Theory of Relativity for school and college students

1. Theory of
Relativity
Aakash Sinha

2. What is the big deal about it?
• If you are moving close to the speed of light or near a black hole:
• Your 1 year could be a 1000 years on earth!
• Distance of a 1000 light years can shrink to 1 km for you
• Your mass/energy could become a 1000 time more , though you won't notice it
• Mass does not really exist; mass is just energy
• Gravity does not really exist as a force, it is just a curvature of space
and time
• Gravity and Acceleration are exactly same things

3. The theory actually
very simple
All laws of universe are same for every
observer- That’s it !
Einstein did a thought experiment
He imagined travelling with a light beam at
the speed of light
He realized that he would notice a static
electric and magnetic fields
This was impossible

4. The speed of Light
• It turns out that the speed of light
is a constant and a universal law
• In fact, all massless particles
move at the speed of light
• Thus, Einstein argued that speed
of light should be the same for
every observer as it is a universal
law
• Irrespective of the observer’s
speed the speed of light would
always be a constant

5. Let us build the
theory ourselves
• Laws of universe are same for every
observer
• Speed of light is a constant universal
law
• Thus speed of light should be same for
all observers
• Now imagine you are shining a torch in
a train
• Your friend is observing from outside
• The train is moving a speed v
• It takes time t for the light to reach the
floor form the celling according to your
friend
Ct
vt
Ct

6. Let us build the
theory ourselves
• Speed of light is constant for
both the observers
• But the hypotenuse of the right
triangle must be larger than the
perpendicular
• So, Einstein argued that the
time pass differently for both
observers
• Let us use Pythagoras theorem
to derive relation between t and
t’
Ct
vt’
Ct’

7. Let us build the
theory ourselves
• Let us use Pythagoras theorem
to derive relation between t and
t’
• (Ct)^2 +(vt’)^2 = (Ct’)^2
• Can you find the value of t’ in
terms of t ? Ct
vt
Ct’
vt

8. Let us build the
theory ourselves
• Let us use Pythagoras theorem
to derive relation between t and
t’
• (Ct)^2 +(vt)^2 = (Ct’)^2
• t’ = t / sqrt(1-v^2/C^2)
• What happens when v =0;
• And when v =C
Ct
vt
Ct’
vt

9. The great value of
relativity
• Used for space missions and GPS
• Used for super precise atomic clock
• One of the few theories that were
designed in a thought experiment
• It was proven much later during a
solar eclipse
• It has been shown by atomic clocks
that time slows down in airplanes as
relativity predicts

10. Questions ?