### Right Again, Einstein

A new experiment by UC Berkley physicist Holger Müller and current Energy Secretary Stephen Chu (where does he find the time?!) confirms through experiment (though not for the first time) one piece of Einstein's theory of relativity (paper here article here).

The concept shown in the experiment is called gravitational redshift, which basically says that gravity can not only bend light, but as a result, slow down time. This slowing only happens to the observer who feels the gravitational pull. So lets say that you and I get matching Flava Flav-sized clocks:
and we take a trip to a black hole. You stand near the black hole where you feel a very large gravitational pull, and I stand far away. From where I'm standing, your clock appears to slow down. From your perspective, mine appears to speed up. You are stuck in a gravitational well, and as a result, time slows down.

The experiment used the principle that everything (yes, EVERYTHING) has both particle and wavelike properties. You may be familiar with the concept that light acts like both a particle and a wave. You can detect individual photons, but also see a wave pattern created by a beam of light. Because photons have no mass, it's easier for us to see both of those characteristics. Atoms and molecules also have a wavelength, but the waves oscillate so incredibly fast (3x10^25 or 30 million billion billion times per second) that we cannot currently detect them. (Remember, for clarity here, that wavelength and frequency are almost the same thing. Frequency is how often the crest of a wave passes by the same point. It's a time. Wavelength is the distance between those crests.)

So experimenters started with a cesium atom and went to work messing with it's wavelike existence. By striking the atom with a pulsed laser, the researchers change the direction of the wave, causing the atom to rise up a tenth of a millimeter away from the Earth's gravitational pull.

On this tiny scale, that very small change in distance reduces the pull of Earth's gravity on the atom (which, remember, is also a wave). That means that a cesium atom not lifted up by the laser experiences a greater gravitational pull, and time should move slower for it than for the atom lifted up.

So time should slow down for the atom that remains closer to the Earth. And the experiment shows that this is true.

Now, figuring out that this is actually happening gets a little too complex for me. Apparently in order to observe the time dilation requires either measuring the oscillation frequency of the cesium atom wave (which, as I said, we cannot currently do) or measuring the interference between the the two waves (which is why you can't just do this with particles). Apparently the cesium atom actually enters an alternate quantum reality, where one reality holds the cesium atom in the gravitational well, and another holds the cesium atom that is lifted up. But you can still measure both. Ugh, quantum mechanics you are kind of a jerk sometimes.

So why is this phenomenon called gravitational REDSHIFT? Redshift is what happens when an ambulance drives by you. Wave, of light or sound, being projected toward you by a moving object (like an ambulance) travel faster than they would be if the moving object weren't moving. This changes the frequency of the wave, which is responsible for the pitch of a sound wave and the color of a light wave. So a sound wave has a higher frequency (higher pitch) as it moves toward you, and a lower pitch as it moves away. Similarly, a light wave appears redder as it moves toward you, and bluer as it moves away. Using the reddening of the light as a cue, we call this phenomenon redshift.

This is not by any means the first time this particular part of Einstein's theory or any other part has been demonstrated through experiment. But talking about all that would be a a whole new blog post.

Hmmm...should I make this week's theme "Incidences where Einstein's theories have been proven through experiment", or should I make it "Kind of awkward pictures of Einstein"?

1. Awkward pictures of Einstein! I think Flavor Flav should be the poster bot of time dilation. Yeaaaah, boeeeey!

### 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?