The Biggest Ideas in the Universe

How do space and time combine to form the reality we live in, and could understanding their essence change how we see ourselves in the universe?

1: Space is more than meets the eye

Space is not simply "the empty void where stuff happens." For centuries, thinkers have debated its true nature. Is it a "container" for everything, or does it only exist due to the relational positions of objects? Isaac Newton believed space was a substance, while Gottfried Wilhelm Leibniz argued that space was purely relational. Today, most physicists see space as a "thing" since it contains fields like gravity and can change independently.

Space has three dimensions, which you can visualize with tied-together pencils forming right angles. Try adding a fourth – you can't. This three-dimensional nature can explain phenomena like why gravitational forces weaken with distance. Imagine the sun's gravitational influence radiating outward. As the radius of the sphere increases, the force becomes weaker per unit of area, aligning with the "inverse-square law" of gravity.

While physicists often model systems as one- or two-dimensional for simplicity, space itself fundamentally remains three-dimensional. Some advanced theories, like string theory, hypothesize extra dimensions, but these concepts remain theoretical. Nonetheless, all of this proves space is far more complex than the simple "void" we imagine.

Examples

• Newton's idea of a "substantial" space encouraged centuries of study into gravity and fields.
• A simple sphere's growing radius shows why gravity grows weaker with distance.
• String theory's wild take on space includes dimensions beyond the familiar three.

2: Time as change and flow

Time is that mysterious aspect of experience that ensures one moment leads into the next. Without it, there would be no change or progression – just static existence. Time allows us to track transformations, from the Earth rotating on its axis to a movie's 120-minute runtime. It's also what enables us to plan our when to complement our where, merging into spacetime.

Unlike space, you cannot opt out of time. It carries you forward endlessly, from past to future. This perceived progression – where the past feels fixed and the future uncertain – is known as the arrow of time. It distinguishes time from space where reverse movement is possible.

Philosophers have come up with different ways of interpreting time's reality. Presentism argues only the "now" is real. Eternalism suggests all of time, past to future, exists simultaneously like a block. Lastly, possibilism compromises, claiming the past and present are real, while the future remains open.

Examples

• Earth's predictable orbit defines a year and functions as a natural "clock."
• The arrow of time explains why days past are concrete while the future is uncharted.
• Ancient thinkers like Heraclitus ("everything flows") leaned presentist, while some preferred eternal static views like Parmenides.

3: Spacetime revolutionized our view of reality

Before Einstein, it was convenient – but inaccurate – to consider space and time separately. By introducing special relativity, Einstein tied time tightly with space, treating them as a single entity: spacetime. Hermann Minkowski's later work unified this further into a four-dimensional "block" concept, within which all events occur.

Special relativity revealed that separate observers can disagree on the timing or spacing of events, depending on how they're moving. General relativity expanded this understanding by showing how spacetime isn't static but instead curves around massive objects, creating what we experience as gravity. This overturned Newton's simpler model of a gravitational pull.

This mixture of curvature and motion means spacetime is incredibly dynamic. To illustrate, think of the twin paradox, where one twin rockets near light speed and ages more slowly than the twin left behind. The differing paths through spacetime lead to different experiences of time.

Examples

• Special relativity introduced ideas like the speed of light as an absolute.
• Curved spacetime explains orbits and why planets "fall" toward the sun.
• The twin paradox demonstrates measurable differences in aging.

4: Three dimensions, but more in theory

Physics assumes space has three dimensions because that's what we observe. However, theorists sometimes work with additional dimensions to accommodate mathematical models. This helps explain certain phenomena but remains unproven in our day-to-day existence.

Even within three dimensions, phenomena arise that feel counterintuitive. For instance, gravitational force depends on the inverse square, as explained earlier. These axes help explain the organization of physical forces in ways we experience constantly but might not consciously notice.

Theories of extra dimensions suggest that tiny, compact dimensions could exist beyond human perception, influencing the natural laws subtly. This might hint at advanced physics waiting to be discovered.

Examples

• A pencil exercise demonstrates the limit of three-dimensional space.
• Gravitational inverses align with three axes radiating outward.
• String theory uses hypothetical extra dimensions to unify forces.

5: Time isn't totally agreed upon

Despite its constance in our lives, time's nature fuels debates among scientists. Does time move like a stream or exist permanently as part of spacetime's whole? There's no consensus, but different theories help us explore.

Presentism sticks to our human perception where each moment replaces the last. Eternalism incorporates Einsteinian spacetime, making all points equally real, seemingly trapping us in four dimensions with no "before" or "after." Possibilism acknowledges time's flow but holds the future unknowable.

Debating time's nature helps us question basic assumptions about life and reality. The more clarity we gain here, the closer we come to understanding "why" as well as "how."

Examples

• Einstein's views support eternalism with block spacetime concepts.
• Heraclitus advanced ancient presentism using "flow."
• Possibilism aligns with practical human concepts about past actions shaping present.

6: Relativity makes time non-absolute

Time's progression depends partly on movement through spacetime. Special relativity proved that two people moving differently will perceive time differently. While subtle in our daily lives, near-light-speed travel magnifies these discrepancies, making time effectively slower for high-speed travelers.

Experiments with particle decay back this up. For particles zooming through accelerators, their lifespan extends, showing how speed stretches time. While we lack the technology to perform human tests, today's physics equations make strong predictions.

This relativity broadens our perspective on time and space—not fixed absolutes, but dependent on perspective.

Examples

• The twin paradox shows why astronauts age differently depending on their journey.
• Particle accelerators "stretch" time for high-speed particles.
• Clock experiments with planes verify relativity in small ways.

7: Gravity is curvature

General relativity introduced the concept that mass curves spacetime itself. The "force" of gravity, in this model, is an effect experienced due to an object following spacetime's curvature, like a ball rolling on a stretched-out trampoline-like surface.

Everyday gravity, like objects falling to Earth, is explained by Earth's mass curving spacetime around it. Without this curve, objects would continue straight. Testing this with precise astronomical measurements consistently affirms Einstein's curvature-based model.

Thus, gravity isn't pulling you; the Earth is subtly "bending" your path.

Examples

• The trampoline analogy shows spacetime's curvature appearing to exert force.
• Orbital paths are predicted by curved spacetime perfectly.
• Astronomical data confirms gravitometer projections.

8: Light's speed isn't negotiable

The speed of light is an absolute constant regardless of perspective. Even if you're moving close to its speed, you'll measure 186,282 miles/second for light. This limitation forms the structure for how spacetime "behaves."

Practical tests and theoretical equations consistently show that light's speed is a ceiling. Beyond travel implications, this constant redefines how electromagnetic waves, mass-energy interconversions, and cosmic scales relate.

We experience light’s invariance less directly, but it touches all natural processes.

Examples

• Maxwell's theory supports light’s constant speed mathematically.
• Real-world observatory data matches speed predictions exactly.
• GPS relies on understanding relativity corrections partly from this constant.

9: Non-intuitive "straight" spacetime paths

Relativity reveals that spacetime treats paths differently than you'd expect. A straight spacetime "line" maximizes elapsed time, not shortening paths. This inversion shapes how objects encounter differing spacetime flows.

Car journeys breaking city grids mirror multi-dimensional strangeness—paths aren't pure linear shortcuts under Einstein. This keeps physics wonderfully mysterious and counter-intuitive!

Examples

• Straight observed light clarifies illusions nearby high gravity.
• Satellites lucky enough obey curved trajectories—space buses!
• Non-linear universal dynamics behaving fully limitlessly defying historic intuitionalism.

Takeaways

1. Use physics thought experiments to question everyday assumptions and stretch your reasoning.
2. Approach "mysteries," like the nature of time or space, as problems with multiple possible answers rather than fixed truths.
3. Read more of Carroll's work to dig deeper into concepts like relative motion or higher dimensions.