How can a photon be massless?

12.07.2013 14:08

How stable is the photon?

Dr. Bernold Feuerstein Press and public relations
Max Planck Institute for Nuclear Physics

Photons, the quanta of electromagnetic radiation, are usually assumed to be massless. Simple extensions of the theory, however, allow a rest mass other than zero. As a consequence, they could break down into even lighter elementary particles. A physicist at the Max Planck Institute for Nuclear Physics in Heidelberg has calculated how such a decay would manifest itself in the very old cosmic background radiation. In comparison with the precise measurement data from the COBE satellite, there is a lower limit for the lifetime of the photon of three years. [Physical Review Letters, July 11, 2013]

If you look in the data collection of the "Particle Data Group" for the properties of the light particle, i. H. of the photon, the remarkably short entry says: "Mass <2 ∙ 10–54 kg, lifetime: stable". In classical electrodynamics, the photon has no rest mass, it always moves at the speed of light in a vacuum and therefore does not 'age' - it is almost timeless.

However, there is no compelling theoretical reason that would prohibit a finite mass of the photon and there is also a mathematical description for this case. Even if it sounds exotic, it is worth considering the consequences of a massive photon, as Julian Heeck, doctoral student in Werner Rodejohann's group at the Max Planck Institute for Nuclear Physics (MPIK) in Heidelberg, did. This includes the possibility that the photon will break down into even lighter elementary particles. A candidate for this is e.g. B. the lightest of the three known neutrinos, which could even be massless. A photon with a - albeit tiny - mass would move in a vacuum almost - but only almost - at the "speed of light". This means that massive photons age, but only extremely slowly for us as observers due to their highly relativistic movement. The greater the energy of the photon or the frequency of the light, the more the relativistic mass dominates over the mass of a photon at rest.

In the search for a measurable effect of the photon mass and a possible decay resulting from it, the cosmic background radiation offers itself. On the one hand, this is very 'old' light, because it comes from the early universe around 13.8 billion years ago - the 'echo' of the Big Bang, so to speak. In addition, it is in the microwave range, so it is relatively low-energy. "In my view, what matters is not what photons decay into, but only their lifespan - it is therefore independent of the model," says Julian Heeck. The cosmic microwave background was measured in the early 1990s by NASA's COBE satellite with an accuracy of 10–4 and therefore offers a good data set for testing the lifetime of the photon. Because of the relativistic time expansion, the possible deficit due to background photons that have decayed since the Big Bang is greater the lower their energy or frequency is.

The figure shows the spectrum of the background radiation measured by COBE in comparison with the calculations by Julian Heeck. The relationship between rest mass and lifetime of the photon is included in the deviation from the ideal spectrum of the microwave background without photon decay. Within the error tolerance of the measurement data, with an assumed mass of 2 ∙ 10–54 kg, the service life is at least 3 years with 95% probability. This seems very brief, but you have to take into account that this value applies to a hypothetical photon at rest. For a highly relativistic microwave photon, the lifespan would be 3 quadrillion years thanks to the time dilation, so that it can survive from the early universe to the present day.

Although masslessness is part of the generally accepted theory of electromagnetism (quantum electrodynamics), simple extensions allow a massive photon. The question is therefore tied to the experimental findings. So far only upper limits can be given and one of the challenges is to understand why this parameter is so small (if not zero). The basic possibility of a photon decaying into neutrinos is in turn closely linked to the still unknown masses of these 'ghost particles', which are being investigated at the MPIK both theoretically (e.g. in the group of Werner Rodejohann) and experimentally (GERDA experiment).

Original publication:
How stable is the Photon?
Julian Heeck; Physical Review Letters 111, 021801 (2013)
doi: 10.1103 / PhysRevLett.111.021801


Julian Heeck
Tel .: 06221 / 516-820
Email: [email protected]

Dr. Werner Rodejohann
Tel .: 06221 / 516-824
Email: [email protected]

Additional Information: Group MANITOP by Werner Rodejohann at the MPIK Lindner department at MPIK NASA's page on COBE

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