how i became a physicist

When I studied physics (decades ago) there were two exams feared by all students.

One was classical mechanics, a course with the (unofficial) goal to eliminate more than 3/4 of the students. If you wonder how classical mechanics can be so difficult, let me only mention that e.g. Arnold Sommerfeld wrote a four volume master piece about the spinning top. Somehow I passed this exam, which only got me to the next big hurdle: inorganic chemistry.

It was common knowledge among students that the professor had a habit of asking 'weird questions' during the exam and one only had a chance to pass by attending the lectures (where he would mention and discuss them). Unfortunately, those lectures were scheduled early in the morning and since I lived outside the city, it would have meant to get up around 6am or even earlier and this was out of the question for me.

My plan was to skip the lectures, read the book of the professor and bluff my way through the exam.



My luck was that he examined three students at a time, so I had one or two questions and struggling students in front of me to understand what was so 'weird' about this.
It turned out that he was a closet philosopher, e.g. when he asked about the size of a hydrogen atom one could not talk about ångström, but the answer was that an atom does not really have a size because the wave function of the electron extends to infinity.

So when he asked me about the pre-requisit of every measurement, I talked about the basic structure of time and the distinction of past and future (because a measurement is not possible if one cannot distinguish 'before the measurement' from 'after the measurement') and we settled on the fact that the world is inhomogeneous as the basic pre-requisit. Subsequently we discussed the logic of Newton's absolute time and finally he accused me of using a Machian approach and I replied that his methods were close to dialectical materialism (I am not kidding). He was thrilled and I suspect it was not too often that he enjoyed an exam as much as mine.

After discussing transubstantiation he actually asked a few questions about inorganic chemistry and my knowledge about it was unfortunately limited (I remember the oxidation of lanthanum as a stumbling block).

He noticed it, but I still passed the exam (to the great astonishment of my colleagues) at the first try.



I wonder if a subconscious trauma from these exams is the reason I keep writing this blog.


consistent histories

This post is from one PA member to the other.



First, let me be clear that the interpretation problem in quantum theory is a real problem. Schroedinger's wave function does not match with our direct experience of reality. Several very different proposals have been made on how to solve this problem and in my opinion none of them is very convincing.



Furthermore, decoherence does not solve the measurement problem and this is all there is to say about that.



As for the many worlds proposal, the big problem is to get probabilities right but
if one is interested in consistent histories, there are two good papers about it. We read that
"their explanation of the apparent persistence of quasiclassicality relies on assumptions about an as yet unknown theory of experience".



I think this is the key. We don't have a theory of (conscious) experience and therefore there is an interpretation problem even in classical physics, but usually overlooked and well hidden.

Ernst Mach was one of the few who noticed it but he was pushed to the side long ago.


recently, at the PA meeting

My name is Wolfgang B. and I am a recovering physicist.



Hi Wolfgang.



Unfortunately, I still need my daily dose of formulas, otherwise my brain begins to come up with crazy ideas.

But recently I found free supply of high quality stuff: quantum field theory and strings.

Perhaps you want to check it out too.



OMG! And with problem sheets...


the last philosopher

I understand Friedrich Nietzsche as the last philosopher.

Of course, some will argue that this honor really belongs to Wittgenstein and indeed his proposition 7 would make for a very slick end to the whole project.

But there is no way around the fact that it was all pretty much said already in Beyond Good and Evil.


1+2+3+4+...

"I told him that the sum of an infinite number of terms of the series: 1 + 2 + 3 + 4 + · · · = −1/12 under my theory. If I tell you this you will at once point out to me the lunatic asylum as my goal."

Ramanujan to Hardy



In my opinion it is easier to see that s = 1 + 2 + 4 + 8 + ... = -1, by writing
s = 1 + 2*( 1 + 2 + 4 + ... ) = 1 + 2s and from s = 1 + 2s it follows that s = -1.


Ernst Mach about blogs

"Man possesses, in its highest form, the power of consciously and arbitrarily
determining his point of view. He can at one time disregard the most salient
features of an object, and immediately thereafter give attention to its smallest
details; ...;he can rise at will to the most general abstractions or bury
himself in the minutest particulars.

...

A further consideration [reveals that] the links to colors, sounds and other sensations appear to be more
real than the colors, sounds and other sensations themselves.

...

But as soon as we have perceived that [blogs] are only makeshifts, designed for provisional
survey and for certain practical ends, we find ourselves obliged ... to abandon them
as insufficient and inappropriate."

source


soggy not crispy

Please explain this to me.

When I take bread from the fridge (cold not frozen) and put it in the microwave for about 20sec, it will warm up, but it will also become soggy and taste like wet paper.

Where does the humidity come from?

However, if I would use the toaster, the same bread would warm up and eventually turn into crispy toasted bread.

What is the difference? Why can I not toast bread with a microwave?

Please, explain this to me with your string theories...


January 17, 2010

In my quest to understand quantum theory and find a reasonable interpretation
I recently came across this paper (pdf!) about 'The Impossibility of Accurate State Self-Measurements' by Thomas Breuer.

It is very similar to how I think about all this.



"It is shown that it is impossible for an observer to distinguish all
present states of a system in which he or she is contained ...

[..]

As far as quantum mechanics is concerned, von Neumann assumed the theory
to be universally valid and thus was led to the measurement problem.
Bohr denied the universal validity of quantum mechanics for 'purely logical reasons',
and thereby avoided confrontation with the measurement problem.
It has often ... been suggested that self-reference problems for universally valid
theories may pose serious difficulties for a quantum mechanical description of
the measurement apparatus. The aim of this paper is to investigate these suggestions."


January 9, 2010

I think it is unlikely that a lattice gravity model will correctly describe the quantum theory of gravitation any time soon,
although the odds have improved somewhat with new results about asymptotic safety.

While such models have interesting properties, as far as I know a reasonable continuum limit has never been demonstrated.

A few days ago Simon Catterall et al. published results for a model which goes back to the simulations of
Menotti & Pelissetto, but with an additional Wilson term in the action and thus an additional coupling
parameter. They find an interesting transition for a certain threshold value of this parameter and conclude that

"It remains to be seen whether this threshold value can be thought of as a true critical value and if so
whether this critical value corresponds to a continuous or discontinuous phase transition. The latter issue is
of course a crucial issue to address in the context of obtaining a non-trivial continuum limit."



Perhaps I should dust off our lattice gravity code(s) and see what they can do on new and better hardware.

It would be a nice (but almost certainly completely useless) project for 2010...


January 3, 2010

I know that some people suspect that I delete blog posts every now and then, but this is not true. As far as I can remember I never deleted anything. It is much more plausible to assume that random 'blog posts' pop up in various RSS readers (and then disappear). Let me explain.



Quantum theory is the best description of physical reality we have and its most convincing aspect is of course the proper interpretation (see also here, here and here). In order to illustrate that, physicists usually consider the gedanken experiment of Schroedinger's cat. Initially the cat is in a state |Cb> (the C stands for cat and b stands for box), which develops into a state w|Hb> + w|Ub>.
The boring Copenhagen interpretation assumes that now a miracle occurs and we can use the coefficient w to calculate the probability to find either a happy or unhappy cat in the box.



However, the modern 'many cats interpretation' does not believe in miracles and instead tells us that the wave function never collapses and therefore ...

But wait a second, if the wave function never collapses then it is wrong to begin this gedanken experiment with the |Cb> state!

Instead we have to assume that initially the state was something like a|Cb> + b|Co>, where the o now indicates a cat
outside the box (because the decision to put the cat in the box never collapsed the wave function) and this state develops into something like w'|Hb> + w'|Ub> + n1|Co1> + n2|Co2> ...

But what if initially there was a choice between a cat and a dog, what about the decision to do the experiment at all, what about McCain winning the election etc. ?
So we have to assume that the wave function really is an infinite sum ... + w|Hb> + w|Ub> + ... ,with w->0 and with everything being possible included(*), even cats and blog posts popping up in RSS feeds etc. (**)

No wonder that Stephen Hawking was reaching for his gun...



Indeed we have to face this situation again (Fatwa and all), with all probabilities different from zero, but the lottery ticket worth nothing.

I am sorry Ponder S.





(*) The idea to look at 'relative probabilities' (i.e. for whatever reason consider |Hb> and |Ub> only, ignoring all branches we don't care about) would face the problem that we have to ignore infinitely many branches which may indeed interfere with the branches we care about; Even if the coefficients are small for each one, there are infinitely many to consider...

And how would we even know what branches we should include? Do we have to consult our memories to determine which branches were initially possible (cat inside or outside the box yes, pink unicorns no)? And the memories of which branch would we use?



(**) Of course, the idea of blog posts popping up randomly would explain a whole lot about the blogosphere...