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Scientists believe that there are more dimensions to our universe, but our limited senses are keeping us from noticing them.This also means that there could be hidden curvatures of space-time or gravitational fields, which could explain a fundamental issue in physics: the hierarchy problem.There are several ongoing experiments searching for evidence that could prove this theory correct—and, if it holds true, it may help explain the nature of our universe.
While humans tend to live in three dimensions (plus time, which is often considered a fourth dimension), many physicists argue our universe has far more planes of existence. To understand how that mind-boggling concept might work, imagine you’re Alice, the wildly curious—and skeptical—protagonist from Lewis Carroll’s 1865 children’s book, Alice’s Adventures in Wonderland.
Alice famously drinks potions that make her grow or shrink. But in what’s known as the “braneworld” model—which explains extra dimensions in terms of a warped geometry—all you need to do is start walking, to change size like Alice would. Move to one side of a room, and you crash into the ceiling. Shift to the opposite corner, and you shrink to the size of an ant.
While we aren’t living in a fantastical land where we change in size while walking from room to room, the thought experiment still has some real-world applications. The braneworld model suggests that there could be other dimensions in our universe—we just can’t see them because of the constraints of our senses. The theory could also imply that our universe is immeasurably close to other worlds, but we can’t see them yet, either.
But what does that mean—and how does it realistically occur outside of Wonderland? To break this down, Popular Mechanics spoke with Raman Sundrum, PhD, a distinguished professor of physics at the University of Maryland. He is coauthor of the Randall-Sundrum Model of the braneworld along with Harvard University professor of science Lisa Randall, PhD.
“We got into it, partly because in the 1990s we were following developments in string theory,” Sundrum says of braneworld. Simply put—although the physics is not simple—string theory is the notion that fundamental particles, the smallest building blocks possible in our universe, are vibrating bits of string. String theory can also be used to try to explain quantum gravity, which attempts to unite general relativity with quantum mechanics.
According to Sundrum, string theory seemed to require extra dimensions than what we use to explain everyday life. “We were contemplating the idea that we, the particles we’re all made of, and all of our atoms are stuck on a three-dimensional plane in a higher-dimensional bulk,” Sundrum says.
“[As to] why we don’t see this higher-dimensional bulk, it’s because even photons of light are stuck on this surface. So we started playing around with what those possibilities allowed, and the centerpiece of our discovery … was that when you have hidden dimensions, you can have hidden forms of space-time curvature or gravitational fields, but they’re hidden—to the same extent that the extra dimensions are hidden.” (Taken even further, it could even mean there are hidden universes near ours, although this theory is not a slam-dunk.)
But coming back to the hidden curvature of space-time, Sundrum continues, this structure could, for example, help scientists approach the so-called “hierarchy problem”—sometimes expressed as the discrepancies in size between different fundamental particles, such as electrons and quarks, or the building blocks of protons and neutrons.
We can perhaps explain the hierarchy issue through a form of higher-dimensional curvature, called warping, which is where the Alice thought experiment comes in. “You walk into a room, and you look around, and it looks perfectly [like] the classic rectangular kind of shape,” Sundrum says, but walking to one edge has your head hitting the ceiling and at the other edge, you find yourself shrunk to the size of an ant.
“This is what it feels like to be in curved space,” he adds. As to why it’s important to the braneworld, he explains the thought experiment illustrates how your apparent size (or length scale) varies as you move across the room.
Now take the same thought experiment, but replace a human with a fundamental particle, like an electron. We already know there’s a hierarchy problem, with different fundamental particles having drastically different sizes, so perhaps this warping through positions in extra-dimensional space is one way of resolving the issue.
Many other researchers have contributed their own opinions since the Randall-Sundrum theory emerged in 1999. But there’s a rub: scientists have yet to find warping, or that invisible curve that makes Alice grow and shrink.
The theory of warping suggests that there are heavier versions of standard particles that occur at higher energies in these extra dimensions, but nobody has spotted those types of particles yet. Of course, there are ongoing searches. Near Geneva, Switzerland, the Large Hadron Collider—the world’s largest particle accelerator, which propels protons and ions at velocities near the speed of light—and other, similar experiments have not yet found the particles that are hypothetically involved. It could be that new colliders are needed to seek these higher-frequency or higher-energy particles, assuming the theory is not wrong in the first place.
More Mysteries of the Universe
Sundrum says another solution could be through studying gravitational waves, which are ripples of space-time. Perhaps we could see a key transition moment in the early universe related to the braneworld, for example. Here’s how that would work:
Sundrum explains that, at some point early on in our cosmic history, there would have been a kind of black hole called a “black brane,” but at some point this black brane would’ve disappeared.
“There’s a transition, a ‘ka-chunk moment,’ in the very early universe very close to the Big Bang, where this sudden transition would have taken place, and you would have had bubbles of one type of phase immersed from the other,” Sundrum says. “These are classic and very powerful sources of gravitational waves, not the kind that we see currently between, say, colliding black holes … but between the colliding bubbles of the different phases in the early universe.”
This phase transition would have taken place in all directions, so in theory we should be able to see these gravitational waves in the background, just like the cosmic microwave background, or the leftover radiation from when light could first travel through our universe. However, we haven’t found that phase transition yet. But Sundrum notes that we could be getting close to a solution. A gravitational wave detector called LISA—the Laser Interferometer Space Antenna—is expected to launch in 2035 and may have the capacity to look for these phase transitions.
The lack of observational evidence may point to, eventually, the Randall-Sundrum theory and braneworld being the wrong way to describe the fundamental universe. But Sundrum says any “counter-narratives” would need to solve the hierarchy problem—the discrepancy between fundamental particle measurements—which mathematically remains difficult to prove outside of his theory.
He says there have been some critiques from quantum gravity experts on aspects such as the specific kind of extra dimensions that are needed. From a mathematical perspective, however, Sundrum says the braneworld appears to make sense. That said, the real question is “whether nature chooses to use it or something else, or [if] there is an entirely different reframing of the hierarchy problem.”
Should the universe “choose” braneworld, it could mean that your senses (which perceive fewer dimensions) are keeping you from experiencing the full number of dimensions surrounding you. This could even suggest that you’re microscopically close to other universes at this very moment, but you just don’t have a way of seeing them—yet.
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Elizabeth Howell (Ph.D., she/her) is one of a few space journalists in Canada. She has written five books, and was Space.com’s former staff reporter in spaceflight. As a freelancer, she has written or edited articles about astronomy and space exploration for outlets such as Payload Space, Air&Space Magazine, Sky & Telescope and Salon. Elizabeth holds university degrees in journalism, science and history and also teaches an astronomy course, with Indigenous content, at Canada’s Algonquin College. Aside from watching several astronaut missions launching from Florida and Kazakhstan, Elizabeth once lived like an astronaut at the Mars Society’s Mars Desert Research Station in Utah.