## Saturday, 20 July 2013

### Time Dilation and Symmetric effects

Let us start by taking an example. If we had a runner run a 100m race in 10 seconds, his speed would be 10m/s (Very tough). Now if another runner ran a 150m race, but still had a speed of 10m/s. There's no way he could have ran it in 10 seconds, because then his speed would be 15m/s, the only way the 150m runner could run the distance at the speed of 10m/s, the time taken would have to be 15 seconds. Thus we can conclude that distances are fixed in all frames of reference.

Now take another example of Light clock (doesn't exist, Just assume that it exists). A light clock ticks one second whenever light passes through it twice. It is a vertical clock. Light enters the clock from the bottom of the clock and exits from the top from where it is reflected back again and enters the clock. This pulse generates 1 second in Light clock.

Now suppose you have accelerated a Light clock in open space at a constant speed. In this case light has to travel a little greater distance in being reflected as the clock is in motion. Since now light has to travel more distance, 1 second would pass slower. Thus a moving light clock is slower than the one at rest. Here one who is at rest would see the time slowing down in the moving clock but the one moving along with the clock won't feel any time change. This phenomenon is called Time Dilation.

 Light has to cover larger distance in scenario 2

"Time Dilation" describes how an observer in reference frame "A" perceives the passing of time in a (different) reference frame "B". In simple words Time is also Relative. It is different in different frames of reference.

Let's take another example to explain Time Dilation. Suppose you were on a Space ship with a similar Light clock with one on Earth. Now suppose you are accelerated with the clock. Suppose Time measured by you in the Space ship is 12 second then the Time measured by your friend on Earth would we 10 second. Who is correct ? Relativity explains that both are correct. Time passes distinguishly in different frames of reference. Time Dilation is however not observed in most cases. It is only observed when a body is moving at a very high speed.

 The blue clock is inertial and red clock is accelerated

Symmetric effect

Suppose your friend in an Space craft is travelling at a speed nearer to that of speed of light. Then you on Earth would observe the Time slow down in the Space ship. The time you measure might be 1 hour whereas more than 2 hour might have passed on the Space Craft. A similar and opposite effect is too observed. Your Time slows down for your Friend on Earth but symmetrically your friend's time also slows down with respect to you. This is called Symmetric effect.

Is Time Dilation Apparent or Actual ?

Time Dilation has been verified. Experiments were performed in which atomic clocks were left on the ground, and atomic clocks were placed in aircraft.

These clocks were then flown around the world, and the 'times' compared when they landed. The clocks in the planes (which were moving relative to the clocks on the ground) seemed to show an 'earlier' time than the clocks on the ground, providing direct experimental evidence for time dilation.

The effect of time dilation is also accounted for in GPS satellites, which are moving at high speed relative to observers on the ground. If you didn't account for that relative motion and time dilation, GPS wouldn't be as accurate as it is. The atomic clocks in GPS satellites run about 7.2 microseconds a day slower than Earth-bound atomic clocks, so their data must be synchronized to maintain accuracy.

Time travel

Time travel is the concept of moving between different points in time in a manner analogous to moving between different points in space.
Time travel could hypothetically involve moving backward in time to a moment earlier than the starting point, or forward to the future of that point without the need for the traveler to experience the intervening period (at least not at the normal rate). Any technological device – whether fictional, hypothetical or actual – that would be used to achieve time travel is commonly known as a time machine. This phenomenon is however hypothetical and fictious. It may not happen.

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