Why photons move

Maybe the particle chooses to play something heavy, like a Higgs boson. Instead, speed is determined by the ratio between energy and momentum. Why this is is trickier to explain. Some of you may be happy with this explanation, but others will accuse me of passing the buck.

Ok, a photon with any energy will move at the speed of light. But why do photons have any energy at all? And even if they must move at the speed of light, what determines which direction? Here I think part of the problem is an old physics metaphor, probably dating back to Newton, of a pool table. A pool table is a decent metaphor for classical physics. You have moving objects following predictable paths, colliding off each other and the walls of the table. Where people go wrong is in projecting this metaphor back to the beginning of the game.

At the beginning of a game of pool, the balls are at rest, racked in the center. The Higgs decays, and turns into two photons. This process is quantum mechanical, and with no preferred direction the photons will emerge in a random one. The water waves do not start out motionless and then slowly pick up speed as they travel away. The water waves are already traveling at their nominal speed the moment you start creating them. That is how waves behave.

Waves are created because a deformation in the material medium or in the field medium causes the medium to snap back towards the equilibrium state, but overshoot this state, and therefore end up oscillating back and forth, all the while yanking neighboring regions into the same motion.

The wave speed is therefore determined by the medium's ability to snap back, and not by an external agent pushing the wave to accelerate it to different speeds. Pushing harder on the medium just makes the crests of the waves taller. It does not make the wave travel faster through space. If the medium is constant across a region of space and across all frequencies of motion, then the wave speed will be constant through this region.

These interactions slow it down, and cause it to move at a speed less than the speed of light as long as they're in a material. The behavior of white light as it passes through a prism demonstrates how light of different Different photons have different energies, which also means their electric and magnetic fields oscillate at different rates.

While the speed of all different types of light is the same in a vacuum, those speeds can be different in any sort of medium. Shine white light made up of all the colors through a drop of water or a prism, and the more energetic photons will slow down even more than the less energetic ones, causing the colors to separate. The primary brightest and secondary outer rainbows are due to sunlight interacting with water Colors separate due to the differing speeds of light of photons of different energies through a medium, in this case, water.

This is how shining light through water droplets creates a rainbow, because photons of different energies interact with the charged particles in a medium and slow down by different amounts. Multiple reflections of light within a water droplet result in light separating at a variety of What's important to remember, though, in all of this, is that nothing is changing about the light itself.

It isn't losing energy; it isn't changing its fundamental, intrinsic properties; it isn't transforming into anything else.

All that's changing is the space around it. When that light exits the medium and goes back into vacuum, it goes back to moving at the speed of light in vacuum: ,, meters per second. In fact, the very definition that we have of both distance and time — what defines a "meter" or a "second" — comes from light itself. Atoms can absorb or emit light, depending on how the electrons within an atom transition.

Cesium, the 55th element on the periodic table, has 55 electrons in a single, stable, neutral atom. The first 54 electrons typically live in the lowest energy state, but the 55th has two possible energy levels it can occupy that are extremely close together. If it transitions from the slightly higher one to the slightly lower one, that energy goes into a photon of a very particular, well-defined energy. If you take 9,,, cycles of that photon, that's how we define one second.

Does light travel through time? If so, what exactly is it that it is travelling through? Or does time itself do the moving and is constantly sweeping past us like the wind while everything else stands still? Does time actually exist as anything or is it just a convenient invention to allow us to talk about how things are moving? To answer this question — or ask a new one — email lastword newscientist. Questions should be scientific enquiries about everyday phenomena, and both questions and answers should be concise.

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