Explaining determinism

HJ Farnsworth

Greetings,

Most of the people I know are not scientific, so they'll often ask me random questions about scientific subjects. One of the questions I get a lot is for a general overview of what quantum mechanics is. What I usually want to communicate to them is that it's a probabilistic theory, and that that's one of the things that are incredibly weird about it, since pretty much all of human instinct says that a theory should be deterministic (of course, some of the interpretations say that there should be a deeper theory that is deterministic, but this is outside of QM as it currently stands, so is besides the point).

Now, here's the problem I keep running into when explaining this to people: when I first heard about the concept of determinism, it was completely obvious to me what it must mean (namely, that given all of the information about a system at one point in time, a determinstic theory will give all of the information about the system for all points in time). It's also very easy for me to see why it's so counterintuitive that our current theory would not be deterministic, since it's counterintuitive to me and, indeed, it took me a long time to allow myself to accept that aspect of QM. However, it's hard for me to explain the concept to people who haven't thought about it before and don't know what it is, and it's even harder for me to explain why a non-deterministic theory is so counterintuitive - it's just so inherently clear to me that it's hard for me to break down and explain.

So my question is: does anyone have a good explanation (analogy, whatever) for the layman of what determinism is, and why it seems "obvious" that a theory should be deterministic, making QM counterintuitive?

-HJ Farnsworth

schaefera

I'd actually add a bit of a complicating factor- even knowing the details of every particle in the world, how could any theory claim to predict the actions of an intelligent being? For example, if you know the details of my brain, you can't know whether in the next second an electrical signal will fire in my brain and make my hand move, say. So how is any theory deterministic in this sense, given the fact that humans and animals can INITIATE behavior that can't actually be predicted.

HJ Farnsworth

Thanks for the reply, schaefera.

Let me know if I misinterpreted what you said, but I think your post is roughly a free will based argument against determinism. Incidentally, when I explain determinism to people and they do get it, often the conversation streamlines to a debate about free will. What I've found is that there are generally three positions of someone to whom I have just explained determinism - they 1) understand it, and it immediately seems highly plausible to them, 2) understand it, but reject it because they don't accept elimination of free will, which is a pretty immediate consequence of it, or 3) don't understand it no matter what I do.

I'm in category 1, and I think you're in category 2 (again, correct me if I'm wrong). The purpose of this thread is for me find a way to explain what determinism is, and why it seems inherently "obvious" that a correct theory of the universe must be deterministic to many scientists (the most prominent example being Einstein), to people in category 3.

Many of the people I attempt to explain it to have never heard the term and, for the most part, are new to science and philosophy, and in some ways abstract thinking in general. So the ideal explanation might be a concrete example, that is as un-abstract as possible.

Thanks again.

-HJ Farnsworth

gmax137

Determinism = things happen for a reason

For example, when that neutron decayed, there was something that changed in it that "made" it decay. And, something happened that made that change happen, and so on.

What could be more intuitive? And yet, QM says that's not the way it works.

the_emi_guy

Interestingly, deterministic does not necessarily mean predictable. See
http://en.wikipedia.org/wiki/Chaos_theory

schaefera

That was just my own musing! I understand determism as you describe, it's just that bit that troubles me. I believe in it, maybe just not on a universal level.

Would this link to the chaos theory article, though, mean that QM is deterministic since at all times the Schrodinger Equation lets us predict the future evolution of a wave function completely? That seems deterministic to me, even if the wave function itself only lets us calculate probabilities for things like position and momentum. The function itself, not the things it describes, are always known perfectly well, no?

the_emi_guy

QM is non-deterministic. The point was that even ignoring QM and assuming we lived in a deterministic world (which we do not), chaos theory still presents a significant barrier to making certain predictions. We may need to measure initial conditions with infinite accuracy, which is not possible.

1MileCrash

I'm not sure I understand that. My understanding of determinism is that it really has nothing to do with what "we" could measure. It only states that if those measurements were known, that any prediction could be made. It doesn't imply that obtaining those measurements is possible by humans or whatever.

My understanding of determinism is simply that given some situation, only one result can follow. Qm is not deterministic but on large scales the probablistic qm "approaches" a deterministic state asymptotically. Or at least thats how I understand it.

HJ Farnsworth

Oops, sorry for the misunderstanding, schaefera.

Diverging from the original topic of the post for a moment, 1MileCrash is exactly correct. A deterministic theory is one that allows you to theoretically calculate outcomes given initial data, a chaos theory is one that says you cannot do so practically. These are not mutually exclusive.

For instance, let's pretend classical physics is perfectly correct, so that we have a deterministic theory that describes the universe, and let's say we want to predict the weather (weather prediction is a chaos theory) for the next thousand years. If we somehow collect data on where every particle everywhere on earth is and how fast it is moving now, then we could do this, in principle (though the calculation might take us more than a thousand years). However, we cannot collect all of the data, so instead we use statistics and say on average, everything is moving at speed X +/- Y, etc. The theory will end up being such that it predicts certain X's, deterministically, but with the uncertainties, Y's, quickly becoming enormous compared with the predicted X's, so that the predictions become meaningless. Basically, a small change in the starting conditions causes huge changes later.

So you can think of a chaos theory as a deterministic theory with the butterfly effect thrown in.

the_emi_guy

It is certainly impractical, but I am thinking that it may, in fact, be impossible to know initial conditions exactly. Let's just consider one particle. Imagine some supreme being wants to bless us with the *exact* speed of this particle. How many significant digits would this be? Wouldn't infinite accuracy mean infinitely many digits in the number?

1MileCrash

I don't see why that's an issue. In beginning physics classes all problems are worked as though the universe is deterministic because for those problems it's a non-issue. The problem with a deterministic universe has nothing to do with it not working mathematically as you're suggesting, it's simply that it's not an accurate description of how the world works.

Look at it in a different way, measurement aside. In a deterministic universe, if the big bang were to happen again, in identical starting conditions, exactly the same way, under the exact same circumstances, everything would unfold in exactly the same way, down to my typing of this message. It's not that this doesn't work out mathematically. It's just that it's wrong.

Vorde

The easiest was to explain determinism to me is by saying that someone could know the entire past and future of the universe by knowing exactly how it is at one moment.

Khashishi

One way to think of it is cause and effect. Every effect has some cause so you should be able to predict all the effects if you know all the causes, if the universe is deterministic. If you think of the universe as one big computer, where every particle follows some specific laws (or programs), then you can just compute what the future is based on these laws. In a non-deterministic universe, there some randomness in the program. More than one effect is possible from a given cause, and there's no way of knowing which one will happen.

HJ Farnsworth

Those are great ways to explain what determinism is, thanks.

Now, any ideas on explaining why it seems intuitively obvious that a theory would be deterministic, and why it disturbs a lot of people that QM is not?

Thanks.

-HJ Farnsworth

bahamagreen

Determinism presents an idea, a concept not just of how things might be, but that they must be - the way they were, the way they are, and the way they will be. Ultimately it leads to a specific conclusion which can be stated a few different ways. It applies to all time, but is usually oriented to the future...

There is only one future (and past and present)
The future is fixed (as is the past and present)
The future is predestined (as is all of time)
The future cannot change from what it will be (kind of an awkward statement since once it happens it becomes the fixed past)
What has happened, what is happening, and what is about to happen could be no other way

The usual implications include additional ideas:

Materialism subject to mechanics - (though not always - like Berkeley)
Strict cause and effect (though this is not always needed for some versions - like Leibniz, or gets redefined, or empirically denied - like Hume)
No free will (this also is tricky and is sometimes preserved with redefinition)

Determinism does not require predictability on our behalf, nor any particular degree of measurement precision... a deterministic world will have been deterministic before we got here, still be deterministic after we are gone, or if we were never here at all.

If one adopts determinism from the bottom up (with whatever hidden variable ideas are needed) then things may seem fine "conceptually" at the sub atomic, atomic, molecular, chemical, and biological levels, but as one rises into considering the levels of human consciousness, volition, and such, lots of problems pop up with determinism.

Here is the main problem with trying to explain determinism to people. Everyones existential experience is that they can make choices, and in fact the whole structure of society is based on learning how to make choices, being responsible for one's choices, and learning to make better choices. The idea that these choices are none other than the determined result of a hierarchy of complex interacting systems does not seem to square with the everyday observation that one can spontaneously make what seems like a totally undetermined action... in spite of some who would say that the conscious experience of determined action will still feel exactly like free will anyway...

1MileCrash

Simple, because on a macroscopic level (all of our experiences) the world is approximately deterministic. It is rooted in our logic, philosophy, mathematics, pretty much everything humans have thought of and developed were based on 'our' experiences of a deterministic world because it wasn't until the quantum level could be studied that we saw that at that level, probability and pure random chance are key factors.

Nature trolled us.

Vorde

Because talking about a theory realistically means talking about a set of equations.

By definition, an equation means if you put an initial state into the equation it should give you a single specific answer back.

Therefore it seems natural that if you start with an initial condition and put that into a physical theory you should get another state which you can iterate back into the theory and get another state and go on and on to potentially know the entire future and past of the universe.

This is the way every classical theory operates (including SR and GR) and is the basis of determinism.

The thing about quantum theory is that it doesn't work that this. No matter how accurate you know something, when you put it through the equations you get a range of answers, not just a specific one.

As a result of this, 'not even god' could know the future. It also implies that there is an innate (albeit predictable) randomness that is in our universe.

A lot of people refuse to believe that you can't know something exactly, and see the fact that you can't within quantum theory as a fatal flaw within QM. But the bottom line is that there is so much good experimental evidence towards QM that mainstream scientists have accepted this innate randomness.