One of Albert Einstein’s most mind-boggling predictions is that gravity slows down time. This so-called gravitational time dilation comes directly from his theory of general relativity. How does it work? Raise your arm up high so that your wristwatch is up over your head. Now take that same watch and hold it close to the ground. Per Einstein, your watch runs slower near the ground than it does when it is over your head!

You see, assuming identical clocks, the lower clock (lower altitude) always runs slower than the higher clock (higher altitude). Why? Because the lower clock is closer to the source of gravity, the Earth. This effect is extremely tiny here on our small planet. That’s why we don’t notice it. But it is real!

How do we know? Physicists at the National Institute of Standards and Technology (NIST) recently used two super-accurate aluminum ion clocks to test Einstein’s wild idea. They placed one clock about 6 inches above the other. The lower clock indeed ran a tiny bit slower than the higher clock; and in just the amount predicted by Einstein’s formula! This came as no surprise to scientists. All previous tests showed the same effect — atomic clocks at sea level ran slower than identical clocks on mountains, airplanes, rockets, and satellites.* Gravity does slow down time.

What does this have to do with time travel? It means we can (and do) travel forward and backwards through time. And we don’t have to climb into some imaginary time machine to do it; we just need to change altitude.

The Twins Paradox and Gravity

To get a feel for how this works, imagine a fictitious super-massive, super-dense stellar object called Neutronium. On its surface is a mountain of great height called Mount Neutron. Now gravity on the surface of Neutronium is much stronger than gravity on the mountain top. So much so that time on the surface runs twice as slow as time on Mount Neutron. So, for every year that goes by on the surface of Neutronium; two years go by on the mountain top.

Say twins are born on the surface and separated at birth. One is immediately sent to live on the top of Mount Neutron. Let’s call her Tina Top. We’ll call the remaining twin Sammy Surface.

On her 30th birthday, Tina Top decides to leave Mount Neutron and visit her twin brother on the surface. But when she arrives down below, Sammy Surface is only 15 years old. After all, time for Sammy has run at only half the rate of time for Tina on top of that mountain.

Say the twins were born in the surface year 2000. Tina Top returns to Sammy in the surface year 2015. But 30 years have gone by for Tina. To her, it should be the year 2030, but it is only the year 2015 here on the surface. Do you see it? Tina Top has effectively traveled 15 years into the past.

Now say after visiting Sammy Surface, Tina Top immediately returns to Mount Neutron. Then on his 30th birthday, Sammy Surface travels to Mount Neutron to visit his twin sister. When he meets Tina, she is now 60 years old. In fact, everything on the mountain-top has aged 60 years since they were born.

Sammy spent 30 years on the surface, but it is now 60 years later on Mount Neutron. Sammy Surface has traveled 30 years into the future!**

Everyday Time Travel with Gravity

Time on the surface of the Earth runs some 0.02 seconds per year (2 seconds per century) slower than time in the zero gravity of outer space. Again the effect is so small here because Earth’s gravity is relatively weak. But we still experience time travel whenever we change altitude.

I live in a two-story house and my bedroom is upstairs. Gravitational time dilation makes the alarm clock in my upstairs bedroom run a very tiny bit faster than the wall clock in my downstairs kitchen. (Assuming identically accurate clocks.) This effect has nothing to do with the clocks themselves. If I switch clocks, gravitational time dilation still makes the upstairs clock run faster than the one downstairs.

It is time itself which is running faster upstairs. All night long, as I sleep upstairs, all aspects of time are moving ever so slightly faster than time downstairs. For example, my heart beat at rest (all other things being equal) is a tiny bit faster upstairs than down. I am aging a tiny bit faster upstairs. When I come down to breakfast in the morning, I am in effect traveling into the past (by a miniscule amount). Similarly, if I spend my day at ground–level, when I return to my upstairs bedroom at night I am traveling a miniscule amount into the future.

So gravitational time travel is real. We do it all the time. Into the future and into the past. If we wore accurate enough watches, we wouldn’t need to read about this effect. Time travel with altitude would be an accepted gravitational effect, just like the proverbial apple falling from the tree.

* Physicsworld reports that these remarkable devices remain accurate to within one second in 3.7 billion years.
** Effects of time dilation due to motion ignored throughout.

© 2011 Ira Mark Egdall
This article is based on a book I am writing which explains relativity in everyday language. For more details go to:
http://marksmodernphysics.com/

Learn more:
http://www.upscale.utoronto.ca/PVB/Harrison/GenRel/TimeDilation.html
http://hyperphysics.phy-astr.gsu.edu/hbase/relativ/airtim.html

Time machines are a favorite ploy in science fiction stories. Climb into one and zap, you’re in another century. Entertaining, sure, but a real possibility? It does seem far-fetched, to say the least. But what most people don’t realize is that time travel is actually a common, everyday experience. It all has to do with a core tenet of Einstein’s theory of special relativity (called time dilation).

In Einstein’s crazy world, time is not rigid. Per the great physicist, time does not go by at the same rate for everyone. Imagine, for example, that you are sitting in a chair and I walk by you at a steady pace. (And assume we are both wearing identical super-accurate watches.) Per Einstein, you see my watch running a tiny bit slower than your watch. Why? Because I am moving with respect to you. Time slows down with motion.

Now at the speed I am walking, this slowing of time is a very, very small effect. That’s why we don’t notice it. But it is real. And according to Einstein’s seminal theory, the effect becomes dramatic at speeds approaching the speed of light. Say, for example, I somehow speed by you at an incredible 580 million miles an hour. This is 87 percent the speed of light. Because of my tremendous speed, you now see my watch running at only half the rate of your watch. That is, for every second that is ticked off on your watch, only a half second ticks off on mine!

Hard to believe? It does sound crazy. We can only imagine how Einstein must have felt when he first thought of it. It took tremendous guts for him to just propose such a thing! But today, we have all kinds of evidence; from atomic clocks on airplanes, rockets and satellites, from the measured lifetimes of subatomic particles, and from numerous laboratory experiments that Einstein was right! Time is relative; time does in fact slow down with motion; and in just the amount Einstein’s formula predicts. Einstein’s crazy universe is our universe.

Into the Future

OK. So what does this have to do with time travel? Imagine it is the year 2950. Let’s say you are now the one who is moving. Say you take a trip into outer space in your hot new rocket-car at 87 % the speed of light. Because of your great motion, time for you passes at half the rate as time for the rest of us back here on Earth.

Say it takes you 5 years round-trip to travel out to a distant star and return to Earth. (This is 5 years rocket time.) To you time appears to be running normally. Every 24 hours you mark off one day. And every 365¼ days, you mark off a year on your rocket calendar. Recall you left Earth in the year 2950. So when you arrive back on Earth, your rocket calendar says it is the year 2955. And indeed you and everything else on the rocket have aged 5 years. But here on good old Earth, time is not running slow, like time on your rocket. So when you return, 10 years have elapsed here on Earth. (This is 10 years Earth time). In fact, everyone on Earth has aged 10 years and Earth calendars now say it is the year 2960.

When you alight from your rocket-car and step onto solid ground, to you it is the year 2955. After all, you have only been away 5 years (in your rocket time). But it is now the year 2960 on Earth. So in effect you have traveled 5 years into the future! And, theoretically, if you traveled fast enough, you could arrive back on the Earth some thousands of years into the future For example, according to Einstein’s formula if your rocket traveled at a speed of 99.99999 % the speed of light for just one year, you would arrive back on Earth 2236 years later!

But we must be careful here. According to special relativity, this time travel is a one-way trip “You may be able to buy a round-trip ticket to (outer space),” wrote relativists Edwin F. Taylor and John Archibald Wheeler in Spacetime Physics, “but you get only a one-way ticket to the future.” So you can (and do) travel into the future, but you cannot travel into the past.

Everyday Time Travel

We don’t have the technology (yet) to accelerate macroscopic objects like rockets and people to so-called relativistic speeds; but even at the relatively slow every day speeds we experience, the effect is still there. Say you go to work or school today, but your room mate stays home sick in bed. You drive your car to your destination, and then return in the evening. Time has run a tiny bit slower for you than for your room mate. Why? Because you were in motion in your car, while your room mate was at rest in her bed. So (big drum roll here) you arrive back at home a tiny bit into the future. Any time you are in motion, you experience this effect. In fact, every time you leave a place, the slowing
of time due to your relative motion means that you return to that same place a tiny fraction of a second into the future. We are all time travelers!

© 2010 Ira Mark Egdall
http://marksmodernphysics.com/

Learn more:
http://galileoandeinstein.physics.virginia.edu/lectures/srelwhat.html
http://www.walter-fendt.de/ph14e/timedilation.htm