# Sorry if this is in the wrong place but im at work so i need to be

Sorry if this is in the wrong place but im at work so i need to be quick.
What is time?is it just a way of measuring events or is it a physical thing?
If time can be affected by travelling at high speeds over great distances, or by the gravitational pull of a large/massive body in space, does this make it physical?

There are many ways to interpret time. What you are asking is a philosophical question. Stephen Hawking wrong a book called "A Brief History of Time", I suggest you read it.

DaveC426913
Gold Member

It is generally considered to be the timelike dimension of 4-dimensional spacetime.

time is the difference between two moments. just as distance is the difference between two points.

DaveC426913
Gold Member

time is the difference between two moments. just as distance is the difference between two points.

Unfortunately, that is self-referential, so sheds no light on the question.

'So what is a moment?'
'A point in time.'

Thanks for your reply.i will keep an eye out for the book.

I can sort of understand why time is considered as the 4th dimension.however, would it not make more sense for time to be the 1st dimension?

Last edited:
DaveC426913
Gold Member

Thanks for your reply.i will keep an eye out for the book.

I can sort of understand why time is considered as the 4th dimension.however, would it not make more sense for time to be the 1st dimension?
There is no order; they are all equal.

Time is one of the four dimensions.

The only reason it is often listed is the fourth is because almost everyone is already familiar with the three physical ones.

DrGreg
Gold Member

I can sort of understand why time is considered as the 4th dimension.however, would it not make more sense for time to be the 1st dimension?
Actually, in the more advanced notations in relativity theory, time's usually counted as the 0th dimension!

Perhaps overly-simplistic, but my simple mind perceives "time" as having an intimate, immutable relationship to "change"
I leave it as that, else my mind will blow-up.

Hello every body
i think so "Time" once gone never came back
So, utilize the "Time" available right now and it will not be available any other Time"
What do you think?

It is generally considered to be the timelike dimension of 4-dimensional spacetime.

For us who are the simple of mind, could you clarify the meaning of "timelike"? Thanks!

For us who are the simple of mind, could you clarify the meaning of "timelike"? Thanks!

Timelike means that the time distance counts negative when calculating distances in 4 dimensional space.

Timelike means that the time distance counts negative when calculating distances in 4 dimensional space.

First, I do sincerely want to thank you for taking the trouble to reply.

And for clarifiying to me, why, in 1955, I switched from physics to mechanical engineering.

And for clarifiying to me, why, in 1955, I switched from physics to mechanical engineering.

Looking back or forward any insight about that choice? Seems like a valid question here....

DrGreg
Gold Member

For us who are the simple of mind, could you clarify the meaning of "timelike"? Thanks!
For a verbal explanation, two events ("points" in 4D spacetime) have

• a timelike separation if it is possible to travel from one to the other slower than the speed of light
• a lightlike or null separation if it is possible to send light in a straight line from one to the other (in vacuum)
• a spacelike separation if, to travel from one to the other, you would need to go faster than the speed of light (which is impossible)

So directions in 4D spacetime can be classified as timelike, spacelike, or null. We can decompose 4D spacetime to have one timelike axis and three spacelike axes, but we have lots of choice over how to do this. So, time is essentially "distance" in 4D spacetime measured in a timelike direction. (But if you measure 4D distance in a spacelike direction, that's the same as distance in 3D space for some observer.)

DaveC426913
Gold Member

For a verbal explanation, two events ("points" in 4D spacetime) have

• a timelike separation if it is possible to travel from one to the other slower than the speed of light
• a lightlike or null separation if it is possible to send light in a straight line from one to the other (in vacuum)
• a spacelike separation if, to travel from one to the other, you would need to go faster than the speed of light (which is impossible)
Are these mutually exclusive? Can two points only be one of the above?

Also, I'm having trouble imagining examples.
So, Earth at 12:00 and the Moon at 12:05 have timelike separation (you can travel from one to the other) as well as null separation (you can send light between them).
But Earth at 12:00 and Moon at 12:00 have spacelike separation because you would need to travel >c.

Does separation mean "causally isolated from"?

Are these mutually exclusive? Can two points only be one of the above?

Also, I'm having trouble imagining examples.
So, Earth at 12:00 and the Moon at 12:05 have timelike separation (you can travel from one to the other) as well as null separation (you can send light between them).
But Earth at 12:00 and Moon at 12:00 have spacelike separation because you would need to travel >c.

Does separation mean "causally isolated from"?

These things are mutually exclusive. Earth at 12:00 and the Moon at 12:05 do not have null separation, light that you send from earth will reach the moon in 1.26 seconds. Only the moon at 12:00:1.26 would have zero separation. For different observers at different speeds close to the speed of light, distances can look completely different. But the nice thing is, that time like separations will always stay time like, zero separation or light like ones will always stay light like (the speed of light is the same for every observer), and space like ones stay space like.

DrGreg
Gold Member

Yes, 3735928559 is correct on all counts.

The mathematical definition is to consider the quantity

$$c^2(t_2-t_1)^2 - (x_2-x_1)^2 - (y_2-y_1)^2 - (z_2-z_1)^2$$​

and see if it is > 0 or = 0 or < 0, which gives timelike, null, or spacelike respectively.

The quantity above is an invariant: it takes the same value relative to any inertial frame of reference.

(All of the above is for special relativity with no gravity. When you add gravity, you have to formulate it slightly differently but it's the same general idea.)

Does separation mean "causally isolated from"?
Spacelike separation does indeed mean "causally isolated from", and in that case observers may disagree over which of the two events occurred first. When events have timelike or null separation, everyone agrees on the temporal order.

________________
In case you were wondering, 3735928559 = 0xDEADBEEF

Last edited: