Philosophical Objections to Time Travel

So all this talk of the fourth dimension and gateways to 600BC has got me thinking we should discuss a few of the standard objections to time travel that have less to do with physics and more to do with logic.  Here are a few I've come across as well as a couple I've thought of myself.

1. Grandfather Paradox.  Pretty much everyone has heard of this or some variation of it.  You go back in time to kill your baby grandpa which results in your not being born to kill him which results in your being born to go back in time to kill him.  Alternatively, you go back in time to "murder" yourself.

2. The universe will allow only some kinds of time travel; for example, the kind that doesn't allow you to create a grandfather paradox.

3. By time traveling, you would be putting matter into the universe that wasn't there before and by doing so, break the law of conservation of energy.  This is not possible therefore neither is time travel.

4. The future does not exist since it hasn't happened yet.  Therefore there is no place to travel to so time travel into the future isn't possible.

5. Einstein theorized and it has been proven that gravity and speed cause time dilation, but can we say that it doesn't affect time with a capital T, but rather just processes that occur within time?

6. Applying the classic Zeno paradox of halving distances in space to time.  That which is in locomotion must arrive at the half-way stage before it arrives at the goal.

7. If time travel was ever invented, then, in an infinite universe, there would be an infinite number of time travel trips that would fill up every instant in the universe.  Since this has not happened yet, it will not happen, regardless of the possibility of time travel.

So respond to any and all of these and throw out some more!

The Fourth Dimension

Kamper brings up the point that we can't just assume the dimension of time behaves the same way as the three spatial dimensions.  Why not?  Let's think about it for a minute.  I can walk to the corner store from my house, and then I can walk back to my house from the corner store.  Or I can run.  I can slowly climb up a ladder, sit up there for a minute, and then slide down a twisty slide.  Point being, in all of the first three dimensions, the direction and speed of motion is not limited - more or less.  

But it would at least seem like time is different in this regard.  If we think again about time as a line, we realize that we can only move along that line in one direction and only at one speed.  The idea of speeding up and slowing down time or going "backward" in time doesn't make sense.  Right?  Wrong.  Or, kind of wrong.  The "pace" of time is effected by gravity.  Time runs slower in areas closer to a massive object and and faster in areas further from a massive object.  The difference can be extremely small, but it is enough that GPS satellites in Earth orbit have to have their on-board clocks routinely resynched with clocks on Earth's surface.  Theoretically, time stops altogether at the center of a black hole, not a small difference at all.

More to the point, it is obvious at least that we humans are not capable of traversing time with as much freedom as we can space, but that doesn't necessarily say anything about the nature of time as much as it can say something about the nature of humans.  Human beings are not capable of flying, but we developed machines that were; and we got inside them.  That's the story of our species.  We encounter a boundary that we can't cross and instead of going somewhere else, we develop a technology that crosses that boundary for us.  That said, we are finite creatures with finite abilities and so it may be that time travel is possible, just not for us - or our machines.

A Fixed Point in Space

In the Welcome post I mentioned that the time machine could possibly open a "so-called 'gateway' at a fixed point in space".  There have been some  questions as to what that phrase would mean.  One sharp individual notes:

"...if we accept that the Earth rotates and moves and such, doesn't time travel also necessarily involve space travel? I mean, "a fixed point in space" is always relative. You walk into a "gateway" on a hill in Buffalo in 2012AD. Why would you expect to exit on a hill in Buffalo in 600BC (or whenever) and not be left out screaming in the "fixed point in space" where the earth is no longer at? Or caught in solid rock or some such?" 

Of course John is right.  I think, though, the problem is just semantic.  Where I used the word space, I should have used the more accurate term space-time.  To stick with his above example, Let's instead say that the gateway connected two points, a current location and a destination; both of which exist in space-time.  It isn't enough to say the location is a hill in Buffalo.  A hill in Buffalo in 2012AD is a totally different location in space-time than a hill in Buffalo in 600BC.  Typically one would think of the hill in Buffalo as the location and then there is the matter of when (600BC vs. 2012AD), but, as John points out, this model doesn't seem to make sense for time travel as we want it.  We have to imagine the fourth dimension as being simultaneous, not sequential; A hill in Buffalo in 2012AD and a hill in Buffalo in 600BC both exist along side each other.  It helps me, when thinking about such things, to reduce all the dimensions.  See the below diagram:

The three spatial dimensions are reduced down to a zero dimensional point.  Therefore, the line, being one dimensional, can represent time because it has one extra dimension than a point.  Point A represents a hill in Buffalo in 600BC and Point B represents a hill in Buffalo in 2012AD.  Points A and B do not exist in the same "dimension" because in a zero-dimensional universe there can be only one point.  The only way to have more than one point is to add at least one more dimension.  Think of our three-dimensional universe the same way.  Let's use "universe" to mean all that exists at any given present moment.  So like the point in  zero dimensions, there can only be one "universe" in three dimensions.  And in the same way that the distance between two zero-dimensional points can be traversed by traveling on a one dimensional path (line), the distance between two three-dimensional universes can be traversed by traveling on a four-dimensional path (time).  Here we are understanding the universe today and the universe tomorrow (or a hill in Buffalo in 600BC and a hill in Buffalo in 2012AD) as two distinct things, in the same way that two points along a line are two different things.  So it's important to think of space and time as not being two independent things, but rather think of time as a sort of a fourth spatial measurement.  So if we we are on a hill in Buffalo in 2012AD and want to get to what we would think of as that same hill, but in 600BC through some sort of a gateway, I guess it would mean that we'd have to somehow give our time machine coordinates and it would then connect the two "points" in space-time by bending the fourth dimension.  Now look at the next diagram:

If we want to get from Point A to Point B we have to travel along the line of time; in other words, we wait.  But if time is bent, the two points can be brought together and the "distance" between them made small.  This would be where the gateway would happen.  This is what I meant by, "a fixed point in space".  It's better to say, "a specific location in space-time", or maybe better yet,  refer to the gateway as a threshold that connects two specific locations in space-time.

As a footnote, we can infer by this method of thought that if time is to be bent in such a way, there must be a dimension above time for it to be bent into.  The two zero-dimensional points on the one-dimensional line are brought closer to each other without affecting other theoretical points along the same line (relative to the line) by using the second dimension.  So if the line is time, then the area it is bent into would be a fifth dimension.  Thoughts?

Welcome

Let me start by telling you a little bit about my vision for this sight.  The hope is to eventually build a working, honest-to-goodness time machine, yes, but that is years away still.  In the meanwhile I want to foster a community that is centered around the sharing of ideas regarding time travel.  You don't have to be an expert on the subject (although experts are excitedly welcomed to share their knowledge!), just an enthusiast who wants to be part of the group.

Let's lay some basic ground rules for the machine itself.  It must be able to travel both forward and backward through time, so we're talking about more than just flying really, really fast.  It may be some sort of vehicle that moves through space, or it may be some kind of device that opens a so-called "gateway" at a fixed point in space.  It must be able to use a power source that is available to humanity right now - none of this Leonardo  da Vinci helicopter nonsense.  In short, it has to work and it has to be possible to build it.

The idea is to actually get this thing built at some point so while new ideas are always welcome, let's try to eventually come up with a plausible theory and then work together to refine a single design.  In all likelihood we'll probably have multiple designs going on at once.  That could work too, but either way, let's stay focused.  Eventually we'll get more into the nitty gritty of building materials and methods etc., but at first, let's not limit our imaginations. 

I also want to expand this beyond the designing and building of the machine.  I welcome submissions of artwork that is inspired by time machines or time travel.  This could be visual art, poetry, story, music, performance, film, installation, whatever.  We can also host conversations about current events as they relate to time travel, new developments in the field of quantum physics, favorite fictional time machines, or anything else that's relevant and interesting. 

So let's start a community!

Feel free to comment, but please submit any new topics / potential posts (including photos, links, video, audio, etc.) to The Communal Time Machine Blog.