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What Is a Femtosecond?

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  • Written By: Daniel Liden
  • Edited By: Bronwyn Harris
  • Last Modified Date: 04 November 2014
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A femtosecond is an incredibly small unit of time that is sometimes used for precise measurements in physical sciences such as chemistry and physics. One quadrillionth of a second is equal to one femtosecond; this means that one quadrillion femtoseconds, which is equal to a one billion times one million femtoseconds, make up one second. In scientific notation form, a femtosecond is equal to 1x10-15 seconds. To put it in context, even the fastest chemical reactions are measured in hundreds of femtoseconds. The number of femtoseconds in a second is far greater than the number of seconds in a human lifespan.

The femtosecond is a part of the International System of Units, called the SI system after the French le Système international d'unités. The International System of Units is used primarily for scientific inquiry and is built around the number ten, which is generally easy to deal with in quantitative situations. While it is similar to the metric system, it is more comprehensive and better defined.

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Femtoseconds are almost never used outside of scientific inquiry; they are simply too small to be convenient. A technique called femtosecond pulse shaping is used in optics to view various extremely rapid processes, such as some chemical reactions, on very short timescales. Pulse shaping is used to alter the duration of a short pulse of light from a laser, which can be used for observation purposes. Shorter pulses allow for shorter observable time frames, allowing for more detailed viewing of a process. Femtosecond pulse shaping gave rise to the field of femtochemistry.

Chemical reactions occur very quickly; generally one only sees the reactants, or substances that exist at the beginning of a reaction, and the products, or substances that exists at the end of a reaction. Reactions occur so quickly that one generally can not discern what happens between the beginning and the end of the reaction. Using femtosecond pulse shaping, scientists can view the intermediate stages of some reactions at incredibly small time intervals measurable in femtoseconds. This level of accuracy allows scientists to watch the stages of a reaction on a time scale that the human mind can actually understand and process.

While the femtosecond is an incredibly small unit of measurement, it is far from the smallest known unit. The smallest size distance about which anything can theoretically be known is called the Planck length and is measured at about 1.62x10-35 meters. The smallest unit of time is called Planck time, which is the amount of time it takes to travel one Planck length at the speed of light, the theoretical universal maximum speed. One Planck time is 5.39x10-44 seconds.

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