# B Time as a dimension

#### tylerfarzam

Is time its own dimension or is it a constant that remains through all dimensions? Also, are there multiple dimensions of time, and how do we know the answers to these questions?

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#### Spacepantz86

I do not think its possible for time to be its on dimension. Time would have to exist on all dimensions, because without it, there would be no differentiation between the dimensions.

Maybe it works differently in different dimensions though?

#### PeroK

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Is time its own dimension or is it a constant that remains through all dimensions? Also, are there multiple dimensions of time, and how do we know the answers to these questions?
I do not think its possible for time to be its on dimension. Time would have to exist on all dimensions, because without it, there would be no differentiation between the dimensions.

Maybe it works differently in different dimensions though?
You could both start here:

https://en.wikipedia.org/wiki/Spacetime

#### Drakkith

Staff Emeritus
2018 Award
and how do we know the answers to these questions?
We observe the universe, perform experiments, and make new models and theories. Basically ask, "what are the consequences if X is true" and then see if your observations and experiments support this over other possibilities. If there were multiple dimensions of time then there would be some serious consequences that would could observe. But we don't observe these things. All our observations and experiments support the idea that time is a single dimension, just one out of the four total dimensions making up spacetime.

I do not think its possible for time to be its on dimension. Time would have to exist on all dimensions, because without it, there would be no differentiation between the dimensions.
To the best of our knowlege, time is a single dimension. Note that I'm using 'dimension' to refer to a particular axis of a coordinate system. I don't mean 'dimension' in the sense of there being alternate dimensions, which might be better described as alternate or parallel universes.

#### klocke

Is time its own dimension or is it a constant that remains through all dimensions? Also, are there multiple dimensions of time, and how do we know the answers to these questions?
Undoubtedly Time is not a dimension but like gravity Time is the only thing that can move across dimensions

#### Periwinkle

To the best of our knowledge, time is a single dimension. Note that I'm using 'dimension' to refer to a particular axis of a coordinate system. I don't mean 'dimension' in the sense of there being alternate dimensions, which might be better described as alternate or parallel universes.
I am always surprised that people accept so quickly that time is a dimension. It should not be forgotten that only with $it$ will we get a number that behaves like a spatial dimension in many ways. However, without the $i$ multiplier, it is impossible to include time as the fourth dimension besides the three spatial dimensions. The $i$ itself has a lot of unique features.

So if anyone asked me this, I'd answer him, but don't forget about imaginary $i$.

#### Drakkith

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2018 Award
Undoubtedly Time is not a dimension but like gravity Time is the only thing that can move across dimensions
This is nonsense. Time is treated as a dimension in physics, and your statement about moving across dimensions is vague and probably meaningless. Since you appear to be a new member, please note that PF does not allow personal theories or ideas that fall outside of mainstream science. You can find a list of the rules under the 'INFO' dropdown menu at the top of the page.

#### Periwinkle

This is nonsense. Time is treated as a dimension in physics, and your statement about moving across dimensions is vague and probably meaningless. Since you appear to be a new member, please note that PF does not allow personal theories or ideas that fall outside of mainstream science. You can find a list of the rules under the 'INFO' dropdown menu at the top of the page.
Interestingly, the article on the physical concept of time in Encyclopedia Britannica begins with time as a continuum with no spatial dimension.

#### Drakkith

Staff Emeritus
2018 Award
Interestingly, the article on the physical concept of time in Encyclopedia Britannica begins with time as a continuum with no spatial dimension.
Of course. Time is a non-spatial dimension, as it has to be.

#### Nugatory

Mentor
However, without the $it$ multiplier, it is impossible to include time as the fourth dimension besides the three spatial dimensions.
That's not quite right; not only is it possible, but $it$ is one of the first things you unlearn as you move from special relativity to general relativity. There's a short section in MTW entitled "Farewell to $ict$" in which the authors explain why our "old friend $ict$.... must be put to the sword".

The $ict$ trick has the modest virtue of making Lorentz boosts in flat four-dimensional spacetime look formally like rotations in four-dimensional Euclidean space, but that's not necessary for treating time as a dimension and it gets in the way when we're working with curved spacetimes modeled by pseudo-Riemannian geometry.

#### Mark44

Mentor
There's been a lot of nonsense spouted here.

I do not think its possible for time to be its on dimension.
Time would have to exist on all dimensions
Undoubtedly Time is not a dimension but like gravity Time is the only thing that can move across dimensions
I am always surprised that people accept so quickly that time is a dimension.
The very first sentence in the wiki article that @PeroK cited in post #3 says:
n physics, spacetime is any mathematical model that fuses the three dimensions of space and the concept of time into a single four-dimensional continuum.
Some sources distinguish the three spatial dimensions, the x, y, and z of Euclidean space, and the temporal dimension, time.

#### David Lewis

If we assume time is inextricably interrelated to the 3 dimensions of space (in a specific way) then the laws of physics become simpler and more unified. At the same time, a theory that rules out the existence of dimensions beyond those four has not yet been developed.

#### Mister T

Gold Member
I am always surprised that people accept so quickly that time is a dimension. It should not be forgotten that only with $it$ will we get a number that behaves like a spatial dimension in many ways.
Dimensions do not have to be spatial. There are many examples of dimensions that are neither spatial nor temporal.

#### Periwinkle

Dimensions do not have to be spatial. There are many examples of dimensions that are neither spatial nor temporal.
Mathematics and physical reality should not be combined. Mathematics knows Hilbert space of infinite dimension, but no one thinks that the physical space itself would be an endless dimension. Of course, I also know, for example, that general theory of relativity uses a space concept in which physical spatial and temporal relationships only appear as spatial relationships. However, time in these geometries also plays a unique role.

The dimension itself is an entirely geometric concept. Until there is no other direct concept of geometry than the three-dimensional Euclidean space experienced, the time as a dimension is challenging to accept. Minkowski also faced this. He believed that switching from the three-dimensional Euclidean to the four-dimensional, by analogy, was acceptable, and if the time were represented by $cit$ quantities, the unification of space and time would be even more evident. (I remember he wrote that space and time as a separate reality will disappear forever.)

So, for example, I can personally accept so much that time is a separate dimension, as much as I assume Hilbert space is space. In a mathematical sense, this is undoubtedly the case.

#### PeroK

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Mathematics and physical reality should not be combined.

The dimension itself is an entirely geometric concept.
By that argument "dimension" is a mathematical concept, with a mathematical definition. Space and a 3D geometry are related by a mathematical relationship. When we say space is 3D, we mean space is modelled by a 3D mathematical space. And when we say spacetime is 4D we mean that spacetime is modelled by a 4D mathematical space.

And, if you say that spacetime is 3+1 dimensional - by acknowledging the +++- nature of the metric - all you're doing is adding a bit more information to the definition of a dimension.

In any case, you are saying nothing of any physical significance.

"Time as a dimension"

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