A layman's view of time.

One layman's view of time.

I would like to say a few words about basic, fundamental , time, mostly because when I look for a formal definition of time, all I can find is nothing more than references to various periodic motions which can be used to measure time, and the practical benefits of everybody referencing the same time system, rather than their ad hocing it locally. I feel a little more needs to be said about time than just that.

It appears to me that when we measure time all we are doing is comparing the progress of one motion with another reference motion (or identical copies of it). We usually pick a periodic motion, of some sort, for the reference motion because it automatically sets up its own units for counting, but I think most of us tend to forget what it is we are counting, namely the total number of units of periodic motion the periodic motioner is doing compared to the other motion (activity) we are evaluating. I feel this forgetfulness tends to give time the apparent abstract character of an independent existence which would remain in space after all matter were, somehow, removed.

Also, some people postulate the possibility of traveling in time (not just figuratively). We can initiate travel in space, but since time is, apparently, just measured amounts of motion, and motion maintains itself (does its own thing), it seems to me that all we can do about
motion (time) is just stand by and watch, and also try to redirect some of this self asserting motion towards our own beneficial uses.

Having described what I think time is I would like to mention a couple of complications we run into when logging the times of events. First of all, if observers are scattered around at many different locations they will record different views of the environment for any given point in their common time. If they are far enough apart for the delay in the arrival of light to be significant in there counting of time they will also record slightly different starting times for any events in their ranges of view.

What about the effect of the propagation delay of light on the appearance of any simultaneity of events? It is obvious that all observers anywhere on a line equidistant from two events will all agree about the apparent relative simultaneity of those two events.

On the other hand, all those observers who are not on a line equidistant from the two considered events will disagree with all those observers who are on the equidistant line, about any appearance of simultaneity. The same goes for any observers who move off the equidistant line during the time of light travel from the two events. (This latter theoretical situation has been erroneously used by some authors as a basis to claim that a different time frame necessarily exists for moving observers, or bodies. A different time frame does necessarily exist for moving bodies, but not because of any altered evaluation of simultaneity, because this also can happen with non moving bodies. Rather is the fact of a different time frame for moving bodies on account of some basic dynamics of physics uncovered by Maxwell and Lorentz, and verified by others.)

All the above observers can, of course, reconcile their disagreements by calculating out the delays in the arrival time of light to their locations from the particular events, and arrive at a common "real time" log of the events, providing nobody is moving fast enough to cause the Special Theory of Relativity to become a significant factor. If the Special Theory of relativity becomes a factor then alternate time (rate) frames are set up which can also be calculated out using the Lorentz Transformation, but it is currently considered to be against the rules to call either one of the time frames the only real one. They are both said to be real. However, since most of the matter in the universe, at least "close by", is in the same time frame, I see no reason why we couldn't refer to a "Common Relative Time" and an "Extrinsic Relative Time", the latter phrase being applicable to more than one possible time frame of course. In this way fictitious (or real) super speed space travelers could be
hooked into a common time system, by using the Lorentz transformation, and agreeing to accept "Common Relative Time" as the basic reference time.

That's all the head scratching regarding a description of time I can do right now, but I feel this description is more adequate than I have been able to find in the literature on the subject.

Any criticism or corrections will be welcome.

 

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Welcome to Sciforums, Fairfield. May your posts be long and varied!

I think that prehaps I do not have the fundamental grasp that I need to answer this topic but I will make a few comments. If necessary I will go research some on this at a later date. It seems to be a reoccuring topic in some form or fashion in most message boards pretaining to science.

The study of time for a large part seems to be thought experiment material. Prehaps it is that we do not fully understand enough to set up the necessary conditions to test it. I have made the observation before that time is the method by which we measure our lives.

It has been likened by some to be like a film with one frame following the next in logical order. Within each a segment of the total continous run. The reference point seems to be the observer. Not that time wouldn't pass on if you where not here but that no observation and therefore no reference motion would be noted without the observer.

I have my doubts that we will travel in time other than the way we do now. If there is one thing that we have noted through the path of humankind, it is that the world around us follows rules. To go back in time would allow us to break the rules so I suspect there will be some preventive barrier that will not allow us to do so.

I have enjoyed reading your specutlations on the subject.

 

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Interesting posts...

Hi Fairfield,

There are various ways of measuring time, and what is probably the most acurate earth-time-measurement system is the atomic clock. These do not directly count periodic "motions" in the classical sense, but use the the possible electron excitation levels of elements (most commonly 133-Cesium) to determine the "countrate".

However, this does not disprove your point that the all-day measurement of time in seconds distracts from the more abstract definition of time

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Loads of facts, and loads of questions. I hope I can answer some

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First of all, the special theory of relativity is required in this discussion, and cannot be ruled out because the observers do not travel at high speeds. You already mentioned one factor that requires a relativistic treatment: light! The appearant discrepancy between the simultanity of events you mention, is exactly the problem that special relativity solves in Newtonian mechanics.

As you mentioned, intertial Lorentz frames are always "real" (or to put it in another way, an observer in a Lorentz frame is always "right" about his measurement of an event). There are some very nice mind-boggeling paradoxes about simultanity of events, and as an illustration of how Lorentz frames are always"right", I'll mention the one I found the most incomprehensible of all here: (it also uses Lorentz contraction - for the people that don't have experience in relativity, this is the effect where a moving object appears to be smaller than it really is).

Suppose you have two knights with a sword who are about to engage in a fight. There's a referee who stands exactly in the middle of the arena, and both knights start running with their swords forward to eachother. The question on the everybody's lips is: who wins the sword fight ? The answer is that they both die, and both lose.

Answer: Knight 1 assumes that his opponent's sword will shorten due to Lorent contraction, and that his own sword (that doesn't Lorentz contract because it's at rest in his frame of reference) will kill his opponent before the opponent's shortened sword reaches him. Conclusion: Knight 1 decides that Knight 2 would die first.
However, Knight 2 makes the same reasoning, so for him, Knight 1 dies first. And for the sake of complexity: for the referee, both their swords are Lorentz-contracted, so they both die at the same time. The tricky part is that they are all right according to the theory of special relativity

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Anyway, I mentioned this example here (it's in every textbook on special relativity) to illustrate that there indeed is no prefered frame to measure events in.

Aaaah... the complexity of time (Crisp wandering off in his own little world of thoughts...

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Bye!

Crisp

 

Very illuminating (eh, eh) Crisp! Could you prehaps take a bit more of your time

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to go into intertial Lorentz frames. I see your point of concept in that each observer will see his world independant of the other. I guess what I am asking is what makes this so?

 

Inertial frames

Hi wet1,

Well, there's not really much to say about inertial frames. You have the formal definition, going something like:

"An inertial frame is a frame of reference that moves at a constant velocity (with respect to the observer)".

And then you have the Lorentz transformation, which relates physical quantities between inertial frames (e.g. time, space, energy, momentum, ... ). A common representation of the Lorentz transformations is the so-called "boost transformation", which relates quantities as follows:

gamma = 1 / sqrt( 1 - (v/c)^2 )
t' = gamma*(t - x * v / c^2)
x' = gamma*(x - v*t)

Here x and t are the position/time measured in the inertial frame of observer2, who moves with a velocity v along the x-axis with respect to observer1. The quantities x' and t' are the position/time measured by observer1.

A special feature of Lorentz transformations is that they take into account effects as time dilatation and length contraction, so this makes them the transformation tool for relativity.

There are several other kinds of transformations that relate quantities between inertial frames, the best known probably being the Galilei transformations, known for their use in Newtonian mechanics:

x' = x - v*t
t = t'

These do not take into account relativistic effects and are therefor not suited for relativity.

I hope this more or less clarifies what them Lorentz transformations are

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Bye!

Crisp

 

That's what I was after! Thanx Crisp.

 

Hey wet1 ...

Just in case you're not quite clear on the Galilei transformations Crisp referred to, this might help:

Once upon a time up was up, down was down, so the symmetry group of the world was E(2), the Euclidean group in two dimensions. This is a 3-dimensional group since it is generated by: translations in the x direction translations in the y direction rotations in xy plane.

Then someone shook up the world by pointing out that it has as symmetries the group E(3), since up and down are in fact merely conventional concepts and one man's up is another woman's down. This bigger symmetry group include rotations that mix up and down! It is 6-dimensional since in addition to the above it includes translations in the z direction rotations in the xz plane rotations in the yz plane.

The laws of physics were thought to be symmetric under E(3). In classical physics (to be precise, "Hamiltonian mechanics" or "Lagrangian mechanics"), to each symmetry which commutes with time evolution one associates a conserved quantity. This is the brilliant Noether's theorem. To the translational symmetries we get conservation of MOMENTUM in the x, y, and z directions, while from the rotational symmetries we get ANGULAR MOMENTUM in the xy, xz, and yz planes.

The laws of physics are also invariant under time evolution itself. Time evolution is a symmetry of spacetime given by

t -> t + c x -> x y -> y z -> z

and the conserved quantity correspoding to time translation is ENERGY. In short, energy tells you how fast things are wiggling around as time passes! The group consisting of E(3) plus time evolution (i.e. the direct sum E(3)+R) doesn't have any common name, but it's the simplest group of spacetime symmetries. Simplest in the sense of most naive, that is.

Galileo pointed out that the laws of physics are the same in a boat moving on a constant speed (on a calm sea!). This gives rise to the notion of Galileo transformations

t -> t x -> x + vt y -> y z -> z

(and similarly for y and z) which express how to transform coordinates into a frame of reference moving at speed z in the x direction. The Galilei group is a group containing E(3)+R but also Galilei transformation in the x direction Galilei transformation in the y direction Galilei transformation in the z direction.

Thus this group, sometimes called G (a fancy script G, please!), is 10-dimensional. This is the group of symmetries of classical mechanics.

(original version posted to sci.physics by John Baez, Aug. 13, 1992)

Chagur

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Ok...

Hi Chagur,

Yep. I agree

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Bye!

Crisp

 

After a period of time the concept of time travel and 4th dimensions will become obsolete and a science fantasy of naïve scientist of past.