What is space time, how can dimensions be fused, and how is time a dimension?
In: Physics
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Imagine you want to meet a friend in the building. You know the longitude and latitude on the map (this is your 2 space dimensions), floor (this is your 3rd space dimension – height). If you go on Wednesday, but him on Saturday, you won’t meet. You need to go to same latitude, longitude floor and hour to meet.
As spacetime is a challenging concept, the best way I know to give an initiutive, kid-level understanding is by playing a game (designed to teach special and general relativity): https://www.testtubegames.com/velocityraptor.html
https://youtu.be/N0WjV6MmCyM
Watch this video, it’s a good starting point on the subject and I believe Sagan explains it very well so that even those who are not familiar with physics can understand.
I’d really love to know a 5year old who can get their head around this level of physics.
This is the kind of request that you need a Phd to understand the answer.
Simply put, our brains only function to a certain level, but the universe is much more complex, and unable to *really* comprehend the hows/whys.
Yeah, sucks, I know.
Spacetime is simply space and time together as one whole. In other words space is just spacetime in any given moment (which is usually “right now”). So you don’t think of space as something that changes, but space as it was in the past and how it will be. Basically the difference between “I’ll meet you in front of the school” and “I’ll meet you in front of the school next Thursday at 3 PM”. If you open up a movie on your computer there will probably be a timeline on the bottom where you can decide _when_ in the movie you want to jump. Imagine if someone looked at a complete recording of the universe and could rewind and fast forward with something like that.
“Dimension” is a term that’s used for very fancy sci-fi things but in practice it’s related to measurements. Grab a tape measure, lay it out flat. That’s one dimension (if you ignore its thickness or just think of its edge) and you can tell where you are on the tape measure with one number, the one that’s written on it.
On a flat sheet of paper you can do something similar with two tape measures laid down at the edges of the sheet so they meet in a corner at 0 – for any point on that sheet of paper you can read from one tape measure how far up/down you are (if you look at your point from that side of the paper) and the other tells you how far left/right you are. So it is said that the surface of a paper has two dimensions.
For space, add a third: how far above/below. For spacetime, add a fourth: how far into the past/future: when.
The dimensions aren’t fused. Think about it in terms of degrees of freedom to move – generally there is an intuition that there are 3 dimensions. There is no “preferential” direction in physical terms – so all 3 are treated equally, which doesn’t mean they are fused but simply recognizing that anything that happens in one dimension can also happen in the other identically.
The idea of spacetime is something that comes from Einstein’s theory which has now been experimentally verified. Before Einstein, time was considered invariant – one second to you means one second to me or to anything else in the universe. Time passes the same for everyone and everything. With relativity proven, it is now clear that time is NOT an absolute or invariant and that how time passes varies depending on the relative speed of the measurer. This ties together motion in space with motion in time – no longer were these two independent – moving quickly in space means moving less quickly in time.
From a mathematical viewpoint, time is no longer “invariant” and enters into the equations governing motion, therefore it is, conceptually, another dimension. Objects “move” through time as well as space.
Dont mind people telling you the question is stupid.
If you have some basic understanding of geometry it doesn’t have to be very complicated. A dimension is really just a number that tells you where something is.
Do describe something in one dimension you need a number, an easy example are the natural numbers: 0,1,2,3,.. If you lived in the universe “natural-numbers”, you could perfectly describe every position with a single number.
Same way with 2 and 3 spatial (room) dimensions, pointing something out on a piece of paper needs 2 numbers (x,y in coordinate system) while pointing something out in a “grid” you need 3 numbers (up/down, back/forward, left/right).
The reason I said you need some basic understanding of geometry is because Pythagoras theorem (a^2 + b^2 = z^2) giving the relation of the length of the sides of a right-angled triangle, also works just as well in three spatial dimensions (a^2 + b^2 + c^2 = z^2).
The same is true for higher “theoretical dimensions” (4,5,etc). It’s maybe trivial to point out but our three spatial dimensions are also “fused” (I dont think this is a scientific term though…).
If you have grasped the things above, adding time as another dimensions is almost as simple as adding another X^2 term on the left side and suddenly, changes in the spatial dimensions are directly related to changes in the time-dimensions, they are no longer separate and you need to take into account how time is affected by moving around in space, hence Space-Time.
As a note generally the time-dimension has some special characters in most physics formulas to make it not work exactly like a spatial-dimensions (I think the term is negative instead of positive amongst other).
EDIT: If you are interested why space and time is “fused” you could google Einsteins Special Relativity or Space elevator thought experiment it will explain in a fairly logical way how all of this follows from the constant speed of light.
Start with Quantum Superpositions, & the Many Worlds Interpretation Theory. After that, have fun!
Okay, other engineering student here.
From what my lecturers told me, space-time is a term used to describe how the 3rd and 4th dimensions are fused together in a “fabric”. Dimensions are “fused” in that they are linked together, one cannot happen without the other. Einstein’s Theory of Time Dilation helps explain this as well. Simplified video explanation [here](https://www.youtube.com/watch?v=yuD34tEpRFw)
For the final one, you gotta go back. So the 0th dimension is a dot, but when you drag the dot in a line, it becomes the 1st dimension. drag that line along a different axis and now you have the 2nd dimension. drag that shape along another axis and you now have a 3D object. But how can you drag that object along another axis. Well, thats where the concept of time being the 4th dimension comes into play. Video also helps explain this around the 4 min mark.
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