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Time Travel is Impossible

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thetabularasa posted on Wed, Nov 28 2012 3:28 PM

The quantum physics discussion in my Holiday Dinner Table thread got me thinking about time and space, what is bendable, what isn't, whether General Relativity is a viable theory and so forth, and naturally I started considering the possibility of time travel. Here's how I know it is impossible:

Time doesn't exist. It is a manifestation of the human imagination. Things change; the world changes, we change and everything seems to be in flux somehow. Even if an object takes millenia to destruct and end, it inevitably does, similar to entropy, I suppose, in the sense that there is a systematic degradation involved in all things. Nevertheless, my point is that things are always changing, and of course distances between objects exist, but time itself does not exist. It is merely a subjective measurement system.

Prove me wrong if you must!

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thetabularasa:

No, causality exists regardless of time.

Please expand on that; I can't imagine that's even close to true.

 
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The subject is called "algorithmic complexity" "algorithmic information theory" "kolmogorov complexity" etc. It is very young (about half a decade), and some of its philosophical implications are hotly contested. I'll give you a springboard to dig around but there really isn't any single book I'm aware of that lays out the case as I've stated it above.

The single best proponent of algorithmic complexity - both in terms of explaining the subject and explaining its philosophical/physical implications - is Gregory Chaitin. I recommend buying one of his books (I own Meta Math! The Search for Omega and I also recommend his latest book on metabiology to give you an idea of how his ideas can be applied).

This page gives a fairly lucid translation between algorithmic information theory and the arguments I presented above.

While studying the topic - which goes far afield into computation theory, which seems to have no obvious implications to the structure of reality - there are some key things to keep in mind:

  • Computers can be built out of any substance... there are even "water-valve" computers.
  • A special kind of computer called a Universal Turing Machine (or any logical equivalent) is capable of simulating the operation of any other computer... it merely needs to be given a special encoding that specifies the logical operation of the computer it is to simulate
  • Because computers can be built out of any physical substance (including computers that are equivalent to a UTM) and because there are problems that are uncomputable (cannot be solved by a computer), it is trivially easy to prove there can be no physical theory of everything: Simply construct a physical computer that is configured to simulate the operation of a computer that is supposed to solve an unsolvable problem and set it running then ask "what are the laws of physics that govern the long-run behavior of this physical device?"
  • While the structure of reality itself does not appear to be "discrete", there is no reason its apparent continuity could not be the consequence of very fine discreteness (think of a digital photograph which is made of discrete elements but appears to convey a continuous image). This subject of research is called digital physics or digital philosophy.
  • Even more fundamentally, there is no reason that continuous systems could not arise which are subject to discrete laws. For example, if we speak of the "set of all computable real numbers", we are actually talking about an infinitely sparse subset of the "real number line" that mathematicians use. Nevertheless, there is no reason to suppose that mathematics could not be constructed completely on the computable real numbers (Godel went some way in this direction with his "constructible set theory"). Mathematicians feel there are "holes" between computable numbers but this could be because we haven't chosen a more natural metric.
  • The laws of statistical physics (e.g. thermodynamics, statistical mechanics, aerodynamics, etc.) are governed by an entropy relation that is formally identical to the information theoretic complexity of abstract codes! This suggests that information plays a crucial role in the physics of the world, whether as a fundamental substrate of some kind or merely as a reflection of the proper way for scientists to circumscribe their own ignorance (uncertainty).
  • Ray Solomonoff's theory of induction has put the Humean problem of induction to rest for all practical intents. While there are still some philosophical "hangnig chads" (see above regarding issues surrounding discreteness, etc.), the fact is that once you understand Solomonoff, you understand why these philosophical objections are irrelevant and miss the larger point.
  • Here's the Bible of Algorithmic Information Theory. I've read a significant portion of the book but I can't say that I've grasped more than 50% of it. A lot of it is simply advanced theorems as they apply to specialized research interests (e.g. the P=NP problem) but a lot of it is very important technical clarifications of the limits to which this subject can be applied (prefix codes, etc.)

I hope you find that helpful. Feel free to ask for further information/guidance as needed.

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Jon Irenicus:

Clayton, do you know of any good books that go into all that?

 
I typed up a long reply and it's pending moderation.
 
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When I was younger I used to believe that time didn't exist, but that what we called time was rather simply the passing of events.

Now I'm older and I hear of things in physics proving that time and space lie on some continuum. I'm not a scientist, so I have no idea, really.

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AJ replied on Thu, Nov 29 2012 9:05 PM
thetabularasa:

hashem:

Ok so I was watching some videos on special relativity and time, both supporting and opposed to it. I'm not an expert by any standard, but it seems at a bare minimum there's a definition game problem. It seems to be conflating perception of light with whatever definition of time supports the theory (the theory that time isn't just a concept about difference and distance, but something mystical, simultaneously not-physical and manipulatable) in any given context.

Again, I could be confused and probably am, but that's what I got from watching a few videos regarding both sides of the issue of special relativity and time.

Yes. If one can commit to the premise that there is no physicality to time, then one must admit that it does not exist. If it exists, it exists; if it does not exist, it does not exist. Sounds silly/sarcastic to say it like that, but I agree, hashem: time doesn't exist, therefore time travel is impossible.

 

Definitely, hashem, it's a definitions game. The theories are held up by malleable definitions that are changed flexibly as needed to support whatever claim is being made at the time. When traveling through space, space=nothing, but when talking about gravity, space=physical object that can bend. Getting a clear definition out of a modern physics proponent is like playing whack-a-mole.

And absolutely, TR, time does not exist. Timing (the action) *happens*.

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Clayton replied on Thu, Nov 29 2012 11:46 PM

Erratum: "half a decade" should be "half a century"

ugh.

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Anenome replied on Fri, Nov 30 2012 12:20 AM

If time's flow weren't a physical feature of reality, you'd have absolute time, where nothing could affect it's speed of flow.

However, that's not what we see. We see gravitational time delay, where atomic clocks running at different altitudes run at different speeds despite using the same unvarying periodic phenomena to meter the passage of time. The slowing of one of those clocks over another cannot be due to any physical interference apart from the property of time itself.

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Anenome replied on Fri, Nov 30 2012 12:25 AM
 
 

thetabularasa:

Yes. If one can commit to the premise that there is no physicality to time, then one must admit that it does not exist. If it exists, it exists; if it does not exist, it does not exist. Sounds silly/sarcastic to say it like that, but I agree, hashem: time doesn't exist, therefore time travel is impossible.

Again, traveling into the future by slowing your own time-frame -is- possible by the following means:

There are various ways in which a person could "travel into the future" in a limited sense: the person could set things up so that in a small amount of his own subjective time, a large amount of subjective time has passed for other people on Earth. For example, an observer might take a trip away from the Earth and back at relativistic velocities, with the trip only lasting a few years according to the observer's own clocks, and return to find that thousands of years had passed on Earth. It should be noted, though, that according to relativity there is no objective answer to the question of how much time "really" passed during the trip; it would be equally valid to say that the trip had lasted only a few years or that the trip had lasted thousands of years, depending on the choice of reference frame.

This form of "travel into the future" is theoretically allowed (and has been demonstrated at very small time scales) using the following methods:[24]

  • Using velocity-based time dilation under the theory of special relativity, for instance:
    • Traveling at almost the speed of light to a distant star, then slowing down, turning around, and traveling at almost the speed of light back to Earth[56] (see the Twin paradox)
  • Using gravitational time dilation under the theory of general relativity, for instance:
    • Residing inside of a hollow, high-mass object;
    • Residing just outside of the event horizon of a black hole, or sufficiently near an object whose mass or density causes the gravitational time dilation near it to be larger than the time dilation factor on Earth.

Additionally, it might be possible to see the distant future of the Earth using methods which do not involve relativity at all, although it is even more debatable whether these should be deemed a form of "time travel":

 
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AJ replied on Fri, Nov 30 2012 8:10 AM
The relativity theorists are very confused indeed. They say that one clock based on unvarying periodic phenomena ran slower, but they cannot possibly visualize this without selecting a reference frame in so doing, thereby privileging one frame. Either that or they visualize the situation from a god-like absolute vantage point, but then they could see no variance in the clocks, violating the conclusion.

Perhaps you can visualize a movie of two clocks running at the same unvarying rate while also NOT running at the same unvarying rate, but I cannot, and therefore can only treat such utterances as incoherent.

Whatever causes the purported differences in the clocks, I hope we can all agree it is not square circles, no matter who claims it or how many people agree that it has been "proven." An incoherent theory is no theory at all.

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It's interesting that there are basically two separate notions of "time" which have been discussed in this thread. One can be described as something like "simultaneity or non-simultaneity of observations". The other can be described as "causality". Relativity theorists use the former notion of "time" exclusively, although they still accept the existence of causality.

Interestingly enough, while relativity theorists think that the simutaneity or non-simultaneity of multiple observations is dependent on the observer's frame of reference, IIRC they conversely think that causality is not. The question then is whether Newton's notion of "absolute time" is actually analogous to causality rather than simultaneity.

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Anenome:

 
 

thetabularasa:

No, causality exists regardless of time.

Please expand on that; I can't imagine that's even close to true.

 

I hope the following isn't too crude. I'm genuinely thinking out loud, but then again, that's why I like this forum.

Causality, or cause-and-effect, is a subjective perception of change. It is the same with time. You are viewing matter changing from one state to another. When this state "started" and "finished" is totally arbitrary on behalf of your perception and what you choose to be the starting and ending phase to some "sequence." Granted astronomical interaction between matter happened before humanity, it is the system of measurement that is subjective. Any distinction between material processes or phases is distinguishable and accomplished by way of basic dimension, but my point is that measurement is purely subjective.

Now in the case of an astronomical unit, for instance, I would say that the perceived 93 million miles between the sun and the earth does exist as a distance but not as a number. It could just as easily be 149 million kilometers. So it's established that there is no absolute measurement, otherwise it would only be in feet or kilometers or some other form absolutely. But absolute distance, of course, did exist just as I know the matter in front of me exists right now. As for its characteristics such as color, length, width and depth, I really don't see anything absolute about it other than its presence and distance between other objects.

My point is that dimension physically exists and measurement does not physically exist. Dimension is objective; measurement is subjective.

Now onto time, which is much more complicated. The issue with time is you have to note that a point in space doesn't exist in our understanding other than by way of picturing objects at that particular point in space. Space is not a grid. Take our solar system. We think of it as a grid of sorts, but I maintain that the grid doesn't exist. Do the dimensions of length, width and depth exist as part of or between the planets and the sun? Of course! But to measure them is arbitrary. To say the object in my living room that I eat dinner on which has 4 legs and a flat surface is a table is not wrong; but you must acknowledge that it can also be called a mesa- it's all in how I choose to define it. But definition doesn't determine the nature of the table. In this case, perceiving the rate at which one object travels from the vacinity of another object to a separate object is calculated by velocity, which equals a measurement in time, doesn't determine the nature of the process. So people say, "This point in space to this point in space, going at a velocity of 60 mph, takes 2 days time." While

And more importantly, dimension does not equal measurement, just as the time it takes an object to travel from a separate object to a different object is subjectively one point in space versus another point in space when in actuality the point in space doesn't exist. Objects and dimensions exist, but we created the idea of one point in space versus another point in space. The object we call a table most certainly exists as an object and independent of all other objects, but to say it is absolutely a table is wrong. As for time, or linear time, being a fourth dimension, I say that it should come into play, and this is actually where I could use some help from any aspiring or actual physicists in here. Time itself is known to be a dimension, and in this case I cannot deny that time exists as a dimension, for if one object traveling at one speed is surpassed by another object traveling at a higher speed, I deductively know the faster one will reach the perceived destination faster than the other object traveling at a slower rate. But I mean to specify that time does not have physical property. 

Time itself is a construct we use to measure rates of objects relative to other perceived objects, thus the measurement of time itself does not exist; however, the dimension of time appears to exist (it must as I mentioned just above), but time being a physical property seems to be an oxymoron. This is where I say time is better viewed as being rate. Time indicates a measurement, and all measurement is subjective; but to deny rate is to deny the truth that the object traveling to another object (location) faster than another object traveling to the same "location" will arrive there sooner than the slower object. I suppose rate and speed would mean the same time, and time would be the subjective interpretation of rate as a dimension of its own.

Thoughts? Am I completely right or wrong? Curious to see some responses.

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AJ replied on Fri, Nov 30 2012 9:47 AM
TR, that's essentially spot on. I'd just be sure to distinguish between spacial dimensions and the more general concept of a dimension (temperature, brightness, etc.). Time (actually timing) is an action requiring an agent to do the comparing, and further this agent must have a memory to discern "before" and "after."
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Clayton replied on Fri, Nov 30 2012 10:12 AM

@Auto: Great observation. In my view, simultaneity only matters to the extent that it affects or can affect causality. Science is about explaining phenomena and explaining phenomena ultimately means identifying causal relations. Simultaneity matters only insofar as only simultaneous states of affairs can interact with one another (have causal relations). A finite propagation-speed of information necessarily entails relativistic mathematics, though relativistic maths do not imply a single, universal "speed limit"! Nor do they justify speaking of the "bending of time" or "bending of space" except in a formalized, mathematical sense.

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AJ:
TR, that's essentially spot on. I'd just be sure to distinguish between spacial dimensions and the more general concept of a dimension (temperature, brightness, etc.). Time (actually timing) is an action requiring an agent to do the comparing, and further this agent must have a memory to discern "before" and "after."

Thanks AJ. Thanks for the clarification on dimension, I hadn't thought about it that way before. That's where my thoughts were leading me, admittedly. I was wondering why we are considered a 3 dimensional universe when linear time is often considered a 4th dimension. Makes me wonder how we would have any knowledge of any other dimensions that we can't directly perceive in the universe. I suppose we may be inappropriately limiting ourselves to 3 dimensions?

 

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AJ replied on Fri, Nov 30 2012 12:05 PM
Forget perceive, we can't even *imagine* four spacial dimensions. One must at this point stop and ask the physicist what activity he or she is actually engaged in in uttering those words.

"Mr. Physicist, I was under the impression that these words you're saying are an attempt to get some idea you have in YOUR head into MY head. You know, an attempt to COMMUNICATE. But here you are claiming that the idea is not even in your own head in the first place, not even part of your own experience or imagination. Was it in someone else's head and you're just relaying some quote you don't understand but take on authority? Who then is this wizard who can vizualize 4D so as to speak about it and actually know what he is talking about? Are you communicating or doing something else? Because in any other context, saying words that don't refer to anything in your mind is known as incomprehensible blathering."

So it's not a matter of whether there is a fourth spatial dimension or not, it's a matter of not even knowing what anyone means by those words.

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