Could any of the photons that were around me when I played outside as a child decades ago still exist travelling through space?
Eric Kvaalen
Les Essarts-le-Roi, France
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Yes. In fact, about half the photons that were reflected off you when you were outdoors went upwards. If they didn’t hit a cloud, most of those would have gone right through the atmosphere into outer space. Almost all of those still exist. A very small percentage hit the moon or the sun and came to an end. Far fewer hit planets or other solar system bodies. An even smaller number hit Alpha Centauri or other nearby stars. The vast majority are still flying, dozens of light years away.
Guy Cox
Sydney, Australia
Photons are easily absorbed, but they don’t decay. Our giant telescopes capture photons that are billions of years old. But those from your childhood will be a long way away. Any surviving photons that were around me as a schoolboy are now at least 60 light years away.
Mike Follows
Sutton Coldfield, West Midlands, UK
What an enchanting idea. Yes, many of the photons that flashed past you as a child will still be hurtling across the galaxy and they will keep going until they are absorbed – perhaps in the eye of a yet-to-be-born life form. After all, the photons that comprise the cosmic microwave background set off on their journey roughly 14 billion years ago, about 400,000 years after the big bang.
Things get complicated when thinking about the photons before they were reflected by the scene around you, because it isn’t clear that reflected photons are the same entities as incident ones. Indeed, perhaps the same could be said of any skyward photons that ricocheted off particles on their journey out of our atmosphere and beyond.
Of course, you won’t have seen any of those photons that are now light decades away. Your visual memories were created by some of the photons that didn’t escape to space, but ended up triggering interactions in the photoreceptors at the back of your eyes.
Chris Daniel
Colwyn Bay, Conwy, UK
The potential lifetime of a photon is thought to exceed the age of the universe. However, whether the exact same photons that were around us decades ago still exist is a difficult question to answer.
A photon is a package, or quantum, of electromagnetic energy, and when one hits an object, its energy is converted into the internal energy of that object. This energy may be absorbed as heat, meaning that the photon no longer exists, or it may be momentarily contained by an excited electron, which then releases it as a new photon of transmitted, reflected or scattered light, depending on the material.
But is it the same photon? Although a reflected photon may not be continuous with the original one, it is identical to it, the quantum information having been preserved with the same energy, wavelength and frequency.
Photons may, therefore, have multiple existences, but they carry the same quantum information as they bounce, scatter and illuminate the world around us. With luck, some from our childhood will have made their way out through the atmosphere into space, where they will keep going until they meet other distant objects that they might be re-emitted from or in which they might finally be absorbed and cease to exist.
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