Can anybody explain the theory of relativity an with example?

Asked on by kiransabne

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amarang9's profile pic

amarang9 | College Teacher | (Level 2) Educator Emeritus

Posted on

This is the basic theory of relativity and then a brief explanation of Einstein's special relativity.  Basic relativity (Galileo and Newton) states that the laws of physics are the same for all things moving in uniform motion.  That is, everything is moving; there is no privileged or absolute "at rest."  So, there is no "One" best place from which to observe things happening.

Everything is moving; you are on Earth, which spins on its axis and revolves around the Sun, yet, standing still, you think you are at rest.  You're not.  Everything moves relative to everything else.

Imagine you are standing on a street.  A car is coming at you at 50 mph.  The driver of the car sees you coming at him at 50 mph.  Who is right?  Both of you. 

Imagine you are standing on the shore. You watch someone on a boat drop a ball on deck.  From the boatsman's point of view, the ball drops straight down.  From your point of view, on the shore (at "rest"), you see the ball drop not straight down, but on a slant to the east because the boat is traveling east.  You're and the boatsman's perceptions of the path of the ball are different because you have different frames of reference. Neither of you has a privileged reference point (no absolute state of rest), so you are both justified in how you see the ball. 

Now, imagine an airplane is struck by lightning at the front and back.  From your position on land (again, seemingly at rest), you see the two bolts strike simultaneously.  But for a person seated in the middle of the plane, and although it is a difference of fractions of a second, she sees the front bolt first because she is moving towards that bolt and she is moving away from the bolt hitting the tail of the airplane.  Classic relativity tells us that the bolts occur simultaneously because time is absolute; she (on the plane) sees the front bolt first because it has less distance to travel to her eyes - because she is moving towards it and away from the rear bolt. Makes sense, right?


Einstein thought the speed of light is constant.  That is, it travels the same speed relative to anything no matter what speed or direction that thing is traveling. So, the light from bothbolts travels at 186,282 mps towards the middle of the airplane regardless of the airplane's movement; even though you'd think the light from the front would have to travel faster or go a shorter distance.

Since the light from the two bolts is is the same speed and since both bolts travel the same distance to the middle of the airplane, what is the remaining variable?  It's not distance and not speed that is relative here, so it's TIME.  Here is where Einstein showed time to be relative! For the person moving, in the plane, she observes the front bolt - Not because it is moving faster and Not because it has less distance to travel.  She sees it first because it takes less time for the front bolt to get to her.  Why?  Same distance, same speed.  Time slows down as you approach the speed of light. The faster you move, the slower time becomes - only noticeable at near light speeds. 

Common sense tells you that you'll experience the event you're moving toward because it has less distance to get to you. Your observation is relative to your motion.  This is classic relativity.

Special relativity states that time is also relative to your motion. Since light moves at the same speed, regardless of speed or direction, it is time that changes!

ravilpfan's profile pic

ravilpfan | Student, Undergraduate | eNotes Newbie

Posted on

If the plane is moving towards first bolts then it simply means that the   

distance is being reduced between middle part and light from bolts till light from first bolts hits the plane and the light from tail bolts will have to travel more and so wud take more time even though light speed is constant.

Correct me if wrong.

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