# B What is time? How can we see it?

1. Aug 22, 2015

### yashwanthippili

how can we describe time

2. Aug 22, 2015

### Abullais Ghazi

Time is measure of duration and also referred as the fourth dimension. No we cannot see it.

3. Aug 22, 2015

### Staff: Mentor

We cannot see it, but we can measure it. There are lots of things (for example, weight and temperature) like that.

4. Aug 22, 2015

### Staff: Mentor

Except insofar as we can see, with our eyes, how "long" something is in both the space dimensions and time dimension...you just have to wait a while to observe something's duration.

5. Aug 22, 2015

### Drakkith

Staff Emeritus
Time is that which a clock measures.

6. Aug 23, 2015

### axmls

This. We describe the quantities we speak of with the measurements we use to determine them. All distance is is what we can measure with a meter stick.

7. Aug 23, 2015

### lightarrow

How do you define "clock"?

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lightarrow

8. Aug 23, 2015

### Drakkith

Staff Emeritus
9. Aug 23, 2015

### Staff: Mentor

I am fond of the following definition from Wikipedia and a two-stage definition of time

Time is a measure in which events can be ordered from the past through the present into the future.

I like to think of that as the basic definition of time. It allows reasoning such as "before and after" and "causality" without reference to the quantity of time.

Next, the "advanced" definition of time adds the quantity of time, i.e. what a clock measures as Drakkith said. Hence the full Wiki definition.

Time is a measure in which events can be ordered from the past through the present into the future, and also the measure of durations of events and the intervals between them.

I also like to think of a basic clock as a machine which generates an ordered list of tick events. Clocks can be fast (small interval between clicks) or slow, or even non-uniform but nevertheless useful in logical reasoning. For example, if event A happens before tick 99 and event B happens after tick 99, then A must be before B in time.

An advanced clock has equal intervals of time between ticks.

10. Aug 23, 2015

### Gaz

Time is a measurement of the rate of change of everything in the universe.
Eg atomic clocks are the measurement of motion of electrons.
We measure motion then give it a time frame based on its speed in relation to other motions.

11. Aug 23, 2015

### ZapperZ

Staff Emeritus
12. Aug 24, 2015

### lightarrow

That doesn't seem a good link to answer my question, because it says:

"A clock is an instrument to indicate, keep, and co-ordinate time."

We can't answer to the question "what is time?" with the answer "it's what a clock measures" if a clock is what written up...
We need a better definition of "clock", a one which of course doesn't use the concept of time.

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lightarrow

13. Aug 24, 2015

### Isaac0427

Just remember, in the technical world of physics things that you thought were basic, are very complicated. Honestly, I think what you and many others have asked about time doesn't fall in our knowlage right now. In fact, there are multiple theories I have seen describing time (or spacetime) that are still disputed. For example, I have had an interesting convorsation on here about the Big Bang and what t (time)=0 meant. What I got from that was nobody knows what happened at t=0, and I don't think you can fully describe what something is if you can't fully describe what happened when it wasn't there.

14. Aug 24, 2015

### Isaac0427

I agree, but clocks need time to work, and time doesn't need a clock to exist. In other words, clocks are dependent on time, and time is independent of clocks. Therefore, the definition of a clock should stay the same, and it is time that we need to have a better definition for.

15. Aug 24, 2015

### Staff: Mentor

Why not? Try the similar question: what is length?

16. Aug 24, 2015

### Drakkith

Staff Emeritus
I don't know of any accepted theories other than GR that describe time. My opinion is that we know a great deal about what time is, just as we know a great deal about what distance is.

My understanding is that we choose t=0 to be at the big bang because it makes the math easier, not because time literally started at the big bang (which isn't what the big bang theory says). But I could be wrong.

17. Aug 25, 2015

### Isaac0427

Look at my other post called the begining of time. I share how two very well-respected physicists (Stephen Hawking and Brian Greene) have written contradictory things about time.
Really? I thought t=0 gives is weird infinites in our math. Also, t=0 means that time actually started.

18. Aug 25, 2015

### Drakkith

Staff Emeritus
Their opinions have nothing to do with the nature of time, but with the question of did time (and anything else) exist prior to the big bang. They certainly don't count as theories. See Nugatory's post (post #2) in your thread.

The ONLY accepted theory that deals with what time is and how it works is General Relativity unless you count the fact that we usually just use the plain old Newtonian idea of time in our everyday lives since it's MUCH easier than GR. The Newtonian concept of time being absolute and the same for all observers is, of course, known to be inaccurate/incomplete, which is where GR comes in.

I don't think you plug a 0 in for time anywhere. I think it's that if you continue to look backwards in time, you reach a point where the density reaches infinity based on current models. As far as I understand it this point in time is labeled t = 0 for convenience. I could very well label it t = -13.7 billion. Most equations that have time as a variable only care about the elapsed time between two events, not the 'absolute' time.

That's my understanding of it at least. Someone correct me if I'm wrong.

19. Aug 25, 2015

### Isaac0427

I don't believe that GR gives us a full understanding of time. It can't answer every question about time, and if it could, there would be no controversy on how time behaved at the Big Bang.

20. Aug 25, 2015

### lightarrow

To define "clock" we need something which allow us to distinguish it from other Instruments, or we could use a ruler to measure durations.

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lightarrow