Will you have an echo on the moon?

Will you have an echo on the moon?

The moon is an echoless wasteland, sadly. Echoes happen when a sound wave bounces off of something and reflects back towards you. This is pretty noticeable in canyons, or other places where there’s a big, relatively flat wall at some distance away from you. If you shout, you’ll hear a faint response, and the delay depends on how far away you are from the wall that’s reflecting the sound.

But echoes are all caused by sound waves, and sound always needs a medium to travel through. Fundamentally, sound is a compression wave; if there’s nothing to compress, there’s no way for the wave to move. On earth, we hear echoes as the sound we create moves through the air. But the moon doesn’t have an atmosphere, so there’s no air for the sound to travel through, so any noise would die instantly. And if you can’t have sound, you can’t have echoes.

That said, if you were on the moon, it’d be quite unwise to remove the helmet from your space suit, so you’d simply get an echo off the front of your helmet instead. Unfortunately, the sound would come back to you so quickly that you wouldn’t be able to hear it as a separate sound.

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Does space go on forever?

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What happens when light hits an atom in space?

This dramatic image offers a peek inside a cavern of roiling dust and gas where thousands of stars are forming. The image, taken by the Advanced Camera for Surveys (ACS) aboard NASA/ESA Hubble Space Telescope, represents the sharpest view ever taken of this region, called the Orion Nebula. Image Credit: NASA, ESA, M. Robberto ( Space Telescope Science Institute/ESA) and the Hubble Space Telescope Orion Treasury Project Team

This dramatic image offers a peek inside a cavern of roiling dust and gas where thousands of stars are forming. The image, taken by the Advanced Camera for Surveys (ACS) aboard NASA/ESA Hubble Space Telescope, represents the sharpest view ever taken of this region, called the Orion Nebula. Image Credit: NASA, ESA, M. Robberto ( Space Telescope Science Institute/ESA) and the Hubble Space Telescope Orion Treasury Project Team

Light is an interesting beast.  We treat it as a particle (the photon) that travels with no mass and at the fastest possible speed through the universe.  But light also behaves as a wave; it has a frequency, or a color, associated with the amount of energy it carries.  The more energy a photon carries, the bluer its color.  (This trend continues far beyond the visible range of light; radio waves are the reddest color of light, and gamma rays are the furthest to the blue.)  When light interacts with an atom, a number of things can happen, depending on the complexity of the atom and the energy of the photon.

Let’s simplify things by dealing with hydrogen.  It’s the simplest possible atom, with one proton and one electron, and it’s what the majority of the universe is filled with.  The electron is bound to a region of space around the proton by a series of electromagnetic barriers, but if you had enough energy to donate to the electron, it could bounce its way over the barriers and wander around space freely.  

This is where light comes in - photons are very effective carriers of energy.  Depending on the source of the photon (normally a nearby star), they can be more or less energetic.  The redder the nearby star, the lower the average energy of the photon you get.  We’ll take one of these photons, and smash it into our atom.

If the energy of the photon is less than the amount you’d need for the electron to hop out of its electromagnetic moat, the electron will remain stuck to its proton.  However, our electromagnetic barrier has a series of little ledges in it where an electron can hop up and stay for a little while.  If the energy of the photon happens to match up with the amount of energy the electron needs to jump up to one of these ledges, the photon will get completely absorbed.  The electron won’t stay there forever, and when the electron drops back down to the bottom of the moat, it will emit light of the color that corresponds to the amount of energy it took to get to the ledge.

If, however, the photon happens to be energetic enough for the electron to break free from its proton, you have successfully ionized an atom!  The electron will wander around aimlessly for some time, until it encounters a lone proton with no electron.  The electron will fall back into the potential well, which undoes the ionization of the atom.  The electron has to lose energy to do this, so it spits out a photon with a color corresponding to the amount of energy lost.  If you have enough hydrogen floating around in the same place, you wind up with a cloud of glowing gas - but only glowing at very specific colours!  This complex sea of escaping and recaptured electrons is what lights up an emission nebula.

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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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