What's the universe collapsing into?

[Derek Potter]

What if you don't register it? A Stern-Gerlach apparatus can run even if no-one's looking. In that case I don't see any trace of that information anymore.

You may not see it but it is there, scrambled in a gazillion entangled microscopic changes in the apparatus and environment. You must distinguish between classical erasure which is just deleting (readable) data and quantum erasure which undoes *all* of the microscopic entanglements. This is why they do quantum erasure experiments with photons, photons don't interact with the environment very much so observations made with simple observers, *can*, with immense effort and care, be erased.

 

[lowkee]

Hey guys new to this forum sorry for derailing the thread, but can some of you guys suggest any books or websites on this type of stuff or quantum physics in general. Trying to get my head around this stuff so layman's terms might help. Cheers

 

[Derek Potter]

Hey guys new to this forum sorry for derailing the thread, but can some of you guys suggest any books or websites on this type of stuff or quantum physics in general. Trying to get my head around this stuff so layman's terms might help. Cheers

This *is* all in layman's terms!
Sorry, couldn't resist.

 

[friend]

The universe doesn't collapse into anything because the universe can't be observed which would requre something outside the universe interacting with it that by the definition of the universe is impossible.

Even if we can't measure anything because there is nothing outside the universe to interact with it to cause a collapse to a particular value,... If all we have is the universal wave function, that would still allow calculation of possible eigenstates and their probabilities, right? The question is: do we have enough information to derive in theory a universal wave function? If not, what do we need?

 

[bhobba]

Even if we can't measure anything because there is nothing outside the universe to interact with it to cause a collapse to a particular value,... If all we have is the universal wave function, that would still allow calculation of possible eigenstates and their probabilities, right?

The probabilities thus obtained would be meaningless.

Its an open question if the state of the universe is a valid concept.

Thanks
Bill

 

[friend]

The probabilities thus obtained would be meaningless.

The whole point of physics is to gain confidence in where the universe is going.

 

[bhobba]

The whole point of physics is to gain confidence in where the universe is going.

The whole point of physics is to understand the fundamental laws of nature.

So far the answer to that has been surprising - symmetry.

Thanks
Bill

 

[Derek Potter]

The probabilities thus obtained would be meaningless.
Its an open question if the state of the universe is a valid concept.

Open in the sense of depending on interpretation and thus inherently unanswerable, or open in the sense of subject to ongoing research which stands a reasonable chance of answering it before the insects take over ?

 

[bhobba]

Open in the sense of depending on interpretation and thus inherently unanswerable, or open in the sense of subject to ongoing research which stands a reasonable chance of answering it before the insects take over ?

Open in the sense of if its a meaningful concept at all - and that has interpretive aspects (eg in that statistical interpretation state is a preparation procedure - what prepares the universe?)- but that will take us way off topic and requires another thread.

Thanks
Bill

 

[friend]

What would it take to write the universal wave function? What do we need to write any wave function? we need the starting state, and then what...? The hamiltonian or lagrangian for each particle, and then what?

 

[bhobba]

What would it take to write the universal wave function? What do we need t write any wave function? we need the starting state, and then what...?

A model eg
http://www.superstringtheory.com/cosmo/cosmo41.html

Some question if such are meaningful.

Thanks
Bill

 

[Derek Potter]

A model eg
http://www.superstringtheory.com/cosmo/cosmo41.html
Some question if such are meaningful.

I choose to interpret it as meaningful.

 

[stevmg]

According to William Thomson (Lord Kelvin) who predates Planck et al, entropy is always increasing implying that the total energy level will asymptotically approach some unspecified universal mean of total neutrality in some very distant future. Of course, Lord Kelvin was a Creationist (not the YEC type, though) who thought the Earth was only, say, 3-4 million years old.)

 

[bhobba]

According to William Thomson (Lord Kelvin) who predates Planck et al, entropy is always increasing implying that the total energy level will asymptotically approach some unspecified universal mean of total neutrality in some very distant future.

There is this law called conservation of energy that means what you wrote above is incorrect. Its entropy that increases - chaos is coming - energy remains the same.

Thanks
Bill

 

[Dr_Zinj]

Friend said, "We are certain of the past but not sure of the future."

I'm not so sure that statement is true. When we talk about the collapse of a wave function, we're talking about what it is observed to be a a single point in time. However, the uncertainty of the result still exists in the past. If we see a photon go through the right slit now, our uncertainty of 5 minutes ago would still exist in the past. 5 minutes ago that collapse hasn't happened. However, if I return to the past of 5 minutes ago knowing which slit it will go through in 5 minutes, will the waveform collapse when I reach 5 minutes ago, or will it continue until we reach the future where we see it go through the right slit? And if it doesn't collapse when I reach 5 minutes ago, and this time goes through the left slit, have I caused a fork in the futures of the universe?

"t" looks so easy in an equation on paper.

 

[friend]

Its entropy that increases - chaos is coming - energy remains the same.

Has entropy been proven on first principles of QM or GR?

 

[bhobba]

Has entropy been proven on first principles of QM

Of course:
https://en.wikipedia.org/wiki/Quantum_statistical_mechanics

Thanks
Bill

 

[friend]

Has entropy been proven on first principles of QM or GR?

Actually, I meant: Can the 2nd law of thermodynamics be derived from first principles?

 

[bhobba]

Actually, I meant: Can the 2nd law of thermodynamics be derived from first principles?

Yes - but the math is deep:
https://terrytao.wordpress.com/2008/01/30/254a-lecture-8-the-mean-ergodic-theorem/

The above is a very famous theorem. The trouble is it isn't quite general enough to fully provide the foundations to statistical physics. The key theorem required is still unproven:
http://library.lanl.gov/cgi-bin/getfile?00285754.pdf
http://www.pnas.org/content/112/7/1907.full.pdf

Thanks
Bill