Some of you guys know that I was in IISSQI08 for the last two weeks. For me, this summer school was a great achievement in two ways: first, taking part in some fantastic lectures on quantum computing given by eminent international professors and secondly, having brilliant conversations with these lecturers and some new friends. One of these conversations was with David Gross who is a positivist physicist.
The conversation started just after his talk. He finished the talk by explaining the GHZ experiment and the Bell’s inequality to conclude that nature cannot have a deterministic AND local description.
Sina: It can only show that nonlocality cannot have a classical description. What if someday we had a local and deterministic physical theory which is not classical?
David: Like what? Look… This would be a classical theory and since you think classically you are not able to realize it! I can accept if somebody brings a physical theory that is deterministic (X)OR local. But it can’t be both deterministic AND local!
(At that time I couldn’t remember the Multiverse theory to bring an example. Actually, I by myself am not yet convinced by this theory. However it could be a good example of non-classical local and deterministic theory)
Sina: Assuming you are right, so the theory of everything won’t give us any better result than unification of the four forces! It is still indeterministic and is not able to describe the reality of nature!
David: This is the same mistake that Einstein did! You think that e physical theory must satisfy these conditions to be a complete description of reality. However, I believe that the nature is essentially not deterministic and a theory which claims to describe everything must be indeterministic too. Although classical mechanics is working well in classical sizes, but it is an approximation of the reality and it is not true to be considered as theory that is capable of making precise predictions.
Sina: Do you also believe that classical propositional logic is also imprecise?
David: You are entering a dangerous field, ha-ha… No! Why should I say such a thing?
Sina: Because of the intuition one can get from observing the relationship between physics and mathematics. Everything in physics has a counterpart in mathematics and vice versa! It can be seen in the relationship between physics and mathematics. Just like non-Euclidean geometry and relativity!
David: It was a great example. Here is where no one has an answer! No one knows why! But it doesn’t mean that we are allowed to build such a relationship. A counter example is that you deal with finite numbers in physics but you have infinity in mathematics.
Sina: There are some physicists who believe the universe in infinite. On the other hand there are mathematicians (intuitionistic logicians) who do not believe in infinity!
David: But you can image the largest length in a finite world while the largest number does not make sense in mathematics!
Sina: OK! Then if our universe is governed by the laws of quantum mechanics and in this theory there shouldn’t be any assignment to any object that is not yet measured, how do you answer this paradox: A star is born out of reach of any conscious observer. Two thousand years later, when we see the light we analyze it and find out that the star was born 2000 years ago, when nobody saw it!
David: This is the state of the star at time t=0
So the state of the system at this time is
Then at t=1 is
Finally at t=2000 you get
There is a superposition of you when you saw the star and the other you. One who is able to measure you from beyond your system will see you in one of these states with a certain probability and the other state in another probability!
The observer must stand out of the system.
Sina: But when I saw the light I realized that the star was born when I couldn’t see!
David: Physical theory doesn’t answer the questions like “when things happened”. It just gives predictions about the nature. You want to know: what really happened?! However, I want to know: what will I observe? I would be very happy if someone could answer the first question, but I believe that no one will do that! So I prefer to choose the second question.
There are people on your side. If you want to choose the first question I suggest you read David Deutsch’s book: The Fabric of Reality.
(I took his advice and just ordered the book from Amazon)
October 7, 2008 at 8:42 am |
Is the moon there when nobody looks ?
October 7, 2008 at 12:26 pm |
I believe so! But note that it has been observed some (!) years ago!
October 7, 2008 at 8:11 pm |
That was a nice chat!
Nature is essentially not deterministic, I agree with him in this case, I think determinism and definiteness are not in fact universal properties of nature but rather properties of macroscopic objects.
You said that, “ Everything in physics has a counter part in mathematics and vice versa! “ I only know that in classical physics there exists a one-to-one correspondence between the physical properties of the object and their formal and mathematical representation in the theory and in quantum physics this property is gained through measurement. But there is no way of saying such a relation does exist!
I think the fact is that science is going in a way that its purpose is to describe human experience only and not to describe what really is, I can say that it’s properly capable of predicting what will be observed in all possible circumstances. As you all know that our knowledge is based on experience which is on observation.
But I can say that although there is a lot of murkiness in quantum theory, it seems a fair bet to say that classicality is at root another quantum phenomenon, and classical physics no longer sits in opposition to quantum mechanics but is demanded by it! And that will lead us to conclude that quantum mechanics is universal!
October 13, 2008 at 7:53 pm |
I’d really like to know what would make a physicist BELIEVE that no one will answer the “what really happened” question?
October 14, 2008 at 5:31 am |
Leila,
you say:
> in quantum physics this property is gained through measurement. But there is no way of saying such a relation does exist!
Heisenberg is telling me that he has just found correspondence between matrix algebra and the quantum theory! Schrödinger is also unhappy with your statement! He has his own picture!
> I think the fact is that science is going in a way that its purpose is to describe human experience only and not to describe what really is
So you are pissing subjective sciences off, ha? I could think about it if you have said “physics” instead of “science”! Even at that particular occasion I wouldn’t agree!
(A not too related comment)_Unfortunately_ science is going in a way where money goes!
> And that will lead us to conclude that quantum mechanics is universal!
I don’t believe so! Ha-ha! What do you mean by universality?
October 14, 2008 at 5:32 am |
Afsoon,
I also believe that it is quite unlike to know what really happens! The argument is very philosophical and the scientific part is much mathematical! However, briefly you can see the Gödel’s incompleteness. It states that you can never prove the consistency AND completeness of a system! Therefore, some of the things are always beyond your capture. While this is true, you can never make sure what really happened!
October 16, 2008 at 9:44 am |
well I see what you’re trying to say & I pretty much I agree with you but let me restate my point this way; for those who want to know what has really happened there are always things beyond their capture & they can never fully understand the nature since we are nature’s subsets, but what will those who have lowered their goals to knowing what they’ll observe, achieve?
is the fact that we can never fully understand the nature reasonable enough to give up on even trying to understand it in the best way possible?
PEACE
October 16, 2008 at 4:41 pm |
If we go back to my first comment, I’ve wrote: our knowledge is based on experience which is on observation. I think this is enough to say that science’s (by science I totally meant physics!!) purpose is to describe human experience. I can understand why David Gross prefers to know what he will observe instead of attempting for the impossible! (As he describes it)
Although we say that quantum mechanics is not complete yet and it does not give us a complete description of reality, but I think there are good reasons to believe that we are as much part of the quantum world as a single atom is (which I meant by universality). Experimental physicists have gain mesoscopic and even macroscopic superpositions, such as superconducting quantum interference devices (SQUID’s) in which superposition of macroscopic currents become observable. There still exists the measurement problem which is the big hole in quantum mechanics but I think quantum mechanics is moving on.