We will now
consider two observers **A** and **B** making
observations of two events. Think of the term
"event" as meaning something which takes a *very*
short time. In other words, an event defines a very precise
point in space and time or a point in space-time as Einstein
would have said. The events will be the arrival of two flashes
of light at two mirrors.

Observer **A** has a long
support on which are mounted two mirrors, p and q, as shown
below. The two mirrors are equidistant from **A**.

**
****A**
sends a flash of light towards each mirror (flashes sent at
the same time).

The light
flashes are reflected and arrive back at **A** at the same
time.

**
****A **must
therefore conclude that the two events, light hitting mirror p
and light hitting mirror q, *were simultaneous*.

Now we will consider another
observer’s view of the same situation.

**
****B**
is very close to **A**.

**
****B**
has his/her own ruler and watch (the diagram shows **B**’s
ruler).

We will first
assume that **A** and **B** have zero relative speed. It
is hoped that the reader will agree that if the experiment is
repeated, **B** will also conclude that the two events
occurred simultaneously.

We will now
consider that **A** and **B** have a very large relative
speed. Again, we will imagine that at the instant the flashes
are sent, **A** and **B** are very close to each other.

A short time later, the
situation, *as seen by ***B,** might be something like
that shown in the following diagram.

As **B**
sees things, mirror p is moving towards the light source and
mirror q is moving away from the light source. In **B**’s
frame of reference, therefore, the light going towards mirror
p will have a shorter distance to travel before it reaches the
mirror than the light going the other way.

Remembering
that the *speed of light is the same* *for all inertial
observers*, we can see that **B** will consider that the
*light hits p before it hits q*.

**B ***must*
therefore conclude that the two events, light hitting mirror p
and light hitting mirror q,** ***were NOT simultaneous*.

**A** and **B**
are both right; simultaneity is relative.