Seven Brief Lessons on Physics

1. Einstein's theory of general relativity revolutionized our understanding of space, time, and gravity

Einstein wrote an equation which says that R is equivalent to the energy of matter. That is to say: space curves where there is matter. That is it. The equation fits into half a line, and there is nothing more.

Gravity as curved spacetime. Einstein's groundbreaking insight was that gravity is not a force, but the curvature of spacetime itself. This elegant concept explains phenomena such as:

• Planetary orbits: Planets move in elliptical paths around the sun because they follow the natural curvature of spacetime created by the sun's mass.
• Light bending: Massive objects like stars can bend light rays passing near them, an effect observed during solar eclipses.
• Time dilation: Time passes more slowly in stronger gravitational fields, a phenomenon confirmed by precise atomic clocks at different altitudes.

Predictions and confirmations. General relativity made several startling predictions that have since been verified:

• Black holes: Regions of spacetime where gravity is so intense that nothing, not even light, can escape
• Gravitational waves: Ripples in spacetime caused by massive accelerating objects, detected for the first time in 2015
• Expanding universe: The theory's equations naturally led to the concept of an expanding universe, later confirmed by observations

2. Quantum mechanics unveiled the strange, probabilistic nature of the subatomic world

Heisenberg imagined that electrons do not always exist. They only exist when someone or something watches them, or better, when they are interacting with something else.

Wave-particle duality. Quantum mechanics revealed that subatomic entities like electrons and photons can behave as both particles and waves, depending on how they are observed. This led to counterintuitive concepts such as:

• Uncertainty principle: It's impossible to simultaneously know both the exact position and momentum of a particle
• Superposition: Particles can exist in multiple states simultaneously until observed
• Quantum tunneling: Particles can pass through barriers that classical physics says they shouldn't be able to

Probabilistic nature. Unlike classical physics, quantum mechanics deals with probabilities rather than certainties:

• Wave function: Describes all possible states of a quantum system
• Collapse of the wave function: Upon measurement, the system "chooses" one of the possible states
• Entanglement: Particles can become "entangled," with the state of one instantly affecting the other, regardless of distance

3. The cosmos: from a flat Earth to an expanding universe of billions of galaxies

The universe began as a small ball and then exploded to its present cosmic dimensions. This is our current image of the universe, on the grandest scale that we know.

Evolving cosmology. Our understanding of the universe has dramatically expanded over time:

1. Flat Earth with sky above
2. Spherical Earth at the center of the cosmos
3. Heliocentric solar system
4. Milky Way as one of many galaxies
5. Expanding universe originating from the Big Bang

Current model. The modern cosmological model includes:

• Big Bang: The universe began as an extremely hot, dense state about 13.8 billion years ago
• Cosmic inflation: A period of rapid expansion in the early universe
• Dark matter and dark energy: Mysterious components that make up most of the universe's mass and energy
• Cosmic microwave background: Leftover radiation from the early universe, providing evidence for the Big Bang

4. Elementary particles: the building blocks of all matter and energy

Everything we touch is therefore made of electrons, and of these quarks.

Standard Model. The current theory of particle physics describes the fundamental particles and forces:

Particles:

• Quarks: Six types, combine to form protons and neutrons
• Leptons: Include electrons and neutrinos
• Bosons: Force-carrying particles like photons and gluons

Forces:

• Strong nuclear force: Binds quarks together
• Weak nuclear force: Responsible for radioactive decay
• Electromagnetic force: Interactions between charged particles
• Gravity: Not yet incorporated into the Standard Model

Quantum field theory. Particles are viewed as excitations of underlying quantum fields that permeate all of space. This framework helps explain phenomena like:

• Virtual particles: Fleeting particles that pop in and out of existence
• Particle-antiparticle pairs: Spontaneous creation and annihilation of matter and antimatter

5. Quantum gravity: the ongoing quest to unite general relativity and quantum mechanics

Loop quantum gravity is an endeavour to combine general relativity and quantum mechanics. It is a cautious attempt because it uses only hypotheses already contained within these theories, suitably rewritten to make them compatible.

The incompatibility problem. General relativity and quantum mechanics, while both highly successful in their domains, are fundamentally incompatible:

• General relativity: Describes gravity as smooth, continuous curvature of spacetime
• Quantum mechanics: Describes the world in terms of discrete quanta and probability

Approaches to quantum gravity:

1. String theory: Proposes that fundamental particles are actually tiny vibrating strings in multiple dimensions
2. Loop quantum gravity: Suggests that space itself is quantized into discrete units called "loops"
3. Causal set theory: Models spacetime as a discrete set of events with causal relationships

Challenges and implications. Developing a theory of quantum gravity could:

• Explain the nature of black hole singularities
• Provide insights into the origin of the universe
• Unify all fundamental forces into a single framework

6. The nature of time: illusion or reality?

People like us, who believe in physics, know that the distinction made between past, present and future is nothing more than a persistent, stubborn illusion.

Time in physics vs. human experience. While we perceive time as flowing from past to future, physics presents a different picture:

• Block universe: In relativity, past, present, and future exist simultaneously
• No universal "now": The concept of simultaneous events breaks down in special relativity
• Reversibility: Many fundamental laws of physics are time-symmetric

Emergence of time's arrow. The apparent flow of time may emerge from:

1. Thermodynamics: Increase in entropy gives a direction to time
2. Quantum mechanics: Measurement and decoherence create apparent irreversibility
3. Cosmology: Expansion of the universe provides a cosmic time direction

Open questions:

• What gives rise to our subjective experience of time?
• Is there a fundamental difference between past and future at the quantum level?
• How does time emerge in theories of quantum gravity?

7. Consciousness and free will: reconciling human experience with the laws of physics

There is not an 'I' and 'the neurons in my brain'. They are the same thing. An individual is a process: complex, tightly integrated.

The hard problem of consciousness. How does subjective experience arise from physical processes in the brain?

• Integrated Information Theory: Proposes that consciousness is a fundamental property of certain information-processing systems
• Global Workspace Theory: Suggests consciousness emerges from the broadcast of information across the brain

Free will in a deterministic universe. Reconciling our sense of free will with the deterministic laws of physics:

• Compatibilism: Free will is compatible with determinism if our actions stem from our own motivations and desires
• Libertarian free will: Proposes that quantum indeterminacy allows for true randomness in decision-making

Implications and reflections:

• Our decisions are the result of complex neural processes, not a separate "self"
• Consciousness and free will may be emergent properties of highly complex systems
• Understanding these phenomena could have profound implications for ethics, law, and our sense of self

Last updated: September 6, 2024