We can only experience time as it moves forward, but is it possible that time moves both ways? Our experience relies on memory and if time moves backwards we would have no memory of an event that has not occured and so we would be condemned to see time only as it surges ahead. Have there been any speculations or theories on this topic?

Our experience relies on memory and if time moves backwards we would have no memory of an event that has not occured and so we would be condemned to see time only as it surges ahead.
To me your theory kind of sounds backwards. Wouldnt our experiences rely on what we are experiencing (present) and if time were to go backwards then we would have memory of it seeing as its the past and has happened already. or maybe i didnt understand your theory correctly?!?!

time can flow backwards, but you get alot of temporal backwash that way, and nobody likes backwash.
seriously though, you both bring up some interesting points. i will be back to comment more latter.

Our experience relies on memory and if time moves backwards we would have no memory of an event that has not occured and so we would be condemned to see time only as it surges ahead. Have there been any speculations or theories on this topic?

Yes, Hawking refers to this as the "psychological arrow of time" in his book "The Universe".

We can only experience time as it moves forward, but is it possible that time moves both ways?

If we are not able to measure it, then we can only say "perhaps it is possible"... but equally... "perhaps not".

no one has yet to explain how we would have no memory of events that have happened already?

no one has yet to explain how we would have no memory of events that have happened already?

Because from your past perspective, those future events wouldn't have happened yet. Your memory would only serve past events and not future ones.

no one has yet to explain how we would have no memory of events that have happened already?

Might have something to do with entropy. I've read some books that deal with the arrow of time. Really, the only thing that would distinguish the two would be entropy. Entropy increases in our time frame, and it would decrease if time were reversed.

If we are not able to measure it, then we can only say "perhaps it is possible"... but equally... "perhaps not".
If we are not able to measure it, even in principle, then we are positing something that is equivalent to nothing. An undetectable dragon in my garage is exactly the same as no dragon.

Might have something to do with entropy. I've read some books that deal with the arrow of time. Really, the only thing that would distinguish the two would be entropy. Entropy increases in our time frame, and it would decrease if time were reversed.

Yes, that is "another" arrow of time. Next to the psychological effect ("we do not remember the future") there is also a prediction made by physics, which is usually linked with entropy and is called the "thermodynamic arrow of time".

If time is reversed, then entropy will not decrease; as a matter of fact, it will increase again. It is not easy to bend your mind around this, it is a matter of statistics (and it requires quite some knowledge of statistical physics to understand too). This was first remarked by Boltzmann if I remember correctly.

this is assuming that your thoughts would not progress in a backward direction, in which case, you would have memories of the "future" which would no longer be the future - as you are traveling back from that future - which becomes the past. :m: i need to do some reading...

quite simply the past becomes your future.......and all language and thought would have to be backward as well....a strange existence for sure...... I suppoose we could get used to it....ha ( sreeb wef a htiw )

I read a sci fi book once that described a child that at the age of 10 started to grow backward. every day she woudl wake up a day younger live a day and wake the next missing two days. She ended up a baby then a foetus in a test tube.....amazing concept but a really crappy book...can't remember the author...and don't want to.

i think the same thing happened to merlin. but here's a question; if the past is predetermined, and as time is moving backward the past becomes the future, doesn't that take away from the notion of the future. i mean, one of the main points of difference between the past and future is that the future has infinite possibilities, whereas the past is "set in stone." wouldn't what we are suggesting here mean that the past be pre-determined? moreover, in the case of the backward-time universe, the future would already be plotted and we are just marionettes controlled by what amounts to fate. or maybe the past isn't set in stone... hmm.

i dont think that the whole future would be predetermined just up to the point where u began to travel backwards from. because every thing after that would be experiences and events that havent been experienced yet. with that in mind here is a question that was discused in another forum if a person were to go back or forward in time what would prevent time from aging or regressing that person?

P.S. who is merlin ? the magician ?!?!?

ha ha this is one amazing discussion but yes teh past that is now teh future would be extremely predictable to a point. allowing for the spontaneous nature of freewill with in abouyt 10 years or so the future would become like it was....becasue every moment of sponanaety would change the locked future. Unfortunately reality would regress but us humans would adapt......crazy thread this....but hey what the heck.

quantum and mr freeze what are your thoughts on my question

The Fabric of the Cosmos by Brian Greene is a good book that covers this topic. I was wondering though, if the arrow of time were to go both ways, would this maybe strengthen the theory for parallel universes and alternate worlds?

It's not determined that with time regression, the same outcome would occur as it had in the same, previous slice of time. Is it possible that with each possible outcome and choice made by observer or nature an alternate world would be created explaining why we would have no accounts of time going backwards?

if a person were to go back or forward in time what would prevent time from aging or regressing that person?


For the theory to be plausible, wouldn't one have to go with the assumption that with time regression, your memory and your age would regress along with everything else.

Does time even flow, or could it spike, undulate..?

Yes, that is "another" arrow of time. Next to the psychological effect ("we do not remember the future") there is also a prediction made by physics, which is usually linked with entropy and is called the "thermodynamic arrow of time".

If time is reversed, then entropy will not decrease; as a matter of fact, it will increase again. It is not easy to bend your mind around this, it is a matter of statistics (and it requires quite some knowledge of statistical physics to understand too). This was first remarked by Boltzmann if I remember correctly.

Crisp: do u agree that in a contracting phase of the universe, entropy would decrease through time? Or is time flowing backwards and so entropy is still increasing. In the latter case, does this change such fundamental things as us counting backwards on an abacus some time in the distant future (provided we are still alive!)

"Does time even flow, or could it spike, undulate..?"
what do you mean? i cant even think of what a spike in time would do can you?

"For the theory to be plausible, wouldn't one have to go with the assumption that with time regression, your memory and your age would regress along with everything else."
I think you're right unless the person found a way to step outside of the time stream go to specific point then rejoin the time stream.

Crisp: do u agree that in a contracting phase of the universe, entropy would decrease through time? Or is time flowing backwards and so entropy is still increasing. In the latter case, does this change such fundamental things as us counting backwards on an abacus some time in the distant future (provided we are still alive!)

Why would a contracting universe be much different from an expanding one (when far away from the singularity at the end/beginning) ? In that sense, "locally" entropy would still increase, regardless if time goes forward or backwards. But I must admit that I have absolutely no scientific foundation for that. I am not sure what to think of the entropy of the universe -- it is something I try to avoid as much as possible in arguments ;).

But when not considering the expansion or the contraction of the universe, then the mathematics is quite clear: the increase of entropy is not coupled to time running backwards or forwards (an argument first used by Boltzmann, I think in the context of time-reversal of the Boltzmann equation... I would have to look it up) ... I really haven't got the slightest clue on how to include the expansion and/or contraction of the universe in this scenario.

who said that time moves at all? suppose it is just entropy that makes time seem to move forward. one thing to point out; entropy has an equal probability of increasing in both the future and the past. however, we must also remember where the order we see around us comes from; and that is the big bang. from this we see that the past could only have been in a more ordered state than the present. therefore, as we look back through spacetime, it is entirely possible that people would, uh, reverse age since people and memories are merely particles in places at times. though i don't see why time would "flow in reverse" and therefore believe this whole thread can amount to nothing more than speculation. i hope that was a satisfactory opinion. :)

but if you are talking about time travel, i think that is based on time passing at different rates for different people [as is the case when "traveling into the future" at near light speed]. aging is a process within the body, i think recently [or perhaps not so recently] a theory has emerged about telomere degeneration; such that, when a cell divides, its telomere caps - which protect the genetic coding in dna - are cut off little by little. therefore, after a certain number of divisions, the dna gets degraded and the new cells don't work as effectively. the result is aging. [if i got that theory wrong, someone please correct me] but a major road block in my answering your question is that i simply do not know how true time travel would work. in the example i gave above, the reason given is a reduction in the velocity through time in accordance with an increase in velocity through space. as you approach the speed of light, therefore, the number of mitotic divisions you undergo in a given amount of time [earth time] is fewer, and you age very slowly. i think there was something in string theory about traveling back in time, but i'll have to check on that first. so that is the best shot in the dark i have for now.

mr. freeze, i like that. :D and merlin is the magician in the king arthur story.

Why would a contracting universe be much different from an expanding one (when far away from the singularity at the end/beginning) ? In that sense, "locally" entropy would still increase,

It probably would not be different. In our own phase, entropy is decreasing on a local scale too. That is not what I am talking about though.

It probably would not be different. In our own phase, entropy is decreasing on a local scale too. That is not what I am talking about though.

It does ? You are not talking about the Evans experiment, are you ? ;)

Haha, no! Maybe I misunderstood what u said. Local entropy can increase as u say but global (universal) entropy will be decreasing right? Just the opposite of today.

An example of entropy decreasing locally is life itself is it not? Life is an active process of reversing the dispersal of energy (that is entropy). I say this coz I do not want to reap the wrath of Nasor by talking about disorder :D

Life is an active process of reversing the dispersal of energy (that is entropy).
what?! :eek:

What happens when life ends? Is the system left with more entropy than the individual reversed during his lifetime? Then one must consider that life feeds on life. We take creatures that are composed of order and disintegrate and take parts of them into our order while other parts pass through or are discarded to rot. What about biological processes? Don't they create entropy? The heat from an organic being is a form of entropy isn't it? It would be interesting for a definitive study to be made of entropy and life.

Local entropy can increase as u say but global (universal) entropy will be decreasing right? Just the opposite of today.

I don't know, I am not that an expert in cosmology. I can't really comment on what the influence of the expansion of the universe on entropy is. It would seem a bit surprising that the total entropy of the universe would decrease, but locally it would decrease (after all, entropy is additive over subsystems).


An example of entropy decreasing locally is life itself is it not? Life is an active process of reversing the dispersal of energy (that is entropy).

Yes, life is such an example, but it is not really a spontaneous proces, surely not a closed system, so the second law does not really apply (in case some people are wondering). But the organisational molecular structure of beings is indeed a decrease in entropy.

The lamp flickers and goes out, it starts sucking heat and light from the room, electricity is flowing back into the power grid, brrr it's cold in here. Oh no! the golden braid of time is unravelling into limitless possibilities. Now the sun is unborn, the repulsive nuclear forces have blown everything into a hot primordial plasma that is contracting everywhere rapidly. It's dark again.

and God says to himself..."Um..ah....oops"

What happens when life ends? Is the system left with more entropy than the individual reversed during his lifetime?

System = universe. Yes.

Then one must consider that life feeds on life. We take creatures that are composed of order and disintegrate and take parts of them into our order while other parts pass through or are discarded to rot.

When we eat we break down that order to release energy most of the time. We then use the energy to create a little more order in our own bodies.

What about biological processes? Don't they create entropy?

U mean increase entropy? That is what we have been talking about. Life is, by definition, a biological process!

The heat from an organic being is a form of entropy isn't it?

No its a form of energy

I don't know, I am not that an expert in cosmology. I can't really comment on what the influence of the expansion of the universe on entropy is. It would seem a bit surprising that the total entropy of the universe would decrease, but locally it would decrease (after all, entropy is additive over subsystems).

I presume u mean increase in bold there :p
Why would it be surprising when I have shown that a similar thing is happening today in our universe?

Yes, life is such an example, but it is not really a spontaneous proces, surely not a closed system, so the second law does not really apply (in case some people are wondering). But the organisational molecular structure of beings is indeed a decrease in entropy.

The law allows subsystems to decrease their entropy as long as those subsystems are (subs) of a system which always increases over time. Take the system of the earth. Maybe the entropy of the earth is decreasing quite rapidly (not going to calculate it of course) but there are reasons to believe so. This is not a closed system (nothing is) but it is a local decrease.

I presume u mean increase in bold there :p

That oughta teach me to post late at night.

Why would it be surprising when I have shown that a similar thing is happening today in our universe?

I seem to have missed this, were you referring to the biological system example ? If you were, then I can only say: there are many other factors on earth which, when added to the "little" decrease caused by biology, will eventually let the entropy on earthscale increase too.

The law allows subsystems to decrease their entropy as long as those subsystems are (subs) of a system which always increases over time. Take the system of the earth. Maybe the entropy of the earth is decreasing quite rapidly (not going to calculate it of course) but there are reasons to believe so. This is not a closed system (nothing is) but it is a local decrease.

Exactly the opposite of what I would say. Let me bring in some help.

Say hello to mister Penrose ;). I took the liberty of looking this up earlier today in Penrose's "The Emperor's New Mind". I forget my copy at work, but if I remember correctly he agrees with me: even when the universe contracts, the entropy will still increase. And I remember clearly how he explains that the sun serves as an input of "low entropy photons (high energy) and the earth as a gigantic source of "high entropy photons" (low energy, read infrared radiation).

I'll look up the pagenumbers and a bit more detailled text tomorrow.

The question is, how would a forward time observer see a backward time observer? Could they tell the difference?

Say hello to mister Penrose ;). I took the liberty of looking this up earlier today in Penrose's "The Emperor's New Mind". I forget my copy at work, but if I remember correctly he agrees with me: even when the universe contracts, the entropy will still increase. And I remember clearly how he explains that the sun serves as an input of "low entropy photons (high energy) and the earth as a gigantic source of "high entropy photons" (low energy, read infrared radiation).

I'll look up the pagenumbers and a bit more detailled text tomorrow.

I am quoting from pages 328 and following in Penrose's "The Emperor's New Mind" (1989, Oxford university press, New York)

---
Does the big bang explain the second law?
... Recall that the primordial fireball was a thermal state - a hot gas in expanding thermal equilibrium. Recall, also, that the term 'thermal equilibrium' refers to a state of maximum entropy. However, the second law demands that in its initial state, the entropy of our universe was at some sort of minimum, not at a maximum!

What has gone wrong? One standard answer would run roughly as follows:

"True, the fireball was effectively in thermal equilibrium at the beginning, but the universe at that time was very tiny. The fireball represented the state of maximum entropy that could be permitted for a universe of that size, but the entrpoy so permitted would have been minute by comparison with that which is allowed for a universe of the size that we find it to be today. As the universe expanded, the permitted maximum entropy increased with the universe's size, but the actual entropy in the universe lagged well behind this permitted maximum. The second law arises because the actual entropy is always striving to catch up with this permitted maximum"

However, a little consideration tells us that this cannot be the correct explanation. If it were, then, in the case of a (spatially closed) universe model which ultimately recollapses to a big crunch, the argument would ultimately apply again -- in the reverse/I] direction in time. When the universe finally reaches a tiny size, there would again be a low ceiling on the possible entropy values. The same constraint which served to give us a low entropy in the very early stages of the expanding universe should apply again in the final stages of the contracting universe. It was a low-entropy constraint at 'the beginning of time' which gave us the second law, according to which the entropy of the universe is increasing with time. If this same low-entropy constraint were to apply at 'the end of time', then we should find that there would have to be gross conflict with the second law of thermodynamics!

Of course, it might well be the case that our actual universe never recollapses this way. Perhaps we are living in a universe with zero overall spatial curvature, or negative curvature. Or perhaps we are living in a (positively curved) recollapsing universe, but the recollapse will occur at such a remote time that no violation of the second law would be discernible to us at the present epoch -- despite the fact that, on this view, the entire entropy of the universe would eventually turn aorund and decrease to a tiny value -- and the second law, as we understand it today, would be grossly violated.

In fact, there are very good reasons for doubting that there could be such a turn-around of entropy in a collapsing universe. Some of the most powerful of these reasons have to do with those mysterious objects known as black holes. In a black hole, we have a microcosm of a collapsing universe; so if the entropy were indeed to reverse in a collapsing universe, then observable gross violations of the second law ought also to occur in the neighbourhood of a black hole. However, there is every reason to believe that, with black holes, the second law powerfully holds sway. The theory of black holes will provide our vital input to our discussion of entropy.

--

I am not going through the entire text on black holes now. He says (p338, figure caption)

For an ordinary gas, increasing entropy tends to make the distribution more uniform. For a system of gravitating bodies the reverse is true. High entropy is achieved by gravitational clumping -- and the highest of all, by collapse to a black hole.

Which would mean that, as the universe collapses back to a singularity, entropy would still be increasing. Penrose argues how actually the the Weyl and Ricci tensors provide a source of low entropy at the big bang (Weyl -> 0 and Ricci -> oo), and not the "tiny size of the universe". With respect to the "common" argument he described above, Penrose concludes (p339)

For some reason, the universe was created in a very special (low entropy) state, with something like the WEYL=0 contraint of [standard cosmology]-models imposed up it. If it were not for a constraint of this nature, it would be 'much more probable' to have a situation in which both the initial and final singularites were of high entropy WEYL -> oo type. In such a 'probable' universe there would, indeed, be [i]no second law of thermodynamics!

I must admit that I do not fully understand the argument that he gives on why exactly black holes increase entropy; I have never taken classes in general relativity so the Weyl and Ricci tensors are strange beasts to me :).

Bye!

Crisp

Crisp, if you can,
What is meant by "Special Low entropy state"?
For some reason, the universe was created in a very special (low entropy) state
Why special?

We can only experience time as it moves forward, but is it possible that time moves both ways? Our experience relies on memory and if time moves backwards we would have no memory of an event that has not occured and so we would be condemned to see time only as it surges ahead. Have there been any speculations or theories on this topic?
When the time progresses (flows forward) it leaves the past (Past1) behind it ; and the future is yet to occur - so future is non-existant.

IF the time 'flows backwards' then it should do that without any past of its own (Past2) by definition (as it nullifies Past) ; That is, at any point it has only its 'present' and yet to start flowing. (If we say it is flowing then that means it already started flowing , but, alas, it has no past to say 'started' ). Hence Time stands still since it is always yet to have started flowing.

Reverse flow of time is not a possibility in reality.

However, there is every reason to believe that, with black holes, the second law powerfully holds sway. --

Really? I wouldn't be so sure. After all, it is not a naked singularity and so one can treat them as seperate universes with seperate laws! The event horizon marks the boundary.

I must admit that I do not fully understand the argument that he gives on why exactly black holes increase entropy; I have never taken classes in general relativity so the Weyl and Ricci tensors are strange beasts to me :).

Do u think it has anything to do with Hawking radiation? I doubt it because that was an explanation for why we cannot decrease the overall entropy of our universe isn't it?!

What is meant by "Special Low entropy state"?

Why special?

Well, because entropy is really a measure on how "special" a state is. For example; a magnet is composed of small tiny little magnets attached to atoms (called "magnetic moments", related to the spin of the particle). If you want a strong magnetic field pointing up, then you have to align all those spins (little magnets) in the "up" direction. If you want a magnetic field zero, then you just have to make sure that there are as many spins pointing up as there are down.

To realise the maximal magnetic field pointing upwards, you have only one option: all spins must point up. To realise a field zero, you have.. many many many options: you can put a spin up, then one down, then another up... or you can put the leftside of the magnet all spins down, right side all spins up, ... You see, there are many possible configurations. Hence, the situation "magnetic field zero" is not special: there are many ways to realize it. The situation "maximal strength magnetic field up" is very special: there is only one way to realize it. Entropy expresses exactly "the number of ways to realize a given configuration".

Now you have to understand that in our universe, entropy has been increasing ever since time started to count. So, if we go back in time, it must have been smaller than today; but because the big bang was such a long time ago, the entropy must have been very very very small to make it possible for it to increase until today. A very low entropy in the past means that the initial state of the universe was very very special; the number that Roger Penrose sites is 10 to the power 10¹²³ (yes, that is a 1 with 10¹²³ zeroes after it -- even if you could write the numbre "0" on every proton on the universe, then there would still be too few protons ;)). By that he means that if you take that number of possible starting states, ours is one of them. That also means that our initial state was quite exceptional.

Crisp: do u agree that in a contracting phase of the universe, entropy would decrease through time? Or is time flowing backwards and so entropy is still increasing. In the latter case, does this change such fundamental things as us counting backwards on an abacus some time in the distant future (provided we are still alive!)

I can see both arguements here. But from my perspective if time were the ultimate controlling factor, I seem to believe that entrophy would also reverse.

Otherwise one would not have actually reversed time. That is if transfer of energy in backwards time still suffers entrophy then you haven't actually reversed time.

It may infact be the strongest arguement against backwards time travel. I have always maintained to do so requires a complete revearsal of energy flow.

Failure to reverse entrophy would make backwards time impossible. A simple example a spinning top on a table. As its energy dissapates and it falls motionless on the table.

If in your time machine you do not restore entrophy the top could never acquire its original stored energy or speed.

The question is more whether entropy would decrease in a contracting universe. I believe it would.

The question is more whether entropy would decrease in a contracting universe. I believe it would.

Penrose believe it doesn't, because black holes are perfectly compatible with the second law of thermodynamics.

Isn't that only because of Hawking radiation though?

Isn't that only because of Hawking radiation though?

It has to do with the curvature of spacetime.

Interesting. Can u expand a little?

Interesting. Can u expand a little?

I said earlier that I don't understand the argument myself ;) ...