Just why would FTL lead to time travel?

Because the simultaneity is relative, for the slower than light
communications?

But why should inertial frames be equivalent when it comes to FTL?

If you can send a ship or a message from point A to point B and back
in zero roundtrip time, you can compare the real simultaneity of
events as established by FTL with the outcome of lightspeed signals
sent through ordinary space. This would enable observation of ether
wind - inertial frames would no longer be equivalent. Only in FTL,
though - all predictions of relativity as to STL effects would still
hold.

Also, what happens if Earth turns out to be subject to strong, indeed
relativistic ether wind?

For example, because Earth turns out to be some distance away from the
epicentre of Big Bang?

Is it possible to come up with a noncontradictory metrics where ether
itself expands?

In rec.arts.sf.written, Crown-Horned Snorkack <chornedsnorkack@hush.ai> wrote:
> Just why would FTL lead to time travel?

Because inertial frames are equivalent in all the physics we know.

> But why should inertial frames be equivalent when it comes to FTL?

If you assume otherwise, you can get FTL without causal problems. Many
authors have done this, notably Charles Sheffield.

> Also, what happens if Earth turns out to be subject to strong, indeed
> relativistic ether wind?

Then FTL travel is asymmetric.

> For example, because Earth turns out to be some distance away from the
> epicentre of Big Bang?

According to the physics we know, there is no epicenter. If the
preferred frame for FTL matches up with isotropic cosmic background
radiation, then we're close to still.

> Is it possible to come up with a noncontradictory metrics where ether
> itself expands?

Now you're past me. :)

--Z

--
"And Aholibamah bare Jeush, and Jaalam, and Korah: these were the borogoves..."
*
If the Bush administration hasn't subjected you to searches without a warrant,
it's for one reason: they don't feel like it. Not because you're innocent.

: Crown-Horned Snorkack <chornedsnorkack@hush.ai>
: But why should inertial frames be equivalent when it comes to FTL?

Because *all* the laws of physics we know of are lorentz invariant.
And the last time we had some laws of physics we thought were not
lorentz invariant, we turned out to be wrong. And even when we thought
the laws of nature were galilean invariant, there was no prefered
inertial frame.

Basically, we've had nigh on 400 years of physics which shows us that
there's no inertial frame, ever since galileo invented relativity,
and there's no reason to suppose otherwise.

Now, if you *want* to suppose otherwise for SFnal purposes,
that's fine. Shrug. No biggie. But if you're talking about
what's plausible, it seems a bit less than.

: Also, what happens if Earth turns out to be subject to strong, indeed
: relativistic ether wind?

Then relativity would be wrong. But it's not the way to bet.

: Is it possible to come up with a noncontradictory metrics where ether
: itself expands?

Not sure what a "metric" would have to do with whether the "ether expands".


Wayne Throop throopw@sheol.org http://sheol.org/throopw

Crown-Horned Snorkack <chornedsnorkack@hush.ai> wrote in
news:d0adcd0e-143e-4e8d-a33a-47c205f1e0b7
@d4g2000prg.googlegroups.com:

> If you can send a ship or a message from point A to point B
and back
> in zero roundtrip time, you can compare the real
simultaneity of
> events as established by FTL with the outcome of lightspeed
signals
> sent through ordinary space. This would enable observation
of ether
> wind - inertial frames would no longer be equivalent. Only
in FTL,
> though - all predictions of relativity as to STL effects
would still
> hold.

You are saying if you assume there is a preferred frame, you
can conclude there is a preferred frame.

On 15 veebr, 22:23, Gene Ward Smith <g...@chewbacca.org> wrote:
> Crown-Horned Snorkack <chornedsnork...@hush.ai> wrote in
> news:d0adcd0e-143e-4e8d-a33a-47c205f1e0b7
> @d4g2000prg.googlegroups.com:
>
>
>
> > If you can send a ship or a message from point A to point B and back in zero roundtrip time, you can compare the
> > real simultaneity of events as established by FTL with the outcome of lightspeed signals sent through ordinary space. > > This would enable observation of ether wind - inertial frames would no longer be equivalent. Only in FTL, though - all
> > predictions of relativity as to STL effects would still hold.
>
> You are saying if you assume there is a preferred frame, you can conclude there is a preferred frame.

If the roundtrip time between A and B by FTL is exactly zero, and
signals sent A to B at FTL and back B to A arrive after a finite time,
how do you avoid a preferred frame?

: Crown-Horned Snorkack <chornedsnorkack@hush.ai>
: If the roundtrip time between A and B by FTL is exactly zero, and
: signals sent A to B at FTL and back B to A arrive after a finite time,
: how do you avoid a preferred frame?

By "round trip time between A and B" you mean the time to get from
A to B and back to A? And is this time the elapsed time, or the
coordinate time, or what exactly?

And if A and B are points, you haven't said anything about what
frames of reference are involved (and if they are frames of referance
you haven't said anything about where the messages go).

So... I don't see how you can conclude anything at all, because your
hypothetical is nearly meaningless as it stands. Taking the statement
at face value, and if we assume that you didn't leave anything out (ie,
about swapping frames), and that A and B are points, then what you've
stated doesn't even come close to proving there's a prefered frame,
since you haven't attempted measurements in more than one frame. You can
"avoid a prefered frame" because all your statements are compatible with
having been done in a single (non-prefered) frame.


Wayne Throop throopw@sheol.org http://sheol.org/throopw

Crown-Horned Snorkack <chornedsnorkack@hush.ai> wrote:
>Just why would FTL lead to time travel?
>
>Because the simultaneity is relative, for the slower than light
>communications?

Because there _is no simultaneity_ as such in relativity. You literally cannot
measure a well-defined time interval between spacelike-separated points. This
means that there are always some frames where ANY pair of such points has one
seen as occurring before the other, and always some other frames where the SAME
pair has the second seen as occurring before the first. As long as one point
is not in either the past or future light cone of the other, you can't escape
this.

As long as you're going slower than light, you're confined to the future (and
past) light cones of any event-point you go through on your trajectory. The
moment you're going FTL, you are _outside_ both light-cones of every event-
point on your FTL trajectory ... so suitable rearrangement of frames (via
acceleration to different velocities) for a pair of FTL trips can get you
ending up inside the past light-cone of the point you started from. Time
travel.

>But why should inertial frames be equivalent when it comes to FTL?

Because if they're not, then you have to figure out what the theory is that
covers FTL trips AND reduces to relativity, _exactly_, when things are NOT
faster than light. Finding a way to go FTL does not invalidate all the data
we already have; any New Theory has to account for all that data PLUS the
new stuff. This is the difficult part of physics, and is why so many crackpots
have No Clue that their magnificent new Theories are trash from the get-go.

>If you can send a ship or a message from point A to point B and back
>in zero roundtrip time, you can compare the real simultaneity of
>events as established by FTL with the outcome of lightspeed signals
>sent through ordinary space.

There's still no simultaneity. I think that word does not mean what you
think it means.

>Also, what happens if Earth turns out to be subject to strong, indeed
>relativistic ether wind?

Then we have to figure out why it never shows up at all in any slower-than-
light experiments. My Michelson and Morley T-shirt wants to talk to you.

>Is it possible to come up with a noncontradictory metrics where ether
>itself expands?

As long as it reduces to what we already know on the stuff we already know
about, knock yourself out. If it doesn't, you HAVE to explain why. (Note that
Lorentz contraction is already an example of "well, things shift themselves
around in such a way that we never see this effect at all however we try to
look for it", so it can be done at times...)

Dave "paint your whiskers green" DeLaney
--
\/David DeLaney posting from dbd@vic.com "It's not the pot that grows the flower
It's not the clock that slows the hour The definition's plain for anyone to see
Love is all it takes to make a family" - R&P. VISUALIZE HAPPYNET VRbeable<BLINK>
http://www.vic.com/~dbd/ - net.legends FAQ & Magic / I WUV you in all CAPS! --K.

On 16 veebr, 00:44, thro...@sheol.org (Wayne Throop) wrote:
> : Crown-Horned Snorkack <chornedsnork...@hush.ai>
> : If the roundtrip time between A and B by FTL is exactly zero, and
> : signals sent A to B at FTL and back B to A arrive after a finite time,
> : how do you avoid a preferred frame?
>
> By "round trip time between A and B" you mean the time to get from
> A to B and back to A? And is this time the elapsed time, or the
> coordinate time, or what exactly?
>
> And if A and B are points, you haven't said anything about what
> frames of reference are involved (and if they are frames of referance
> you haven't said anything about where the messages go).
>
> So... I don't see how you can conclude anything at all, because your
> hypothetical is nearly meaningless as it stands. Taking the statement
> at face value, and if we assume that you didn't leave anything out (ie,
> about swapping frames), and that A and B are points, then what you've
> stated doesn't even come close to proving there's a prefered frame,
> since you haven't attempted measurements in more than one frame. You can
> "avoid a prefered frame" because all your statements are compatible with
> having been done in a single (non-prefered) frame.
>
Very well.

Let us imagine that we establish back and forth FTL signalling between
Earth and Alpha Centauri.

These two are not exactly in the same inertial frame. However, the
relative speed is not relativistic.

It is relatively easy to establish that a signal exchanged at the
speed of light travels for 8,6 years return trip.

Once we establish FTL signalling, we can establish that round trip
time is exactly zero, and never negative.

Now assume we then examine signals sent FTL one way and at speed of
light the other way.

Suppose we establish that signals sent Earth to Alpha Centauri FTL and
back at the speed of light take 3,3 years, while the opposite takes
5,3 years.

Have we then observed ether wind?

: Crown-Horned Snorkack <chornedsnorkack@hush.ai>
: Let us imagine that we establish back and forth FTL signalling between
: Earth and Alpha Centauri.
:
: These two are not exactly in the same inertial frame. However, the
: relative speed is not relativistic.
:
: It is relatively easy to establish that a signal exchanged at the
: speed of light travels for 8,6 years return trip.
:
: Once we establish FTL signalling, we can establish that round trip
: time is exactly zero, and never negative.
:
: Now assume we then examine signals sent FTL one way and at speed of
: light the other way.
:
: Suppose we establish that signals sent Earth to Alpha Centauri FTL and
: back at the speed of light take 3,3 years, while the opposite takes
: 5,3 years.
:
: Have we then observed ether wind?

You've observed that your FTL seems to be occuring wrt a particular frame,
in which both earth and AC are in pretty brisk motion, something like
20 percent lightspeed (iicc). This doesn't establish that that frame is
the only frame in which FTL can occur, nor that this is an "ether wind",
but it is certainly suggestive.


Wayne Throop throopw@sheol.org http://sheol.org/throopw

On 16 veebr, 22:38, thro...@sheol.org (Wayne Throop) wrote:
> : Crown-Horned Snorkack <chornedsnork...@hush.ai>
> : Let us imagine that we establish back and forth FTL signalling between
> : Earth and Alpha Centauri.
> :
> : These two are not exactly in the same inertial frame. However, the
> : relative speed is not relativistic.
> :
> : It is relatively easy to establish that a signal exchanged at the
> : speed of light travels for 8,6 years return trip.
> :
> : Once we establish FTL signalling, we can establish that round trip
> : time is exactly zero, and never negative.
> :
> : Now assume we then examine signals sent FTL one way and at speed of
> : light the other way.
> :
> : Suppose we establish that signals sent Earth to Alpha Centauri FTL and
> : back at the speed of light take 3,3 years, while the opposite takes
> : 5,3 years.
> :
> : Have we then observed ether wind?
>
> You've observed that your FTL seems to be occuring wrt a particular frame,
> in which both earth and AC are in pretty brisk motion, something like
> 20 percent lightspeed (iicc). This doesn't establish that that frame is
> the only frame in which FTL can occur, nor that this is an "ether wind",
> but it is certainly suggestive.

If you have multiple frames where FTL could occur, how do you avoid
time travel?

If in frame A, sending a signal FTL to Alpha Centauri and back at
speed of light arrive 3,3 years after departure

and in frame B, sending a signal FTL to Alpha Centauri and back at
light speed takes 4,3 years, as does sending a signal at light speed
to Alpha Centauri and back FTL

then a signal sent from Earth to Alpha Centauri FTL in frame A and
back from Alpha Centauri to Earth in frame B should arrive a year
before departure.

So we have time travel.

: Crown-Horned Snorkack <chornedsnorkack@hush.ai>
: If you have multiple frames where FTL could occur, how do you avoid
: time travel?

Is that the goal now? Well, OK, but note that your originnal question
was
If the roundtrip time between A and B by FTL is exactly zero, and
signals sent A to B at FTL and back B to A arrive after a finite
time, how do you avoid a preferred frame?

which doesn't involve avoiding time travel. But if that's the goal,
then the usual way is some mechanism embodying the so-called
chronological protection conjecture. Often illustrated by wormholes;
you can connect any two spacelike separated events with wormholes,
so there are "multiple frames where FTL could occur", but if you try
to move your wormhole ends to have a timelike separation (or indeed
establish any network of wormholes with a closed timelike curve), you
must first have a closed lightlike curve, and that would collapse your
wormhole network. There are reasons to suppose this would indeed occur.

So. You "avoid timetravel" because if you try it, your FTL widdget
implodes before you accomplish it. (Note that this is not the same issue
Niven raised in "theory and practice of time travel", and turned into
a short story, but it has much the same effect).


Wayne Throop throopw@sheol.org http://sheol.org/throopw

On 2008-02-17 04:30:57 -0700, Crown-Horned Snorkack
<chornedsnorkack@hush.ai> said:

> If you have multiple frames where FTL could occur, how do you avoid
> time travel?

If multiple frames where FTL could occur can't exist, then you can
avoid time travel by fiat. Have your fictional universe work under
different rules.

Otherwise convince us that time travel can exist.

Or let us assume you created alternate physics even if you didn't mean to.

On 17 veebr, 22:25, thro...@sheol.org (Wayne Throop) wrote:
> : Crown-Horned Snorkack <chornedsnork...@hush.ai>
> : If you have multiple frames where FTL could occur, how do you avoid
> : time travel?
>
> Is that the goal now? Well, OK, but note that your originnal question
> was
> If the roundtrip time between A and B by FTL is exactly zero, and
> signals sent A to B at FTL and back B to A arrive after a finite
> time, how do you avoid a preferred frame?
>
> which doesn't involve avoiding time travel. But if that's the goal,
> then the usual way is some mechanism embodying the so-called
> chronological protection conjecture.

Ah, I see.

Often illustrated by wormholes;
> you can connect any two spacelike separated events with wormholes,
> so there are "multiple frames where FTL could occur", but if you try
> to move your wormhole ends to have a timelike separation (or indeed
> establish any network of wormholes with a closed timelike curve), you
> must first have a closed lightlike curve, and that would collapse your
> wormhole network. There are reasons to suppose this would indeed occur.
>
> So. You "avoid timetravel" because if you try it, your FTL widdget
> implodes before you accomplish it. (Note that this is not the same issue
> Niven raised in "theory and practice of time travel", and turned into
> a short story, but it has much the same effect).

So... you have a pair of wormholes whose ends have spacelike
separation, like one end (A) at Earth and the other (B) 3 years in
past on Alpha Centauri - getting through wormhole and back to Earth at
lightspeed takes 1,3 years.

Now, you have another pair of wormholes whose ends also have spacelike
separation - like C near Earth and D 3 years in future 4,3 light years
from Earth, but in a direction different from Alpha Centauri, such
that B is more than 6 light years away from B.

Now you move D. Does it mean that when D reaches 6 light years from B,
A, B, C and D all collapse? Or just C and D?

And that the otherwise unexplained collapse of A and B proves that the
otherwise unknown wormholes C and D must have existed and been moved?

Chronological protection conjecture does, in any case, look more
complex that the existence of an unique preferred frame for all FTL
travel, which would easily assure chronological protection.

: Crown-Horned Snorkack <chornedsnorkack@hush.ai>
: So... you have a pair of wormholes whose ends have spacelike
: separation, like one end (A) at Earth and the other (B) 3 years in
: past on Alpha Centauri - getting through wormhole and back to Earth at
: lightspeed takes 1,3 years.
:
: Now, you have another pair of wormholes whose ends also have spacelike
: separation - like C near Earth and D 3 years in future 4,3 light years
: from Earth, but in a direction different from Alpha Centauri, such
: that B is more than 6 light years away from B.
:
: Now you move D. Does it mean that when D reaches 6 light years from
: B, A, B, C and D all collapse? Or just C and D?

If I'm tracking what you say correctly, all four collapse. If you thread
a photon on a closed loop, it goes through all four wormhole ends, and
every wormhole on that path collapses.

: And that the otherwise unexplained collapse of A and B proves that the
: otherwise unknown wormholes C and D must have existed and been moved?

If there were nothing else that could collapse a wormhole, perhaps so.
But I presume there are other things, such as trying to stuff too much
mass into one end, that could do the trick.

: Chronological protection conjecture does, in any case, look more
: complex that the existence of an unique preferred frame for all FTL
: travel, which would easily assure chronological protection.

True, from an "explain why and how it works" point of view.
But in terms of "coming up with new mechanisms" point of view,
the prefered frame is more speculative. There are already some
reasons to suppose lightlike pathways must collapse.


Wayne Throop throopw@sheol.org http://sheol.org/throopw

On Mon, 18 Feb 2008 09:05:23 -0800 (PST), Crown-Horned Snorkack
<chornedsnorkack@hush.ai> wrote:

>On 17 veebr, 22:25, thro...@sheol.org (Wayne Throop) wrote:
>> : Crown-Horned Snorkack <chornedsnork...@hush.ai>
>> : If you have multiple frames where FTL could occur, how do you avoid
>> : time travel?

>> Is that the goal now? Well, OK, but note that your originnal question
>> was
>> If the roundtrip time between A and B by FTL is exactly zero, and
>> signals sent A to B at FTL and back B to A arrive after a finite
>> time, how do you avoid a preferred frame?

>> which doesn't involve avoiding time travel. But if that's the goal,
>> then the usual way is some mechanism embodying the so-called
>> chronological protection conjecture.

>Ah, I see.

> Often illustrated by wormholes;
>> you can connect any two spacelike separated events with wormholes,
>> so there are "multiple frames where FTL could occur", but if you try
>> to move your wormhole ends to have a timelike separation (or indeed
>> establish any network of wormholes with a closed timelike curve), you
>> must first have a closed lightlike curve, and that would collapse your
>> wormhole network. There are reasons to suppose this would indeed occur.

>> So. You "avoid timetravel" because if you try it, your FTL widdget
>> implodes before you accomplish it. (Note that this is not the same issue
>> Niven raised in "theory and practice of time travel", and turned into
>> a short story, but it has much the same effect).

>So... you have a pair of wormholes whose ends have spacelike
>separation, like one end (A) at Earth and the other (B) 3 years in
>past on Alpha Centauri - getting through wormhole and back to Earth at
>lightspeed takes 1,3 years.

>Now, you have another pair of wormholes whose ends also have spacelike
>separation - like C near Earth and D 3 years in future 4,3 light years
>from Earth, but in a direction different from Alpha Centauri, such
>that B is more than 6 light years away from B.

>Now you move D. Does it mean that when D reaches 6 light years from B,
>A, B, C and D all collapse? Or just C and D?

If D actually reaches six light-years from B (and all numbers cited are
absolutely precise), then A,B,C,and D all collapse due to instantaneous,
infinite virtual-quantum-fluctuation feedback.

In practice, I suspect actual wormholes would collapse due to finite
feedback when D is 6+epsilon light-years from B. And if one wormhole
pair is more resilient to collapse than another, it could be that the
"epsilon" associated with an A-B collapse is smaller than that for a
C-D collapse, and so C and D collapse first.

Or vice versa, and this would have nothing to do with which wormhole
you think you are moving. Instead, it would be a function of things
like the wormhole throat cross-section and the ammount of excess
negative-energy bracing, and anything more than WAG handwaving is
beyond my Math-fu at the moment.


>Chronological protection conjecture does, in any case, look more
>complex that the existence of an unique preferred frame for all FTL
>travel, which would easily assure chronological protection.

Right. What you'd wind up with, is a manifold which locally divides
FTL-possible and FTL-impossible reference frames, varying in time and
space such that things like "wormhole to a point 4.3 light-years to
galactic North and 3 years into the local past" is possible in some
places and not in others.

And this manifold is purely an artifact of wormhole construction or
other FTL activity. If you're the first one on the block with a
spiffy new warp drive, you can fire it up for its first trip in any
direction at any speed you like.

Just be careful planning your return trip. And note that "first"
and "block" can be globally ambiguous, so you'll want to coordinate
with your neighbors before making any long trips. Indeed, you may
want to get together with your neighbors and hire a traffic cop to
non-destructively regulate such activity. Speeding tickets can be
unpleasant, but much less so than spontaneously-exploding starships.


--
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