Image credit: pixabay user StockSnap. Imagine you stand still on the ground, shining a flashlight in one direction at an object one light-second away. If your clock is at rest, you see the photon bouncing up-and-down, and the seconds pass as normal. But if your clock is moving, and you look on it, how will the seconds pass, now?
A light clock moving close to the speed of light will appear to run slower relative to an observer Image credit: John D. Quite clearly, it takes longer for the bounces to occur if the speed of light is always a constant. Light, in a vacuum, always appears to move at the same speed -- the speed of light -- regardless of Image credit: pixabay user Melmak. In , Einstein put forth his theory of special relativity, noting that the failed Michelson-Morley experiment and the phenomena of length contraction and time dilation would all be explained if the speed of light in a vacuum were a universal constant, c.
Image credit: NASA. The warping of spacetime by gravitational masses. The views of space and time which I wish to lay before you have sprung from the soil of experimental physics, and therein lies their strength. They are radical. Henceforth space by itself, and time by itself, are doomed to fade away into mere shadows, and only a kind of union of the two will preserve an independent reality. The way this works is that everyone and everything that exists at all always moves through spacetime, and they always move through spacetime with a very particular relationship: you move a certain amount through the combination of the two no matter how you move relative to anything else.
Time dilation L and length contraction R show how time appears to run slower and distances If you moved faster, your clock would be even farther ahead. A relativistic journey toward the constellation of Orion. Now, by studying ancient light radiated shortly after the big bang , a physicist has calculated the minimum lifetime of photons, showing that they must live for at least one billion billion years, if not forever.
That lifetime may sound like an eternity, but to a photon traveling at light speed, it passes in a relative blink. Because of the time-dilation effect predicted by Einstein's special theory of relativity , a billion billion years on Earth feels like only three years to a photon, because it's traveling so fast. Photons released simultaneously from distant stars would arrive at Earth at different times, depending on their wavelengths. A photon with mass would also necessitate modifications to the Standard Model of particle physics which posits a massless photon , the Maxwell equations that describe electromagnetic waves and fields photons are the carrier particles for electromagnetic force and the laws describing interactions between charged particles.
The current experimental limit on the possible mass of the photon is 10 kilogram. To find the limit on the photonic lifetime, Heeck analyzed observations of the cosmic microwave background radiation—light pervading the universe that dates from a few hundred thousand years after the big bang—gleaned from the now defunct NASA's Cosmic Background Explorer COBE satellite, launched in This light fits a very specific pattern—called blackbody radiation, that tells scientists how intense the light should be, based on its wavelength.
If any photons were decaying as they traveled across the universe, however, COBE would see less low-energy redder light than predicted by the blackbody radiation law, because red light would be expected to decay sooner than blue light.
But according to COBE's measurements, the cosmic microwave background appears to behave like a perfect blackbody. No low-energy light seems to be missing, indicating that very few photons, if any, have decayed since the big bang some This analysis enabled Heeck to calculate that the minimum lifetime of a photon is 10 18 , or one billion billion, years.
Although the simple model of a massless photon may turn out to be correct, the prospect of one with mass raises some intriguing possibilities. Typically, photons are said to have zero mass. If a photon did have a non-zero rest mass, that means that it can decay into lighter elements, so the photon would breakdown into either some known elementary particles that are lighter, such as a neutrino and antineutrino, or an as-yet-undiscovered-particle.
Notably, the problem with this idea is that, according to our current understandings, photons cannot be brought to rest. As a result, the idea of rest mass does not really apply to them. Thanks to previous experiments, we know what the upper limit of this mass is, so taking this into consideration, how long can photons live?
Julian Heeck of the Max Planck Institute for Nuclear Physics set out the tackle this issue, and in research that was published in Physics Review Letters B, Heeck calculated how long photos can live at minimum. Since photons are moving at such excessive speeds, time dilation comes into play and must be accounted for. Once this is taken into consideration, according to the photons frame of reference, Heeck found that its lifetime would be a rather short three years; however, from our frame of reference, light would live about one billion billion 10 18 years.
That looks a little something like this: 10,,,,,, For comparison, the universe is only 13,,, years old. Notice the great disparity in those numbers?
Well, this excessive gap means that, for all intents and purposes, the photon lives forever. In order to come up with the figure, as previously mentioned, Heeck needed to know what the upper limit rest mass is for photons.
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