# Time at t = 0

1. May 8, 2014

Hi,

It is a mystery that is complicated to figure out.

What does t = 0, (time 0) actually mean in Physics?

Suppose a device's rate of processing is modelled by

Q(t) = 467√(t) [where "t" is the time in minutes after the device began working]

Why is the instantaneous moment when the device began working t = 0?

In addition, what would

"0 minutes after the device began working" mean? What does this represent?

Thanks

2. May 8, 2014

### UltrafastPED

It's when you press start, or when the engagement mechanism is released.

In general it is an arbitrary point on the time scale which corresponds to your beginning of the process of interest ... like in a count down, but going in the opposite direction.

3. May 8, 2014

### Staff: Mentor

No great mystery here, just convention. No special physical meaning. What does have meaning is the amount of time elapsed--which is easier to represent if you start from 0.

Because you want to measure the elapsed time from the moment the device began working. You could call that time Ti, if you like. But then you'd have to describe your model in terms of T - Ti, to get the time since the device began working. Much easier to let Ti = 0.

It just represents the instant that the device began working. Nothing more.

4. May 8, 2014

### dauto

You can chose t = 0 to be any time whatsoever. It's an arbitrary choice made to our convenience. The idea is to make a choice that helps understand the problem and solve it. A different choice might be more difficult but it won't be wrong.

5. May 9, 2014

How would you force the device to start at t = 1?

But think about this, WHY does it even start at t= 0

Once you turn the device on, at least 1 trillionth of a trillionth of a trillionth of a picosecond must have passed? Some very small infinitesimal time MUST have passed right?

Thanks

6. May 9, 2014

### UltrafastPED

It's an idealization in the experimental world - part of my research was to experimentally find the "time zero" which was defined as the "time when a laser pulse passed through an electron pulse"; with the expenditure of time, ingenuity, and sweat a repeatable experimental technique was developed - it was good to less than a picosecond; the limitation was about double the pulse duration.

The "error" then contributes to the experimental spread.

But for a theoretical model you can just set t=0; a mathematical analysis will tell you the amount of error which will accumulate for small, random variations.

7. May 9, 2014

I can sort of see where you are getting at.

The device DOESNT start at t = 0, but rather some infinitesimal time after that?
Devices starting at t= 0 is just an assumption in the mathematical world?

So because it doesnt actually start at t = 0, you get the error right?

This could mean that

it could have start at t =

1 planck time of 1 planck time of 1 planck time.......

8. May 9, 2014

### phinds

You are SERIOUSLY over-thinking this. t=0 is a mathematical ideal as has been explained. Depending on the characteristics of a physical device, its actual "start" time will be some amount of time later. SO WHAT? You have to assign some value to the start and as long as you are consistent, it doesn't really matter.

9. May 9, 2014

### ZapperZ

Staff Emeritus
Do you also have a problem with following a recipe? If the recipe tells you to bake the cake for 45 minutes, how much do you over-think this? Or do you simply put the cake into the oven, and then start your timer or look at your clock?

Zz.

10. May 9, 2014

### ChrisVer

you can say t=0 the time that it actually starts to to work- problem solved.
No matter what, what is important is the elapsed time, and not a single instant time (you need two ("tick tack") to measure time, not just one ("tick")....)
After that it depends on your MEASUREMENT device- how close together are the "tick tacks" it can realize... speaking of planck time for example, for any device we have, is totally meaningless....Don't forget that the clocks we use, are using some process to measure time (eg atomic clocks), so even they measure "time differences" and not instant time values...

Last edited: May 9, 2014
11. May 9, 2014