We Have No Idea: A Guide to the Unknown Universe

by Jorge Cham

Matter as we know it is composed of atoms of the elements listed in the periodic table. Each atom has a nucleus surrounded by a cloud of electrons. The nucleus contains protons and neutrons, each of which is built from up quarks and down quarks. So, with up quarks, down quarks, and electrons, we can build any element from the periodic table.

How Matter Interacts

Here’s where we stand: in the everyday business of physics exploration, we’ve discovered twelve matter particles. Six of those we call “quarks” and the other six we call “leptons.”

Yet you only need three of those twelve to make up everything around you: the up quark, the down quark, and the electron (one of the leptons). Remember that with the up and down quarks, you can make protons and neutrons, and together with the electron, you can make any atom.

Now we know that the patterns in the periodic table are due to the arrangement of electron orbitals, and we know that there is an element for every spot and that some elements are rarer than others because they decay radioactively. It’s all just a matter of putting together the right number of neutrons, protons, and electrons to get every element.

quarks feel the strong nuclear force, but leptons do not.

the six quarks and six leptons—there are particles that transmit forces.

Wrong! Okay, mostly right. For n = 2, 4, 8 … up to n = 1023 or so, it works. But then it doesn’t. The reason is going to sound very strange: the total mass of the llama is not just the mass of the stuff inside of it. It also includes the energy that holds that stuff together.

Mass is the property of objects that makes them resist changes in velocity.

The masses of the individual quarks only account for about 1 percent of the mass of the proton or neutron. The rest is in the energy that’s keeping those quarks together.

Particles—in our current theory—are actually indivisible points in space. That means that in theory they take up zero volume and they are located at exactly one infinitesimal location in three-dimensional space. There’s actually no size to them at all.32 And since you’re made of particles, that means you’re not mostly empty space, you are entirely empty space!

We like to think of particles as tiny little balls of stuff.

the particles that transmit forces—the photon, the W boson, and the Z

the weight of an object, which means the gravitational pull of the Earth on it. That’s very closely related to mass, because the more mass something has, the stronger the Earth pulls on it.

simple experiment to confirm this. Drop two objects with different masses (like a cat and a llama) inside of a vacuum (so there is no air resistance) and you will see that they fall at the same speed. Why does that happen? If the gravitational mass of the llama is larger, then it gets pulled by a larger force from the Earth; but since the llama also has a larger inertial mass, it takes a larger force to get it moving. The two effects perfectly cancel out each other, and the cat and llama fall at the same speed.

It’s weird because the mass of something is not just the mass of the stuff inside of it. Mass also includes the energy that binds the stuff together. And we don’t know why that is.

It’s weird because mass is actually like a label or a charge (it’s not really “stuff”), and we don’t know why some particles have it (or feel the Higgs field) and others don’t.

And it’s weird because mass is exactly the same whether you measure it via inertia or gravity. And we don’t know why that is either!

When an electron pushes on another electron, it doesn’t use the Force or some form of invisible telekinesis to cause the other electron to move. Physicists think of that interaction as one electron tossing another particle at the other electron to transfer some of its momentum.

It even explains why the weak force has such a short range: its force particles have a lot of mass, which limits how far they can travel.

we already have a great theory of gravity, one that Einstein came up with in 1915. It’s called general relativity, and it works pretty well on its own.

General relativity tells us that at the heart of a black hole there exists a singularity, a point where matter is so dense that the gravitational field becomes infinite. This would be a (literally) mind-bending experience because space-time would distort you beyond any intuitive understanding. General relativity has no problem with such a thing existing, but quantum mechanics disagrees.

In 2016, after $620 million and decades of watching, scientists spotted their first gravitational wave. This beautifully confirmed Einstein’s picture that gravity bends space itself.

we mentioned some of the difficulties in trying to merge quantum mechanics and general relativity, and detecting a graviton,

Remember that gravity is basically the only force that works on grand scales,

The current theory of particles is based on quantum fields that fill all of space. A field just means there is a number, or a value, associated with every point in that space. In this view, particles are just excited states of these fields.

here are the major unresolved mysteries about space thus far:

you can’t be on your couch eating ice cream one moment and then be halfway through a marathon in the next.

you can’t be on your couch eating ice cream one moment and then be halfway through a marathon in the next. a from rovelli's book about time vs heat

This is exactly what our mathematical language for physics—calculus—was invented to do: convert many tiny little slices into a smooth variation.

why do these macroscopic processes seem to happen in only one direction? The reason is the amount of disorder in the system, known as entropy, which has a very strong preference for one direction in time.

Entropy always increases with time.This is known as the second law of thermodynamics.

In science and math, the word “dimension” means a possible direction of motion.

Mathematically speaking, there’s no reason why there should be only three dimensions.

Think of what happens when someone farts at a party. If you are very close to the source, the smell is strong. But as you backpedal from the culprit, the stink molecules (i.e., fart particles, or “farticles”) spread out into the air and get diluted.

Particles, especially ones with the same electric charge (like protons), don’t like being that close to one another.

String Theory

The theory says that these strings can vibrate in lots of ways, and each vibrational mode corresponds to a different particle.

it would take you a really long time to get in the neighborhood of the speed of light. Even if you accelerate at 10g (ten times the force of gravity, or about 100 m/s2), which is the maximum even top fighter pilots can briefly withstand, it would take you months just to get anywhere close to 300 million meters per second.

this speed limit has some profound implications for our view of the universe. Namely, we have to give up on the idea that time, and even the order that events happen, is the same for everyone everywhere.

The speed limit of the universe says that nothing can be seen to be moving faster than the speed of light.

But that is different from what YOU see. You see the two photons leave the flashlights at the speed of light (relative to you), but you also see Bertha (and the targets) moving. So while the photons are making their way to their targets, you will see one of the targets move closer to the photons, while the other target moves away from the photons. As a result, you’ll see one of the photons (the left one) hit its target before the other photon reaches the other target.

But that is different from what YOU see. You see the two photons leave the flashlights at the speed of light (relative to you), but you also see Bertha (and the targets) moving. So while the photons are making their way to their targets, you will see one of the targets move closer to the photons, while the other target moves away from the photons. As a result, you’ll see one of the photons (the left one) hit its target before the other photon reaches the other target. Cham, Jorge. We Have No Idea: A Guide to the Unknown Universe (p. 171). Hodder & Stoughton. Kindle Edition.

you can change the order of events by watching them at different speeds.

broken

What if we could squeeze the very space between us and some distant location so we get there in a reasonable amount of time without having to go very fast through space? Could that be done?

wormholes.

if space has more dimensions than just three, it is possible that places that seem far apart in 3-D space are actually next to each other in other dimensions.

Other than making the obvious visible light for which it became famous, the Sun also makes high-energy photons

Most of the high-energy particles that hit the Earth slam into the air and gas molecules covering the surface of the Earth and break up, causing massive showers of lower-energy particles. If you ever wondered where the aurora borealis or aurora australis

That’s right, we are being bombarded by millions of extremely high-energy particles on a daily basis, and we have no idea what could be creating them.

That is the case with cosmic rays. There are cosmic rays hitting the Earth at energy levels that cannot be explained by anything we know in the universe,