[ATAG_Bliss]

Two identical machines (even down to the molecular level) can never perform truly identical unless they resided in their own foot print.

Basically, because of this:

When Albert Einstein introduced his Theory of Relativity in the early 20th century, however, he suggested that time wasn't separate from space but connected to it. Time and space combined to form space-time, and everyone measures his or her own experience in it differently because the speed of light (300,000 km per second) is the same for all observers. In other words, if all observers have to agree on the speed of light being 300,000 km per second, then they can't agree on the time it takes for other objects to travel relative to them.

Einstein also suggested that space-time wasn't flat, but curved or "warped" by the existence of matter and energy. Large bodies in space-time, like the Earth, aren't just floating in orbit. Instead, imagine an apple resting on a stretched out blanket -- the weight of the apple warps the sheet. If the Earth is an apple, then we can imagine the Earth's blanket as space-time.

This means that someone moving through space-time will experience it differently at various points. Time will actually appear to move slower near massive objects, because space-time is warped by the weight. These predictions have actually been proven. In 1962, scientists placed two atomic clocks at the bottom and top of a water tower. The clock at the bottom, the one closer to the massive center of the Earth, was running slower than the clock at the top. Einstein called this phenomenon time dilation.

A further explanation of the bending of space-time and time dilation came in the form of a thought experiment called the twin paradox, devised in 1911 by French physicist Paul Langevin. If one twin lives at the foot of a mountain and the other lives at the top, the twin closer to the Earth will age more slowly. He or she would turn out younger than the other twin, though by a very small amount. If you sent one twin in a spaceship accelerating close to the speed of light, however, he or she would return much younger than the other twin, because high acceleration and large gravitational masses are the same in relativity. Of course, no one's gone so far as to send somebody's twin into high-speed orbit, but scientists proved the hypothesis true in the '70s by sending an atomic clock into orbit. It returned to Earth having run much slower than grounded atomic clocks.