"Education is what remains after one has forgotten what one has learned in school." - Albert Einstein


The Unexpected Hanging Paradox

Source: https://en.wikipedia.org/wiki/Unexpected_hanging_paradox

A judge tells a condemned prisoner that he will be hanged at noon on one weekday in the following week
but that the execution will be a surprise to the prisoner. He will not know the day of the hanging until
the executioner knocks on his cell door at noon that day.

Having reflected on his sentence, the prisoner draws the conclusion that he will escape from the hanging.
His reasoning is in several parts. He begins by concluding that the "surprise hanging" can't be on Friday,
as if he hasn't been hanged by Thursday, there is only one day left - and so it won't be a surprise if he's
hanged on Friday. Since the judge's sentence stipulated that the hanging would be a surprise to him, he
concludes it cannot occur on Friday.

He then reasons that the surprise hanging cannot be on Thursday either, because Friday has already been
eliminated and if he hasn't been hanged by Wednesday noon, the hanging must occur on Thursday, making a
Thursday hanging not a surprise either. By similar reasoning he concludes that the hanging can also not
occur on Wednesday, Tuesday or Monday. Joyfully he retires to his cell confident that the hanging will not
occur at all.
The next week, the executioner knocks on the prisoner's door at noon on Wednesday - which, despite all
the above, was an utter surprise to him. Everything the judge said came true.

The Twin paradox

Imagine two twins: 'Bob' and 'Tom'.
Bob travels away from Earth at a very high speed and returns
after a fews have passed. Tom remains on Earth for all of the
time that Bob is away from Earth. When Bob returns to Earth
Tom is older than Bob.


Intuitively, you would think that Bob
and Tom would be the same age... this is true in a Newtonian
frame of reference. However, under Einstein's Relativistic frame
of reference Tom is actually older than Bob.

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