Lesson 11: The Special Theory Of Relativity Part III

Draw that picture.  Teach someone.

The Theory of

Special Relativity III

Get ready for another mind-blowing statement.

Two events that occur at the same time in one frame of reference may not occur at the same time in another frame of reference.  Let’s say I have two signs, one with the number one and one with the number two. If I hold up both signs at the same time, that is only in my frame of reference.  Someone in another frame of reference could see me hold up sign one first and then sign two.  And someone in another frame of reference could see me hold up sign two and then sign one.

Who is correct? You might be tempted to say it is me because I held up the signs.  But no, we are all correct.  Remember, there is no special frame of reference.  I can not say I am right and you are wrong.  That would be saying the laws of physics are correct in my reference frame and wrong in yours.  Remember, the laws of physics apply to everyone equally moving in uniform motion.

How can we see events in a different order?

It is because of length contraction.  We have already seen that the closer you get to moving at the speed of light, the slower time moves for you.  But not only is time affected but so is space.  The closer you travel to the speed of light, the shorter things become. This is not because objects are shrinking.  All measures of space are changed.  Again it’s like time. You don’t notice any shrinkage.  Everything is normal for you.

Consider two spaceships moving towards each other at the same speed.

There is no contraction relative to each other, so the front and back of the spaceships will meet each other at equal times.

Now let’s say the top ship is speeding close to the speed of light.  Its length will be contracted.  The back of the top ship will meet the front of the bottom ship, and later in time…

…the front of the top ship meets the back of the bottom ship.  It is not simultaneous, like when the ships are moving at the same speed. 

Now let’s make the bottom ship move at close to the speed of light, so it contracts.  This time things change again.  The front of the top ship will meet the back of the bottom ship, and then later in time…

.. the back of the top ship will meet the front of the bottom ship.

The time order is changed.  There were three different views.

1. Simultaneous 
2. Back/front, then front/back
3. Front/back then back/front

We have talked a lot about the speed of light now we need to discuss two more facts about light.

Light is information, and nothing can move faster than the speed of light.  These two facts create what is known as The Elsewhere.  Let me give you an example of The Elsewhere.

It takes about 11 minutes for light to reach Mars from Earth.  You cannot get any message (information) to Mars in less than 11 minutes because nothing can move faster than light.  There is a time span in which we have no influence.  It is the same for Mars to us. So there are actually 22 minutes of no influence.  Coming from Mars, there are 11 minutes of nothing we can do about the past, and if going from Earth to Mars, there are 11 minutes of nothing we can do about the future.

These 22 minutes are in the elsewhere.  They are not in the past or future. They are not in the past because they cannot influence events on the earth, and they are not in the future because we can’t influence them.

The past is those events that can influence the present moment.

The future is those events we can influence.

We have no influence because nothing can move faster than the speed of light.   To understand why nothing moves faster than the speed of light, it will first be helpful to discuss Einstein’s most famous equation – E=mc².

What is E=mc²?  

Energy equals mass times the speed of light squared.

Mass is anything that has matter.  Matter is just about anything physical you can see and touch.  Energy is power derived from the utilization of physical or chemical resources, especially to provide light and heat or to work machines.

It was actually an afterthought that Einstein added to relativity.  It says matter is equivalent to energy.  Energy and matter are one and the same.  

The equation shows matter and energy can be converted into each other.  Notice that matter converts to energy multiplied by the speed of light squared. We already know the speed of light is a huge number. That means a huge amount of energy can be created from just a small amount of matter.

It is often mistakenly believed that Einstein created the nuclear bomb with this equation.  This is not true.  The equation just shows why a nuclear bomb has such a big explosion.  A nuclear bomb creates a log of destructive energy from very little matter.  Nuclear bombs get their explosive energy by splitting atoms and creating energy.  Think about how small an atom is and how big a nuclear explosion is.

Because energy and matter are equivalent, it is hard to accelerate energy as well as mass.  The faster energy moves, its energy increases its inertia which is a tendency to remain unchanged.  So it becomes harder and harder to accelerate, and it would take an infinite amount of energy to accelerate faster than the speed of light.

This lesson may have been a little more difficult.  But if you grasped these two points, you got it.


1. Two events that occur at the same time in one frame of reference may not occur at the same time in another frame of reference.
2. Nothing can move faster than the speed of light.  This means the time information can reach you is limited by the speed of light.

Ready to take a break? 

In the next lesson, we will be talking about sight.

You know what to do now.  You can do it.