Rietdijk, Putnam, and Penrose independently proposed a thought-experiment to explore the concept of simultaneity in special relativity. The Andromeda galaxy is approximately 2.5 million light years from Earth (≈2.5×m) . Assume, for simplicity, that the galaxy and the Earth remain momentarily at this fixed distance, with no relative motion. Suppose an Earthling is slowly strolling in the direction of Andromeda. Then events on Andromeda that occur, in concept, simultaneously in the Earthling's frame of reference, depend rather sensitively on his or her walking speed. Roughly, an increase in walking speed of one foot per second corresponds to a simultaneous event occurring on Andromeda about an Earth day later! This can be deduced from the Lorentz-transformation equation , where is the time advance on Andromeda, which can be considered simultaneous with an event on Earth occurring at , most conveniently set equal to 0. Note that the observer on Earth can only "infer" what is happening simultaneously on Andromeda. He would not actually "see" what is occurring until 2.5 million years later.