On 2/25/2012 3:46 AM, Warp wrote:
>  even air resistance is virtually inexistent because the rod is
> perfectly vertical and falling straight down

At high speeds this won't necessarily hold due to the viscosity of the 
air.  A common, and generally accurate, assumption in fluid dynamics is 
that fluid velocity along a surface is equal to the velocity of the 
surface, and thus the viscosity of the air would lead to potentially 
significant drag and the rod would almost certainly have a terminal 
velocity well below relativistic speeds.


>    Where is all this energy coming from? Clearly the portal technology itself
> must supply this energy in order to transfer matter from one portal to
> another, and it was supplying all these kilotons to the rod when it was
> falling down. But where does the portal gun get all this energy from?
>

I've always wanted the answer to be that the power is supplied by the 
walls, which is why you can't put a portal on all of the walls. 
Unfortunately this seems to be inconsistent with some bits in the game, 
so I can't think of an answer better than technobabble magic.

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On 2/26/2012 7:07 AM, clipka wrote:
> Am 25.02.2012 12:46, schrieb Warp:
>>
>> The rod will start falling.
>
> Why should it? Downward motion won't put the endless rod into a state of
> lower potential energy, so it will not happen.

What you say is of course true, but there are so many issues raised with 
portals and potential energy (or, in general, the interaction of portals 
with fields) that I'm inclined to throw up my hands and just assume that 
things somehow work like you'd expect in the "Newtonian physics" sense 
and just go from there.


> To the contrary, every coaxial acceleration of the rod would impose an
> ever so slight length contraction due to relativistic effects, leading
> to buildup of internal stress (and hence internal energy - not sure how
> the expert would call this type), while deceleration would reduce the
> internal stress, so deceleration is likely to happen spontaneously while
> acceleration would require external energy input (and I mean energy
> input, not just some force). So even if the rod was falling in the first
> place, given sufficient time its motion will actually /stop/.
>
> (You /could/ force it into motion by heating it up though: As the
> material would try to expand in all directions, again internal stress
> would be induced, and as any increase in speed - whether up- or
> downwards - would reduce this stress due to length contraction, a tiny
> push /would/ eventually get it up to relativistic speed - provided the
> rod doesn't melt long before due to air friction. Or at least that's how
> the thought experiment goes - maybe there's a flaw in it as well.)

There's one bit of your reasoning that I don't follow.  If you put 
yourself in the rod's reference frame, then the velocity of the rod does 
nothing to change the internal stresses, so whence the acceleration?  It 
seems like you're calculating the elastic energy of the rod in purely in 
rest coordinates without modeling how elastic energy behaves in 
relativistic settings.  I think this is probably most clearly shown by 
that fact that your argument doesn't obey conservation of momentum. 
Note, however, that I'm pretty weak on relativistic physics and I 
haven't calculated any stress-energy tensors or anything for this, so I 
might be wrong.

In addition, the argument you've stated would seem to apply equally well 
to inducing a rotation in a ring.  It's possible there's some subtle 
reason why a ring is different than a linear rod here, but I suspect 
it's an indication that the original analysis doesn't really work.

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On 2/27/2012 9:49 AM, Warp wrote:
>    I haven't done the math but I assume that if you had a manageable mass
> as a black hole, such as 1 kilogram, it would evaporate in microseconds
> or faster (and still release a rather big amount of energy).

Much faster actually, I believe it'd evaporate in a fraction of a 
femtosecond.  As you say, there's pretty much nothing which could 
possibly be done to make that anything other than an extremely powerful 
bomb.
>
>    Also if the black hole is not kept constantly charged it could escape
> the magnetic field and fall to the ground, and again it would explode
> (although this time probably somewhere inside the Earth).

I read a science fiction book once with a somewhat similar premise (only 
the black hole was larger and non-manmade -- still charged though).  It 
was an interesting read partially because the author took some obvious 
pains to get the science at least largely correct.  If you listen to 
audiobooks, you can listen to it for free: 
http://www.podiobooks.com/title/singularity I found it pretty enjoyable, 
even if not particularly exceptional.

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Kevin Wampler <wam### [at] uwashingtonedu> wrote:
> I've always wanted the answer to be that the power is supplied by the 
> walls, which is why you can't put a portal on all of the walls. 
> Unfortunately this seems to be inconsistent with some bits in the game, 
> so I can't think of an answer better than technobabble magic.

  I think there has never been an explanation attempt at why you can make
portals only on some surfaces but not others. Seems like the rule is "if
the surface is white and smooth, then it's portal-friendly, else it isn't".
No explanation why. (Smoothness could perhaps be handwaved, but not the
whiteness. There certainly are many surfaces in both games that are smooth
but non-white, and you can't put portals on them.)

-- 
                                                          - Warp

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On 2/27/2012 9:28 AM, Warp wrote:
> clipka<ano### [at] anonymousorg>  wrote:
>> That's /one/ way to interpret the observable effects of gravity, but the
>> force of gravity is not necessarily the /cause/ of the movement, but may
>> instead be just the result of trying to /counter/ that movement.
>> Remember how inertia is not caused by some force of inertia, but instead
>> a force of inertia is caused by trying to counter the inertial movement.
>
>    I don't understand.
>
>    At every single point in the rod you have an acceleration downwards of
> about 9.8 m/s^2 due to gravity.
>
>    What exactly is the force in the opposite direction counteracting this?

I don't think it's any more odd to have a mysterious "force" holding the 
rod up than it is to have a mysterious energy input allowing it to fall. 
  You're really just giving slightly different answers to the same 
paradox.  Which answer is the most "natural" depends on how you treat 
gravity, and formulations of gravity which are mathematically isomorphic 
under normal circumstances seem to differ here.  For instance:

1) An object of mass m in a gravitational field of acceleration g feels 
a force of strength m*g, and by Newton's law moves according to m*g = 
m*dx^2/d^2x

2) An object of mass m in a gravitational field which gives potential 
m*g*x to an object at position x moves along a spacetime path which 
minimizes 0.5*m*(dx/dt)^2 - m*g*h

Under view (1) it seems that the rod should move, but under view (2) it 
seems the rod should stay still.  Of course, based on how portals work 
in the game I think it's clear that things operate more like (1), but 
this is just a fact about the game design and isn't the sort of thing 
you're going to be able to reason about like there's a correct answer 
for in any sort of "real" sense (unless you want to treat the portals as 
wormholes and solve the GR equations on the resulting topology, which I 
sure don't want to do, and which I suspect would turn out like (2) anyway).

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On Tue, 28 Feb 2012 10:38:27 -0500, Warp wrote:

> Kevin Wampler <wam### [at] uwashingtonedu> wrote:
>> I've always wanted the answer to be that the power is supplied by the
>> walls, which is why you can't put a portal on all of the walls.
>> Unfortunately this seems to be inconsistent with some bits in the game,
>> so I can't think of an answer better than technobabble magic.
> 
>   I think there has never been an explanation attempt at why you can
>   make
> portals only on some surfaces but not others. Seems like the rule is "if
> the surface is white and smooth, then it's portal-friendly, else it
> isn't".
> No explanation why. (Smoothness could perhaps be handwaved, but not the
> whiteness. There certainly are many surfaces in both games that are
> smooth but non-white, and you can't put portals on them.)

I think the whiteness is nothing more than a visual cue for the player.

Jim

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On 2/27/2012 4:08 AM, Le_Forgeron wrote:
> Le 27/02/2012 01:23, Darren New a écrit :
>> Unless you're assuming the power is not coming from the gun itself.
>
>
> Or, if you opened the gun, you would find a set of miniportals with
> their infinite rods falling all over again and again.

I actually really like this theory that the portal gun is powered by 
internal portals.  The paradoxical chicken and egg aspect seems like it 
would fit well within the humor style of the game.

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Kevin Wampler <wam### [at] uwashingtonedu> wrote:
> I actually really like this theory that the portal gun is powered by 
> internal portals.  The paradoxical chicken and egg aspect seems like it 
> would fit well within the humor style of the game.

  Actually a phenomenon feeding itself isn't all that paradoxical.

  For example stars work via fusion, and the fusion keeps the star doing
the fusion. It's an endless loop (which only stops when there's too much
iron, which does not fuse, in the core of the star).

  Or more precisely: What stops a star from collapsing due to gravity?
The continuous fusion that happens in its core. But what causes the fusion?
Gravity (and temperature, which the fusion itself produces). So the fusion
itself allows the fusion to continue.

-- 
                                                          - Warp

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On 2/27/2012 4:08, Le_Forgeron wrote:
> Or, if you opened the gun, you would find a set of miniportals with
> their infinite rods falling all over again and again.

I think that's the most meta concept I've heard in a year.

-- 
Darren New, San Diego CA, USA (PST)
   People tell me I am the counter-example.

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On 2/28/2012 7:38, Warp wrote:
>    I think there has never been an explanation attempt at why you can make
> portals only on some surfaces but not others. Seems like the rule is "if

In the first game, the walls you could portal were tiles and the walls you 
couldn't were clearly metallic. I.e., it was evident it was due to the 
material of which the walls were made.

In the second game, you have the same sort of thing, except with moon rocks 
too, which strengthens that argument a bit.

-- 
Darren New, San Diego CA, USA (PST)
   People tell me I am the counter-example.

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