Role of quantum mechanics in cosmology

[pvk21]

According to laplace,universe is totally deterministic.(one can tell position of object future position of object if he knows current position and velocity)but Heisenbergs's tells us that there is always uncertainty in position of particle and velocity.but Heisenberg's uncertainty only applies to small particles.well we can predict position and velocity of large object.so we can still say that universe is deterministic.but it isn't.why is that?

 

[rootone]

I think what you are asking is, why is it that if things are not predictable on a quantum scale, then what makes them predictable in large scales?
Roughly I would say that this is because at large scales quantum effects tend to cancel out statistically, so we can make generalised statements that will always be true without having to take the uncertainty principle into account.

 

[pvk21]

Well ok that's the case.so can we predict the future of universe?

 

[rootone]

Well obviously we can't ever know in practice the exact state of everything on a macroscopic scale.
If we could, then we should be able to make a reasonable forecast of what will happen on a macroscopic scale, (if the theories applied are true).
We should be able to draw a conclusion while still be no wiser concerning the position of a particular electron.

The computer required to make this calculation would probably need to be bigger than the universe though ;)

 

[pvk21]

So we will be able to predict the future.if we have computer powerful enough.well in the book I was reading,there was one statement that we can't predict future due to uncertainty principle.that what my point is 'can we predict the future if we have all required resources or we can't because of uncertainty in position and velocity'

 

[Drakkith]

Well ok that's the case.so can we predict the future of universe?

To some degree of accuracy, based on the time and size scales involved in our predictions, sure. For example, we can be sure that the Andromeda galaxy is going to approach the Milky Way over the next few billion years, but we can't accurately predict where any individual star may end up. (More due to difficulties doing the staggeringly huge amount of calculations needed than any quantum uncertainty) If you want to talk about where an individual proton in the Sun's stellar wind is, then you may run into quantum uncertainty.

So we will be able to predict the future.if we have computer powerful enough.well in the book I was reading,there was one statement that we can't predict future due to uncertainty principle.that what my point is 'can we predict the future if we have all required resources or we can't because of uncertainty in position and velocity'

You can't predict the future to perfect accuracy, no, but predicting the future with greater than 99.999999% certainty, at minimum, still counts as 'predicting the future' in my book.

 

[pvk21]

So there is really no use of quantum mechanics in cosmology?except predicting what happened at big bang.

 

[Drakkith]

So there is really no use of quantum mechanics in cosmology?except predicting what happened at big bang.

I wouldn't say that. I know it's important in nucleosynthesis. I'm sure there's other areas too.

 

[pvk21]

Cool.thanx drakkith.quantum mechanics is pretty exciting field.

 

[wabbit]

Cosmology does predict a lot of things about the future (e.g. lots of expansion and an increasingly lonely galaxy for us), like Newtonian mechanics predict the evolution of the solar system. I think that the scales concerned aren't very much affected by quantum uncertainty, probably more by chaotic unpredictability in deterministic systems. But does it matter if we can't tell exactly what the distance of the sun to a certain star in Andromeda will be in a billion year ? The prediction of a future collision of Andromeda with the Milky Way and how it might look at large scale surely is at least as interesting : )

One thing we can't predict as well is the weather next week.

 

[wabbit]

I would also venture that the most interesting predictions tend to be about some coarse grained model of a complex system, and these can be accurate in some aspects long after they lose the ability to track every individual component, depending on the system under consideration.

 

[Chalnoth]

According to laplace,universe is totally deterministic.(one can tell position of object future position of object if he knows current position and velocity)but Heisenbergs's tells us that there is always uncertainty in position of particle and velocity.but Heisenberg's uncertainty only applies to small particles.well we can predict position and velocity of large object.so we can still say that universe is deterministic.but it isn't.why is that?

One way to understand this is to state that the fundamental laws are perfectly-deterministic, but observations are not. This is what happens when you just use the wave equations of quantum mechanics: the appearance of collapse that arises from systems interacting with one another gives the appearance of a random result.