# Is time travel possible? Teaching the Theory of Relativity

Seth Robey
6th July 2015 at 12:06

Is time travel possible? Can we someday change the course of the present and future by altering events in the past? Probably not, even though many movies and books have suggested some clever methods.

That being said, one of the greatest theories ever conceived was the Theory of Relativity by Albert Einstein. One of its basic assertions being that time passage is not absolute. In other words, in certain situations, time passage of an object can be slowed down relative to other objects.

We tend to think of time as a linear voyage from the past to the present and on to the future, each second ticking by at even intervals. While we talk about events that occurred millions and even billions of years ago, nobody can really comprehend that amount of time passage. So what if we could speed through time, perhaps only experiencing the passage of a few hours, while years pass by for the rest of the world?

Well, whether this is a future possibility remains unclear.  However, it has been demonstrated that when the speed of an object is increased significantly, time passage for that object actually slows down.  In other words, a clock moving at a very high speed will have less time pass than a clock moving at a slower speed.  If an object could be accelerated to near the speed of light (about 300 million meters per second), time could be slowed down significantly.

So how can you illustrate this concept to students?  A theoretical example of how this could work is described in the following story:

The year is 2115.  “Thermonuke”, the most advanced spacecraft ever built, is on a voyage around the solar system, traveling near light speed. Towards the end of the journey, one of the astronauts, Lieutenant John, breaks his wrist by bumping into a wall.  John immediately sends a message (by light speed radio waves) to Earth reporting the injury.

The same day, the spaceship immediately heads back to Earth, continuing near the speed of light.  About 3 months later, according to John’s calendar, the ship arrives back on Earth.

Upon arrival, John speaks with Lieutenant Rick, one of the engineers on Earth who received the message from the spaceship.

Rick:  “Yes, we received that exactly a year ago.”

John:  “But according to my calendar, I sent the message only 3 months ago. That would mean you received my message almost 9 months before I sent it!  How could that be?”

While travelling at such a high rate of speed, John experienced only three months of time passage.  Situated on Earth, Rick experienced an entire year, plus the time it took for the message to reach Earth.  From John’s perspective, much less time elapsed than for Rick.  Because of this discrepancy, it would appear that the message was actually received BEFORE it was sent, and there would not be any clear way to dispute this paradox.

So, aside from changing how we think of time passage, what does the theory of relativity mean for us today?  Well, while it is unclear whether a spaceship that moves this fast could ever be built, it has been shown that fast moving satellites and spaceships that exist now do experience a different rate of time passage than on Earth, although the difference is quite small. For instance, the clocks on the satellites that form the Global Positioning System (GPS), which many of us rely on for directions when driving, move at a different rate from clocks on earth and have to be adjusted by about 38 millionths of a second per day in order to maintain precise measurements.  While this might not sound like much, to neglect this discrepancy would cause the system to lose accuracy by about 10 kilometers per day!

A second example is that faster moving subatomic particles age more slowly and therefore, last longer than slower moving particles.  This can make a difference in applications of nuclear science.

Finally, the theory of relativity also adds to the truth of statements like “Exercises like running and cycling will make you age more slowly.”

Some TES resources:

Einstein Quotes

Albert Einstein Word Search