-
Free energy, FTL paradoxes and so on.
by
KelvinZero
on 03 Aug, 2014 04:50
-
I have noticed a few threads getting effectively derailed by repeated explanations of these general issues, till the point the specific idea being claimed is totally lost in the noise. However I think any proponent of those ideas has to fully understand these issues and be able to defend their claim with respect to it. The defense should not be a phrase or word salad. They should be able to explain how it behaves in specific scenarios.
If you do not know how you claim behaves, you do not know what you are claiming.
For example there are many phenomena that are FTL yet do not provide FTL communication. The spot of a torch beam flicked across a distant wall could easily move faster than light, for example. I have also heard descriptions of FTL warp travel where, yes, but the theory also predicts you can not exit the bubble at the other end. You have FTL, paradoxes are avoided, but it is not the Star trek drive that you want.
So therefore I thought we could move this to a general thread. We can put nice explanations here and then link to them instead of repeating them endlessly in specific threads.
Another use for this thread is that I would like to gather some nice simple paradox examples. For example I would like a nice example of how to produce a paradox from communicating only slightly faster than light.
Finally..There may be loopholes! Im quite prepared to admit that, in fact I enjoy proposing them. A general issue stated here does not kill every possible variation of the idea. I just think that they are an excellent framework for investigating what a given claim actually means, or if it has any defined meaning at all.
-
#1
by
KelvinZero
on 03 Aug, 2014 05:06
-
A common claim of propellantless propulsion is that you can get a constant acceleration from a constant source of power. This would provide free energy.
(*) Propellantless propulsion lets you increase velocity proportional to the energy you put in.
(*) Classical physics lets you extract energy proportional to the velocity
squared.
Possible workarounds:
(*) Firstly why not free energy? In for a penny, in for a pound as they say. Also perhaps the energy is coming from somewhere else. To this I would just say: at least stop talking as if the only application is a better ion drive. It is still freaking free energy, man.
(*) How about if it is somehow tied to a particular inertial frame. In this case you have something that is propellantless in the sense of a plane with a propeller. You won't get the same advantage but it could still be way better than a rocket which requires exponential amounts of fuel to achieve a given velocity. Another interesting result is that it may also provide a form of free energy, but only the well known type exploited by sailing ships that act in two mediums with different average velocity.
(*) frobnicat points out that a photon rocket is a special case that fits within my definition, and obviously does not deliver free energy. We can't just assume an infinite power source that we can carry along with us. So check your maths isn't just a fancy way of describing a photon rocket!
I would add a third workaround : if the propellantless propulsion ship has not a higher Thrust/Power ratio than a perfect photon rocket engine (that is not much efficiency, T/P<1/c) then it is not giving free energy,
-
#2
by
Stormbringer
on 03 Aug, 2014 05:14
-
i think 2 or more reference frames kind of muddy the issue a bit when it comes to FTL and time travel.
the best example i can think of is Dr Woodward's and Dr Cramer's talks on Wormholes. in the home reference frame the results are absurd. but Physicists in the audience do not object on the ground that present understandings of physics forbid it. they instead caveat that when we get a theory of Quantum gravity it will probably forbid it. that's not a strong critique. we don't even know much of anything about a formal framework for QG. but we do have one for relativity and apparently even relativity with all it's restrictions on FTL and time travel does not forbid absurd results.
i am speaking specifically of opening a worm hole and sending the distal opening towards a star 2000 light years away with the distal end traveling at fractionally bellow c. on the near end frame of reference the wormhole is usable within weeks even though the other end will not arrive for 2000 years according to common sense. you can look through it and see the destination as if the distal end were already there. you can step through it and be at the destination AND then step back through and be back nearly instantly. the wormhole clearly allows you to travel 2000 years into the future and then back to the present as well as travel 2000 light years FTL.
You'd think a room full of troubleshooting physicists would cream the idea. but the best they did was speak of a future theory of Quantum Gravity will most likely forbid it later. not relativity forbids it. not present day understanding of physics forbids it.
-
#3
by
ChrisWilson68
on 03 Aug, 2014 05:21
-
For example I would like a nice example of how to produce a paradox from communicating only slightly faster than light.
The standard paradox from being able to communicate faster than light is that you can send messages back in time, thanks to relativity. If it's only slightly faster than light, it requires moving your communication apparatus at very close to the speed of light. As your communication gets faster, the speed at which you need to get your communication setup moving becomes lower and lower. But for any communication speed greater than that of light, there is some speed less than light speed at which you can move your communication gear to be able to send your message backward in time.
This effect comes from one of the main points of relativity -- that two events that are simultaneous but at different points in space in one inertial reference frame happen at different times in a reference frame moving with respect to the original one along the axis connecting the two points.
Here's the setup. Send out four spaceships, ships A, B, C, and D. A has a faster-than-light transmitter to a receiver on B and C has a faster-than-light transmitter to D. Put ships A and D in the vicinity of Earth and B and C in the vicinity of Mars. Accelerate A toward Mars and D away Mars at a significant fraction of light speed and arrange that A and D pass close to each other at noon by Earth clocks. Also accelerate B away from Earth and C toward Earth and arrange for B and C to pass by each other going opposite directions at the same speeds as A and D, and make sure they pass at about the same time that A and D pass.
A and B are going the same speed in the same direction. Have A send a message to B faster than light in the inertial frame they share. If they're going fast enough and the message goes fast enough over light speed, the message will arrive from A (near Earth) at B (near Mars) at a point in time that is before it was sent from Earth's reference frame. B is near C, so have B flash a light in Morse code for C to see. Now C and D are also in the same reference frame, so have C send the same message to D using the same faster-than-light communication method. From the point of view of someone on Earth, the message arrives at D at a point in time before it was sent by C. So, round trip, the message goes from A to B to C to D and goes back in time from the point of view of the Earth on the A to B leg and on the C to D leg. B and C are near each other, so arbitrarily little time can be taken by that leg. And the same is true of A and D, so the message can be sent to A and arrive before it was sent from A.
Now if you can send a message back in time, you have causality issues. What if you decide that if you get the message "yes" you will send the message "no" and if you get the message "no" you will send the message "yes"? Since you get the message before you send it, you can choose to do that, and you have a paradox.
-
#4
by
KelvinZero
on 03 Aug, 2014 05:35
-
@Stormbringer
Just think in terms of explaining it to a science fiction writer.
They won't particularly care whether it is permitted by quantum gravity, they will just want to be able to describe to their audience what actually happens when, for example, someone tries to murder their grandfather before they are born. What do they experience? What do observers experience?
My position is that if someone claims to have a time machine but cannot describe what happens in this case, then they cannot describe what the word time machine actually meant when they used it.
@ChrisWilson68
I will have to try drawing that out on paper
.. I know I can demonstrate it with an 'instantaneous communicator' but this is a bit tougher.
-
#5
by
Stormbringer
on 03 Aug, 2014 05:48
-
@Stormbringer
Just think in terms of explaining it to a science fiction writer.
They won't particularly care whether it is permitted by quantum gravity, they will just want to be able to describe to their audience what actually happens when, for example, someone tries to murder their grandfather before they are born. What do they experience? What do observers experience?
My position is that if someone claims to have a time machine but cannot describe what happens in this case, then they cannot describe what the word time machine actually meant when they used it.
i can buy that.
we don't know because there are several theories that allow or disallow paradox situations from arising. until we know which theory is correct we cannot describe the outcome of such an effort. E.G. you cannot effect your own past because any time traveling spits you into a parallel timeline. in this type of solution you could actually surveill the past or return with an extinct critter because most things would still be the same. some solve it by saying no form of time travel is permitted. period. some say your action creates a paradox it self nullifes your action.
but like you said until we nail the correct theory we cannot describe what happens truthfully.
-
#6
by
CW
on 03 Aug, 2014 09:33
-
Concerning free energy coming from somewhere else.. it would be 'free' in the sense that we don't have to, well, pay for it in any way. But this seems just like drilling a 'hole' somewhere in physical reality and extracting some stored up energy potential. That potential would still be missing elsewhere, after we made use of it. So, looking at the whole picture, I think this would qualify as 'free' energy in only relative terms. Maybe it would be like going to a foreign country and robbing that place of its resources - for 'free'. Who knows
.
-
#7
by
MP99
on 03 Aug, 2014 09:40
-
@Stormbringer
Just think in terms of explaining it to a science fiction writer.
They won't particularly care whether it is permitted by quantum gravity, they will just want to be able to describe to their audience what actually happens when, for example, someone tries to murder their grandfather before they are born. What do they experience? What do observers experience?
My position is that if someone claims to have a time machine but cannot describe what happens in this case, then they cannot describe what the word time machine actually meant when they used it.
I find the "many worlds" hypothesis easiest to swallow.
Every second of the grandfather's life, some of the "worlds" include him dieing in various ways.
So, congrats to the murderer - they just created another branch where the grandfather died, and the grandchild never existed.
If this is correct, then "a" grandfather dies, but not the one in the grandchild's past. From the murderer's POV, they then just need to kill themselves in all universes where the grandfather didn't die in order that they get to experience the world they tried to create.
Cheers, Martin
-
#8
by
MP99
on 03 Aug, 2014 10:13
-
I have also heard descriptions of FTL warp travel where, yes, but the theory also predicts you can not exit the bubble at the other end. You have FTL, paradoxes are avoided, but it is not the Star trek drive that you want.
But, surely this is sending at least one "bit" of information to an outside observer - the presence / transit of the bubble itself. Something like "hurrah, we made it there", or "send help".
Imagine an ET sending such a device barrelling through our Solar System. Assuming a natural origin can be eliminated, confirmation of the existence of ETs would be one hell of a "bit" of information to impart.
Actually, more than that - the origin can also pass further information about the location of the sender. Also, changes of speed & direction might be modulated to pass further information?
You could also imagine a stream of such objects (miniaturised, presumably), passing more complex messages.
Or... is the warp bubble somehow invisible to outside observers?
Also, does "can not exit" mean can never stop - must keep travelling forever? Doesn't that need continuous energy to maintain the warp bubble?
Or does it mean the collapse of the warp bubble is too energetic to be survivable? If so, would that make for an incredibly potent "interstellar ICBM" if ET fired it towards Earth and collapsed the bubble in our vicinity?
Cheers, Martin
-
#9
by
MP99
on 03 Aug, 2014 10:22
-
A and B are going the same speed in the same direction. Have A send a message to B faster than light in the inertial frame they share. If they're going fast enough and the message goes fast enough over light speed, the message will arrive from A (near Earth) at B (near Mars) at a point in time that is before it was sent from Earth's reference frame. B is near C, so have B flash a light in Morse code for C to see. Now C and D are also in the same reference frame, so have C send the same message to D using the same faster-than-light communication method. From the point of view of someone on Earth, the message arrives at D at a point in time before it was sent by C. So, round trip, the message goes from A to B to C to D and goes back in time from the point of view of the Earth on the A to B leg and on the C to D leg. B and C are near each other, so arbitrarily little time can be taken by that leg. And the same is true of A and D, so the message can be sent to A and arrive before it was sent from A.
Per my previous post, A sends a "can not exit" warp object to/past B, and C sends another back to/past D.
Only one bit, but that's enough for paradoxes, yes?
(Do you actually need objects B & D, or can C & A just observe the message objects whiz past?)
Cheers, Martin
-
#10
by
john smith 19
on 03 Aug, 2014 10:40
-
They won't particularly care whether it is permitted by quantum gravity, they will just want to be able to describe to their audience what actually happens when, for example, someone tries to murder their grandfather before they are born. What do they experience? What do observers experience?
Well not murder but certainly trying to change the future is the subject of Gregory Benford's novel "Timescape."
It's my impression the SF writers of the 1930's understood the actual quantum thinking of time travel quite well. Another classic short story from that time is "Sidewise in time."
Bottom line. You cannot change
your past. You can create a
new universe where those events happened and the new universe has the resulting follow on. However a pure time machine would only take you back through
your time line (its a time machine, not an inter universe transporter).
-
#11
by
KelvinZero
on 03 Aug, 2014 11:17
-
I wasn't really asking for solutions to the grandfather paradox:)
However, since FTL apparently is time travel, we can expect FTL to be just as strange.
What I really wanted to do was separate this really general stuff that everyone should know from specific threads about propellentless propulsion and FTL. Then we can ask on those threads just how those particular claims work around a specific paradox, and not have to explain the issue again and again to each new person who joins the conversation.. just point them here.
I don't expect to understand the theory behind any particular claim, If it is correct but not obvious to the physics community it must be mindbogglingly difficult. But I should be able to understand what I experience if I had such a device and tried to recreate one of the classic paradoxes.
-
#12
by
Nilof
on 03 Aug, 2014 13:15
-
I wouldn't dismiss what physicists mention about quantum gravity affecting wormholes just yet. You can actually get quite far and make some useful predictions with semiclassical quantum corrections to GR. Essentially, using a fixed curved metric as a background for a quantum field theory and seeing if anything happens.
If all the fields stay at similar values to what is found in flat space for a given metric, you don't get much out of it. But if you see significant changes, you get a warning that the considered metric breaks down completely as soon as quantum mecanics is considered.
The best known example is Hawking radiation around a black hole, which implies that black holes are not static and will evaporate in a finite ammount of time. But some wormhole metrics see vastly more dramatic effects. The example above of two wormhole mouths being moved so that a closed timelike curve is formed will have streams of virtual particles moving along that curve leading to infinite stress-energy densities. In other words, for that system, GR breaks down completely and the classical solution is useless.
This is the background of the
chronology protection conjecture by Stephen Hawking. It may or may not be true in quantum gravity for more complex systems, but the background is certainly worth considering. Metrics that have CTCs but no stream of virtual particles in semiclassical gravity exist.
Note that such a pulse of virtual particles could actually be quite useful for some applications, especially if it can be tuned so that the mouths do not collapse. If momentum is exchanged between the mouths by the pulse, as a repelling force, it could be a good way to move extremely heavy stuff around. One of the very few things that is better than a reactionless drive is a reactionless drive that moves wormholes.
By "riding the edge" of the future lightcone of a stationary wormhole mouth, you could in theory move a wormhole mouth to a distant point in a very small amount of proper time. The means that the distant destination would be available in an equally short amount of time to the people residing near the stationary wormhole mouth because they would be traveling into the future(in their reference frame).
-
#13
by
Chris Bergin
on 03 Aug, 2014 13:50
-
Isn't this oogey boogy?
If my post gets more than five likes, that'll be a yes and a lock. If the opening post gets more than five likes, it stays.
Who wins? You decide!
-
#14
by
DMeader
on 03 Aug, 2014 14:32
-
By "riding the edge" of the future lightcone of a stationary wormhole mouth, you could in theory move a wormhole mouth to a distant point in a very small amount of proper time. The means that the distant destination would be available in an equally short amount of time to the people residing near the stationary wormhole mouth because they would be traveling into the future(in their reference frame).
Didn't Seven Of Nine say that in the Voyager episode where the ship was trapped on the event horizon of a quantum filament and Janeway wanted to reverse the polarity of the main deflector to yadda yadda yadda........
Can we have some discussions of real-world spaceflight-related advanced concepts and not yet more bad Star trek technobabble? Please?
-
#15
by
Nilof
on 03 Aug, 2014 15:36
-
@ DMeader:
General relativity isn't exactly near term "real life" spaceflight, but it is a roughly 100 year old and well established scientific theory. Semiclassical corrections from QFT to it is a rich subject with almost 50 years of history. If common physics terminology sounds like technobabble to you, I can provide equations:
For a wormhole mouth traveling at velocity v, we have dτ = dt/sqrt(1-v2) = cosh(r)dt where r is the rapidity(which is defined by r = atanh(v), replaces delta-v in the relativistic rocket equation and is generally more useful than the velocity).
The proper distance between the two synced mouths is given by s2 = x2 - (t - τ)2. What I was referring to as "riding the future lightcone" of the fixed mouth is a trajectory of constant s, since a lorentz invariant and symmetric interaction between the two mouths will depend only on s2. If s2 ever becomes negative, we get a CTC.
So we have the algebraic equation (t - τ)^2 = x2 - s2. Taking the τ derivative and rearranging, we get x/(t - τ) = (cosh(r) - 1)/sinh(r).
Looking at the LHS, the ratio x/(t - τ) will get closer and closer to one as you get closer you are to the lightcone edge. Similarily, the RHS is a strictly increasing function of r and tends to one as r goes to infinity. So if there is a repelling force between wormhole mouths that may cause a stable constant s trajectory, it can be used to propell one mouth at arbitrarily high final speeds away from the stationary mouth as the two push against each other.
(Using units with c = 1 throughout the post.)
-
#16
by
DMeader
on 03 Aug, 2014 19:43
-
Ok, and that has anything to do with spaceflight in the near future (like our lifetimes), how?
Maybe we are working off of two very different definitions of what "advanced concepts" means. The header for the thread states "In-works and future conceptual ideas of space flight, from Nuclear Propulsion to Tethers and beyond". I don't see "free energy, FTL paradoxes, dark energy, wormholes, warp travel, FTL communications", any of that being within that definition.
I think that there are those who take the term "advanced concepts" as an excuse to go off into oogie-boogie sci-fi bordering-on-nonsense that will have no application while any of us are on this earth and long beyond this until several laws of physics can be broken. Of course, if this forum were to be renamed "Sci-Fi Advanced Concepts" I'd go away and never say another word about it.
-
#17
by
Lee Jay
on 03 Aug, 2014 19:48
-
I'm not fond of the term "free energy" to describe something that has an efficiency higher than 1 or otherwise violates conservation of energy. One could argue that the Bussard ram jet is a free energy device since it harvests fuel it finds along the way, the way a propeller harvests propellant (and oxidizer, if not an electric) along the way.
-
#18
by
MP99
on 03 Aug, 2014 19:58
-
Isn't this oogey boogy?
If my post gets more than five likes, that'll be a yes and a lock. If the opening post gets more than five likes, it stays.
Who wins? You decide!
Chris,
please, don't!!!
This is the science counter aurgument to possible oogey boogy stuff.
How do I record an un-like for this???
cheers, Martin
Edit: there are probably many threads where a link here may be a valid alternative to closing some anti-science thread.
-
#19
by
IslandPlaya
on 03 Aug, 2014 20:35
-
Isn't this oogey boogy?
If my post gets more than five likes, that'll be a yes and a lock. If the opening post gets more than five likes, it stays.
Who wins? You decide!
Chris,
please, don't!!!
This is the science counter aurgument to possible oogey boogy stuff.
How do I record an un-like for this???
cheers, Martin
Edit: there are probably many threads where a link here may be a valid alternative to closing some anti-science thread.
Thanks Chris.
.. I know I can demonstrate it with an 'instantaneous communicator' but this is a bit tougher.
