Physics Help Forum Spacetime dimentions.

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 Jul 5th 2018, 01:23 AM #1 Senior Member     Join Date: Feb 2017 Posts: 203 Spacetime dimentions. Well space has 3 dimensions: length, width and depth. But these are the dimensions of the object not of space. Space which exists without the matter does not have any dimensions then. Then how can these be dimensions of space as they appear to be dimensions of the object?
 Jul 5th 2018, 04:03 AM #2 Senior Member   Join Date: Oct 2017 Location: Glasgow Posts: 269 Dimensions are just the number of axes in your coordinate system. That's it.
 Jul 5th 2018, 04:09 AM #3 Senior Member     Join Date: Jun 2016 Location: England Posts: 688 I would suggest that the dimensions are set by the way the fundamental pieces of existence (particles, bosons, photons, whatever...) interact. These interactions seem to happen in a 3 space 1 time dimensional manner. Why that may be requires a better brain than mine! __________________ ~\o/~
 Jul 5th 2018, 11:01 AM #4 Forum Admin     Join Date: Apr 2008 Location: On the dance floor, baby! Posts: 2,491 Consider the property of the position of an object. Position is a vector with three values that place the location of an object in reference to an assigned origin. Thus we have three dimensions that have nothing to do with the geometric sizes of an object. So even though a line in space only has one dimension we still need to locate it (specifically locate a point on it) using the three position axes. If you want to get all Relativistic then replace the word "position" with "event" and then the location has four values: x, y, z, t. -Dan lancew561 likes this. __________________ Do not meddle in the affairs of dragons for you are crunchy and taste good with ketchup. See the forum rules here. Last edited by topsquark; Jul 5th 2018 at 11:03 AM.
 Jul 5th 2018, 02:57 PM #5 Senior Member   Join Date: Apr 2015 Location: Somerset, England Posts: 995 Just remember that mathematical dimensions are not necessarily the same as physical ones. How many dimensions are needed for A Peano curve? A Fractal curve? topsquark likes this.
 Jul 6th 2018, 08:52 AM #6 Senior Member     Join Date: Jun 2016 Location: England Posts: 688 3, 4 or more? Looking at my previous post on this thread, I suggested that the universes fundamental interactions happen in 4D. There are however string theorists who will happily claim that these interactions happen in 11 dimensions. The claim then is that we humans have limited 3 + 1 Dimensional perception and that we then try to force the universe into this 4D straitjacket. I'm not sure where I personally stand on this, I have too little evidence (that I can understand) to form a firm opinion. __________________ ~\o/~ Last edited by Woody; Jul 8th 2018 at 10:11 AM. Reason: spelin
 Jul 7th 2018, 05:03 AM #7 Senior Member     Join Date: Feb 2017 Posts: 203 Well then you mean mathematical dimensions are length breadth and depth whereas physical dimensions are latitude, longitude and height above sea level. Is that right? Well another thing I noticed was that when the spacial dimensions of length breadth and depth are taken then the material is irrelevant as these dimensions remain same when we immerse a body in water as oposed to when placed in air.Why so? Now if we consider the dimensions of length breadth and space as woody says it is the interaction where space is defined. Now then this makes sense as to how Einstein discovered his new gravity. Since we can now imagine space to be a rubber sheet and the massive object causing a deviation in space. Look at it this way just as when you have a cube so this cube if we consider space to be a rubber sheet has to displace some of the space it interacts. Is this how Einstein knew gravity?
Jul 7th 2018, 05:10 AM   #8
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 Originally Posted by avito009 Well then you mean mathematical dimensions are length breadth and depth whereas physical dimensions are latitude, longitude and height above sea level. Is that right?
No, that's not at all what was meant.

 Well another thing I noticed was that when the spacial dimensions of length breadth and depth are taken then the material is irrelevant as these dimensions remain same when we immerse a body in water as oposed to when placed in air.Why so?
Are you sure that's true? When you put a rectangular sponge in water, each dimension becomes a little larger.

 Now if we consider the dimensions of length breadth and space as woody says it is the interaction where space is defined. Now then this makes sense as to how Einstein discovered his new gravity. Since we can now imagine space to be a rubber sheet and the massive object causing a deviation in space.
The "rubber sheet" is a very crude analogy.

 Look at it this way just as when you have a cube so this cube if we consider space to be a rubber sheet has to displace some of the space it interacts. Is this how Einstein knew gravity?

Jul 7th 2018, 05:18 AM   #9
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 Well then you mean mathematical dimensions are length breadth and depth whereas physical dimensions are latitude, longitude and height above sea level. Is that right?
No I didn't mean that at all.

Dimensions in mathematics are an abstract construct, some correspond to physical reality, some do not.

Dimensions in Science and Engineering correspond to something physical.

By physical I mean something we can measure.

Your examples of {length, breadth, height} and {elevation, latitude, longitude} are Science and Engineering use, not mathematical ones.

Before you consider mathematical dimensions I suggest you look up the word 'embedded'

This is one crucial difference.

Jul 9th 2018, 04:05 AM   #10
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 Originally Posted by avito009 Well then you mean mathematical dimensions are length breadth and depth whereas physical dimensions are latitude, longitude and height above sea level. Is that right?
No. There's no distinction between mathematical dimensions and physical dimensions. Dimensions are just names for the orthogonal bases in coordinate systems.

 Well another thing I noticed was that when the spacial dimensions of length breadth and depth are taken then the material is irrelevant as these dimensions remain same when we immerse a body in water as oposed to when placed in air.Why so?
They're not irrelevant. The object can still be described using the 3D Cartesian coordinate system you adopted.

There are physical phenomena that we can model using the laws of physics. Those laws of physics are framed using well known coordinate systems, like Cartesian, cylindrical and spherical polar coordinates, but there's nothing stopping you from defining your own coordinate systems and using those. For example, solid state physics makes use of reciprocal space, relativity can be framed using time as a dimension. Nuclear physicists sometimes use complex forms for things that are not complex, which can sort of be seen as a new 'dimension'.

Yes, most of nature can be described by using three spatial dimensions in your coordinate system, but that's not a fundamental property of the Universe. You could, for example, use a 1D or 2D system of coordinates if you want to describe a certain situation, which might be fine. However, there are other situations where one or two dimensions are not good enough. Similarly, some scientists find it easier to describe certain phenomena using many dimensions, but that doesn't mean that they are 'real'.

 Now if we consider the dimensions of length breadth and space as woody says it is the interaction where space is defined. Now then this makes sense as to how Einstein discovered his new gravity. Since we can now imagine space to be a rubber sheet and the massive object causing a deviation in space. Look at it this way just as when you have a cube so this cube if we consider space to be a rubber sheet has to displace some of the space it interacts. Is this how Einstein knew gravity?
I'm not very good at relativity, but from what I understand, I think the answer is no. Einstein developed his theories by investigating the properties of objects as they approach the speed of light and never assumed or treated space as a material that can be warped and bent.

Last edited by benit13; Jul 9th 2018 at 04:11 AM.

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