Every person on Earth is living in the past. This may sound like the plot to some sci-fi,
time-travel thriller, but it’s actually a fact of human biology and the trickiness of time.
Our brains don’t perceive events until about 80 milliseconds until after they’ve happened.
This fine line between the present and the past is part of the reason why some physicists argue
that there’s no such thing as “now” and that the present moment is no more than an illusion.
Hi, I’m Erin McCarthy, editor in chief of Mental Floss, and welcome to the List
Show from my living room. The potential impossibility of “'living in the present”
is just one head-scrambling fact about time I’m going to share with you today.
Throughout history, different cultures around the world have experienced time in different ways.
In the Western world, we tend to think of time as linear and flowing from left to right.
But this isn’t the case for everyone. Language affects how people conceptualize time,
particularly the spatial metaphors they use to describe and map it.
Those who read languages that flow from right to left, such as Arabic and Hebrew, generally view
time as flowing in the same direction. The Aymara, who live in the Andes Mountains in South America,
consider the future to be behind them, while the past is ahead. In their view, because the future
is unknown, it’s behind you, where you can’t see it. Some indigenous Australian cultures, which
rely heavily on direction terms like north, south, east, and west in their languages, visualize the
passage of time as moving from east to west. If they’re facing north, for example, the past would
be to their right, or east, whereas the future would be to their left, which would be west.
Individual people can experience time differently, too. You’ve probably noticed how time seems to
speed up when you’re racing against a deadline or having fun, and how it tends to drag when
you’re bored. This is because when you’re focused on something, like a big work project or a party,
your brain pays less attention to how time passes. But when you’re bored, or your brain is less
stimulated, you become more aware of the passing of time, making it feel slower. One study proposed
that dopamine—the neurotransmitter and hormone that helps us feel happy—may be an additional
culprit. It showed that increased dopamine production, which happens when you’re enjoying
something, may slow down your body’s internal clock, making time feel like it’s flying by.
Science has a number of different ways of defining time. To discuss just a couple: There’s
astronomical time, which is measured in relation to how long it takes Earth to rotate on its axis.
In astronomical time, a second is 1/60th of a minute. And then there’s atomic time,
which dictates the numbers that you’ll see on a clock. According to atomic time, one second
equals 9,192,631,770 oscillations of a cesium-133 atom. Measuring the vibration of an atom—which,
in simple terms, is the gist of what oscillation is—is the most accurate way to track time.
We can thank Albert Einstein for a lot of our current understanding of the physics of time.
Rather than viewing time as a set order, he proved that it’s actually relative. For example,
according to Einstein’s theory of special relativity, there’s an inverse relationship
between your speed and the speed of time. The faster you move, the slower time moves. This
is why someone blasting through space will age slower than the people still hanging out on Earth.
Astronaut Scott Kelly was born several minutes after his twin brother, Mark,
but after Scott spent 340 days living on the International Space Station, he returned to Earth
around an extra 5 milliseconds younger than his “big” brother. Had Scott been traveling at a speed
close to the speed of light, that age difference would have become much more pronounced.
Einstein’s theory also states that gravity can warp time. If you’ve seen the 2014 movie
Interstellar, this concept may seem familiar. The closer you are to a massive body—which,
in the case of Interstellar, is a giant black hole—the slower time would pass for you.
Gravity’s effect on time isn’t limited to intergalactic travel. Here on Earth,
gravity can vary for a number of reasons, including your altitude, since you’re changing
your distance from the center of the earth. That means if you put a bunch of synchronized atomic
clocks at various altitudes, eventually those clocks would fall out of sync. A clock at the top
of Mount Everest and one at sea level would, over the entire 4.5 billion year history of the planet,
have diverged by about a day and a half. Gravity is also the reason why our days are
getting longer. Over a billion years ago, a day on Earth lasted around 18 hours. Our days are longer
now because the moon’s gravity is causing Earth’s spin to slow down. In Earth’s earlier days,
the moon wasn’t as far away, which caused Earth to spin much faster than it currently does.
Longer days also mean shorter years. Kind of. The time it takes the Earth to orbit the
sun hasn’t changed, but the amount of days within a year has. Back when the dinosaurs
ruled 70 million years ago, days were only around 23.5 hours long, and a year was made
up of 372 of those slightly shorter days. There are two ways to think of the length
of a day on Earth. Though you probably learned that one day on Earth is 24 hours,
it actually takes the planet 23 hours, 56 minutes and 4.0916 seconds to rotate on its axis. This is
the difference between a solar day and a sidereal day—a solar day is 24 hours, whereas a sidereal
day is roughly four minutes shorter. We measure solar time based on the sun’s position in the sky,
whereas a sidereal day is measured based on the location of the “fixed” stars. In other words,
a sidereal day is the time it takes for a distant star or constellation
to appear on the same meridian. Because astronomical time and atomic
time don’t always line up, every so often, we get a leap second. Earth’s spin speed can
be a bit unpredictable. Atmospheric winds, Northern Hemisphere winters with heavy snow,
and other big weather systems can affect how fast the planet rotates. In order to keep the
difference between astronomical time and atomic time to less than .9 seconds, the International
Earth Rotation and Reference Systems Service will occasionally announce the need for a leap second.
Most people won’t notice a leap second, but they can be a huge pain for tech companies. Because
leap seconds are added irregularly, developers have no way of working them into their codes,
which has caused websites like LinkedIn and Reddit to crash in the past. A bug caused by 2012’s leap
second created so much chaos on Qantas’s servers, more than 400 flights wound up being delayed.
By more than a second, presumably. The length of a year on Earth can also get
a bit complicated. The original Roman calendar was a bit of a mess, so much so that in 46 BCE
Julius Caesar mandated a 445 day long year to help bring the calendar back in sync with the seasons.
At the same time, Caesar asked the astronomer Sosigenes to help reform the calendar.
Most years were set at 365 days, but to make up for the fact that the earth's revolution
around the sun doesn't take exactly 365 days, leap years were implemented. Every
four years the month of February was given an extra day to make up for what
is a sort of rounding error in the calendar. But Sosigenes made a bit of a miscalculation,
so the calendar continued to be a little off. He thought a year lasted 365.25 days. It’s actually
around 365 days, five hours, 48 minutes, and 45 seconds, equivalent to about 365.242 days.
This tiny error had some pretty big consequences: By 1577, the Julian calendar was off by 10 days,
meaning key Christian holidays were being celebrated on incorrect dates.
Pope Gregory XIII took issue with this and established a commission to get the calendar
back on track. In 1582, the Gregorian calendar was created. Rather than having an extra day
every four years without exception, years that are divisible by 100—like 1700 or 1900—skip leap year.
UNLESS the year is also divisible by 400, like the year 2000, in which case the Leap Year is back on!
Even this system isn’t perfect, though: It has an error of one day in 3236 years.
We can thank the railroad industry for standardizing our time zones. Up until
the 19th century, towns and villages synchronized their clocks to the local solar noon. This created
thousands of local times that all varied and made scheduling transportation a major headache. Train
schedules in different cities had to list dozens of arrival and departure times for each train to
account for all the mini time zones. On November 18, 1883, railroad companies in the United States
and Canada began using a system very similar to the standardized time zones we still use today.
In the UK, the railroad companies began using a standard London-based time in 1840.
After an engineer named Sandford Fleming missed a train in 1876, he set out to change the way time
worked. He originally proposed a concept he called “Cosmic Time,” in which the world would run off an
imaginary clock located at the planet’s center, essentially a line from the center of the planet
to the sun. He then suggested splitting the world into 24 time zones labeled with a letter of the
alphabet, with each zone spanning 15 degrees of longitude. His original plan to create a
standard “Cosmic Time,” was rejected, but it did lay the groundwork for a similar standardization,
so-called Universal Time. And nations present at the 1884 International Meridian Conference
laid the groundwork for dividing the world into 24 time zones, with the Prime Meridian,
also known as Longitude 0°, running through Greenwich, England.
Even with the advent of standardized time, people still struggled to keep their clocks in sync.
One London family used this to their advantage, and made a living by selling people the time.
An astronomer named John Belville would set his pocket watch to the time at the Royal Observatory
Greenwich. He would then travel around the city and visit his network of subscribers,
who paid to set their own clocks by Belville’s pocket watch.
After Belville died in 1856, his wife, and then later their daughter Ruth,
carried on the tradition. Ruth continued to sell the time until World War II. By then
she was in her 80s, and a couple of factors led to her timely retirement: improved technology
had made her role less important, and the War was making treks around London too dangerous.
Time zones can still be a bit complicated. Big countries like Canada and the United States
have multiple time zones, whereas China, another large country, only has one. China adopted the
Beijing Standard Time to foster unity, but the effect can feel a bit uncanny—two cities in the
country can be at roughly the same latitude, but experience sunrise hours apart, according to their
clocks. In some parts of China, for example, the sun doesn’t rise until nearly 10 a.m.
Speaking of confusing time changes, here are a few facts about Daylight Saving Time. Though
a lot of people believe the biannual time change was adopted to keep farmers happy, that’s a myth.
The first person to seriously advocate for Daylight Saving Time was an entomologist who
wanted more sunlit hours to look for insects after work in the summer. He proposed his idea
to a scientific society in New Zealand in 1895. Daylight Saving Time wasn’t officially implemented
until 1916, when Germany became the first country to adopt it in an effort to conserve
coal during World War I. The United States didn’t follow suit until 1918.
Daylight Saving Time ended on a national level after the war, but individual states
and municipalities kept it going until World War II, when the entire nation went on what
was effectively a year-round Daylight Saving Time. After World War II the entire nation was
again picking and choosing Daylight Saving Time. It’s been reported that in Iowa, back in 1964,
there were 23 different combinations of dates that communities turned
on and off Daylight Saving Time. In 1966 the government officially mandated a standardized
Daylight Saving Time for the entire United States, though individual states can opt out.
Up until 2007, Daylight Saving Time ended in October. It’s been reported that the
candy industry lobbied to wait until after Halloween to change the clocks back an hour.
Daylight Saving Time does more than make people lose an hour of sleep.
It can have some pretty concerning health effects. Studies have linked Daylight Saving Time with an
uptick in heart attacks, car crashes, and mining injuries. The extra hour of daylight
is good for koalas, though. Researchers found that koala-car collisions went down
by up to 11 percent during Daylight Saving Time. We can’t talk about time without mentioning a few
clock facts. People have been tracking time for thousands of years. In 2013, archaeologists found
what’s thought to be the world's oldest lunar calendar while excavating a field in Scotland.
The calendar, which is made of a series of 12 pits that mimic the moon’s phases,
dates back to around 8000 BCE. Sundials read differently depending
on the hemisphere you’re in. In the northern hemisphere, the sun casts
a shadow that moves from north, to east, to south, to west. In the southern hemisphere,
the shadow moves in the opposite direction. Our concept of “clockwise” is based on the way
sundials in the northern hemisphere told time. In 1090, an innovative clock was built in China.
A man named Su Song created a water-powered clock tower that measured time and tracked the
movements of the planets and stars in the night sky. Su Song built a giant water wheel within
the clock tower. Buckets attached to the wheel would fill with water and then tip once full,
causing the wheel to rotate, demarcating time. The Maya had multiple calendars to measure time,
but the most familiar to apocalyptic crackpots is the Long Count Calendar. These calendars measured
around 5125 years, beginning around August, 3114 BCE. The Long Count calendar’s cycle came
to an end around December 21, 2012, sparking a craze of Armageddon conspiracy theories.
You’ll find the most accurate clock at the National Institute of Standards and Technology
in Boulder, Colorado. The clock keeps time by measuring the vibration of a single aluminum ion,
and should remain accurate for 33 billion years.
The clock sitting on your bedside table isn’t quite as precise. If you’ve bought
a new clock or watch recently, you may have noticed that the default setting was 10:10,
give or take a few minutes. There are various theories behind this particular choice of time,
but really, it all comes down to aesthetics. Setting the time to around 10:10 allows the
hands of an analog clock to be displayed in a neat, symmetrical way that doesn’t obscure any
logos in the center of the clock’s face. Clocks were once set to 8:20, and occasionally still are,
but the hands’ downward angles can make it look like the timepieces are frowning.
Traveling back in time is possible—theoretically, at least. According to Einstein’s theory,
you could travel back in time by moving faster than the speed of light, as long as you could
somehow have infinite mass. Since that probably won’t work, you could create “wormholes” between
two points in space-time. This would also be tough, since humanity still hasn’t invented the
technology to actually build a wormhole. Or, you could try bending space-time by plucking
some “cosmic strings.” Two of these theoretical strings, which are thin streams of pure energy,
that are moving in opposite directions at very very near the speed of light could theoretically
warp space-time enough to create a closed timelike curve, also known as a time machine.
Our next episode is all about the infamous body parts of historical figures.
If you know of a strange, funny, or otherwise interesting story behind a historical body part,
leave it in the comments for a chance to be featured in that episode. Thanks for watching!