The Science of Interstellar

Incidentally, I’m impressed by the explosion in the movie. An explosion in space makes no sound, as there is no air to transmit the sound waves. The Endurance explosion has no sound. The flames in such an explosion must quench quickly, as the oxygen that feeds them is quickly disbursed into space. The flames in the movie quench quickly. Paul Franklin tells me that his team worked hard to achieve this, as the explosion was a real one, on a movie set, and not a computer-generated visual effect. Another example of Christopher Nolan’s commitment to science accuracy.

Our discussion of Gargantua’s environs has taken us from the physics of planets (tidal deformation, tsunamis, tidal bores,…), through Gargantua’s vibrations and the search for organic signs of life, to engineering issues (the Endurance’s robust design and its damaging explosion). As much as I enjoy these topics—and I’ve done research or textbook writing on most of them—they are not my greatest passion. My passion is extreme physics; physics at the edge of human knowledge and just beyond. That’s where I take us next.

VI EXTREME PHYSICS

21 The Fourth and Fifth Dimensions

Time as the Fourth Dimension

In our universe, space has three dimensions: up-down, east-west, and north-south. But to schedule lunch with a friend, we must tell her not only where, but also when. In this sense, time is a fourth dimension.

However, time is a different kind of dimension than space. We have no trouble traveling westward as well as eastward; we make our choice and go. But having arrived at our luncheon, we cannot immediately, then and there, travel backward in time. No matter how hard we may try, we can only travel forward. The relativistic laws guarantee it. They enforce it.[34]

Nevertheless, time is a fourth dimension; it is the fourth dimension of our universe. The arena for our lives is four-dimensional spacetime, three space dimensions plus one time dimension.

When we physicists explore this spacetime arena by experiments and by mathematics, we discover that space and time are unified in several ways. At the simplest level, when we look out in space, we are automatically looking backward in time because of how long it takes light to reach us. We see a quasar a billion light-years away as it was a billion years ago, when the light that enters our telescope was launched on its journey to us.

At a much deeper level, if you move relative to me at high speed, then we disagree on what events are simultaneous. You may think that two explosions, one on the Sun and the other on Earth are simultaneous, while I think the Earth explosion was five minutes before the one on the Sun. In this sense, what you regard as purely spatial (the separation of the explosions) I see as a mixture of space and time.

This mixing of space and time may seem counterintuitive, but it is fundamental to the very fabric of our universe. Fortunately, we can pretty much ignore it in this book except for Chapter 30.

The Bulk: Is It Real?

Throughout this book, I visualize warped space by picturing our universe as a two-dimensional warped membrane, or brane, that resides in a bulk with three space dimensions—as in Figure 21.1, for example. Of course, in reality our brane has three space dimensions and the bulk has four, but I’m not very good at drawing that, so in my pictures I usually throw one dimension away.

^{Fig. 21.1. A small black hole spiraling into a large black hole, as viewed from the bulk with one space dimension removed. [Drawing by Don Davis based on a sketch by me.]}

Does the bulk really and truly exist, or is it just a figment of our imaginations? Until the 1980s, most physicists, including me, thought it a figment.

How could it be a figment? Don’t we know for sure that our universe’s space is warped? Don’t the radio signals sent to the Viking spacecraft reveal its warpage to high precision (Chapter 4)? Yes…. And since our space is truly warped, doesn’t it have to be warped inside some higher-dimensional space, inside some bulk?

No. It is perfectly possible for our universe to be warped without there really existing a higher-dimensional bulk. We physicists can describe our universe’s warping, in mathematics, without the aid of a bulk. We can formulate Einstein’s relativistic laws, which govern the warping, without the aid of a bulk. In fact, that’s how we almost always do it, in our research. The bulk, for us, until the 1980s, was just a visual aid. An aid to give us intuition about what’s going on in our mathematics, and to help us communicate with each other and with nonphysicists. A visual aid. Not a real thing.