The second postulate of Einstein's Special Relativity Theory states that the speed of light stays the same for every observer regardless of RF's (reference frames). Now, this somewhat conflicts with the laws of RF's because speeds of objects observed in difference RF's are always different, right? Why is light any different? The answer can be explained by understanding the effects of Lorentz Contraction.
"Objects moving at or near the speed of light appear to be contracted relative to a stationary observer"
This means that, from the perspective of a stationary observer, objects traveling near the speed of light literally appear to be physically contracted, or squished. This phenomena can be calculated using the factor of gamma.
The effect of the physical contraction has little effect until approaching the speed of light, but it's a natural occurrence that effects any object moving in relation to another. If I'm walking down the street faster than you, then your size is contracted in my RF (perspective). However, the contraction is so minuscule, that it's not even noticeable. The closer to light speed an object travels, the higher the rate of it's contraction. So, if an object travels at 10% the speed of light (67 million mph), a stationary observer notices a .5% size contraction on the object. That's not really a huge difference in size. However, if the object is traveling at 80% the speed of light(536 million mph), the observer notices a 40% size contraction on the object.
The physical contraction of an object is a phenomenon that may be hard to believe for the common individual. People ask:
"so why do objects contract when approaching the speed of light?"
The reason is due to the contraction of the 3D fabric of space. The moving object is not really contracting; The 3D fabric of space is contracting and giving the impression of a squished object. Since the 3D space around the object is contracting, then the distance the object has to travel to get from one point to another decreases. This decrease in distance makes an object take less time to travel to its destination. The time that is saved by the contracted distance is the reason why objects are perceived by stationary observers to travel into the future. This phenomenon is called Time Dilation.
As explained, time dilation is not independent of Lorentz Contraction, it is just a result of the effects of contraction..
Imagine yourself embarking on a high speed journey through space for one day. You are traveling at a high speed of about 90% C (C=speed of light), which is roughly 603 million mph. You return to Earth after one day of travel, but 2.25 days have passed on Earth. This phenomenon has been tested and confirmed by NASA and the US Military.
The rate of dilation dramatically increases when an object approaches C. Traveling at 90%C, 2.25 days pass on Earth for every day that passes for the object. However, if you increase the speed to 99.9999%C, almost two years pass on the Earth for every day of the object. At 99.999999999999%C, for every day the object travels, nearly twenty thousand years pass for the observer at rest. As you can see, The dramatic effects of time dilation really start kicking in around 99%C.
So, back to the original question. "Why is the speed of light the same in all reference frames?". The effects of Lorentz Contraction has really complex effects on objects traveling near the speed of light. However, things get even more complex when traveling at the exact speed of light and understanding these effects will answer the question.....