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Podcast:Yoctonewtons: The Smallest Recorded Force

This week on the Physics Central Podcast, I talk with physicist Dan Stamper-Kurn about making the smallest measurement of a force ever recorded.  He and his group (including lead author Sydney Schreppler) applied a force to a cloud of 1200 atoms, using a laser. Their measurement came out to 40 yoctonewtons: that's 40 x 10-24 newtons (if you drop an apple from a third story window, it hits the ground with about 1 newton of force).

The reason this measurement is significant is because it gets to within a factor of 4 of the standard quantum limit, or SQL. This is a natural limit to how precisely scientists can measure certain variables. (The proof for this is in the Heisenberg Uncertainty Principle).  The limit arises through various means, but scientists reach it when the system itself has an uncertainty greater than the measurement. In many cases, the observer imposes this limit: for example, if a scientist uses photons to study a single atom, the photons may start to influence the motion of the atom. So at some point the scientist can't discern the natural motion of the atom from the motion imposed on it by the photons.

Reaching this limit is important for many experiments, including LIGO, the Laser Interferometer Gravitational Wave Observatory. LIGO is searching for ripples in space time, known as gravitational waves. When a gravitational wave passes by, it may stretch or contract space itself. A distance of 1 meter may suddenly be shorter by something like 10-21 meters. LIGO scientists want to measure these changes, but they are bumping up against the standard quantum limit. Nergis Mavalvala also chats on the podcast about how experiments like the one by the Stamper-Kurn group will help LIGO anticipate challenges that may arise as they approach the standard quantum limit.


  1. How do you figure that an apple dropped out of a third story window hits the ground with a force of about 1N? A medium apple weighs about 1N anyway. Why not just say that?? The force to stop its fall from any height ought to be dependent on the ground materials and conditions (how much time the ground takes stopping the fall), which were not specified. Landing on cement would be quite different from landing on lawn turf which would be different from landing on freshly tilled garden soil, and so on and so forth. Whether the apple bounces or "spats" or breaks apart upon impact will also influence the force of impact.

  2. that's impulse. different from force, kiddo. F=ma. Mass of an apple with acceleration. Voila


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