Originally posted by syrinx high priest
roth ! bravo ! you have come to the side of reason ! Thank you for illustrating my point that travelling in time first BACKWARDS to 1976, then
FORWARDS to 1999, and then again to 2036 seems extremely unlikley given the arguments you were kind enough to post (too bad sagan wasn't specifically
referring to titor, like maku was)
NOVA: Such as?
Sagan: First of all, it might be that you can build a time machine to
go into the future, but not into the past, and we don't know about it
because we haven't yet invented that time machine. ( strike 1 for JT) Secondly, it might
be that time travel into the past is possible, but they haven't
gotten to our time yet, they're very far in the future and the
further back in time you go, the more expensive it is.(strike 2,) Thirdly, maybe
backward time travel is possible, but only up to the moment that time
travel is invented. We haven't invented it yet, so they can't come to
us. They can come to as far back as whatever it would be, say A.D.
2300, but not further back in time.
strike 3, you're out !
i mean really, going backwards and then forwards, only 21 years after the nuclear war that kiled 3 billion people.
and to top it off, the trip was basically an errand to pick up a computer part, and he has time to kill before he goes back, so he hangs out of some
chat boards to tell us how lazy we are.... tsk tsk...we need a stern lecture....
I'm off to research the paradox thingy, when he meets himself.....
Carl Sagan merely was summing up some examples as to why we are not invaded yet by time travellers (in our perception) if time travel would be
possible. He simply weakens that weak argument and confirms time travel is very well possible. Here are his words again (sigh):
"I can think half a dozen ways in which we could not be awash
in time travelers, and still time travel is possible.
Then there's the possibility that they're here alright, but we don't
see them. They have perfect invisibility cloaks or something. If they
have such highly developed technology, then why not?
Finally, there's the possibility that time travel is perfectly possible, but
it requires a great advance in our technology...
I'm sure there are other possibilities as well, but if you just think
of that range of possibilities, I don't think the fact that we're not
obviously being visited by time travelers shows that time travel is
So "striker," apparently you have a severe reading comprehension problem. Either that or you just like to play dumb and dumber.
Doesn't the incomprehensible world of the Quantum Theory allow particles moving backwards as well as forwards in time and appear in all possible
places at once
? Ofcourse they do!
Here's an interesting article:
Taming the multiverse
Parallel universes are no longer a figment of our imagination.
They're so real that we can reach out and touch them, and even use
them to change our world, says Marcus Chown.
FLICKING through New Scientist, you stop at this page, think "that's
interesting" and read these words. Another you thinks "what
nonsense", and moves on. Yet another lets out a cry, keels over and
Is this an insane vision? Not according to David Deutsch of the
University of Oxford. Deutsch believes that our Universe is part of
the multiverse, a domain of parallel universes that comprises
Until now, the multiverse was a hazy, ill-defined concept-little more
than a philosophical trick. But in a paper yet to be published,
Deutsch has worked out the structure of the multiverse. With it, he
claims, he has answered the last criticism of the sceptics. "For 70
years physicists have been hiding from it, but they can hide no
longer." If he's right, the multiverse is no trick. It is real. So
real that we can mould the fate of the universes and exploit them.
Why believe in something so extraordinary? Because it can explain one
of the greatest mysteries of modern science: why the world of atoms
behaves so very differently from the everyday world of trees and
The theory that describes atoms and their constituents is quantum
mechanics. It is hugely successful. It has led to computers, lasers
and nuclear reactors, and it tells us why the Sun shines and why the
ground beneath our feet is solid. But quantum theory also tells us
something very disturbing about atoms and their like: they can be in
many places at once. This isn't just a crazy theory-it has observable
consequences (see "Interfering with the multiverse").
But how is it that atoms can be in many places at once whereas big
things made out of atoms-tables, trees and pencils-apparently cannot?
Reconciling the difference between the microscopic and the
macroscopic is the central problem in quantum theory.
The many worlds interpretation is one way to do it. This idea was
proposed by Princeton graduate student Hugh Everett III in 1957.
According to many worlds, quantum theory doesn't just apply to atoms,
says Deutsch. "The world of tables is exactly the same as the world
But surely this means tables can be in many places at once. Right.
But nobody has ever seen such a schizophrenic table. So what gives?
The idea is that if you observe a table that is in two places at
once, there are also two versions of you-one that sees the table in
one place and one that sees it in another place.
The consequences are remarkable. A universe must exist for every
physical possibility. There are Earths where the Nazis prevailed in
the Second World War, where Marilyn Monroe married Einstein, and
where the dinosaurs survived and evolved into intelligent beings who
read New Scientist.
However, many worlds is not the only interpretation of quantum
theory. Physicists can choose between half a dozen interpretations,
all of which predict identical outcomes for all conceivable
Deutsch dismisses them all. "Some are gibberish, like the Copenhagen
interpretation," he says-and the rest are just variations on the many
For example, according to the Copenhagen interpretation, the act of
observing is crucial. Observation forces an atom to make up its mind,
and plump for being in only one place out of all the possible places
it could be. But the Copenhagen interpretation is itself open to
interpretation. What constitutes an observation? For some people,
this only requires a large-scale object such as a particle detector.
For others it means an interaction with some kind of conscious being.
Worse still, says Deutsch, is that in this type of interpretation you
have to abandon the idea of reality. Before observation, the atom
doesn't have a real position. To Deutsch, the whole thing is
mysticism-throwing up our hands and saying there are some things we
are not allowed to ask.
Some interpretations do try to give the microscopic world reality,
but they are all disguised versions of the many worlds idea, says
Deutsch. "Their proponents have fallen over backwards to talk about
the many worlds in a way that makes it appear as if they are not."
In this category, Deutsch includes David Bohm's "pilot-wave"
interpretation. Bohm's idea is that a quantum wave guides particles
along their trajectories. Then the strange shape of the pilot wave
can be used to explain all the odd quantum behaviours, such as
interference patterns. In effect, says Deutsch, Bohm's single
universe occupies one groove in an immensely complicated multi-
dimensional wave function.
"The question that pilot-wave theorists must address is: what are the
unoccupied grooves?" says Deutsch. "It is no good saying they are
merely theoretical and do not exist physically, for they continually
jostle each other and the occupied groove, affecting its trajectory.
What's really being talked about here is parallel universes. Pilot-
wave theories are parallel-universe theories in a state of chronic
Back and forth
Another disguised many worlds theory, says Deutsch, is John
Cramer's "transactional" interpretation in which information passes
backwards and forwards through time. When you measure the position of
an atom, it sends a message back to its earlier self to change its
But as the system gets more complicated, the number of messages
explodes. Soon, says Deutsch, it becomes vastly greater than the
number of particles in the Universe. The full quantum evolution of a
system as big as the Universe consists of an exponentially large
number of classical processes, each of which contains the information
to describe a whole universe. So Cramer's idea forces the multiverse
on you, says Deutsch.
So do other interpretations, according to Deutsch. "Quantum theory
leaves no doubt that other universes exist in exactly the same sense
that the single Universe that we see exists," he says. "This is not a
matter of interpretation. It is a logical consequence of quantum
Yet many physicists still refuse to accept the multiverse. "People
say the many worlds is simply too crazy, too wasteful, too mind-
blowing," says Deutsch. "But this is an emotional not a scientific
reaction. We have to take what nature gives us."
A much more legitimate objection is that many worlds is vague and has
no firm mathematical basis. Proponents talk of a multiverse that is
like a stack of parallel universes. The critics point out that it
cannot be that simple-quantum phenomena occur precisely because the
universes interact. "What is needed is a precise mathematical model
of the multiverse," says Deutsch. And now he's made one.
The key to Deutsch's model sounds peculiar. He treats the multiverse
as if it were a quantum computer. Quantum computers exploit the
strangeness of quantum systems-their ability to be in many states at
once-to do certain kinds of calculation at ludicrously high speed.
For example, they could quickly search huge databases that would take
an ordinary computer the lifetime of the Universe. Although the
hardware is still at a very basic stage, the theory of how quantum
computers process information is well advanced.
In 1985, Deutsch proved that such a machine can simulate any
conceivable quantum system, and that includes the Universe itself. So
to work out the basic structure of the multiverse, all you need to do
is analyse a general quantum calculation. "The set of all programs
that can be run on a quantum computer includes programs that would
simulate the multiverse," says Deutsch. "So we don't have to include
any details of stars and galaxies in the real Universe, we can just
analyse quantum computers and look at how information flows inside
If information could flow freely from one part of the multiverse to
another, we'd live in a chaotic world where all possibilities would
overlap. We really would see two tables at once, and worse,
everything imaginable would be happening everywhere at the same time.
Deutsch found that, almost all the time, information flows only
within small pieces of the quantum calculation, and not in between
those pieces. These pieces, he says, are separate universes. They
feel separate and autonomous because all the information we receive
through our senses has come from within one universe. As Oxford
philosopher Michael Lockwood put it, "We cannot look sideways,
through the multiverse, any more than we can look into the future."
Sometimes universes in Deutsch's model peel apart only locally and
fleetingly, and then slap back together again. This is the cause of
quantum interference, which is at the root of everything from the two-
slit experiment to the basic structure of atoms.
Other physicists are still digesting what Deutsch has to say. Anton
Zeilinger of the University of Vienna remains unconvinced. "The
multiverse interpretation is not the only possible one, and it is not
even the simplest," he says. Zeilinger instead uses information
theory to come to very different conclusions. He thinks that quantum
theory comes from limits on the information we get out of
measurements (New Scientist, 17 February, p 26). As in the Copenhagen
interpretation, there is no reality to what goes on before the
But Deutsch insists that his picture is more profound than
Zeilinger's. "I hope he'll come round, and realise that the many
worlds theory explains where the information in his measurements
Why are physicists reluctant to accept many worlds? Deutsch blames
logical positivism, the idea that science should concern itself only
with objects that can be observed. In the early 20th century, some
logical positivists even denied the existence of atoms-until the
evidence became overwhelming. The evidence for the multiverse,
according to Deutsch, is equally overwhelming. "Admittedly, it's
indirect," he says. "But then, we can detect pterodactyls and quarks
only indirectly too. The evidence that other universes exist is at
least as strong as the evidence for pterodactyls or quarks."
Perhaps the sceptics will be convinced by a practical demonstration
of the multiverse. And Deutsch thinks he knows how. By building a
quantum computer, he says, we can reach out and mould the multiverse.
"One day, a quantum computer will be built which does more
simultaneous calculations than there are particles in the Universe,"
says Deutsch. "Since the Universe as we see it lacks the
computational resources to do the calculations, where are they being
done?" It can only be in other universes, he says. "Quantum computers
share information with huge numbers of versions of themselves
throughout the multiverse."
Imagine that you have a quantum PC and you set it a problem. What
happens is that a huge number of versions of your PC split off from
this Universe into their own separate, local universes, and work on
parallel strands of the problem. A split second later, the pocket
universes recombine into one, and those strands are pulled together
to provide the answer that pops up on your screen. "Quantum computers
are the first machines humans have ever built to exploit the
multiverse directly," says Deutsch.
At the moment, even the biggest quantum computers can only work their
magic on about 6 bits of information, which in Deutsch's view means
they exploit copies of themselves in 26 universes-that's just 64 of
them. Because the computational feats of such computers are puny,
people can choose to ignore the multiverse. "But something will
happen when the number of parallel calculations becomes very large,"
says Deutsch. "If the number is 64, people can shut their eyes but if
it's 1064, they will no longer be able to pretend."
What would it mean for you and me to know there are inconceivably
many yous and mes living out all possible histories? Surely, there is
no point in making any choices for the better if all possible
outcomes happen? We might as well stay in bed or commit suicide.
Deutsch does not agree. In fact, he thinks it could make real choice
possible. In classical physics, he says, there is no such thing
as "if"; the future is determined absolutely by the past. So there
can be no free will. In the multiverse, however, there are
alternatives; the quantum possibilities really happen.
Free will might have a sensible definition, Deutsch thinks, because
the alternatives don't have to occur within equally large slices of
the multiverse. "By making good choices, doing the right thing, we
thicken the stack of universes in which versions of us live
reasonable lives," he says. "When you succeed, all the copies of you
who made the same decision succeed too. What you do for the better
increases the portion of the multiverse where good things happen."
Let's hope that deciding to read this article was the right choice.
[edit on 29-7-2005 by Roth Joint]