### The Physics of Crowds

What do the grains of sand in an avalanche and the people at the 2009 Presidential Inauguration have in common? There are so many small parts that the best way to understand how they all flow is through the physics of fluid dynamics.

The news is reporting that close to 2 million people crowded the lawn of the National Mall for the inauguration of Barack Obama. At the end of the ceremony, the throngs of people surged out seeking to get somewhere warm. I know because I was right in the middle of it, caught up in the flow.
That's what fluid dynamics is, the study of the flow of liquids or large numbers of particles, or even large crowds of people. Whenever there is a whole bunch of anything moving together, like a running river or an avalanche, certain patterns and organizations tend to emerge. Areas with a smooth flow or turbulence can be predicted accuratly based on the fluid's properties like viscosity or particle size.

The same is true with large crowds of people. Properties like how densely packed the area is or what people are trying to get to, allow programmers to predict how a crowd will act in any environment. In large groups, people really behave just like a "smart fluid." Modeling the behavior of thousands of individual people (or particles) takes a lot of processor power. But, as computers continue to improve, more detailed information about each person in the crowd, like line of sight, or even individual personalities, can be programmed in.

This kind of crowd mapping can literally save lives. Architects used this technique to completely redesign an especially dangerous bridge in Saudi Arabia. Until its redesign, pilgrims traveling to Mecca would overwhelm the bridge, resulting inlarge numbers of trampling deaths.

Planners for yesterday's inauguration used similar kinds of models to make sure people would flow smoothly in and out of the National Mall. Where turbulence started to form, was where people were in danger. By keeping the flow running smooth, giving everyone a little room and avoiding crowd surges, the enormous crowd was able to disperse without major incident.

[UPDATE]

Here's a couple of photos from the inauguration. You can see how closely packed everyone is. Not too hard to see how the flow of an avalanche might be similar to everyone trying to move at once.

### How 4,000 Physicists Gave a Vegas Casino its Worst Week Ever

What happens when several thousand distinguished physicists, researchers, and students descend on the nation’s gambling capital for a conference? The answer is "a bad week for the casino"—but you'd never guess why.

### Ask a Physicist: Phone Flash Sharpie Shock!

Lexie and Xavier, from Orlando, FL want to know:
"What's going on in this video? Our science teacher claims that the pain comes from a small electrical shock, but we believe that this is due to the absorption of light. Please help us resolve this dispute!"

### The Science of Ice Cream: Part One

Even though it's been a warm couple of months already, it's officially summer. A delicious, science-filled way to beat the heat? Making homemade ice cream.

(We've since updated this article to include the science behind vegan ice cream. To learn more about ice cream science, check out The Science of Ice Cream, Redux)

Over at Physics@Home there's an easy recipe for homemade ice cream. But what kind of milk should you use to make ice cream? And do you really need to chill the ice cream base before making it? Why do ice cream recipes always call for salt on ice?