Question

Discussion on Time as a Concept/Theory

Answer 1

Basically trying to capture the discussion that's been going on in chat regarding Time. I'm totally behind and just catching up, but I love hearing about this sort of stuff.

(BTW I *know* it's not a question, but hey, I get to bend the rules *sometimes* right?)

Answer 2

We're kind of jumping in halfway through the discussion, so converting to this format might be a bit messy, but here is my assertion summarized.

Time is a dimension of reality, along with the three spatial dimensions. We can define forwards in time as the direction in which total entropy increases. Correspondingly, then, backwards in time is the direction in which total entropy decreases. This produces a coherent system and axiomatization of the notion of time.

Answer 3

The real question that there actually is concerns consciousness moreso than time itself. We recognize that we are consciousness put into a particular narrative, that combination of space and time that we view as our reality. It is arguable that consciousness could exist independent of that narrative, but then you're talking more about philosophy than you are about physics.

Answer 4

The debate began here:

nbouscal: color is just a human construct
agentx5: I agree, nbouscal
nbouscal: because we just happen to be able to visually interpret a particular slice of the electromagnetic spectrum
agentx5: As is time itself


Alternate point of view:

Time is an abstract concept, non-real. It is a measured value, it's an extrinsic property, sure I can agree with that. But to say time is concrete is something I strong disagree with.

Why? Goes back to the definition of the word first:
1: naming a real thing or class of things
2: formed by coalition of particles into one solid mass
3: or...
a): characterized by or belonging to immediate experience of actual things or events
b): specific, particular <i.e.: a concrete proposal>
c): real, tangible <i.e.: concrete evidence>


What is not concrete, is abstract is it not?


"Backwards in time := Direction in which entropy decreases." -- my distinguished colleague, @nbouscal's , in describing his point of view
This is a concept I disagree with. Entropy is defined by the Gibbs Free Energy Equation as:
\(\Delta G = \Delta H - T \Delta S_{intial} \)
where
\(\Delta G < 0 \), "Spontaneous"
\(\Delta G = 0 \), "Equilibrium"
\(\Delta G > 0 \), "Nonspontaneous"
\(\Delta H =\) change in heat energy (in Joules, BTU, etc.)

In layman's terms I could said -\(\Delta\)H means a release of heat from the system/reaction/etc out into the environment (heat release by a fire, steam condensing into water, etc.), +\(\Delta\)H is heat energy taken out of the environment by the system (energy put into boiling water to steam, chemical coldpacks that you administer as First Aid to reduce swelling). T is temperature (average thermal kinetic energy) and \(\Delta\)S is what is known as entropy.

Other ways of writing the metric standard for energy (the Joule), in US units it's ft-lbs or BTU:
\(\rm 1 \ J = \rm 1\ \frac{kg \cdot m^2}{s^2} = 1\ N \cdot m = \rm 1\ Pa \cdot m^3={}\rm 1\ W \cdot s \)



How is time measured?

The base unit of time (in all systems we use) is the second. The official defintion since 1967, the second has been defined to be:
"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 (\(\large {}^{133}_{55}Cs\)) atom."

It's measured as an abstract unit of change. Change itself is what's being measured, not time.



Finally I'd point to why "time travel" is bogus:
* Special Relativity shows us that time should slow for the traveler as they travel at a faster speed (magnitude of velocity), and indeed this had held true in experiments so far.
* Distance = | Displacement | or Distance = \(\large \sum_{1}^{n}\) | Displacement\(_n\) |

In the same way that distances can't be negative, neither can time. You cannot therefore say that just because relative entropy is negative that this represents a change back in time! In fact I can show that with a simple equation:
2n CO\(_2\) + 4n H\(_2\)O + photons → 2(CH\(_2\)O)n + 2n O\(_2\) + 2n H\(_2\)O
This is better known as the general reaction equation for photosynthesis, which is endothermic. There is no space time distortion or reversal going on where leaves are.


Metaphor:
"Burn a forest to ash and it can be regrown, but it will never be like it was before"
Even if you CAN reverse everything exactly, it still doesn't change the fact that change occurred. Change occurred and TIME is said to have passed. There are in fact many instances in nature that would make the case that time moves in spiraling cycles, a combination of both linear and cyclical behaviors. This would agree with the notion of time having an "arrow", a direction if you will, but it would not PROVE time as concrete as my college claims.

Answer 5

Thank you for making this topic/question @cshalvey :-) It should prove to be an interesting discussion, as long as it's taken strictly as a matter of logic and philosophy. I would expect that some tempers might flare in this, as this is one of the harder things to explain about the world we perceive as human beings.

Answer 6

@agentx5 Completely agree. Also, in situations where there isn't a universally agreed upon answer, opinions, emotions, and personal philosophy are bound to get involved.

For example, I don't see people having these arguments about the conversion equation between Fahrenheit and Celsius ;)

Answer 7

Your definition of second is not an abstract measurement of change, it's a duration. Duration only makes sense within the context of time as a dimension. Duration is a norm defined on that dimension, and the direction is provided by increase in entropy. You can't say that time is an abstract concept any more than you can say that space is an abstract concept.

Throughout the discussion you've been talking about reversals, about negative distances, and about a bunch of other stuff that is completely unrelated to anything I've said. I'm not talking about traveling backwards in time. I'm not talking about reversing time. I'm talking about time as a dimensional basis for reality, and defining that basis in terms of entropy. To draw a direct analogy: we can define the x-axis of a coordinate graph in terms of a unit vector. We can then talk about the negative unit vector, which goes in the opposite direction. Despite that definition, we still have the case where all norms are positive. The same thing applies to time. We can talk about what it means to go in the opposite direction of the basis vector for time without needing negative norms.

Answer 8

"Let us draw an arrow arbitrarily. If as we follow the arrow we find more and more of the random element in the state of the world, then the arrow is pointing towards the future; if the random element decreases the arrow points towards the past. That is the only distinction known to physics. This follows at once if our fundamental contention is admitted that the introduction of randomness is the only thing which cannot be undone. I shall use the phrase ‘time’s arrow’ to express this one-way property of time which has no analogue in space." - Arthur Eddington

Answer 9

I attest time cannot be drawn as a vector. The "arrow of time" Eddington says here is an abstraction.

First let's go back to how we define vectors in space in the first place with a little airplane :-)


Now if the airplane flies to one airport an back it's displacement is zero. Using what I said previously:

100 km heading forward to + 100 km heading back from
if we use sign conventions letting +x be going to, and -x being heading the other way.
100 km - 100km
for a net change (displacement)
0 km

But distance?
| 100 km | + | -100 km | = 100 km + 100 km = 0 km

Question, if this was in distance per time how would it change? See what I mean?







Here's another example, a pendulum's period varies only base on the force of gravity and the length of the rotating arm.


It is said to sweep out equal arc lengths in the same time. Think if a metronome if you need a more concrete example.
\(\large T \approx 2\pi \sqrt{\frac{L}{g}}\)
T = period in units of time
L = length of the rod/arm
g = local acceleration due to gravity

Now, question. According to my interpretation I proposed, are we not REALLY just measuring the effect for the change to repeat itself? This is very hard to put in words, I don't have anything in the English language I can do to properly describe this. But when we said period we're really measuring the effect of gravity to make a sequence of event repeat in a cycle, over and over.