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Old 13th November 2012, 01:13 PM   #41
Squeegee Beckenheim
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Originally Posted by ben m View Post
I propose a perpetual motion machine: you will haul a heavy weight to the top of Pike's Peak via the tunnel, then roll it back down via the long main road. Is that a source of infinite energy? No.
Okay, that makes sense, mostly. What I'm not sure about is how the force of gravity acts upon the ball while it's in the wormhole.
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Old 13th November 2012, 01:15 PM   #42
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Originally Posted by Loss Leader View Post
Do wormholes lack mass?
Yeah, they're literally holes.
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Old 13th November 2012, 01:20 PM   #43
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Originally Posted by Squeegee Beckenheim View Post
Well, firstly, Cuddles didn't say anything whatsoever about wormholes requiring energy to remain open.
Indeed he did not, because that's not the problem.

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And, secondly, I've already addressed what I see as problems with Cuddles' explanation and am waiting for a reply to those. It doesn't have to be Cuddles who addresses them, you can if you like.
Alright. First off, a worm hole doesn't need to have an event horizon. The idea that it does comes from the fact that the first mathematical wormhole solution found was essentially a black hole. But that kind of wormhole will necessarily collapse. Traversable wormholes do not have event horizons, and can be crossed in either direction.

Second, your objection about the energy requirements for travel are wrong. Let me preface that by saying that the energy we're talking about here is assuming perfect efficiency. In real life, we never come close to that, and that makes a giant difference to our intuitive understanding. For example, the energy required to travel from LA to New York is basically zero: they're both at the same gravitational potential (sea level), so the only energy we actually expend is used to overcome friction, which in principle could all be eliminated. So if you had a frictionless railway with perfectly efficient regenerative brakes, you could spend energy accelerating and then recapture all of that energy decelerating on your trip from LA to New York. But a trip from LA to Denver would always cost energy since Denver is higher.

Now, your wormhole doesn't help you with overcoming gravitational potential differences. All it does is shorten the path. For space travel that's a big friggin' deal, because 1) the time scales really matter, 2) space travel is really inefficient with current and foreseeable technology, and 3) efficiency gets worse the faster you try to go. So if you can make the trip take less time, you can save a hell of a lot of energy. But you're shaving it off from the inefficiency of space travel, making it less inefficient, but you aren't getting free energy from anywhere. But if you're trying to generate energy well, there's no energy to be had. Both routes (through ordinary space or through the wormhole) will experience the same gravitational potential difference, regardless of the path you choose.
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Old 13th November 2012, 02:04 PM   #44
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Originally Posted by Ziggurat View Post
Now, your wormhole doesn't help you with overcoming gravitational potential differences. All it does is shorten the path. For space travel that's a big friggin' deal, because 1) the time scales really matter, 2) space travel is really inefficient with current and foreseeable technology, and 3) efficiency gets worse the faster you try to go. So if you can make the trip take less time, you can save a hell of a lot of energy. But you're shaving it off from the inefficiency of space travel, making it less inefficient, but you aren't getting free energy from anywhere. But if you're trying to generate energy well, there's no energy to be had. Both routes (through ordinary space or through the wormhole) will experience the same gravitational potential difference, regardless of the path you choose.
This brings up an interesting question, slightly off-topic.

So if it's the difference in gravitational potential that matters, does that mean you could get "free" travel if you wormhole (for example) connected from Earth's surface (at 1G) to the surface of an alien world (also at 1G)? In other words, is it the difference in potential that matters, or would you have to leave one gravity well and eneter another?

In your moutain example, would we be digging a tunnel or building a bridge?
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Old 13th November 2012, 02:20 PM   #45
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Originally Posted by Hellbound View Post
This brings up an interesting question, slightly off-topic.

So if it's the difference in gravitational potential that matters, does that mean you could get "free" travel if you wormhole (for example) connected from Earth's surface (at 1G) to the surface of an alien world (also at 1G)? In other words, is it the difference in potential that matters, or would you have to leave one gravity well and eneter another?

In your moutain example, would we be digging a tunnel or building a bridge?
Minor quibble: the *strength* of gravity doesn't tell you the depth of the potential. The potential well depth at the Earth's surface, where the acceleration is 1g, is not the same as the potential well depth at the surface of a (hypothetical) 3000-km-radius ball of lead, which would have the same 1-g acceleration.

I think I parse your question as: let's assume we're traveling between two points at the same potential, say Earth and Counter-Earth. Normal travel requires us to climb waaaay up (to escape Earth) and *then* fall waaay down (to enter Counter-Earth), both of which are technologically difficult, not that GR cares about that. Would wormhole travel look like "up then down", or like "down then up", or would it actually look like a flat, zero-gravity traverse---the latter being technologically easy? You know, I can vaguely imagine it either way, and you'd have to actually specify the geometry of this wormhole in order to tell the difference.
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Old 13th November 2012, 02:25 PM   #46
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Originally Posted by Squeegee Beckenheim View Post
I'm sure it would take a lot of energy to create a wormhole. I'm not sure it'd take any to maintain it, although I'll defer to anyone with greater knowledge of the physics than me on that point.
Wormholes are theory. We don't know if they exist, but more importantly, we don't know what their physics are.

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But the point is that this machine could theoretically run until the universe ended or the gravity source was destroyed. And there's no reason why you have to restrict your thinking to a small metal ball or just the one, or even a metal ball at all. The physical apparatus is an illustration, really, of the fact that when the ball moves from the bottom of the tube to the top that it gains potential energy for seemingly no cost.
Highlighted the crunch: That the wormhole omves the ball to the top of the tube at no energy cost is a putative property of a theoretical phenomenon. I might as well assume that the cost of moving the ball is exactly equivalent to the potential energy gain.

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Can we know that there's no way that such a device could make back the energy it'd take to create the wormhole and more?
No. That is another putative property of wormholes: That they might circumvent conventional physics, like thermodynamics.

It is speculation. Perhaps interesting speculation, but .... speculation.

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Old 13th November 2012, 02:35 PM   #47
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M.C. Escher knew how to draw a good hydro-electric power plant.
First you fool the eye; then you fool the investors.

There is, seemingly, an inexhaustible supply of chumps to exploit for power and money.
This may be as close as we get to a perpetual engine.
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Old 13th November 2012, 03:59 PM   #48
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Originally Posted by Hellbound View Post
This brings up an interesting question, slightly off-topic.

So if it's the difference in gravitational potential that matters, does that mean you could get "free" travel if you wormhole (for example) connected from Earth's surface (at 1G) to the surface of an alien world (also at 1G)? In other words, is it the difference in potential that matters, or would you have to leave one gravity well and eneter another?

In your moutain example, would we be digging a tunnel or building a bridge?
I've been wondering this as well.

Let's say that we've got 2 tubes on 2 different but identical planets, on opposite sides of the galaxy. The hole at the bottom of tube A connects to the top of tube B, and the bottom of tube B connects to the top of tube A. Otherwise it's exactly the same set-up.

Do we still encounter the same problem of having to overcome gravity?
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Old 13th November 2012, 04:01 PM   #49
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Originally Posted by MRC_Hans View Post
Wormholes are theory. We don't know if they exist, but more importantly, we don't know what their physics are.
So, then, the people in this thread who have been telling me objections based on physics may not be correct? If so, that rather takes it back to Miles' point.
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Old 13th November 2012, 05:50 PM   #50
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Originally Posted by Squeegee Beckenheim View Post
So, then, the people in this thread who have been telling me objections based on physics may not be correct? If so, that rather takes it back to Miles' point.
Apparently he uses 'wormholes' to mean 'magic'.
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Old 13th November 2012, 06:06 PM   #51
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Old 13th November 2012, 06:49 PM   #52
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Originally Posted by Squeegee Beckenheim View Post
So, then, the people in this thread who have been telling me objections based on physics may not be correct? If so, that rather takes it back to Miles' point.
We have a picture of wormholes that come up in the following procedure.

a) Imagine hypothetical types of matter that obey certain basic laws of physics. b) Imagine engineer them (using unspecified technology) in ways that obey all known laws of physics.
c) Apply GR to the resulting matter and energy.

Bang, that's a wormhole. This kind of wormhole obviously and completely forbids the "free energy" waterwheels you're discussing.

If you want to throw out some part of the above---like "hey, let's use something other than GR to do part C"---well, you're no longer talking about the same thing scientists are talking about when they say "wormhole". You're no longer talking about the thing scientists mean when they say "wormholes can exist and we study them using physics theory". Instead, you're talking about something made up in sci-fi, with whatever properties the author wants. That's fun sometimes, but that's not how wormholes work.
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Old 13th November 2012, 09:48 PM   #53
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Originally Posted by Squeegee Beckenheim View Post
Okay, that makes sense, mostly. What I'm not sure about is how the force of gravity acts upon the ball while it's in the wormhole.
It does not matter in any way.

As soon as the ball is on top of the machine and ready to fall down to the wheel it has more potential energy than it had when it entered the wormhole at the bottom. And this energy must come from somewhere.

Basically there are several possiilities:

1) there is an energy source inside the wormhole and the source is powered from the energy source which maintains the wormhole. In this case we have a third order PM

2) the wormhole takes energy from its inside, which may be in a different universe or at the other end of the universe or in the mass which forms the wormhole. Again we have a third order PM

3) the ball must enter the wormhole with enough kinetic energy to reach the top of the machine and this ehergy is at least m*g*h where h is the difference in height between top and bottom of the machine

Besides that - even teleportation or simple magic would not work. They both must also in some way add potential energy to the ball - and be it chemical energy from the teleporters lunch or the magicians liver

This is not just a problem with this machine, which is nothing but an obfuscated Bessler Wheel. Maxwlles demon stumbles over the same problem as well as all known attempts to build a second order PM. They all try to violate the first (not only the second) law of thermodynamics.

Just my $0,02
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Old 14th November 2012, 02:46 AM   #54
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Originally Posted by Squeegee Beckenheim View Post
So, then, the people in this thread who have been telling me objections based on physics may not be correct? If so, that rather takes it back to Miles' point.
Only if Larry's point is that he can make stuff up.
Which we know he can do.
I've read some of it.
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Old 14th November 2012, 05:00 AM   #55
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Originally Posted by Squeegee Beckenheim View Post
Well, firstly, Cuddles didn't say anything whatsoever about wormholes requiring energy to remain open. And, secondly, I've already addressed what I see as problems with Cuddles' explanation and am waiting for a reply to those. It doesn't have to be Cuddles who addresses them, you can if you like.
Originally Posted by Squeegee Beckenheim View Post
It gains the same amount of potential energy, but it certainly costs more energy to take it to the moon and back.



So would that mean that it'd get stuck in the wormhole? Because it wouldn't have enough kinetic energy to get back to the top were there, say, a ramp at the bottom. So if it necessarily would take the same amount of kinetic energy to get to the top again, then it couldn't reach the top. The gravity would pull it in to the wormhole (unless wormhole event horizons block gravity), and it'd travel a little way in to the wormhole through momentum, but couldn't reach the top, as there's not enough energy to carry it there.

I'm not entirely sure I see how this works. I can't believe that it must cost the same amount of energy to make a journey with a wormhole as it does without. Wormholes are often talked about with a view to travelling interstellar distances - from galaxy to galaxy, even. If there was a wormhole just the other side of the moon which led to the opposite side of the universe, would it really take as much energy to travel through it as it would to just point a rocket at the other side of the universe and let it travel on for trillions upon trillions of years? That doesn't make sense to me.
Err no. Disbelief is not addressing an argument.
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Old 14th November 2012, 06:22 AM   #56
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Originally Posted by ben m View Post
We have a picture of wormholes that come up in the following procedure.

a) Imagine hypothetical types of matter that obey certain basic laws of physics. b) Imagine engineer them (using unspecified technology) in ways that obey all known laws of physics.
c) Apply GR to the resulting matter and energy.

Bang, that's a wormhole. This kind of wormhole obviously and completely forbids the "free energy" waterwheels you're discussing.
I'm a bit fuzzy on the last part. Can you show that?
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Old 14th November 2012, 07:20 AM   #57
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Squeegee Beckenheim

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I'm not entirely sure I see how this works. I can't believe that it must cost the same amount of energy to make a journey with a wormhole as it does without. Wormholes are often talked about with a view to travelling interstellar distances - from galaxy to galaxy, even. If there was a wormhole just the other side of the moon which led to the opposite side of the universe, would it really take as much energy to travel through it as it would to just point a rocket at the other side of the universe and let it travel on for trillions upon trillions of years? That doesn't make sense to me.
No. To travel from here to the other end of the galaxy does not cost any enrgy if your destination point is at the same horizon of potential energy as the starting point. The energy you need if you travel by a rocket ist totally wasted because we have no means of getting the energy which we invested during acceleration back, thus we blow twice the energy used for acceleration in form of heat into space.

If we travel through a (ideal) wormhole there is no need for acceleration and deceleration (starting and ending point are identical) and the only energy needed is to overcome the difference in potential energy between the starting and destination point.
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Old 14th November 2012, 08:20 AM   #58
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Originally Posted by ben m View Post
Minor quibble: the *strength* of gravity doesn't tell you the depth of the potential. The potential well depth at the Earth's surface, where the acceleration is 1g, is not the same as the potential well depth at the surface of a (hypothetical) 3000-km-radius ball of lead, which would have the same 1-g acceleration.

I think I parse your question as: let's assume we're traveling between two points at the same potential, say Earth and Counter-Earth. Normal travel requires us to climb waaaay up (to escape Earth) and *then* fall waaay down (to enter Counter-Earth), both of which are technologically difficult, not that GR cares about that. Would wormhole travel look like "up then down", or like "down then up", or would it actually look like a flat, zero-gravity traverse---the latter being technologically easy? You know, I can vaguely imagine it either way, and you'd have to actually specify the geometry of this wormhole in order to tell the difference.
Yes, I'm glad you understood what I meant. Nice to know my lack of knowledge is not an impedement to removing itself .

If I'm understanding you correctly, it's the geometry of the wormhole that would make the difference. So, that geometry is obviously determined by GR and the relevent theories. But is it static (i.e.-any wormhole from point a to point b is going to be shaped like x) or dynamic (i.e.-if you make it like so, it will be x, but if you make it this way it's y)? Or do we even know enough to make an educated guess on that?
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Old 14th November 2012, 08:39 AM   #59
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Squeegee: I know why this doesn't work. It's because the kinetic energy of a falling body comes from that falling body. If you raise a ball, you do work on it. You give it gravitational potential energy. This is in the ball, not in the gravitational field. You can work this out by firing a ball upwards so fast that it's got escape velocity. The kinetic energy of the ball is gradually converted into gravitational potential energy, and as the ball departs into space it takes the kinetic/potential energy with it. The Earth's mass is then reduced a little, as is the Earth's gravitational field.

When you drop a ball, the gravitational potential energy within it is converted into the macroscopic kinetic energy of the falling ball which drives the waterwheel. Even with an idealised wormhole, the energy you're extracting is coming from the ball. The first thing you will notice is that it goes cold. If you keep cycling it round, its mass has to be reducing. In the end the particles have to start breaking down. The electron has a minimum energy of around 511keV; an electron can't be an electron at 411keV. Then you might find that this device isn't a perpetual motion machine at all. Instead, it's a bomb.

Last edited by Farsight; 14th November 2012 at 08:55 AM. Reason: Removed reference to Bose-Einstein condensate
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Old 14th November 2012, 09:00 AM   #60
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Originally Posted by Hellbound View Post
This brings up an interesting question, slightly off-topic.

So if it's the difference in gravitational potential that matters, does that mean you could get "free" travel if you wormhole (for example) connected from Earth's surface (at 1G) to the surface of an alien world (also at 1G)? In other words, is it the difference in potential that matters, or would you have to leave one gravity well and eneter another?

In your moutain example, would we be digging a tunnel or building a bridge?
Think of it this way. You have two cities on opposite sides of a mountain range, a road over the mountains, and a tunnel through the mountains.

The road over the mountain is like normal space travel. You have to expend a lot of energy getting up the mountain, which is like climbing out of the gravity well of earth. The tunnel is like a wormhole, in that you can coast through it as long as it's level and both cities are the same altitude.

The machine in the OP would be like a tunnel to the top of the mountain. Sure, it might be a shorter route than the road, but the change in elevation, and therefore potential energy, is the same either way,
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Old 14th November 2012, 09:01 AM   #61
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I think Farsight's more than a bit mistaken again. You can't move something on a closed path through a gravitational potential and have it end up at a different potential energy than it started with. If you keep cycling it round, it will end up the same at the end of the loop as at the start. Particles won't break down or any such thing.
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Old 14th November 2012, 09:15 AM   #62
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Originally Posted by Farsight View Post
Squeegee: I know why this doesn't work. It's because the kinetic energy of a falling body comes from that falling body. If you raise a ball, you do work on it. You give it gravitational potential energy. This is in the ball, not in the gravitational field.
Actually, it's not in the ball, it's in the whole ball / earth / gravitational field system.

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You can work this out by firing a ball upwards so fast that it's got escape velocity. The kinetic energy of the ball is gradually converted into gravitational potential energy, and as the ball departs into space it takes the kinetic/potential energy with it. The Earth's mass is then reduced a little, as is the Earth's gravitational field.
When you fire a ball upwards above escape velocity, it doesn't take potential energy with it. It takes only the kinetic energy it was fired with, minus some which is lost and added to the earth's kinetic energy.

Quote:
When you drop a ball, the gravitational potential energy within it is converted into the macroscopic kinetic energy of the falling ball which drives the waterwheel. Even with an idealised wormhole, the energy you're extracting is coming from the ball. The first thing you will notice is that it goes cold. If you keep cycling it round, its mass has to be reducing. In the end the particles have to start breaking down. The electron has a minimum energy of around 511keV; an electron can't be an electron at 411keV. Then you might find that this device isn't a perpetual motion machine at all. Instead, it's a bomb.
The first sentence is correct except that the GPE is not contained in the ball, the rest is just nonsense.

By the way, if the machine in the OP worked the way it implies, it would violate the conservation of momentum. It would be a reactionless engine.
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Old 14th November 2012, 09:32 AM   #63
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Originally Posted by phunk View Post
Think of it this way. You have two cities on opposite sides of a mountain range, a road over the mountains, and a tunnel through the mountains.

The road over the mountain is like normal space travel. You have to expend a lot of energy getting up the mountain, which is like climbing out of the gravity well of earth. The tunnel is like a wormhole, in that you can coast through it as long as it's level and both cities are the same altitude.

The machine in the OP would be like a tunnel to the top of the mountain. Sure, it might be a shorter route than the road, but the change in elevation, and therefore potential energy, is the same either way,
How does potential energy work in GR? Can you show how energy is conserved, specifically in the context of some wormhole solution?
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Old 14th November 2012, 09:49 AM   #64
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I think all your points have already been addressed, but just for the record...
Originally Posted by Squeegee Beckenheim View Post
It gains the same amount of potential energy, but it certainly costs more energy to take it to the moon and back.
It actually doesn't. As Ziggurat says, the only reason it costs energy is because of inefficiencies in our technology. Looked at from a purely energetics point of view and ignoring the details of how things happen, a body that starts and ends at specific points gains or loses exactly the same amount of energy. If you start on the floor and end exactly one metre above the same point on the floor, you've gained mg of gravitational potential energy regardless of what route you took to get there. If someone came and examined the rock on the shelf in my example, there would be no possible way for them to find out what path it had taken to get there.

Quote:
So would that mean that it'd get stuck in the wormhole? Because it wouldn't have enough kinetic energy to get back to the top were there, say, a ramp at the bottom. So if it necessarily would take the same amount of kinetic energy to get to the top again, then it couldn't reach the top. The gravity would pull it in to the wormhole (unless wormhole event horizons block gravity), and it'd travel a little way in to the wormhole through momentum, but couldn't reach the top, as there's not enough energy to carry it there.
Something like that. It's not possible to say exactly what would happen without knowing all the details of the wormhole. For example, given a wormhole with zero length (which I'm not sure is possible, but you should be able to get arbitrarily close to zero) it would effectively act as a solid surface. If you hit it with enough energy to overcome the gravitational potential you'll pass through and come out the top, but if you don't quite have enough energy you'll just hit it.

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I can't believe that it must cost the same amount of energy to make a journey with a wormhole as it does without.
As already noted, it takes the same amount of energy to travel up a tunnel as it does to follow a road (again, ignoring inefficiencies which aren't relevant to the fundamental physics). There's nothing special about a wormhole, it's just a shortcut.

Quote:
Wormholes are often talked about with a view to travelling interstellar distances - from galaxy to galaxy, even. If there was a wormhole just the other side of the moon which led to the opposite side of the universe, would it really take as much energy to travel through it as it would to just point a rocket at the other side of the universe and let it travel on for trillions upon trillions of years? That doesn't make sense to me.
Why do you think travelling to the other side of the universe would take a lot of energy? Forget the Moon and lets go with interstellar space where we can effectively ignore gravity. How much energy does it take to get to a similar region of space on the other side of the universe? None. You have to put some energy in to get moving, but then you get that back out at the other end.

As already noted, the reason travel costs energy is because of waste. If we want to travel a long way, we have to spend a lot of time accelerating, and that means wasting a lot of energy while doing so. If you take out all that waste and just look at the pure physics, even long distance travel hardly takes any energy at all. You convert a bit of energy into kinetic energy, then change it back into a different kind of energy again at the destination. Depending on the difference between energy potentials of the start and end points, given some equivalent of regenerative breaking you could end up not actually using up any energy at all. And if you're happy to take a very long time travelling, you hardly even need any kinetic energy either.

Wormholes are a popular idea, as already mentioned, for two reasons. Firstly, we're not willing to spend that long travelling, and secondly we're not capable of actually eliminating inefficiencies. Not only do we have to use a ton of energy overcoming friction and the like, by going fast we need a lot of kinetic energy that we don't have any way to get back again. A wormhole means a shorter trip, so much less waste and therefore less energy. The absolute minimum energy required to make a trip set by the laws of physics doesn't change, just how much we waste on top of that.

Originally Posted by edd View Post
I think Farsight's more than a bit mistaken again.
I'm shocked.
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Old 14th November 2012, 10:29 AM   #65
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So, are these wormholes moving? Or stationary? And if they're stationary, then with reference what?

If you have two wormholes, each on opposite arms of the galaxy, then when you send something through it in, say, the western spiral arm, when it comes out of the other end in, perhaps, an easter spiral arm, isn't it moving at a pretty impressive rate relative to everything else in it's local area. And in the opposite direction?


(ETA - Sorry, total derail while musing about wormholes)
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Old 14th November 2012, 10:32 AM   #66
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"It's really very simple". LOL

It's really oversimplified...
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Old 14th November 2012, 12:44 PM   #67
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Originally Posted by Cuddles View Post
Something like that. It's not possible to say exactly what would happen without knowing all the details of the wormhole. For example, given a wormhole with zero length (which I'm not sure is possible, but you should be able to get arbitrarily close to zero) it would effectively act as a solid surface. If you hit it with enough energy to overcome the gravitational potential you'll pass through and come out the top, but if you don't quite have enough energy you'll just hit it.
Fair enough.
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Old 14th November 2012, 01:27 PM   #68
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I know some stuff about taking short-cuts.

My s.o. claims that they require more energy than the normal, paved road.

I sure hope this thread can prove her wrong.
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Old 14th November 2012, 01:36 PM   #69
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Why it doesn't work. I like Cuddles explanations but I'm going to take an entirely different tact. But first I need to draw a realistic analogy between worm hole physics and more mundane physics.

If you are parked in orbit above Earth and measure the distance a traveler goes to get from their house to grandma's then that distance, from your perspective will be ever so slightly shorter than the distance the traveler measures to grandma's house. This is a result of the traveler being at a greater gravitational depth. The fundamental physics is essentially the same reason the distance through a wormhole is shorter than the distance around it. Note that this is "gravitational depth" rather than curvature, or gravity itself. For instance, inside a uniform hollow massive sphere there is no gravity. Yet the time dilation will remain slowed to match what is on the surface where gravity is at a maximum. From this, from the perspective of someone at higher altitude, we are living in a very low grade distorted wormhole.

The reason you can travel through a wormhole so much faster is not because anything flows, like what is implied by the water wheel analogy. Rather, the distance through the wormhole is merely shorter. So sticking a paddle wheel partly into such a wormhole will have no more effect than sticking that same paddle wheel into a partly into a gravitational potential, such as in orbit. There is simply nothing "flowing" with which to turn the wheel.

On the other hand, you need no wormhole, or gravitational potential, at all to spin your wheel in space. And once you set it to spinning it will never stop, at least until you try to take energy from that spin to use for energy. Or otherwise acted on by an outside force. I suppose you can call the perpetual motion if you want, but tapping it for energy for your use drains the perpetual motion through a process called entropy.

On the theoretical side physicist play with all kinds of variations of "energy condition" assumption. General Relativity provides no energy conditions because it makes no sense to in a general theory, by virtue of it being a 'general' theory. Yet specifying energy conditions in any particular application is required to be taken seriously. Whether we are talking abercrombie drives or wormholes. To many physicist, the energy condition assumptions required to make many of these scenarios work are not very realistic. Instead of trying to explain these various energy condition assumptions, I'll merely describe the consequences giving what some (including me) would consider more realistic energy conditions. Though there are more ways to choose these energy conditions than what I'll cover.

Suppose you created a topology of space that provided an effective wormhole one light year end to end. Yet from the inside the distance through the wormhole was only six light months, such that at any given velocity it would be twice as fast to go through the wormhole than around it. The consequences (energy conditions) that I find most reasonable is that even if the spaceship takes half as long to get through, from their point of view, twice as much time will pass for observers outside the wormhole as did for the spaceship crew. Hence, from a relativistic standpoint, it's no different from the distance between Earth and a star one light year away and that same distance being six light months away from the perspective of a spaceship near Earth traveling toward that star at ~86% the speed of light. Same end result as the standard relativistic contraction.

Basically, to get around this, you need create energy conditions under which space and time are not strictly inversely related to each other. This is hard to justify given the following thought experiment. Take two separate uniform hollow massive spheres, such that their gravitational potentials vary significantly enough to measure the difference in clock rates on their respective surfaces, where gravity is at a respective and different maximum. Now another pair of clocks, one inside each sphere, will maintain the same clock rates of the clock on the surface, even though there is no gravity inside these spheres. This means that two clocks, each respectively in flat spacetime, can nonetheless differ in clock rates (and inversely space). Though the fact that the clock rates differ still dictate an observable gravitational gradient exist somewhere in the space between the pair of spheres. In the wormhole case this would be the surface of the wormhole. Bottom line is that creating a flat region of space is not in itself enough to get you out of the so called clock paradox. Two flat regions of space do not necessarily share a common clock rate even if both regions are motionless with respect to each other!

Might it be possible that the variables can be manipulated to get around this? Perhaps, but based on anything we actually know you have to play very fast and loose with the energy conditions to even speculate theoretically.
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Old 14th November 2012, 01:42 PM   #70
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Old 14th November 2012, 02:09 PM   #71
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Would a wormhole be subject to gravity? Is a wormhole actually an object, moving through space, like the planet providing the gravity for this device? Or is it a feature of space/time? If the latter, than how would you move the wormhole to remain at the top of the gravity well you're using to power the device? Wouldn't the wormhole just be left behind as the planet moves through space?
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Old 14th November 2012, 03:36 PM   #72
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Originally Posted by Prometheus View Post
Would a wormhole be subject to gravity? Is a wormhole actually an object, moving through space, like the planet providing the gravity for this device? Or is it a feature of space/time? If the latter, than how would you move the wormhole to remain at the top of the gravity well you're using to power the device? Wouldn't the wormhole just be left behind as the planet moves through space?
Would a wormhole be subject to gravity? Yes. However, it is possible to make it safe to approach in the same way a black hole could be safely approached if it was big enough. It doesn't matter what the total gravitational depth is as long as the curvature isn't too extreme in space you occupy. Much like the hollow sphere I spoke of, where the gravity goes to zero on the inside it makes no difference how strong the gravity is on the surface.

Is a wormhole an object or a feature of space/time? It's more a feature of spacetime, but requires a certain distribution of stuff to maintain it. Back to this in a moment. Physicist playing with wormholes will generally try to map out the spacial topology they want and then try to and distribute stuff, often a dust model, after the fact to maintain their topology of choice.

As far as powering "the device" I tried to explain why it wouldn't power the device described by the OP. You could still use a gravitational field as a gravitational slingshot, but you are basically transferring momentum of some object to some other object. Like a planet and a spacecraft. The total energy of both system put together remain the same, even if the planet ends up with less while the spaceship gets more. Yet even that is relativistic and only meaningful with respect to a particular coordinate choice.

Now if we go back and consider the "stuff" generally required to have a stable wormhole you need some form of exotic matter. The most common type needed has negative mass. To give some idea of how strange this is, if you found a rock made of negative mass, in order to bring it to your house you would have to push it away from your house. Just one of those consequences of playing fast and loose with the energy conditions I spoke of.

This is why saying wormholes are described by "valid solutions to the equations of the theory of general relativity" is misleading. It's not wrong because general relativity doesn't provide energy conditions because it's a general theory that applies to ALL cases. Yet once you want to speak about any single (particular) case then you need to specify those energy conditions. Hence, under the definitions used with the term "valid solutions", any energy conditions you can dream up qualify as "valid" under GR. Some of theses energy conditions that are dreamed up can, in heavily oversimplified terms, be analogous to assuming you can build a house with negative size by the builders measuring right to left instead of left to right. Problem is, we don't know that such stuff is actually impossible. To understand why take a look at what's called the vacuum catastrophe.
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Old 15th November 2012, 04:51 AM   #73
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Originally Posted by phunk View Post
Actually, it's not in the ball, it's in the whole ball / earth / gravitational field system.
No it isn't. It's in the ball. See for example hyperphysics which says "Gravitational potential energy is energy an object possesses because of its position in a gravitational field." The object posseses it. You can work it out from conservation of energy. Imagine you're in space, light years away from Earth, and you come across a ball. You give it a nudge towards the Earth, whereupon the total mass-energy plus kinetic energy of the ball is x. Meanwhile the total energy of the Earth+gravitational field is y. Aeons later, the ball falls to Earth with considerable extra kinetic energy k because it impacts at 11km/s. You capture that kinetic energy by heating the Earth. Thereafter the total energy of the Earth+ball+gravitational field is now x+y, not x+y+k.

Originally Posted by phunk View Post
When you fire a ball upwards above escape velocity, it doesn't take potential energy with it. It takes only the kinetic energy it was fired with, minus some which is lost and added to the earth's kinetic energy.
It isn't true phunk. Do your own research on this and you can find plenty of websites that tell you how it is. Here's one picked at random, I've bolded a few bits to bring them to your attention:

"When an object is released above the ground, it accelerates in response to the gravitational force of the Earth and its potential energy is converted into kinetic energy. When the brick strikes the ground the kinetic energy is converted, partly into the sound of the impact, but mostly into heat, slightly warming the ground and the brick. The most important principles of physics are the conservation laws. This means that as the brick falls its increase in kinetic energy is exactly balanced by its decrease in gravitational potential energy.

Originally Posted by phunk View Post
The first sentence is correct except that the GPE is not contained in the ball, the rest is just nonsense.
It isn't nonsense at all. Saying the gravitational potential energy is not contained within the ball is nonsense. I really am not kidding you about this, phunk. Look carefully at what Cuddles said: a body that starts and ends at specific points gains or loses exactly the same amount of energy. A ball is a body. If it starts off on the floor and ends up six foot off the floor, its gains energy. Sadly Cuddles doesn't have the grace to say actually, Farsight is right.

Originally Posted by phunk View Post
By the way, if the machine in the OP worked the way it implies, it would violate the conservation of momentum. It would be a reactionless engine.
Sure. A perpetual motion machine violates conservation of energy too, and both are aspects of conservation of four-momentum.

Squeegee: you could replace the wormhole with something else hypothetical, like an antigravity field which you can switch on and off. You still run into the same problem. Conservation of energy means any energy you extract comes from the ball, with potentially disastrous results.

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Old 15th November 2012, 05:06 AM   #74
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Originally Posted by edd View Post
I think Farsight's more than a bit mistaken again.
I'm not at all mistaken.

Originally Posted by edd View Post
You can't move something on a closed path through a gravitational potential and have it end up at a different potential energy than it started with.
The wormhole is just a hypothetical artifice that shortcuts that closed path and joins the top of the vertical path to the bottom. We could use some other hypothetical artifice to achieve the same result, which is to get the ball back to the top without doing any work on it. The ball at the top has more gravitational potential energy than the ball at the bottom. Gravity extracted it in the form of kinetic energy which drives the waterwheel. The energy comes from the ball, and we can't keep on doing this forever. Replace the ball with a 511keV electron, if you keep on taking energy out of it, it cannot continue to exist as an electron. 510keV electrons do not exist.
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Old 15th November 2012, 05:41 AM   #75
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Originally Posted by Farsight View Post
No it isn't. It's in the ball. See for example hyperphysics which says "Gravitational potential energy is energy an object possesses because of its position in a gravitational field." The object posseses it. You can work it out from conservation of energy. Imagine you're in space, light years away from Earth, and you come across a ball. You give it a nudge towards the Earth, whereupon the total mass-energy plus kinetic energy of the ball is x. Meanwhile the total energy of the Earth+gravitational field is y. Aeons later, the ball falls to Earth with considerable extra kinetic energy k because it impacts at 11km/s. You capture that kinetic energy by heating the Earth. Thereafter the total energy of the Earth+ball+gravitational field is now x+y, not x+y+k.

It isn't true phunk. Do your own research on this and you can find plenty of websites that tell you how it is. Here's one picked at random, I've bolded a few bits to bring them to your attention:

"When an object is released above the ground, it accelerates in response to the gravitational force of the Earth and its potential energy is converted into kinetic energy. When the brick strikes the ground the kinetic energy is converted, partly into the sound of the impact, but mostly into heat, slightly warming the ground and the brick. The most important principles of physics are the conservation laws. This means that as the brick falls its increase in kinetic energy is exactly balanced by its decrease in gravitational potential energy.

It isn't nonsense at all. Saying the gravitational potential energy is not contained within the ball is nonsense. I really am not kidding you about this, phunk. Look carefully at what Cuddles said: a body that starts and ends at specific points gains or loses exactly the same amount of energy. A ball is a body. If it starts off on the floor and ends up six foot off the floor, its gains energy. Sadly Cuddles doesn't have the grace to say actually, Farsight is right.

Sure. A perpetual motion machine violates conservation of energy too, and both are aspects of conservation of four-momentum.

Squeegee: you could replace the wormhole with something else hypothetical, like an antigravity field which you can switch on and off. You still run into the same problem. Conservation of energy means any energy you extract comes from the ball, with potentially disastrous results.
Saying that the ball has the energy is a convention. The energy is in the system.

Think about the Potential Energy of the Earth with regards to the ball.

Alternatively, imagine you have a ball in a gravitational field, so that it has a certain potential energy. Now remove the gravitational field. Does the ball still possess the same gravitational potential energy?
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Old 15th November 2012, 07:40 AM   #76
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If you have the ability to create a worm hole, is this what you would do with it?
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Old 15th November 2012, 08:09 AM   #77
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Originally Posted by RenaissanceBiker View Post
If you have the ability to create a worm hole, is this what you would do with it?
If I could create free energy with a wormhole, that's exactly what I would do with it. Get our whole planet on free energy and every country on Earth can have a First World standard of living in less than a hundred years.
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Old 15th November 2012, 08:13 AM   #78
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Originally Posted by RenaissanceBiker View Post
If you have the ability to create a worm hole, is this what you would do with it?
Originally Posted by AvalonXQ View Post
If I could create free energy with a wormhole, that's exactly what I would do with it. Get our whole planet on free energy and every country on Earth can have a First World standard of living in less than a hundred years.
While this might be an interesting theoretical question, I'd have another plan.

I'd open one end of the wormhole inside the sun.

Now we have cheap, clean fusion power that will last for another 5 billion years or so. Plenty of time and energy to waste on trying out perpetual motion ideas and various other projects
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Old 15th November 2012, 09:33 AM   #79
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Originally Posted by Hellbound View Post
I'd open one end of the wormhole inside the sun.
That was my thought as well. The energy you could get from a pinhole sized wormhole to the sun would be much greater than a marble sized wormhole that causes a water wheel to turn.
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Old 15th November 2012, 09:40 AM   #80
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Originally Posted by Farsight View Post
No it isn't. It's in the ball. See for example hyperphysics which says "Gravitational potential energy is energy an object possesses because of its position in a gravitational field." The object posseses it.
You're reading too much into an oversimplfied explanation. GPE is not contained in the ball. It's contained in the whole system. Think of it this way: instead of gravity, there is a spring connecting the ball to earth. Would you still say that the potential energy of the ball when it's pulled farther away is contained in the ball, or would you agree it's in the whole ball, earth, spring system?
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