Einstein’s theory of special relativity reflects the
unchanging character of the speed of light, among other things. Before Einstein
space and time had appeared to be separate and absolute, but after Einstein’s
work, space and time became interwoven and relative. The most famous equation
of Einstein is mass-energy equation. In this equation Einstein stated that the
energy (E) of an object and its mass (m) are not independent perceptions and we
can conclude the energy from a known mass by multiplying it twice by the speed
of light, or we can conclude the mass from known energy simply by dividing it
twice by the speed of light. You can say mass and energy are interchangeable
like currency, but one thing we should note here is unlike money, the exchange
rate given by two factors of the speed of light is permanent and forever
constant.

Image source: Infographic

Two observers one is standing and another is moving can
never agree on a same statement about an event. Suppose there is rail track and
an observer with a triangular mirror is standing precisely at equal distance
between two poles on the rail track. Imagine that the lightning hit the two
poles at once. The observer standing still beside the rail track could see
lightning bolt hitting the poles and agrees that the lightning hit the poles at
the same time. Now imagine another observer who is in a moving train on the
rail track with the same mirror and can see the both poles with a triangular
mirror. When second observer, while in moving train, reaches the point, where
the first observer was standing, the lightning hits again the two poles and
observer on the moving train observers this event. The moving observer does see
the lightning bolt hitting the poles simultaneously. He sees the lightning
hitting the pole, towards which he is moving, first. Light takes time to move
from the pole towards mirror. In that time train travels towards the forward
pole and the light from that pole has the shorter distance to cross to reach
the mirror. So the two observers one standing still and another is moving
cannot agree on “when the light hits the poles. That’s how Einstein proved
that time is relative. Time flows at different rates for each of us. The faster
you move the slower your clock will tick as compared to that of a stationary
observer.

The effect of this phenomena is hardly observable in our
daily life, but if you can move with the 90% of the speed of light the effects
will become striking. You would shrink to 44% shorter than your usual length
from the point of view of someone watching you outside. At the speed of light
length of an object becomes zero and time stands still. So there is no way of
crossing this speed limit.