the Michelson-Morley experiments for a starting point. These experiments were designed to measure the effects of the presumed ether on the speed of light. Instead, they found that the speed of light is constant from any direction, which upset the apple cart quite a bit - not just for the existence of the ether, but for the general principle that if object A is travelling at velocity v_a and B is travelling at velocity v_b, then B sees A travelling at velocity v_a - v_b. This led to the Lorentz contractions as a way to explain these results in terms of geometric distortions when travelling near light speed, which meant that time and space terms were linked; and ultimately to Einstein's special and general theories of relativity, in which time and space are deeply linked.

Every point in space-time is simply an event. Now, for a cause-effect relationship to apply the effect must be in the future light cone of the cause. So, for instance, for the event of "being at point X at time T1" to have the effect "being at X at time T2 and having a memory of being at X at T1", we must have T1 < T2. Or, more simply, if an object is at X at both T1 and T2, and T1 < T2, then the event at (X, T1) must be the cause and (X, T2) must be the effect. Perceptually this means that we perceive T1 as happening before T2. Indeed, an observer in any inertial reference frame must perceive (X, T1) happening before (X, T2).

More generally, if we have two events (X, T1) and (Y, T2) and (X-Y)^2 < c^2(T2-T1)^2 then this is a timelike interval; an observer in any inertial reference frame will agree with us as to which event comes first and there is potentially a cause-effect relationship between them; everyone agrees on which is the cause and which the effect.

Contrast this with the situation where we have two events (X, T1) and (Y, T2) with (X-Y)^2 > c^2(T2-T1)^2. This is a spacelike interval, and observers may disagree on which one came first; there can be no cause-effect relationship between them.

Answer 5

I actually saw an interesting video based off of a book regarding dimensions. I can't answer your question, but perhaps the video will shed some light...