Physics distinguishing future from past

[watchamadoodle]

(Sorry in advance if this topic has already been covered recently.)

What physical laws distinguish future from past? For example, I imagine some of the laws about entropy would distinguish future from past (like heat spreading). Is there anything besides entropy, and which rules of entropy specifically?

I was wondering if this relates to causality somehow? If the universe is completely deterministic, would there be a way to distinguish future from past or cause from effect?

I'm thinking about the "first cause" argument for God. Some Gnostics believed that God was a "unity". What would entropy and thermodynamics suggest as the starting point - a single bit?

[Surgenator]

1) 2nd law of thermodynamics.
2) Retarded potential (<-- not a joke, that is what it is actually called)

[Alex K]

Surgenator already said it, in classical physics, the arrow of time arises as a statistical phenomenon. The classical laws of physics are time reversal invariant. If you look at a system of masses moving according to newtonian laws, and play it backwards, you will again see a system of masses moving according to the same newtonian laws, but with different initial conditions. Incidentally, Surge, it is exactly the choice of inital conditions which tells you whether to use retarded or advanced potentials (since they are both perfectly fine Greens functions of the wave equations).

My favourite example is as follows: imagine an otherwise empty space with only two masses which attract each other with newtonian gravity for example. They will have some mutual kepler orbit. If you let run time backwards in this system, you will not be able to tell that it's backwards. There is no arrow of time.

If you add one more mass, the situation already changes: if you start with a constellation of three gravitationally bound masses and let the system run, it can happen that one of the masses gets a lot of kinetic energy by accident and is ejected from the system. If you look at this film backwards, you will see a particle coming from infinity and hitting a two body system at just the right energy to get integrated in it.

You'd see that and say: what are the odds!

And that's exactly the point, as soon as you have a more complicated system, the arrow of time is defined by going from less to more likely states. And that's what entropy is.
The analogy in electromagnetism: if you calculate the emissions of a radio antenna using advanced potentials, you'll get a radio programme coming in from the infinities of space and hitting the antenna at just the right point and moment to correspond to the electrons moving in the antenna. A very unlikely scenario.

If you have physics going on depending on a time parameter, future is the direction of increasing total entropy. If you have a minimum of entropy at some point and it goes up in both directions, you have two universes in which time runs the opposite way, which are connected at their temporal origins. Cool, huh.

Now, there are two more things that boggle my mind: first of all, quantum mechanical measurements in the copenhagen interpretation break this invariance as far as I understand it, because the wave function collapse happens only when going to the future. In the MWI, there is no problem afaik, it's again perfectly invariant under time reversal.

Secondly, the weak interactions have a term which violates time reversal, but it's extremely weak and can't possibly be the source of the arrow of time now. However, it could have something to do with the reason why a low entropy state was created.

---

1) 2nd law of thermodynamics.
2) Retarded potential (<-- not a joke, that is what it is actually called)

"Retarded potential" is my middle name.
I'll look it up and see if I understand it... thanks.

The retarded potential is given as that solution of maxwells equations where a movement of charges produces a wave which moves away as time progresses. The advanced potential is the opposite: a wave comes in and cancels with the movement of the charges. The retarded potentials are needed to describe the usual situation of an antenna emitting something.

---

I was wondering if this relates to causality somehow? If the universe is completely deterministic, would there be a way to distinguish future from past or cause from effect?

To answer that question, yes, also via entropy

I'm thinking about the "first cause" argument for God. Some Gnostics believed that God was a "unity". What would entropy and thermodynamics suggest as the starting point - a single bit?

The first cause argument for God is nonsense anyway. You'd first have to do their work for them and come up with a proper definition of God which allows you to say something about its thermodynamics

[watchamadoodle]

1) 2nd law of thermodynamics.
2) Retarded potential (<-- not a joke, that is what it is actually called)

"Retarded potential" is my middle name.
I'll look it up and see if I understand it... thanks.

[vorlon13]

As time goes forward the universe expands. It has been speculated with the right astronomical equipment, the change in the redshift of extremely distant objects would be discernable in regards to their increasing distance over a period of some decades. (If I recall the details correctly, and such observations might commence in 10 to 20 years)

[Alex K]

As time goes forward the universe expands. It has been speculated with the right astronomical equipment, the change in the redshift of extremely distant objects would be discernable in regards to their increasing distance over a period of some decades. (If I recall the details correctly, and such observations might commence in 10 to 20 years)

I didn't know that, and that's extremely awesome!

[vorlon13]

Watching the redshift of an extremely distant object change is heady stuff.

Wish I could remember where I saw it.

How long ago is the past, and how long do you have to wait for the future?

I think it is something like 10^-43 seconds, or some such.

Ooops, you missed it . . . .

[watchamadoodle]

If you add one more mass, the situation already changes: if you start with a constellation of three gravitationally bound masses and let the system run, it can happen that one of the masses gets a lot of kinetic energy by accident and is ejected from the system. If you look at this film backwards, you will see a particle coming from infinity and hitting a two body system at just the right energy to get integrated in it.

You'd see that and say: what are the odds!

The problem I see with this idea, is that I could watch the third body zip past the other two bodies with a deflection, and I could say "what are the odds! That's just exactly the right deflection to start forming the "w" on "watchamadoodle". Intuitively what you said makes sense, but it reminds me of the argument for God where people say "what are the odds!"

That's really interesting on the QM interpretations. I don't understand the details, but it seems the copenhagen interpetation might have an advantage by mirroring the way that thermodynamics distinguishes past and future?

[watchamadoodle]

Is there any evidence that the future influences the past?
"Science Set Free" by Rupert Sheldrake mentioned that Whitehead ( http://en.wikipedia.org/wiki/Alfred_North_Whitehead ) had an interpretation of QM that suggested the future influencing the past. That book by Sheldrake makes other claims that I googled and found were false, so I'm curious what you guys think. (I can google, but the physics and math is all Greek to me. )

[Surgenator]

Can you post the link to the "math." The wikipedia article doesn't seem to mention the 'future influencing the past.'