The Science of Interstellar

It would seem easy to prevent this. Just shield the wormhole from photons. However, there is something you cannot shield out: quantum fluctuations of light with ultrahigh frequency—fluctuations that inevitably exist, according to the quantum laws (Chapter 26). In 1990, Sung-Won Kim (a postdoctoral student in my research group) and I used the quantum laws to compute the fate of such fluctuations. We found a growing explosion (Figure 30.4). We thought, at first, that the explosion was too weak to destroy the wormhole. The wormhole would become a time machine despite the explosion, we thought. Stephen Hawking convinced us otherwise. The fate of the explosion is controlled by the laws of quantum gravity, he convinced us. Only when those laws are well understood will we know for sure whether backward time travel is possible.

Stephen, however, was so convinced that the ultimate answer will be no time machines, that he codified this in what he calls his “chronology protection conjecture”: The laws of physics will always prevent backward time travel, thereby “keeping the universe safe for historians.”

^{Fig. 30.4. Quantum fluctuations of light, traveling along the red path, build up into a crescendo explosion at the moment the wormhole is becoming a time machine.}

Many researchers have struggled, over the past twenty years, to prove or disprove Hawking’s chronology protection conjecture. The bottom line today, I think, remains the same as in the early 1990s, when he and I were debating the issue: Only the laws of quantum gravity know for sure.

Time Travel with a Bulk

All this research and conclusions—educated guesses—are based on the laws of physics that prevail if there is no bulk with a large fifth dimension. What happens to time travel if a large bulk does exist, as in Interstellar?

We physicists find Einstein’s relativistic laws so compelling that we suspect they hold in the bulk as well as in our brane. So Lisa Randall, Raman Sundrum, and others have extended his laws into the five-dimensional bulk by one simple step: adding a new dimension to space. That extension proceeds mathematically in a straightforward and beautiful manner, which makes us physicists think we may be on the right track. In my interpretation of the movie, Professor Brand uses this extension as a foundation for his equation and for his struggle to understand gravitational anomalies (Chapter 25).

If this speculative extension is correct, then time behaves fundamentally the same in the bulk as in our brane. In particular, objects and signals in the bulk, like those in our brane, can only move in one direction through locally measured time (local bulk time): toward the future. They cannot move backward, locally. If backward time travel is possible in the bulk, it can be achieved only by journeying out through the bulk’s space and returning before the journey started while always moving forward in local bulk time. This is a bulk analog of the round trip in Figure 30.3.

Messaging Murph: My Physicist’s Interpretation

This description of time underlies my physicist’s interpretation of Cooper’s messaging Murph.

Recall that the tesseract is an object whose faces have three space dimensions and interior has four. The interior is part of the bulk. Everything we see in the movie’s tesseract scenes lies in the faces: Cooper, Murph, Murph’s bedroom, the bedroom’s extrusions, the world tubes of the book and watch—all lie in tesseract faces. We never see the tesseract’s bulk interior. We can’t see it, since light can’t travel through four space dimensions, only three. However, gravity can do so.

In my interpretation, when Cooper sees a book in Murph’s bedroom, he does so via a light ray that travels in faces of the tesseract (for example, the red dashed ray in Figure 30.5). And when he pushes on a book’s world tube, or on the world tube of the watch’s second hand, he generates a gravitational signal (a gravitational wave in the bulk) that spirals into and through the tesseract’s bulk interior, along the violet curve in Figure 30.5. The signal travels forward in local, bulk time, but backward in bedroom time, arriving before it started out.[57] It is this gravitational signal that pushes the book out of the bookcase and twitches the watch’s second hand.

^{Fig. 30.5. A Cooper icon sees a book via the red dashed light ray and exerts a force on the book via a gravitational signal that spirals along the violet curve. I’ve suppressed one of our brane’s spatial dimensions.}