The Physics arXiv Blog is 10 years old and we’re celebrating by publishing links to the top stories from each year of its existence.
Today, 2009.
The Physics arXiv Blog is 10 years old today. Over the next few days, we’ll be celebrating by publishing links to the top stories from each year of its existence.
Today, 2008.
From September 2013, The Physics arXiv Blog is moving to Medium.com. Keep up to date with the latest ideas in physics and astronomy at: https://medium.com/the-physics-arxiv-blog
For those of you more interested in technology, I’ll be covering this at Technology Review in a blog called “Emerging Technology from the arXiv” at: http://www.technologyreview.com/contributor/emerging-technology-from-the-arxiv/
Follow the headlines on Twitter at: https://twitter.com/arxivblog
See you there!
…this week’s peaches from the physics arXiv blog:
How Hawking radiation may explain dark energy
When does a quantum measurement end?
Surprisingly, quantum physicists cannot agree. Some say the measurement ends when you register a result on a piece of classical equipment such as a photomultiplier. Others says the measurement ends when the information in the quantum system has irreversibly leaked into the environment. There are still more who believe in the manyworlds interpretation of quantum mechanics and say a quantum measurement never ends but exists ad infinitum in several parallel universes.
This may sound like an ineffectual academic scrap but it actually has hugely important consequences for the quantum property of entanglement.
Entanglement is the state in which two physically separated particles share the same quantum existence, so that a measurement on one instantaneously affects the other. Yep, that’s instantaneously. It’s what Einstein described as “spooky action at distance”.
For some years, physicists have been measuring this “spooky action at a distance” in tests known as Bell experiments.
These tests depend crucially on the measurement ending quickly. Because if it were to drag on, the particles might be able to communicate at light speed by some currently unknown mechanism.
But because nobody has actually determined when a measurement ends, all the experiments to date are potentially open to this loophole.
Perhaps there is no spooky action at a distance after all, just long quantum measurements during which the particles communicate at the speed of light in some quite ordinary way.
Now Nicolas Gisin and colleagues at the University of Geneva have closed this loophole using the ideas of the Oxford theorist Roger Penrose. A few years ago, he suggested that the end of a quantum measurement is realted to the gravitational energy of the mass distribution of the resulting quantum superposition. In other words, the measurement ends when a massive object receives a decent kick.
So Gisin and buddies set up a Bell experiment which involved sending entangled photons in each direction from the midpoint of an 18 km fibre. At the ends of the fibre were piezoelectric actuators attached to small but massive mirrors. When the photons hit, they triggered the actuators causing the mirrors to move and deflect a beam of light.
The experiment was carefully set up so that the mirrors were heavy enough to please Penrose and far enough apart that no light speed signal could travel between them in the time it took for a pair of entangled photons to “kick” them.
The result? Gisin’s team confirmed that “spooky action at a distance” still governs the behaviour of the entangled photons.
If you believe Penrose, this is the first experiment to ever prove “spooky action at a distance”. Impressive, huh?
More interesting, is the idea of gravity and quantum mechanics coming under the microscope in the same experiment for the first time.
It won’t be the last. There are plenty of other mysteries about gravity that quantum mechanics can probe. It’s about time physicists bit the bullet and started testing them.
Ref: arxiv.org/abs/0803.2425: Space-like Separation in a Bell Test assuming Gravitationally Induced Collapses
This week’s pearls from the physics arXiv blog:
The best of the rest from the arXiv this week:
Plasma Treatment Advantages for Textiles
Electron Mass Shift in Nonthermal Systems
How Efficient is Towing a Cargo by a Micro-swimmer?
Movie Recommendation Systems Using An Artificial Immune System
Dark matter is highly sought-after but like the unicorn, it is an elusive match for its hunters.
Physics bods have been a-huntin’ for dark matter here on Earth for some time now. They’ve set an impressive number of traps all over the planet and and found zilch.
So they’ve asked the astrobods to help out by looking for indirect evidence. If dark matter particles ever bang into each other, they might annihilate leaving a signature that we can see (or so the thinking goes).
Today, Dan “Super” Hooper from the Fermi National Accelerator in Illinois gives us the low down on the various candidates that the eggheads are looking at.
They’ve looked at the unexplained excess of positrons in cosmic rays, at the unexplained excess of 511KeV photons coming out of the galactic bulge, at the unexplained excess of gamma rays at energies above 1GeV coming from within our galaxy and from outside it and the unexplained excess of microwave emissions from the centre of the Milky Way as measured by the WMAP spacecraft.
None of these signals is particularly strong but that don’t bother dark matter theorists: if it don’t have a current explanation, it’s gotta be dark matter, right?
Whatever happened to that old fashioned notion of hypothesis testing.
I know what ya’ll thinkin and I’ll wager it includes the words “clutching” and “straws”. Myself? I’m rootin’ for the unicorn.
Ref: arxiv.org/abs/0710.2062: Indirect Searches For Dark Matter
Today’s highlights from the preprint server
Coherent ultrafast core-hole correlation spectroscopy: x-ray analogues of multidimensional NMR
Secure Transmission with Multiple Antennas: The MISOME Wiretap Channel
Relationship between degree of efficiency and prediction in stock price changes
Tracing the early development of harmful algal blooms with the aid of Lagrangian coherent structures
