Ask Ethan, Are My Atoms Really "Touching"? (2023)

One of the most counterintuitive things about existence is the idea of ​​forces. To experience a force—that is, to feel the influence of something else on us—two objects don't even have to be touching or in contact. Objects on the earth's surface feel the earth's gravity, but so do airplanes, satellites and even the moon. An electrically charged object attracts and repels other electrical charges, no matter how far apart they are. And, more commonly known, two magnets inverted so that their north poles face each other repel each other so strongly that even the strongest people cannot bring them fully together.

So what happens when you try to bring your thumb and forefinger together? How close do they actually get, and do they ever actually "touch" each other? That's what Peter Mead wants to know and writes:

"If I stretch out both hands in front of me with my two index fingers pointing at each other and then bring them together, the distance between my fingers becomes smaller and smaller. I can see them and hold them a little less than a millimeter apart before they touch. Is there a moment just before they touch where my fingers are just an atom (or subatom) width apart? Or does space behave somehow differently on such a small scale?”

There is obviously a large range between what we can see (a little less than a millimeter) and the size of an atom (about a ten billionth of a meter). Let's find out what's happening on these tiny scales.

Ask Ethan, Are My Atoms Really "Touching"? (1)

Although by volume an atom is mostly empty space dominated by the electron cloud, the dense atomic nucleus, which accounts for only 1 part in 10^15 of an atom's volume, contains ~99.95% of an atom's mass. Reactions between inner components of a nucleus can be more precise, occur on shorter timescales, and at different energies than transitions that are confined to the electrons of an atom.

Although we'll go down to very small scales to fully answer this question, it's important to realize that "small" doesn't necessarily mean "quantum" as you might think. Yes, quantum effects typically occur in isolated single or few particle systems and tend to disappear when many particles interact frequently, which is a hallmark of (most) macroscopic phenomena. While quantum effects usually occur on atomic scales or below, the more classical effects - including gravitational and electromagnetic effects - can never be ignored and often even dominate over inherently quantum effects, even on the smallest scales of all.

So the first step is to realize that your body is made up of atoms, and that while the atoms in your fingers are connected to form molecules that make up the organelles that make up your cells, they're still basically all atoms: Electrons orbiting atomic nuclei. Although it is a long way from the macroscopic world (fingers) to the atoms and the subatomic particles that make up even the atoms, this is what the structure of matter really looks like.

Ask Ethan, Are My Atoms Really "Touching"? (2)

The journey from macroscopic to subatomic scales involves many orders of magnitude, but going down in small increments can make each new scale more accessible than the previous one. Humans are made up of organs, cells, organelles, molecules, atoms, then electrons and nuclei, then protons and neutrons, and then quarks and gluons inside them. This is the limit of how far we have ever explored nature.

The atoms bound together — into molecules and then into larger structures — have restrictions on how their electrons can move. Even when shared by multiple atoms, the electrons orbit in cloud-like envelopes and have a smeared distribution over time depending on what specific energy level (and molecular/atomic orbitals) they occupy. Whether you are looking at a single atom or a larger structure of atoms, this is the basic picture: there is a negatively charged cloud of electrons orbiting a single or a series of positively charged atomic nuclei.

So what happens when you bring two atoms close together, as you might imagine what happens when you bring your thumb and forefinger close together, but not close enough to touch?

It's an interesting problem that most physics students learn to solve in grad school, where we all get the same answers if we get our calculations right: the shape of the electron cloud orbiting the nucleus changes in response to its presence of the atomic nucleus other atom nearby. Although atoms (and molecules) are themselves neutral entities, the fact that they are made up of negatively and positively charged components allows them to do something extremely important: polarize.

Ask Ethan, Are My Atoms Really "Touching"? (3)

When an external electric field is applied to a neutral atom, it polarizes and behaves overall like a dipole: with one side that is more positively charged and one side that is more negatively charged. The atom also deviates from a spherical shape as shown below.

Polarization is a classic electromagnetic phenomenon that occurs wherever positive and negative charges are together and these charges can move and redistribute relative to each other depending on the external forces acting on them. It turns out that while the presence of a nearby positive or negative charge is an easy-to-visualize "external force," simply bringing two uncharged but polarizable objects together can actually cause not only both objects to become polarized, but a net arises force that arises between the two.

For example, consider two simple atoms approaching each other. Everyone has a positively charged nucleus and a diffuse cloud of negative charge around it. At first, when you bring one close to the other, they remain spherical: with no net attraction or repulsion. However, the closer you bring them together, the more the electron clouds distort in shape, creating a tiny dipole: with a positively charged nucleus being slightly off-center relative to the negatively charged spherical distribution of negative charges.

Ask Ethan, Are My Atoms Really "Touching"? (4)

In this diagram, two atoms are brought close together and (i) they are initially unpolarized. When one of the atoms (ii) becomes polarized, the neighboring atom experiences the electrostatic forces from the positive and negative components of the nearby atom (iii), causing it to also become polarized, resulting in an attractive van der Waals power leads.

As soon as an atom behaves as an electric dipole - to become polarized - it begins to create its own electric field, which polarizes all atoms in its vicinity. When the "positive" end is closer to the other atom, it pushes the "positive" nucleus farther away and pulls the "negative" electron cloud closer, resulting in a gravitational attraction between the two atoms. This attraction, felt at short range, is calledVan der Waal's Kraft, and explains why if you rub an inflated balloon on your shirt (and transfer some electrons to it), you can "stick" the balloon to the wall where you rubbed it: because the charged balloon polarizes the atoms on the wall .

But that was the story for two free, unbonded atoms. What if the atoms are bound together in a network of atoms—i.e. H. in a molecular or larger structure - in which the electrons aren't completely free to move around, but have some restrictions on where they can go / where not? When one is brought close to the other, the following now happens:

  • The negatively charged electrons, where the electron clouds overlap, repel each other, creating an oval distribution that bulges "away from each other" on the side.
  • Also the positively charged nuclei, because they are now relatively "closer" to each other due to the polarization of the electron clouds, push themselves away from each other.
  • And the closer you force them together, the more you amplify this effect, increasing the repulsive forces even further.
Ask Ethan, Are My Atoms Really "Touching"? (5)

When two atoms are part of a larger structure in which each is tightly bound, the electrons and nuclei in the outermost atoms are not necessarily free to polarize as they would if they were not bound to each other. This can create a repulsive electrostatic force that becomes stronger as the atoms get closer to each other.

It may seem counterintuitive, but when you bring your thumb and forefinger close together, then touch them, and then squeeze them together with increasing force, that's what happens at the atomic/molecular level. However, there is an extremely important caveat here: This only works as far as "touching" is concerned, because the atoms in your thumb are bound together much more strongly and securely than they can be "touched" by the atoms in your index finger. Likewise, the atoms in your index finger - in molecules, cell membranes, etc. - are more strongly bound together than they are "touched" by your thumb.

This is the main reason why when you touch two typical objects together, they remain two independent objects instead of either merging or blending together. Solid objects, like your finger, have strong atomic bonds — covalent molecular bonds where electrons are shared between atoms — that are easily left intact and difficult to break. When you push two separate objects together, each object is much more likely to hold on to its own electrons than they are to exchange electrons between themselves or form new side-to-side covalent bonds.

Ask Ethan, Are My Atoms Really "Touching"? (6)

Although the electron wavefunctions of two atoms can easily overlap and bind to each other, this is generally only true for free atoms. When each atom is bound together as part of a much larger structure, the intermolecular forces can often keep the atoms at considerable distances from each other, preventing strong bonds from forming except in very special circumstances.

However, there are exceptions to this. If you go outside in cold temperatures below freezing and lick your finger and then touch your finger to a cold metal surface (do itNotlick the surface with your tongue!), the water freezes, binding the frozen water to the metal and water molecules in your finger. Once you start forming those strong bonds including:

  • ionic bonds,
  • covalent bonds,
  • or most severely form a lattice structure that overlaps both objects,

It is no longer certain that individual objects retain their integrity.

This may seem like an extreme example that couldn't possibly happen if you simply touch your thumb with your forefinger, but if you've ever performed an extraordinary amount of physical activity with your feet compressed, either by taping them or in a very tight one Having clamped your shoe – like a ballet dancer – you may actually be familiar with this phenomenon. Your individual toes can become painfully jammed in a variety of ways, which is why many dancers have started using toe spacers to counteract the foot deformities that can result from these mechanical stresses.

Ask Ethan, Are My Atoms Really "Touching"? (7)

Although ballet dancers are known for their elegance and grace and for making everything seem effortless, the reality is that ballet dancers' toes and feet often sustain severe trauma, often leaving the dancer with life-long injuries and even deformities.

Luckily, most people don't have to worry about this when doing something mundane like putting your thumb and forefinger together. While you may be able to visually discern separation distances down to a tenth of a millimeter (0.0001 meter), it's a long way down the size of a typical atomic electron cloud, which arrives at an angstrom or a ten-billionth meter (0.0000000001 meter) .

If you want to know how close you have to get two atoms for one to start polarizing or in some way "react" to the presence of another, we can estimate that to be about one hundred millionth of a meter: 0.00000001 meters, or ~10 nanometers: the size of a fairly large molecule. Hydrogen bonds can form on this scale, meaning that atoms polarized one way or the other within molecules can exert forces that you can very well "feel" with your body.

However, as you squeeze your fingers tighter and tighter, the atoms in your thumb and forefinger don't get much closer.

Ask Ethan, Are My Atoms Really "Touching"? (8)

When your two fingers touch, the atoms in your two fingers never come nearly as close together as the actual atoms that make up each finger. The pressure or force on a surface remains small in almost all cases.

Instead, the bonded structures in each of your fingers—your molecules, the cells that make them up, and the overall cellular structure that makes up each finger—are very strongly (covalently) linked together. As you squeeze your thumb and forefinger together, you bring more and more of these surface atoms into close proximity, and those atoms that are connected to everything else in your thumb and forefinger, respectively, push against each other.

Ask Ethan, Are My Atoms Really "Touching"? (9)

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Although you can press and apply significant force to your thumb and forefinger by pressing them together - enough to visibly discolor your skin - that force is distributed over a significant area: the area where your thumb and forefinger touch itself. Forces acting across an area create a pressure, and although the force is very large, the pressure is relatively small because the area is also large. As a result, the individual atoms that make up your thumb and the atoms that make up your index finger never get extremely close compared to the bond lengths between atoms in your thumb and index finger individually.

Ask Ethan, Are My Atoms Really "Touching"? (10)

Although at a fundamental level the universe is made up of point-like quantum particles, they combine to create objects of finite size and mass that occupy a specific volume.

This also answers a question that many people often ask themselves: if myAtoms are mostly empty space, why don't my thumb and forefinger ever get mixed up when I put them together? Although many people pounce on one quantum rule – thePauli exclusion principle- that's actually not necessary. The integrity of atoms, the fact that they are covalently (strongly) bonded to each other in molecules, and the fact that negative electron charges are spread over a large volume of space is more than sufficient to prevent two atomic-based structures from happening confused. Electron-based chemical bonds and the large spatial distribution that the electrons occupy are enough to make matter occupy space.

But this is the key: When we say “touch” each other, we really only mean “How close does something have to be for its properties to become something my sense of touch, or the nerves in my body that are sensitive to that sensation, respond to it?” And although we have different neurons that are sensitive to temperature, pressure and pain, they are all triggered by either electrons or photons interacting with matter in our body. In the case of pressure-based touch, a distance significantly smaller than your eye can see but still significantly larger than the size of an atom is all that is required to elicit a response!

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