We Have No Idea

Author: Jorge Cham

• In other words, the stars should be flying off the edges of the galaxies, just like the Ping-Pong balls in the merry-go-round.

• And as the famous astronomer Carl Sagan once said, “Extraordinary claims require extraordinary evidence.”

• Seeing the same galaxy twice makes perfect sense if there is something heavy (and invisible) sitting between you and this galaxy; this invisible heavy blob can act like a giant lens, bending the light from the galaxy so it appears to be coming from two directions.

• The most convincing single piece of evidence for dark matter came when we observed a giant galactic collision in space.

• Sometimes physics is like botany in that way. We don’t understand why any of these forces exists. This is just a list of the things we’ve observed.

• will discuss in gory detail in chapter 7, space is not a static empty backdrop on which the theater of the universe plays out. It is a physical thing that can bend

• space is not a static empty backdrop on which the theater of the universe plays out.

• space is not a static empty backdrop on which the theater of the universe plays out. It is a physical thing that can bend

• space is not a static empty backdrop on which the theater of the universe plays out. It is a physical thing that can bend (in the presence of massive objects), ripple (called gravitational waves), or expand.

• We call these particles “fundamental” not because they are fun (they totally are) but because we can’t yet see if they are made of even smaller particles.

• When two electrons repel each other, they are actually exchanging a photon.

• And so on. If you chop the llama into n pieces, you can measure its mass by adding up the masses of the n pieces. Right? Wrong! Okay, mostly right. For n = 2, 4, 8 … up to n = 1023 or so,

• And so on. If you chop the llama into n pieces, you can measure its mass by adding up the masses of the n pieces. Right? 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.

• What’s crazy is that most of your body is made out of these bags of beans (protons and neutrons), which means most of your mass doesn’t come from the “stuff” you’re made of (quarks, electrons) but from the energy needed to hold your “stuff” together.

• In our universe, the mass of something includes the energy needed to keep that stuff together.

• In our universe, the mass of something includes the energy needed to keep that stuff together. And the mind-blowing part is that we don’t really know why.

• What is the density of an electron? An electron has nonzero mass and it exists in zero volume, so the density (mass divided by volume) is actually … undefined?

• But if electric charge means a particle can feel electrical forces (like getting repelled by other electrons), what does mass mean for a particle? Mass is the thing that gives a particle inertia (resistance to motion).

• One of them (inertial mass) is how resistant something is to being moved, and the other (gravitational mass) is how much it wants to be moved by gravity.

• The answer is that gravity is very important at huge scales and when dealing with enormous masses.

• Gravity doesn’t repel particles with mass. This is important because it means that gravity can’t be canceled

• Gravity doesn’t repel particles with mass. This is important because it means that gravity can’t be canceled out.

• This is why gravity dominates at the scale of planets and galaxies: because all the other forces are in balance.

• So there are two curious and as-yet-unexplained properties of gravity: first, it is really, really weak compared to the other fundamental forces.

• The other curious property about gravity is that it only attracts.

• 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. In the case of electrons, these force-carrying particles are called photons. In the case of the weak force, particles exchange W and Z bosons. Particles that feel the strong force exchange gluons.

• According to general relativity, the reason the Earth goes around the Sun rather than flying off into space is not because there is a force that pulls it around in an orbit. It goes around the Sun because the space around the Sun is distorted in such a way that what feels like a straight line to the Earth is actually a circle (or an ellipse).

• Two cosmically massive objects slamming into each other—that’s what you’d need to probe gravity at the quantum level.

• Gravitons make neutrinos seem like social butterflies that like to talk to everyone at the party.

• Although we mentioned some of the difficulties in trying to merge quantum mechanics and general relativity, and detecting a graviton, it doesn’t mean that physicists have given up on the idea of finding a unified theory that can explain all the forces we know about.

• In this view, space is literally the lack of stuff.

• And we know that space can expand because we have seen it expanding—this is how dark energy was discovered.

• But we seem to have just the right amount to make space perfectly flat as far as we can tell. In fact, the exact amount is about five hydrogen atoms per cubic meter of space. If we had had six hydrogen atoms per cubic meter of space, or four, the entire universe would have been a lot different

• Some even suspect that the relationships between nodes of space are formed by the quantum entanglement of particles, but this is mathematical speculation by a bunch of overcaffeinated theorists.

• Like when you realize that chocolate and peanut butter taste so good together, they must be part of some deep universal chocolate-peanut continuum.

• Even though you can be in the same position in space at different times, you can’t be in different positions in space at the same time.

• While thinking about time as another dimension is mathematically convenient in our theories, it’s important to keep in mind that there are significant differences that make time unique.

• First, moving backward in time can break causality.

• The moral of the story is that time travel is not possible because it violates causality

• According to the second law, it is impossible, on average, to decrease the total entropy in the forward direction of time.

• Why did the universe start in such a highly organized low-entropy configuration in the first place? We have no idea.

• Even if you accept that entropy increases with time, it still doesn’t explain why time goes only forward. For example, you can imagine a universe where time goes backward and entropy decreases with negative time, which would maintain the relationship and not violate the second law of thermodynamics!

• For the most part, you could not tell which direction time flows in our universe by watching individual particles interact.

• The weak force, the one responsible for nuclear decay and mediated by the W and Z bosons, has a part that prefers one direction.

• Einstein famously predicted that moving clocks run more slowly. If you take a trip to a nearby star by traveling close to the speed of light, you will experience less time than those left back on Earth.

• The GPS receiver on your phone (or your car or airplane) assumes that time moves slower for the GPS satellites orbiting the Earth (which are traveling at thousands of miles per hour in space curved by the gigantic mass of the Earth). Without this information, your GPS device would not be able to accurately synchronize and triangulate your position from the signals transmitted by these satellites.

• Ultimately, we have to give up on the idea of time as an absolute single clock for the universe. Sometimes that leads us into areas that make no sense intuitively, but the amazing part is that this way of looking at time has been tested and shown to be true.

• When you learn that the meaning of “dimension” in science doesn’t mean an alternate universe where everything is made of chocolate and debts are paid in marshmallows, you might be tempted to wag your finger at those pesky scientists for stealing the word and giving it a different meaning.

• On top of that, these dimensions can hide in plain sight by being a little different. How different? Imagine that these extra dimensions are actually curved and form little circles or loops.

• Why is gravity so much weaker than electromagnetism and all the other forces? Well, extra dimensions might be the explanation.

• But here is the thing about physicists: if you say something is “almost impossible,” that’s only going to get them riled up.

• This is how we can use particle colliders to detect extra dimensions. If we smash particles together, and one day we see a particle that looks, for example, exactly like an electron (same charge, same spin, etc.) except it is much heavier, we could reasonably suspect that it is actually an electron that is also moving in other dimensions.

• In fact, if extra dimensions do exist, we could reasonably expect to find exact copies of all the particles we know about except they would be heavier due to their motion in extra dimensions. The theory predicts we would find “towers” (called Kaluza-Klein towers) of identical particles with heavier and heavier masses at regular intervals.

• So, yes, the Large Hadron Collider in Geneva could create black holes. If the scale of extra dimensions is about one millimeter, it is possible that the LHC makes one black hole per second.

• Happily for our continued existence, the little black holes that the LHC might potentially be creating are different than the massive cosmic black holes made from collapsing stars. These are cute little black holes that will evaporate very quickly rather than gobble up Switzerland and the rest of the planet.

• One of them is string theory, which suggests that the universe is not built out of zero-dimensional point particles but instead is constructed from tiny one-dimensional strings—not

• One of them is string theory, which suggests that the universe is not built out of zero-dimensional point particles but instead is constructed from tiny one-dimensional strings—not tiny like minimarshmallow tiny, but tiny like 10−35 meters tiny.

• When you look at the strings from far enough away (a resolution of only 10−20 meters) they look like point particles because you can’t see their true stringy nature.

• Superstring theory prefers to work in a universe that has ten spatial dimensions. Bosonic string theory likes a universe with twenty-six dimensions.

• Wouldn’t you like to know if there was more to space than what you see and experience in your everyday life?

• over by the galactic police. Your engine is not going to suddenly explode. Your Scottish

• Think of it another way: you can change the order of events by watching them at different speeds.

• The concept of a universal clock or universal simultaneity is gone—all as a consequence of having light travel at the same speed for everyone, which follows from having a maximum speed limit to the universe.

• And breaking causality is not a minor thing even for first-time offenders. The universe tends to take it pretty seriously.

• A speed limit is useful in order to have a universe that is local and causal.

• The question of why the universe is causal is very difficult even to discuss, not to mention answer in a satisfactory way. Causality is built so deeply into our pattern

• The question of why the universe is causal is very difficult even to discuss, not to mention answer in a satisfactory way.

• Right now we have no idea why the speed limit is what it is. But we can speculate about different possibilities. It could be that this is the only

• Right now we have no idea why the speed limit is what it is. But we can speculate about different possibilities.

• When a speedboat moves along the surface of a lake faster than the waves it makes in the water, those waves add up to make a wake. If an airplane travels faster than the speed of sound, it creates a shockwave of air called a sonic boom. What happens when a muon travels faster than light through a block of ice? It creates a light boom! This is also known as Cherenkov radiation, and the faint blue rings of light generated by this boom are routinely used by physicists to detect such particles and measure their speed.

• If you ever wondered where the aurora borealis or aurora australis (i.e., the northern and southern lights) come from, they are the glow that comes from the stream of cosmic rays diverted by the Earth’s magnetic field to the North and South poles.

• Some scientists have recently proposed that if enough people (tens of millions) ran the app at night when their phones were not in use then the resulting network could see a lot more of those high-energy cosmic rays that we might otherwise be missing.

• you might have heard that when a particle touches its antiparticle they explode. That sounds ridiculous, doesn’t it? Actually, this one turns out to be true.

• a raisin plus antiraisin combination would be a dehydrated weapon of mass fruitation.

• Why does the universe have these weird rules? We don’t know. Maybe one day we’ll be able to show that these rules are a natural consequence of some underlying simpler theory of particles.

• Does it make sense that our universe was once a single infinitesimal point? That all the stuff that exists today was once at the exact same location, squished down to zero volume? Actually, according to general relativity, it does make sense.

• Did that blob lead inevitably to a universe like ours, or could it have been different? Could

• Did that blob lead inevitably to a universe like ours, or could it have been different? Could the blob happen again? Has it happened before? The answer is that, as usual, we have no idea.

• Stephen Hawking has suggested that asking “What came before the Big Bang?”

• Stephen Hawking has suggested that asking “What came before the Big Bang?” is like asking “What is north of the North Pole?”

• It turns out physicists have a pretty good story for how we ended up in a nonbland universe full of structure. Here’s the theory: small quantum fluctuations in the early universe were stretched by the rapid expansion of space-time (i.e., inflation) into huge enormous wrinkles that seeded the formation of stars and galaxies by gravity, which was aided by dark matter; and at some point in there, dark energy started stretching space out even farther.

• It seems like we live in a special moment given our current understanding. But the truth is that we don’t know for sure because we can’t confidently predict the future of dark energy.

• So the fact that we have any structure at all—rather than perfect smoothness—is due to the quantum fluctuations that created the first wrinkles, which were then blown out of proportion by inflation, creating the seeds that led to our current universe.

• The atoms of a rock don’t like to get squeezed together too tightly (Ever tried to squeeze a rock into a diamond? Not easy) and they resist. What you end up with is a balance between the squeezing of gravity and the internal pressure of the rock.

• The reason the center of the Earth is hot and liquid is due entirely to gravity.

• Stars are essentially fusion bombs that are continually exploding; the only thing that contains them is their gravity.

• Once they burn their fuel and can no longer provide the pressure to resist the relentless pull of gravity, some stars collapse into black holes.

• Galaxies are also pulled together by gravity, but they resist a total collapse into a massive black hole by various kinds of pressures, depending on the galaxy. Spiral galaxies don’t collapse because they are spinning very fast, and the resulting angular momentum effectively keeps all the stars apart.

• So the emptiness of our universe comes from the interplay between these two quantities: the speed of light that defines the distance scales and the expansion of space, which is pulling everything apart.

• In other words, you can accurately predict the collective action of all the little parts of something even if you don’t know anything about what those little parts are doing

• The first of these is the quantum mechanical constant, h, known as Planck’s constant. This is a very important number because it is connected to the fundamental quantization of energy, which is like the pixelation of energy.

• How will we know what is inside of electrons and quarks? We have to keep smashing things together.