Refuting Einstein in 4 Easy Steps: Physicists Measure Brownian Motion

By Joseph Calamia | May 21, 2010 4:22 pm

brownian-motionA team of scientists led by Mark Raizen at the University of Texas at Austin had the gumption to take on Einstein. And according to their new paper in Science, they won. The point of contention? The lovechild of statistical mechanics and thermodynamics: Brownian motion.

Here’s how they did it.

Step 1. Learning the Moves

In the 1820s, Scottish botanist Robert Brown looked through a microscope at plant bits floating in water, and wrote [PDF]:

“I observed many of them very evidently in motion . . . [these motions] arose neither from currents in the fluid, nor from its gradual evaporation, but belonged to the particle itself.”

To make sure that the pollen wasn’t alive–actually swimming around–Brown tried it with coal dust. Dust had the same moves.

Today, we understand that Brownian motion, the random break dance of these tiny particles, comes from the water molecules bumping against them. In 1905, Einstein determined the properties of the liquid and the particles that would help describe their wanderings and the motion of molecules. But he also said that it was “impossible” to determine at any moment the speed and direction of a single particle during this dance.

Step 2. Water Into Air

The reason for Einstein’s doubt? The particles bumped around too quickly to ever measure their speed and direction:

He believed that it would be impossible in practice to track this motion, given the incredibly short timescales over which the Brownian fluctuations take place. [PhysicsWorld]

How quick is too quick? A very tiny glass sphere (think micrometers) in water would change direction almost every 100 nanoseconds (about the time it takes light to travel 30 meters). Raizen wanted to make the time between moves longer, so they didn’t use water. They put the glass beads on a dance floor with fewer partners, using a medium whose molecules are farther apart: air.

Step 3. Floating on Air

Pollen doesn’t float on air. Neither does a micrometer-sized glass bead. Raizen’s team needed something to hold the glass up. They decided that the answer was light particles in a pair of laser chopsticks:

In 1907, Einstein likely did not foresee a time when dust-sized particles of glass could be trapped and suspended in air by dual laser beam “optical tweezers.” Nor would he have known that ultrasonic vibrations . . . would shake those glass beads into the air to be tweezed and measured as they moved in suspension. [ScienceDaily]

They could control a glass bead’s motion to the precise point where it was still dancing the Brownian, but not too fast to follow. But the lasers allowed them to do more than suspend the glass: By looking at how the glass bead deflected the light while it was buffeted by air molecules and bounced about on the chopsticks, the researchers could determine what Einstein dubbed impossible, a bead’s instantaneous direction and speed.

Step 4. Future Directions

Understanding these discrete steps will help wherever Brownian motion rules: everywhere from cell guts to the scent of perfume wafting through apparently stagnant air.

“It is certainly an important achievement to be able to directly measure the velocity of the Brownian particle at these short times,” says Christoph Schmidt of the University of Göttingen in Germany. “Technically it is now becoming possible to track individual particles with very high time and spatial resolution, limited in the end only by how many photons per second one can get to interact with the particle.” [New Scientist]

Related content:
80beats: Putting “Ears” on a Microscope Lets Reseachers Listen to Bacteria
Cosmic Variance: The Cell is Like Tron!
DISCOVER: Einstein’s Gift for Simplicity
DISCOVER: Einstein’s Lonely Path

Image: Science / AAAS

  • NewEnglandBob

    How is it equivalent for particles in water vs particles in air suspended by lasers? They probably move at different speeds, no?

  • rapidmob

    Bob, from the article, “Raizen wanted to make the time between moves longer, so they didn’t use water.”

  • John Grigni

    The only thing they disproved from Einstein was his grasp of the technically feasible. Or did he say it was unknowable as well?

  • Bill door

    This doesn’t refute Einstein’s theory. It only refutes his assumptions about our ability to measure something.

    Leave it to a popular source masquerading as peer-reviewed scientific literature to use a misleading, sexy title on a scientific article.

  • Amos Zeeberg (Discover Web Editor)

    John: Einstein said the measurement was not technically feasible, not that it was unknowable. From the post: “He believed that it would be impossible in practice to track this motion.”

    Bill: The headline and post are correct; Joe never said the experiment disproved Einstein’s “theory,” just that he thought Brownian motion would be “impossible in practice” to see.

  • entrekken

    I read the headline, rushed to read the article and well Bill door Says’ post at
    May 23rd, 2010 at 11:17 pm summarized my sentiments exactly:
    Leave it to a popular source masquerading as peer-reviewed scientific literature to use a misleading, sexy title on a scientific article.

  • tc

    When someone says “this is not technically feasible” they usually mean it can’t be done with the then-current technology, not that it will never be possible. Sorry, the experiment didn’t “refute” (to use the term in the title) Einstein at all.

  • WhiskeyEchoPapa

    It’s actually quite a neat experiment. my question now is, what about the light pressure of the lasers, and the ambient light? I assume that has been factored in?

  • Saph

    @ WhiskeyEchoPapa: I’m wondering about this as well, what about the photon pressure? How does the coherency of the laser affect things?

  • Llor

    And how much is the precission of this speed (and direction)? 0.0000001m/s? perhaps 0.0000000001m/s? or perhaps Einstein knew the truth? Are you able of catch an atom and paint it in green colour?

  • mike

    True Brownian motion is non-differentiable! If you graph the position of an object over time, you have to find the slope to get velocity, but the slope is UNDEFINED at EVERY point for a graph of Brownian motion, so there is no such thing as the “instantaneous” velocity! When reading the actual paper and not this garbage write-up, they mention that, in fact. they are actually looking at ballistic movement.


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