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I wonder how the fact that teleportation is revealed to be limited by the
speed of light affects this, tho.
--
Darren New, San Diego CA, USA (PST)
People tell me I am the counter-example.
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Am 26.02.2012 17:35, schrieb Warp:
> clipka<ano### [at] anonymous org> wrote:
>> Why should it?
>
> Because gravity points down at all points in the rod. There is no force
> in the opposite direction counteracting gravity.
>
> If the rod were not welded to itself, it would definitely fall (until
> it collides with the borders of the portal or whatever), no? Welding it
> to itself doesn't change this. (Basically the only thing the welding
> achieves is that it stops the rod from tilting.)
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.
The movement due to gravity may instead be a result of quantum
fluctuations in the position of the object, which favor positions with
lower potential energy.
>> Downward motion won't put the endless rod into a state of
>> lower potential energy, so it will not happen.
>
> The rod is not literally endless. It's just that the parts that go
> thrown the portal on the floor are transported to the portal on the ceiling.
> (From a physics point of view this would mean that the energy required to
> transport matter adds to the potential energy of said matter if the endpoint
> is higher than the startpoint.)
Actually that's the one weak point of all the portal stuff: Rather than
allowing objects to fall through without a change in speed, a real
working portal technology would have to result in a repelling force at
the lower portal itself, proportional to the difference in potential energy.
So no automatic "speedy thing goes in, speedy thing comes out".
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Am 26.02.2012 18:56, schrieb Darren New:
> I wonder how the fact that teleportation is revealed to be limited by
> the speed of light affects this, tho.
... or is it?
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On 2/26/2012 15:21, clipka wrote:
> Am 26.02.2012 18:56, schrieb Darren New:
>> I wonder how the fact that teleportation is revealed to be limited by
>> the speed of light affects this, tho.
>
> ... or is it?
Have you played Portal 2?
Answer: Yes, the developers say so in the commentary. :-)
--
Darren New, San Diego CA, USA (PST)
People tell me I am the counter-example.
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On 2/26/2012 15:20, clipka wrote:
> The movement due to gravity may instead be a result of quantum fluctuations
> in the position of the object, which favor positions with lower potential
> energy.
That's kind of a non-answer, tho, given that the potential energy is indeed
mass.
> Actually that's the one weak point of all the portal stuff: Rather than
> allowing objects to fall through without a change in speed, a real working
> portal technology would have to result in a repelling force at the lower
> portal itself, proportional to the difference in potential energy.
Unless you're assuming the power is not coming from the gun itself. I would
think anything powered by a black hole could probably generate as much power
as it needs to via hawking radiation. Certainly as much power as we've seen
it use.
--
Darren New, San Diego CA, USA (PST)
People tell me I am the counter-example.
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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.
That will answer "where does the infinite energy come from, for the new
portal." (now, it's a chicken & egg... how was the first portal done...
maybe with some failed experiment in time travel or space travel)
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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?
--
- Warp
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Darren New <dne### [at] sanrrcom> wrote:
> Unless you're assuming the power is not coming from the gun itself. I would
> think anything powered by a black hole could probably generate as much power
> as it needs to via hawking radiation.
You can't use a black hole to generate energy because you have no way
of controlling the amount of energy it produces.
Rather ironically, the smaller the black hole, the more energy it produces
(assuming the Hawking radiation hypothesis is true) and the faster it
evaporates, and very small black holes produce enormous amounts of energy
and evaporate extremely fast.
For example, a black hole with a mass of 200 thousand kilograms would
evaporate in just 1 second and release an amount of energy equivalent
to 5 million megatons of TNT. (The largest hydrogen bomb ever detonated
was "only" 50 megatons.)
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).
In order to keep the black hole at a constant mass and producing energy
you would need to feed it matter continuously. At the rate of evaporation
of a 1-kilogram black hole I wouldn't be surprised if you had to feed it
matter faster than c in order to keep it from evaporating.
A much larger black hole takes more time to evaporate and releases less
energy. The problem now becomes how to handle that black hole and stop it
from plummeting towards the center of the Earth. A black hole (probably)
can have an electromagnetic charge, which means you could keep it hovering
inside a magnetic field. But we are talking about millions and millions of
tons here. The magnetic field required to keep a mass of that size hovering
is probably so strong that it would melt everything around it (including
the hardware used to create the magnetic field in the first place).
Of course with such massive black hole you also get the problem of its
gravity becoming a significant hazard factor close to it. It would have to
be kept hovering quite far away from all other objects, and in a vacuum
(else it would constantly suck atmosphere).
Then we have the problem of what to do with the excess energy. Not only
do you have to feed the black hole a constant stream of matter to stop it
from evaporating in a multi-million-megaton explosion, but you also have
to do something with the energy that it's producing. It has to go somewhere.
And there's a lot of it. (Basically you are converting matter into pure
energy, and matter has incredible amounts of it.)
The hazards of having such a source of energy are quite great. You only
need a small disruption in the inflow of matter and you could end up with
a runaway evaporation that you cannot stop, and which would make the Tsar
Bomba look like a firecracker.
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).
And this assuming you can produce such a black hole in the frist place...
--
- Warp
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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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