It's About Time.

[peter]

[Sorry, professor Hawking for that outright bit of thievery.]

I have OCD in respect of time so humour me. I correct my watch to GMT every day [well, every four days actually - it gains a second in that period], but it occors to me 'who watches the watchers'. What extra-terrestrial 'fixed point' do we use in order to start and finish a complete revolution of the earth from? This is the basic unit of time is it not, around which the length of seconds [in atomic pulses or whatever] is calibrated.

The most inportant thing [I'd guess] in 'keeping time', is going to be ensuring the seasons stay in place in respect of the dates in any given year. The leap year arrangement thing does this pretty well, with a bit of tweaking here and there, but below this level, we have to make sure that when the 1st Jan 2015 starts, it does so in respect to some exact position in respect of the Eaths relationship to the Sun as it was in on 1st Jan 2014. And then the interim period has to be divided up into it's 365 and whatever days. But is the earths rotation regular to that degree of accuracy. Throughout those days, will everyone of them be exactly equal down to the n'th nanosecond [even allowing for a gradual increase in the revolutionary period of the globe as a result of slowing]. I'm guessing that the 'solstices' would be the best point at which to fix the position of Earth wrt the sun [why then does the 1st Jan not fall on the winter solstice if this represents the greatest angle of asscencion [or declination ?] beween the earth and suns eqitorial plane. and really in the computation of the duration of the second, there's no real need to go beyond this.......is there?

[wayfriend]

Yes, the solstices are the metric by which we base time.

There is also "sidereal time", which is based on the relative positions of fixed stars. However, I don't think we set clocks by that. If we did, it would be just "time", I would think.

[Vraith]

The most inportant thing [I'd guess] in 'keeping time', is going to be ensuring the seasons stay in place in respect of the dates in any given year. The leap year arrangement thing does this pretty well

Well...depends on what you mean by "pretty well."
Cuz the seasons are not fixed according to our position in orbit.
They are due to the orientation of the axis in relation to the sun...and that axis precesses.
If we adjust to match revolutionary position with calendar date, in a few thousands of years the seasons will occur opposite to what they do now. [Jan 1, 15,015 will be summer where it is winter today and vice versa].

I'm pretty sure we DO, nowadays, set the calendar by sidereal time...though perhaps they take all the motions into account, and "round" everything to keep as consistent as possible over human lifetimes.
But they'll have to make some adjustments incrementally if they want to keep the seasons and the "real" passage of a year [one exact orbit] and the calendar roughly aligned.
Cuz there is no way to keep them all.

[peter]

I'm not making my question clear - sorry. The Grenwich Meridian is the 'great [half] circle' that passes from the poles through the Grenwhich Observatory [or whatever] in the UK and one 24 hour day represents a single revolution of the Earth on it's own axis. If we were looking down on the Sun with the Earth spinning around it [and revolving on it's own axis at the same time], would midnight [of say the start of day 1] be the point at which the Grenwhich merridian was absolutely in line with the straight line drawn from the center of the earth to the center of the sun - but on the side of the Earth facing away from the Sun [ie in darkness]. And would 1 day exactly have passed when, on the start of day 2, the Grenwhich merridian was again in that exact straight line position exactly opposit said line above Or is the rotation of the Earth just not that 'regular' and so the days are actually only really ever just close approximations to 'one revolution' but are actually fixed against only two 'earth/sun' relative positions each year [ie the equinoxes]

[I think I've just sussed it out myself a bit; the G Meridian can't be in that straight line position away from the sun every day because there is not a whole number of days in a year, so the leap days have to be put in to bring the GM line back to it's straight opposit the sum position [but wait - I thought that was to keep the earth in the same position on it's orbit round the sun on given times of the year, or is that the same thing? Gosh - I don't know - it's complicated!]

[I'm Murrin]

High noon wanders throughout the year, relative to our clocks. I think I recall it was by 16 minutes either direction?

[peter]

I think I'm going to have to go back to the drawing board on this one; I've got a big book on astronomy somewhere - looks like I'll have to try and dig it out .

[Vraith]

I think I'm going to have to go back to the drawing board on this one; I've got a big book on astronomy somewhere - looks like I'll have to try and dig it out .

Nothing lines up exactly on any axis, and all the axes are shifting in various ways all the time.
Time isn't "set" by any one thing. It is defined by relationships among things.
One "day" is neither exactly one rotation of the Earth, nor the distance along its orbit the Earth has moved in that time. It is a relationship between the two.
One rotation and one day are just slightly different [by just less than a degree, if I've made the right assumptions], so we don't use that. If we did, and didn't define/adjust it, I believe, time would slip around through the year...so that "noon" would be "midnight" 6 months from now. [[I think...I just did a picture in my head, and that's not my strong point]].

[Hashi Lebwohl]

We don't rely on the Meridian any more, at least not for having one set standard against which other clocks are calibrated. However, most scientists and military organizations do run off the U. S. Navy atomic clock found via the USNO/NIST site, presuming they aren't using their own atomic clocks (which were probably originally calibrated via the USNO clock, anyway).

I suspect astronomers measure our year by the relative positions of sufficiently distant stars, ones which are so far away that they may as well be truly fixed points in space.

To the best of my recollection, a year is actually 365.2422 days. This means that years which are divisible by 4 are leap years, years which are divisible by 100 are not but years which are divisible by 500 are leap years and, finally, years which are divisible by 5000 are "extra" leap years (we haven't had one of those yet) which would require 30 February. I doubt our current calendar will be in existence when one of those would occur. Anyway, this explains why 1900 was not a leap year but 2000 was and why 2100 won't be (but they may go ahead and do it, anyway).

Absolute time doesn't matter, either, so we don't really need a fixed point which qualifies as "when such-and-such happens it will be exactly noon". Only relative or elapsed time has much meaning--5 minutes from now, in 3.72 seconds the alarm will ring, or it took 8.4 nanoseconds for the particles to leave their traces in the chamber.

[peter]

Forgot to mention this earlier, but clearly the guys who 'mark-time' at the Grenwich Observatory [or wherever GMT is 'set' from] must use the 'turning of the year' as some kind of point for correction of any discrepancy that has built up over the year. Howso? Well [from above] I keep a pretty accurate track on the time - I mean the correct time - to the second - at all times [I believe that if you are responsible for closing a shop at night you close on time, not earlier, not later] and I have a watch that gains one second per week [ish]. This year, between December 31st and 1st Jan the GMT website 'jumped' by about 3 seconds in relation to my watch pretty much overnight. My watch is still gaining at the rate it always has, ergo the GMT website time was altered. QED.

[Hashi Lebwohl]

Small corrections to the "official" time are often made. In fact, we are due for a leap second this coming June (or is it July?). There is no such thing as "one year" because we base the length of time which we call a year on the planet's rotation and that rotation is not static. The planet speeds up and slows down due to influence from the other planets, the Moon, the Sun, its own inner workings, etc. In fact, the core and the surface don't even rotate at the same rate--if I recall correctly the core spins faster (it is heavier/more dense and smaller thus rotates faster).

[Zarathustra]

I'm not sure what you're asking, Peter.

The earth's rotation isn't constant. The moon's tidal effects create friction that slows it down. This is why we add leap seconds.

[peter]

That makes two of us Z! [Two attempts to explain it later, I'm abandoning the task. King Soloman said that there are four things that are beyond understanding - the way of a bird in the air, the way of a ship on the sea, the way of a serpant on a rock and the way of a man with a maid - I think I've just added a fith in my case!]

[peter]

Time is the sequential experience of moments of 'now'. it really is what stops everything happening at once - and is an illusion that exists only within the cognating human brain. We should look at the past and future as 'out there' in just the same way we look at the distant regions of space as being out there too. In relation to the illusion of the passage of time that we experience, we should rather see our ultimate reality as if the brain had evolved us as a 'viewer' that is leafing through the sequential moments of 'now', and is utterly absorbed in them in the same way that a viewer can become utterly absorbed while wiewing a film, but to the nth degree. As with the film analogy, the frames that have run through the projector, and those still to run are just as existant as the one that is currently on the screen.

Given this fact - and it is a fact according to the multiple tests that Einsteins Theorie's have been put to and sucessfully passed - how can the world be other than deterministic?

[wayfriend]

Well, I don't believe that time is an illusion that only exists within the human brain. Time is real. It's just so fundamental that, like many things that are fundamental, it's hard to truly grasp.

But the past IS gone. The future is NOT YET. You can imagine that they are "out there" ... but they are not. You can imagine a viewpoint that encompasses past, present, and future ... but there isn't one. It's a thought exercise. The notion of time being a path, a line from here to there, is the thing that exists only in the human brain. It's just a way to think about time. But it's not time.

You can't confuse evidence of past events as the actual events. We can see light that was emitted from stars millions of years ago ... but we are not actually seeing into the past, we are only seeing reflections of the past that survived into the present, like old photos.

Meanwhile, water falls, radioactive elements decay, waves propagate, people age, and planets orbit. All the proof you need that time is real. It is rather immune to our investigations into it's nature. But that's our limitation as time-bound beings, and not a quality of Time.

[Fist and Faith]

I'm with wf. Everything does not happen at once. We don't merely view events in a certain order; they actually happen in order.

[Avatar]

Do we differentiate between the measurement of time and its passage? Certainly its measurement is pretty arbitrary and human-constructed.

I've always personally considered time to just be another direction.

--A

[peter]

I'm sorry I can't link it guys, but there is a short you tube film called The illusion of time : past present and future all exist together in which, if I begin to understand it, the physicists talking are stating the case pretty much exactly as I have summarised it apart from the last 'deterministic' bit. The clip gives a comprehensible account of the notion of space-time, together with a fine explanation of its larger implications re what is meant by 'now'. If any of you have the time, please track it down and you will really see where I am coming from.

edit; Managed to get the adress - my android tablet didn't seem to show it but hopefully this will get you to the vid if you want to see it:
https://www.youtube.com/watch?v=vrqmMol0wks
Alas I've just tried the link and it doesn't seem to work (for me at least), but if you Google the title I gave above it should pull up the YouTube video as the first result.

[wayfriend]

It appears therefore more natural to think of physical reality as a four dimensional existence, instead of, as hitherto, the evolution of a three dimensional existence.
-- Einstein

Notice the critical words: think of. It's a model - a useful way to think about it, but only a way to think about it.

[SoulBiter]

Well, I don't believe that time is an illusion that only exists within the human brain. Time is real. It's just so fundamental that, like many things that are fundamental, it's hard to truly grasp.

But the past IS gone. The future is NOT YET. You can imagine that they are "out there" ... but they are not. You can imagine a viewpoint that encompasses past, present, and future ... but there isn't one. It's a thought exercise. The notion of time being a path, a line from here to there, is the thing that exists only in the human brain. It's just a way to think about time. But it's not time.

You can't confuse evidence of past events as the actual events. We can see light that was emitted from stars millions of years ago ... but we are not actually seeing into the past, we are only seeing reflections of the past that survived into the present, like old photos.

Meanwhile, water falls, radioactive elements decay, waves propagate, people age, and planets orbit. All the proof you need that time is real. It is rather immune to our investigations into it's nature. But that's our limitation as time-bound beings, and not a quality of Time.

Agree with WF... its creative to think of Time as malleable in that its not a constant. But unless and until someone can prove differently, time remains that way. Now if someone could go back in time and change time, then you end up with multiple timelines from a human perspective and that opens up a whole can 'o worms in determining past/present/future. But that's all science fiction. But again as WF said, time just is. The past has gone, the present is, and the future isn't here yet.

[Hashi Lebwohl]

Only at the macro level, though. It is possible, mathematically, for objects at the quantum level to travel backwards in time. Given what else we know about quantum mechanics if the probability of an event is not exactly 0 then at some point it does happen. Causality doesn't exist at the quantum level like we experience it here.