# Why was the idea of space-time created?

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For years I have been plagued by an inability to understand the concept of space-time. Specifically I was never able to understand the logic behind the concept itself. I am not an expert but my understanding of space is that it is the area in which matter exists. Time is the immaterial progression of existence and we measure time using a system centered around the Earth's orbit around the Sun. (If my understanding is wrong please provide the correct answer below).

From what I understand, space-time is considered to be the continuum upon which the universe exists. (Again if I am wrong please inform me). What I do not understand is how Einstein and other physicists are able to use time, an immaterial measurement found by charting movement of stellar bodies, as a component in their theories. I just do not understand how a concept like time can be joined with a physical and measurable substance like space to form the idea of space-time. I would prefer the answers to use as many layman terms as possible because I have always had trouble understanding mathematical terms.

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You'r mistaking the general use of time with what the abstract concept of time really is, time doesnt exist as the earth spinning around the sun, we just put dates and times on them as a record for outselves. Time in its most simplest form can be expressed as a "moment" in which something changed.

The idea behind space-time was to entwine the dimensions we see and perceive x,y,z with this abstract notion of time, in doing so your encompassing time as an indistinguishable property of the universe (unlike the idea that time is based on the earth spinning around the sun), Also when you define a 4th dimension like this you extend physics to the extent that new theories and rules can be conceived from such a notion.

Essentially if time didnt exist, we would all be stationary objects that cant and will never move, by adding time to the previous dimensions it then allows the universe to move and interact. There are other more specific rules drawn up by space time such as how time can bend around space and objects and such but im not physicist so i dont know the whole concept.

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Perhaps I worded it badly but I already knew time was an abstract concept that was different from the system we set clocks to. What I'm asking is how physcists somehow use this abstract concept that cannot be altered with by any man-made methods to expand our kowledge of physics. I mean, time has no inherent mathematical value to it, (so far as I know) so I don't understand how anyond can incorporate it into a system with something measurable like space. Actually, under that definition of time, wouldn't that make the idea of time dilation a misnomer? If time is an abstract concept then wouldn't the time difference between two objects moving relative to one another be due to a difference in the rate of movement of the very molecules of the two objects with time not actually playing factor into it?

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The idea of space-time is very old, Newton and Galileo had some notion of 'space-time'. That is, they understood that every event needs to be specified by 4 numbers: a position in space, which is 3 numbers and then one more the time. They also new that all measurements in space are relative to some reference event, but they thought that time was absolute.

It was Einstein and others that realised that we need to take into account 'mixing' of space and time. In the Newtonian picture we can mix what we mean by x,y,z by changing coordinates, but time is absolute and we can just allow for shifts in time. Special and general relativity tell us that we also have transformations that mix all x,y,z, and t!

Edited by ajb
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Time is a very real thing. It's not abstract, and it's not a concept people made up. Well, technically it is. Time is actually much more specific scientifically. We refer to the ARROW OF TIME or causality. The fact that causes always precede effects and so on. The reason why it's better to think of space and time as one thing is because we do not actually distinguish the two of them. Even you don't much as you don't realize it. Imagine any set of coordinates, say you are going to go meet your friend for a burger or something. You know the X,Y,Z (Z almost never matters because on earth we interact with the surfaces of things primarily), but the unspoken agreement is that you will met your friend at T which is a specified time. There is no describing space without describing time, we just assume time in most of the cases. If you wanted to describe an event in lithuania happening NOW you normally disregard time (now is assumed), but if you were talking about 200 years ago you specify that. For all intents and purposes space and time are the same thing. You cannot move through one without the other.

I guess another way of saying it is Time is just a coordinate of space. It's not even really good to think of them as Space-Time IMO. I personally find it better to think of it as just SPACE with time being an inherent property of space.

Edited by TheGeckomancer
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For years I have been plagued by an inability to understand the concept of space-time. Specifically I was never able to understand the logic behind the concept itself. I am not an expert but my understanding of space is that it is the area in which matter exists. Time is the immaterial progression of existence and we measure time using a system centered around the Earth's orbit around the Sun. (If my understanding is wrong please provide the correct answer below).

From what I understand, space-time is considered to be the continuum upon which the universe exists. (Again if I am wrong please inform me). What I do not understand is how Einstein and other physicists are able to use time, an immaterial measurement found by charting movement of stellar bodies, as a component in their theories. I just do not understand how a concept like time can be joined with a physical and measurable substance like space to form the idea of space-time. I would prefer the answers to use as many layman terms as possible because I have always had trouble understanding mathematical terms.

Time is no longer based on the earth's movement around the sun.

Space is a physical and measurable substance? What measurable properties does it have?

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Space is the "region" in which matter exists. Area is a subset of the region.

A field is represented by a distribution of acceleration over a region. Time is part of acceleration, and therefore is also distributed over the region.

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Space is the "region" in which matter exists. Area is a subset of the region.

Time is the "period" in which matter exists. Moment is a subset of the period...

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For years I have been plagued by an inability to understand the concept of space-time. Specifically I was never able to understand the logic behind the concept itself. I am not an expert but my understanding of space is that it is the area in which matter exists. Time is the immaterial progression of existence and we measure time using a system centered around the Earth's orbit around the Sun. (If my understanding is wrong please provide the correct answer below).

From what I understand, space-time is considered to be the continuum upon which the universe exists. (Again if I am wrong please inform me). What I do not understand is how Einstein and other physicists are able to use time, an immaterial measurement found by charting movement of stellar bodies, as a component in their theories. I just do not understand how a concept like time can be joined with a physical and measurable substance like space to form the idea of space-time. I would prefer the answers to use as many layman terms as possible because I have always had trouble understanding mathematical terms.

You need to know two things,

1) Space is information without more information, or matter, basically it's not non-existence, its just void of extra information, or matter.

2) Time is only a non-simultaneousness of events, it is why a and b are a and b and not ab.

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You need to know two things,

1) Space is information without more information, or matter, basically it's not non-existence, its just void of extra information, or matter.

2) Time is only a non-simultaneousness of events, it is why a and b are a and b and not ab.

!

Moderator Note

TJ, you've been told repeatedly NOT TO SPECULATE IN THE MAINSTREAM SECTIONS. Since you refuse, the next step is to put you in an approval queue. From now on, your posts need to be approved by staff before they'll appear to everyone else. We don't want the reputation for giving students bad information.

No more speculation outside of Speculations, please.

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• 2 weeks later...
"Space is different for different observers. Time is different for different observers. Spacetime is the same for everyone."
- E. F. Taylor and J. A. Wheeler

To answer SimplyCurious, I also wondered what connects time and space into spacetime. Then I learned about the "spacetime interval." Its a little involved, but I know of no better way to understand the connection.

1) Imagine you are seated in a classroom with other students. One student claps their hands together once. A little later, a second student also claps their hand together once.

Say you and the other students measure the distance between hand-claps. We call this distance the "space interval." Assuming you all have the same measuring devices and methods, you get the same value for this distance or space interval.

We call the elapsed time between the two hand-claps the "time interval." Using the same devices and methods, you and the other students also measure the same elapsed time or time interval between the two hand-claps.

You all get the same values for the space and time intervals because you are all at rest with respect to each other.

2) Now imagine astronaut Laura flies in a rocket past the classroom at half the speed of light, relative to the classroom.

According to special relativity, Laura measures a different value than the you for the time interval between the hand-claps. She also measures a different value than you for the space interval between the hand claps.

Why? Because Laura is moving relative to you. Space and time are relative -- they are affected by relative motion.

3) Now you multiply the value you got for space interval by itself to get the square of the space interval. And you multiply the value for the time interval by itself to get the square of the time interval. Then you take the difference between the two squares. This difference is called the square of the "spacetime interval."

The formula for the spacetime interval is:

(the spacetime interval squared) equals (the time interval squared) minus (the space interval squared)

Astronaut Laura also squares the value she got for the space and time intervals, and calculates the difference. This is her spacetime interval squared.

You get together with Laura and compare calculations. You find you have measured different space intervals, and different time intervals, but have come up with the same spacetime interval.

Space and time are relative -- they are both affected by relative motion. But spacetime -- as represented by the spacetime interval -- is absolute. It is not affected by (uniform) motion.

Space and time are inextricably linked. We call this connection "spacetime".

4) Where does the spacetime interval formula come from? It is derived from Einstein's light postulate -- the speed of light is absolute or unaffected by relative (uniform) motion. The absolute nature of the speed of light leads to the Lorentz transform, the relativity of space and time, and to absolute spacetime.

The above is based on the explaination in Einstein Relatively Simple

Edited by IM Egdall

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