### memories

Cosma links to this article about a 'quantum solution to the arrow-of-time dilemma' and suggests to read it carefully.

I think I read it already a year ago as this preprint and it has several interesting ideas; In particular I liked the passages about Borel's argument.

But I was also thinking about the hidden assumption of this paper.

As far as I understand it, the author (implicitly) assumes that memories are about the past and in a paper about the 'arrow-of-time dilemma' this should be explained not assumed.

In other words, the author suggests that we could live in a time-symmetric world and we just do not have memories about "phenomena where the entropy decreases"; This is all fine, but in my opinion one needs to explain then why we only have memories about the past but not the future, without "an ad hoc assumption about low entropy initial states".

E.g. in the conclusions we read that "we could define the past as that of which we

have memories of, and the future as that of which we do not have any memories" (*), but the author does not ask or answer the question how there can even be such a thing as the future, of which we have no memories, in a time-symmetric world.

update: Sean wrote about the paper also and in the comments the author responds.

I am glad that at least one comment agrees with my own reading; And it seems that Nick Huggett has some interesting papers I should read.

update2: The author responds to Huw Price (who raised the same objection I did).

update3: Last, but not least, Dieter Zeh comments on the paper as well. I wrote about his book about 'the direction of time' previously.

PS: The question "if the world would run 'backwards', would we even notice it?" was raised and discussed previously on this blog.

(*) As it stands this statement is of course contrary to the usual convention(s). From what we know about physics, most of those events of which we have no memory are space-like to us and not in our future. And of course there are even events in our past of which we have no memories. E.g. we know for sure that Philip Augustus of France spent a night with Ingeborg of Denmark, but we have no documents or memories about the mysterious events of that night.

### Fermi event

The Fermi experiment detected a 31 GeV photon emitted by a short gamma-ray burst and "this photon sets limits on a possible linear energy dependence of the propagation speed of photons (Lorentz-invariance violation) requiring for the first time a quantum-gravity mass scale significantly above the Planck mass".

In other words, the observed event suggests that Lorentz invariance holds up to (and in fact above) the Planck scale and thus provides an empirical argument to rule out several proposals for quantum gravity. As far as I know, this is the first time direct empirical evidence about the Planck scale was obtained!

Via Lubos.

So what does this tell us about string theory?

### quantum Hamlet

The arXiv blog reports about a new proposed effect called Quantum Hamlet Effect. According to the author "It represents a complete destruction of the quantum predictions on the decay probability of an unstable quantum system by frequent measurement".

But I think there is a problem with it. The Hamlet state is prepared by a series of subsequent measurements, happening at decreasing time intervals tau/sqrt(n), with n

going to infinity. This leads to a divergent sum in the probability, which then leads to the "complete destruction of predictability".

But, of course, in reality the limit n to infinity cannot be taken (e.g. as the author notices himself he neglects time - energy uncertainty!), so we have to assume the procedure stops at n = N, with some finite but perhaps large N. Unfortunately, the divergent Hamlet term is the harmonic series, which increases only with log(N),

i.e. it increases much slower than sqrt(N).

Therefore I doubt that the Hamlet effect will "turn out to be more useful and famous than [the Zeno effect]" as suggested on the arXiv blog.

### truth

The statement "p is an unknown truth" cannot be both known and true at the same time.

Therefore, if all truths are knowable, the set of all truths must not include any of the form

"p is an unknown truth"; Thus there must be no unknown truths, and thus all truths must be known.

At least according to Frederic Fitch.

Later, after a heated debate about the question if all true logical statements are indeed tautological, one

of the philosophers finally screamed "enough is enough" and stormed out of the room.

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