How physics and biology work together to understand cell organization

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Cells get organized

How researchers probe the physics of motion, communication and organization in cell networks, and how understanding these systems could help us tackle serious issues in medicine and biology

self-organized bacterial community

A colony of bacteria organize with each other under certain conditions to maximize nutrient intake. [Credit: Eshel Ben-Jacob]

From Physics World:

Consider this scenario: in your haste to grab the latest issue of Physics World, you scrape your hand on your postbox. It’s nothing severe, just a little scratch, but if your immune system is functioning as it should, your body will perform an amazing feat of microscopic organization. Your body assesses the level of damage and threat from infection, sending security cells to the site to hoover up intrusive bacteria and seal the wound. Within a few days you’d hardly know the scrape was ever there: your skin and blood vessels repair themselves.

Except of course there’s no mind behind this repair. Your brain isn’t required to heal a wound: there’s no local oversight from any intelligent agent, and the cells involved don’t think. Instead, cells interact with their neighbours, and a larger pattern emerges from those small-scale interactions. That’s the key to “self-organization”, whether it occurs in the human immune system, swarms of locusts, water molecules in a snowflake or electrons in a magnetic material.

For that reason, researchers studying biological self-organization draw heavily on physics. Some directly investigate the physical interactions between cells and their environments; others use theoretical models drawn or adapted from physics to understand emergent behaviours in biological systems. It’s an interdisciplinary field, involving physicists, computer scientists, biologists, mathematicians and medical doctors.

The rest of this story is in the print edition of Physics World, which you can subscribe to through membership in the Institute of Physics, which costs £15, €20, or $25 per year. You can join by clicking here. You can also get a nice mobile- and tablet-formatted version of the story using the Physics World app, available in the Google Play and iTunes stores. However, if you just want to read the rest of this article, Physics World has kindly allowed me to offer it to you as a PDF download, which looks exactly like the printed version!

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The physics of dinosaurs!

[ This blog is dedicated to tracking my most recent publications. Subscribe to the feed to keep up with all the science stories I write! ]

Computer model for the swing of a Stegosaurus tail-spike assembly, also known as a thagomizer from a classic Gary Larson cartoon. (Alas, we didn't get permission to reprint this cartoon.)

Computer model for the swing of a Stegosaurus tail-spike assembly, also known as a thagomizer from a classic Gary Larson cartoon. (Alas, we didn’t get permission to reprint this cartoon.)

Like many (most?) of us, I was a huge dinosaur fan as a kid. I read every horrible, outdated book I could get my hands on. I read Robert Bakker’s book The Dinosaur Heresies not long after it was published, with its often-wrong but very provocative reimagining of how dinosaurs lived, moved, and interacted with their environments. My primary scientific love was space, and so I pursued physics as a career, but I never completely forgot my dinosaur obsession. Now in the February 2017 issue of Physics World, I get to combine the two interests!

Deducing how dinosaurs moved

How did dinosaurs dash and their cousins the pterosaurs take flight? Physics-based modelling is helping to solve these mysteries of movement

For Physics World:

Jurassic Park and its sequels are best thought of as monster movies. But they do make dinosaurs look and act like real animals – which, of course, they were. For more than 100 million years, various groups of dinosaur were the largest predators and herbivores on the planet. There were many smaller species too, though we only know about a fraction of them, since fossils of them are rare, and we’re aware of many only through fragments.

Scientists have been able to answer the biggest scientific question posed by Jurassic Park in one of its most tense chase scenes: could a Tyrannosaurus rex outrun a Jeep? (Answer: no.) Knowing the top speed of an apex predator is vital as it tells us what sorts of prey it could catch. To better understand these creatures, scientists also want to know if a Stegosaurus’ fearsome spike-wielding tail could be used as a weapon, and what damage it could do. Another question is how pterosaurs (cousins of the dinosaurs) could evolve to become the largest flying animals.

Answering all of these questions involves understanding what forces and torques these creatures’ skeletons could withstand. [Read the rest at Physics World]

Could gravity have mass?

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Might gravity have mass?

Click on the image to read the whole article for free, courtesy of Physics World.

Click on the image to read the whole article for free, courtesy of Physics World.

From Physics World:

When confronted with something unexplained in the data, scientists face several possibilities. Maybe there’s an error and the result is spurious. Maybe there’s a more mundane explanation they simply overlooked. Or perhaps the unexplained is a sign that a theory needs to be revised or supplanted. That last option is the rarest, at least when the theory in ques- tion is a successful one. After all, any new theory must explain all the same phenomena an old theory explained, and predict something new that can’t be handled with the old.

One unexplained result that’s been bugging physicists for more than 15 years is dark energy, which is the name we give to our ignorance. The universe is expanding at an accelerating rate, but we don’t know why. To make matters worse, dark energy comprises roughly three-quarters of the total energy content of the cosmos, so it’s not a minor thing we don’t get. For that reason, a small but dogged group of physicists thinks the existence of dark energy might be a clue that we need to revise one of the most successful theories we have: general relativity.

One way to revise general relativity is to modify the nature of the gravitational force so that it behaves as though it has mass.

The rest of this story is in the print edition of Physics World, which you can subscribe to through membership in the Institute of Physics, which costs £15, €20, or $25 per year. You can join by clicking here. You can also get a nice mobile- and tablet-formatted version of the story using the Physics World app, available in the Google Play and iTunes stores. However, if you just want to read the rest of this article, Physics World has kindly allowed me to offer it to you as a PDF download, which looks exactly like the printed version!

How standard are “standard candles”?

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Not-so-standard candles

From Physics World:

The story is already legendary. In the late 1980s and early 1990s, two groups of rival researchers set out to measure the deceleration of the expanding universe. These groups often used the same observatory, sometimes even using the same telescope on consecutive nights. And they both found the same thing, publishing their results at roughly the same time in 1998–1999: the expansion of space–time isn’t slowing down at all. In fact, it’s getting faster. The leaders of those collaborations – Saul Perlmutter and Brian Schmidt – along with Adam Riess of the latter’s group, won the Nobel Prize for Physics in 2011 for this discovery. The implication of the result was that the universe consists not only of visible matter and dark matter, but also a gravitationally repulsive substance. Known as dark energy, the nature of this weird stuff remains as mysterious today as when it was first discovered.

Both groups used certain kinds of exploding stars called type Ia supernovae for their measurements. These supernovae brighten and fade in very similar ways and the current thinking is that this is because they have a common source: the explosion of either one or two white dwarfs, which are the stellar remnants of small-to-medium-mass stars such as the Sun. This consistent brightness allows astronomers to determine how far away the object was when the light left it and for that reason, type Ia supernovae are known as “standard candles” – reliable light- houses in the measurement of cosmic distances.

Or so we all thought.

The rest of this story is in the print edition of Physics World, which you can subscribe to through membership in the Institute of Physics, which costs £15, €20, or $25 per year. You can join by clicking here. You can also get a nice mobile- and tablet-formatted version of the story using the Physics World app, available in the Google Play and iTunes stores. However, if you just want to read the rest of this article, Physics World has kindly allowed me to offer it to you as a PDF download, which looks exactly like the printed version!

I Love Q, and now you can too!

I wrote a feature story for Physics World on an interesting little discovery about neutron stars, but unfortunately the article wasn’t in their free online edition. HOWEVER, the editors have kindly let me repost the article here in PDF format for free download! (Here’s the summary I wrote a few weeks ago.)

Physics World is a glossy magazine published by the Institute of Physics (IoP) in Europe. My articles are in the print version, but you can access them online by joining IoP (US$25 per year) and see everything they publish either through the Physics World website (which also has tons of free content) or the app, available on iTunes or Google Play.

The three little words every pulsar wants to hear

[ This blog is dedicated to tracking my most recent publications. Subscribe to the feed to keep up with all the science stories I write! UPDATE: you can now download this article in PDF format! See the follow-up post or the update below.]

I can’t help falling in Love with Q

The first page of my latest print article in Physics World. Unfortunately, there doesn't seem to be an online version.

The first page of my latest print article in Physics World. Unfortunately, there doesn’t seem to be an online version.

From Physics World:

The dancers are an elegant pair. Clothed in the fabric of space–time, they are driven by the music of gravity and make a stately orbit around one another once every two-and-a-half hours. They pirouette as they move – one spins once every few seconds while the other spins many times per second – and each one of their twirls is marked by an intense flash of light. The dancing partners are pulsars – spinning neutron stars that send a regular blip of light our way.

Named PSR J0737-3039, this duo is one of a kind. More commonly known as the “double-pulsar system”, it is the only two-pulsar system where we have observed both partners. Other binary-pulsar systems exist, consisting of a pulsar and, for example, a white dwarf or a (non-radiative) neutron star. However, astronomers find the double-pulsar system particularly valuable because it consists of two flashing beacons rather than one, and the more information they can glean to test their theories, the better.

Unfortunately, this article is currently only available in print, and Physics World isn’t a typical newsstand offering. Update: the editors have kindly let me repost the article here in PDF format for free download! You can also access all the content online by joining IoP (US$25 per year) and see everything they publish either through the Physics World website (which also has tons of free content) or the app, available on iTunes or Google Play.

I am overly proud of the headline, and the concepts I described in the article are very interesting. In brief, measurable properties of neutron star exteriors are independent of the particular physics going on inside. Since neutron stars are some of the most complex objects we know of — they are the density of an atomic nucleus, the mass of a star, and the size of a city on Earth — anything we can learn to help study them is a good thing. A few theorists figured out how to relate observable properties to each other, in particular three parameters labeled I, Q, and the “Love number” (named for a person, not the emotion). The I-Love-Q relations in combination with sophisticated neutron star observations could hopefully help us solve the deep mystery of what’s going inside an object that’s like nothing we can create in the lab.

(If you want some more technical information, here’s the main paper I drew on for background.)

I'm in a magazine!

I’m in a magazine!

Physics is largely a matter of finding patterns in natural processes and translating that to mathematical expression. That’s a horribly oversimplified view, of course, but there’s no question that physics (and other branches of science) seeks to find symmetries. The huge successes of modern particle physics have largely arisen from identifying symmetries — and when those symmetries break down. To cite just one: physicists understand the weak force, which governs neutrinos and processes like nuclear beta decay, using a mathematical symmetry. That symmetry isn’t perfect, however, and one outward manifestation of that imperfection is the Higgs boson.

This pattern-seeking behavior among physicists is the theme of Dave Goldberg’s book The Universe in the Rearview Mirror: How Hidden Symmetries Shape Reality. I reviewed the book for Physics World, which marks my first publication in a print magazine. (It also may be the first time The Decemberists were quoted in Physics World.) You can read my review online, though the site requires a free registration to do so. In brief, I enjoyed the book, but found a few problems with it as well.

Inevitably, Goldberg’s explanations vary in quality. I found his discussion of the Casimir and Unruh effects (weird quantum phenomena in the vacuum) to be very good introductions for non-specialists. He also provides an excellent summary of the problems facing attempts to unify the different forces of nature, and specifically the question of pro- ton decay. On the other hand, his explanation of Lagrangians and the principle of least action (both essential topics in a mathematical sense) falls short, since it requires him to define a lot of new terminology in just a few pages, most of it barely mentioned again. The book also misses an opportunity to explain how specific symmetries shaped the development of the Standard Model; while it outlines a few of the important symmetries (including parity or reflection symmetry, time-reversal, time-translation, and exchange of matter and antimatter) early on, it fails to bring them back into the picture when the Standard Model is discussed. [Read more…]

We are bound by symmetry