# Are there 2 kinds of Time?

1. Jul 27, 2011

### riezer

Are there 2 kinds of time? One from Spacetime and the other time is subjective time where we experience time? Or could our time and the time in Spacetime the same?

2. Jul 27, 2011

### atyy

3. Jul 27, 2011

### pervect

Staff Emeritus
The two different sorts of time I'm most familiar with are proper time, and coordinate time. Proper time is what you actually measure with a clock, it would also be "what you experience". Coordinate time is just a label you stick on an event to determine its "position" in time, just as a spatial coordinate determines its "position" in space.

At any particular point in space, there is usually some relationship between the rate at which coordinate time changes, and the rate at which clocks actually tick - this is known as "time dilation". For some reason, people tend to emphasize coordinate time, but in general there are many different coordinate systems possible, in my view that makes it less fundamental.

4. Jul 27, 2011

### riezer

5. Jul 27, 2011

### nitsuj

If proper time is what a clock measures, is proper time what is constant? To say it differently, no matter the velocity, a clock will always appear* to tick at the same rate. The "flow/rate/speed" of force is always constant c.

I don't understand "Coordinate time is just a label you stick on an event to determine its "position" in time, just as a spatial coordinate determines its "position" in space." Can you say it differently? I understand position in space, I don't understand "position" in time and how it differs from a position in space. If we are at the same place at the same time, imo we are in the same space.

These two different kinds of time seem to be one of QM and one of GR. Place to fine a point on measurement and well, uncertainty of where/when.

*at same velocity as clock

6. Jul 27, 2011

### Staff: Mentor

These two concepts exist in both modern QM (QED, QFT) and SR/GR.

7. Jul 28, 2011

### ghwellsjr

I have no idea what you mean by your last sentence but proper time is not constant unless you simply mean that two clocks, always next to each other will always have the same times on them no matter how they accelerate together. Otherwise, two clocks that start out next to each other but accelerate differently taking separate paths and finally end up next to each other may not have the same time on them and neither one of them is incorrect or wrong in any sense.
Prior to Einstein, coordinate systems only included postitions of space, made up of three parameters, x, y and z. I'm sure you're familiar with the concept. But in Einstein's Special Relativity, he introduces a new kind of coordinate system which is called a Frame of Reference, which includes the three spatial coordinates but also a time coordinate, which is just an ordinary elapsed time as we are all familiar with. So in a FoR, it takes 4 parameters to define what is called an "event" which is a particular location at a particular time. The reason why time is included in the FoR is that when you want to use a different FoR, one that is moving at a constant speed relative to the first one, there are a set of equations called the Lorentz Transforms that allow you to convert the four parameters from the first FoR to the second FoR and they include time as well as location in the set of equations. So you could have an event in the first FoR that has a time of, say, 20 seconds at location 40, 50, 60 that ends up with a time of 15 seconds at location 30, 50, 60 in the new FoR. They are the same event but with different coordinates for the two Frames of Reference.

8. Jul 28, 2011

### nitsuj

The constancy of c is not simple, it's kinda odd. And yes that is what I meant, I added the * to clearify the obvious point of relativity.

Your second comment clearified things for me, thanks.

9. Jul 28, 2011

### nitsuj

Yea ns,

I meant to say "it's as if...."

10. Jul 28, 2011

### ZealScience

In my opinion proper time is like the distance of four dimensional space time of Minkowski space-time (as it is the sum of components squared). But I don't think that space time and our time are very different...we just experience the same time always.