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*From different reference frame there can never be an agreement on the**simultaneity**of events..*"
The concept of events happening at the
same time is called the “simultaneous events”. In Newtonian physics there was a notion of the
universal time, and hence simultaneous events in all inertial frame was a consequences of such
assumption. In simple words, observers in all reference frame would agree on particular events irrespective of their motion. However, as we have seen that from the postulate of special relativity that the concept of time is relative
one, we would thus expect the need for the revision of the concept of
simultaneity.

As you can guess from the equations for the Lorentz
transformation, the concept of time becomes a relative concept since we no
longer have the simple relation $t=t'$. What we actually mean here
is that the concept of simultaneity will depend on which frame you are in.
This is a simple consequence of the fact that the speed
of light is the same no
matter which inertial frame you are in.

Consider a moving train at speed close to
speed of light $c$.Imagine a
long platform with an observer located at its midpoint. At either end, at the
places marked $A$ and $B$, there are two momentary lightning flashes. The light
propagates from these events to the observer. Noticing that they arrive at the
same moment as they come from places of equal distances away, the observer will
decide that the two
events happened simultaneously.

However, from the point of view of the
person on the train, the front of the train is moving towards the lightning coming from the point $B$, while the back of
the train moves away from the point of the lightning flash point $A$. This
means that the distance covered by light coming from point $B$ will be shorter
than the light coming from the point $A$. And
since the speed of light is $c$ in both
directions for the observer on the train too, the light will reach the front of the train before it reaches the back of the train. Hence, the observer on the train will claim the two
events were not simultaneous events.

In short

**, Simultaneity**is the property of two events happening at the same time in a frame of reference. According to Einstein's Theory of Relativity, simultaneity is not an absolute property between events; what is simultaneous in one frame of reference will not necessarily be simultaneous in another. For inertial frames moving at speeds small compared to the speed of light with respect to one another this effect is small and can for practical matters be ignored such that simultaneity can be treated as an absolute property.
A short video on the simultaneity can be found here.

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