Question of the Month

Dec 172019
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Due to technical improvements during the last years, machines outcompete humans in a couple of specialized tasks: Whereas it can take a human person very long to calculate the square root of a (non-square) number, a computer can finish this calculation at high precision within a fraction of a second. However, there are some areas in which machines still cannot compete with nature (yet). One of them is olfaction: Currently, no device is available that could replace police dogs with the ability to detect trace amounts of molecules. Similarly, farmers sometimes even train pigs to search for truffles hidden in the soil. Of course, the ability to detect relevant molecules in low amounts offers an enormous advantage and is thus subject to extensive optimization by evolution.
L’odorat, Honoré Daumier (circa 1839, public domain – wikimedia)

How exactly olfaction works in higher organisms has not been known for a long time. Nonetheless, it had been intuitively clear that there must be specific receptors interacting with the corresponding odours. This simple assumption has a remarkable consequence: Since mammals can distinguish a high number of odours, there also must be a high number of different receptors encoded in the genome. Indeed, the two scientists Linda Buck and Richard Axel discovered a comparatively large family of genes encoding for odorant receptors [1]. For this discovery, they were awarded the Nobel Prize in Physiology or Medicine in 2004. The activation of these receptors on the cell surface always results in similar intracellular reactions. If a cell had receptors for different odour molecules on its surface, it could therefore not distinguish these odours. In accordance to this consideration, it turned out that each olfactory cell only carries one type of all the different odorant receptors encoded in its genome. Why exactly this is the case is still not known in detail to date. Even more surprisingly, it even turned out that the axons of cells, which carry the same type of odorant receptor on their surface, end on the same set of cells.

An odour can of course consist of several kinds of molecules. The activation of different combinations of olfactory sensory neurons further increases the number of differentiable odours. A phenomenon seemingly similar to the exclusive expression of a single odorant receptor by an olfactory sensory neuron is the generation of only one type of antigen receptor by immune cells. They achieve this by a complicated recombination of genes, which is clearly not observed in olfactory neurons.

Investigating how a biological structure develops is often very helpful: In a later work, Linda Buck could show that in contrast to mature olfactory neurons, there are multiple mRNAs for different odorant receptors in immature neurons [2]. Why cells of our body can have entirely different morphologies and properties even though they all carry a copy of the same genome is a fundamental question which keeps many biologists busy. It is the differential expression of the genes in a cell, which causes these differences. This gives muscle cells the ability to contract and enables neurons to generate action potentials.

However, all olfactory neurons express a very similar pattern of genes except for their odorant receptor. One of the reasons for the transcription of different amounts of RNAs from different genes is the spatial arrangement of the DNA in the nucleus. Had it not been tightly packed into the nucleus, the DNA in each cell would have a total length of 1.8 m and highly condensed sections of DNA are usually not accessible for transcription into RNA. Stavros Lomvardas, a former member of the group of Richard Axel, could show that DNA segments encoding for odorant receptors on different chromosomes get pulled close to each other in a small spatial region in the nucleus. Interactions between the different DNA segments encoding for odorant receptors could contribute to the exclusive transcription of one specific odorant receptor gene [3,4].

The relevance of the spatial arrangement of the DNA within the nucleus for gene expression is an open question of major interest beyond olfaction. To which degree there is a specific nuclear arrangement of DNA and how this is established after cell division would then be further important for other unsolved questions in biology.

— Tobias Ruff


  • [1] Buck, L. and Axel, R. , A novel multigene family may encode odorant receptors: a molecular basis for odor recognition. Cell 1991, 65-1 PP175-187 DOI:10.1016/0092-8674(91)90418-x
  • [2] Hanchate, N. K. and Kondoh, K. and Lu, Z. and Kuang, D. and Ye, X. and Qiu, X. and Pachter, L. and Trapnell, C. and Buck, L. B. , Science 2015, 350-6265 PP1251–1255
  • [3] Clowney, E. J. and LeGros, M. A. and Mosley, C. P. and Clowney, F. G. and Markenskoff-Papadimitriou, E. C. and Myllys, M. and Barnea, G. and Larabell, C. A. and Lomvardas, S., Cell 2012, 151-4 PP724–737
  • [4] Markenscoff-Papadimitriou, E. and Allen, W. E. and Colquitt, B. M. and Goh, T. and Murphy, K. K. and Monahan, K. and Mosley, C. P. and Ahituv, N. and Lomvardas, S., Cell 2014, 159-3 PP543–557

Sep 102019
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Curious things happen around us all the time – and sometimes we are so familiar with them that we do not even notice them anymore.

If you read the title you might now think that this article was about the Leidenfrost effect [1], that is, this little funny dance water droplets perform on a hot surface such as a frying pan. It is not, though. The Leidenfrost effect occurs when a material – usually a liquid – meets a surface far above its boiling temperature. A thin layer of the droplet’s surface will then evaporate rapidly, causing a protective gas coating to appear that effectively insulates the droplet and lets it last longer on the hot surface. Similar effects can also be seen with liquid nitrogen on a material at room temperature. These droplets appear to travel around due to ejected gasses. But does a similar phenomenon also occur without the necessity of a hot surface?

There is in fact a location where such an effect occurs regularly without us usually noticing: The bathroom. Under certain conditions water droplets can be seen moving on a surface of water as if they had hydrophobic properties. The easiest way to see them is in the shower, when the shower floor is already covered in a thin layer of water. If new water droplets now impact on this surface at certain angles and speeds, they can be seen rushing around for a while before disappearing. It turns out that in recent years a few scientific publications were dedicated to investigating this effect more closely. [2,3] With a high-speed camera, the bouncing effect can be visualized rather easily, as shown in Fig. 1: The droplet appears to cause a dent in the water surface and then bounce off without merging with the rest of the liquid. Of course, the first idea that comes into mind now is the Leidenfrost effect, where a similar behavior can be seen caused by a layer of vapor. However, here no high temperatures are involved and thus the generation of water vapor is negligible.

Figure 2: A schematic depiction of the resistance time phenomenon. On impact, a thin layer of gas (air) is compressed on the surface, causing a protection from immediate coalescence. However, eventually, the air escapes and the lower periphery of the droplet merges with the rest of the liquid. The surface tension can then rapidly squeeze the edges of the droplet together, causing the upper half of the droplet to be cut off from the rest. It can then repeat the bouncing process if the conditions are right. Reproduced from [4].

The first intuition of an air coating to protect the water droplet is still standing, though, and thus the scientists tried to model the behavior. It turns out that there is indeed a protective coating of air, which can get compressed when the droplet approaches the surface of the liquid underneath. The air simply cannot escape quickly enough and therefore protects the droplet on impact and pushes away from the water surface. This phenomenon causes what is called the residence time of a droplet, that is, the time a droplet can sit on top of a pool of the same liquid before coalescing (see Fig. 2). The theory was confirmed by lowering the ambient air pressure around the experiment, which caused the residence time to decrease. [4] However, one would expect that this thin layer of gas should not withstand a heavy impact of a droplet coming from e.g. the shower head with a lot of speed and thus kinetic energy.

An explanation can be found using a simple speaker membrane: When the droplets are put in contact with an oscillation surface, like water on an oscillating speaker, the bouncing is facilitated, and the droplets can remain intact for much longer. Moreover, the droplets now travel around just like they do in a shower! High-speed camera footage can show the reason for this change in behavior: The surface of the water pool, excited into periodic up- and down-movement patterns, gently catches the droplet if the surface is moving downwards in the moment of impact and therefore prevents the impact from destroying the protective gas layer. It is just like gently catching a water balloon with your hand by grabbing it in motion and then slowing it down. Additionally, the continuous movement of the surface seems to stabilize the gas layer and therefore massively increases the residence time, all while allowing the droplet to travel from minimum to minimum, thus creating the “walking water” effect. [6] In a shower, the impact of many, many droplets cause the surface of the water pool on the ground to oscillate in a similar manner, creating landing spots for some droplets that then move around the surface. The phenomenon can thus be explained by the residence time of a droplet together with an oscillating surface.

Finally, one can reproduce a similar behavior in space, where microgravity does not pull the droplets down. An air bubble inside of a water bubble can thus act like an isolated system where droplets can form and move… excited by the sound of a cello! If you got curious, please check out the beautiful footage in Ref. [6] where much of the inspiration of this article came from.

As stated initially, the most curious things happen around us and we simply have to notice them.

— Kai Litzius



[2] Y. Couder et al., From Bouncing to Floating: Noncoalescence of Drops on a Fluid Bath, Phys. Rev. Lett. 94, 177801 (2005).

[3] J. Molácek & J. W. M. Bush, Drops bouncing on a vibrating bath, J. Fluid Mech. 727, 582-611 (2013).

[4] I. Klyuzhin et al., Persisting Water Droplets on Water Surfaces, J. Phys. Chem. B 114, 14020-14027 (2010).


[6] (Water bubble in space at time index 8:18).

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Sep 042019
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Superstitions are having hard times in our modern always progressing world. It is no longer easy to fool someone with a myth or a beautiful legend from childhood. But how about this one: have you ever heard that a thunderstorm could curdle milk

A correlation between thunderstorms and the souring or curdling of milk has been observed for centuries. As early as in 1685 the first clue was written down in the book “The Paradoxal Discourses of F. M. Van Helmont: Concerning the Macrocosm and Microcosm, Or the Greater and Lesser World, and Their Union” [1]:

“Now that the Thunder hath its peculiar working, may be partly perceived from hence, that at the time when it thunders, Beer, Milk, &c. turn sower in the Cellars … the Thunder doth everywhere introduce corruption and putrefaction”.

By the beginning of the 19th century there had been numerous attempts to find theories of a causal relationship. [2-7] They all were not plausible, many even contradicting. Later, after refrigeration and pasteurization became widespread, eliminating bacteria growth, interest in this phenomenon almost disappeared. While the most popular explanation remains that these occasions are only a correlation, we would like to draw the reader’s attention to some of the suggested theories. 

In order to understand what actually happens with milk during a thunderstorm we would need to know (i) what processes are behind the milk souring and (ii) what accompanies thunderstorm, e.g. lightning. While the latter is not yet entirely clear to scientists, [8] the simplified picture of the first point we will cover in the next few paragraphs.     

Figure1: Schematic image of casein micelles covering fat globules within milk as a colloid solution.

Fresh milk is a textbook example of colloid – a solution consisting of fat and protein molecules, mainly casein, floating in a water-based fluid. [9] The structure of milk is schematically illustrated in Fig. 1. Fat globules are coated with protein and charged phospholipids. Such a formation protects the fat from being quickly digested by bacteria, which also exist in milk. Casein proteins under normal conditions are negatively charged and repel each other so that these formations naturally distribute evenly through the liquid. Normally, milk is slightly acidic (pH ca. 6.4-6.8), [10] being sweet at the same time due to lactose, one of the other carbohydrates within the milk. When the acidity increases to pH lower than 4, proteins denature and are no longer charged. Thus, they bind to each other or coagulate into the clumps known as curds. The watery liquid that remains is called whey.

The acidity of milk is determined by the bacteria which produce lactic acid. The acids lower the pH of milk so the proteins can clump together. The bacteria living in milk naturally produce lactic acid as they digest lactose so they can grow and reproduce. This occurs for raw milk as well as for pasteurized milk. Refrigerating milk slows the growth of bacteria. Similarly, warm milk accommodates bacteria thrive and also increases the rate of the clumping reaction.

Now, we can think of a few things that may speed up the souring process. The first one could be ozone that is formed during a thunderstorm. In one of the works it was shown that a sufficient amount of ozone is generated at such times to coagulate milk by direct oxidation and a consequent production of lactic acids. [2] However, if this were the case, a similar effect would occur for sterilized milk. The corresponding studies were carried out by A. L. Treadwell, reporting that, indeed, the action of oxygen or oxygen and ozone coagulated milk faster Ref. [2]. But the effect was not observed if the milk had been sterilized. The conclusion drawn from this study was that the souring was produced by unusually rapid growth of bacteria in an oxygen rich environment.

In the meantime, a number of other investigations suggested that a rapid souring of milk was most likely due to the atmosphere that is well known to become sultry or hot just prior to a thunderstorm. This warm condition of the air is very favourable for the development of lactic acid in the milk. [3, 4] Thus, these studies were also in favour of thunderstorms affecting the bacteria.

A fundamentally different explanation was tested by e.g. A. Chizhevsky in Ref. [5]. It was suggested that the electric fields with particular characteristics produced during thunderstorms could stimulate a souring process. To check this hypothesis the coagulation of milk was studied under the influence of electric discharges of different strength. Importantly, in these experiments the electric pulses were kept short to eliminate any thermal phenomena. Eventually, the observed coagulation for certain parameter ranges was explained by breaking of protein-colloid system in milk due to the influence of the electric field.

Other experiments investigating the effect of electricity on the coagulation process in milk turned out to be astonishing. [6] When an electric current was passed directly through milk in a container, in all the test variations, the level of acidity rose less quickly in the ‘electrified’ milk samples compared with the ‘control’ sample. Which contradicted all the previous reports.

To conclude, while there is no established theory explaining why milk turns sour during thunderstorms, we cannot disregard numerous occasions of this curious phenomenon. [7] What scientists definitely know is that milk goes sour due to bacteria – bacilli acidi lactici – which produce lactic acid. These bacteria are known to be fairly inactive at low temperatures. Which is why having a fridge is very convenient for milk-lovers. However, when the temperature rises, the bacteria multiply with increasing rapidity until at ca. 50°C it becomes too hot for them to survive. Thus, in pre-refrigerator days, when this phenomenon was most popular, in thundery weather with its anomalous conditions the milk would often go off within a short time after being opened. Independently of the exact mechanism, i.e. increased bacteria activity or breaking of the protein-colloid system, the result is – curdled milk.

Should you ever witness this phenomenon yourself, do not be sad immediately. Try adding a bit brown sugar into your fresh milk curds…

— Mariia Filianina

Read more:

[1] F. M. van Helmont Franciscus “The Paradoxal Discourses of F. M. Van Helmont, Concerning the Macrocosm And Microcosm, Or The Greater and Lesser World, And their Union” set down in writing by J.B. and now published, London, 1685.

[2] A. L. Treadwell, “The Souring of Milk During Thunder-StormsScience Vol. XVIII, No. 425, 178 (1891).

[3] “Lightning and Milk”, Scientific American 13, 40, 315 (1858). doi:10.1038/scientificamerican06121858-315

[4] H. McClure, “Thunder and Sour Milk.” British Medical Journal vol. 2, 651 (1890).

[5]V. V. Fedynskii (Ed.), The earth in the universe” (orig. “Zemlya vo vselnnoi”), Moscow 1964, Translated from Russian by the Israel Program for Scientific Translations in 1968.

[6] W. G. Duffield and J. A. Murray, “Milk and Electrical Discharges”, Journal of the Röntgen Society 10(38), 9 (1914). doi:10.1459/jrs.194.0004

[7] “Influence of Thunderstorms on MilkThe Creamery and Milk Plant Monthly 11, 40 (1922).

[8] K. Litzius, “How does a lightning bolt find its target?” Journal of Unsolved Questions 9(2) (2019).

[9] R. Jost (Ed.), “Milk and Dairy Products.” In Ullmann’s Encyclopedia of Industrial Chemistry (2007). doi: 10.1002/14356007.a16_589.pub3


May 222019
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Once, thunderstorms with thunder and lightning were interpreted as signs of the god’s wrath; nowadays, we are taught the mechanics behind a thunderstorm in school. You are probably already thinking about ice crystals that are smashed together by strong winds inside clouds, creating static charges in the process. How does a lightning bolt, though, find its way from the cloud to the ground? This question still keeps scientists awake at night – and there is still not a clear answer to how exactly the formation and movement of a lightning bolt work. This Question of the Month will give a brief summary on how a lightning bolt selects its target.

Lightning [1,2] occurs always when a large thunderstorm cloud with strong winds generates sufficient electrostatic charge that it must discharge towards the ground. The discharge itself occurs (simplified) in a twostep process, consisting of a main lightning bold and a preflash: The preflash travels as comparably weak (but still dangerous!) current downwards from the cloud. This usually happens in little jumps, which have been investigated with high-speed cameras. They show that the current path is apparently selected randomly by slowing down at a given position and then randomly selecting the next to jump to. This random selection appears to happen within a sphere of a few tens of meters in diameter around the tip of the growing lightning bolt. The process also involves growing many tendrils with individual tips and thus covers a large area (see also Fig. 1). With this procedure, the lightning bold eventually “feels” its way to the ground until it reaches it either directly or via a structure connected to it.

Figure 1: Lightning bolts are branching off into many tendrils. [3]

Therefore, if a conductive object reaches into such a sphere, the bolt will immediately jump to it and use it as a low-resistance shortcut to the ground – as a result, if possible, shortening the path for the discharge. This behavior leads to the curious effect of exclusion areas around structures that are protected with lightning rods, in which practically no ground strike will occur, and a person will not be hit directly. Unfortunately, this will not completely protect the person, as the electricity can still be dangerous within the ground.

Now that the preflash has found a path to the ground, the second phase starts, and the majority of the charge starts to flow with up to 20 000 A along the path found by the preflash. This is also the portion of the discharge that is visible by bare eye. It can consist of several distinct discharges that all follow the path of ionized air of the previous one, creating the characteristic flickering of a lightning bolt.

How the entire process from preflash to main discharge works is still not completely understood today and much of the presented insights were simply gathered phenomenologically by camera imaging. Additionally, there are many more types of and effects related to lightning bolts, which are relevant for our understanding of a variety of weather phenomena. All in all, thunderstorms are still something magical today, even if only figuratively.

— Kai Litzius

Further reading:




[4] Chem. Unserer Zeit, 2019, 53. DOI: 10.1002/ciuz.201980045

Mar 012019
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Just a few years before Dolly was born as the first surviving clone of a sheep in 1996, the movie Jurassic Park was launched, based on the same-named novel by Michael Crichton.[1,2] In this story scientists insert genetic material derived from fossils into amphibious eggs to bring all sorts of dinosaurs back to life. The actual cloning of animals follows a quite similar approach called somatic cell nuclear transfer or SCNT (fig 1): a nucleus with the desired DNA is isolated from a somatic (body) cell and introduced into an emptied ovum of the same species. Several electrical impulses excite the cell and stimulate proliferation in a nutritional medium. The most stable cell clusters, called blastomeres, can then be transferred to a host mother and grow into an embryo.[1] Dolly managed to fully develop into a lamb and lived 13 years until she died of an infection. She even gave birth to a lamb, proving the viability of cloned creatures.[3] Blastomeres that are dissected instead of implanted can be used to treat diseases or might enable the growth of tissue. Maybe in the future we will be even able to grow a whole surrogate organ ‒ an approach that is highly controversial since human somatic cells are mostly derived from embryotic tissue.[4]

Fig 1: Schematic depiction of the SCNT process: The nucleus with the desired genetic material is inserted into an empty egg cell which is growing into a blastomere.[5]

According to a report from the Intergovernmental Science-Policy Platform on Biodiversity and Ecosystem Services (IPBES) about one million species of an estimated number of around 8 million species (only counting eukaryotes) on earth are currently endangered or threatened with loss of habitat.[6,7] In the history of Earth extinction has mostly been a consequence of natural disasters like climate change, volcanic eruptions, or meteorite impacts until human population started to expand.[8,9] The IPBES report demonstrates the present impact of human behaviour on biodiversity and it seems that we are facing many more extinctions caused by anthropogenic reasons in the next decades. It has become a growing interest to not only preserve existing species but also to revive those that have already died out.

One attempt is currently being made to revive Quaggas, a subspecies of the living plain zebra that has died out in the 1880s (fig 2), by selective breeding. Due to their close genetic relation some plain zebras that resemble the characteristic pattern of the quaggas have been selected in the hope to one day give birth to a zebra that looks just like them and shows similar genetic information.[10,11,12]

Fig 2: Taxidermied Quagga foal in the Museum of Natural History in Mainz, Germany. (© Tatjana Dänzer)

More demanding is the CRISPR Cas9 method: the DNA that can be extracted from most fossils like the woolly mammoth could be much too old to produce a healthy individuum. But their DNA might be partially recovered by replacing some sequences in the DNA of their closest living relative, the elephant, with extracted mammoth DNA. The genome will not be the same as it was millions of years ago and no one really knows how this will influence the livability of the animals.[13]

But most of the extinct species do not have such close relatives anymore. Interspecies nuclear transfer like in Jurassic Park can be another possibility for de-extinction, that means to revive species that have gone extinct or are on the verge of extinction. The San Diego Zoo Institute for Conservation Research maintains a large collection of cells and embryos called Frozen Zoo®.[14] By using reproductive technologies they develop methods to prevent endangered species like the northern white rhino or the Przewalski horse from extinction or inbreeding.[ 15] The first animal of an endangered species that was successfully cloned was a gaur (bos gaurus), an Asian ox, in 2001 by Advanced Cell Technology using genetic material from the San Diego Zoo. DNA from the skin cells of a male gaur were implanted into empty cow egg cells, grown into blastomeres that were then transferred into the wombs of domestic cows. One of eight embryos developed to a full-grown calf. Unfortunately, after being born, the gaur did not live for more than two days. However, the cause of death is considered to be an infection and not the fact that it is a trans-species clone.[16] The second clone that was created with the very same method had a higher life expectance. It was a banteng (bos javanicus), another endangered Asian cattle. Also remarkable is, that the used fibroblasts were taken and frozen 25 years before, in 1978.[17] An attempt to clone a species that has already gone extinct, the Pyrenean ibex (capra pyrenaica pyrenaica) failed since the kid was born with a deformed lung.[18]

The fact that cloned cells do in principle develop to embryos and even prolific adult animals (like Dolly) gives hope that one day species that have recently been wiped out could come back to life. But besides the challenging and time-consuming scientific research these plans also evoke a lot of critical questions in the society:

How is decided which species will be revived and which stays extinct?

It is clearly difficult to revive every species that we know has ever lived on this planet. There would just not be enough space and food and we might soon experience another wave of mass extinction. Since DNA from fossils might be too old, mammoths and dinosaurs are still out of question. This is shifting the focus on species of the recent past. But how can we select which species can live again and which won’t? We surely must consider the preservation of still existing species as a priority.

Where should they live?

If it is possible to clone many animals of one kind that can even mate, there must be a safe and nourishing environment, most likely captivity. Who knows how an entire species that has been created in captivity will develop? And the knowledge about the behaviour and needs of most of those animals is very little.[13]

Who is going to pay?

The scientist’s motivation might surely be an idealistic one but somehow all the research and maintenance must be financed. Innovations will always attract temporizers that try to exploit it financially. Zoos and wildlife parks that exhibit animals are the lesser problem. Some worry that wealthy poachers and “gourmets” who don’t withhold from hunting and eating endangered species now will just as much be attracted by the thought of getting hold of a cloned specimen. Paying to hunt an endangered species to support the protection financially is already practised in southern Africa and raises a lot of ethical issues.[19,20]

To see living “fossils” like dinosaurs, mammoths, dodos and all the others is surely an exciting thought. But if mankind proceeds like this, in just a few decades there might be much less animals on earth than there are now. Let’s hope that combined common sense, technical progress, and less vanity will lead to a preserved and healthy nature in our future.

‒Tatjana Dänzer

Read more:

[1] I. Wilmut, A. E. Schnieke, J. McWhir, A. J. Kind, K. H. S. Campbell, Nature 1997, 385, 810–813.

[2] M. Crichton, Jurassic Park, Alfred A. Knopf, Inc., 1990.


[4] S. Lü, Y. Li, S. Gao, S. Liu, H. Wang, W. He, J. Zhou, Z. Liu, Y. Zhang, Q. Lin, C. Duan, X. Yang, C. Wang, J. Cell. Mol. Med. 2010, 14, 2771‒2779.

[5] By en: converted to SVG by Belkorin, modified and translated by Wikibob – derived from image drawn by / de: Quelle: Zeichner: Schorschski / Dr. Jürgen Groth, with text translated, CC BY-SA 3.0,


[7] C. Mora, D. P. Tittensor, S. Adl, A. g. B. Simpson, B. Worm, PLoS Biology, 2011, 9, 1‒8.

[8] D. B. Weishampel, P. Dodson, H. Osmólksa, The Dinosauria, 2nd ed., University of California, 2004.

[9] D. P. G. Bond, P. B. Wignall, Geological Society of America Special Papers, 2014, 505, 29–55.



[12] J. A. Leonard, N. Rohland, S. Glaberman, R. C. Fleischer, A. Caccone, M. Hofreiter, Biol. Lett., 2005, 1, 291‒295.

[13] B. Shapiro, Genome Biology, 2015, 16, 1‒3.

[14], [15]


[17] D. L. Janssen, A. L. Edwards, J. A. Koster, R. P. Lanza, O. A. Ryder, Reproduction, Fertility and Development, 2004, 16, 224‒224.




Feb 052019
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Imagine you are on an airplane, ten thousand meters up in the sky. Now, if you close your eyes you know exactly which way the airplane has started moving, whether it has begun to manoeuvre to the right or to descend. This ability we owe to our inner ear as a part the humans’ vestibular system.

The vestibular system is designed to send information about the position of the head to the brain’s movement control centre, that is the cerebellum. It is made up of three semi-circular canals and two pockets called the otolith organs (Fig. 1), which together provide constant feedback to the cerebellum about head movement. Each of the semi-circular canals is orthogonal to the two others so that they detect the variety of movements in three independent directions: rotation around the neck (horizontal canal), nodding (superior canal) and tilting to the sides (posterior canal). Movement of fluid inside these canals due to the head movement stimulates tiny hairs that send signals via the vestibular nerve to the cerebellum. The two otolith organs (called the saccule and utricle) signal to the brain about linear movements (backwards/forwards or upwards/downwards) and also about where the head is in relation to gravity. These organs contain small crystals that are displaced during linear movements and stimulate tiny hairs communicating via the vestibular, or balance nerve to the cerebellum.

So why is that, even equipped with such a tool, sometimes we get a feeling sitting on an airplane that it is falling down when in fact it is not? Why is that some people, particularly underwater divers, may lose direction and no longer know which way is up?[1] Surely, typical divers should still have the inner ear, unless a shark has bitten their heads off. Is it all caused by stress? Actually, there is much more to it!

Humans have evolved to maintain spatial orientation on the ground, whereas the three-dimensional environment of flight or underwater is unfamiliar to the human body, creating sensory conflicts and illusions that make spatial orientation difficult. Normally, changes in linear and angular accelerations and gravity, detected by the vestibular system, and the relative position of parts of our own bodies, provided by muscles and joints to the proprioceptive system, are compared in the brain with visual information. In unusual conditions, these sensory stimuli vary in magnitude, direction, and frequency. Any differences or discrepancies between visual, vestibular, and proprioceptive sensory inputs result in a sensory mismatch that can produce illusions. Often the result of these various visual and nonvisual illusions is spatial disorientation.

For example, fighter pilots who turn and climb at the same time (they call it “bank and yank”), feel a strong sensation of heaviness. That feeling, caused by their acceleration, surpasses the pull of gravity. Now, if you asked them while blindfolded to tell which way was down using only their vestibular organ, they would point to the cues provided by the turn, not to the cues provided by the earth’s gravity. [2]

Furthermore, the vestibular system detects only changes in acceleration, thus a prolonged rotation of 15-20 seconds [3] results in a cessation of semi-circular output. As a result, the brain adjusts and does not feel the acceleration anymore which can even result in the perception of motion in the opposite direction. In other words, it is possible to gradually climb or descend without a noticeable change in pressure against the seat. Moreover, in some airplanes, it is even possible to execute a loop without exerting negative G-forces so that, without visual reference, the pilot could be upside down without being aware of it.

Another interesting example is the phenomenon of loopy walking. When lost in a desert or a thick forest terrain without landmarks people tend to walk in circles. Recent studies performed by researchers of Max Planck Institute for Biological Cybernetics, Germany, revealed that blindfolded people show the same tendency. Lacking external reference points, they curve around in loops as tight as 20 meters in diameter while believing they are walking in straight lines. [4]

Seemingly the vestibular system is quite easy to trick by eliminating other sensory inputs. However, even when visual information is accessible, e.g. underwater, spatial disorientation can still occur [any scuba diving forum – for the reference]. The obvious fact that water changes visual and proprioceptive perception is crucial here: humans move slower, see differently and let’s not forget the Archimedes’ principle. It happened a lot, that a confused diver thought that the surface was down, especially when the bottom seemed brighter because of reflections. This can be a dangerous mirage in such an unusual gravity. On top of it, water can affect the vestibular system directly through the outer ear. When the cold water penetrates and reaches the vestibular system, it can cause thermal effects on the walls of the semi-circular canals, leading to slight movements of the fluid inside, which are enough to be detected by the brain.[5] Just like in the situations described before this causes the symptoms of spatial disorientation and dizziness.

Fig. 1. Schematic structure of a humans’ inner ear [6].

The vestibular system is indeed frightfully complicated. We can trick it for fun riding roller coasters in an adventure park, but when incorrect interpretation of the signals coming from the vestibular system occurs at the wrong moment this can lead to serious consequences. Luckily, nowadays the airplanes and even divers are equipped with precise instruments used to complement the awareness of the situation and thus avert dangerous situations.

P.S. If you are interested, try riding an elevator while seated on a bike.

— Mariia Filianina


  1. The Editors of Encyclopaedia Britannica, (2012). Spatial disorientation, Encyclopædia Britannica, inc.,
  2. L. King, (2017). The science of psychology: An appreciative view. (4th. ed.) McGraw-Hill, New York.
  3. Previc, F. H., & Ercoline, W. R. (2004). Spatial disorientation in aviation. Reston, VA: American Institute of Astronautics and Aeronautics.
  4. J. L. Souman, I. Frissen, M. N. Sreenivasa and M. O. Ernst,Walking straight into circles, Current Biology 19, 1538 (2009).
Jan 052019
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Certainly, most of us enjoy an occasional nice bowl of spaghetti. Some of us use a spoon along with the fork, some don’t. Doesn’t matter, as long as you enjoy and don’t make a mess. But have you ever wondered whether there is a preferred direction to turn the fork? And is it related to where you live? We did!

In our last issue (Vol 2, 2018), we launched a survey asking our readers exactly this question (Figure 1).

Figure 1: The Spaghetti Turn survey as it appeared on the webpage.

Our survey was advertised in social media (Facebook, LinkedIn, Twitter, ResearchGate) and via QR codes on flyers. The survey reached a total number of n=160 readers, 132 of them found their way directly to our website. The results are shown in Table 1 and Figure 2.

Table 1: Results of the survey “The Spaghetti Turn”.

Northern hemisphere Southern hemisphere worldwide
n % n % n %
right-handed clockwise 117 75.5 3 60 120 75.0
right-handed counter clockwise 12 7.7 1 20 13 8.1
left-handed clockwise 10 6.5 0 0 10 6.3
left-handed counter clockwise 10 6.5 0 0 10 6.3
both-handed clockwise 0 0 0 0 0 0
both-handed counter clockwise 1 0.6 0 0 1 0.6
shovel 4 2.6 1 20 5 3.1
other 1 0.6 0 0 1 0.6
sum 155 96.9 5 3.1 160 100

Figure 2: Worldwide percentage of the preferred direction to turn the fork when eating spaghetti related to the handedness (values in %).

The option „no preferred direction” remained unanswered. One single participant chose “I am right-handed and turn clockwise” and “I am right-handed and turn counter clockwise”, depicted as “other”. Assuming that this is no miss-click one out of a total number of 160 participants has no preferred direction when using the fork with their right hand. This underlines that most people on earth indeed have a favourite direction to screw the fork.

Although there is no clear definition to determine handedness, some publications claim that 70–95 % of human population worldwide are right-handed, 5–30% are left-handed and a small minority is ambidextrous.[1] This is consistent with our findings: the survey was answered by 133 right-handed people, which is 86.9% of all 154 participants who revealed their handedness. 20 participants are left-handed (13.1% of all 154 participants who revealed their handedness). One participant (<1%) is ambidextrous and turns the fork counter clockwise with both hands.

75.0% of all participants are right handed and turn the fork in clockwise direction. Only 8.1% turn it counter clockwise. Surprisingly, there seems to be no preference about the turning direction among left-handed people. Their numbers equal (each ten or 6.3%), while 90.2% of all right-handed people turn clockwise. Fortunately (or shockingly?), 3.1% of spaghetti eaters worldwide shovel.

Unfortunately, we did not reach a significant number of readers from the southern hemisphere. Four participants out of five are right-handed, one shovels. 60% of the right-handed southerners turn the fork clockwise, 20% turn it counter clockwise. Considered that only five participants (3.1% of all) do not represent the whole ~10% of the human population living on the southern hemisphere,[2] the preference of turning counter clockwise shows the same tendency for both hemispheres. There is therefore supposedly no relation to where you live on this planet.

But why is the clockwise direction so obviously favoured?

Time and therefore clocks have a powerful influence in our daily lives. Also, in a lot of cultures texts are written from left to right (as the clockhand moves). Moving and looking to the right is very often linked to the future and openness. An experiment from Sascha Topolinski and Peggy Sparenberg from 2012 suggests, that the preferred direction to turn objects could be determined by one’s conservative or open personality.[3] Or is it just for handling reasons only and it is a little easier to apply force on the edge of the fork while turning it clockwise?

With a simple survey like our’s it is impossible to determine whether the habit to turn the fork left or right is a matter of education, subconsciousness or technique.

Throughout the active survey it was possible to answer the poll via the Facebook “Surveys for Pages” and our webpage. Hence, we cannot entirely assure the integrity of the results. Also, we hope our readers understand humour but also answer the survey genuinely. We simply trust in the scientific spirit of our readers. We also did not consider that for cultural habits in certain cultures spaghetti dishes might not be available or forks might not be part of the traditional cutlery. Although it is very often a cause for heavy crossfires during meals, the use of a spoon along with the fork is discounted in the evaluation of the results too. With this survey we just aim to give a picture about the general turning behaviour of spaghetti eaters. To the best of our knowledge there has not been a similar survey until now.

We are now smarter than before but still missing the details of the big picture. Let’s see what the new year brings…

Tatjana Dänzer, Mariia Filianina, Alexander Kronenberg, Kai Litzius, Adrien Thurotte

The editorial team of the Journal of Unsolved Questions thanks all 160 participants of the survey and wishes Bon Appetit and a very happy start into the year 2019!

Read more:

[1] [ (last access 31.12.18, 15:20).

[2] (last access 31.12.18, 15:40).

[3] Sascha Topolinski, Peggy Sparenberg, Social Psychological and Personality Science, 2012, 3, 308–314.

Dec 042018
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When Francis Guthrie took on the task to colour a map of England in 1852 he needed four colours to ensure that no neighbouring shires had the same colour. Is this the case for any map imaginable, he wondered.

As it turns out, five colours do suffice, as mathematically proven in 1890 in the five-colour theorem [1]. That indeed four colours are enough to colour a map if every country is a connected region took until 1967 to prove [2] and required computer assistance. It abstracted the idea to geometric graph theory where regions are represented by vertices connected by an edge if they share a border (see fig. 1).

Fig 1: Illustration of the abstraction of the map colouring problem to graph theory.

The four-colour theorem was then proven by demonstrating the absence of a map with the smallest number of regions requiring at least five colours. In its long history the theorem attracted numerous false proofs and disproofs. The simplest versions of counterexamples focus on painting extensive regions that bordering many others, thereby forcing the other regions to be painted with only three colours. The focus on the large region might cause people’s inability to see that colouring the remaining regions with three colours is actually possible.

Even before the four-colour theorem was proven, the abstraction to graph theory evoked the question as to how many colours would be needed to colour a plane so that no two points on that plane with distance 1 do have the same colour. This is also known as the Hadwiger–Nelson problem. Note that we are not colouring continuous areas in this case, but instead each individual point of the plane, rendering it extremely more complex. In the 1950s it was known that this sought number, the chromatic number of the plane, had to be between four and seven.

The upper border is known from the existing tessellation of a plane by regular hexagons that can be seven-coloured [4] (fig. 2). The maximal distance within one hexagon, the diameter, needs to be smaller than one to comply with the requirement. Additionally one needs to ensure that the distance to the next hexagon of the same colour is larger than one. These constraints imply that the hexagon edge length a has to be between 0.5 and $\sqrt(7)/2$ for an allowed colouring of the plane, where no two points with distance one have the same colour.

Fig. 2: Colouring of a plane in a seven colour tessellation pattern of regular hexagons.

As to the lower border for the chromatic number of the plane, it is obvious that two colours will not suffice to colour even the simple unit-distance path of an equilateral triangle (see fig. 3 a). To demonstrate that three colours do not suffice either and therefore at least four colours a needed, we take a look at the Moser spindle shown in fig. 3 b. The seven vertices (all eleven edges / connections have unit-distance) cannot be coloured with three colours, say green, blue, and yellow. Assigning green to vertex A, its neighbours B and C need to be blue and yellow, respectively, or vice versa, enforcing D to be green again. A’s other neighbouring vertices E and F analogously are assigned blue and yellow, or vice versa, enforcing in turn G to be green. This conflicts with G’s neighbour D to be green, too, thus demonstrating that arbitrary unit-distance graphs require at least four colours.

Fig 3: a) An equilateral triangle as a simple example for a unit-distance graph. b) The Moser spindle is a four-colourable unit distance graph [3].

After many years of intractability only this year there was some significant progress in closing in on the Hadwiger–Nelson problem. It was demonstrated that “the chromatic number of the plane is at least 5” [5], by finding two non-four-colourable unit-distance graphs (with 20425 and 1581 vertices). The smallest unit-distance graph with chromatic number five found this year has 553 vertices [6] and is shown in fig. 4. Whether the chromatic number of the plane is five, six, or seven still remains to be shown.

Fig 4: Five-colourable unit distance graph with 533 vertices. The fifth colour (white) is only used in the centre. [6]


— Alexander Kronenberg

[1] Heawood, (1890), “Map-Colour Theorems”, Quarterly Journal of Mathematics 24, pp. 332–338

[2] Appel, Haken, (1989), “Every Planar Map is Four-Colorable”, Contemporary Mathematics 98, With the collaboration of J. Koch., doi:10.1090/conm/098

[3] Soifer, (2009) “The Mathematical Coloring Book”, Springer

[4] Hadwiger, (1945), “?berdeckung des euklidischen Raumes durch kongruente Mengen”, Portugal. Math. 4 ,pp. 238–242

[5] de Grey, (2018), “The chromatic number of the plane is at least 5”, arXiv:1804.02385

[6] Heule, (2018), “Computing Small Unit-Distance Graphs with Chromatic Number 5”, arXiv:1805.12181

Nov 222018
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It is one of the most common educational experiments in school and straight from the books: The reaction of an alkali metal with water. During this reaction significant amounts of hydrogen gas are produced, which can ignite and thus explode due to the strongly exothermic reaction – at least that is the explanation one finds pretty much everywhere. However, there is something odd about this reasoning. On the one hand, a complete immersion of the metal within water should then prevent the explosion from happening as no oxygen is present to ignite the hydrogen gas. On the other hand, it is surprising that the solid-liquid interface of this heterogeneous reaction creates enough physical contact to drive the reaction. Additionally, the produced gas tends to separate the educts and therefore stop the reaction. Overall, there are quite a few unclear details in this proposed reaction mechanism.

A study of the Czech Academy of Sciences in Prague and the Technical University of Braunschweig, however, showed that even in presumably clear textbook reactions a lot of surprises may be found sometimes. [1,2] The scientists used drops of sodium-potassium alloy that is liquid at room temperature and filmed the reaction with high speed cameras. They could show that the explosive reaction also happens under water when the metal is completely immersed, thus ruling out the ignition of the hydrogen gas as the main driving mechanism for the explosion. Supported by molecular dynamics simulations, they instead showed what mechanism actually drives the reaction: A Coulomb explosion! During the reaction of a clean metal surface with the adjacent water molecules, electrons move quickly from the metal atoms into the water. This also explains why a solid piece of an alkali metal does not always explode in water: it needs a clean interface without significant oxidation. After the electrons left the metal surface and moved into the water, a strongly charged surface is left. On this surface, the ionized atoms strongly repel each other, and thus open up a path to more inner atoms that have not taken part in the reaction yet. On a time scale of about 0.1 ms, metal dendrites shoot into the water (see figure) and suddenly increase the surface area of the metal. [1-3] This happens extremely fast with giant charge currents flowing in the interface region. The surface tension is pretty much nullified in this case [2,3] and the expanding surface provides more reactive area. As a result, large amounts of hydrogen gas are suddenly produced. Together, these effects drive the explosion, while the ignition of the gas is not directly necessary for the explosion to occur. Instead, the hydrogen gas can also burn off later. [2]

Further results of the study could lead to approaches to avoid metal-water explosions and thus gain application relevance in industry. What is however most unusual about this study is that parts of it got funded by the YouTube science channel of the lead author of the paper, which he explicitly acknowledges. In this exciting case, science and media are really in a close relationship.

As soon as a drop of NaK-alloy gets in contact with water (top left), fine metal fingers are protruding into the water (middle). These are driven by the Coulomb explosion that massively increases the surface area and therefore the reactive interface. As a result, a fast production of hydrogen becomes possible, which further drives the explosion (bottom left). The right column depicts the impact of a water droplet for reference. [1,3]

— Kai Litzius


[1] P. E. Mason et al., Nature Chemistry 7, 250–254 (2015).