Wesley Clark Support Warp Drive, Time Travel

Peter Fairbrother wrote:
Take a laser pulse, split it, and bounce off two equidistant
surfaces. From a position nearer one surface you see the beam hit
that surface first. From nearer the other you see it hit the other
surface first.

Yes, but if you compensate for the speed of light delay, you can
figure out that it hit both surfaces at the same time. Assuming
that you and both surfaces are all at rest relative to each other.

That's no problem. It might look like "time travel", or reversal
of order, but it doesn't violate causality, or locality, or special
relativity.

If, on the other hand, the observer is *moving*, he will correctly
conclude that the light hit one surface first. An observer moving
in the opposite direction will correctly conclude that the light hit
the *other* surface first. Which observer is right? They *all* are.
This causes no paradox *only* because no signal can get from the one
event to the other, since such a signal would have to travel faster
than light.

So it's possible to choose a particular frame of reference, leave
Earth in 2003, arrive at Sirius in 2005, choose a different frame
of reference, leave Sirius in 2006, and arrive on Earth in 2000,

You say "leave Sirius in 2005", but what does that mean?

What, indeed? You can only synchronize a clock on Sirius to a clock
on Earth after you decide on a frame of reference. Any frame of
reference is equally valid. Since Sirius is nine light years away,
one person on Sirius can insist it's 2009, and another insist it's
1993, with equal validity. As long as FTL travel isn't allowed, this
doesn't lead to any paradox. No signal can make a round trip which
ends before it begins. But allow FTL signalling, and someone in one
frame of reference can get a signal from 2003 Earth to 2004 Sirius.
The person on Sirius then retunes his transmitter to a different
frame of reference, in which it's only 1986 on Sirius. He then
sends an FTL signal to Earth. It's received on Earth in 1987.

Assuming Sirius is 20 light years away, there is a "now" here, and a
corrsponding "now" at Sirius. In 20 years the light from that "now"
at Sirius will arrive.

Nine years. No. Someone traveling past the Earth very rapidly, but
who is right next to Earth when the light arrives (in, lets say,
2003), would correctly conclude that Sirius isn't nine light years
away, but, due to relativistic contraction, only one. Meaning that
the light he and Earth are seeing right now left Sirius not in 1994
but in 2002. And they're *both* right. There's no reason to prefer
"1994" to "2002" or vice versa as what to set clocks to on Sirius.

When the takeoff and landing points are moving relative to each
other it gets more complicated, but the math always works out so
that you can't arrive back where you started before you leave, no
matter what frame or frames you choose to look at it from.

You are mistaken. You want math? Ok. Velocities add, not as w = u + v,
as they do in Newtonian physics, but as w = (u + v)/(1 + uv/c^2). We
can get rid of the c^2 by measuring velocities on a scale where 1 equals
the speed of light. So the equation becomes w = (u + v)/(1 + uv).

Play with it. You'll soon see that:

* If u and v are much less than 1, it approximates the Newtonian
w = u + v. (Even at the speed of sound, the error is only about
one part in a *trillion*.)

* If u an v are both less than 1 (slower than light), the sum is
always less than 1, no matter how close they are to 1.

* If u is more than 1 (FTL), and v less than 1, w will be more than 1
(FTL). *And* has a *direction* that depends on v. Meaning that in
some perfectly valid frames of reference, an FTL voyage will arrive
before it leaves. By switching frames at turnaround, it's possible
to make a round trip that arrives back at the starting point before
it left that starting point.
--
Keith F. Lynch - - http://keithlynch.net/
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On Sun, 05 Oct 2003 13:57:10 GMT, Scott Lowther
wrote, in part:

But they fought down increased taxation. While it's Not Good that Repubs
are also spending whores, at least they're not as bad tax whores.

Nope. Spending borrowed money is WORSE than *paying for* things that
the American people have decided are best paid for by the community as
a whole instead of being left to the market.

Even the U. S. Constitution acknowledges that some things - like
lighthouses and libraries - can sometimes be best paid for with taxes.
Maybe going from lighthouses to the Interstate Highway System, and
going from libraries to public schools, was a mistake, but at present,
very few people agree with that viewpoint.

John Savard

John Savard
http://home.ecn.ab.ca/~jsavard/index.html

Since Sirius is nine light years away,
one person on Sirius can insist it's 2009, and another insist it's
1993, with equal validity. As long as FTL travel isn't allowed, this
doesn't lead to any paradox. No signal can make a round trip which
ends before it begins.

One way to measure time however is to synchronize your clock with the Universe.
You could measure how old the Universe is from Sirius and measure how old the
Universe is from Earth. If you can measure the age of the Universe precisely
enough you could tell what year it was. In earlier years the cosmic background
radiation is greater than in later years. This would be easier to do with
objects that are more remote such as quasars. Einstein didn't like universes
that have a definite beginning.

Tom

TKalbfus wrote:

Since Sirius is nine light years away,
one person on Sirius can insist it's 2009, and another insist it's
1993, with equal validity. As long as FTL travel isn't allowed, this
doesn't lead to any paradox. No signal can make a round trip which
ends before it begins.



One way to measure time however is to synchronize your clock with the Universe.
You could measure how old the Universe is from Sirius and measure how old the
Universe is from Earth. If you can measure the age of the Universe precisely
enough you could tell what year it was. In earlier years the cosmic background
radiation is greater than in later years. This would be easier to do with
objects that are more remote such as quasars. Einstein didn't like universes
that have a definite beginning.


You can't do that. The Universe isn't the same age everywhere. The age
of the Universe
depends upon your reference frame.

Einstein thought that the Universe was stable when he wrote special and
general relativity.
But when Hubble measured the expansion of the Universe he quickly
embraced the idea.
He had nothing against a Universe with a beginning, he just thought that
what he had been
taught was right.

Alain Fournier

John Savard wrote:
On Sun, 05 Oct 2003 13:57:10 GMT, Scott Lowther
wrote, in part:


But they fought down increased taxation. While it's Not Good that Repubs
are also spending whores, at least they're not as bad tax whores.


Nope. Spending borrowed money is WORSE than *paying for* things that
the American people have decided are best paid for by the community as
a whole instead of being left to the market.

On some occasions I've seen deferred payment work. Individuals maxing
out their credit cards to buy the latest software and hardware increase
their productivity. With the increased productivity they can pay off
their maxed credit cards.

But more often, it seems to me, the above scenario leads to a tragic
bankruptcy with the latest hardware and software being repo'd.

Most of the businessmen I know prefer to pay cash and not subject
themselves to interest expense.

ISTR reading that Bill Gates has always been extremely debt-phobic.

Hop
http://clowder.net/hop/index.html

Keith F. Lynch wrote

Meaning that in
some perfectly valid frames of reference, an FTL voyage will arrive
before it leaves.

Yes. And from another valid frame it will leave before it arrives.

By switching frames at turnaround, it's possible
to make a round trip that arrives back at the starting point before
it left that starting point.

No. If we limit it to "near-instantaneous" travel: Both journeys are not
possible from the same frame.



Let's define a "fastest possible" ship. Take a point halfway between here
and Sirius, and send light from it in both directions. Our ship takes off
when the light hits Earth, and lands, by our definition, after the light
reaches Sirius. It can't land earlier, that's the fastest it can go. It
still goes well faster than light, viewed from any frame.


Now consider the combined forward and return journey, taking that into
account. There is no way, no matter how you change or shift frame, that the
ship can arrive back at Earth before it left Earth.


I don't blame you for thinking it could - it's widely taught in schools and
elsewhere. More than a few physicists would agree with you. It's almost a
paradigm. It just happens to be wrong.




--
Peter Fairbrother

(I have a feeling that it may become possible to build a quantum ansible
quite soon. So do some other quantum physicists. We don't know for sure just
yet)

You can't do that. The Universe isn't the same age everywhere. The age
of the Universe
depends upon your reference frame.

Earlier in the Universe's history, the Universe is younger, later in the
Universe's history it is older. With FTL travel it would be possible to travel
to that distand quasar in the sky as we now see it, even if its 10 billion
years old and 10 billion light years away. Its even possible to travel to early
in the Universe's history, so long as its further away in light years than it
is in years. So even if the Universe has a finite life span you can just keep
on going back in time and relive the Universe's history at another location,
all without violating causality. Since light signals cannot communicate at
these widely separated points.

Tom

Peter Fairbrother wrote:
"Keith F. Lynch" wrote:

Meaning that in some perfectly valid frames of reference, an FTL
voyage will arrive before it leaves.

Yes. And from another valid frame it will leave before it arrives.

Yes. So?

I don't understand why we don't seem to be communicating.

By switching frames at turnaround, it's possible to make a round
trip that arrives back at the starting point before it left that
starting point.

No.

Yes.

If we limit it to "near-instantaneous" travel: Both journeys are not
possible from the same frame.

So? We switch frames at turnaround.

Let's define a "fastest possible" ship. Take a point halfway
between here and Sirius, and send light from it in both directions.
Our ship takes off when the light hits Earth,

Ok.

and lands, by our definition, after the light reaches Sirius.

Not necessarily.

It can't land earlier, that's the fastest it can go.

How do you figure?

Consider the frame in which Earth and Sirius are moving in the
Earth-Sirius direction at 90% of the speed of light. In other words,
the rest frame of an astronaut who is passing Earth on the way to
Sirius at 90% the speed of light. And who, for simplicity, I'll say
just happens to be passing by Earth at the instant the light from the
point halfway between Earth and Sirius reaches Earth.

By "halfway between Earth and Sirius" I assume you mean halfway in the
rest frame of that light source, which is at rest relative to both
Earth and Sirius, which are at rest relative to each other (or pretty
close to at rest, anyway).

That astronaut's frams is just as valid a frame as any. At the
instant that light from reaches Earth, you take off from Earth at
twice the speed of light *relative to that frame*, heading in the
direction of Sirius.

Note that in that frame, Earth and Sirius are only four light years
apart, rather than nine, due to the Fitzgerald contraction. When the
light arrives on Earth, it had been emitted only 0.2 years earlier,
not two years earlier, since in that frame the source is receding at
90% the speed of light, meaning it must have only been 0.2 light years
away when the light was emitted. (Remember that the speed of light
is *not* subtracted from the speed of the source. It's 299,792,458
meters per second relative to the observer in that frame, just as it
is for any other frame.)

But when does the light reach Sirius? Since Sirius is receding at
90% the speed of light, it takes the light twenty years to reach it.
(Remember that the speed of light is *not* added to the speed of the
source, so it doesn't matter that the source is also heading in the
same direction at the same speed.)

So going at twice the speed of light relative to this astronaut,
starting on Earth when the light reaches Earth, will cause you to
arrive on Sirius 19.8 years (twenty minus 0.2) years before the light
arrives there.

It's not quite as extreme as it sounds, since you have to account for
the clocks running slower in that frame. It will actually only be 8.8
years before the light reaches you, once you come to rest on Sirius.
(Or on a planet around Sirius. Whatever.)

Instead of waiting the 8.8 years, suppose you immediately take off,
and head back to Earth at twice the speed of light *relative to the
frame in which Earth and Sirius are stationary*. In that case you
will arrive on Earth 4.5 years later in that frame. Which will be 4.3
years before the light arrives on Earth. In other words, 4.3 years
before you started your round trip!

Or, if you took off from Sirius at twice the speed of light relative
to the frame in which Earth and Sirius are moving in the Sirius-Earth
direction at 90% the speed of light (i.e. the "opposite" of the first
frame), you'd arrive on Earth 17.6 years before you started your round
trip.

Unless some reference frames are less valid than others (which there's
no evidence for), if you can go faster than light relative to one of
them, you can go faster than light relative to any other of them. In
which case you can "tack" your way up the stream of time, and end up
as far in the past as you like. Ever wonder what trilobite tastes like?

I don't blame you for thinking it could - it's widely taught in
schools and elsewhere.

Yes, because it's true.

More than a few physicists would agree with you.

Yes, because they're right.

(I have a feeling that it may become possible to build a quantum
ansible quite soon. So do some other quantum physicists. We don't
know for sure just yet)

I doubt it. Sure, using quantum entanglement you can do what looks
sort of like sending a message faster than light, or back through
time. However, the message always consists of completely random bits.
You can't even signal by destroying the "transmitter" and hoping that
someone at the "receiver" will notice the difference. They won't.
All you're guaranteed is that if both the "transmitter" and the
"receiver" are operating properly, they'll receive the *same* random
bits at the same time. And you can define "same time" any way you
like it, even in ways such that paradoxes would result if the signal
were anything but random bits.
--
Keith F. Lynch - - http://keithlynch.net/
I always welcome replies to my e-mail, postings, and web pages, but
unsolicited bulk e-mail (spam) is not acceptable. Please do not send me
HTML, "rich text," or attachments, as all such email is discarded unread.