# Physics motivated POVs in the time problem

1. Jan 18, 2008

### jostpuur

The problem:

Which one of the presentism and eternalism is the correct picture of the nature of time?

Simple Newtonian systems:

It doesn't really matter. They are mathematically equivalent, the problem is destined to remain purely philosophical, and there is no way of figuring out the truth, if it exists.

Relativistic systems:

The presentism becomes problematic, becomes simultaneity isn't absolute anymore. If the presentism was true, we would need to have some inertial frame to be the correct rest frame. The relativity seems to suggest, that the eternalism is the correct answer.

Statistical physics:

The eternalism becomes problematic. The laws of physics are invariant in time reversal, so there doesn't seem to be any obvious reason for the entropy to increase in some particular direction in an eternal space time. So the statistical physics suggests, that presentism is the correct answer.

What I think is mostly puzzling, is that there are physical arguments to the both directions. Is the correct answer, that both of these alternatives is incorrect? What other alternatives are there?

Last edited: Jan 18, 2008
2. Jan 19, 2008

### Quatl

These propositions are functionally identical.
This to me seems to hold up regardless....

Relativity most certainly does not get rid of simultaneity. What relativity actually says is that the rate of recurrence of events (what we use to measure time) is related to the relative difference in acceleration between ourselves and the clock.

That is to say: As long as I observe you to be accelerating relative to myself, when I look at your clock with one eye through my telescope, while looking at my own identical clock through the other eye, your clock will be counting at a different rate than mine.

To clarify my point; in the same moment (that is simultaneously) our clocks show a different count of how much time has passed. It is still possible to create a chart of sequential moments that lists each tick of both of our clocks (it doesn't even mater if we use your clock ticks or mine as the basis for this chart, the order of the events will still be the same.) A Sequence is not always the same idea as "time."

The two options are still identical in terms of consequences. As the deep past and future do not add forces to the situation. The equations themselves are built to deal with instants. As an aside, the current descriptions of our universe do not include an "eternal space time." Big bang theory places bounds on what we normally speak of as the universe.

The statement that entropy always increases with the flow of time is an extrapolation of the equations as related to the situation in our universe. Locally speaking, input energy is required to maintain order. The universe we are observing is in transition, thus experiencing thermodynamic effects (that is we see "change.") Along these lines picture an experiment where there is no change in what you are observing over some interval of "time." From your perspective you can still create a chart of the lack of change (as in: Time=0 X=5;@Time=1 X=5;Time=2 X=5;@Time=x... X=5) That is, the concept "time" is not identical with the concept "sequentiality." Folks often lose this in their logic somewhere.

Another response to this sort of statement, is that there is a sequence here that is not "time symmetric", but the laws in operation can still have this quality. In fact most "interesting" sequences will not be time symmetric (that's pretty much the definition of boring :) .)

I doubt you'll find an emotionally satisfying answer to this.

As for me, "If there are no differences in consequences between two propositions, then they are irrelevant to the subject at hand," works for me. Now if you can find such differences then you can attempt to measure them, and as a result answer your question. (From an epistemological perspective, if there is no change in results between two descriptions, then the descriptions can not be meaningfully differentiated. That is to say they are identical.)

3. Jan 19, 2008

### jostpuur

I didn't say that there would not be simultaneity in relativity, but instead that simultaneity isn't absolute. It becomes relative. I assume you missed this detail, because...

you know something about time dilation, but have messed acceleration into it, and...

you do not know relativity very well, yet at least. Do you know, that events that are simultaneous in some inertial frame, are not simultaneous in other inertial frames? It does mess the presentism view pretty badly.

4. Jan 19, 2008

### jostpuur

The increase of entropy isn't necessarily in contradiction with the eternalism view, because it could be, that for the God, our Universe was not a boundary value problem at the beginning and at the end, but an initial value problem instead. This is loosely speaking, of course But I wouldn't say that this thought solves the whole problem. If the eternalism view is correct, it is still difficult to understand, what precisely are the principles, according to which the world chooses the entropy to increase in some direction. After all, the Universe probably has not been a PDE problem with some boundary conditions for the God, at least.

5. Jan 19, 2008

### Quatl

Please elaborate as I think I'm misunderstanding you here. An example of a scenario which demonstrates relative simultaneity vs the regular kind would help.

I have "messed acceleration into it" because a difference in acceleration between observers is one of the main situations in which relativity describes time dilation. I could have as easily used gravitational fields of varying strengths between them.

It is very possible that I don't. I think I have a better handle on the subject that the average, but then relativity is often a subject muddled by various folks, I'd appreciate your assistance if you think I'm mislead on the topic. Please enlighten me as to the relevant point(s) you think I'm missing. (I also have a habit of skipping steps in explanations that I think are obvious, so it's possible that that's the problem too.)

6. Jan 19, 2008

### jostpuur

lesson in relativity

Suppose you are doing something on the earth now, and we call it an event A, and at the same time at some space shuttle, that is moving with some great velocity, your friend is doing something else, and we call it an event B. It makes sense for us to say that the events A and B occur at the same time, but this is only our point of view. From your friends' point of view, the event A and B occur at different times. So there is no absolute meaning for a claim "A and B occur at the same time". The claim can only be true in some particular inertial frame.

It is true that acceleration will cause lot of things, but for time dilation, a constant velocity is sufficient.

7. Jan 20, 2008

### Quatl

Maybe this example will illustrate what I mean.

We have 2 clocks, #1 and #2. We synchronize them while they are in the same reference frame.

A ship departs from a deep space station with clock #1. It's mission is to accelerate to 50% of light speed, and then fire a laser in a predetermined direction relative to it's direction of movement. Assume for argument's sake that the beam is emitted at the exact moment that the ship believes it is traveling at 50% of c. (all speeds from this ships perspective, measuring when needed relative to the station. All times are readings from clock 1, on the ship.)

A second ship is sent out, with clock #2. It's mission is to intercept the laser's beam, using relativity theory to determine a place and time. That is ship 2 must figure out a course that will place it at a a point in space and "time" which intersects the beam that ship two plans to emit.

It is either possible or impossible for ship 2 to complete it's mission. If you think it is possible then read what I said the first time again. If you think it's not possible then take another look at relativity theory.

8. Jan 21, 2008

### moving finger

There is an obvious reason for a temporal entropy gradient in any universe which supports complex sentient beings (such as ourselves) - we exist only BECAUSE the temporal entropy gradient exists, and our psychological "arrow of time" is automatically aligned with the entropy "arrow of time". Complex sentient beings could not exist (naturally) in a universe where there is no entropy gradient.

The question I think you seek an answer to is not "why is entropy increasing in a particular direction?", but rather "why is there any temporal entropy gradient in the first place?”.

The answer to this lies in the sudden “switching on” of gravitational attraction shortly after the inflationary expansion which followed the Big Bang. In absence of gravity, the entropy of the universe would have been at a maximum immediately following the Big Bang, and would have remained so ever since. The sudden presence of gravitational attraction, however, in a universe where there was no gravitational attraction at the outset, creates the conditions for a temporal entropy gradient.

Whether presentism or eternalism is the “correct” view has no bearing on this.

9. Jan 21, 2008

### jostpuur

I don't know enough cosmology to say anything to the details of this claim, but I have one response anyway. I understand, that if the entropy had been already in the maximum right after Big Bang, then entropy could not have started increasing anymore, but I still don't understand why the entropy is increasing now.

10. Jan 21, 2008

### Quatl

From a cosmological stand point entropy is increasing still because thermodynamic activity hasn't finished it's work. Remember that order in a thermodynamic sense more or less means "lumpiness." Maintaining local lumpiness requires energy to be added to the system from a non-local source. (note that for a given scenario, the maximum possible entropy state may still contain some lumpiness)

The expanding universe, along with gravity present an energy gradient, which thermodynamics' constituent laws can work on. Keep in mind that the statement "systems seek a state of maximum entropy" is a metaphor, or summery of the action of various physical laws. Entropy seeking is not a "force", it is a handy concept though.

The argument Moving Finger offers here is referred to as "The Anthropic Principle" (TAP.) It's not so much an explanation as a statement that imposes limits on the types of universes that can be observed by native intelligences (obviously only those universes which have native intelligences.)

Personally I find the response "we don't yet know why the initial conditions of our universe were as they were" to be more satisfying. But TAP is often invoked by cosmologists, when their favored theory is challenged on similar grounds. String theory for example can describe a great many very different universes, and you hear TAP often reading in that area.

11. Jan 22, 2008

### moving finger

Whether entropy is at a maximum or not for any given configuration of the universe depends on whether we have universal gravitational attraction (UGA, as we have today), or universal gravitational repulsion (UGR, my hypothesis for conditions during the initial inflationary epoch just after the Big Bang). UGR would produce a universe with matter uniformly distributed - this would be the "high entropy" state under such conditions. If UGR continued then the universe would have continued in this maximal entropy, uniform state.

However, if UGR suddenly switches off, and UGA switches on (my hypothesis for sometime near the end of the inflationary period) then the uniform-matter state of the universe is no longer the highest entropy state (in presence of UGA) - it is in fact a low-entropy state, and henceforth the universe moves from this (uniform-matter) low entropy state to a more clumpy higher-entropy state.

The key is: Whether entropy is at a maximum or not for any given configuration of the universe depends on whether you have UGA or UGR. Switch from one to the other and a high entropy state can suddenly become a low entropy state.

12. Jan 22, 2008

### gullapalli

your Idea about the grational repulsion is correct. But the repulsion does not disappear suddenly. This theory is already proposed by Stephen Hawkings. I will post it later.... I am in a little busy now !