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How long does a Photon take to Form?

Category: Science & Technology
Posted: 11:29AM

The photoelectric effect describes a connection between electricity and optics. The energy of an electron can change based on an interaction with a photon of a proper frequency. For example, when an electron with a lot of energy wants to give some up, it will do so by releasing a photon as it falls to a state of lower energy. Researchers have been wondering how long it takes for this photon to form, and some at Aarhus University have figured it out.

Most people would think that the photon is instantaneously made, because the electron that produces it instantaneously moves from one place to another, without existing anywhere in between. Despite this quantum mechanical fact and reasonable hypothesis, the researchers devised a simple mechanical way to measure the formation time. They shot high energy electrons at two carefully positioned pieces of gold foil. When the electron enters the first foil, it bounces around as it interacts with the gold atoms. This bouncing causes the electron to slow down and release photons. The frequency of the emitted photons depends on how much the electron is slowed, which then depends on how thick the foil is. Here is the clever part.

If it takes time for the photon to form and be released, then the process could start within the first piece of foil, without completing. When the electron enters the second foil though, enough time may pass for the photon to form and be emitted. When the photon is emitted in the second foil, its frequency and how the foil scatters it will be different than if it had completely originated in the second foil. By knowing the distance between the two foils, and the speed of the electron, the researchers could then find the formation time of the photons emitted this way.

By varying the distance between the two foils, the researchers were able to tune into the time it takes to form photons, of a specific energy. Next the researchers want to repeat the experiment, but with a laser in the place of the second foil, to allow for better analysis. Knowing how long it takes for a photon to form from an electron could affect future optoelectronics as the materials could be made to the proper size.

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