Why doesn a photon have the same mass as the Universe?”
It’s not really true that an object “gains mass”. In fact, its “mass” turns out not to be an intrinsic property of the object at all. You can only measure it relative to its speed. It’s similar to the way you can’t ask how far “left” an object is; it’s only meaningful relative to your position. That, an object moving very fast relative to you has more mass than an object that’s at rest, i.e. right where you are. That “right where you are” mass is called the “rest mass”, and that IS an intrinsic property of the object. If you know the rest mass of an object, and you know how fast it’s moving, you can determine its mass relative (its “relative mass”) to you by this formula: where “m” is the rest mass, “v” is the speed, and “c” is the speed of light. We humans move slowly, and nothing we see without a telescope is moving anywhere near c. So v^2/c^2 is very close to 0, and (we call that part of the equation “gamma”, or γ) is very close to 1, and mrel is equal to m. A photon, however, has v
