Archive for quantum physics

Inflation and the Multiverse

Posted in Astrohype, The Universe and Stuff with tags , , , , , , on January 6, 2014 by telescoper

I was quite excited when I discovered, via Twitter, a paper on the arXiv with the title Quantum Fluctuations in Cosmology and How They Lead to a Multiverse, which was written by one of the architects of the inflationary universe scenario, Alan Guth. Despite numerous attempts to understand the argument how inflation leads to a Multiverse I’ve never really succeeded. To me it always seemed like  a version of the Mind Projection Fallacy inspired by a frequentist interpretation of probability: the construction of notional ensembles for the purposes of calculation in quantum mechanics does not imply that such ensembles are realized in nature. In fact I’ve never found much more substance in articles about this issue than the assertion that Quantum Physics = Woo! = Multiverse.

Anyway, since the paper I found is a review article I hoped it would help teach me the error of my ways. Here is the abstract

This article discusses density perturbations in inflationary models, offering a pedagogical description of how these perturbations are generated by quantum fluctuations in the early universe. A key feature of inflation is that that rapid expansion can stretch microscopic fluctuations to cosmological proportions. I discuss also another important conseqence of quantum fluctuations: the fact that almost all inflationary models become eternal, so that once inflation starts, it never stops.

My eye was drawn to the phrase “almost all inflationary models”.  I had hoped to see “almost all” used in its strict mathematical sense, ie “apart from a set of measure zero” with the measure being fully specified. Disappointingly, it isn’t.   Guth discusses the consequences of the tail  the inflationary potential V (for large values of the inflaton field ϕ) on the long-term evolution of inflationary dynamics and then states

Since V3/2/|V ′| grows without bound as ϕ → ∞ for most potentials under consideration, almost all models allow for eternal inflation.

This means, to me, most models people have constructed but doesn’t mean all possible models. I don’t doubt that some inflationary models  become eternal, but would have preferred a more rigorous statement.  This is particularly strange because Guth spends the last section of his paper discussing the “measure problem”:

While the multiverse picture looks very plausible in the context of inflationary cosmology — at least to me — it raises a thorny and unsolved problem, known as the “measure problem.” Specifically, we do not know how to define probabilities in the multiverse.

The measure problem to my mind also extends to the space of all possible inflationary theories.

And then there’s the title, which, I remind you, is Quantum Fluctuations in Cosmology and How They Lead to a Multiverse. Guth’s argument consists of going through the (standard) calculation of the spectrum of cosmological density fluctuations (which does fit a host of observational data). He then states:

Since the density perturbation calculations have been incredibly successful, it seems to make sense to take seriously the assumptions behind these calculations, and follow them where they lead. I have to admit that there is no clear consensus among cosmologists, but to many of us the assumptions seem to be pointing to eternal inflation, and the multiverse.

I have to admit that I get a bit annoyed when I read a paper in which the actual conclusions are much weaker than implied by the title, but that seems to be par for the course in this field.

For the record, I’ll state that I am an agnostic about the multiverse. It may be a correct idea, it may not. I will say, however, that I still haven’t found any article that puts it on a firm scientific footing. That may well, of course, just be a measure of my ignorance. If you know of one, please let me know through the comments box.

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Punch and Judy meet Quantum Technology

Posted in The Universe and Stuff with tags , , , , , , on June 28, 2013 by telescoper

It’s an Open Day here on campus, and there’s quite a crowd of potential students and parents gathering in the School of Mathematical and Physical Sciences here at the University of Sussex to find out a bit more about the School in advance of making decisions about where to apply next year.

I noticed the other day that quite a few of these have appeared on campus over the last few days:

IMG-20130627-00139

Apparently they’re information points manned by various helpers to help visitors find their way around the place. When I first saw this one, I thought it was a Punch and Judy box, so assumed that there was some sort of conference of Punch and Judy performers going on. That wouldn’t be inappropriate for a University campus, actually, because the traditional name for a Punch & Judy puppeteer is a “Professor”. Not a lot of people know that.

Anyway, none of that is really relevant to what I wanted to post today. I stumbled across this video featuring Winfried Hensinger (one of my colleagues from the Department of Physics & Astronomy within the School of Mathematical and Physical Sciences). I thought it would be fun to share it here, just to give an idea of some of the work that’s going on here outside my own speciality of astrophysics. I hope this will complement the real open day with a mini virtual open day on the blog.

Winfried is Reader in Quantum, Atomic and Optical Physics at the University of Sussex and he works in the group we generally call “AMO” (Atomic, Molecular and Optical). In this TEDX lecture he talks about the future of quantum computers and the role the team he is part of, at Sussex University, plays as they develop large scale quantum computers using ions cooled to extremely low temperatures using lasers. Enjoy!

Are Quantum States “Real”?

Posted in The Universe and Stuff with tags , , on November 20, 2011 by telescoper

Busy day today, despite it being a Sunday, so I’ve only got time for a quick post, by way of a diversion while I take a break for a cup of tea.

There’s been an attack of the hyperbolics this week arising from a paper entitled “The quantum state cannot be interpreted statistically”.  The abstract of the paper reads:

Quantum states are the key mathematical objects in quantum theory. It is therefore surprising that physicists have been unable to agree on what a quantum state represents. There are at least two opposing schools of thought, each almost as old as quantum theory itself. One is that a pure state is a physical property of system, much like position and momentum in classical mechanics. Another is that even a pure state has only a statistical significance, akin to a probability distribution in statistical mechanics. Here we show that, given only very mild assumptions, the statistical interpretation of the quantum state is inconsistent with the predictions of quantum theory. This result holds even in the presence of small amounts of experimental noise, and is therefore amenable to experimental test using present or near-future technology. If the predictions of quantum theory are confirmed, such a test would show that distinct quantum states must correspond to physically distinct states of reality.

According to a commentary published in the journal Nature, this paper could “rock quantum theory to the core” and a number of quantum physicists have reacted, e.g.:

“I don’t like to sound hyperbolic, but I think the word ‘seismic’ is likely to apply to this paper,” says Antony Valentini, a theoretical physicist specializing in quantum foundations at Clemson University in South Carolina.

I have to admit I haven’t had time to read the paper in detail yet, so I’m just passing this on as something fresh that may be of wider interest, rather than something I’ve got particularly strong views about. I have to admit, though, that I find the way quantum theorists use words, especially what is meant by “physically real” and what “states of reality” could be. Can a mathematical theorem ever prove itself to be applicable to the physical world anyway? You’ll see that ontology was never my strong suit.

However, if anyone out there in blogland has read this paper and would like to pass on their thoughts for the edification of me and my readers I’d be delighted. I might return to it in a longer post if and when I’ve been able to digest it fully.

Now, back to reality…

 

A little knowledge….

Posted in Uncategorized with tags , , , on March 2, 2011 by telescoper

A little knowledge is a dangerous thing, but for a homeopath no knowledge at all will apparently do just as well.

No satire  is necessary (or indeed possible) for the following clip, although you could try making a list of the basic conceptual errors until you feel obliged to switch off your computer in order to stop yourself from throwing it out of the window, and even if that only takes a few seconds you’ll still need a  lot of paper…


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