Driving Makes You Age Slower.

Here is a sentence that sounds like something a guy with a podcast would tell you, but he wouldn't technically be wrong.

If two people start next to each other, one goes for a drive while the other stays put, and they later meet back up, the person who spent more time moving will have experienced a tiny bit less elapsed time.

Actual time.

The difference at normal car speeds is so unbelievably small that it is useless for anything except making an otherwise normal commute sound like experimental physics. But it is there, and we have clocks precise enough to measure effects like it.

That is the part about relativity I think gets lost when the conversation immediately jumps to spaceships moving at 99.999 percent the speed of light.

Einstein's ideas about time are not something that switch on when you reach space. You experience them in everything you do in your day-to-day life.

And somewhere above you right now, GPS satellites are actively correcting for them so your phone does not send you several miles away from the Taco Bell you were actually trying to find.

The easiest way I have found to think about relativity is to stop imagining time as one giant invisible clock hanging over the universe.

There is no universal clock that everybody, everywhere, is secretly synchronized to. You carry your own time with you.

Physicists call the amount of time measured along your own path through spacetime "proper time." That is about as deep into the terminology as I want to get here, because the interesting part is what it actually means.

Two people can leave the same place, take different paths through spacetime, meet again later, compare clocks, and discover that different amounts of time passed for each of them.

There are two big ways to make that happen:

Move differently.

Experience gravity differently.

Higher speed makes your clock accumulate slightly less time compared with a slower-moving clock.

Stronger gravity also makes your clock accumulate less time compared with a clock sitting in weaker gravity.

You do not notice either effect happening to yourself. Your watch still ticks normally. Your heart beats normally. Your thoughts do not suddenly start playing at 0.75x speed.

From your own perspective, one second is still one second.

The disagreement only appears when you compare your clock with somebody else's.

Which is where normal life starts getting stupid.

Driving Is Technically an Anti-Aging Strategy

Suppose you and an identical copy of yourself synchronize watches.

Your copy stays home.

You get in a car, drive around at 60 mph for one hour, return, park next to them, and compare clocks.

Ignoring tiny complications like changes in elevation and Earth's rotation, you would have experienced roughly 14 picoseconds less time.

A picosecond is one trillionth of a second.

So, admittedly, we are not exactly threatening the skincare industry here.

But it is not zero.

Now stretch the experiment out.

Suppose you somehow averaged two hours of 60 mph driving every single day for 50 years while your extremely boring twin spent those same hours sitting still at essentially the same elevation.

At the end of that extremely specific lifestyle experiment, you would be roughly half of one microsecond younger from the speed difference alone.

Half a microsecond.

Fifty years.

The highway has defeated aging by approximately the amount of time it takes light to travel a few hundred meters.

Still counts.

More importantly, this is not just us plugging numbers into Einstein's equations and trusting the math because his hair looked convincing. NIST has directly measured motion-based time dilation using extremely precise atomic clocks at speeds comparable to a car traveling around 20 mph.

Relativity works at highway speeds.

The effect is just tiny enough that humans spent most of history having absolutely no chance of noticing it.

Your Head Is Older Than Your Feet

Gravity also changes how much time passes.

Closer to a massive object, where gravity is stronger, time passes slightly more slowly compared with a location higher up where gravity is weaker.

That means a clock on the first floor of a building runs very slightly slower than a clock on the second floor.

In 2010, NIST scientists compared two extremely precise atomic clocks separated vertically by only about 33 centimeters (about 1 foot for us imperials).

The higher clock ticked faster.

Over a 79-year lifetime, a 33-centimeter height difference would add up to only around 90 billionths of a second, but the clocks were good enough to see it.

When you are standing up, your head is technically aging faster than your feet.

Your face has experienced slightly more time than your ankles.

I do not know what anyone is supposed to do with that information.

Sleeping Upstairs Is Also Technically Aging You Faster

The same effect keeps going as you move to larger differences in elevation.

NIST gives a nice example: if two identical clocks are separated vertically by one kilometer near Earth's surface, the higher clock gains roughly three seconds on the lower clock over one million years.

That sounds useless until you convert it into normal-human numbers.

It works out to around three microseconds per year.

Spend 80 years living one kilometer higher than somebody else, with everything else magically held equal, and you would accumulate roughly a quarter of a millisecond more time.

You would be older, by a quarter of a millisecond. (Congratulations on your mountain property, though)

This means that, strictly from the relativity side of things, someone living at sea level ages ever so slightly more slowly than somebody living high in the mountains.

A person sleeping in a basement ages slightly more slowly than somebody sleeping upstairs.

A person at the bottom of a skyscraper ages slightly more slowly than somebody at the top.

Again, none of these differences matter biologically. Weather, exercise, diet, stress, air quality and about ten thousand other things are going to bulldoze straight over a relativistic age difference measured in microseconds.

Airplanes Get Both Effects at the Same Time

They move fast, which makes their clocks run slower relative to slower clocks.

But they also climb thousands of meters above the ground, where Earth's gravity is slightly weaker, which makes their clocks run faster.

So does flying make you younger or older?

Your speed matters.

Your altitude matters.

Your direction matters because Earth itself is rotating.

Your route matters.

This was tested in one of my favorite "somebody actually paid for this" experiments.

In 1971, physicists Joseph Hafele and Richard Keating took cesium atomic clocks onto commercial airliners and flew them around the world, once eastward and once westward.

They then compared the traveling clocks with atomic clocks that stayed behind, and they disagreed with eachother.

More importantly, they disagreed by amounts consistent with the combined effects predicted by relativity.

The eastbound and westbound flights did not produce identical results because the airplanes were moving with or against Earth's rotation, changing their total speeds relative to the relevant reference frame.

They bought atomic clocks plane tickets and came home with different times.

Now we can finally leave the airport.

The International Space Station moves around Earth at roughly 28,800 kilometers per hour.

At that speed, motion-based time dilation is no longer living way down in picosecond territory.

Astronauts aboard the ISS are also farther from Earth, so weaker gravity tries to make their clocks run faster than clocks on the surface.

Once again, both effects are fighting each other.

At the altitude and speed of the ISS, the speed effect wins overall.

ESA has estimated that an astronaut returning from a six-month stay aboard the station would be roughly 0.007 seconds younger than if they had simply stayed on Earth.

That is still useless as an actual anti-aging technique, especially when you consider that microgravity has much larger effects on muscles, bones, vision and the rest of the human body.

Relativity hands the astronaut a few milliseconds back.

Spaceflight sends a much larger invoice.

But there is something incredible about the fact that "astronaut returns younger than everyone else" is not purely science fiction.

GPS systems have to care about this or they stops working.

This is probably the best example of why relativity should not be treated like a weird philosophical side quest in physics.

GPS satellites carry extremely precise clocks.

They are moving very quickly, so special relativity makes those clocks lose about 7 microseconds per day compared with clocks on Earth.

But GPS satellites also orbit far above Earth's surface, where gravity is weaker.

That makes their clocks gain about 45 microseconds per day.

Put the two together and the satellite clocks run about 38 microseconds per day faster than comparable clocks on Earth.

Thirty-eight millionths of a second per day does not sound like much, but for GPS, it is enormous.

The entire system figures out your location using the travel time of radio signals moving at the speed of light. Tiny timing errors turn into large position errors very quickly.

NASA notes that without relativistic corrections, GPS navigation would drift enough to guide you miles away from where you were actually trying to go.

Your phone's blue location dot works because engineers had to account for the fact that time does not pass at the same rate on the satellite as it does down here.

Einstein is basically doing tech support for Google Maps from 1905.

Now Turn the Speed Knob Until Things Get Stupid

At car speeds, relativity is a rounding error.

At orbital speeds, it becomes milliseconds.

Get close enough to the speed of light and the entire situation changes.

Imagine a future spacecraft leaves Earth, accelerates to 90 percent the speed of light, cruises for a while, turns around, and eventually comes home.

During the high-speed portion of the trip, if Earth measures ten years passing, the people aboard would experience only about 4.36 years.

Now push the ship to 99.9 percent the speed of light.

Ten years on Earth becomes about 163 days aboard the ship during that same idealized cruising time.

At 99.9999 percent the speed of light, ten Earth-years of cruising corresponds to only about five days for the travelers.

That is where the phrase "time dilation" stops sounding like a microscopic clock correction and starts becoming an actual method of traveling into the future.

Nobody aboard feels time slow down.

They eat normally, sleep normally, watch five days pass, then they come home and discover Earth got ten years older.

There are approximately several enormous engineering problems hiding inside the phrase "just accelerate a spacecraft to 99.9999 percent the speed of light," starting with the obscene amount of energy required and continuing through the fact that hitting a grain of dust at that speed becomes a very exciting problem for whoever designed the windshield.

The point is that the effect causing our imaginary traveler to skip years of Earth's history is the same basic effect that makes a moving atomic clock tick differently at 20 mph.

There is no moment where physics changes the rules.

We just finally move fast enough for the rules to become obvious.

Gravity can play the same game.

The stronger the gravitational field, the more slowly time passes relative to somewhere with weaker gravity.

Earth gives us differences measured in nanoseconds and microseconds because Earth is not particularly extreme.

A black hole has no such restraint.

NASA released a visualization in 2024 that modeled a trip near a supermassive black hole. In one version of the scenario, a spacecraft makes a six-hour round trip near the black hole while colleagues remain far away.

The traveler returns 36 minutes younger than the people who stayed farther from the black hole.

Same basic idea as the staircase.

Just with a black hole involved because apparently 33 centimeters was not dramatic enough.

Near a rapidly rotating black hole, the difference could become much larger under the right conditions.

That does not mean the traveler goes backward in time.

Everybody keeps moving forward.

They just accumulate different amounts of it.

Relativity is not giving you a rewind button, it is giving you different fast-forward settings.

I can already hear the relativity people clearing their throats, so there is one important detail worth adding.

There is no absolute definition of "moving."

Earth rotates and orbits the Sun.

The Sun moves through the Milky Way.

The Milky Way is moving relative to other galaxies.

So saying "the driver is moving and the person at home is stationary" is only shorthand for comparing them within roughly the same local Earth-based frame.

What makes the thought experiment meaningful is that the two people begin together, take different paths, and eventually reunite.

At that point they can put the clocks next to each other and compare how much time each clock actually recorded.

Different path through spacetime.

Different elapsed time.

That is the thing we can physically measure.

The effects around us are microscopic because our everyday speeds are pathetic compared with light and Earth's gravity is fairly tame compared with something like a neutron star or black hole.

That means there is no perfectly shared "now" that everybody in the universe is moving through at exactly the same rate.

Every person, planet, spacecraft and satellite is following its own path through spacetime and accumulating its own elapsed time along the way.

Usually those differences are nanoseconds.

Get fast enough or close enough to something massive and they can become years.

The same universe that can let a near-light-speed astronaut skip years of Earth's future is already doing the bargain-bin version of that experiment every time you pull out of your driveway.

Your commute is technically time travel.

It is just the worst time machine ever built.

SOURCES

• https://www.nist.gov/news-events/news/2010/09/nist-pair-aluminum-atomic-clocks-reveal-einsteins-relativity-personal-scale
• https://www.nist.gov/news-events/news/2010/09/nist-clock-experiment-demonstrates-your-head-older-your-feet
• https://www.nist.gov/publications/relativity-and-optical-clocks
• https://www.nist.gov/blogs/taking-measure/putting-einstein-test-worlds-most-accurate-clocks
• https://www.nist.gov/atomic-clocks/a-powerful-tool-for-science/putting-einstein-test
• https://www.esa.int/ESA_Multimedia/Images/2019/02/Timing_is_everything
• https://science.nasa.gov/solar-system/10-things-einstein-got-right/
• https://science.nasa.gov/universe/black-holes/supermassive-black-holes/new-nasa-black-hole-visualization-takes-viewers-beyond-the-brink/
• https://science.nasa.gov/universe/what-happens-when-something-gets-too-close-to-a-black-hole/