# I Types of time in relativity

1. Jan 2, 2017

### windy miller

What different types of time are there is relativity. I have heard phrases like "proper time" "co-oridnate time" and "conformal time". but what would be great to get a layman friendly definition. Thanks.

2. Jan 2, 2017

### Orodruin

Staff Emeritus
Proper time is the time measured by a clock following a given world line. It is the only physically meaningful time that can be measured directly.

Coordinate time is just a nomenclature for any time-like space-time coordinate and conformal time is a special case of a coordinate time that is used in cosmology.

3. Jan 3, 2017

### Battlemage!

I'd say proper time, laymen term speaking, is the time measured by a clock right at (and moving at the same speed as) the events being looked at. Or rather, your proper time is the time measured by the clock you are holding, rather than looking at some clock 3 miles away and measuring your time with that. Likewise the proper time of Superman as he flies by is the time measured by his watch (since he is at the location in question and moving exactly the same way as his watch).

4. Jan 3, 2017

### pervect

Staff Emeritus
Proper time is sometimes called wristwatch time. Perhaps a stopwatch would be a better analogy, you basically measure the number of seconds elapsed on the stopwatch between when it is started and when it is stopped.

Note that it's implicitly understood that one understands how the observer moves through space when talking about proper time. The abstract concept is called a "worldline".

Coordinate times are arbitrary labels given to describe when events happen. UTC time and GPS time (which are very similar, but I don't recall the minute specifics) are two examples of coordinate time. https://www.timeanddate.com/time/aboututc.html . Note that due to gravitational time dilation, the amount of proper time elapsed between two UTC times 1 hour apart will depend on the position of the person or stopwatch doing the measurement. Specifically, it will depend on the altitude of the clock.

I know conformal time is used in cosmology, but I'm not sure of the definition. So I'll let someone else fill in the blank there.

5. Jan 3, 2017

### windy miller

Hi thanks for the explanation. Can anyone elaborate a bit more on conformal time? I understand that it works in a way that it invariant under a scale transformation. So a photon for example doesn't have any proper time but still has conformal time. but how does this work?

6. Jan 4, 2017

### Mister T

The best approach to understanding this is to recognize the phenomenology. That is, there's a reason physicists, technicians, and engineers have invented and adopted the use of these different terms. There are lots of conditions under which it's necessary to use them if one is to understand how clocks actually behave.

In Einstein's native German proper time is eigenzeit. It comes from 'eigen', meaning one's own, and 'zeit', meaning time. It is the time each of us measures with the clocks we carry with us. The central issue is that the amount of proper time that elapses on your clock won't necessarily match the amount of proper time that elapses on my clock. The reason for the invention of things like coordinate time is to help us understand and predict the differences in proper time.

One famous example of this is the twin paradox. Engineers who calibrate the GPS clocks have to contend with this issue. So do lots of other scientists, technicians, and engineers who deal with precision clocks, fast-moving particles, or both.

7. Jan 7, 2017

### Stephanus

Proper time is 1 minute, 2 minutes or 1 month, 2 months, etc...
So if you see a rocket travelling toward (away from you) and you see its clock shows, some thing like this.
I'll make it two simple tables below.
(Y)our clock 15:00, (R)ocket clock 01:00
Y: 15:05, R: 01:01
Y: 15:10, R: 01:02
Y: 15:15, R: 01:03,
so you'll see that the rocket proper time is slowed by 20% compared to your proper time.

While 15:00 and 01:00 is meaningless here.
Your clock might shows 15:00, January 1st 2017
Rocket: 01:00, February 2nd 2016
Y: 15:05 January 1st 2017, R: 01:01, February 2nd 2016, etc...
Your calendar and watch don't matter. All you see is the rocket proper time is slower by 20%.

Whilst 15:00 January 1st 2017 is the coordinate time.
And also 15:05, January 1st 2017 is the coordinate time.

Perhaps a simple example is this.
Y: 5 minutes, R: 1 minute
Y: 5 minutes, R: 1 minute
Y: 5 minutes, R: 1 minute

I'm sorry I can't explain it further, but I hope you know what I mean. Last year (two years ago?) I also had confusion with this, but now I understand. You can ask again if you don't understand, perhaps I can explain it to you interactively.