# Is the past uncertain?

1. Mar 20, 2013

### DrZoidberg

As far as I know the future is uncertain because of random events like e.g. decaying atoms. But what about the past? If the current state of all the particles in the universe was known could the past in theory be calculated?
If all laws of physics are completely time reversal symmetric does that mean the past is equally uncertain as the future?

2. Mar 20, 2013

### DrChinese

It is possible, in some interpretations of quantum physics, to see the past as having an element of "uncertainty". You could in fact say that the universe we experience as "now" could have come from many different universes. I don't know that there is any way to evaluate whether the future is more or less uncertain than the past.

3. Apr 19, 2013

### bcrelling

You're right- the laws of physics are time symmetrical.

The exception is the 2nd law of thermodynamics- entropy may be a clue to the why there is less uncertainty in the past than the future. Unfortunately I cannot say more than that.

4. Apr 19, 2013

### tekNiqueAU

To put it simply. The past must be fixed in our reality much like video, u can only rewind it but it will play out the same. But there could be a catch............

The only way the past could be different if time actually went into reverse and wasnt Rewound so to speak...stick with me.

Much like u reverse your car. Take for example you reverse u car back 10 meters from the current spot you drove too, in all likeness your wheels would take slight different track then what you came in on. If "Time" was the car you'd be creating a different timeline in theroy. So the only way would be to be able to reverse time in my mind.

Last edited: Apr 19, 2013
5. Apr 19, 2013

### Crazymechanic

Technically speaking if you would want to be perfectly sure about a given moment in past you would have to know all the particle positions , states and so on at that given moment to build up a precise copy of what the state of things in universe once was , now in big terms we can make approximations about how it unfolded and so but we can never know for sure as just like you said who can track every single decaying atom or change of entropy we usually observe those things only on "overall" levels.
So from that point of view the past is lost once the moment goes away and the next one comes.

6. Apr 19, 2013

### Khashishi

There's a difference between randomness and uncertainty. Quantum mechanics is deterministic at a microscopic scale, yet there is always uncertainty (which is represented as a superposition of measurable states). When a measurement occurs, somehow one of these states is chosen randomly, and we don't understand this. Uncertainty exists in the past, present, and future, but the wavefunction collapse seems to only occur in one direction.

7. Apr 19, 2013

### Digitalism

In addition there is not one local time, but rather as many as there are independent observers because events don't impact a particular reference frame until information travels to it (it enters the light cone of the event).

8. Apr 19, 2013

### DrZoidberg

Thank you. Those were interesting answers.
But one more question - if you are thinking in terms of the multiple worlds interpretation, is the universe more like a tree or like a network(graph theory)?
I'm not asking if the MWI is correct, just want to know how branching works in that theory. If the past is uncertain it should be a network, right?

9. Apr 19, 2013

### Digitalism

I am not very familiar with computer science, but you may be able to represent the branes of m theory using graph theory, however there are higher order levels of connection that I am not sure how one would represent in graph theory (as I am unfamiliar with it). I am not sure what you mean by "like a tree."

10. Apr 19, 2013

### Khashishi

Multiple worlds normally treats the universe as a sort of tree, but I see no reason why a network would be any less valid. Your consciousness would somehow be moving along this network seemingly randomly, where each connection is a physically possible future or past. Perhaps the only difference between future and past is that it's more probable to move to adjacent "future" universes due to some entropy argument. But this is just speculation.

11. May 4, 2013

### mrspeedybob

It depends on what you mean by an "uncertain past"

An uncertain past could mean that the present moment is the result of any one of a number of possible pasts. I'm fairly sure this is wrong. Consider the double slit experiment. If a photon arriving at the screen had an uncertain past in the sense that it could have come via either of 2 routes then there would be no interference pattern. In order to get the interference pattern it had to come via BOTH possible routes.

Extending this principle to a macroscopic scale would mean that the universe, as it is now, is a result of all possible histories. If that is what you mean by "uncertain past" then the answer is yes.