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**These are excerpts and elaborations from my book "The Nature of Consciousness"**

The paradox of the twins (devised by Einstein in person) is due to the
fact that... everything is relative. If a twin leaves the Earth, travels to
another planet with a speed close to the speed of light, and comes back to the
Earth, this twin will be younger than the one that stayed on Earth. The reason
is that clocks slow down as speed increases (time dilation). However, according to Relativity, one can also run the experiment
the other way around: from the point of view of the twin that departs the
Earth, it is the Earth that travels away and then comes back. In this case, the
twin who travels at high speed, and therefore who is younger, is the twin who
stayed on the Earth. Thus the second
twin is younger if measured from the first twin, but the first twin is younger
if measured from the second twin: these measurements cannot both be true at the
same time. Depending on which reference
frame you use, you get two contradictory results. Einstein solved the paradox by
pointing out that the two situations are not symmetric. The twin who leaves the
Earth has to apply an acceleration to get out of the Earth; then decelerate,
turn and accelerate again to return to the Earth. All of this violates the
principle of Relativity: the twin that departed the Earth has done something
absolute. Even if one assumes that the twin does not accelerate and
decelerate, the fact remains that it changes direction. In a sense, there are
three (not just two) inertial frames: the twin that stays on Earth, the twin
that travels away from the Earth, and the twin that travels towards the Earth. Thus the elapsed time for the first twin is
calculated by adding up two motions referred to the same frame (the Earth),
whereas the elapsed time for the second twin must be calculated by adding up
two motions referred to two different frames (the one moving away from the
Earth and the one moving towards the Earth).
Thus there is an absolute difference between the first twin measuring
the second twin and the second twin measuring the first twin. The twin who becomes younger is the one
leaving the Earth. That said, it is important to remember that this "becoming
younger" has nothing to do with bodily aging: it is only referred to time
measured by clocks. You can in fact dream up several "paradoxes" based on
the same idea of going back and forth. Imagine, for example, that i cut a 1 cm circular hole from a sheet
of paper. Now i move the sheet of paper far away, and move the circular piece
high up in the air. Then i move the sheet of paper at very high speed towards
the point where it will meet the circular piece that i am letting fall at a
point in time such that it perfectly hits the hole. From the point of view of
the circular piece, the sheet of paper is traveling at a very high speed,
therefore it is shrinking, and, in particular, the hole in the middle is
shrinking: therefore the circular piece will no longer be able to go through
the hole. From the point of view of the sheet of paper, it is the circular
piece that is traveling at very high speed, and thus shrinking: therefore the
circular piece will easily pass through the hole. Imagine if instead of paper,
you used spaceships: depending on which reference frame you use, the spaceships
collide or they smoothly pass each other. This is not just a detail. The solution of this second paradox is similar to the first one:
we have done something at the very beginning, i.e. moving the sheet of paper
far away. No matter how slowly we did that, we caused a change in its size
relative to the circular hole (and viceversa). Thus, when we start moving the
sheet of paper in the opposite direction, we cannot use its original size to
compute the shrinking. When the sheet of paper and the circular piece meet,
they are again the exact same size that they were at the beginning of the
experiment. Back to the beginning of the chapter "The New Physics" | Back to the index of all chapters |