[Astronomiya]

One of the biggest mysteries left to man.

What is it?

Physics should be time symmetric. Relativity implies all of the universe should exist as one enormous block, past present and future already fixed and complete.

Why do we experience it as moving down a path with a 'now'? Why is there an arrow to causality?

For a while I pondered the possibility that there's simply a 'now', a stage, no past or future to reach - just one instant in which the universe exists, coupled to the past and future only inasmuch as the laws of physics dictate how the layout of the stage will change. But relativity requires time to be accessible in a cross-sliced fashion.

This question is critical because it ties into the mystery of consciousness. We do not yet understand why we experience experience the way we do, moment to moment. If we were able to resolve the arrow of time and understand why the now is the now, we might have a better idea. If the mind is a physical system, and the mind experiences time, there must be a physical reason for time.

Ideas?

First off, why must physics be time-symmetric, as redsniper noted?  I mean, for the most part it is, sure, but there is no objective reason why this should be the case.  There are also at least two cases of physics not being time symmetric at all:  thermodynamics and quantum mechanics.  Entropy is an obvious one, but QM also is not time-symmetric.  Rather, it follows CPT symmetry (if you simultaneously reverse time, charge, and parity, you end up where you were).  A less obvious example of time assymmetry is GR.  If you time-reverse a black hole, you get a white hole, not another black hole.

Second, I'm not sure what you mean here by "relativity requires time to be accessible in a cross-sliced fashion."  It also, to my mind, does not imply that past present and future are fixed and complete; what in the equations leads you to believe this to be so?

Third, why must there be a physical reason for time itself?  Why not simply the perception of time?  Having the latter is an obvious benefit to the organism.  I would argue that the mind is a physical system, and it gives us the experience of time because it was evolutionarily advantageous to do so.  It chooses irreversible processes for what are again seemingly obvious reasons (this is where it gets a bit circular).

Just an aside, doesn't relativity also state that there also no real such thing as "now"?  (Relativity of simultaneity)

No, it states that there is no concept of a universal now.  If there was no now at all, you couldn't talk about the time coordinate having any value in any frame whatever.  Rather, what is happening "now" is frame-dependent, but "now" is not an obsolete concept.

Hmmm, can the anthropic principle be called upon here?  Is it possible for life to exist in a universe where time flows backwards or has no preferential direction at all?

...is it even possible for such a universe to exist?!

¯\(°_o)/¯

What would "time running backwards" be?  If you flip time around, in QM at least you flip the antimatter and matter states, so there's nothing necessarily special there.  You don't really run into problems in GR, either (black holes become white holes, etc.).  The Friedman equations look weird, but if the universe is either inhomogenous or non-isotropic, they don't apply anyway.  Time having no particular direction is also possible (I think), though it involves changing how entropy works (e.g., ALL processes are now reversible, perpetual motion machines are possible, etc.), and all the wonderfully weird stuff you get out of GR if you invent negative matter and such would probably be possible as well.  Well, QM would also be radically different, but I still think such a universe is at least conceptually possible.

[Herra Tohtori]

If the future was set in stone, then a lot of the probabilistic properties of quantum physics would not work at all.

As a good basic example - if we take the double-slit diffraction experiment and run it in reverse time, we would see photons being emitted by the shader towards the double slit, go through it, and converge into the original laser emitter.

This would, of course, never be predicted by quantum physics. If you plug variables with appropriate uncertainties into Schrödinger equation, it outputs wave function which is always probabilistic in nature, it won't predict all the photons that go through the double slit to converge in one spot. Hell, it won't even be able to say where one photon came from (without knowing beforehand that it came from the laser emitter). That'd be completely uncharacteristic of quantum mechanics.


It is my view that the inherent randomness of quantum physics makes it unsymmetric on time axis. Every quantum tumbler can go either way (or more), and if you backtrack the process it should only be able to go one way - it's original state. Even if you look at CPT symmetry, quantum mechanics still isn't time symmetric because of the statistical qualities.

Thermodynamics and one-way trip to increasing enthropy is really more about losing information in the process and the fact that running the simulation from END STATE in reverse time and expect it to end up in START STATE is impossible because the original start state information has been lost.

If you have Zoidberg in a pool and he releases the ink in his pouch as the START STATE of an experiment, and END STATE is ink spread evenly into the water and Zoidberg hiding in the middle of the pool, then even if you run the simulation backward from END STATE, you can't really know where all the ink came from just based on the END STATE alone. Time symmetric thermodynamics would be able to recreate the START STATE information from the END STATE (all the ink in Zoidberg's pouch), but that's impossible as the ink is evenly spread to the pool; you can with good reason assume the ink came from the pouch, but you can't know for sure that the ink even became from Zoidberg's pouch, or what was Zoidberg's position at START STATE or his movements mixing the water during the experiment...


Conclusion: Future is fluid and not set in stone. Deterministic, discrete models such as classical mechanics and relativity are outside their application range in this context.

[swashmebuckle]

So it goes.

/Vonnegut

[Snail]

woopwoopwoop

[JCDNWarrior]

Perhaps it also has to do with the movements of the galaxy, and our planet around the sun, combined with a need to address the past with fitting words and dates? Weeks, months and years are things we thought of ourselves, but time itself, well, the only way we can really compare time is what we can perceive because of the four seasons and the birth and death of others, as well as the time they would have generally left to biologically live.

[Luis Dias]

One of the biggest mysteries left to man.

What is it?

Physics should be time symmetric. Relativity implies all of the universe should exist as one enormous block, past present and future already fixed and complete.

Physics implies none of this. The arrow of time is defined by the presence of a low entropy in the universe.

That's it. Simple as that.

Why do we experience it as moving down a path with a 'now'? Why is there an arrow to causality?

These are two different questions. One is about conscience, another is about the arrow itself.

Let's stick to physics, shall we.

It is very simple, stick with me. Imagine a wave splashin on a beach. All of that action seems quite directed towards one direction of time. Yet, as yourself noted, microscopically one could metaphorically say that it is like "Billiards", where the movements aren't assymetrical at all. So what causes this assymetry in our own scale of perception?

Entropy is the answer. Let's invoke a classical example. We have two boxes of gas. One is a red gas, another is a blue gas. They have "low entropy" in the sense that they are each one neatly arranged in each box. Now we open a gate that was between these boxes, and the gases mix. What happens? The resulting gas becomes a mix between blue and red. No matter how hard you wait, you will never* see the reverse happening. So, although microscopically, all the gas balls are floating and hitting the walls of the boxes as they always were, macroscopically we do recognize a difference in the arrow of time. This happens because there is a million more opportunities for these gas balls to intermix than to magically "divide themselves" into their own boxes again.

This difference is all that is needed to get an arrow of time.

Ideas?

There are more things to say than this, since other questions that are now really interesting begin to form. Why was Entropy so "low" in the beggining of the Big Bang? The answer seems to be that it was at its maximum possible! Entropy depends upon the size of the "pool" so to speak, and the crazy fast inflation of the universe since the big bang lowered Entropy to very small levels.

The future is defined by heat death. It's when even all the mesons degenerate, and all that remains is radiation.... fully homogeneized, with maximum entropy. In that moment, "Time" is no longer meaningful. When all is homogeneous noise, "Time" stops. There is no causality any more, there are no more events to report.

[0rph3u5]

One of the biggest mysteries left to man.

[...]

Why do we experience it as moving down a path with a 'now'? Why is there an arrow to causality?

[...]

Ideas?

maybe this is helping:

Kant, Critique of Prue Reason
Edition B, p. 13f [Causality as example of a "synthetisches Urteils apriori"; contains a short explaination what Casaulity is]
Edition B, p. 46f ["Trancendental Aethetiscs - Second Part - On the Time"]

=> Both the fact and the way we percieve time and causality are hardwired into our conciousness... It is a necessary fact to constitutes our (human) understanding of reality - reality itself by itself maybe entirely different (but we can't know - we are human)
The fact what we notice slip-ups as Science advances is not a flaw of Reality or Science but our limitation being reached...

"Critique of Pure Reason" contains the most convinient attempt on explainations on the way we percieve reality I've yet encountered and its's from 1787 ...

Though I've admit I've not read the whole book (you can see it that I have to pull the part for Causality form Introduction-chapter of Edition B, while it is actually fully explained in somewhere in "Trancendental Logics") ... but reading CoPR in its full and understanding it is a life-long task as I can confirm from experience: even understanding the indroduction chapters (of both Edition A and B) in full strains the mind (and that's not just because Kant is unable to formulate short sentences)...

ps. I still wonder how I managed to pass the exam on that lecture... maybe I'll find out once I get the exam back (the results have been released but actual sheets of paper are stuck somewhere in my university's administration - common german problem)

[Mikes]

Yes, I made that point in the original (and current) version of the first post. It is the primary objection to the 'stage of the now' conception of time.

The following is more about conciousness in general than about the passage of time specifically, but it seemed relevant as you touched on the former as well.

Recent scientifice research discusses that we make our choices before our consciousness knows what we choose... what always irked me is that this does not begin to answer the question what comes before the decision... which may as well be conciousness... again.

Wouldn't it be rather plausible if conscious review of decisions was just one stage in a circular decision loop.
Would it then even matter if we "only" review our choices after we made them when that constant review could as well influence the next decision cycle(s)?
I.e. Does it matter if conciousness comes after decision if it also comes before every decision?

We may as well make our decisions before we become aware of them, but is there any reason other than blind assumption that decisions are completely unrelated to any conscious thought that we had *before* we made them? (It's been a while since i digged into the field, so please enlighten me if I'm missing something.)

That line of thinking goes hand in hand with linguistic theory and the "monitor hypothesis" btw.
I.e.: If foreign language learners actively/conciously review their language according to the grammatic rules they learned while the speak/write, the language they produce is very different than when they simply write or speak "without thinking" about it: So conscious appliance of rules to actions while performing those actions does obviously influence the outcome.

Couldn't conciousness well have a similar effect on decisions?

[Mika]

The thing is that all processes in physics should be totally time-symmetric. You could turn the clock back and the universe would neatly run in reverse. So what establishes the direction of causality? Why do we go only one way?

Quite a lot of macroscopic processes are actually not time symmetric. Also, don't mix up that Mathematics allow for a negative time solution which might or might not happen in a physical real world. There might be negative time, or there might not.

Nevertheless, you are now meditating on Physics issue that even Physicists themselves are not very familiar with. What is time is a fundamental question in Physics, and currently there's no answer.

[Scotty]

People assume that time is a strict progression of cause to effect, but *actually* from a non-linear, non-subjective viewpoint - it's more like a big ball of wibbly wobbly... time-y wimey... stuff.

[General Battuta]

The thing is that all processes in physics should be totally time-symmetric. You could turn the clock back and the universe would neatly run in reverse. So what establishes the direction of causality? Why do we go only one way?

Quite a lot of macroscopic processes are actually not time symmetric. Also, don't mix up that Mathematics allow for a negative time solution which might or might not happen in a physical real world. There might be negative time, or there might not.

Nevertheless, you are now meditating on Physics issue that even Physicists themselves are not very familiar with. What is time is a fundamental question in Physics, and currently there's no answer.

Yeah, that's exactly why we have this thread, to talk about it.

[Mustang19]

Consciousness is not an objectively observable phenomenon. Your question in the OP is an unfalsifiable intellectual neurosis with no meaningful answer.  Good luck. 

[Luis Dias]

The thing is that all processes in physics should be totally time-symmetric. You could turn the clock back and the universe would neatly run in reverse. So what establishes the direction of causality? Why do we go only one way?

Quite a lot of macroscopic processes are actually not time symmetric. Also, don't mix up that Mathematics allow for a negative time solution which might or might not happen in a physical real world. There might be negative time, or there might not.

Negative time? Lol. That's as meaningless as saying there might be a "negative down slope".

Nevertheless, you are now meditating on Physics issue that even Physicists themselves are not very familiar with. What is time is a fundamental question in Physics, and currently there's no answer.

Utter bollocks. Time is simple to understand, and if you had only avoided certain physicists who are seemingly mesmerized with the obvious (I'm looking at you, Sean Carroll), and instead went to listen to the real giants like Feynmann, you'd see that this is not even a puzzle in physics.

What is indeed puzzling is our own perception of time. But this is going to be solved by neuroscience, not physics.

[Mika]

Negative time? Lol. That's as meaningless as saying there might be a "negative down slope".

Utter bollocks. Time is simple to understand, and if you had only avoided certain physicists who are seemingly mesmerized with the obvious (I'm looking at you, Sean Carroll), and instead went to listen to the real giants like Feynmann, you'd see that this is not even a puzzle in physics.

What is indeed puzzling is our own perception of time. But this is going to be solved by neuroscience, not physics.

Observation: You seem to have pretty bold statements of the topic considering your education. Which begs the question are you absolutely sure you understood the underlying theories?

For some reason, there seems to be a minimum time step that can possibly observed by the current knowledge. The current understanding is that there is no way to know whether time is even continuous or stepwise and same applies for the usual three dimensions. I can't even fathom what step like time progression would mean in the context of the world. Nor can I get my head around how do the proposed extra dimensions work in quantum world, and have their existence been confirmed.

There's quite a lot of research work to be done to take out those orders of magnitude to be able to observe processes that happen within Planck time, whether that is even possible or not I don't know.

[Luis Dias]

Negative time? Lol. That's as meaningless as saying there might be a "negative down slope".

Utter bollocks. Time is simple to understand, and if you had only avoided certain physicists who are seemingly mesmerized with the obvious (I'm looking at you, Sean Carroll), and instead went to listen to the real giants like Feynmann, you'd see that this is not even a puzzle in physics.

What is indeed puzzling is our own perception of time. But this is going to be solved by neuroscience, not physics.

Observation: You seem to have pretty bold statements of the topic considering your education. Which begs the question are you absolutely sure you understood the underlying theories?

What do you know about my education? I've been a finalist in physics olympiads in my country, to which I had to learn quantum physics. Thermodynamics is ****ing easy in comparison.

For some reason, there seems to be a minimum time step that can possibly observed by the current knowledge. The current understanding is that there is no way to know whether time is even continuous or stepwise and same applies for the usual three dimensions. I can't even fathom what step like time progression would mean in the context of the world. Nor can I get my head around how do the proposed extra dimensions work in quantum world, and have their existence been confirmed.

No, their existence has not been confirmed, they are astonishingly tiny. Imagine something tinier than an electron. Then imagine something more tinier than that. There yet? No, not even close. It's megatiny. And things that are so tiny as that require huge swaths of power to unravel. Solar System sized particle accelerators.

The fact that time is not continuous, but rather "bubbly" in planck's scale, is a direct consequence of quantum mechanics. There are no problems here, except if you have too many classical prejudices runnning in your brain. Even then, you'll be fine, if you think that "time" as measured by, say, the scale of atoms, is too far away from planck's scale for these bubbliness to matter in the equations.

In these scales, the arrow of time is meaningless. These are "billiard balls" doing their thing, symetrically in time.

There's quite a lot of research work to be done to take out those orders of magnitude to be able to observe processes that happen within Planck time, whether that is even possible or not I don't know.

Solar System Scale. Everything is possible, but everything has a cost. And this is irrelevant to the issue of the arrow of time (there are questions whose answer is not located at the deep end of everything, be them strings, branes, or whatever).


EDIT: Here, there are good videos on the net about these issues, specially when they are lectures by actual top physicists. And there were no greater physics lecturers than Feynman, so I'll give you this:

http://www.youtube.com/watch?v=_Kab9dkDZJY

"Richard Feynman - The distinction between past and future"

I hope it is relevant to the thread

[Astronomiya]

Negative time? Lol. That's as meaningless as saying there might be a "negative down slope".

Why?  Time is dealt with in most areas physics by ascribing it a coordinate.  This coordinate may have any possible value; the zero point is one of convenience. So negative time is certainly possible.

Utter bollocks. Time is simple to understand, and if you had only avoided certain physicists who are seemingly mesmerized with the obvious (I'm looking at you, Sean Carroll), and instead went to listen to the real giants like Feynmann, you'd see that this is not even a puzzle in physics.

What is indeed puzzling is our own perception of time. But this is going to be solved by neuroscience, not physics.

Oh?  Then, pray tell, what is time?  What does Feynman say about time?  I'm not familiar with his writings/discussions on the subject; I want to see direct quotes.  EDIT:  Didn't see your edit.

These are "billiard balls" doing their thing, symetrically in time.

NO interpretation of QFT (and necessarily, string theory) of any sort is time-symmetric in ANY way, shape, or form.

[Luis Dias]

Negative time? Lol. That's as meaningless as saying there might be a "negative down slope".

Why?  Time is dealt with in most areas physics by ascribing it a coordinate.  This coordinate may have any possible value; the zero point is one of convenience. So negative time is certainly possible.

Yeah, it's called "The Past".

Utter bollocks. Time is simple to understand, and if you had only avoided certain physicists who are seemingly mesmerized with the obvious (I'm looking at you, Sean Carroll), and instead went to listen to the real giants like Feynmann, you'd see that this is not even a puzzle in physics.

What is indeed puzzling is our own perception of time. But this is going to be solved by neuroscience, not physics.

Oh?  Then, pray tell, what is time?  What does Feynman say about time?  I'm not familiar with his writings/discussions on the subject; I want to see direct quotes.  EDIT:  Didn't see your edit.

Yeah well sorry. I didn't mean to say "TIME" is simple, I meant to say that understanding the arrow of time is simple enough, so the claims that this question is still puzzling physicists is just ludicrous.

[Luis Dias]

NO interpretation of QFT (and necessarily, string theory) of any sort is time-symmetric in ANY way, shape, or form.

Oh **** now you open a can of worms.... (let them rest quiet!)

[watsisname]

No no, let them speak, please. 

Negative time? Lol. That's as meaningless as saying there might be a "negative down slope".

The negative of a negative slope is a positive slope...

And Astronomiya, could you describe why reversing time would involve flipping matter and antimatter states as you mentioned earlier?

[Astronomiya]

And Astronomiya, could you describe why reversing time would involve flipping matter and antimatter states as you mentioned earlier?

In QFT, if you imagine a normal particle running backwards in time, it turns out to be exactly mathematically equivalent to an antiparticle running forwards in time, and vice versa.  So, if time suddenly reversed, matter could be said to become what is now antimatter, and vice versa.  It is an interesting interpretation of the underlying physics, and I feel it could very well be reality.  I should have been more clear it was only an interpretation, though.  The first answer there says it better.