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Author Topic: How do we know what the exact correct time is?  (Read 6265 times)

NOREEN

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How do we know what the exact correct time is?
« on: 21/07/2009 16:30:02 »
NOREEN asked the Naked Scientists:
   
HI  THERE I RESIDE IN JHB AND LISTEN TO YOU EVERY FRIDAY  AND WHAT A  JOY YOU ARE

MY QUESTION IS HOW DO WE KNOW WHAT THE REAL TIME IS.

MY CLOCK IN MY CAR IS EXACTLY 11MINUTES FAST AS ACCORDING TO THE RADIO BUT ON ODD DAYS THE TIME GIVEN ON THE RADIO IS BETWEEN 1 TO 2 MINS FASTER OR SLOWER THAN MY CLOCK i ALSO HEARD THE EARTH IS SLIGHTLY OFF IT,S AXIS AGAIN GIVING A VARIANT IN TIME MAYBE ONLY A COUPLE OF SECONDS IS THAT 24HOURLY OR DAILY OR YEARLY WHO KNOWS WHAT THE CORRECT TIME IS AND DO ALL THE LOST SECONDS EVENTUALLY ADD UP WHO KNOWS WE COULD BE A YEAR OR TWO OUT OR DOES THIS SOUND SILLY TO YOU MANY THANKS
NOREEN...JHB

What do you think?


 

Offline johnspannenburg

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How do we know what the exact correct time is?
« Reply #1 on: 23/07/2009 15:55:49 »
Well, by 'real time' I suppose you must mean at a certain precise location as it makes a difference.

Firstly if you really want to be finicky about the real time you can forget about your car clock or your time from the radio - none of these are very accurate and could be significantly incorrect at any time. 

The highest level of time keeping would be with various atomic clocks around the planet and these measure time as a function of the vibration of atoms such as cesium . 

Generally these clocks will measure a universal time (perhaps it's GMT I'm not 100% on that) from which, in theory, you should be able to get your local time if you really wanted to be super precise.  I live in Australia and I have to add 10 hours to GMT time to get my local time. 

In relation to the rest of your query there are many things that effect our measures of time such as years / months / days / hours etc as these are generally functions of the rotation of the earth on it's axis and around the sun both of which are totally local phenomena and really would have no meaning somewhere else in the universe. 

Of course the speed of rotation of the earth is changing over time as is it's speed around the sun (in accordance with well established scientific principles) and it wobbles on it's axis and so on.  These changes do add up over time and are the reason we have leap years and special rules about leap years (it's not just every 4 years), we have leap seconds also to account for these varying things.

Basically , at high levels of accuracy, all your normal measures of time Y/M/D/H etc are changing and somewhat fluid thing.  A second used to be 1/60 of a minute which was 1/60 of an hour which was 1/24 of a day and so on .. I believe a second is now defined as "the duration of 9,192,631,770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium 133 atom" (from wikipedia).  What this means is that in the future a day / month / year (whatever) as per the above will have a different number of seconds than is the case now.

In short 'exact time' or 'real time' are very slippery things - your best bet is to use the time from one of the atomic clocks (many place on the web - use google)and ajust for your locality.

Hopefully this sort of clears it up for you.
Cheers.
 

Offline Databit

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How do we know what the exact correct time is?
« Reply #2 on: 23/07/2009 16:45:40 »
Time as it is measured is a human construct.

Our measurement of time would not work in other situations due to various situations.  My favorite example of time being used where it does not fit is in time keeping in space, i.e.: Star Trek.  It may take them several days to get somewhere, but whose days?  Earthís days?  The target planetís days?  So when you hear it is 9:11am on your radio, that time is relative to just our situation.

There is also the matter of time flowing differently in different situations.  Specifically time appears to be effected by gravity.  A clock in space in orbit around the earth will keep very slightly different time than a clock on Earth.  Even if both clocks are atomic clocks.  The difference between them is extremely small, but it is still a difference.

So back to your question.  How do we know what time it is?

We created time measurements specific to our situation.  Originally time was kept through solar or stellar positioning, later devices were devised to track the passage of time.  The positioning of the moons around Jupiter have even been used to keep track of time.  Now days our most precise time measuring devices are atomic clocks.  By measuring the vibrations of the crystals contained in them, we can measure the passage of time in a very accurate manner.  But why is 9:11am exactly 9:11am?  Because we said so.  We made up time, we made up how to measure it, and at some point in time we decided when midnight would be and thus we defined what all other times would be.
 

lyner

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How do we know what the exact correct time is?
« Reply #3 on: 23/07/2009 16:48:45 »
Hi Noreen
We are SO RUDE, ignoring you for so long!
Days with nothing, now there are three replies - one whilst I was in the middle of this-

There are two issues here. (I'm really only concerning myself with time on Earth)

The RATE at which time progresses can be defined very accurately. 'Clocks', based  on the vibrations of atoms, can easily be in agreement to one part in a million million (costing only a few hundred quid)

'The Actual Time' is another problem entirely. The root of the problem is that the Earth revolves erratically and that light (and radio waves etc) travel at a very finite speed.
Local Noon is different as you move East or West. The clocks we use now are set to Universal time, based on the rate that the Earth rotates - but its rotation varies measurably - so Universal Time has been developed, based on the poshest atomic clocks. This gives us a 24 hour (exact) day but Noon may not be when the Sun is exactly overhead at Greenwich every day.

If you use a radio to get your time information, then beware! Listen to the 'pips' with your digital, your FM and your AM radio all switched on (even worse if you listen to internet radio) - they all arrive at different times because of the processing and transmission delays. Some local radio stations used to broadcast TAPES of the pips (really rude!)
You can get a good version of the right time using a radio controlled clock / watch which listens to a time signal from MSF, in the UK or Frankfurt in Germany.

Sat Nav satellites have a particular problem because they work using calculations of times taken by radio signals to get to your receiver. They are moving (at significant speed) and also affected by gravity to varying extents, which, along with their changing positions, means that their time has to be constantly updated. The time you can read off your sat nav will be pretty accurate but afaik it relates to their own time standard 'amongst themselves'. Anyone with a sat nav has access to that time and it's pretty good.
BUT your time is, necessarily, affected by the distance you are away from the reference you use. No need to sweat about it too much - Radio Waves travel at 300,000km every second so, even if your are 1000km away from the transmitter, you're only out by 1/300s.

Databit is right when he says that time (in the sense of ticking clocks) is a totally human construct and only needed so that we can all turn up at the station in time for the 7:50 from Woking.
« Last Edit: 23/07/2009 16:51:45 by sophiecentaur »
 

Offline LeeE

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How do we know what the exact correct time is?
« Reply #4 on: 23/07/2009 16:59:17 »
The exact time is Now.
 

lyner

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How do we know what the exact correct time is?
« Reply #5 on: 23/07/2009 23:55:42 »
Do you mean "now" - when you wrote that or "now" when I am reading it?
 

Offline LeeE

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How do we know what the exact correct time is?
« Reply #6 on: 24/07/2009 00:21:14 »
It's always now.

Sounds a bit twee, I know, but it's an important thing to remember, especially in the context of time-dilation where, regardless of the rate of time/duration for different observers, 'now' appears to be the same for everyone/thing.
 

lyner

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How do we know what the exact correct time is?
« Reply #7 on: 24/07/2009 15:40:44 »
But your consciousness lags by nearly half a second. So the Now you are aware of is already HISTORY.  ;D
 

Ethos

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How do we know what the exact correct time is?
« Reply #8 on: 24/07/2009 16:09:20 »
Time??????????? It's all relative my dear.
 

Offline LeeE

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How do we know what the exact correct time is?
« Reply #9 on: 24/07/2009 16:52:46 »
But your consciousness lags by nearly half a second. So the Now you are aware of is already HISTORY.  ;D

'You have just started reading the sentence that you have just finished reading.'  ;D

Half a second is way too long.  When I was young, (so much younger than today) I had a reaction time of about 1/8th of a second, and even today it's still around 1/4-5th of a second.
 

lyner

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How do we know what the exact correct time is?
« Reply #10 on: 24/07/2009 20:33:15 »
Reaction time is not the same thing. You may well have reacted long before you were aware of it. You know how you made the brilliant kick but you don't necessarily know WHEN you made it.

I have read / listened to a lot of stuff about this and, it seems, our poor old Conscious Mind is not as much 'the boss' as it would like to think. It's more like the Chancellor of the Exchequer, who thinks his decisions made to Economy go that way. In fact, he's (like your Conscious Mind) more like the little chap sitting on the elephant's neck, giving it the occasional nudge. Mostly, the elephant just gets on with it.

To keep our self respect, we believe we made a conscious decision / reaction but it's very often all done and dusted before we know anything about it. Just like any big organisation, we write the history to put us in the right, whenever possible.

SO, I ask again "which particular Now are we discussing?

Don't waste too much time waiting to see when you 'make a decision'. I don't think you can ever know WHEN - just that you HAVE made it.
 

Offline LeeE

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How do we know what the exact correct time is?
« Reply #11 on: 25/07/2009 15:08:48 »
Hmm...  while I'd certainly admit that sometimes I've reacted without thinking about it, there have also been times when I've consciously had to wait for something to happen before I could react, so it can seem to work both ways.  I've no definite conclusions about it.

As to which 'Now' we're discussing, well, there's only one Now; all else is past or future.
 

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How do we know what the exact correct time is?
« Reply #11 on: 25/07/2009 15:08:48 »

 

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