How Many Dimensions Are There?


Ask someone to name every dimension they know of and they'll likely list the following: length, width, and depth. They might also add time if they’re thinking outside the three-dimensional box. But asking a string theorist, “How many dimensions are there?” would elicit a very different response. According to this branch of theoretical physics, there are at least 10 dimensions of space, most of which are impossible for humans to perceive.

Dimensions are the metrics that physicists use to describe reality. Sounds broad, right? Let's start with the three dimensions most people learn in grade school. The spatial dimensions—width, height, and depth—are the easiest to visualize. A horizontal line exists in one dimension because it only has length; a square is two-dimensional because it has length and width. Add depth and we get a cube, or a three-dimensional shape.

These three coordinates are used to pinpoint an object's location in space. But space isn’t the only plane we exist on; we also exist in time, which is where the fourth dimension comes in. Once we know a dot's altitude, longitude, latitude, and position in time, we have the tools needed to plot its existence in the universe as we know it.

But some physicists who subscribe to string theory argue there’s more to reality than the observable universe. String theory, also known as "superstring theory," aims to unify two main theories describing how the universe works: general relativity (which applies to very large objects) and quantum mechanics (which applies to very small ones). In a four-dimensional universe, this theory wouldn’t be possible, but once scientists tweaked the math to include 10 dimensions—11 including time—their equations worked.

After coming up with a theory that hinges on the existence of 10 space dimensions, string theorists then had the job of explaining where those new dimensions were hiding. Their answer: They are just as real as the "big" dimensions we can see, but the extra dimensions are curled up so tightly that they're too small for us to notice directly.

Our basic understanding of physics makes this hard to process, but string theorist Brian Greene does a great job of framing the concept in terms most people can understand. In his 2005 TED Talk, Greene compares these invisible dimensions to the cables connected to telephone poles: From a window, a wire looks like a one-dimensional line. But if we were to study it up close we'd see that the cord is actually round, making it three-dimensional. No analogy comparing unobservable dimensions to objects in the observable world can ever be perfect, but this illustrates how something so fundamental to reality could be hiding in plain sight.

String theory states there must be at least 10 dimensions of space plus one dimension for time, but there are physicists who argue that there are more. Some posit a universe composed of 11 space dimensions. But to really blow someone's mind when they ask how many dimensions there are, say 26: That's the magic number according to Bosonic string theory, and it's as high as mainstream physicists are willing to go for the time being.

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Friday’s Best Amazon Deals Include Digital Projectors, Ugly Christmas Sweaters, and Speakers

As a recurring feature, our team combs the web and shares some amazing Amazon deals we’ve turned up. Here’s what caught our eye today, December 4. Mental Floss has affiliate relationships with certain retailers, including Amazon, and may receive a small percentage of any sale. But we only get commission on items you buy and don’t return, so we’re only happy if you’re happy. Good luck deal hunting!

Why Do We Celebrate Christmas on December 25?

If Jesus wasn't born on December 25, does this rule still apply?
If Jesus wasn't born on December 25, does this rule still apply?
Jon Tyson, Unsplash

Each December, Christians throw a collective birthday bash to celebrate the anniversary of Jesus’s arrival on Earth. But without a birth certificate—or any other official record of his actual birthdate—in existence, December 25 seems like an arbitrary day for all our Christmas traditions. So how did early observers choose it?

When Was Jesus Really Born?

Since the Bible doesn’t name a month or even a season for Jesus’s birth, historians have relied on other context clues to estimate when it occurred. Shepherds tend sheep in the Nativity story, which people often cite as evidence that Jesus was more likely born during the spring. Others argue that Israel’s mild winter temperatures allow sheep to graze even in December. According to Slate, it’s also possible that sheep set aside for religious sacrifices may have been given free rein, frigid night or not.

The Adoration of the Shepherds by Sebastiano Conca, 1720.J. Paul Getty Museum // Public Domain

One clue pointing specifically to December 25 comes from the story of Mary’s cousin Elizabeth, who approached old age without having given birth to any children. One day, her husband, a priest named Zacharias, was burning incense in the temple when the angel Gabriel appeared to him with good news: Elizabeth would bear a son. Early Christians guessed that Zacharias was probably in the temple for Yom Kippur, which they believed always took place on September 24 (it actually shifts year to year based on the Jewish lunisolar calendar). Nine months after September 24 is June 24, so they chose that as the birthdate—and feast day—of Elizabeth and Zacharias’s son, John the Baptist. When Gabriel later visited Mary to let her know that she’d bear a son, too, he mentioned that Elizabeth was in her sixth month of pregnancy. That means Jesus would’ve been conceived in late March, and born in late December—the night of December 24, to be exact, or the early hours of December 25.

Another theory suggests that Christians arrived at December 25 based on an ancient Jewish idea that prophets die on their birthday. During the 3rd century CE, theologists like Tertullian and Hippolytus dated Jesus’s crucifixion to March 25, since it happened around Passover. But to Sextus Julius Africanus, it was less about when Jesus was born and more about when he first came to Earth; in other words, he believed Jesus’s death and conception coincided on March 25, and thus his birth occurred on December 25 [PDF].

The Early History of Christmas

Even if Zacharias was in the temple on September 24, Gabriel did visit Mary exactly six months later, and Jesus was born right on his due date, it’s still possible that we celebrate Christmas on December 25 for a different reason altogether.

While 3rd-century Christians were busy worshiping the Son of God, some of their pagan counterparts were busy worshiping the Sun God. In the 270s, Roman emperor Aurelian popularized the cult of Sol Invictus, or “The Unconquered Sun,” whose feast day was celebrated on December 25. According to John Carroll University history professor Joseph F. Kelly, other Romans revered a Persian god, Mithra, whose feast day also may have fallen on December 25. There was also Saturnalia, an annual Roman festival that ran from December 17 to December 23. In short, many ancient Romans were well-accustomed to celebrating something in late December by the time Christianity entered the mainstream.

A painting of Saturnalia festivities by Antoine Callet, 1783.Themadchopper, Wikimedia Commons // Public Domain

That happened during Constantine’s rule over Rome in the early 4th century. In 313, Constantine and his fellow ruler Licinius issued the Edict of Milan, which basically legalized Christianity and condemned the ongoing persecution of anyone who practiced it. Constantine was a devout Christian himself, and he spent the rest of his reign spreading the religion throughout the empire. The first known record of December 25 as Jesus’s official birthday is from 336, the year before Constantine died. Because it’s mentioned in a volume containing other important religious dates, some have assumed that a celebration probably occurred on that day, and 336 is often cited as the first known “Christmas.”

Whether Christians celebrated Christmas on December 25 before 336 may forever be unknown, but we do know that the custom quickly caught on (spending the holiday watching A Christmas Story marathon wouldn't come until much later). By the end of the 4th century, Christian bishops were holding Christmas Mass all over Rome, and pagan festivals soon fell out of fashion. The fact that Christmas essentially replaced those earlier December traditions could be a coincidence, but some believe it was by design: Since Romans were already primed for parties on December 25, the Church could’ve been trying to co-opt a built-in subscriber base.

In summary, the origins of Christmas are just as subject to interpretation as Jesus’s actual birthdate—so feel free to play Christmas music whenever you want.

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