FTL (Faster Than Light) and Thermodynamics
In a thread a couple of weeks ago that started out talking about presidential candidate General Clark and time travel (even remembering Clark as being a candidate is beginning to feel like time travel to me), a side topic spun off about the implications of faster-than-light (FTL) phenomena for physics as we know it today.
The discussion wasn't about whether or not there's any reality to FTL (specifically causal FTL like communication or travel), but simply to what extent our current understanding of physics ruled out such phenomena or not.
I got sidetracked rewriting a bunch of Perl code for my web site, so I had to drop the discussion for a while, but I did take the time to write to Dr. Jason W. Hinson, PhD., a physicist who works at CNAC, and who has some discussion of FTL on his web site.
I got a reply from him today, and he gave me permission to quote him here. (It turns out he's a Mac enthusiast, and he says he might sign up for AppleInsider and show up here himself.) Here's what he had to say:
I'm not trying to put forth what this guy is saying as absolute authority or the last word on the subject, just as evidence that someone with a bit more knowledge of the subject of physics than myself, or than certain people I was previously arguing with, can look at the issue of FTL and not find any glaring or unresolvable conflict with thermodynamics.
The discussion wasn't about whether or not there's any reality to FTL (specifically causal FTL like communication or travel), but simply to what extent our current understanding of physics ruled out such phenomena or not.
I got sidetracked rewriting a bunch of Perl code for my web site, so I had to drop the discussion for a while, but I did take the time to write to Dr. Jason W. Hinson, PhD., a physicist who works at CNAC, and who has some discussion of FTL on his web site.
I got a reply from him today, and he gave me permission to quote him here. (It turns out he's a Mac enthusiast, and he says he might sign up for AppleInsider and show up here himself.) Here's what he had to say:
Quote:
Hi there...
Sorry for the delay in getting back to you.
In any case, entropy is more of a statistical reality than a hard physics rule. Let's consider the often-used example of a gas particles in a box. We start by putting all the gas in one half of the box, separated from the other half by a partition, and then remove the partition. Over time, the gas will disperse into the entire chamber. If we ran the clock backwards, we'd know something wasn't right by how all the particles suddenly bunch up on one side of the chamber (and then get captured when--in the backwards-running scene--we put the partition back in place).
But if we think of this as a classical example in which the gas molecules are little balls bouncing off one another, then what we have are a bunch of inelastic collisions between the individual gas particles and collisions between them and the walls of the box. For any of those interactions, the physics involved is completely time-reversible (that is, if you reverse the arrow of time after the interaction, the resulting reverse interaction is physically allowed as if it were a forward-in-time interaction). So, classically, if you took all the particles and the box at some point after the partition were removed and reversed each of their momentum vectors, the particles would indeed re-assemble themselves on one side of the box. There is nothing about the physics that makes it impossible. But all the particles and the box itself have to be in just the right state to cause that assembly over on one side of the box. The number of situations that would cause that is extremely small compared to the total number of situations in which you might find the particles. It's very unlikely, statistically, to find the particles in just the right state. Therefore, if you see a film showing gas in a box all moving to just one side of the box, you assume that the film is running backwards--that's the more plausible explanation, but its not the only one allowed by the laws of physics.
That's why entropy works. Reversing the time arrow doesn't make for a situation that's physically impossible, it just makes for a situation that's highly unlikely to occur statistically.
(By the way, if we go from the classical explanation to a quantum explanation, the reasoning is much the same. Just about every type of interaction in quantum mechanics is time-reversible. The few that aren't are few and far between. In general, nothing about "running the film backwards" is physically disallowed. Its the improbability of finding such a situation that makes it so odd.)
So, in short, I don't see entropy causing a problem with FTL travel. It's not a law involving what is really allowed/disallowed physically, its just a matter of probability. If FTL travel changes the probabilities, that's not really a big deal.
Hope your friend doesn't find my answer too upsetting
Hi there...
Sorry for the delay in getting back to you.
In any case, entropy is more of a statistical reality than a hard physics rule. Let's consider the often-used example of a gas particles in a box. We start by putting all the gas in one half of the box, separated from the other half by a partition, and then remove the partition. Over time, the gas will disperse into the entire chamber. If we ran the clock backwards, we'd know something wasn't right by how all the particles suddenly bunch up on one side of the chamber (and then get captured when--in the backwards-running scene--we put the partition back in place).
But if we think of this as a classical example in which the gas molecules are little balls bouncing off one another, then what we have are a bunch of inelastic collisions between the individual gas particles and collisions between them and the walls of the box. For any of those interactions, the physics involved is completely time-reversible (that is, if you reverse the arrow of time after the interaction, the resulting reverse interaction is physically allowed as if it were a forward-in-time interaction). So, classically, if you took all the particles and the box at some point after the partition were removed and reversed each of their momentum vectors, the particles would indeed re-assemble themselves on one side of the box. There is nothing about the physics that makes it impossible. But all the particles and the box itself have to be in just the right state to cause that assembly over on one side of the box. The number of situations that would cause that is extremely small compared to the total number of situations in which you might find the particles. It's very unlikely, statistically, to find the particles in just the right state. Therefore, if you see a film showing gas in a box all moving to just one side of the box, you assume that the film is running backwards--that's the more plausible explanation, but its not the only one allowed by the laws of physics.
That's why entropy works. Reversing the time arrow doesn't make for a situation that's physically impossible, it just makes for a situation that's highly unlikely to occur statistically.
(By the way, if we go from the classical explanation to a quantum explanation, the reasoning is much the same. Just about every type of interaction in quantum mechanics is time-reversible. The few that aren't are few and far between. In general, nothing about "running the film backwards" is physically disallowed. Its the improbability of finding such a situation that makes it so odd.)
So, in short, I don't see entropy causing a problem with FTL travel. It's not a law involving what is really allowed/disallowed physically, its just a matter of probability. If FTL travel changes the probabilities, that's not really a big deal.
Hope your friend doesn't find my answer too upsetting
I'm not trying to put forth what this guy is saying as absolute authority or the last word on the subject, just as evidence that someone with a bit more knowledge of the subject of physics than myself, or than certain people I was previously arguing with, can look at the issue of FTL and not find any glaring or unresolvable conflict with thermodynamics.
Comments
Originally posted by Ganondorf
So where does the Flux Capacitor fit into all of this?
Shall I suggest where?
Originally posted by Ganondorf
No, I know it goes in the DeLorean.
Damn, no wonder why I'm still stuck in 2004.
The following is from wikipedia:
Using spooky action at a distance
Quantum mechanical phenomena such as quantum teleportation, the EPR paradox, or quantum entanglement might appear to create a mechanism that allows for faster-than-light (FTL) communication or time travel, and in fact some interpretations of quantum mechanics such as the Bohm interpretation presumes that some information is being exchanged between particles instantaneously in order to maintain correlations between particles. This effect was referred to as "spooky action at a distance" by Einstein.
Nevertheless, the rules of quantum mechanics curiously appear to prevent an outsider from using these methods to actually transmit useful information, and therefore do not appear to allow for time travel or FTL communication. This misunderstanding seems to be widespread in popular press coverage of quantum teleportation experiments. The assumption that time travel or superluminal communications is impossible allows one to derive interesting results such as the no cloning theorem, and how the rules of quantum mechanics work to preserve causality is an active area of research.
Entry
The fact that it has been "proven" that useful information cannot be transmitted through FTL communication indicates that this is intimately tied to entropy... I will not argue any further...
Originally posted by billybobsky
The fact that it has been "proven" that useful information cannot be transmitted through FTL communication indicates that this is intimately tied to entropy... I will not argue any further...
I'm aware of what you speak of, which is not the same thing as accepting your overstatement of the facts.
There's a difference between the fact that known QM phenomena don't allow for the the transmission of useful information faster than light, and proving that no form of FTL transmission is possible by any means, or proving that a conflict must exist between QM and FTL.
Where we left off on this discussion before, thermo was your big thing, and that's mainly what I addressed in my e-mail with Dr. Hinson. I'll gladly provide you with the text of my original e-mail, here or by PM if you like.
Since it is obvious that I cannot convince you that FTL signal transfer is not possible by quantum, that your arguments of a special frame in the relativistic sense make no sense, let me at least point out a few things... Thermodynamics is not a fundamental science in that the properties observed arise from even more fundamental science on the level of quantum and such... When I speak entropy, I am not merely talking about the conventional entropy with which the good professor gave the most obvious example, but I am talking about the total entropy of the entire system down to the energy of the electrons. One brief point about the example the professor used: inelastic collisions are forbidden by quantum, I believe. Both bodies in the collision must show signs that their electron clouds interacted and this in turn appears changes the energy levels of those electrons making the collision at least always partially elastic. While all of physics depends on the reversing the clock phenomena, every physical system also tends towards increased entropy with time. There are no real exceptions...
One more comment and this is a repeat of one you have ignored many times: in at least one frame your FTL signal will be observed as sending mass backwards in time. This is strictly forbidden...
The cosmic microwave background radiation that permeates our universe, a remnant of the primordial fireball just a few thousand years after the big bang, can be considered a "frame of the universe". One can measure how fast one is moving "relative to the rest frame of the universe" using doppler shifts and whatnot (cf. dipole anisotropy).
Now the question one should naturally ask is that is this frame preferred in any way? That is to say, is physics not invariant under the Lorentz transformation? The answer by Einstein relativity is no, but more advanced theories such as string theory and others predict some symmetry breaking. There is already a hint of this with the GZK paradox (Greisen-Zatsepin-Kuzmin).
Look up italized terms and you'll have some ammo. Just some help from an indifferent physicist.
I really don't see how that helps him any...
Originally posted by billybobsky
Shetline,
Since it is obvious that I cannot convince you that FTL signal transfer is not possible by quantum, that your arguments of a special frame in the relativistic sense make no sense, let me at least point out a few things... Thermodynamics is not a fundamental science in that the properties observed arise from even more fundamental science on the level of quantum and such... When I speak entropy, I am not merely talking about the conventional entropy with which the good professor gave the most obvious example, but I am talking about the total entropy of the entire system down to the energy of the electrons.
Since you have not elaborated on how this concept of "total entropy" is defined, it's impossible to say how FTL would affect it. Does it break totally? Does it break in a tweakable way where FTL could be worked in, but known experimental data would still be properly accounted for?
One brief point about the example the professor used: inelastic collisions are forbidden by quantum, I believe. Both bodies in the collision must show signs that their electron clouds interacted and this in turn appears changes the energy levels of those electrons making the collision at least always partially elastic. While all of physics depends on the reversing the clock phenomena, every physical system also tends towards increased entropy with time. There are no real exceptions...
Elasticity or lack thereof wasn't an important or pivotal point in what Dr. Hinson wrote.
One more comment and this is a repeat of one you have ignored many times: in at least one frame your FTL signal will be observed as sending mass backwards in time. This is strictly forbidden...
"Strictly forbidden" is what's at issue here. How strict is strict? Conservation of mass/energy is certainly a well-tested and solid idea, but since we don't know of any real information-bearing FTL phenomena yet, we obviously haven't tested the conservation notion against FTL.
My guess is that non-local violations of the conservation of mass/energy, if inherent to FTL, could be absorbed into current theory in such a way as to see be in line with known experimental data, probably still retaining the notion of conservation of mass/energy, but killing off its total frame invariance (and some frame invariances are expected to suffer here).
(Dr. Hinson's web site brought up the idea of localized FTL fields as opposed to Universe-permeating aether-like frames... also another idea to consider.)
The way you said "Think of this as a conservation of mass issue" I thought you were merely striving for an analogy, not that you were actually making the claim that mass would be somehow "sent into the past" -- I thought you were still on about entropy.
Okay... so, there's a non-local violation. The mass/energy embodied in an FTL message could travel into my [i]non-local[i] past, so then my [i]non-local[i] concept of "now" could end up containing both the mass/energy that I'm about to put into sending an FTL message, and a "repeat" of that same mass/energy absorbed and/or radiated at the message's reception point, carrying forward through time into my "now".
This is a curious result, but not daunting when you're already playing with assumptions about the absoluteness of frame invariance. Mass/energy would still be conserved in the preferred frame, and I don't think the mathematical pillars of modern physics would crumble trying to deal with this loss of total invariance.
Actually it is a key point that the collisions be inelastic in the profs example. Otherwise you would have to deal with the energetics of each collision.
I still really don't get what you mean by prefered frame...
I am "the good professor" in question, though in actuality, I'm "the good Dr.," not a professor.
I wanted to throw out three things:
(1) The question addressed to me asked, in essence, whether I had reason to think that entropy prohibits FTL travel. Both in classical physics and in quantum mechanics, entropy is a statistical phenomenon, not a hard-and-fast physics requirement. Though I used inelastic collisions in my classic example, as I noted, just about every quantum mechanical system is also time-reversible. That is, almost every quantum mechanical system is invariant under time reversal. I thus pointed out that entropy is about the statistical probability of finding a system naturally moving towards order rather than disorder. If FTL travel changed the probabilities involved for certain special cases, that wouldn?t necessarily violate any physical reality.
(2) My web page is largely dedicated to convincing the "non-scientist" sci-fi fan that the problems that physics poses to FTL travel are not easily overcome. It points out that in addition to the often-noted problem of the light-speed barrier (can?t accelerate past c due to--among other things--the need for ever increasing energy to increase speed), a more insidious problem comes from the fact that, as has been pointed out here, an FTL trip in one frame of reference will always be a trip backwards in time for other frames of reference. That, in and of itself, does not undeniably create a logically _unsolvable_ paradox.
[A quick aside on wording: in general, "a paradox" does not have to mean "a situation that contradicts itself," but rather it usually means "a situation that _seems_ to contradict itself until one understands the proper explanation." (E.g., the twin paradox)]
But anyway... An undeniably unsolvable paradox _can_ occur if one is allowed to travel FTL in one frame, use normal thrust to "enter" a frame of reference in which that trip took him back in time, and then make a second FTL trip in that second frame of reference. Then a person can go back to a time before they left and at a location where they can use slower-than-light signals to affect events that lead to the first FTL trip. Thus, they can produce a situation that logically contradicts itself (irregardless of whether a physical law is known to have been "broken").
(3) However, I do go on to then provide purely speculative reasoning as to how we might imagine getting around even that limitation--noting that such a feat would require the universe to provide some sort of special provisions (such as parallel universes, consistency protection, or unique-use special frames of reference). Obviously, none of these are required attributes of the universe according to our current understanding of physics, and I for one think that the universe is pretty happy to tell us that we simply can't travel faster than light. However, for a sci-fi fan/writer, its at least useful to know what sort of mumbo-jumbo COULD be discovered at some point in our future that would allow for FTL travel given all we know now about its problems.
Hope that helps a bit.
-Jay
Originally posted by PhysGuyJWH
Hi there...
I am "the good professor" in question, though in actuality, I'm "the good Dr.," not a professor.
I wanted to throw out three things:
Let me throw back a few things:
0
1 - Nice to have an opinion on quantum issues from a real expert.
OT: Quark Researcher must look cool on a business card (without the negative PR that the software Quark carries).
Although... Lepton/Pion Tracker has a certain Saint Patrick's Day charm.
I know a guy who used to do Meson research at TRIUMF who loved answering "what do you do".
2 - I'd be interested to hear your perspective on BEC and Fermionic Condensates.
AI thread from a few weeks back
3 - While we're pursuing opinions and past threads... Searching for 'Faster and LIght' returned
Ye olde 'Future Space Travel for the Next 100 years thread
Good books on QM/Physics? from the Archives
4 - Thoughts on Super String Theory? (I'm watching Elegant Universe )
5 - Although I've lent out my copy of The Holographic Paradigm and can't go quoting Bohm, Pribham, et al., my understanding of the Holographic Model proposes that not only do all pieces have all the information for the total picture, this includes all past and future states.
This would seem to suggest that Time Travel or FTL may have more to do with tweaking the reference frame at some quantum level than dumping more energy into the proverbial engine.
And I probably didn't phrase that as well as I could've. Hope it makes sense.
Originally posted by curiousuburb
1 - Nice to have an opinion on quantum issues from a real expert.
OT: Quark Researcher must look cool on a business card (without the negative PR that the software Quark carries).
Although... Lepton/Pion Tracker has a certain Saint Patrick's Day charm.
I know a guy who used to do Meson research at TRIUMF who loved answering "what do you do".
I'm an aspiring physicist myself (just starting my Master), and I love telling my aunts and grandparents what I do: "Well, I'm studying the properties of Bottom quarks" "Bot-what??" - After a few minutes of trying to give them an idea of what a quark might be - "And what's the point of studying whatever you just said?" They can't understand that some science might have absolutely no possible applications within short or midterm.
More on topic. I haven't studied much of general relativity , and therefore I'm no specialist of time travel. But I fail to see how entropy would have any impact. Like that nice doctor said above, entropy is a statistical entity. It is in fact directly equivalent of the probability of a system being in a state. And quantum mechanics told us that no matter how low some probabilities are, really improbable stuff do happen. For instance, quantum tunnelling is quite improbable in pretty much any circumstances, but its the only way some fusion reactions in the sun are possible.
I haven't read the other thread so I don't know the exact problems that were brought regarding entropy, so maybe it was a useless reply. Sorry if that is the case
Now it's 3h30 in the morning, I'm tired, so let's go to bed!