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How Time Travel Works

From millennium-skipping Victorians to phone

booth-hopping teenagers, the term

time travel

often summons our most fantastic visions of what it

means to move through the fourth dimension. But of

cour you don’t need a time machine or a fancy

wormhole to jaunt through the years.

As you’ve probably noticed, we’re all constantly

engaged in the act of time travel. At its most basic

level, time is the rate of change in the univer --

and like it or not, we are constantly undergoing

change. We age, the planets move around the sun,

and things fall apart.

We measure the passage of time in conds,

minutes, hours and years, but this doesn’t mean

time flows at a constant rate. Just as the water in a

river rushes or slows depending on the size of the

channel, time flows at different rates in different

places. In other words, time is relative.

But what caus this fluctuation along our

one-way trek from the cradle to the grave? It all

comes down to the relationship between time and

space. Human beings frolic about in the three spatial

dimensions of length, width and depth. Time joins

the party as that most crucial fourth dimension. Time

can’t exist without space, and space can’t exist

without time. The two exist as one: the space-time

continuum. Any event that occurs in the univer

has to involve both space and time.

Time Travel Into the Future

If you want to advance through the years a little

faster than the next person, you’ll need to exploit

space-time. Global positioning satellites pull this off

every day, accruing an extra third-of-a-billionth of a

cond daily. Time pass faster in orbit, becau

satellites are farther away from the mass of the

Earth. Down here on the surface, the planet’s mass

drags on time and slows it down in small measures.

We call this effect gravitational time

dilation. According to Einstein’s theory of

general relativity, gravity is a curve in

space-time and astronomers regularly

obrve this phenomenon when they study

light moving near a sufficiently massive

object. Particularly large suns, for instance,

can cau an otherwi straight beam of

light to curve in what we call the

gravitational lensing effect.

What does this have to do with time?

Remember: Any event that occurs in the univer

has to involve both space and time. Gravity doesn’t

just pull on space; it also pulls on time.

You wouldn’t be able to notice minute changes

in the flow of time, but a sufficiently massive object

would make a huge difference -- say, like the

supermassive black hole Sagittarius A at the center

of our galaxy. Here, the mass of 4 million suns exists

as a single, infinitely den point, known as a

singularity [source: ]. Circle this

NASA

black hole for a while (without falling in) and

you’d experience time at half the Earth rate.

In other words, you’d round out a five-year

journey to discover an entire decade had

pasd on Earth [source: ].

Davies

Speed also plays a role in the rate at which we

experience time. Time pass more slowly the

clor you approach the unbreakable cosmic speed

limit we call the speed of light. For instance, the

hands of a clock in a speeding train move more

slowly than tho of a stationary clock. A human

pasnger wouldn’t feel the difference, but at the

end of the trip the speeding clock would be slowed

by billionths of a cond. If such a train could attain

99.999 percent of light speed, only one year would

pass onboard for every 223 years back at the train

station [source: Davies].

In effect, this hypothetical commuter would

have traveled into the future. But what about the

past? Could the fastest starship imaginable turn

back the clock?

Time Travel Into the Past

We’ve established that time travel into the future

happens all the time. Scientists have proven it in

experiments, and the idea is a fundamental aspect

of Einstein’s theory of relativity. You’ll make it to the

future; it’s just a question of how fast the trip will be.

But what about travel into the past? A glance into

the night sky should supply an answer.

The Milky Way galaxy is roughly 100,000

light-years wide, so light from its more distant stars

can take thousands upon thousands of years to

reach Earth. Glimp that light, and you’re

esntially looking back in time. When astronomers

measure the cosmic microwave background

radiation, they stare back more than 10 billion years

into a primordial cosmic age. But can we do better

than this?

There’s nothing in Einstein’s theory that

precludes time travel into the past, but the very

premi of pushing a button and going back to

yesterday violates the

law of causality, or

cau and effect. One event happens in our

univer, and it leads to yet another in an

endless one-way string of events. In every

instance, the cau occurs before the effect.

Just try to imagine a different reality, say, in

which a murder victim dies of his or her

gunshot wound before being shot. It

violates reality as we know it; thus, many

scientists dismiss time travel into the past

as an impossibility.

Some scientists have propod the idea of

using faster-than-light travel to journey back in time.

After all, if time slows as an object approaches the

speed of light, then might exceeding that speed

cau time to flow backward? Of cour, as an

object nears the speed of light, its relativistic mass

increas until, at the speed of light, it becomes

infinite. Accelerating an infinite mass any faster than

that is impossible. Warp speed technology could

theoretically cheat the universal speed limit by

propelling a bubble of space-time across the

univer, but even this would come with colossal,

far-future energy costs.

But what if time travel into the past and future

depends less on speculative space propulsion

technology and more on existing cosmic

phenomena? Set a cour for the black hole.

Plea do Ex.1 or Ex.2 or both.

1. What is time travel? (20 points)

2. Make up a time-travel story. (50 points)