According to Einstein’s famous Theory of Relativity, everything is relative to the speed of light which is absolute regardless of the motion of the light source or observers. Contrary to the speed of light, time is not a universal quantity, time is what we think it is relative to our velocities or the velocity of Earth and speed of light.

An accurate clock at rest with respect to one observer maybe measured to tick at a different rate when compared to a second observer’s own equally accurate clocks. This phenomenon is called ‘Time Dilation‘. Time dilation is why astronauts age less. Also due to relative time measurement, based on the regions we live in, we have different time zones. Quite confusing, isn’t it?

According to Einstein’s Theory of Relativity – space and time are relative concepts rather than absolute concepts, but do we know what time in universe actually is? Or, did Einstein fail to explain it?

Consider this physics says everything is relative except the speed of light, but if I stand still and measure the speed of light in all directions, I will measure it to be “c” – speed of light in vacuum; and this explains everything is relative (not except the speed of light). So no matter where you are or how fast you are moving, you are going relative to the things around you.

Also, Special Relativity explains: 1) that the laws of physics are invariant (i.e., identical) in all inertial systems (non-accelerating frames of reference); and (2) that the speed of light in a vacuum is the same for all observers, regardless of the motion of the light source. But when it comes to Newtonian physics coupled with Galilean transformation, the theory becomes counter-intuitive because when you are running at a speed X, chasing an object moving at a speed Y, the observer will see it as moving at X-Y. Fraser Cain from Universe Today has a good explanation regarding this complication.

He said, ” If you are moving at 200,000 km/s faster than me, and shine a flashlight ahead of you, I will see the light from your flashlight moving at the 300,000 km/s. It will appear to me, as though the light from your flashlight is moving away from you at 100,000 km/s. But when you will measure the speed of that light, relative to you, you’d think it’d be moving at 100,000 km/s as well, but instead from your perspective it will also clock in at 300,000 km/s. The speed of light. How is this even possible? It is possible in part because the rate at which you experience time relative to me changes. For you, time will seem normal, but from my perspective your time will seem slower.”

At the end, he concluded that because of relativity, each point in the Universe experiences time at a slightly different rate and when we observe the cosmic microwave background, we find that we are moving at a speed of about 630 km/s relative to the background, which is a little bit slower, but when we add the entire age of Universe, our cosmic clock is found to be 30,000 years behind the times.

So, does this mean assuming speed of light to be an absolute quantity is a flaw in Theory of Relativity and nothing is absolute? No, light is absolute, its space and time that are relative to it. So, no time is actual whether you’re on North pole or thousands of light-years away in some other galaxy.