Name: The Biggest Ideas in the Universe
Rating: 3.9 (74 reviews)
Author: Sean Carroll

What’s in it for me? Understand the concepts of space, time, and spacetime.

When was the last time you had a conversation with your neighbor about inflationary cosmology? How often do you discuss superstring theory with your partner? What? Never?

Well, if Sean Carroll had his wish, we’d all be talking about physics and offering our opinions on the latest theories in the same way that we discuss politics or the economy.

Although that’s not likely to ever happen, Carroll hopes that his book series The Biggest Ideas in the Universe will help people really learn about modern physics – even if they want nothing more than to remain amateur physicists. The first of the three books, Space, Time, and Motion, focuses on nine topics and explores each through a breakdown of the theory and analysis of equations.

Maybe Einstein’s equation would be a good place to start.

You’re probably thinking E=mc², right? Energy equals mass times the speed of light squared. Well, that might be important – but it’s just small beans.

When professional physicists talk about Einstein’s equation, they mean Einstein’s field equation for general relativity, or

Rµv - ½Rgµv = 8πGTµv

But don’t worry! In this Blink, we’re not going to present you with any other complex equations, what they mean, or how to solve them; that level of detail lies firmly in Carroll’s sphere. Instead, we’ll offer a taste of three of the biggest ideas in the universe – space, time, and spacetime – and perhaps whet your appetite to become an amateur, or even professional, physicist.

Space

You’re probably aware of the concept of space. It’s that “thing” where everything takes place. Where “objects” are located. But have you ever really thought about what space actually is? To start unpacking the concept, it might help to wind the clock back some 300 years.

In the early 1700s, there were two schools of thought. One posited that space was a substance and had an existence of its own – it was the “container” for everything else. The other considered space to be nothing at all. There was much discussion at the time, especially in a series of letters between Samuel Clarke in England – who agreed with Isaac Newton that space was indeed a substance – and the German Gottfried Wilhelm Leibniz, an adversary of Newton who believed space was relational. Leibniz’s death brought the exchange to an abrupt end in 1716, but by then, they’d talked about everything from space to free will to God.

Most physicists today agree with Newton: space is indeed a “thing.” Why do they believe that? Well, first, space isn’t empty. There are fields of one type or another – gravity, for instance – operating within it. And second, space (as part of spacetime) can change on its own.

At the moment, the whole picture still isn’t completely understood. We do know some basics, though. Take space’s dimensionality, for instance. Grab a couple of pencils, and tie them together at right angles. Then pick up a third, and tie it at the same intersection so it’s at right angles to the other two. Now pick up a fourth and tie it at the same intersection at right angles to the other three. That’s right! It’s not possible. You’ve just effectively demonstrated that space is three-dimensional.

Occasionally, physicists treat some things as having only one dimension – a stream of electrons moving rapidly along the length of a long wire, for example. Or having two – a thin film or the surface of a three-dimensional object. But although they can model systems as if they have just one or two dimensions, they in fact have all three. And when it comes to string theory or some other models, physicists envision even more than three dimensions. Not quite as simple as it first seems, huh? Well, you’ll be pleased to know that’s all beyond the scope of this Blink!

Having three-dimensional space, though, provides us with a neat explanation as to why the gravitational force between two objects is inversely proportional to the square of the distance between them. Imagine lines radiating out from the center of the sun – these represent lines of gravitational force. Now imagine another bigger sphere with the sun at its center. All the lines radiating from the sun pass through this sphere. Finally, imagine another even bigger sphere even further out. All the lines also pass through the radius of this megasphere. But due to its size, fewer lines pass through any given area compared with the first. The area of a sphere is proportional to its radius squared. So it follows that the gravitational force is inversely proportional to the square of the distance between two objects.

OK, enough about space for now. Let’s turn our attention to the next big idea in the universe: time.

Time

Consider two questions for a moment. First, “Can we meet up at 9:00 p.m.?” Second, “Want to watch this 120-minute-long movie?” Both utilize the concept of time, and we know exactly what the person asking means in terms of time.

But let’s push that up a notch and look at time in terms of the universe. Here, we can consider time to be a convenient way of labeling the slight changes that happen from moment to moment. Something “happens,” and there’s a transformation that takes place over time. Without time, there could be no transformation. No evolution from one moment to the next. No change.

Spacetime

Back in Newton’s day, it would have been quite feasible to put space and time together and talk about a four-dimensional concept of spacetime. But it wasn’t until the theory of relativity was proposed in the early twentieth century that it really came into its element. With relativity, spacetime is reality; separating it into space and time is just a convenience for us mortals.

In his theory of special relativity, Einstein introduced new ways of thinking about length and duration. He also posited that the speed of light – the speed at which light travels through “empty” space – was an absolute limit; it would always be the same, even if the observer were also moving. But it wasn’t Einstein who ultimately proposed unified spacetime. It was his former professor, Hermann Minkowski.

Final Summary

It’s sometimes convenient to think of space as one- or two-dimensional – or, for physicists, to consider many more dimensions. But it can be demonstrated that space essentially has just three dimensions.

Most of us understand the concept of time in our day-to-day lives; unlike with space, we have no option but to travel through it, and our direction of movement is always forward. There are three views on which aspects of time are real. Presentism considers only the present to be real; possibilism builds on this and says the past is real too; and eternalism considers past, present, and future to be equally real. Each view has its own merits.

Worum geht es in The Biggest Ideas in the Universe?

Space, Time, and Motion (2022) is the first of a three-part series titled The Biggest Ideas in the Universe. During the COVID-19 pandemic, Sean Carroll began producing videos that explained some of the biggest ideas and concepts of modern physics – and the equations which support them. He produced 24 videos in all and then developed the book series from that material.

Über den Autor

Sean Carroll is the Homewood Professor of Natural Philosophy at Johns Hopkins University. He’s an expert in cosmology, field theory, and gravitation. He’s written several other books including Something Deeply Hidden, Spacetime and Geometry, and The Big Picture. He’s also host of the podcast Mindscape, in which he discusses science, philosophy, culture, and the arts.