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> Two people make a bet based on the toss of a coin while a third =
> unobserved person looks on discretely from the corner. The probability =
> of heads or tails is fifty-fifty, 0.5. The coin lands heads, one of them =
> loses, fair enough.
>
> Having seen this, our voyeur steps out of the room, gets in his time =
> machine, and returns to witness the same event. But what are the =
> probabilities of heads and tails now? Are they still fifty-fifty, or are =
> they now 1 and 0 respectively?
>
> This little argument seems to pose 3 possible conclusions.
> 1. Time travel is impossible
> 2. Probability (and therefore our human concept of free will) is an =
> illusion (assuming of course, that someone with a capital S is watching =
> over the proceedings).
> 3. There is a fundamental flaw in this line of reasoning.
>
> Does anyone have any comments?
There is a 4th conclusion, which the MWI opens up.
4. Time travel is possible, probability is meaningful. In this case,
the observer travels back in time to the original point (the unsplit
universe). This time, there is a 50% chance that the observer will
find erself in the same future that e came from, and 50% chance that e
finds erself in the future with the opposite outcome.
Of course this is only a cartoon picture - in reality, events with
multiple outcomes happen all the time, and the probability of the
observer finding history repeating itself exactly the same to the time
that e left it diminishes exponentially fast with the time
separation. Thus, time travel is not paradoxical, and in fact General
Relativity allows for the possibility of time travel (although it is
technologically out of our reach for the forseable future). Special
Relativity conversely expressly forbids time travel (or faster than
light travel, which is the same thing).
Cheers
>
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> <HEAD>
>
> <META content=3Dtext/html;charset=3Diso-8859-1 =
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> <BODY bgColor=3D#ffffff>
> <DIV><FONT face=3DArial size=3D2>Two people make a bet based on the toss =
> of a coin=20
> while a third unobserved person looks on discretely from the corner. The =
>
> probability of heads or tails is fifty-fifty, 0.5. The coin lands heads, =
> one of=20
> them loses, fair enough.</FONT></DIV>
> <DIV><FONT face=3DArial size=3D2></FONT> </DIV>
> <DIV><FONT face=3DArial size=3D2>Having seen this, our voyeur steps out =
> of the room,=20
> gets in his time machine, and returns to witness the same event. But =
> what are=20
> the probabilities of heads and tails now? Are they still fifty-fifty, or =
> are=20
> they now 1 and 0 respectively?</FONT></DIV>
> <DIV> </DIV>
> <DIV><FONT color=3D#000000 face=3DArial size=3D2>This little argument =
> seems to pose 3=20
> possible conclusions.</FONT></DIV>
> <DIV><FONT color=3D#000000 face=3DArial size=3D2>1. Time travel is=20
> impossible</FONT></DIV>
> <DIV><FONT color=3D#000000 face=3DArial size=3D2>2. Probability (and =
> therefore our=20
> human concept of free will) is an illusion (assuming of course, that =
> someone=20
> with a capital S is watching over the proceedings).</FONT></DIV>
> <DIV><FONT color=3D#000000 face=3DArial size=3D2>3. There is a =
> fundamental flaw in=20
> this line of reasoning.</FONT></DIV>
> <DIV><FONT color=3D#000000 face=3DArial size=3D2></FONT> </DIV>
> <DIV><FONT color=3D#000000 face=3DArial size=3D2>Does anyone have any=20
> comments?</FONT></DIV></BODY></HTML>
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Dr. Russell Standish Director
High Performance Computing Support Unit,
University of NSW Phone 9385 6967
Sydney 2052 Fax 9385 7123
Australia R.Standish.domain.name.hidden
Room 2075, Red Centre
http://parallel.hpc.unsw.edu.au/rks
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Received on Wed Jan 20 1999 - 20:05:19 PST