If Time Doesn't Exist For Photons, How Does Anything Happen To It?

Images showing the expansion of the light echo of V838 Monocerotis. Image credit: NASA, ESA, H.E. Bond (STScI) and The Hubble Heritage Team (STScI/AURA)

Images showing the expansion of the light echo of V838 Monocerotis. Image credit: NASA, ESA, H.E. Bond (STScI) and The Hubble Heritage Team (STScI/AURA)

Originally posted at Forbes!

The concept of photons running with stopped clocks is something that is pulled straight out of relativity; the faster you’re moving, the slower your onboard clocks are moving, and the closer to the speed of light you’re operating, the more sluggish they get. Once you reach the speed of light, your clock runs infinitely slow - for practical purposes, we can say that time doesn't flow for the photon. As with all things relativity, this isn’t an absolute statement- light still has a finite speed, and we can observe light taking fixed amounts of time to traverse large distances.

When light goes zipping around our Universe, it is physically moving through space at a speed of 186,000 miles every second.  But if you could affix a clock to it, an observer that’s not moving at the speed of light would not see the clock moving forwards the way their own clocks do. A hypothetical person moving at the speed of light wouldn’t notice anything weird with their clock, but what they might notice is that the Universe is full of things to smash into.

This artist's impression shows how photons from the early universe are deflected by the gravitational lensing effect of massive cosmic structures as they travel across the universe. Image credit: ESA

This artist's impression shows how photons from the early universe are deflected by the gravitational lensing effect of massive cosmic structures as they travel across the universe. Image credit: ESA

No matter how fast you’re going, if there’s something in front of you, and you can’t dodge it, you will hit it. This is as true for humans as it is for light, and light is even less capable of dodging an oncoming object than we humans are.  Light always travels in locally straight lines - the only way to bend light is to make a curve in the shape of space. A photon will then follow that curve, but there’s no onboard navigation.

Photons are effectively stuck playing the world’s most obnoxious game of bumper cars, continually bouncing from impact to impact. From our non-speedy perspective, the clocks on photons do not tick forward between impacts, so if the photon has the good fortune to get re-emitted by whatever it ran into, it will, from our viewpoint, instantaneously smash directly into something else without its onboard clock ticking onwards at all.

The photon may not get re-emitted by whatever it ran into, (this is one way to get rid of a photon). The energy of whatever it hit will increase, so the energy isn’t lost. However, if it hits something particularly cold, the object won’t be radiating much, and the photon’s energy will be a convenient donation.  More commonly, after some amount of time, a new photon will be produced, at a different energy level, carrying energy away from whatever the photon punched itself into earlier. That new photon has an equally short apparent flight until it smashes into something else.

It’s not the most glamorous of paths through the Universe, but a continual ricocheting from solid matter to solid matter is how photons in our Universe go about it.

The bright cloud is a reflection nebula known as [B77] 63, a cloud of interstellar gas that is reflecting light from the stars embedded within it. There are actually a number of bright stars within [B77] 63. Image credit: ESA

The bright cloud is a reflection nebula known as [B77] 63, a cloud of interstellar gas that is reflecting light from the stars embedded within it. There are actually a number of bright stars within [B77] 63. Image credit: ESA

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