# What is Time?

#### quant

Could someone give me a detailed explaination of *what* the Time dimention is? I don't mean for someone to start telling me about a clock, but rather to understand relativity you must understand Time as a dimension. I was wondering if anyone could help me out with that? Thanks!

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#### Mentat

Originally posted by quant
Could someone give me a detailed explaination of *what* the Time dimention is? I don't mean for someone to start telling me about a clock, but rather to understand relativity you must understand Time as a dimension. I was wondering if anyone could help me out with that? Thanks!
This thread should, perhaps, be in the Theoretical Physics Forum.

It is a good question that you ask. First off, a dimension is coordinate, which can be used to ascertain the position, size, and/or direction of movement, of an object. Position doesn't just refer to "where" something is, as many think. "Where" you are, refers to your position relative to the spacial dimensions. "When" you are, refers to your position on the time dimension. So, I can tell you the size of the Great Wall of China (for example), but unless it has also existed for a certain amount of time.

One common mistake is saying that the T dimension is just a measurement - of planets spinning, clocks ticking, atoms decaying, etc. Actually, time is a dimension of it's own, and does not measure, but is measured.

I hope I have been clear, and if you need clarification, or if I didn't adequately answer your question, just say so. But, I am in a hurry now, so I can't do my usual proof-reading, sorry.

#### quant

Sorry about it being in the wrong topic.

Well, everything you just told me, i already knew. But, i guess i just need an image of it. Is there anything further anyone could add to explain it a bit more please?

Thanks.

#### Loren Booda

Time in special relativity's spacetime is a zeroth (space being first, second, and third) dimension that juxtaposes spatial dimensions in a four-dimensional (conical) Pythagorean theorem:

x^2+y^2+z^2=(ct)^2 for "lightlike" surfaces,

x^2+y^2+z^2<(ct)^2 for "timelike" regions,

x^2+y^2+z^2>(ct)^2 for "spacelike" regions.

All light comes to observers through their contiguous lightlike surface. Inertial events that can affect them mechanically exist in relative timelike regions, and events out of direct communication exist in spacelike regions.

Physics also allows for time defined by entropy (the second law of thermodynamics), cosmology (the big bang), and radiation (quantum decay).

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#### Phobos

Staff Emeritus
Gold Member
Welcome to Physics Forums, quant!

Originally posted by quant
Could someone give me a detailed explaination of *what* the Time dimention is? I don't mean for someone to start telling me about a clock, but rather to understand relativity you must understand Time as a dimension.
To add to what the others said so well...

Time is part of the very fabric of the universe itself. It is an aspect of the universe that has a certain behavior that is directly tied to the other dimensions (3D space). A full understanding of its nature is not available at this time (just like a full understanding of space or gravity is not known). Whe can measure it and see its effects, but is it continuous/quantized? How does it work? Don't know.

See sig.

#### RuroumiKenshin

Originally posted by quant
Could someone give me a detailed explaination of *what* the Time dimention is? I don't mean for someone to start telling me about a clock, but rather to understand relativity you must understand Time as a dimension. I was wondering if anyone could help me out with that? Thanks!
quite simply, time is an increase of entropy (and what Mentat said is of great importance, esp. in relativity).

BTW, should relativity be capatilized or not?? I am very frustrated; I see it captilized one day, and the next, lower cased!!

#### Chagur

Huhhh? What has entropy got to do with it?

#### RuroumiKenshin

That how time's defined. It never occured to me that there would be relationship..

Maybe, it has something to do with the expansion of the universe. For example, time-space are one in relativity. So naturally, as the universe's entropy increases, so does time.

#### Phobos

Staff Emeritus
Gold Member
Personally, I don't see the benefit in defining time via entropy. Sure, both are on a continuous one-way line, but I think they proceed at different rates.

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