What's in a beam of white light?

Does a beam of white light contain photons that propagate at alternating visible frequencies, or does the beam contain an equal amount of photons corresponding to each visible frequency? I’m pretty sure the answer is the latter, but I went my whole life being told that white light contained all of the colors of the rainbow, and when I thought of a beam, I interpreted that as just one photon. (And I know there are other non-visible frequencies to consider, but I just am wondering about visible.)
A little like Newton's famous experiment, with white light split up into a full visible spectrum.  Image credit: Adam Hart-Davis

A little like Newton's famous experiment, with white light split up into a full visible spectrum. Image credit: Adam Hart-Davis

Naturally created white light contains photons of every visible frequency, so you’re correct in thinking it’s the latter. The difference is that a beam of light is usually a LOT of photons, especially if it’s bright enough to see! (The lower limit for your eye’s sensitivity is generally about 100 photons per second.) By definition, your eye is only sensitive to the visible range of light, so to define “white” light as we see it, ignoring the rest is a fair thing to do. 

Perhaps a better term would be a ‘stream’ of photons; the 'beam’ is usually referring to the fact that the light is focused to a very narrow region of space. Single photons are extremely difficult to isolate, and usually if you go to that effort, you’re trying to test quantum mechanics in some form.

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If the universe is expanding, how can galaxies collide?

The NASA/ESA Hubble Space Telescope has snapped the best ever image of the Antennae Galaxies. The galaxies — also known as NGC 4038 and NGC 4039 — are locked in a deadly embrace. Once normal, sedate spiral galaxies like the Milky Way, the pair have spent the past few hundred million years sparring with one another. This clash is so violent that stars have been ripped from their host galaxies to form a streaming arc between the two. Image credit: ESA/Hubble

The NASA/ESA Hubble Space Telescope has snapped the best ever image of the Antennae Galaxies. The galaxies — also known as NGC 4038 and NGC 4039 — are locked in a deadly embrace. Once normal, sedate spiral galaxies like the Milky Way, the pair have spent the past few hundred million years sparring with one another. This clash is so violent that stars have been ripped from their host galaxies to form a streaming arc between the two. Image credit: ESA/Hubble

This apparent contradiction comes from the way that scientists commonly explain the expansion of the universe; we say, “Imagine a balloon with a series of dots on the outside of it. Now inflate the balloon. All the dots move away from each other as the balloon, which is space, grows in size.”

Or, “Imagine you have a loaf of bread with raisins in the surface. As the dough rises, the raisins will spread further apart from each other.”

This is an accurate metaphor- the fabric of space is expanding as the universe ages. However, when we make these metaphors, we draw out our objects in space - the dots on the surface of the balloon, or our raisins in bread dough - in regular patterns. We put everything on a grid, so that the effects of the universe’s expansion are easier to spot. This is convenient because it means we’re only looking at one effect (the expansion), but it’s a big oversimplification of the universe.

Objects in the real universe aren’t laid out on a grid. The universe doesn’t do grids. Real galaxies are scattered randomly across the fabric of space, which means that sometimes you’re going to wind up with one or two or 50 galaxies pretty close to each other. Sometimes you’ll wind up with a galaxy with nothing around it at all.

When you have two enormously massive objects relatively close in the universe, another force takes over. Gravity. Gravity is an extremely powerful attractive force, and if two objects are near enough to each other to feel the gravitational pull of the other galaxy, it doesn’t matter that the universe is expanding; it’s not expanding fast enough to counteract the attractive force of gravity, and these two objects are going to fall towards each other. When they do, there’s a good chance they will eventually become a single, larger galaxy, and the process gives us magnificent images.

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