Laws of Physics and Predetermination

Call Me Jimmy

Layman here, but if the universe is governed by laws of physics (of which we don't know all but say we did) would that mean it is all predetermined, that there is only truly one path forward in time? (Does time ever move backwards?)

Once say the big bang happened, things would move along based on the laws and the laws would only have one outcome based on one initial condition. I'm thinking in terms of a perfect simulation, with all the initial conditions and laws known or knowable.

I don't know much about physics but interested to hear.

endacl

Brownian motion ftw.

You can't avoid random.

Call Me Jimmy

Is random not a concept that we have to paper over things we don't know or incomplete information?

EDIT: just reading about brownian motion now

endacl

On a grand scale, at the macro level, we can predict what'll most likely happen. Although the question is rather moot, as we won't be around to see our predictions validated or otherwise. The further down the scale you go, the less predictable physics gets.

Food for thought: if the universe emerged from a singularity expanding at an exponential rate into nothingness, why is it not uniformly flat? Because of a little unpredictability, a little imbalance, and a little random. The fact that you're here to ask the question is your answer!

srsly78

http://en.wikipedia.org/wiki/Maxwell%27s_demon

Carawaystick

Call Me Jimmy wrote: »

Is random not a concept that we have to paper over things we don't know or incomplete information?

EDIT: just reading about brownian motion now

Heisenburg Uncertainty applies
you can't know where you are and where you're going below a certain level of accuracy, so definite initial conditions are unknowable.

endacl

Carawaystick wrote: »

Heisenburg Uncertainty applies
you can't know where you are and where you're going below a certain level of accuracy, so definite initial conditions are unknowable.

Story of my life...

seamus

So, you broadly have two branches of physics - Classical Physics (often called Newtonian Physics) and Modern Physics.

Classical physics are the rules and formulae discovered before the 20th century by the likes of Newton and Galileo. Modern Physics are those discovered after by the likes of Einstein, Plack and Hawking.

The two aren't mututally exclusive, but by and large classical physics is best at describing how things happen on a human scale.
So big and slow-moving objects. This is predictable motion, predeterminable in the way that you describe.

Once you get into smaller objects (molecules) and/or faster speeds (> 0.1% of lightspeed), classical physics starts to break down and become less accurate, and the ability to make deterministic predictions falls apart.

Calculations then become more about probability than about black-and-white outcomes. So while at a macro level it seems to make sense that the motion of particles is predictable and therefore the universe is deterministic, at an atomic scale motion is not predictable and therefore the universe cannot be deterministic.

But obviously this makes it far more difficult to explain and work with, so for most everyday applications to do with engineering and teaching core physics to people, classical physics is perfectly adequate and accurate.

Call Me Jimmy

Re the heisenberg uncertainty, 'the more precisely the position of some particle is determined, the less precisely its momentum can be known'

Okay does this mean below a certain threshold there are no laws governing the movement or just that we cannot measure precisely and hence can't derive laws (other than to say things are seemingly random)?

Can someone address my point on randomness? e.g. Before we knew about meteorology we would say that the weather is 'random'.

Thanks

Call Me Jimmy

seamus wrote: »

So, you broadly have two branches of physics - Classical Physics (often called Newtonian Physics) and Modern Physics.

Classical physics are the rules and formulae discovered before the 20th century by the likes of Newton and Galileo. Modern Physics are those discovered after by the likes of Einstein, Plack and Hawking.

The two aren't mututally exclusive, but by and large classical physics is best at describing how things happen on a human scale.
So big and slow-moving objects. This is predictable motion, predeterminable in the way that you describe.

Once you get into smaller objects (molecules) and/or faster speeds (> 0.1% of lightspeed), classical physics starts to break down and become less accurate, and the ability to make deterministic predictions falls apart.

Calculations then become more about probability than about black-and-white outcomes. So while at a macro level it seems to make sense that the motion of particles is predictable and therefore the universe is deterministic, at an atomic scale motion is not predictable and therefore the universe cannot be deterministic.

But obviously this makes it far more difficult to explain and work with, so for most everyday applications to do with engineering and teaching core physics to people, classical physics is perfectly adequate and accurate.

at an atomic scale motion is not predictable, is it possible that is from a lack of information (or ability to measure) or is it more a mathematical certainty?

endacl

Call Me Jimmy wrote: »

Re the heisenberg uncertainty, 'the more precisely the position of some particle is determined, the less precisely its momentum can be known'

Okay does this mean below a certain threshold there are no laws governing the movement or just that we cannot measure precisely and hence can't derive laws (other than to say things are seemingly random)?

Can someone address my point on randomness? e.g. Before we knew about meteorology we would say that the weather is 'random'.

Thanks

For concept purposes, you don't have to think of uncertainty in fancy shmancy sciencey talk. Imagine a car belting down the M50. A measurement of its speed is a product of time x distance. If you want to measure its location precisely, at a single point, it's speed will be zero, as you won't have two points to measure the distance travelled in a set time period.

srsly78

Think if it as the act of measurement itself interferes with the system.

Maximus Alexander

Call Me Jimmy wrote: »

Re the heisenberg uncertainty, 'the more precisely the position of some particle is determined, the less precisely its momentum can be known'

Okay does this mean below a certain threshold there are no laws governing the movement or just that we cannot measure precisely and hence can't derive laws (other than to say things are seemingly random)?

Can someone address my point on randomness? e.g. Before we knew about meteorology we would say that the weather is 'random'.

Thanks

It's not a measurement issue. It's not a case that it can be known but we don't have the tools. When you get to a certain scale it actually becomes unknowable. It's fundamentally not possible to know both an electron's position and momentum, for example. It's not that there are no rules, the rules just become governed by probabilities.

Or as wiki puts it:

Thus, the uncertainty principle actually states a fundamental property of quantum systems, and is not a statement about the observational success of current technology. It must be emphasized that measurement does not mean only a process in which a physicist-observer takes part, but rather any interaction between classical and quantum objects regardless of any observer.

Morbert

Call Me Jimmy wrote: »

Layman here, but if the universe is governed by laws of physics (of which we don't know all but say we did) would that mean it is all predetermined, that there is only truly one path forward in time? (Does time ever move backwards?)

Once say the big bang happened, things would move along based on the laws and the laws would only have one outcome based on one initial condition. I'm thinking in terms of a perfect simulation, with all the initial conditions and laws known or knowable.

I don't know much about physics but interested to hear.

That is correct. In a deterministic universe, if the state of a system at a particular time is perfectly known, the state at any other time in the future or past can be derived from the laws of physics. It is a somewhat more philosophical question to ask if the present "determines" the future, or whether the past present and future are all just nomologically related.

The laws of quantum mechanics, however, have a fundamentally different character. In classical mechanics, we are used to thinking of a system as having absolute properties evolving with time. In quantum mechanics, properties of a system are defined by how the system interacts and correlates with its environment. This "operational" quality of quantum mechanics means we can't talk about properties independent from observation. We can only talk about correlations between properties, and the probabilities of observing different values.

For the sake of balance, I should say there are attempts by reputable scientists to recover a deterministic or objective conceptualisation of reality from quantum mechanics (Bohmian mechanics, Many Worlds, etc).

Call Me Jimmy

Very interesting. Maybe in the back of my mind was this article about a new paper https://www.quantamagazine.org/20140122-a-new-physics-theory-of-life/. I was thinking there may be some force or tendency that everything has (even on the quantum) level that would mean that it was deterministic.

I still can't wrap my head around randomness at the quantum level. It seems like a very unscientific concept.

Call Me Jimmy

Morbert wrote: »

The laws of quantum mechanics, however, have a fundamentally different character. In classical mechanics, we are used to thinking of a system as having absolute properties evolving with time. In quantum mechanics, properties of a system are defined by how the system interacts and correlates with its environment.

Are these systems conceptual or real? As a human I'm a system but I can't exist without my environment, so we are the one system strictly speaking? If that's correct, why treat quantum systems as seperate from their environment (which includes observers and observing actions)?

Morbert

Call Me Jimmy wrote: »

why treat quantum systems as seperate from their environment (which includes observers and observing actions)?

This is where it's useful to make a distinction between the observable properties of a system and the system itself.

In a way, textbook QM always describes observable properties of a quantum system in the context of its environment. When the spin of a particle is described as "a|up>+b|down>", what is meant is, if an observation is made of the particle's spin, there is an |a|^2 probability that 'up' will be observed and a |b|^2 probability that down will be observed. This is why I don't like the pop-science explanation of the particle having "both up and down superposition" spin, which amounts to adding a layer of interpretation to QM in an effort to talk about observables outside the context of an observation. It is an attempt to restore counterfactual definiteness.

But it is still useful to talk about the quantum system itself as separate from its environment, provided we don't try to say it has well defined observables. For example, if we do not place detectors by the slits of a double-slit experiment, we do not force the particles to correlate with the environment before they strike the screen, and so an interference pattern is seen. And isolation of a quantum system from its environment is crucial for exploiting quantum computation. Also, the universe as a whole might be a closed system, and would therefore be an isolated quantum system.

sponsoredwalk

endacl wrote: »

For concept purposes, you don't have to think of uncertainty in fancy shmancy sciencey talk. Imagine a car belting down the M50. A measurement of its speed is a product of time x distance. If you want to measure its location precisely, at a single point, it's speed will be zero, as you won't have two points to measure the distance travelled in a set time period.

The Heisenberg uncertainty principle, "there's no concept of the path of a particle", says that even in the case where you take two points to compute a velocity that velocity still wont exist because the particle did not follow a path from one point to the next, so it's like a layer on top of your example lol

Call Me Jimmy wrote: »

I still can't wrap my head around randomness at the quantum level. It seems like a very unscientific concept.

All you need to remember is that a particle does not follow a path between two points (Heisenberg), but that it should look like the particle follows some path viewed from a large distance scale (quasi-classical approximation). This signifies the end of classical mechanics and classical intuition, but says it should roughly hold. All we can do at this stage is use the mathematical notion of 'expected value'

https://www.youtube.com/watch?v=j__Kredt7vY
https://www.youtube.com/watch?v=EmDEr9r1bn0

to ask where do we expect to find the particle (measurement & the Born rule). Using two ideas from 'linear algebra' we can just re-write the expected value like this



and this is the origin of the mathematical formalism of quantum mechanics, all that matters is the expected value of the operator we just measured, and the math I use is up to me (density matrices, wave functions, Hilbert space version of probability, etc...). So, once we have this, we just pick up a classical mechanics book and ask how those ideas are modified to ensure no path exists except in the quasi-classical approximation.

Morbert wrote: »

For the sake of balance, I should say there are attempts by reputable scientists to recover a deterministic or objective conceptualisation of reality from quantum mechanics (Bohmian mechanics, Many Worlds, etc).

But they are either massively flawed
http://motls.blogspot.ie/2009/01/bohmists-segregation-of-primitive-and.html
too ill-founded
http://motls.blogspot.ie/2014/07/many-worlds-pseudoscience-again.html
https://ndpr.nd.edu/news/24515-many-worlds-everett-quantum-theory-and-reality/
and pretty much lead to more ill-founded craziness if taken seriously
http://phys.org/news/2015-02-big-quantum-equation-universe.html