# How to Understand Time Travel

Are you weirded out by wormholes? Flummoxed by your flux capacitor? Strap yourself in for this time-travel primer.

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#### You Will Need

• A basic knowledge of the theory of relativity
• An open mind
• A PhD in Physics (optional)

## Steps

1. Time-travel theories are riddled with paradoxes that may make your brain explode.

2. Step 1

#### Understand "special relativity"

Brush up on your Einstein: His special theory of relativity revolutionized physics by suggesting that traveling into the future is possible. The idea is that time slows down when you're traveling at high speeds relative to stationary objects. So if you're in a spaceship going very close to the speed of light for five years, you could return home to find that it's 50 years later on Earth.

3. Even on a normal transatlantic flight, time slows down with speed. In one experiment using two atomic clocks, the clock sent into space fell 40 nanoseconds behind the one left on the ground.

4. Step 2

#### Rule out traveling faster than light speed

Going back in time is a far trickier proposition – but might be possible if you could travel faster than the speed of light. Theoretically, if you were moving faster than light speed, you could receive a signal from Earth before it was even sent. Upon returning home, you'd find yourself back in time. The only problem is that traveling that fast is scientifically impossible – at least for now.

5. Step 3

#### Make the quantum leap

Make the quantum leap. Some quantum physicists believe that traveling back in time could be possible on a subatomic level. But unless you're a subatomic particle, you're out of luck.

6. Step 4

#### Say hello to wormholes

Currently, the most popular time-travel theory involves wormholes, or tunnels through the fabric of space and time. If you were to tow one end of a wormhole into space close to the speed of light, then return that end to Earth, you would theoretically be able to enter the wormhole today and come out of the other end in yesterday.

7. Be skeptical. Wormholes are so tiny and close so rapidly that it would take a great leap of science to be able to harness them for time travel.

8. Step 5

Get to know your granddad. The so-called Grandfather Paradox – going back in time and killing your grandfather, thus preventing your own birth and making it impossible for you to travel back in the first place – has been used to help illustrate why backward time travel is impossible.

9. Step 6

#### Understand that you can't change the past

Understand what's done is done. Most ideas about backward time travel include the "chronology protection conjecture," meaning that nature would prevent you from changing the past. For example, you could try to go back in time to shoot your grandfather, but the gun wouldn't fire. It also means that all the embarrassing moments you'd love to erase are going to happen all over again.

10. Step 7

#### Be open to the many-worlds interpretation

Open your mind to the "many-worlds" interpretation. Some theorists have gotten around the rigid "whatever happened, happened" idea by claiming that every different action you take in the past splits off into a parallel universe. So there might be hope yet for preventing those embarrassing moments, at least in one universe.

11. Theoretical physicist Stephen Hawking once said, "Time travel might be possible, but if that is the case why haven't we been overrun by tourists from the future?"