Arrow of Time? I don't understand.

[ThomasT]

There's no dispute, afaik, about the existence of an arrow of time. You might think of the nature of time as changes in the configuration of a group of objects. Suppose you take a bunch of snapshots of a group of objects (eg., a basketball game) and number them sequentially.

As you review your snapshots after the game you notice that they're all different.
This individual uniqueness and the fact that each snapshot more closely resembles the ones that immediately precede and follow it in the sequence than any others suggests an irreversibility wrt the evolution of the game, and this apparent irreversibility is called the arrow of time.

A clearer way to see the arrow of time is via the evolution of an expanding wavefront. Waves in any medium always proceed omnidirectionally (more or less) away from a point (or points) of disturbance. Just drop a pebble or whatever into some calm water. Or drop a hula hoop flatly into some calm water and you'll see waves traveling both inward and outward away from the disturbance. This is called the radiative arrow of time, and it's also apparent in the motion of very large scale cosmological structures.

The inferred isotropic expansion of the universe is called the cosmological arrow of time and it carries with it a number of auxilliary inferences -- one of which is that the universe was denser and hotter at an earlier time and as it expands it gets less dense and cooler. No end and no reversal of this trend is in sight and this sort of dispersal and dissipation is called the thermodynamic arrow of time as any system tends to evolve toward equilibrium.

Although there's little doubt that there is an arrow of time, its deep cause is a matter of speculation.

 

[john 8]

You are using symbols for your arguments rather than arguments themselves. Because of your enlightening symbols, I conclude your arguments don't exist, but are only meaningless symbols. They're not real.

The symbols that I am using represent real ideas. Ideas are real things. Your logic is completely faulty.

If my arguments do not exist then how do you know of them? The symbols that I am using are so completely meaningful and understandable that you can know what I am communicating and can respond with similar symbols to communicate your ideas.

You are being foolish.

The symbols that I use are so real they used by every English speaking person on this planet. These symbols represent ideas and concepts and are universally used by man to convey ideas and concepts.

So let's try this one more time. The symbol "t" that is used to represent time in math is representing what real thing in the physical universe? Does the letter "t" as it is used represent an idea or a physical thing? It is as simple as that. Make up your mind and let me know.

 

[atyy]

So let's try this one more time. The symbol "t" that is used to represent time in math is representing what real thing in the physical universe? Does the letter "t" as it is used represent an idea or a physical thing? It is as simple as that. Make up your mind and let me know.

This guy defines time as "The timing pulses in question can be thought of as places in the transmitted wave trains where there is a particular phase reversal of the circularly polarized electromagnetic signals. At such places the electromagnetic field tensor passes through zero and therefore provides relatively moving observers with sequences of events that they can agree on, at least in principle." http://relativity.livingreviews.org/Articles/lrr-2003-1/

 

[cesiumfrog]

Alright, tell me the meaning of physical that is used by physicists.

Consider four symbols: spacetime, molecule, electron, vacuum. To us each represents a specific shared idea, and all are equally real because none are unnecessary to the "best" mathematica/physical models that fit/predict our experience of the universe. (But by your definition, only one of the three is real or physical, since neither electrons, nor a volume that lacks atoms, nor spacetime itself are made out of atoms).

Your argument is semantic or philosophical; there is certainly no physical basis for saying that an atom (which you cannot directly see) is more real than space or time (which everyone experiences). It just has more weight. At any rate, it's off-topic to the the thread you've zombified.

 

[ThomasT]

There's no 'arrow of time'. There's no reversal of time. Time is what we use to measure one form of motion (change) against a yardstick of motion/change (a clock).

Time is motion is configurational change. You say that there's no reversal of time. Isn't that sort of the same as saying that there is an arrow of time?

That there is an arrow of time (a preferred or common 'pattern' wrt the evolution of natural processes) is clearly seen in radiative processes such as the evolution of water waves expanding out from a point source. Insofar as a water wave evolves (disperses and dissipates) toward equilibrium, this wave mechanical process is clearly connected to the thermodynamic arrow of time. (The statistical thermodynamic analog of the wave mechanical arrow of time is a simplification, for calculational convenience, of what are actually wave mechanical processes in nature.)


That there is an arrow of time wrt every physical process that can be produced or can be observed (ie., the idea that nature/reality is an irreversible process) is one of the most important scientific inferences of all time. On a universal scale it's supported by observations (beginning with Hubble's observations) that the distance between large scale cosmological structures is increasing. This isn't just a little bit important, it's right up there with the most important discoveries of modern science.

Do you think time is a physical thing or a concept?

It's both. The concept is that time is configurational change. The physical things are the objects of some (more or less arbitrary) grouping whose relative positions are observed to change. You might quantitatively track these changes by comparing the indexed configurations of one group of objects (usually some sort of periodic process like the vibrations of a quartz crystal which are enumerated wrt regular intervals -- eg., increment an integer accumulator when, say, 1000 oscillations have been counted) with configurations of one or more other groups of objects.

The symbol "t" that is used to represent time in math is representing what real thing in the physical universe? Does the letter "t" as it is used represent an idea or a physical thing? It is as simple as that.

The hardware and indexing protocals used to time things has been more or less standardized for quite a while. So, if you're a physicist and you want to describe a process as a function of time, then what does the 't' refer to?

Readouts on a clock that are associated with events in the process?