"If I build a time machine I could make billions on the
stock market!! Let's fund that sucker now!"
I see two bogus assumptions right off, one is that sure, only
you and your development team will ever make a time machine, and two,
that no machine will ever be built and used nefariously! i.e. you are
thinking with a very limited scope.
Another assumption is that information has meaning outside of its
context. In the stock market application of such a device,
the information passed back through the time machine is assumed to
have the same context both in the moment when the message is sent,
and in the moment when the message arrives.
But in reality the context is slightly different. If you get a
message from yourself in the future saying "buy Microsoft"
this message is based on the fact that you were not going to invest in
Microsoft on this date. If you do invest in Microsoft you are
about to render the source context of the message
invalid. "Big deal" you say, "I can't affect
the stock market enough to
change the results, so what is the problem?" The problem is
the first assumption - that time machines have limited scope - that
you are the only one who will ever build or have access to a time
machine! When we remove both of these invalid assumptions,
the ability to pass information back to the past has no other possible
conclusion than to saturate the medium to the point of unusability,
and thus completely alter the target environment.
Here is an alternate (and simpler argument). The stock market
functions correctly, therefore the ability to pass knowledge through
time must be impossible. If a communication time machine can be
built, at least one person in the infinity of the future will build
a machine and try to gain wealth from it by communicating with the
past. (and we know that at least one person will not exercise
restraint) If one tries it, others will follow,
mistakes will be made, or deliberate sabotage will be attempted.
The result would be that the world would have already have felt the
completely overwhelming impact of such a possibility. And it
hasn't, therefore passing even information back in time must
be impossible.