Time moving backward - isn't it a nonsense?

I keep reading in pop science books how the authors marvel about why the arrow of time only has the forward direction and not backward. I don't get it. What would it mean that something is happening backward in time? Isn't it a nonsense, like saying that I have minus three apples? How could an object move backwards in time? If forward movement in time means that an object passes through points A, B, C, D, does backward movement in time mean that the object passes through these points in the opposite sequence (D, C, B, A)? It seems that even if the object passes through these points in the opposite sequence it is still moving forward in time, doesn't it? It is simply going back in space while the time still moves forward.

 

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Yes. But with the sole exception, as far as I am aware of course, that the entropy increases in the direction that we know as going 'forward in time' while entropy decreases in the other direction.

Other than this, physics doesn't seem to favor a direction of time. The equations work equally well in both directions and this is where the issue of the 'arrow of time' lies.

 

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See, I don't understand how an equation can work backwards in time. I mean, what physical meaning does it have to "work backwards in time"?

As for the entropy... could we define "movement backward in time" as a movement in which the entropy of a closed system decreases? I'm not sure I understand the meaning of entropy correctly. Is increasing entropy equivalent to the fact that when a more energetic particle meets a less energetic particle then the more energetic particle will give some of its energy to the less energetic particle (and never the other way round)?

 

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Yes, your basic understand of entropy is correct. And the only way to decrease entropy is at the expense of increasing it somewhere else.

As for reversing the time arrow, yes, that's very easily done mathematically. But in the real world it's recognized as nothing more than a math trick. People have been dreaming that dream as long as there have been people.

 

That is not fundamentally true, but always the case in the 14billion years of the universe, once you are speaking of more than a few particles. For example, If you had a sealed box of 4 hydrogen and 4 nitrogen molecules in very short time all the H2 would briefly be on one side and and the N2s on the other, but only for an instant, until it happened again. If there were 40 of each, you might need to watch for 100 years to see this happen, but it is only probabilities that are the foundation of "total entropy always increases."

Again the truth is not quite what you say.

P.M.Drac, even suggested a reason why all electrons are identical based on frequent "reversing times arrow" for the only electron that exists. Argument goes something like this:

At some point it the future the only electron that exists begins to travel backwards in time. It "looks like" (is indistinguishable from) a positron as it passes thru "now." Back in the past it again begins to travel forward and looks like an electron (number 1) "now." After passing thru "now" where we see it as electron 1, into the future, it again scatters back from the future and a long time ago it scatter forward in time passing thur "now" as electron (2). The positron keeps coming back from the future and in the ancient (or recent) past, scatters forward in time again to appear in the present "now" as electron (3) As charge magnitude, spin and mass are conserved in scatteings, electron 1, 2 and 3 are all identical. As so it goes, on and on, until "now" is filled by many electrons.*


Strange as it sounds, this is (as far as I know) the only reason why all electrons are identical. There is a muon which is identical to the electron in everything except the rest mass, sometimes even called a "heavy electron."

A more conventional POV assumes that we simply do not know why only these two "electrons" exist instead zillions with different masses etc.
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*If Drac explained why now is not also filled with positrons, I forget the reason. Perhaps there is a slight "asymmetry" in their physics like that one which "explains" where all the "anti-matter" went to?
This "crazy" playing around with time makes "God's work" a little easier. (Less stuff to make.) It fits in very well with a deterministic future. In such a world the POV that the future determines the past is justs a valid as the convention one - sort of like universe is a giant movie, all pre-recorded. I.e. no problem with "effects" preceding "causes." I. e. the universe movie has been "in the can" for all eternity. - We only experience our "now" and remember the part of the movie that we have seen. This POV does lack any explanation for the increase we observe, however. It would be nice if a broken and scrambled egg could appear whole in the ice box again, would it not?

 

I think you can slightly modify this explanation by saying that all the electrons in the universe are just one object that manifests itself approximately as separate electrons locally.

 

Possibly, but neither Drac, nor I know how "manifests itself" fits into physics. - Smacks too much like angel's work etc. We like mechanistic explainations, but that does not mean others do not exist. Just that we prefer to try to understand universe without any miracles and admit we don't know everything (yet?) when we can not.

 

Probably I chose a wrong word. What I meant was that if you consider all the electrons in the universe as just copies of one electron going back and forth in time, then all the electrons are connected through their history and this supports the view that all the electrons are just one object. In fact, in four dimensional picture of spacetime this object would be a worldline connecting (in far future or past) world trajectories of every electron (an positron) in the universe.

 

So the fact that entropy increases is not equivalent to the fact that when a more energetic particle collides with a less energetic one it passes energy to the less energetic one? That seems to be an obvious explanation of why energy spreads from high-energy (high temperature) regions to low-energy (low temperature) regions, i.e. total entropy increases. And the fact that a high-energy particle passes energy to a low-energy particle is always true (in each individual case), not just a statistical tendency, isn't it?

So an electron traveling backwards in time is identical to a positron traveling "normally", i.e. forwards in time? Switching time direction is equivalent to switching electrical charge? How so?

 

Hi litewave,
Here is an animation of a bouncing ball:

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.

Now think about this:
Can you tell if the animation is being played forward or backward? You can't, right? In fact, for all simple interactions, you can't tell from a recording whether they are happening in forward or reverse.

But for most complex interactions, you can easily tell which way is forward, and which way is backward... and that is what the authors are marvelling at.

 

This is beyond me, but the relevant term to look for is CPT symmetry

 

Hi Pete, it is always being played forward in time. It is irrelevant whether you play a recording forward or backward -- time is always ticking forward while you're playing it. My point is that time cannot really go backward, it necessarily goes only forward, and so there is no point in asking why it doesn't go backward, why something doesn't happen backwards in time. What once happens cannot un-happen.

To give a minus sign before t in physics equations seems to me just like a mathematical operation that doesn't correspond to real passage of time. You may say that an object traveling from A to B is actually traveling from B to A backwards in time, or that an electron is actually a positron traveling backwards in time, but that just seems like a redundant statement that assumes that time can go backwards.

 

Yes, of course.
But that's what they mean when they talk about the arrow of time... some interactions look the same in reverse, some don't. The interesting question is why? If basic interactions look the same in reverse, then why don't complex interactions?

Considering whether time can go backwards or not is a fun way of looking at that question, but it's not necessary.

It seems to me that you're assuming time can't go backwards, and then concluding from that assumption that time can't go backwards.

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Pete

I think time can't go backwards because new physics would be needed to predict observations. For example, an explosion played forward obeys physical laws. Can you devise physical laws that would predict that explosion in reverse. I think they would necessarily be different than the physical laws we know about.

 

Hi Vern,
Yes, that's the interesting question - Why do the laws of thermodynamics have a built-in arrow of time?

With a reverse explosion, there is no problem with energy, motion, gravity, or any physical law except the second law of thermodynamics... and that is interesting!

 

Ok, I can go with that.

Can't the phenomenon of increasing entropy be explained as a result of the fact that in a collision of two particles the higher-energy particle always passes energy to the lower-energy particle (and never vice versa)? Hence energy becomes more evenly distributed in space...

 

Much of the confusion, mystery here is due to excessive reliance on everyday human experience, which tends to makes us think of "time" as sort like a "non-material river" flowing from past thru the present into the future. Some statement made are at best misleading. The truth is as I said in post 5, namely:

"it is only probabilities that are the foundation of "total entropy always increases."

This is also the foundation of the equal partition of energy "law" which makes heat flow from hot to colder objects as litewave has noted a couple of times. Again, it is a "law" of probability not mechanics which makes this the result we observed. I will first show this:
I use capital letter to indicate the particle with more energy and italics for particles of approximately the same energy. (If in italics, ignore the case, I.e. I must use either cap or lower case but ignore it.)
Consider the following collision between two particles:

A & b --> a & b During this "type 1" collision both energy and momentum are conserved. Now consider the following "type 2" collision"

a & b --> A & b Again both energy and momentum are conserved. In fact this is just the first collision with "time reversed"

Now sets up the equations for a "parameterized collision" of two identical hard perfectly elastic spheres. I.e. One has energy E and the other energy e and the separation between their initial trajectory lines (called the "impact parameter") is d where d < D, their diameter.

Now chose initial E1, e1, and d at random and compute the after collision results, E2 and e2. If E2 < E1 (and then of course e2 >e1) AND e2 is not greater than E1 then the collision was of "type 1" Namely the after collision energy is more equally shared between the two particles. Otherwise, the collision was a "type 2" collision. What you will find after investigating a larger number of random cases is that most collisions are of type 1.

This all there is too it!!! In all probability molecules A, B, & C placed in a box with molecules d, e, & f will after a few collision become molecules a,b,c,d,e,&f This is why heat flows from hot to cold. Probability is all there is to it!!!!

I will address the "entropy increases" mystery in another post soon. It is also just probability as I stated in post 5, but perhaps I will need to discuss “time also.”

 

So, "type 2" collisions are those where the energy of the two particles after collision is not more equally shared between them (or perhaps even less equally than before collision)? Hm, I didn't think about this case but I did a numerical example and this is what actually happened. If the transfer of energy is very big, the two particles can end up with an even more unequally shared energy than before the collision... It seems that these "type 2" collisions would therefore actually decrease entropy of the system... So it can be shown statistically that "type 1" collisions prevail enough to increase the system's entropy?

 

Please read post 17 first.

If you see a movie of a old coal burning train with smoke going back down into the smoke stack, you know that the movie is being shown “backwards”, with “time reversed.” It is so improbable that zillions of molecules CO2 molecules and specks of carbon will collide with each other in the air in such a way as to enter (note I did not say “re-enter”) the train’s smoke stack that one can bet their life that even if the train had been running for the 14 billion years of the universe, it would not have happened; HOWEVER, if it did, it would not violate any law of mechanics related to collisions.

Thus, one can not be absolutely sure that the movie is being shown “backwards.

I.e. There is not absolute “arrow of time.”

What is time then, if not this “non-material river“ flowing from past, thur the present, into the future?

To answer this question, we need to carefully examine how we observe “time.” With only a brief consideration of this, we recognize that we need to add the adjective “invisible.” I.e. we can assume or think of time as an “invisible, non-material, river” flowing….

Fact of the mater is we only observe events, a & b and presume that a is “before” b. This presumption is very well founded in human experience, but not physical law. It is based on the same highly improbably possibility smoke going back into the smoke stack etc. that lets us conclude which is cause and which is effect. I.e. we know from our experience that a caused b and not b caused a. We have never the broken glass on the floor leap up as a whole glass on the table. There is nothing in physics that makes this impossible, -it is just such a small set of initial conditions (compared to the total of all possible initial conditions set) that it will never be observed in in 100 billion times the life of the universe even is a glass falls to the floor (as we assume time flows) every second for all this time.

Thus probability, not physics, gives time its “arrow” and makes the determination of which is “cause” and which is “effect” possible.

It is easy to show that “time” is only a very useful convention, not necessary. I have posted the proof of this before, and will not now repeat it all (Omitting the fact that one must make “index” to cover the possibility of multiple occurrence of the same conditions, like Earth taking the same position roughly every 365.25 days. In this case the current “index” is 2007.)

Conventionally the parameters describing ever atom etc of universe are of the form:
L = l(t) and V =v(t) where t is of course “time” the “independent parameter” L is the location in space, l is the functional form giving L, and V the velocity and v its functional form. There are of course lots of other things, like what internal state each atom is in etc. that must also be described to completely specify the universe in a set of N equations. (I am ignoring the quantum uncertainty to may my point, but including it is also possible.) N is a very large, but finite number.

Now in principle, one can solve one of the N equations for t and insert this “t solution” into all the other N-1 equations where t appears. By this one now has a complete description of the universe in N-1 equations. (All physics is is an attempt to completely describe the universe.) Note that time does not exist in this N-1 set describing the universe. Thus time is not essential, only very convenient, in a description of the universe or in physics! If time is not essential, is non-material, invisible etc, in some sense it is only a “notational convenience“, not any thing real.

Recall also that cause and effect are only determined on the extremely high probability that a caused b instead of b caused a. Thus it really only a high probably (even if one wants to use the convenience we call “time”) that a is “before” b. What is certainly true is that event and event b are related to each other by the laws of mechanics (or more generally quantum mechanics or even more generally “the laws of physics“)

We can not really be sure which way the movie of the train with smoke is being shown. The same true of the universe movie. It is just that there are zillions and zillions raised to the zillion and zillions power more way for the present universe to have come from an earlier “big bang” than that the present universe will “expand a little more” reverse and return to the big bang. I.e. even in the case of the whole universe, it is not impossible that what we call the future is what is making the present and the present is what will make what we call the past.

Now that all think I am crazy, I will quit, with just one last word: Probabilities.

 

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M*W: Regarding the movement of time... I am no physicist, so I can't comment physio-scientifically about this subject. However, I will say that the Moon affects the perception of time. I'm trying to figure out if this is only a 'perception' or 'reality' of time. The two weeks following a Full Moon, time 'seems' to speed up. I don't know if it really speeds up or if it only feels like it does. I seem to have more energy, and less time to get things, because the clock seems to get ahead of me when I'm trying to get things accomplished. The two weeks following a New Moon feels like time slows down. I tend to get sluggish, time seems to stand still. I have more than enough time to get things finished, but I don't have the energy to get them done! I've noticed this for years. A New Moon seems to lift me up as if I weighed less, and a Full Moon weighs me down. I retain fluid, feel heavy and sleepy. Is this my imagination, or could there be some truth to it! I swear by it!