Gravity Could Be Spreading Into Hidden Dimensions—Suggesting an Invisible Reality Is All Around Us

• The force of a tiny magnet beats the gravitational pull of our entire planet.
• That makes gravity weak—or does it? Gravity may be diluted. A scientific hypothesis suggests gravity only appears weak because it could be spreading into extra spatial dimensions.
• Physicists are looking for tiny black holes to test for the possibility of miniscule hidden dimensions. Such black holes could be evidence of gravity becoming strong at extremely small scales.

Imagine using a magnet to pick up a paper clip. The magnetic force of a tiny magnet pulls the paper clip upward, while the gravitational force of an entire planet pulls it down—and the magnet wins. You may take the magnet’s action for granted, but it’s evidence that reveals why scientists consider gravity to be a very weak force. This may seem counterintuitive, but one explanation for the apparent weakness of gravity might be that gravity isn’t restricted to the three dimensions of space we know.

Consider this: We can quantify the difference in the strength of electromagnetism versus gravity in a simple way, by calculating the ratio of the two forces in an atom, where they connect an electron to a proton. Electromagnetism outmuscles gravity by a whopping factor of 1,039. That’s a thousand, trillion, trillion, trillion times stronger.

However, comparing electromagnetism and gravity is only the start of understanding a big—and very strange—picture. Over the past century, scientists have come to realize that all of the phenomena you experience in your life, from a push on a swing, to the static cling of socks out of the dryer, to the nuclear fusion that powers the sun, can be explained as originating from four “fundamental” forces. They are electromagnetism, gravity, and two forces that only have an effect on length scales the size of an atomic nucleus. These other two forces are called, rather unimaginatively, the strong and weak nuclear forces.

When we compare the strength of these forces, a startling contrast arises for gravity. Using weight as an analogy, the strong, electromagnetic, and weak forces correspond roughly to the Great Pyramid of Giza, the Empire State Building, and a midsize two-bedroom American house, respectively. Admittedly, these objects have noticeably different weights, but when we compare them to the weight analogous to the strength of gravity, they seem pretty similar. That’s because the force of gravity is equal only to the weight of a single electron—incredibly smaller than the other three.

Just why gravity is so much weaker than the other known fundamental forces is an enduring mystery of modern physics. Bluntly, nobody knows why. However, in 1998 physicists Nima Arkani-Hamed, Savas Dimopoulos, and Gia Dvali made a daring proposal, which is called ADD for short. Perhaps the apparent weakness of gravity was not due to the nature of gravity itself, but rather due to the nature of space.

Both gravity and electromagnetism have a familiar feature: They are stronger when two objects are close together, and weaker as the distance between them grows. This is most commonly experienced when you play with two magnets. The source of how these forces vary with distance is perhaps surprising. It’s simply because of the geometry of three-dimensional space.

Consider a point electrical charge, which is to say a charge with no size, concentrated at a single location in space. It can affect its surroundings, and this effect depends entirely on the magnitude of the charge. Now surround that charge with a sphere. The effect from that charge is spread out evenly over the area of the surface of that sphere.

The area of a sphere depends on the square of the radius (which is half the sphere’s diameter). So if we replace that sphere with one that has double the radius, the area of the bigger sphere is four times bigger. For a radius three times bigger, the area goes up by a factor of 9. For bigger spheres, the effect of the charge has more space to “spread out,” and as the area gets larger, the strength of the force on objects at that radius shrinks.

A sphere is a three-dimensional object. But what if we lived in a two-dimensional world? In that case, we would surround a point charge not by a sphere, but by a circle, and the effect of the charge would spread out through the circle’s circumference. Unlike the area of a sphere, the circumference of a circle grows in a manner proportional to the radius. If you double the radius, the circumference doubles, too. If you triple the radius, the circumference triples. So we see that in a 3D universe, the strength of electromagnetism drops faster than in a 2D world.

The ADD hypothesis suggests that perhaps the reason that gravity appears so weak to us is that, unlike the other three forces, gravity is not restricted to three dimensions of space. Perhaps it can expand into four, five, six, or more. Gravity might be just as strong as the other forces, but with more dimensions to expand into, it simply has more opportunity to spread out.

While the ADD conjecture is very provocative; , it has a seemingly fatal flaw. We clearly live in a three-dimensional reality, and both Newton and Einstein’s theories of gravity depend on a three-dimensional universe. Since those theories very accurately predict the motion of the planets, the suggestion that more dimensions of space exist seems to be rather convincingly disproved.

However, there is one possible way to save ADD. Perhaps those extra dimensions of space are not infinitely large like our familiar dimensions, but instead are extremely tiny.

Now that might sound like a super weird idea, but consider a tightrope walker. They are restricted to walking in only one dimension, forward or backward. But now let’s think about an ant walking on that same tightrope. For the ant, the tightrope isn’t a one-dimensional world. It’s two-dimensional. The ant can walk along the rope in the same manner as a human, but the ant can also walk around the rope. And around the rope is a much smaller dimension than along the length.

The idea that gravity can spread into more spatial dimensions than three is an intriguing one, but how could we test that experimentally? In fact, two such tests exist. The first is to bring two very small masses close to one another and see if the force between them varies with distance following the same relationship that governs how the planets are attracted to the sun. Experiments performed at the University of Washington agree well with Newton’s familiar law of gravity—and thereby validate a three-dimensional behavior of the universe—down to a distance of 50 micrometers.

To investigate even smaller length scales requires a particle accelerator. Assuming gravity is actually a strong force, masked by the existence of extra dimensions, then probing distances roughly the size of those extra dimensions will reveal gravity’s strong nature. If gravity is as strong as the other forces, that means it will be possible to make microscopic black holes that will decay via Hawking radiation. Using a particle accelerator to create very high-energy particles—which implies very small distances—we could look for a burst of photons escaping from a single location and traveling in all directions. That is one of the possible signatures of an evaporating black hole.

The Compact Muon Solenoid (CMS) detector where I conduct research is used (among other purposes) to look for the Hawking radiation that would accompany an evaporating microscopic black hole. That detector is one part of CERN, the European Organization for Nuclear Research, whose Large Hadron Collider (LHC) is the most powerful particle accelerator in existence.

Beginning as early as 1999, scientists followed this approach and set strict limits on the size of extra dimensions. The actual result depends on whether two, three, four, or more dimensions exist, but the ballpark limit from current data says that if extra dimensions exist, they are smaller than about 5 × 10^-20 meters.

With such a strict limit, the scientific enthusiasm for this idea has waned somewhat in the ensuing quarter-century. However, researchers continue to try to discover or exclude spatial dimensions of even smaller size.

The LHC is scheduled to resume operations in 2030. More experiments here might finally explain why gravity seems so weak. Or not. Either way, this future research will provide crucial information that will shed more light on this longstanding mystery of modern physics.