Why the speed of light is constant in every reference point?Train going with a speed of 200m/sec emitting light from the front of it,then there the speed of light would be 300000200m/sec but there also the speed of light is constant (300000000m/sec)

2 Answers



It's not until you read theory of relativity, you can understand the answer. Special and General theory of relativity are objectionable but yet are in use.

If you see your question for a while again and think about it for once, your question is absolutely correct. Let me give you a simple answer to it.

Time about which most of us know is just what is shown by our wall/wrist clock. But time is much more than we know.
We consider time to be independent but time is relative. It is dependent on velocity as well as gravity too.

Here we have to just focus on velocity time dilation (if you are curious enough that you want to know about gravity time dilation I would recommend you to either ask me or some good content is available at Physics Stack Exchange too).
So, let's get back to our question.

Speed of light remains constant , as said by Albert Einstein!

A few examples could be like if measure speed in situation given in question. It comes to be #3×10^(8) m/s# both the times.

NOTE: #3×10^(8) m/s# Is an approximate velocity of light in vacuum. It's accurate value is #2.99792458 × 10^8 m/s#

As soon as we start moving with higher velocity the time for us changes or say it dilates. An example is that astronomers age less because they travel super fast orbiting earth. So time for them seems to flow a bit slowly. So even if we measure speed of light travelling in train or even if we are at rest, it tends to be same.

For your curiosity I would like to inform you that mass to increases with increase in velocity and that variant mass is called relativistic mass.


Aug 7, 2017

The speed of light has to be the same in every reference frame to make the laws of physics consistent. This is special relativity.


It was thought that light travelled through a medium called the aether. This would provide a reference frame from which all speeds could be calculated. Experiments later proved that the aether doesn't exits. This means that there is no at rest frame of reference. Each frame of reference has a speed relative to every other frame. Hence the term relativity.

Einstein's special theory of relativity are based on two postulates:

  1. The laws of physics are the same for every observer. So, two observers travelling at different speeds, and not accelerating, observe identical laws of physics.
  2. The speed of light in a vacuum is the same for all observers, no matter what the speed of the light source is.

The two postulates were derived as a consequence of Maxwell's equations. If you use the Newtonian concept of absolute space and time Maxwell's equations give different results in frames of reference at different velocities. This violates the principle that the laws of physics are the same for every observer.

The need to make Maxwell's equations the same for all observers drove the progress towards Special Relativity. A major consequence of this was that the speed of light in a vacuum has to be the same for all observers.

Relativity may sound weird, but it has been extensively tested and all predicted effects have been observed.