Project Hail Mary’s Relativity Problem Is More Complicated Than You Think
How Ryland Grace nails the science of time dilation and why it’s reasonable that Rocky doesn’t understand the concept at all.
When Ryland Grace reaches Tau Ceti in Project Hail Mary, about four years have passed for his human body and fragile psyche. But, because of the effects of time dilation, about 12 to 14 years have passed on Earth. And that’s just at the beginning of the story. By the end of Project Hail Mary, decades have passed on Earth, but Grace is barely older. In the book, he says he’s 54 years-old, even though 71 years have passed on Earth since he was born. In both book and movie, Grace is younger than he should be considering how much time has passed on Earth.
For those who have not read Andy Weir’s novel, there’s a lot more math and science-y explanations about Grace’s trip in the prose than the movie reveals. As Grace tells us at the end of the book: “I’ve done a lot of time-dilated travel.” This refers to travel with extremely high gravity, or approaching the speed of light, which causes time to move relatively slower for the traveler. So, does the math check out in Project Hail Mary? How does this all work? And, does Rocky really not understand anything about relativity?
Inverse got in touch with Dr. Becky Smethurst, an astrophysicist at the University of Oxford, author of A Brief History of Black Holes, and popular YouTuber, to answer your burning questions about the time differences in Project Hail Mary and how physics can explain it all.
Why Time Is Not the Same Everywhere
One of the basic principles of Project Hail Mary relies on something called the Lorentz transformation, which helps calculate time differences when one is approaching the speed of light. As Smethurst said to Ryan Gosling in one of her interviews, the most basic way to understand time dilation in the movie is that “space and time are relative, and it depends on how fast you’re traveling.” The closer you’re traveling to the speed of light, the slower time moves for you. But for those who are moving at much less fast speeds — like, say, the 67,000 mph (0.00999104 percent of the speed of light) those of us on Earth move around the sun, time will tick away at a faster rate.
Even with a million and one YouTube pop science explanations of this aspect of relativity at our fingertips, the concept still boggles the mind. Doc Brown traveling at 88 miles-per-hour allowed you to into the past in the first Back to the Future film, but in reality, the quicker you move, the more likely you are to end-up in “the future,” because time will move slower for you. Atomic clocks in orbit have proven that this phenomenon exists, and if it didn’t GPS wouldn’t work at all.
Still, many of our brains are hardwired not to accept the notion that an increased velocity that approaches the speed of light can alter the experience of the passage of time. In the novel, Weir highlights this idea by revealing that Rocky and all the Erdians “don’t know about relativistic physics.” In the book, Rocky goes as far as to say, “of course, time is same. Time is same everywhere.”
Rocky’s point of view is pervasive, even in interstellar sci-fi stories like Star Trek, whose Stardate logs expose flawed science, even if they also offer some background relativity explanations. When asked about it, Dr. Smethurst essentially affirms that the opposite of this trope is true. “I think it's interesting to think about how that [sci-fi lightspeed tropes] gets into people's psyches,” Smethurst tells Inverse. “Because even in the search for alien life, people don't realize how much that comes down to timing.”
In this video, Smethurst explained to Ryan Gosling that the most basic way to understand time dilation is that “space and time are relative, and it depends on how fast you’re traveling.”
So, while Rocky and Grace got lucky meeting when they did, Smethurst also points out that what makes Project Hail Mary got to that point without cheating on the space science too much.
Obviously, the near-limitless energy potential of astrophage as a fuel source is fiction, but if we accept that the astrophage-powered spin drive on the Hail Mary can constantly keep accelerating, approaching 94 percent the speed of light, then, Smethurst confirms, yes, all of this makes sense in the realm of speculative science. “The timelines for the book, in terms of what actually happens, and the fact that in terms of the relative ages of Ryland and Stratt, come the end of the book, fully checks out,” Smethurst says.
Ryland Grace spent roughly four years of his experienced time traveling to Tau Ceti, while about a decade and a half passed on Earth. Weir got these numbers by looking at the Lorentz transformation, but also, had to get deep into the weeds of how the constants in that equation changed because of the perpetual acceleration of the Hail Mary itself. If the ship had been traveling at a constant speed, the time dilation equation, from a sci-fi point-of-view, would have been easier. But, Weir, to make it our realistic, also made it more complicated.
“It's really a kudos to Andy in doing those calculations and not just the back-of-the-envelope calculations,” Smethurst explains. “He could have just assumed Grace is traveling at this speed the entire time, when in reality, we know he’s not ... If you're accelerating, your speed is constantly changing, which means the duration of your experience is constantly changing. All of a sudden, it's not a nice, simple equation; it's a lot of calculus.”
Would a Spacefaring Alien Really Not Understand Relativity?
Although it’s not explored in detail in the film, in the book, the fact that the Erdians have no concept of relativity is a major plot point that helps explain Rocky’s backstory. But how realistic is this idea? How could a spacefaring species build an awesome spaceship and harness the power of astrophage without knowing about the impacts of gravity and speed on time?
Smethurst tells Inverse that it’s actually fairly easy to imagine this kind of gap in knowledge, and that’s because technical know-how is not the same as scientific discovery.
In this video, Smethurst spoke to Andy Weir about the time dilation equations in Project Hail Mary.
“We sent people to the Moon before we knew black holes existed,” Smethurst points out. “On Earth, the science has almost outpaced the engineering, whereas on Rocky's planet, the engineering has outpaced the science. For the Moon landings in 1969, we had the science of thrust. What we didn’t have were the computers. So the science outpaced the engineering, whereas on Rocky's planet, it's the opposite way around.”
So, although Rocky himself may not exist in real life out there on the planet Erid, it’s possible that his science and technology problems could happen in reality. And, if you’re watching Project Hail Mary, and you start wondering if Rocky and Grace really are as good at math, engineering, and science as they seem to be, the answer is yes. But it’s all relative.
