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From left to right: apparition of bubble, slow expansion, quick and sudden contraction, emission of light

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Sonoluminescence is the emission of short bursts of light from imploding bubbles in a liquid when excited by sound.

The Royal Navy employed the services of physicist Lord Rayleigh in the early 20th century to understand the degradation of ship propellers. Lord Rayleigh concluded that collapsing air bubbles were the cause.[1]

The sonoluminescence effect was first discovered at the University of Cologne in 1934 as a result of work on sonar. H. Frenzel and H. Schultes put an ultrasound transducer in a tank of photographic developer fluid. They hoped to speed up the development process. Instead, they noticed tiny dots on the film after developing and realized that the bubbles in the fluid were emitting light with the ultrasound turned on. It was too difficult to analyze the effect in early experiments because of the complex environment of a large number of short-lived bubbles. (This experiment is also ascribed to N. Marinesco and J.J. Trillat in 1933, which also credits them with independent discovery). This phenomenon is now referred to as multi-bubble sonoluminescence (MBSL).


Single-bubble sonoluminescence - A single, cavitating bubble.

In 1989 a major experimental advance was introduced by Felipe Gaitan and Lawrence Crum, who produced stable single-bubble sonoluminescence (SBSL). In SBSL, a single bubble trapped in an acoustic standing wave, emits a pulse of light with each compression of the bubble within the standing wave. This technique allowed a more systematic study of the phenomenon, because it isolated the complex effects into one stable, predictable bubble. It was realized that the temperature inside the bubble was hot enough to melt steel. Interest in sonoluminescence was renewed when an inner temperature of such a bubble well above one million kelvins was postulated. This temperature is thus far not conclusively proven, though recent experiments conducted by the University of Illinois at Urbana–Champaign indicate temperatures around 20,000 K. Research has also been carried out by Dr. Klaus Fritsch of John Carroll University, University Heights, Ohio.


Long exposure image of multi-bubble sonoluminescence created by a high-intensity ultrasonic horn immersed in a beaker of liquid.

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In 2002, M. Brenner, S. Hilgenfeldt, and D. Lohse published a 60-page review "Single bubble sonoluminescence" (Reviews of Modern Physics 74, 425) that contains a detailed explanation of the mechanism. An important factor is that the bubble contains mainly inert noble gas such as argon or xenon (air contains about 1% argon, and the amount dissolved in water is too great; for sonoluminescence to occur, the concentration must be reduced to 20–40% of its equilibrium value) and varying amounts of water vapor. Chemical reactions cause nitrogen and oxygen to be removed from the bubble after about one hundred expansion-collapse cycles. The bubble will then begin to emit light "Evidence for Gas Exchange in Single-Bubble Sonoluminescence", Matula and Crum, Phys. Rev. Lett. 80 (1998), 865-868). The light emission of highly compressed noble gas is exploited technologically in the argon flash devices.

During bubble collapse, the inertia of the surrounding water causes high pressure and high temperature, reaching around 10,000 kelvins in the interior of the bubble, causing the ionization of a small fraction of the noble gas present. The amount ionized is small enough for the bubble to remain transparent, allowing volume emission; surface emission would produce more intense light of longer duration, dependent on wavelength, contradicting experimental results. Electrons from ionized atoms interact mainly with neutral atoms, causing thermal bremsstrahlung radiation. As the wave hits a low energy trough, the pressure drops, allowing electrons to recombine with atoms and light emission to cease due to this lack of free electrons. This makes for a 160-picosecond light pulse for argon (even a small drop in temperature causes a large drop in ionization, due to the large ionization energy relative to photon energy). This description is simplified from the literature above, which details various steps of differing duration from 15 microseconds (expansion) to 100 picoseconds (emission).

Computations based on the theory presented in the review produce radiation parameters (intensity and duration time versus wavelength) that match experimental results with errors no larger than expected due to some simplifications (e.g., assuming a uniform temperature in the entire bubble), so it seems the phenomenon of sonoluminescence is at least roughly explained, although some details of the process remain obscure.

Any discussion of sonoluminescence must include a detailed analysis of metastability. Sonoluminescence in this respect is what is physically termed a bounded phenomenon meaning that the sonoluminescence exists in a bounded region of parameter space for the bubble; a coupled magnetic field being one such parameter. The magnetic aspects of sonoluminescence are very well documented.[6]

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The 40-year friendship of Charles Darwin and Joseph Hooker, the most significant and scientifically important of Darwin’s life, can now be explored by anyone in the world with access to the Internet.


Their decades of correspondence include Darwin’s most famous letter, where he first cautiously reveals not only that he thinks species change, but also that he has worked out a completely new theory as to how. Giving voice to such a theory, he admits, is like ‘confessing a murder’.

The 1,200 letters between Darwin and Hooker, 300 of which have not been published before, are being made available in more than 5,000 images by Cambridge’s Digital Library ( Cambridge Digital Library - University of Cambridge ) - which launched to millions of ‘hits’ with the online publication of Isaac Newton’s archive in 2011 .

They have joined forces with the Darwin Correspondence Project ( Darwin Correspondence Project ) to present the images alongside the Project's transcriptions in order to bring Darwin more vividly to life, as both a man and scientist, than ever before.

Cambridge University Library is home to the world’s largest and most important collection of Darwin’s personal papers, and hopes eventually to make far more of them available in this way.

Anne Jarvis, University Librarian, said: "Through the linking together of the Darwin Correspondence Project’s superb transcriptions of the letters with high-quality photographic reproductions from the Digital Library, online viewers of the Darwin-Hooker correspondence can now experience something of the immediacy and intimacy of this long exchange of letters, whilst still being able to easily read and search the text. And this is just the start - we are planning to release further letters and scientific manuscripts from the Darwin Papers in this way over the coming months and years."

Dr Alison Pearn of the Darwin Correspondence Project said: "No single set of letters was more important to Darwin, or is more important now, than those exchanged with Hooker over 40 years – a period that encompasses almost the entirety of Darwin’s mature working life. It is unusual for a single repository to hold both sides of any correspondence, so this is a rare opportunity to see one of the longest running and most wide-ranging conversations of the nineteenth-century unfold."

Not only did the pair discuss their sometimes differing views on a spectrum of different subjects, from science to slavery and the US Civil War [aka The Late Woah of Nawthen Aggression ], the letters between the two illuminate moments of great happiness and tragic loss in both their lives, revealing a much more personal and emotional side to Darwin the man.

It was to Hooker that Darwin sent the manuscript of On the Origin of Species and the pair also traded news and gossip – as well as revealing to one another their grief and anguish at the loss of loved ones.

Nowhere is this more heartbreakingly evident than in a previously unpublished account by Darwin of watching his daughter-in-law Amy die following childbirth. "There are very few people to whom Darwin would have written in this way" continues Pearn. "It gives us a new and unique insight into his attachment to Amy who from the earliest days of her engagement to his son, was recruited by Darwin to help him collect plant specimens and make observations.”

Elsewhere, in a letter dated June 30, 1862, Darwin relates to Hooker how much their correspondence means to him.

The letter reads:

My dear old friend,
You speak of my "warming the cockles of your heart", but you will never know how often you have warmed mine. It is not your approbation of my scientific work (though I care for that more than for any one's); it is something deeper. To this day I remember keenly a letter you wrote to me from Oxford, when I was at the water-cure, & how it cheered me, when I was utterly weary of life.

Their correspondence began in 1843 when Hooker was approached about working on Darwin's collection of plants from the Beagle voyage. Just the previous year Darwin had written out his first coherent account of the main elements of his species theory, and within a few months Hooker was admitted into the small and select group of those with whom Darwin felt able to discuss his emerging ideas. The correspondence flowed back and forth between the men until Darwin’s death in 1882.

The 300 previously unpublished letters cover the last decade of Darwin’s life and give almost day-to-day detail on the experiments that led to his books on insectivorous plants and plants that move – both crucial evidence of the relatedness of plants and animals (and humans). And also to his final and most popular book , on earthworms, published shortly before he died.

Although this is Darwin's longest-running and most detailed conversation, there are many others: the University Library collection includes letters exchanged with more than 100 other correspondents, including Robert FitzRoy, captain of the Beagle; Alfred Russel Wallace; John Murray, Darwin's publisher; his first love, Fanny Owen; his wife, his children; other scientists including the Harvard botanist Asa Gray, the geologist Charles Lyell, and the zoologist Thomas Huxley.

For more information, and links to selected letters see: Darwin Correspondence Project » Darwin-Hooker letters a

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Borrowing Energy from the Future

The uncertainty principle does not necessarily have to refer to that one pair of values which are velocity and position. It can refer to energy and time as well. The shorter the time interval, the wilder the fluctuations in energy. This results in a simple and beautiful consequence: that particles can be created out of nothing.

In the quantum realm, the time scale of events is very short. The number of times that a proton can travel from one side of the atomic nucleus to the other in a single second is 1022. This short time scale allows particles such as protons and neutrons to utilize the uncertainty principle in a crucial way. They can borrow energy from the future (literally nowhere) for a short duration provided the energy gets paid back before the uncertainty principle is violated. The shorter the time the energy is needed for, the more can be borrowed.

A particle called a pion, which is responsible for holding protons and neutrons together can be created by a nucleon (either proton or neutron) which borrows enough energy from its surroundings to create it. The pion then jumps to another nucleon where it vanishes again.

In a similar way, the EM force between charged particles can be seen as the exchange of such a photon, which can be called a messenger particle or virtual particle. It is different from a real photon which may keep its energy for as long as it likes.

Even a vacuum is full of activity and is not exactly empty. Energy is borrowed from the future to create a matter particle and antimatter particle, which collide in a process called pair annihilation to form energy again (through e=mc2), returning the energy borrowed. A vacuum can be said to contain matter and antimatter particles which are constantly created and destroyed all the time.

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Often when reading about cutting edge physics and the amazing feats of the Large Hadron Collider, we are treated to crazy scenarios involving “virtual particles”, also variously referred to as “ghost particles” or worse. These labels clearly distinguish the involved concepts from "real particles" like atoms. Not being bound by restrictions of reality, virtual particles “borrow” energy from nothing, go faster than light, travel back in time, do an infinite amount of loops creating an infinity of other virtual particles during every single infinitesimal moment.

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Dr. Krauss delineates three different kinds of nothingness. First is what may have passed muster as nothing with the ancient Greeks: empty space. But we now know that even empty space is filled with energy, vibrating with electromagnetic fields and so-called virtual particles dancing in and out of existence on borrowed energy courtesy of the randomness that characterizes reality on the smallest scales, according to the rules of quantum theory.

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