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(SeaPRwire) –   When Ryland Grace arrives at Tau Ceti in Project Hail Mary, roughly four years have passed for his human body and mind. But, due to the effects of time dilation, between 12 and 14 years have gone by on Earth. And this is only the opening of the story. By the end of Project Hail Mary, decades have elapsed on Earth, but Grace has barely aged at all. In the book, he states he is 54 years old, even though 71 full years have passed on Earth since his birth. In both the book and the film adaptation, Grace is younger than he should be when you account for how much time has gone by for people on Earth.

For those who have not read Andy Weir’s original novel, the book includes far more mathematical and science-focused explanations of Grace’s journey than the movie reveals. As Grace says at the end of the book: “I’ve done a lot of time-dilated travel.” This refers to travel that occurs under extremely high gravity, or near the speed of light, which makes time pass comparatively slower for the traveler. That brings up key questions: does the math actually add up in Project Hail Mary? How does all of this physics work? And does Rocky really not understand anything about relativity?

Inverse reached out to Dr. Becky Smethurst, an astrophysicist at the University of Oxford, author of A Brief History of Black Holes, and a popular science YouTuber, to answer all your pressing questions about the time differences in Project Hail Mary and how physics explains the whole concept.

Why Time Doesn’t Move at the Same Rate Everywhere

One of the core principles of Project Hail Mary relies on the Lorentz transformation, a mathematical tool that calculates time differences when an object is approaching the speed of light. As Smethurst explained to Ryan Gosling in one of her interviews, the simplest way to understand the movie’s take on time dilation is that “space and time are relative, and it depends on how fast you’re traveling.” The closer you travel to the speed of light, the slower time moves for you. But for those moving at far slower speeds — like, for example, the 67,000 mph (0.00999104 percent of the speed of light) that Earth travels around the sun — time ticks away at a much faster rate.

Even with thousands of YouTube pop science explanations of this relativity concept available at our fingertips, the idea still confounds most people. In the first Back to the Future film, Doc Brown hitting 88 miles-per-hour let characters travel to the past, but in real life, the faster you move, the more likely you are to end up in “the future,” because time moves slower for you. Atomic clocks placed in orbit have already proven this phenomenon is real, and GPS systems would not work at all without it.

Even so, most of our brains are inherently wired not to accept that increasing velocity close to the speed of light can change how we experience the passage of time. In the novel, Weir highlights this common intuition by writing that Rocky and all Erdians “don’t know about relativistic physics.” In the book, Rocky even goes as far as to say, “of course, time is same. Time is same everywhere.”

Rocky’s point of view is actually very common, even in interstellar sci-fi stories like Star Trek, whose Stardate system relies on flawed science, even when it includes basic background explanations of relativity. When asked about this trope, Dr. Smethurst confirms that the opposite is true in real life. “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.”

So, while Rocky and Grace got lucky to meet at exactly the right time, Smethurst also points out that Project Hail Mary reaches this plot point without cutting too many corners on space science.

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 accelerate, approaching 94 percent of the speed of light, then, Smethurst confirms, all of this makes sense within the bounds 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 experiences roughly four years of travel time to reach Tau Ceti, while about a decade and a half passes on Earth. Weir arrived at these numbers using the Lorentz transformation, but also had to dive deep into how the equation’s constants changed because of the Hail Mary’s constant acceleration. If the ship had traveled at a fixed constant speed, the time dilation equation would have been much simpler for a sci-fi story. But to make the story as realistic as possible, Weir also made the math far more complicated.

“It’s really a kudos to Andy in doing those full calculations and not just rough back-of-the-envelope math,” Smethurst explains. “He could have just assumed Grace traveled at a fixed 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.”

Could a Spacefaring Alien Really Not Understand Relativity?

Although this plot point is not explored in detail in the film, in the book, the Erdians’ lack of any concept of relativity is a major story beat that explains much of Rocky’s backstory. But how realistic is this idea? How could a spacefaring species build an advanced spaceship and harness the power of astrophage without knowing how gravity and speed impact time?

Smethurst tells Inverse that this kind of knowledge gap is actually fairly easy to imagine, because technical know-how is not the same thing as fundamental scientific discovery.

“We sent people to the Moon before we understood that black holes existed,” Smethurst points out. “On Earth, science has almost always outpaced engineering, whereas on Rocky’s planet, engineering has outpaced science. For the 1969 Moon landings, we already had the science of thrust worked out. What we didn’t have were powerful enough computers. So science outpaced engineering here on Earth, whereas on Rocky’s planet, it’s the opposite way around.”

So, although Rocky himself probably does not exist on the real planet Erid, this kind of mismatch between science and technology could absolutely happen in reality. And, if you’re watching Project Hail Mary, and you start wondering if Rocky and Grace really are as skilled at math, engineering, and science as they seem to be, the answer is yes. But of course, it’s all relative.

Project Hail Mary is in theaters now.