Why does an electron have the mass it does?
Physicists can measure it, but current particle theory takes that value as an input. The same is true of other numbers that determine how particles behave.
One proposal is that those numbers vary between universes. String theory allows many arrangements of extra dimensions and fields. This collection, called the landscape, could give different universes different physics. The theory also contains branes, objects with several dimensions. In a braneworld model, our universe could be one such object inside a larger space, like the bulk in Interstellar.
The constants of nature
Physics describes the particles and forces we know with a theory called the Standard Model. It's one of the most precisely tested theories in physics. It also contains a list of numbers that the theory can't predict. You have to measure them and put them in by hand: the masses of the particles, the strengths of the forces, and a few more. Fred Adams counts them in The degree of fine-tuning in our universe, and others: 19 numbers, or 26 once you include the masses of neutrinos.
The best known of these numbers is the fine structure constant, which sets the strength of electromagnetism. Its value is about 1/137.036. Change it a little and atoms and chemistry would work differently.
Some of these numbers seem to sit in a narrow range that allows complex structures, like stars, carbon, and people. Physicists call this fine tuning. In 1953, the astronomer Fred Hoyle argued that stars could only make as much carbon as we observe if the carbon nucleus had a specific excited state at a specific energy. Ward Whaling's group at Caltech looked for it and found it the same year, at 7.68 MeV, in The 7.68-Mev State in C12. Hoyle started from the amount of carbon in stars and predicted something about nuclear physics. The historian Helge Kragh points out in When is a prediction anthropic? that Hoyle didn't reason from the existence of life. That reading of his prediction came in the 1980s.
The cosmological constant problem
The energy of empty space poses a harder problem.
In 1998, two teams of astronomers measured the light from distant exploding stars and found that the expansion of the universe is speeding up. The leaders of those teams, Saul Perlmutter, Brian Schmidt, and Adam Riess, won the Nobel Prize in Physics in 2011 "for the discovery of the accelerating expansion of the Universe through observations of distant supernovae". The simplest explanation for the speed up is that empty space itself has a small amount of energy, called the cosmological constant, or more loosely, dark energy.
Quantum physics also says that empty space should have energy. When physicists try to estimate how much, the simple estimate comes out larger than the measured value by about 120 powers of ten. Sean Carroll's review The Cosmological Constant calls it "the famous discrepancy of 120 orders of magnitude". That's a 1 followed by 120 zeros.
That discrepancy means the estimate leaves something unexplained. Physicists don't know why the measured value is so small.
If the cosmological constant were much larger, space would have expanded so fast that matter could never clump into galaxies. Without galaxies you get no stars, no planets, and nobody around to measure anything. In 1987, more than ten years before the measurement, Steven Weinberg used that argument in Anthropic bound on the cosmological constant to set an upper limit on the cosmological constant. Joseph Polchinski later listed "the cosmological constant is not zero" among the predictions of this kind of reasoning around 1987, in The Cosmological Constant and the String Landscape. The 1998 measurement found a small value that isn't zero, below Weinberg's limit.
As an explanation for the value we observe, this argument assumes that different regions can have different cosmological constants. Observers would find themselves in regions where galaxies can form. That is a different argument from Hoyle's prediction, which started from the observed abundance of carbon.
String theory
String theory is a candidate for a deeper theory of physics. In it, different vibrations of tiny strings appear as different particles. Comparison: one guitar string can produce different notes depending on how it vibrates. The string theory proposal applies that idea to particles and their properties.
One reason physicists study string theory is that it treats gravity within quantum physics. The usual superstring theories have ten spacetime dimensions: one of time and nine of space. Edward Witten's 1995 work on their connections helped establish an eleven dimensional framework called M theory.
The six extra spatial dimensions in superstring theory could be curled up, too small to see. Comparison: a garden hose looks like a line from far away. An ant on its surface can also walk around its circumference. That second direction is there even when you can't resolve it. Brian Greene uses the hose in The Elegant Universe, his 1999 book.
In many string models, the six extra dimensions would be curled into a tiny, complicated shape at every point of our space.
String theory has no direct experimental confirmation. A Quanta Magazine podcast from May 2025 described the situation as "despite its mathematical elegance, the theory still lacks empirical evidence." Strings are often associated with a length near 10−35 meters, roughly 15 powers of ten below the scale the Large Hadron Collider can probe, though their size depends on the model. Peter Woit's Not Even Wrong and Lee Smolin's The Trouble with Physics, both published in 2006, criticize the lack of experimental tests.
The landscape
How extra dimensions set the laws
The way the six extra dimensions curl up matters. The shape sets which particles exist at everyday energies, how strong the forces are, and how much energy empty space has. So it sets the numbers that the Standard Model can't explain.
The extra dimensions can take many shapes. Fields through those dimensions can also take different values. Together they determine possible vacua, states of the theory around which particles and forces behave in particular ways. Physicists study which of those states can remain stable long enough to describe a universe.
In 2000, Raphael Bousso and Joseph Polchinski showed how fields in extra dimensions could give many values of the cosmological constant, including very small ones. In 2003, Shamit Kachru, Renata Kallosh, Andrei Linde, and Sandip Trivedi proposed states with a positive cosmological constant that could last for cosmological times.
Frederik Denef and Michael Douglas studied how to count such possibilities in 2004. The frequently quoted 10500 is an estimate, not a count of observed universes. In 2015, Washington Taylor and Yi Nan Wang estimated about 10272,000 possibilities in one F theory construction, a framework related to string theory.
In 2003, Leonard Susskind gave this collection a name in The anthropic landscape of string theory: "I will from now on call the space of all such string theory vacua the landscape." Picture an energy landscape with an enormous number of valleys. Each valley is a possible set of laws of physics.
The landscape plus eternal inflation
A list of possible vacua doesn't tell us which ones occur in nature. Eternal inflation offers a way for different regions to settle into different states.
If inflation can produce bubbles in several of those states, the bubbles can have different particles, force strengths, and cosmological constants. Which states are reachable depends on the model. An infinite number of bubbles alone doesn't guarantee that every possible state occurs.
Some states would prevent galaxies from forming. Others might not allow stable atoms. A universe containing observers needs conditions that support those observers.
The anthropic principle
The anthropic principle says our observations are limited by the conditions needed for us to exist. A universe that can't contain observers won't have anyone measuring its constants.
Comparison: Earth's distance from the Sun allows liquid water on its surface under the conditions here. Other planets have other orbits. In September 2025, NASA's tally of confirmed planets around other stars passed 6,000. Our need for suitable conditions helps explain why we observe from this planet. It doesn't tell us how many other planets support life.
Applied to the landscape, this would explain why we observe a cosmological constant small enough for galaxies to form. It still needs a theory that produces those different values and a rule for comparing how often observers would find each one.
The swampland objection
In 2018, Georges Obied, Hirosi Ooguri, Lev Spodyneiko, and Cumrun Vafa proposed a restriction that would exclude stable or long lived states with a positive cosmological constant. If correct, it would challenge the states used in this explanation of our universe. Their proposal belongs to the swampland program, which studies theories that look consistent at low energies but may be incompatible with quantum gravity.
The proposal is a conjecture. It challenges those particular states, not the existence of every vacuum in string theory.
Branes
What a brane is
String theory doesn't only contain strings. It also contains objects with more dimensions, called branes, short for membranes. Jin Dai, Robert Leigh, and Joseph Polchinski introduced one kind in 1989. In 1995, in Dirichlet-Branes and Ramond-Ramond Charges, Polchinski showed that branes are essential objects of the theory. A brane can have any number of dimensions up to the number of dimensions of space. A one dimensional brane is a string. A two dimensional brane is like a sheet. A three dimensional brane is like a whole space.
In string theory, some strings have their ends stuck on a brane. They can move anywhere along the brane but can't leave it. Other strings are closed loops with no ends, and they can travel anywhere, including off the brane. The particle that carries gravity would be a closed string.
The braneworld idea
Maybe our entire three dimensional universe is a brane, floating in a bigger space with at least one more dimension. That bigger space is called the bulk.
In the models where ordinary matter and light are confined to a brane, they can move along it but can't leave it. Gravity can extend into the bulk.
Comparison: picture ants living on a sheet of paper. They can walk anywhere on the sheet, but they can't step off it. Another sheet of paper could float a millimeter above them, full of other ants, and they would never see it. Now imagine the ants can feel vibrations through the air between the sheets. That vibration is gravity. It's the only thing that connects the two sheets.
Matter on another brane could therefore be invisible to us while still affecting us through gravity. Whether such a brane exists, and how strongly it interacts with ours, depends on the model.
Why gravity is so weak
Braneworlds also offer an explanation for something odd: gravity is far weaker than the other forces. A small magnet can lift a paperclip against the gravitational pull of the entire Earth.
In 1998, Nima Arkani-Hamed, Savas Dimopoulos, and Gia Dvali suggested in The hierarchy problem and new dimensions at a millimeter that gravity might look weak because it leaks into extra dimensions, while the other forces stay on our brane. In 1999, Lisa Randall and Raman Sundrum proposed a version with one warped extra dimension, which could explain the weakness of gravity in a different way.
If gravity spreads into an extra dimension that's large enough, it should become stronger than Newton's law predicts at very short distances. A 2020 experiment at the University of Washington found that Newton's law fit measurements down to 52 micrometers. For extra dimensions wrapped into circles, the result put the largest radius below about 30 micrometers at 95 percent confidence. Other shapes need their own calculations.
Particle colliders also search for signs of extra dimensions and tiny black holes that some models predict. In a 2018 search, the CMS experiment at the Large Hadron Collider found no significant excess above the expected background. CERN describes these signals as possibilities being tested.
These results constrain particular models and sizes of extra dimensions. They don't rule out every braneworld.
The ekpyrotic universe
In 2001, Justin Khoury, Burt Ovrut, Paul Steinhardt, and Neil Turok proposed in The ekpyrotic universe: colliding branes and the origin of the hot big bang that the Big Bang might have been a collision between two branes moving through the bulk. They called it the ekpyrotic universe, from the Greek word for conflagration. It was designed as an alternative to inflation. If it were right, our universe would have a partner brane, and the hot Big Bang would be the moment they hit.
Interstellar and the bulk
Interstellar came out in November 2014. Its science advisor and executive producer was Kip Thorne, a physicist who later shared the 2017 Nobel Prize in Physics for the detection of gravitational waves. Thorne wrote a book alongside the film, The Science of Interstellar (W. W. Norton, 2014), with chapters called "Bulk Beings" and "The Tesseract". Throughout the book he marks each idea as truth, educated guess, or speculation.
The film borrows the braneworld idea. Our universe is a brane in a bulk with one extra dimension. Beings who live in the bulk, the film calls them "they", can move through that extra dimension. Near the end, Cooper falls into a black hole and ends up in a tesseract, a structure the bulk beings built, where he can see moments of his daughter's room from the outside. He sends a message back to her using gravity, because in the braneworld picture gravity is the one thing that can travel through the bulk.
Thorne sorted these ideas himself in an article for Science Friday in November 2014. He called the bulk an educated guess: "it seems likely from the quest to understand quantum gravity that our universe is a membrane". The gravitational anomalies the film depends on, he called speculation, and he noted that "no convincing gravitational anomalies have ever been seen on Earth".
What I think
The small cosmological constant needs an explanation. The landscape offers one if the relevant states exist and inflation can produce regions in those states.
Braneworlds make a different proposal: extra spatial dimensions that gravity can enter. Both ideas need experimental support before we can say they describe our universe.
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