Photons, also known as "light quanta", are quantum units of light and other electromagnetic radiation. It is generally believed that photons have no mass. However, in some theories, photons are allowed to have very small rest mass, but this will have a consequence that photons will eventually decay into lighter particles. And if this decay is really possible, photons have a lifetime. So how long is this life worth? This is a question that German physicists have tried to answer recently. After calculation, he thinks that the lower limit of photon lifetime is about 3 years in his reference system. Converted into our reference system, the lower limit is 18th power year 10 (18 zeros after10, that is,10 billion years). The research paper has been published in the recently published Physical Review Letters.

About quality

Photons have mass, so they have a finite lifetime, which is incredible. As we all know, when astronomers observe distant celestial bodies, all they see are extremely old photons emitted billions of years ago. But in some theories, the rest mass of photons is not zero, although this rest mass is very small. Thanks to the progress of electromagnetic measurement means and instruments, the upper limit of its static mass value is limited to about negative 18 electron volts of 10 or negative 54 kilograms of 10.

Although the mass is so small, as long as it is not zero, photons will face the fate of decay and become other lighter particles, such as neutrinos or antineutrinos, or even other unknown particle types beyond the scope of the existing standard model of particle physics.

Now, Julian Hick, a physicist at the Max Planck Institute for Nuclear Physics in Germany, has turned his hope for photon decay to cosmic observation. He observed the cosmic microwave background radiation (CMB), which is the afterglow of the Big Bang when the universe was born.

background radiation

In the early days of the universe, matter and radiation were closely related. However, with the rapid expansion of the universe, that is, the so-called "skyrocketing" stage, the hot electron plasma and light nuclei gradually cooled down, thus allowing the formation of neutral atoms for the first time. This "decoupling" between matter and radiation allows photons to travel freely in the universe for the first time. With the passage of time, the wavelengths of these photons are gradually lengthened by the expansion effect of the universe and enter the microwave band-this is a nearly uniform blackbody heat energy, which exists in all directions of the universe and constitutes the radiation background of the universe.

Since the discovery of cosmic microwave background radiation, people have conducted more than 65,438+000 experiments on this phenomenon, including the Cosmic Background Detector (COBE) launched by NASA, the Wilkinson Anisotropy Detector (WMAP) and the Planck Space Telescope recently developed and launched by the European Space Agency. All these detectors further make our measurement data of this cosmic background radiation more accurate. In fact, cosmic microwave background radiation (CMB) has become the most accurate blackbody radiation in nature.

Long life cycle

Heck used these data as a constraint for his calculation. He compared the COBE data with his calculated spectra with high accuracy, in which the attenuation factor of photons was considered.

If photons have mass and decay into lighter particles, the number density of photons in CMB will gradually decrease with time. But it also means that CMB spectrum will no longer match the observed near-perfect thermal curve. Heck speculated that because CMB is almost a perfect blackbody, there should be almost no photon decay in the history of the universe of about 65.438+038 billion years, so the measurement data of CMB can be used to constrain the lifetime range of photons.

Using the constraints given by mass and CMB, Heck calculated that the lifetime of photons in its own reference frame is 3 years. However, because these photons travel at a speed close to the speed of light in the universe, the clock slowness effect must be considered in order to correct this lifetime value to the value in our reference system. After doing this, the value obtained is 10 to the eighth power year, that is, 10 billion years. It will be very difficult to further revise this value unless future updated research can explore the earlier universe.