According to the relativity of simultaneity, which is a concept in special relativity theory, it is physically impossible for two events to occur simultaneously for two observers if they are separated by distance or position. 

Time dilation is the apparent difference in the time interval between two events as measured by two clocks. Albert Einstein concluded that the faster you move through space, the slower one moves through time. This concept is best demonstrated through another thought experiment. 

Length contraction is the phenomenon in which an observer at rest would observe a moving object’s length to be shorter than its proper length. In everyday lives, length contraction, just like time dilation, is negligible. 

One of Albert Einstein’s most famous equations, the mass-energy equivalence, is the principle that concludes that anything that has a mass also has an equal amount of energy. 

This equation suggests that all objects with mass, including those at rest, have inherent energies (kinetic energy, chemical energy, mechanical energy, nuclear energy, etc.) since the value of c is very huge. These energies can be used to power an engine or do other tasks, like illuminating a bulb. 

Because no substantial object can ever achieve or surpass the speed of light for reasons that are basic to physics, this equation also proves that this speed is the absolute and ultimate speed.

For centuries, scientists thought that speed has no limit. But Albert Einstein showed that in fact that there is a speed limit, and it is the speed of light in a vacuum. Based on his calculations, as an object travels faster, the more massive it needs to be. And as objects become more massive, more energy will be required for it to move faster. Therefore, it will take an infinite amount of energy for an object to reach the speed of light.