**A Photon’s Universe**

Special relativity gives us the famous formula that tells us energy can be equated to mass: *e=mc^2*, where *e* is energy, *m* is an object’s mass, and *c* is the speed of light in a vacuum—the maximum speed limit of the universe. Basically, this simple equation tells us that the faster an object travels, the more massive it becomes. As is approaches the speed of light, an object becomes more and more massive and so more and more energy is required to keep it in motion—which is why it’s impossible for anything with mass to reach the speed of light. Only the particles that carry light, photons, can travel at the speed of light—because they’re massless. Special relativity also tells us that time is relative to the observer. If Person A is standing still and observing Person B run a race, the time that elapses between the start and finish isn’t the same for both of them—time depends on the relative speeds of their reference frames. The faster you travel, the slower time goes for you—if you’re holding a clock, it will tick slower. The closer you are to the speed of light, the slower time ticks, and when you’re actually travelling *at* the speed of light, time won’t pass at all. Photons, therefore, don’t experience time. To us, light travel extremely fast, but it still takes *time* for it to travel (299,792,458 m/s)—but that’s relative to our reference frame; our place as an observer. To photons, they leave one place and arrive at their destination in the exact same instant. To photons, the universe begins and ends at the same time.

(Image Credit: BBC)