Light and Matter

Light is both a particle and a wave. Why? Because physics says so.

Light is a Wave/Particle

Also, because the universe likes to screw with us. (Credit: Douglas Hofstadter)

Light acts both as an electromagnetic wave, and as particles called photons. For a long time, scientists thought light was just a wave, but experiments involving the photoelectric effect (this is what Einstein won his Nobel Prize for!) established that light acts like a particle too. In fact, wave-particle duality theorizes that waves sometimes act like particles and particles sometimes act like waves. We could call it an atomic example of a bipolar disorder, but that may take it too far.

After all, light is basically the only way we can truly explore the universe, since the distances are far too great for us to traverse. Assuming we could travel at the speed of light, it would take nearly a decade for a space probe to reach the nearest star and transmit information back to Earth, and it would take 50 000 years to travel to the nearest galaxy and transmit information back. Heck, human civilization hasn’t been around for 50 000 years, and it’s debatable whether we’ll survive the next one thousand without blowing ourselves up. But we can’t travel at anything close to the speed of light — the highest speed achieved by the Helios II solar probe, the fastest man-made object ever, is only a tiny fraction of the speed of light (about .00023 c) — so it would take many, many times longer.

Would it surprise you to know that astronomers use time travel on a regular basis? The distances involved in astronomy can be so astronomically huge that it takes light a very significant amount of time to reach us. So we see distant stars as they appeared thousands of years ago; they could have gone supernova in the meantime and we wouldn’t know until the light from the supernova reaches us. The most distant galaxies we can observe are seen as they would appear billions of years ago, nearly at the beginning of the universe. That’s right, light is a time machine.

It’s not just visible light, though. There’s radio waves, microwaves, infrared, ultraviolet, x-rays, and gamma rays, oh my!  But all these forms of radiation, along with our boring old visible light, are just part of the electromagnetic spectrum — all the fancy names are because scientists didn’t realize they were just naming different versions of the same thing.

Electromagnetic spectrum

The electromagnetic spectrum in all its glory.

And then there’s matter, which is stuff. You know, matter, made up of protons, neutrons, electrons, weird bosons (possibly of the Higgs variety)… It’s also mostly empty space, since atoms are basically a tiny speck of a nucleus and a whole lot of nothingness with a couple electrons zooming around in it. Yes, that means if you punch a brick wall, you are punching mostly empty space. We shouldn’t have to tell you this, but don’t try that at home. Please.

Matter can be converted into energy with perhaps the most famous physics equation of all time, E = mc^2, which is derived from Einstein’s Theory of General Relativity. Unfortunately, neither of the two astronomy geeks who write this blog understand relativity well enough to explain how exactly one would derive the equation. Suffice it to say that E = mc^2 works, and it is very relevant to stars, which release massive amounts of energy as they smash together nuclei and fuse elements into heavier elements.

Everyone knows that there are three states of matter — solid, liquid, and gas. Well, not exactly. Under the extreme conditions out there in the universe, matter can exist in all sorts of strange states, such as plasma or degenerate matter. Plasma is basically ionized gas, where electrons have been added or stripped away, turning regular atoms/molecules into ions. Degenerate matter is where plasma has been crammed into such a tiny space that it refuses to be compressed any further because all the “available” energy states fill up (Pauli Exclusion Principle). These two unusual states of matter are especially relevant to astronomy because stars are made of plasma, and white dwarfs and neutron stars are made of degenerate matter.

NGC 2440

The white dwarf at the center of this planetary nebula (the bright dot near the center) is made of electron-degenerate matter. Science Olympians who did Astronomy in 2012 should recognize this image.

But that’s just boring old regular matter. There’s also antimatter, and dark matter.

Antimatter is really just the opposite of matter. Regular matter is made of protons and electrons, while antimatter is made of antiprotons and antielectrons; regular subatomic particles are made of quarks, while antimatter subatomic particles are made of, you guessed it, antiquarks. A word of advice: if you ever meet your antimatter counterpart, do not high-five each other, as both you and anti-you will be annihilated in a blast of energy and leave behind nothing but a giant crater. (Being science geeks, the two of us have since been distracted by trying to figure out how big said crater would be…)

As for dark matter, we only know it exists because it has effects on regular matter, particularly on the galactic scale. We have absolutely no idea what the heck it is, but we wonder, is dark matter a very MACHO type of matter or is it just a WIMP? Yes, these are actual candidates for dark matter. MACHO stands for MAssive Compact Halo Object, and WIMP for Weakly Interacting Massive Particle — if you can’t tell, astronomers like to have fun with acronyms.

I do hope this post, shall we say, “shed a little light on the matter”.


TL;DR — Astronomers use light, as well as other non-visible forms of electromagnetic radiation, to explore the universe. It even allows us to look back in time! Many interesting objects in astronomy are made of exotic states of matter, or perhaps not even made of ordinary matter.

This entry was posted in General.

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