The fourth dimension

[JerryB]

I've been thinking about time as the fourth dimension.
Many people find it incomprehensible or impossible to
imagine. I was thinking that maybe it just hasn't been
explained well. Maybe I don't really understand it either
but this is how I understand it and how I would explain it.
If this seems incorrect or difficult to understand or incomplete
please let me know.

Scientists say that the world is made up of 4 dimensions,
three dimensions in space and one dimension in time. Probably
saying it that way is confusing. All four dimensions are in space.
The first three dimensions describe an object in space. The fourth
dimension designates the location of that object. The location is in
space. Time is only used as an indicator of the location.

Think about a train going from city A to city B. The trip takes an hour.
The train leaves at 6 p.m. and arrives at 7 p.m. There is no way to
identify any location along the track. At 6:30 p.m. the train is halfway
between the cities. So we call that location the 6:30 p.m. point.
When we call it the 6:30 p.m. point, we mean its location at 6:30 p.m.
We use the time to indicate the location just because there isn't
anything else available.

The only reason it's confusing is because we say time is the fourth
dimension. It would be easier to understand if we said location is the
fourth dimension.

Then there may be the question of why we use time as an indicator
instead of space. Why don't we put markers along the track and then
we could say we're at the 5 mile point or 10 mile point etc? Well we
could do that with a train on earth, but when we're talking about the
whole universe where the stars and planets are always moving and
there is no way to mark a particular spot, we can only designate a
location according to where something is at a certain time.

So that's it. Is there any mistake? Is it difficult to understand?
Did I leave anything out?

[Qfwfq]

Who talks about it as incomprehensible or impossible to imagine? What is strange about four or more dimensions?

Originally Posted by JerryB

Well we could do that with a train on earth, but when we're talking about the whole universe where the stars and planets are always moving and there is no way to mark a particular spot, we can only designate a location according to where something is at a certain time.

Doesn't that bust up your way of seeing it?

[Dark Mind]

Time, The Fourth Dimension

© Copyright 1999, Jim Loy

We live in a world of three dimensions. Well, we only perceive three dimensions. We can hypothesize many more dimensions. But, they are difficult to imagine.

Because of Einstein, we often call time the fourth dimension. Special relativity shows that time behaves surprisingly like the three spatial dimensions. The Lorenz equations show this. Length contracts as speed increases. Time expands as speed increases.

Scientists have been graphing time, as if it were a length, for hundreds of years. To the left is a typical graph, showing two things in motion at the same speed, one to the left and one to the right. Time never behaves exactly like a spatial dimension. You cannot go backward in time. And you normally cannot go forward at different rates. But, there are surprising parallels. For some purposes, it is handy to call time a fourth dimension. For other purposes, it is not.

Pretend, for a moment, that there are more than three spatial dimensions. What is a four or five-dimensional cube like? It is hard to visualize. But, we can make a few deductions about such an object. What if a 5-dimensional cube is 2 centimeters on a side, what is its 5-dimensional volume? Well, we can easily generalize from the first three dimensions. A 2x2 square is 4 (2x2) square centimeters in area. A 2x2x2 cube is 8 (2x2x2) cubic centimeters in volume. A 2x2x2x2x2 5-dimensional cube is 32 centimeters-to-the-5th-power in 5-dimensional volume. None of that can be visualized. But, it makes sense. What is the distance between two points in 5-space? You can easily deduce a 5-dimensional Pythagorean Theorem.

Addendum:

I once read in a science fiction story that time is the fourth dimension, and space is the fifth. That's pretty poor; space is the first three dimensions, and there may be more spatial dimensions that we cannot observe or interact with. One would expect a science fiction author to get the simplest science right.



(Source: http://www.jimloy.com/physics/4d.htm )


I don't know if this helps much...

[Dark Mind]

Originally Posted by JerryB

... If this seems incorrect or difficult to understand or incomplete
please let me know.

Letting you know, it seems a little incomplete.

Originally Posted by JerryB

Scientists say that the world is made up of 4 dimensions,
three dimensions in space and one dimension in time. Probably
saying it that way is confusing. All four dimensions are in space.
The first three dimensions describe an object in space. The fourth
dimension designates the location of that object. The location is in
space. Time is only used as an indicator of the location.

Actually, the first three dimensions designate the location of the object ( Length, width, depth). Time is used to measure the location of the object at a certain point in... well... time (10 hours ago I was asleep in a bed, right now I'm typing on the keyboard in front of a computer). Time is used to measure the distances traversed by objects through the other dimensions.

"What we can do is physically experience the effects of time."
(Quote from: http://www.coolphysics.com/4d/)

Originally Posted by JerryB

Think about a train going from city A to city B. The trip takes an hour.
The train leaves at 6 p.m. and arrives at 7 p.m. There is no way to
identify any location along the track. At 6:30 p.m. the train is halfway
between the cities. So we call that location the 6:30 p.m. point.
When we call it the 6:30 p.m. point, we mean its location at 6:30 p.m.
We use the time to indicate the location just because there isn't
anything else available.

We can use the distance it covered.
It has travelled x number of miles after y number of minutes.
It isn't generally standard to use the exact point in time that something occured (e.g. If you are talking about the location of a star, then, yes, you would use the exact time, date included). If you're talking about everyday events then generally it is said "I ate breakfast this morning." or "I ate breakfast about three hours ago.".

Originally Posted by JerryB

The only reason it's confusing is because we say time is the fourth
dimension. It would be easier to understand if we said location is the
fourth dimension.

But location isn't the fourth dimension, the first three dimensions refer to location (Length, width, depth), the fourth dimension is generally used as a reference to where something is or will be in space and time or spacetime (I think, I'm sure Qfwfq can explain this better than I).

Originally Posted by JerryB

Then there may be the question of why we use time as an indicator
instead of space. Why don't we put markers along the track and then
we could say we're at the 5 mile point or 10 mile point etc? Well we
could do that with a train on earth, but when we're talking about the
whole universe where the stars and planets are always moving and
there is no way to mark a particular spot, we can only designate a
location according to where something is at a certain time.

And we usually refer to these points as follows: Half-way point, 1/3-way point, 3/4-way point.

We use the distance covered and the distance left to cover as the final destination points (A, B) and anywhere in between can be measured as fractions of the distance covered.
<--------------------------------------1/3-----------------------------1/2------------------------------------3/4----------------------------------->

Originally Posted by JerryB

So that's it. Is there any mistake? Is it difficult to understand?
Did I leave anything out?

I'm not sure if there's any mistake (There isn't in spelling at least ), it's only difficult to understand if you make it so, but I found it fairly simple to grasp what you were trying to say.... I think.

Hope I helped.

[Dark Mind]

Well, that (above) was my single longest post .

[Tormod]

Originally Posted by JerryB

All four dimensions are in space.
The first three dimensions describe an object in space. The fourth
dimension designates the location of that object. The location is in
space. Time is only used as an indicator of the location.

In fact, this is incorrect.

Time is not a spatial dimension. The three spatial dimensions add up to create depth, like Dark Mind pointed out. There is no fourth spatial dimension providing location (we need only three).

Time is not a spatial dimension. It doesn't have a fixed axis, nor a fixed speed. We talk about "the arrow of time" because we assume that time can only flow in one direction (from the past towards the future) but this is pure theory. There is no good definition of what time is.

Time is one of the dimensions in space-time, where a location in space PLUS the time is taken into account. So 3D gives us anywhere in the universe, whereas 4D gives us anywhere, anywhen.

[infamous]

Originally Posted by Tormod

In fact, this is incorrect.

Time is not a spatial dimension. The three spatial dimensions add up to create depth, like Dark Mind pointed out. There is no fourth spatial dimension providing location (we need only three).

Time is not a spatial dimension. It doesn't have a fixed axis, nor a fixed speed. We talk about "the arrow of time" because we assume that time can only flow in one direction (from the past towards the future) but this is pure theory. There is no good definition of what time is.

Time is one of the dimensions in space-time, where a location in space PLUS the time is taken into account. So 3D gives us anywhere in the universe, whereas 4D gives us anywhere, anywhen.

I'll take this opportunity to pose another question for you Tormod.

Theorists speak of this direction in time on the one hand as the future pointed in one direction and, the past pointed in the opposite. Is this a misunderstanding on my part, or is this a correct interpretation? If this point of view is correct, would it not be true that time has two directions, one for the future and one for the past? I remember reading somewhere that the concept of moving through time in reverse doesn't violate any laws in physics. Just because we move through time from the present into the future doesn't mean that moving through time from present into the past is forbidden by physical law.

[Erasmus00]

Originally Posted by infamous

I'll take this opportunity to pose another question for you Tormod.

Theorists speak of this direction in time on the one hand as the future pointed in one direction and, the past pointed in the opposite. Is this a misunderstanding on my part, or is this a correct interpretation? If this point of view is correct, would it not be true that time has two directions, one for the future and one for the past? I remember reading somewhere that the concept of moving through time in reverse doesn't violate any laws in physics. Just because we move through time from the present into the future doesn't mean that moving through time from present into the past is forbidden by physical law.

While addressed to Tormod, I hope you don't mind if I take a stab at answering. First and foremost, time isn't necessarily a linear dimension, as massive objects cause time and space to bend. As part of that bending, a time dimension can become mixed with spatial ones. The time dimension is already laid out, same as the spatial ones. Our direction in time seems to have something to do with, perhaps, the way we percieve the world, the way our brains work.
However, you are correct in that an "arrow" of time is often talked about, and physical laws are often said to obey time reversal symmetries. The physical law which seems to set our arrow of time is the entropy law(2nd of thermodynamics). Systems move to increase entropy, so broken glasses don't reassmble, all the air molecules don't move to one side of the room.
Some theorists have been quoted as wondering if at some point the arrow of time won't begin to run backwards, favoring less entropy. Personally, I doubt it.
-Will

[UncleAl]

I've been thinking about time as the fourth dimension. Many people find it incomprehensible or impossible to imagine

It's already been done, in exquisite detail, to a fare-thee-well,

Annalen der Physik 4 XVII 891-921 (1905)
Annalen der Physik 4 XLIX 769-822 (1916)

Nobody has found even a single empirical exception at any scale (sub-nuclear to cosmological) in any venue over 100 years of intense looking.

Why don't we put markers along the track and then we could say we're at the 5 mile point or 10 mile point etc?

A relativistic universe has four distinct distances: luminosity (inverse square), angular diameter, parallax, and proper motion. No two of them need agree to maintain consistency. Clocks can only be synchronized by being local (touching).

http://relativity.livingreviews.org/...1-4/index.html
http://arXiv.org/abs/gr-qc/0311039
http://www.weburbia.demon.co.uk/phys...periments.html
Experimental constraints on General Relativity