View Full Version : SpaceTime

Chook

2003-Dec-25, 10:35 PM

A bit like others - I've read many astronomy, physics and science books over the years, but I have NEVER got within coo-ee of understanding SPACETIME.

Space = what is between objects;

Time = rate of change.

I can't seem to get any advancement on that, :blink: and would really appreciate it if someone could explain what SPACETIME means in the context of astronomy.

kashi

2003-Dec-26, 01:13 AM

I just moved your topic Chook. People are more likely to respond here.

I would respond myself but I am tired and inarticulate after Christmas, and I have go to my grandmother's house now.

Kashi

Josh

2003-Dec-26, 01:43 AM

Instead of writing a long winded answer, I'll 'borrow" someone elses. Caltech has a great answer found here. (http://www.theory.caltech.edu/people/patricia/st101.html)

Good luck.

Chook

2003-Dec-26, 01:52 AM

Thanks Kashi. Hullo Grandma :D

damienpaul

2003-Dec-26, 02:58 AM

thank you Kashi also, interesting post, hello Kashi's grandma

Chook

2003-Dec-27, 05:30 AM

Josh:

What did you say that ...

0 = (1/R0) cos(Df/2) + 2 (G M/c2)/R02,

- cos(p/2 + df/2) = sin(df/2) ~ df/2 = 2 (G M/c2)/R0.

meant? :( :huh: :o :( :angry: :blink:

Matthew

2003-Dec-27, 06:49 AM

Space time is 4 dimensional. Its normal 3D with time dimension included. Its pretty important for reletivity.

rahuldandekar

2003-Dec-27, 11:57 AM

Chook, what difficulty do you have in understanding spacetime. The key here is Imagination.

( As I am not using any equations over here, I may go a little wrong, but this is what I understand as spacetime.)

It is a little difficult to imagine four dimensional space, so imagine a three dimensional block.Now, the flat two dimensional base of it represents two dimensions of space. Okay?

Now, suppose the time is represented by height. As Time passes, it's height increases. When no time had passed, it was just a two dimensional plane. As time begins and passes, A new dimension - height here - is introduced. The space turns from two dimensional space to three dimensional space-time.

Now, if you have understood uptil now, just think about our three dimensional world. Here time introduces a fourth dimension. Three dimensional space turn to four dimensional space-time.

P.S. Try to imagine the concepts as you read the sentences. For eg. While reading "as time begins and passes, A new dimension - height here - is introduced", try to imagine a three dimensional block emerging and increasing in height from a flat two dimensional plane.

Any problems? :)

Chook

2003-Dec-28, 10:12 AM

Matthew thanks for your succinct definition.

Rahuldandekar: Thanks for your more detailed explanation - very helpful.

Unfortunately - no help from Josh's reference.

Progressing from your imagery, Rahuldandekar, what is the significance of this concept on the normal view we have of the universe. I mean - even the simplest of us have a 3-dimentional view of the universe, plus a clear appreciation of time (it takes a day to get to the moon; 180 days to get to mars).

So what is new or revolutionary (or "realational" as in Relativity) about SPACETIME?

Still :blink:

rahuldandekar

2003-Dec-28, 10:22 AM

In relativity, Gravity is defined in terms of spacetime. Heavy objects bend space-time. So, light rays get bent when passing heavy stars, and many curious things happen. This gave a very exact theory of gravity, which we haven't contradicted uptil now.

Anyway, next time I come, I'll bring a more exact and detailed answer.

Josh

2003-Dec-28, 10:43 AM

Okay ... what about this for a definition? spacetime is a continuum of four dimensions - three spatial and one temporal - in which any object or event can be located. simple and to the point. the dimensions are synonymous with coordinates or the cartesian plane but with an added "plane" for time.

The importance of spacetime can be seen, for eg, in its warping. Gravity in Einstein's theories differs greatly from that of Newtonian physics. Gravity is actually caused by the warping of spacetime (therefore the points on the 4 planes are warped) rather than a simple force acting at a distance. Because light travels at a constant speed (pretty much) Newton's theory about space and time was essentially wrong - it worked on the small or common scale but not on the macro scale. The theory of relativity, from where spacetime stems, answers the problems of the newtonian model. Spacetime itself is relative depending on where you are and what you're doing. None of this was known before and hence its revolutionary-ness. Before relativity there was no known symmetry between space and time. Now there is.

that eqn is the deflection eqn for light around an object as defined by the general theory of relativity. It makes sense when reading it in context ... and if you don;'t have a mind for equations then the actual explanations that accompany the eqns give all the information you'd need.

Chook

2003-Dec-28, 11:09 AM

Gee - thanks fellas!

I'm beginning to see things clearer (after all this time).

Funny how the "penny doesn't drop" sometimes until somebody explains in a simple way. I now relate to those "rubber sheet" representations that are often used to demonstrate spacetime.

Thanks again.

Matthew

2003-Dec-30, 04:21 AM

Spacetime shows how it is possible for gravity to affect time.

rahuldandekar

2003-Dec-30, 01:15 PM

Time does not exist independently, but is an internal charecteristic of spacetime.

Eg. Imagine a spacetime (of two dimensions of space and one dimension of time) shaped like a cone. If we cut the cone horizontally into many slices, the 'Time' of each slice can be defined by a property intrinsic to each slide, for example, it's radius. But a cone can be cut into slides at many different angles and in many different ways. So, it is always better to imagine Space-time than space and time differently.

Prime

2003-Dec-30, 07:54 PM

Some say that time is simply another aspect of activity, motion, or dynamics in the guise of a symbol.

It actually denotes the amount of activity in terms of a definite unit using space as abackground.

This makes it possible the correlating of events, and bringing order to the physical Universe.

This means that being an integral part of activity, of all forms, it is inseparaple from them, and cannot take on any of weird properties attributed to it by Einstein and other fuzzy thinkers, to explain unusual properties, beyond their comprehension.

Time compliments Space, and the interface, projected, allows physical events that require measurment to happen.

This present instant of now, or interface, where the future, intercepts the past, in forward motion,has been transformed into a past instant of now, or history while being

read by the observer.

Prime

Some say that time is simply another aspect of activity, motion, or dynamics in the guise of a symbol.

It actually denotes the amount of activity in terms of a definite unit using space as abackground.

This makes it possible the correlating of events, and bringing order to the physical Universe.

This means that being an integral part of activity, of all forms, it is inseparaple from them, and cannot take on any of weird properties attributed to it by Einstein and other fuzzy thinkers, to explain unusual properties, beyond their comprehension.

Time compliments Space, and the interface, projected, allows physical events that require measurment to happen.

This present instant of now, or interface, where the future, intercepts the past, in forward motion,has been transformed into a past instant of now, or history while being

read by the observer.,

Prime

Kootenaistar

2003-Dec-30, 08:20 PM

Here's just one more little thought. Remember the term "space-time continuum", as, (space-universe) is a (yes) three dimensional setup, "continually" (that fourth dimension). Keep in mind that our universe is always in motion and changing. Change makes a difference in the space over a period of time. Change any one of those dimensions and it makes a change elsewhere also.

For my poor mind, that is the simplest explanation anyone has given to me. Hope it is at least decent! :unsure:

Chook

2004-Jan-03, 09:40 AM

Prime:

What? :blink: :blink: :blink:

lazserus

2004-Jan-03, 05:39 PM

Another way to explain it (which doesn't deviate from what Josh has said, just adds to it) is by considering the progression of our concept of time. During Newton's time physicists believed that time was an unchanging constant. The problem was that, is there was no reference point in space to measure velocity. When measuring velocity it has to be measured in a frame of reference. This mean that when you tell me your car is driving at 45mph - we ask, 45mph relative to what? I hope that didn't confuse you, but it's vital to understanding space-time. Carrying on, classical physicists pre-Einstein believed that the universe was filled with an ether. They used the ether (only in equations, there was never actual physical evidence of such a substance) as the reference point in measuring velocity and a number of other things. Thus, we have time as a constant, unchanging at any given point in the universe; and we have our ether.

Well, Einstein comes along and publishes a paper in 1905 on special relativity. Einstein says this about the ether:

"... the introduction of a light-ether will prove to be superfluous since, according to the view to be developed here, neither will a space in absolute rest endowed with special properties be introduced nor will a velocity vector be associated with a point of empty space in which electromagnetic processes take place."

Concluding:

1. The laws of physics take the same form in all inertial frames.

2. In any inertial frame, the velocity of light c is the same whether the light is emitted by a body at rest or by a body in uniform motion.

This basically means that there is no need for an ether, because time is not absolute. Thus, he concludes that space and time are one in the same. He doesn't get in much detail about that until he publishes his paper on general relativity in 1915. General relativity concludes that space and time are intertwined in the universe and gravity affects them both. The more mass an abject, the more it warps space and time around it. Hence, existing in a curved space-time. Gravity from a massive celestial object affects time. Time runs faster on Earth than in its orbit. Also, when an object accelerates to near light speeds, time slows for that object (hence, relativistic time dilation).

The very fabric of our universe is space and time combined into space-time. Gravity warps space around a massive object, and time is also affected in the same way. We've experimented for decades on this and so far, Einstein's equations hold up. The easiest way to experiment and test relativity for yourself is to take two clock and place one on the ground and one on a very high structure (water tower, skyscraper). Allow an hour to pass and retrieve the clocks. The clock that was elevated will be behind the one that was at sea level.

I know this post was a bit long winded, but perhaps that's what you needed. Knowing the history helps the present make more sense. Let me know if you have any questions.

Chook

2004-Jan-03, 07:30 PM

Sorry Lazserus - (duh! :blink: ):

What is an "inertia frame"?

(Thanks for you help - I'm still struggling.) :)

lazserus

2004-Jan-03, 07:37 PM

Inertia is basically the act of resisting acceleration. So, when Einstein talks about measurements in inertial frames, he's just saying that the unit was measured while the observer was standing still. In all actuallity, there is no such thing as absolute rest. Everything in the universe is in some form of motion, even the atoms in your desk are moving. Since everything is in motion, Einstein uses inertial frames of reference, meaning the observer is as close to rest as possible to make the measurement.

Don't worry, dude. You'll understand this before we're done with you. I've never failed at making someone understand the basic principles of relativity and I've got a lot of people here to lend me a hand.

note - this post was edited because I goofed and gave inaccurate information. the correct information is present.

damienpaul

2004-Jan-06, 01:39 AM

count me in, i will help and learn as much as i can

slinki75

2004-Jan-13, 06:42 PM

If you are trying to understand how space and time are linked, try this. Say this out loud

"I'm here, now"

Chook

2004-Jan-13, 11:31 PM

Dear Lazserus, Josh, Kootenaistar, rahuldandekar, Prime et al:

I KNOW that I am not alone in trying to get a "common sense" comprehension of space-time continuum and relativity. The problem for us, I suspect, is in trying to put it all together in a common-sense model.

I can easily visualize the rubber-sheet representation of gravity warping space geometry, and velocity/gravity time-change, and that there is no need for ether - but how to put it all together to make a whole image is still beyond me.

I mean - I have good spacial appreciation, I think; I can see curved space (and why); I can see that "faster clocks run slower"; I can even visualise black holes, neutron stars and supernovas (and even your Plasma Universe) - but I've NEVER got a handle on relativity.

Help :unsure:

anewton

2004-Jan-14, 12:53 AM

Time is a way of measuring movement through space. An hour, a minute, etc. One dimension is a foot, cm., ect. Two dimensions is length and width--three dimensions is in the cube. This is on our Earth. Spacetime is under the conditions of space. Even here we have nautical miles and air (pilot ) measurements. To add another dimension illustration to our existence besides Time. Try to imagine that our emotions could have a reality of it's own. Somepeople think our dreams do.

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