Time Moves Forward for Obvious Reason?

bobc2

The idea expressed here is in direct contradiction to established relativity theory. If a given observer, A, changed his past events there would be no possibility of all other observers observing the same event, since those events are observed at different times for different observers (moving at different relativistic speeds) within their differing frames. Special relativity theory requires all observers to agree on the fact of the event. If there is a collision, all observers agree that the collision occured.

If an object is moving in a straight line (as opposed to a curved line) in spacetime, then all observers will agree that the path is a straight line, even though their observations may not be observed at the same times in their own frames.

Past events are never altered.

"...unraveling the log’s past..." What does a burning log have to do with the past? That is perhaps a little poetic, but as James_Harford says, it has nothing to do with physics.

nitsuj

I disagree, "now" has a very very "objective" meaning. The idea echoed in the first quote is this is not absolute; it is relative.

Again I disagree, they (physics) have to rely upon an objective now, this is how it is defined...right....now. It of course is not an absolute "now". distance across space has an invariant isotropic speed limit.

Paul.Dent

I think you are on the right track. Independent estimates of the size and mass of our universe place it entirely within its own Schwartzschild radius, where the radial dimension and time switch roles due to a sign change in the metric. My theory is therefore that we are in a humongous Black Hole, and that the inexorable chute towards the central singulaity is the inexorable passage of time, while the unbounded outside time dimension becomes unbounded space inside. An interetsing consequence is that "where" quesions and "when" questions have to be interchanged. So the answer to "Where is the event horizon?" is "13.5 billion years ago", and the answer to "What came before the Big Bang" is "The Outside".

Thus I equate the instant of the Big Bang with the event horizon, "seen" from the inside. From that instant, everything evolved more or less as we know it.
I have done the math and the interior universe is one that exhibts an initial infinite rate of expansion, falling to 70Km/s/Mpc at the plateau phase, and then accelerating towards the Big Rip, and infinite rate expansion which will occur of the order of 10 billion years hence.

Now, such Robertson-Walker-like metrics are normally associated with a uniform density of matter. This matter density has to be an order greater than the visible matter density to explain the observed expansion rate; hence the Dark Matter hypothesis. But I don't need any Dark Matter. I get the expansion due to the mass of the Black Hole, 90% of which is already at the central singulaity, or at least nearer to it than we are - which means it is displaced in time from us to our future, and that is why we can't interact with it.

Now, the orbits of stars around galaxies follow geodesics of the spacetime, and they in turn are totally determined by the metric. So, they will conform to the geodesics of a universe filled with Dark Matter even though it's not, as the math can't tell the difference. Thus anomalous galactic rotation explained.

Paul.Dent

"The only way that spacetime can evolve on its own is to be part of the physics of a larger 5-dimensional space with a second dimension of time in which it can evolve. That would be bizarre, since it suggests a past that is not static. But since this is not the case, spacetime merely "exists" and does not "evolve". "

I am reminded of Spielberg's SciFi movie "The Langoliers". These were monsters that followed behind us in time to chomp up the spacetime past we left behind us.

Now, with reference to my previous post, where I equate the passage of time with the our inexorable chute along the radial dimension of the interior of a humongous Black Hole, we can place numbers along the radial dimension such as Jan 1 1900, Jan 1 1901, Jan 1 1920 .... Jan 1 2012. Now we ask, if an observer using H.G. Wells' time machine could change his time coodinate from today to Jan 1 1900, what would he see? I say he would not see our universe as it was then, because he did not take our universe back with him. He will see the stuff that is following us 112 years behind in time along the radial dimension. That is a different universe that is now occupying the spacetime coordinates that we once occupied - The Langoliers, no less! So we really do need another dimension along which we can plot the evolution of events in our universe. We occupy a certain 4D region of spacetime at one value of this new axis, and something else occupies the same 4D spacetime at a different value along this axis.


We shouldn't worry about that. Heck, string theorists are into 7 and 11 dimensions and I have also seen suggestions that the number of dimensions must be a large Fermat prime!

James_Harford

But what objectively distinguishes past from future?

- Regards.

jfy4

Hi James,

I have been thinking about our exchange, but I have also been very busy, sorry for a late replay. After considering our topic, I do have to agree with you, but unfortunately (and if you can believe it), not for the reason/definitions you give. First off in this quote
I don't often consider existence and dynamics mutually exclusive things. I tend to think that there are things that exist, and are also dynamic, so I don't interpret that sentence as a point towards the static nature of the universe.

You also seem to be saying that the static nature of our universe is a consequence of no apparent outside time variable, which seems weird to me through and through, as I said last time. So before I say I why I interpret the universe as static, I guess I want you to answer me a question, and perhaps we can talk about this question specifically in another thread; If the universe is static, who is it static with respect to?

My understanding of the universe being static comes from the Hamiltonian formulation of GR. In this, GR is a completely constrained system, that is, there is no dynamics term. The whole hamiltonian vanishes. Each constraint lends itself to a geometric meaning, that describes the shape of spacetime.

Talk to you later,

nitsuj

I don't want to get caught up in semantics.

If instead of the terms "objectively" and "subjectively", I think it would be best to use the words "relative" and "invariant".

If "subjective = relative" & "objective = invariant" then yes I agree with you.

I also feel there is no disputing measurements of time or length with in an FoR, both of which require a "now" (in particular length).

In any case this is silly to interprut such simple comments to such depth, even if they are Einstiens, they are so loosely worded interpretation to this detail is well...subjective :), unlike a ruler or clock.

James_Harford

That is remarkable! Glad to hear from you jfy4.

Then what do you suppose Einstein mean by "exist" vs. "evolve"?

Yes I am, but it seems a tad weirder to postulate an additional outside time variable. Maybe I am mistaken. What function does this variable have?

It's 1+3 dimensional space description is static down to whatever level of detail you wish to take it. It cannot change, except by pencil and eraser, in which case I suppose you could say our physical time is a meta-time of no significance to the physics of that description, or model.

Now that is very interesting! Does H=0 thoroughly weird you as well, then? I wish my physics knowledge was on a par with your own, but it is nowhere near. So, although I very much welcome any conversation in this direction, I am the student and will need to crack open my copy of MTW. (PM me if you think of an appropriate venue.)

It is interesting that you do seem to see this as primarily a GR question.

- Regards.

James_Harford

Then lets don't.

petm1

I am trying, in my own words, please feel free to address them. Please do not scold I may be slow and simple minded but

The temporal image carried within the duration of outgoing photons is the persistence illusion we are all seeing. In my mind the end of the logs duration, as it burned is set, but where is my evidence because now it is as if the log never existed.


Over all I would say matter in the form of atoms, are the most deterministic duration, or maybe dark matter.

James_Harford

No, I was out of line Petm1. Please do accept my apology. I am new here and trying to find the proper "stance".

I realize that you have a very poetic way of seeing the universe --a way which the discipline of physics tends to strip away. Yet the magic is always present, albeit unremarked, and that is what to which I suspect you are especially attuned. For sure, the evocative nature of poetic vision is very heady stuff, and the essence of what makes life worth living.

- Regards.

James_Harford

For anyone reading this thread who hasn't already noticed: The question, "is spacetime static" that this thread morphed into has independently arisen in the "is time mapped out" thread at http://www.physicsforums.com/showthread.php?t=578434.

Perhaps future discussion on this question should be under that thread?

lalbatros

PMichaud,

When your milch dissolves in your cofee, you see something irreversible that shows a preferential direction of time (for all things of these same kind).

I can't see, however, how milch dissolving in cofee can in any way be related to the assymetric initial conditions of the Big Bang.

This does not mean that specific initial conditions could not, indeed, imply some irreversibility in time. There could be several sources of irreversibility, like also at the particle physics level.

However, the most familiar arrow of time, the one related to the second law of thermodynamics can hardly be related to the big bang.

You might be interrested in this book:

The Physical Basis of The Direction of Time, by H. Dieter Zeh.

yuiop

I get the impression from this thread that there is a consensus that objects move through time rather than time moving forward and I can agree with that. I think the point that the OP was getting at is that generally things only move in one direction through the time dimension and this makes time special as objects can generally move in either direction, backwards or forwards, through the spatial dimensions. Now the question is, how do we determine this allowed direction for travel through time and is there any sort of "law" that prohibits travel in the negative time direction? Others have already mentioned the thermodynamic arrow of time. This is a statistical probability that systems evolve from a state of low entropy to a state of high entropy and very rarely in the opposite direction. Bahamagreen gave this example:

The point is that though statistically unlikely, it not impossible for all the molecules to end up back in the bottle. Now if we allow the universe to run long enough it is possible (even highly likely) that we will experience events where a system randomly evolves from a state of high entropy to state of low entropy. When this happens, will we declare that "time has run backwards" or that a system has moved backwards through time? Personally I do not think we can claim that. The thermodynamic arrow of time gives a probability of how systems will evolve but it does not prohibit a temporary reversal of the arrow of time. I think there should be some higher principle or law that clearly states that objects can only move forward through the time dimension even though the equations do not that. Most laws are based on observation and if a huge number of observations support a rule and if that rule has NEVER been observed to have been broken then that rule should have the status of being a physical law. Has anyone ever observed a system going backwards through time? The equations of Special Relativity appear to be symmetrical as far as time is concerned, but it has its own "arrow of time" in that for any two causally connected events, the cause always precedes the effect. This could be one way to define the law of time. If the effect is observed to precede the cause of two causally connected events, then that would be a violation of that law. As far as know that condition has never been violated. I know that sometimes antiparticles are considered as ordinary particles going backwards in time, but equally they can be be considered as antiparticles going forward in time and so long as we accept the existence of anti-particles there is no violation of the order of cause and effect. Tachyons would also violate the law of cause and effect but as far as I know they have never been observed. Anyway, the point is that the thermodynamic arrow of time allows the occasional reversal of a system from high entropy to low entropy but in my opinion that does not amount to a reversal of time and there should be (IMHO) a higher law that is much stronger and prohibits the reversal of time. This agrees with everything we have actually observed, but perhaps people are afraid to reach that conclusion for romantic reasons such as closing the door on the possibility of time travel.