Time travel: how to be your own mother and father - Page 3

**RandomGuy** · 01-07-2007, 08:24 PM

If traveling back in time were possible wouldn't we be up to our ears in time-travelers?

**sordavie** · 01-07-2007, 08:50 PM

Quote:

Originally Posted by Kerostasis

I'm more interested in a slightly different aspect of your argument.

One of the other arguments people bring against this logic is that the very notion of a non-contingent being is counterintuitive. Its very hard to grasp exactly what a non-contingent being even is. However, your closed causal loop appears to be an example of a non-contingent being, in effect, because it is contingent only on itself.

Yes. Actually contingency and noncontingency pretty easy to understand once you understand some modal logic and possible worlds semantics. But, it would probably be a miserable failure if I tried to explain that here.

A being is noncontingent just in case it is impossible that the being does not exist.

A being is contingent just in case it isn't impossible that the being does not exist. (In other words, it is possible that the being does not exist).

The being in the closed causal loop is contingent, since the loop itself is contingent. The loop doesn't necessarily exist.

So, compare these two possible worlds A and B

time travel and functional sex changes are possible at both.

But in world A, Andy does his thing and brings himself into existence.

In world B nobody time travels.

So, since it's possible that Andy doesn't exist (namely possibility B) Andy is a contingent being.

**sordavie** · 01-07-2007, 08:51 PM

Quote:

Originally Posted by RandomGuy

If traveling back in time were possible wouldn't we be up to our ears in time-travelers?

Not if nobody traveled back in time, or if they did, none or few traveled to our time. And, certainly not if nobody ever figured out how to do it.

**Rincewind** · 01-08-2007, 12:44 PM

Quote:

Originally Posted by sordavie

Scientists take the possibility of closed causal loops seriously and we can conceive of them (again no contraditions), so it appears that these are possible.

I conceive a being so mighty and powerful as to make all other beings appear worthless. I am a scientist: what I conceive is possible. But to limit this conceptual being to a mere possibility is damning yourself to an eternity of pain and blindness!

Release yourself from your chains--look no longer at the shadows of the world cast upon a wall of grief and sorrow; turn around a see the truth in the blinding light of reality. All that you see is not what you believe it to be! Realize that His Mighty Tendrils of Soaked Duram contort and control the world; your perception of reality shadows of His Masterful Puppet Show!

HE LIVES

He is the one true creator.

LET HIS NOODLY APPENDAGE EMBRACE YOU.

And don't get yourself pregnant.

**leejo** · 01-08-2007, 03:19 PM

Quote:

Originally Posted by sordavie

Not if nobody traveled back in time, or if they did, none or few traveled to our time. And, certainly not if nobody ever figured out how to do it.

Sorry I haven't been following this thread too closely. Do any of your links discuss the physics involved? My understanding is that current theories are pretty solid regarding particles with mass - a massless particle may not be constrained by time but a person certainly would be. I have read papers in which it appeared that particles were traveling faster than light but closer review showed that all was within the tolerance of the particles' wavelength. In other words, if you measure the start at the trailing edge of the packet and the finish at the front edge of the packet, it may appear that a particle is traveling slightly faster than light but in fact it is traveling slightly slower than light.

These are of course subatomic particles that have been accelerated enough to be measurable. If instead of a subatomic particle you attempt to chuck a person back in time...gosh how much mass would that person have? Infinite. That's how much. Or relativity is wrong, and I'd be interested in reading about that.

So I guess I'm confused. Sorry to ask for a link that you may have already provided, but is there something that addresses the physics of time travel or is all of this a mind game that is unconstrained by the practicalities of reality?

"Paradoxes" exist all the time in physics. It's more an indication that we haven't achieved the correct mental model than anything else, IMO. Now when the MATH is contradictory, you have a problem.

Edit: OK I've skimmed through this thread and I don't see any physics links discussed in any detail. Sordavie, you mention that time travel is a possibility that interests physicists. I have never seen this. I can imagine discussions that wonder if time is like the 4 forces (electromagnetic, strong, weak, and gravity) that each have a carrier particle, or if time is a dimension like the other three.

I think it's important to distinguish between real time, which always moves in one direction, and imaginary time, which may move freely either forwards or backwards. Imaginary time can even pause! (If someone can teach imaginary time to fetch beer or money, we may be on to something.)

REAL time plays an important role in all sorts of physical laws. The second law of thermodynamics relies on time moving in a single direction, for example, and it has a BIG problem with time travel. If it takes energy to move a body from rest, and suddenly that body has moved from its reference position (x,y,z, and time) where is the energy? Real time provides an order to physical processes. That order flies out the window with imaginary time.

If you look at time travel within the context of energy conservation, then the arrow of time ensures that you can't use energy on credit. With time travel, or with any system that allows particles and energy to drift back and forth in time, it's possible to have a process "owe" the universe energy. Something tells me the universe is the worst possible loan shark: owe it energy, and it takes it. Immediately. So maybe time travel is possible but the particles that travel "owe" the universe energy, which it collects at the moment of travel, destroying the particle. Then the question changes from "can I travel through time" to "can I blow myself up in a nuclear reaction"?

Imaginary time may be more fun, but real time gets work done.

**leejo** · 01-08-2007, 04:48 PM

Ok NOW I have read (shall read? Did read? Shall have read? This is another argument against time travel: the added verb tenses will have been creating so many tenses!) a physicist talking about the possibility of time travel, albeit with the sincere hope that it's impossible: http://www.hawking.org.uk/lectures/warps3.html

And in the process read about states in which the universe tolerates VERY SMALL negative energy states - i.e. within the Heisenberg principle's tolerances.

I know that you're approaching this from a philosophical angle, but boy howdy would time travel be a royal PITA for physicists. On the other hand I would completely wreck Liz Wooten if given another trip to the plate, so keep fighting the good fight.

**sordavie** · 01-08-2007, 05:34 PM

Lol Leejo. I can't tell you what the physics says. I don't hold an advanced degree in physics. I can only report to you what the physicists say.

But, even if actual physics doesn't allow for the kind of time travel I'm talking about, the laws of nature could have been different. This sort of possibility is termed metaphysical possibility by philosophers. Actually wikipedia's article on modal logic is decent enough for most people to understand the difference in sorts of possibilities. http://en.wikipedia.org/wiki/Modal_logic (in particular the first part on alethic modalities.

All I need for my story to be philosophically useful is the metaphysical possibility that it can happen. Most people, however, when thinking about speculative stories like this get stuck thinging it's impossible because they think it's physically impossible, or inconsistent with the actual laws of nature.

That said, the physics of time travel is an interesting topic. And, physicists do disagree. But, probably none of us have the resources necessary to decide who is right and who is wrong. Still, no matter who is right about the physical possibility or impossibility of time travel, if time travel is metaphysically possible, then we can continue to use these examples in metaphysics (http://www.rep.routledge.com/article/N095)

I don't know if I'd prefer to travel to the future or the past.

**leejo** · 01-08-2007, 05:44 PM

yeah, I understand. I'm asking you to report on some of the things you're seeing that the physicists are saying. You keep referring to these things but you're not showing them. I'm curious. I'd like to see what you're reading that's leading you on this goose chase!

It occurred to me a few minutes ago that when people speak of time travel, what they really mean is wanting a way to have the rest of the universe travel backwards through time while excluding time's affect on THEM. I do NOT want the atoms in my body to reverse their paths through time. I want ever other atom in the universe to reverse its path through time, though. Practically speaking, this complicates things.

"Philosophically useful". I like that term.

"Lacking the means or probability of being 'actually useful' but sufficient nonetheless to keep people employed."

**Rincewind** · 01-08-2007, 05:51 PM

**sordavie** · 01-08-2007, 05:54 PM

http://plato.stanford.edu/entries/time-travel-phys/

The best way to find out more information is to go through the bibliography here as well as the people these authors cite in their articles.

I don't know whether I should refer you to public lectures, like the one you referenced. As I mentioned in the intelligent design thread, a lot of what the public gets is quite dumbed down. If you want to really understand what's going on, you're going to have to goto the academic journals, where articles are not written for the public audience.

Earman's (1995) Bangs, Crunches, Whimpers and Shrieks: Singularities and Acausalities in Relativistic Spacetimes is a good place to start.

[edit] Just because something is philosophically useful doesn't mean it can't be practically useful. Philosophically useful just means theoretically useful in philosophy. There are a lot of thought experiments that are theoretically useful for mathematics, logic, and philosophy which turn out to have lots of practical uses. Without them you wouldn't have a computer to play games on.

**Rincewind** · 01-08-2007, 06:15 PM

Quote:

Originally Posted by sordavie

http://plato.stanford.edu/entries/time-travel-phys/

The best way to find out more information is to go through the bibliography here as well as the people these authors cite in their articles.

I find the conclusions of the encyclopaedic entry fit well enough here:

Quote:

Originally Posted by http://plato.stanford.edu/entries/time-travel-phys/

We have been interested not in logical possibility but in physical possibility. But even so, our conditions have been relatively weak: we have asked only whether time-travel is consistent with the universal validity of certain fundamental physical laws and with the notion that the physical state on a surface prior to the time travel region be unconstrained. It is perfectly possible that the physical laws obey this condition, but still that time travel is not metaphysically possible because of the nature of time itself. Consider an analogy. Aristotle believed that water is homoiomerous and infinitely divisible: any bit of water could be subdivided, in principle, into smaller bits of water. Aristotle's view contains no logical contradiction. It was certainly consistent with Aristotle's conception of water that it be homoiomerous, so this was, for him, a conceptual possibility. But if chemistry is right, Aristotle was wrong both about what water is like and what is possible for it. It can't be infinitely divided, even though no logical or conceptual analysis would reveal that.

Similarly, even if all of our consistency conditions can be met, it does not follow that time travel is physically possible, only that some specific physical considerations cannot rule it out. The only serious proof of the possibility of time travel would be a demonstration of its actuality. For if we agree that there is no actual time travel in our universe, the supposition that there might have been involves postulating a substantial difference from actuality, a difference unlike in kind from anything we could know if firsthand. It is unclear to us exactly what the content of possible would be if one were to either maintain or deny the possibility of time travel in these circumstances, unless one merely meant that the possibility is not ruled out by some delineated set of constraints. As the example of Aristotle's theory of water shows, conceptual and logical “possibility” do not entail possibility in a full-blooded sense. What exactly such a full-blooded sense would be in case of time travel, and whether one could have reason to believe it to obtain, remain to us obscure.

And it saves you from reading their works cited.

**sordavie** · 01-08-2007, 06:24 PM

Um. That's the conclusion of the two authors who wrote that entry. That's not necessarily the conclusion of any of the authors they cite.

By the way, what do you think of their conclusion? Their conclusion isn't one about why some physicists think it's possible--which is what Leejo wanted references to. Their's is a philosophical conclusion about appropriate and inappropriate philosophical methods of figuring out whether something is metaphysically possible.

**leejo** · 01-08-2007, 06:29 PM

I was just kidding. Mental games are useful sometimes. The EPR experiment is a personal favorite.

Thanks for this link. I'll read it. First glance indicates that it's written by a philosopher, not a physicist, and that it's falling into a common problem that the laity have with quantum mechanics - relying on a cursory understanding of the theory and mental modeling of the subatomic world instead of the mathematics. When you approach quantum mechanics by trying to "see" how the systems work in your mind and extrapolating those behaviors to the big world...well it just doesn't work that way.

Another problem is that it contemplates physical states like "Quantum mechanics near closed timelike curves" that really only exist inside singularities (or near singularities) and only for near-massless particles at the quantum scale. If we were having a discussion about whether or not subatomic particles may experience quantum blurring through time as well as space, I'd be very interested (albeit still skeptical). Why not? But you can't conclude that since a boson may appear to blur time at the milisecond level that I could therefor screw my mother, much less become my own mother. I can only assume that this is heartbreaking news for "some in these forums".

**sordavie** · 01-08-2007, 06:35 PM

Um. Tim Maudlin specializes in the philosophy of physics. He couldn't do that, and be one of the best in his field, without knowing a good deal of physics. Don't mistake him for the laity. He knows just as much mathematics as any theoretical physicist and he's well versed in quantum mechanics.

And, while it's written by a philosopher, he cites a number of physicists. If you don't want to trust a philosopher of physics interpretation of some things that the physicists say, then go look at the materials he's cited.

I like how you're so skeptical about what a professional philosopher of physics has to say about physics, claiming that the laity don't have a good grasp on what's going on, and then going on to make claims of your own about physics.

**Rincewind** · 01-08-2007, 06:53 PM

leejo is actually far from being a layman in physics. As for laity, his views on metaphysical religion were not fully disclosed to me.

And: I'll bandwagon their conclusions, as I don't have the stamina to form my own.

01-07-2007, 08:24 PM	#31 (permalink)
RandomGuy Join Date: May 2003 Location: K-W, Ont. Age: 27 Posts: 1,727	Re: Time travel: how to be your own mother and father If traveling back in time were possible wouldn't we be up to our ears in time-travelers?

01-08-2007, 04:48 PM	#36 (permalink)
leejo Join Date: Sep 2003 Age: 39 Posts: 7,676	Re: Time travel: how to be your own mother and father Ok NOW I have read (shall read? Did read? Shall have read? This is another argument against time travel: the added verb tenses will have been creating so many tenses!) a physicist talking about the possibility of time travel, albeit with the sincere hope that it's impossible: http://www.hawking.org.uk/lectures/warps3.html And in the process read about states in which the universe tolerates VERY SMALL negative energy states - i.e. within the Heisenberg principle's tolerances. I know that you're approaching this from a philosophical angle, but boy howdy would time travel be a royal PITA for physicists. On the other hand I would completely wreck Liz Wooten if given another trip to the plate, so keep fighting the good fight.

01-08-2007, 05:34 PM	#37 (permalink)
sordavie Join Date: Nov 2005 Age: 27 Posts: 2,089	Re: Time travel: how to be your own mother and father Lol Leejo. I can't tell you what the physics says. I don't hold an advanced degree in physics. I can only report to you what the physicists say. But, even if actual physics doesn't allow for the kind of time travel I'm talking about, the laws of nature could have been different. This sort of possibility is termed metaphysical possibility by philosophers. Actually wikipedia's article on modal logic is decent enough for most people to understand the difference in sorts of possibilities. http://en.wikipedia.org/wiki/Modal_logic (in particular the first part on alethic modalities. All I need for my story to be philosophically useful is the metaphysical possibility that it can happen. Most people, however, when thinking about speculative stories like this get stuck thinging it's impossible because they think it's physically impossible, or inconsistent with the actual laws of nature. That said, the physics of time travel is an interesting topic. And, physicists do disagree. But, probably none of us have the resources necessary to decide who is right and who is wrong. Still, no matter who is right about the physical possibility or impossibility of time travel, if time travel is metaphysically possible, then we can continue to use these examples in metaphysics (http://www.rep.routledge.com/article/N095) I don't know if I'd prefer to travel to the future or the past. __________________

01-08-2007, 05:44 PM	#38 (permalink)
leejo Join Date: Sep 2003 Age: 39 Posts: 7,676	Re: Time travel: how to be your own mother and father yeah, I understand. I'm asking you to report on some of the things you're seeing that the physicists are saying. You keep referring to these things but you're not showing them. I'm curious. I'd like to see what you're reading that's leading you on this goose chase! It occurred to me a few minutes ago that when people speak of time travel, what they really mean is wanting a way to have the rest of the universe travel backwards through time while excluding time's affect on THEM. I do NOT want the atoms in my body to reverse their paths through time. I want ever other atom in the universe to reverse its path through time, though. Practically speaking, this complicates things. "Philosophically useful". I like that term. "Lacking the means or probability of being 'actually useful' but sufficient nonetheless to keep people employed."

01-08-2007, 05:51 PM	#39 (permalink)
Rincewind Join Date: May 2005 Location: Casting useless spells in Oklahoma. Age: 27 Posts: 2,804	Re: Time travel: how to be your own mother and father Lol __________________ ~~ Veritas simplex oratio est ~~ No matter how far a wizard goes, he will always come back for his hat. --T. Pratchett <---- You know you're getting old when you rely on your forum meta-data to remind you how old you are. Required Reading for all TG sandboxers

01-08-2007, 05:54 PM	#40 (permalink)
sordavie Join Date: Nov 2005 Age: 27 Posts: 2,089	Re: Time travel: how to be your own mother and father http://plato.stanford.edu/entries/time-travel-phys/ The best way to find out more information is to go through the bibliography here as well as the people these authors cite in their articles. I don't know whether I should refer you to public lectures, like the one you referenced. As I mentioned in the intelligent design thread, a lot of what the public gets is quite dumbed down. If you want to really understand what's going on, you're going to have to goto the academic journals, where articles are not written for the public audience. Earman's (1995) Bangs, Crunches, Whimpers and Shrieks: Singularities and Acausalities in Relativistic Spacetimes is a good place to start. [edit] Just because something is philosophically useful doesn't mean it can't be practically useful. Philosophically useful just means theoretically useful in philosophy. There are a lot of thought experiments that are theoretically useful for mathematics, logic, and philosophy which turn out to have lots of practical uses. Without them you wouldn't have a computer to play games on. __________________

01-08-2007, 06:24 PM	#42 (permalink)
sordavie Join Date: Nov 2005 Age: 27 Posts: 2,089	Re: Time travel: how to be your own mother and father Um. That's the conclusion of the two authors who wrote that entry. That's not necessarily the conclusion of any of the authors they cite. By the way, what do you think of their conclusion? Their conclusion isn't one about why some physicists think it's possible--which is what Leejo wanted references to. Their's is a philosophical conclusion about appropriate and inappropriate philosophical methods of figuring out whether something is metaphysically possible. __________________

01-08-2007, 06:29 PM	#43 (permalink)
leejo Join Date: Sep 2003 Age: 39 Posts: 7,676	Re: Time travel: how to be your own mother and father I was just kidding. Mental games are useful sometimes. The EPR experiment is a personal favorite. Thanks for this link. I'll read it. First glance indicates that it's written by a philosopher, not a physicist, and that it's falling into a common problem that the laity have with quantum mechanics - relying on a cursory understanding of the theory and mental modeling of the subatomic world instead of the mathematics. When you approach quantum mechanics by trying to "see" how the systems work in your mind and extrapolating those behaviors to the big world...well it just doesn't work that way. Another problem is that it contemplates physical states like "Quantum mechanics near closed timelike curves" that really only exist inside singularities (or near singularities) and only for near-massless particles at the quantum scale. If we were having a discussion about whether or not subatomic particles may experience quantum blurring through time as well as space, I'd be very interested (albeit still skeptical). Why not? But you can't conclude that since a boson may appear to blur time at the milisecond level that I could therefor screw my mother, much less become my own mother. I can only assume that this is heartbreaking news for "some in these forums".

01-08-2007, 06:35 PM	#44 (permalink)
sordavie Join Date: Nov 2005 Age: 27 Posts: 2,089	Re: Time travel: how to be your own mother and father Um. Tim Maudlin specializes in the philosophy of physics. He couldn't do that, and be one of the best in his field, without knowing a good deal of physics. Don't mistake him for the laity. He knows just as much mathematics as any theoretical physicist and he's well versed in quantum mechanics. And, while it's written by a philosopher, he cites a number of physicists. If you don't want to trust a philosopher of physics interpretation of some things that the physicists say, then go look at the materials he's cited. I like how you're so skeptical about what a professional philosopher of physics has to say about physics, claiming that the laity don't have a good grasp on what's going on, and then going on to make claims of your own about physics. __________________

01-08-2007, 06:53 PM	#45 (permalink)
Rincewind Join Date: May 2005 Location: Casting useless spells in Oklahoma. Age: 27 Posts: 2,804	Re: Time travel: how to be your own mother and father leejo is actually far from being a layman in physics. As for laity, his views on metaphysical religion were not fully disclosed to me. And: I'll bandwagon their conclusions, as I don't have the stamina to form my own. __________________ ~~ Veritas simplex oratio est ~~ No matter how far a wizard goes, he will always come back for his hat. --T. Pratchett <---- You know you're getting old when you rely on your forum meta-data to remind you how old you are. Required Reading for all TG sandboxers

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