images

You are mostly empty space. Yes, there is a lot of space between all those electrons in your body and the nuclei they surround. So much so that if all that space were eliminated, you would collapse to a size smaller than a freckle on an ant.

That isn't the only weird thing about atoms. When you touch anything made of atoms, you are not really touching anything. And yet, you can touch and be touched; hands don't pass through you. What you feel is the electromagnetic force.

Take this book, for example. The electrons orbiting the atoms of your fingertips feel only the repulsion from the book's electrons. What you feel is the repulsion force, but what you think you felt is at the discretion of your powerful brain. This is a good thing. Your hand probably does not want to build a molecule with the book, so it repulses it away.

Actually there is a bit of chemical bonding in the form of friction. You wouldn't want the book to slip from your hand. That said, atoms chemically joining is a good thing. It holds mass together.

If you cut a slice of bread with a knife, the knife does not actually touch the bread. The atoms of the knife push aside the atoms of the bread.

All of this is thanks to electromagnetic force, one of the four fundamental forces of the universe. These forces are responsible for all the interactions of the Standard Model of particle physics, the model that to date is the best description of how the building blocks of matter (particles) play along with each other in our universal playground.

THE FOUR UNIVERSAL FORCES

Technical note: Each of the four forces has a carrier particle. These are particles that act like messengers between other particles. For example, a photon (a quanta of light, which is the term for a particle of light) is the messenger for electromagnetism. When two electrons get close, they send each other photonic “keep away” messages. The messages are forceful and push apart the electrons.

If a force particle exists for gravity, it travels incognito. Or worse, it's in witness protection and hidden away from scientists. And yet, to fit into the Standard Model, it must exert its force on all matter. There are whispers that it might be a graviton, the quantized embodiment of gravity. The graviton is the holy grail of quantum mechanics. If it ever comes out of hiding, scientists might finally be able to reconcile relativity with quantum mechanics.

WHAT IS AN ATOM?

The answer might not be as simple as you might imagine. Back in the day (around 465 BCE), Democritus talked about how everything we observe is composed of indivisible particles called atoms (a word derived from the Greek atomos, which means “indivisible”). He believed that if you kept cutting something in half, there must come a point where you will not be able to cut it anymore.

These indivisible elementary particles are what make up everything around us. Given what we call an atom today, Democritus was both right and wrong. He was correct that everything is made up of atoms. But alas, they are not elementary because they can be broken down further. As you might have learned in middle school or high school (or earlier), an atom can be broken down into negatively charged electrons, positively charged protons, and neutrally charged neutrons.

All normal atoms have a neutral charge, meaning they all have the same number of electrons as protons. Atoms that do not have the same number of electrons relative to protons are called ions. The different charges of various ions cause atoms to combine into molecules. That's chemistry.

Now, things can get weird even here. Consider a helium atom. This little bit of “something” comes with two negatively charged electrons orbiting two positively charged protons. (Because of this, the atom has a neutral charge. The two negative electrons cancel the charges of the two positive protons. Their charges arise from the electromagnetic force.)

Have you ever heard about opposites attracting? That's how the electromagnetic force likes to roll. A positive charge can't resist the seductive attractiveness of a negative charge. Also, the like minds of two negative charges can't stand to hang out together. According to this intuition, a helium atom doesn't make sense. First off, from what we know of magnets, shouldn't the electrons crash down upon the positively charged protons? And second, why don't the two positively charged protons repulse each other?

The answer to the first question is that an electron's orbit doesn't decay because of wave-particle duality. If you don't remember this topic, take a refresher peek at chapter 2 where we learned how everything contains its own wave with a frequency. For a mental picture, think of the wave as a spring (from the side, it would look like a wave). As an electron gets closer to its nucleus, the spring grows tighter until there is a width at which it cannot be compressed any further. This keeps them from plunging into the nucleus.

The answer to the question of why protons in a nucleus stick together is the strong nuclear force. At very short distances, say the width of an atomic nucleus, the strong force is stronger than the electromagnetic force trying to separate them. In the interlude bonus, you learn how strong the forces are relative to each other.

HOW OLD IS THE OLDEST ATOM?

The answer is about 380,000 years after the big bang. The first atoms in our universe are hydrogen and helium atoms, the lightest of the atoms. The heavier ones did not come around until about 1.6 million years later when the first stars formed. The newbies, the heavier atoms (elements) such as iron, did not come around until after the first supernova. The larger the atom, the younger it probably is. This is because they are formed from the fusion of lighter ones.1

WHAT'S THE MATTER WITH ANTIMATTER?

Antimatter is matter, only different. It's different because it's made of bizarro particles that have the opposite electric charge from ordinary matter. In other words, the atoms inside antimatter have positive electrons, called positrons, hovering in an excited cloud above a nucleus composed of negatively charged protons called antiprotons. When a particle contacts its anti-twin, they annihilate each other. That energy has to go somewhere. In science fiction, this energy could be used as energy for engines or weapons.

Theoretically, for every bit of matter in our universe, there should be an equal amount of antimatter. And they should have eliminated each other…and yet, here we are. For some reason, in our neighborhood of the universe, matter edged out antimatter during the primordial era. This is not necessarily true in other regions of the universe. Somewhere out there might be an antimatter galaxy, which is something science fiction writers should consider. How about a starship coming across an antimatter galaxy, able only to watch and not touch?

The Nobel Prize–winning physicist Paul Dirac derived the equation that discovered antimatter in 1928.2 This wasn't his intent, just happenstance, which is the motif of many discoveries. It all began when he had this really weird idea that the laws of nature should apply to all. Go figure. At the time, quantum mechanics as formulated by Schrödinger violated relativity, and relativity ignored quantum mechanics entirely. Dirac's equation successfully reconciled the two when he showed what happened to an electron traveling close to the speed of light.

The funny thing was that the equation also had a second consistent solution, the positron. In 1932, Carl D. Anderson discovered the positron, the first direct evidence of antimatter.3 He won the Nobel Prize in 1936 for the discovery. Today positrons are already used in positron-emission tomography (PET) scanners.

How about something a little more down to earth? Although still in the concept stage for cancer treatment, using antimatter instead of traditional radiotherapy (which shoots X-rays or protons at a tumor) might be much safer for the patient. Antiprotons could be used to annihilate the protons in the nuclei of tumor atoms. Plus the release of energy would do even more damage to the tumor cells.4

GOING DEEP

Protons and neutrons are made up of quarks. Quarks come in what physicists call different flavors: up quark, down quark, charm quark, strange quark, top quark, and bottom quark.

To keep this interlude (almost) simple, I will limit the discussion to the up quark and the down quark because they are the most stable. Brace yourselves because I'm about to get a bit mathy, but it's nothing more than addition.

To review, a proton has a positive +1 charge, the electron a negative -1 charge, and the neutron a 0 charge. Now for the bizarre: the up quark has +2/3 charge and the down quarks are -1/3. Yep, these are fractional. I know, crazy. This isn't anything for non-physicists to worry about because, thanks to the strong nuclear force, they are never found alone in nature.

How quarks charge up the particles in an atom's nucleus:

1. A proton is made up of two up quarks and one down quark. Now for the math:

+2/3 (up) +2/3 (up) -1/3 (down) = 1, a positive charge.

2. A neutron is composed of one up quark and two down quarks.

+2/3 (up) -1/3 (down) -1/3 (down) = 0, a neutral charge.

To complicate this, no two-quark combination can get you to -1, 0, or 1. Well, almost no combination. For a two-quark combination to work, you need what is known as antiquarks (with -2/3 and +1/3 charges).

These next two paragraphs are for completeness so that you can nod the next time someone tries to impress you with nuclear physics.

Anything held together by the strong nuclear force is called a hadron. Any hadron that is made up of three quarks is called a baryon (such as protons and neutrons). Mesons, on the other hand, are hadrons made up of two quarks.

Electrons are elementary particles, meaning they can't be divided any further. They come from a different family unrelated to quarks, called leptons. Unlike hadrons, leptons do not interact with the strong nuclear force. Their moderator of choice is the electromagnetic force. Electrons are the lightest of the charged leptons. This is very cool for our existence because only the lightest of charged leptons are stable. It is this stability that allows chemistry to happen.

FIRST INTERLUDE BONUS MATERIALS

BONUS 1: THE RANGE OF INFLUENCE OF THE FOUR UNIVERSAL FORCES

In both strength and range of influence, these four forces differ greatly. The strongest of the forces is (surprise!) the strong nuclear force. For reference, imagine the strength of the strong nuclear force defined as equaling one. The next strongest force is electromagnetism, which has only a strength 1/137 of the strong nuclear force. Then comes the weak nuclear force at an astounding 0.0000001 of the strong nuclear force. Finally, the weakest of all, gravity, is 0.00000000000000000000000000000000000001 of the strong force.5

Yes, gravity is relatively puny. Our entire planet is pulling down on you, but all it takes is a kitchen magnet to make a paperclip jump off the ground. And you must have noticed how a wee bit of static electricity makes a piece of paper stick to your hand and overwhelms the entire planet's gravity exertion.

Both gravity and electromagnetism have an infinite range of influence, the strong and weak nuclear forces, less so. The strong nuclear force's influence is not more than the width of an atom's nucleus (0.000000000000001 of a meter, or 1 femtometer). The weak force is limited to about one-tenth of 1 percent of the diameter of a proton.6

BONUS 2: NUCLEAR FUSION VERSUS NUCLEAR FISSION

Fission is when heavy atomic nuclei release energy upon splitting. Fusion is from combining nuclei and moving up the periodic table of elements into heavier atoms. For example, in the early universe, hydrogen atoms fused together to form helium atoms. When two atoms fuse, the mass of the new atom is less than the sum of the two original atoms. The missing mass becomes energy via E=mc2.

As a power source, the advantage of fusion is that no long-lived radioactive waste is produced as what happens with fission.