h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

http://www.bbc.co.uk/h2g2/guide/A274105

This is a page I made with my theories about time travel. I'll admit, it's slightly opinionated, and I slipped into first person here and there, but I believe that it does a good job of explaining many of the more popular theories about time travel. If you have any specific comments, reply, or email me. My address is available on my personal space. I may not check this forum frequently, but I check my email at least twice a week.

Thank you
PhysicsMan

There's a lot of good stuff in this entry, but I would not be happy with it as a Guide Entry in the form it is at the moment. It seems to be more of a "I can't see how they could ever get time travel to work" article.

Your discussion of the Wormhole theory talks about the need for a fourth spatial dimension to set up wormholes. This is not so. It is possible according to Einstein's theories to have wormholes with only three spatial and one time dimension. All of Einstein's warping of spacetime occurs within these four dimensions. There is no need for extra dimensions.

I think the rotating cylinder would have to be longer than a mile. I thought they were talking about a cylinder about the size of our solar system and with a mass equal to a small galaxy.

What can you do with this article?

I suggest that you state each possible method of time travel, then in a small separate section with a subheader, list the problems associated with the method. This would be much clearer than putting them all in together as it is now.

A longer introduction might be worth doing, presenting some of the paradoxes, then launch into the "is time travel possible?" section.

I don't know if you've seen it or not, but a really good book on the topic is 'Time Travel: A writer's guide to the real science of plausible time travel', by Paul Nahin. I actually have used chapters from this book in teaching physics -- it's excellently written with good resources/references. Utilizing books like this and citing outside sources would make your entry less subjective.


Mikey

Hmm.

Interesting topic - and I'm glad you've covered it - but I feel that more info and more backup for what you say is probably a requirement... anywho, here's my thoughts...

general
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You need to incorporate the material here: http://www.bbc.co.uk/h2g2/guide/A398955 - you've already got most of it, but there's a few bits you've missed. Also, some of his explanations - in particular on time travel using wormholes, were somewhat clearer to me - you might want to replace some of your text with some of his in strategic places, and certainly to include his links. Whicheverwhat, you need to give the original researcher a credit: he's at U130314.

I'd like to see each section split into a bit explaining what and how, and a bit explaining the problems. I also think putting the "what would happen" bit before the "how to do it" bit would be good.

On a bigger scale, I don't think any entry on time travel should avoid mentioning that certain experiments appear to have things going back in time on a quantum scale. Your entry is perfectly valid on a large scale, but behaviour on a small scale may be different. You could deal with this by restricting the focus of your entry, or by writing about small scale time travel (if only briefly: it's not as interesting).

You'd also do better to mention the other objection to time travel: the "where are the tourists" objection. You touch on it somewhat at the start, but I think it warrants it'ss own section.

I don't think you have any big problem with balance - if you read it while trying to think like a time travel advocate, you might spot a few bits here and there which are a little off: "As you can see, it is unlikely that time travel would work". Generally, though, I think you deal with the controversy pretty well, and you're pretty fair throughout. Have no worries on that front: it's fine.

wormholes - there's an entry on this topic in the guide - "Black Holes and Wormholes" - can't remember the entry number, but it's searchable... there's also an entry on tesseracts, in the sense of 4-cubes. Unless you mean something else by a tesseract? Worth linking so you don't need to explain so much yourself - don't reinvent the ice cream sundae...

A wormhole would exist not "in the fourth dimension", but in (some number of) dimensions in addition to the normal ones we directly experience: three of space and one of time. Note that if there's one extra spatial dimension, there may be more (10 is often given as a number - dunno why), and there may too be more timelike dimensions.

re: 2d beings making 3d items. We don't have regular access to the extra dimensions postulated, but there's no reason why we should be completely barred - the things are supposedly there: it's just a question of making things able to go in that direction. It'll probably involve building giant doughnuts under the ground, most things do in physics...

Umm - the point of time travel with wormholes is that it is entirely possible to open wormholes to a different time as well as a different space (Einstein called such phenomena 'closed time-like loops'): never mind this weirdness with the end of the wormhole jittering about at super-light speed...

It's inevitably worth adding "according to mainstream theory" when you say that something is impossible. This goes for what you write on wormholes and FTL travel both. It's likewise sensible to add "probably" when you say that a technical challenge won't be met in our lifetime: futurology is notoriously inaccurate, hedging your bets is only sensible.

FTL - worth mentioning the postulated "tachyons" that go faster than the speed of light?

tippler method: worth explaining why they won't collapse into a black hole? I think the cylinder has to be hollow, or some such...??

rotating black holes: last I checked they existed, and we had seen them. A rotating black hole happens whenever something which is rotating collapses into a black hole, or swallows rotating matter. Both cases are pretty common.

You missed the "something we don't know about yet" theory, which is, to me, the most plausable. After all, we know very little about how stuff works, and most of our major theories have only been around for a few decades. Whose to say we aren't wrong?

free will is an illusion: Don't say that unless you can convince me that you know what free will is, and it isn't already an illusion. Free will is kinda controversial. You can get away with saying that "nondeterminism is an illusion", though - that'd be more accurate.

I'd call the 'any voyage into the past would cause the universe to end' theory something like "The End of the Universe". Incidentally, less with the word "theory" after everything: It's clear it's all speculation, why overemphasise?

'you are in the past therefore you were in the past' has been given a fancy name on star trek: they call it "The Predestination Paradox". Use that name: trekkies and sci-fi peops will know what you mean, and everyone else will appreciate the snappy title.

> "One thing to note about these universes is that, without inter-universe interaction, all parallel universes would be the same"

umm - unless the universe(s) is/are nondeterministic. In which case they may well be wildly different.

You could also note that prophecy is essentially time travel too: specifically, it's information travelling backwards in time. (prophecy as opposed to prediction). It'd be interesting to speculate on that: could you send information down a rotating black hole, for example, rather than going yourself and risking atomisation...

Good stuff - I learnt a lot reading the entry, and it was really fascinating. Nice going...

Killer article . I really like it. See if you can make those paragraphs a little smaller though, it's real easy to lose your place in there.

- Matt

An increadibly cool article. Lots of great info there, very well researched and presented. If I were a scout I'd recommend it (hint hint scouts!)

About the "The Slower than Anything Else Theory":

Abtually, this seems wrong to me. Going slower is impossible, it is just going faster in another direction.
Think of Einstein's train station watch thingie:
If a person rides on a train that goes close to lightspeed while it approaches the station, a person in the station sees the passenger's watch as being slow, while the passenger sees the person in the station looking at a slow wath as well.

It is mind boggling, but fast or slow is a reletive thing, and you can't say: this is moving and this isn't.

Well, I've emailed the author and asked him to stop by and respond to some of our comments.


Mikey

I hope you asked him to response yesterday.

Now for something a bit more serious.
There are allready theories that time travel not only is possible, but they are happening now. I refer to the field of parapsychology.
Whether the results are real, humbug, tricks or a hint of reality, I do not know, but it is possible to describe it in scientific terms.
You have to postulate a non-electromagnetic or -gravity field that encompasses at least the four dimensional space-time continuum.
The human mind then should somehow be able to receive information from distant points in his field, or even be able to influence distant points.
In the past there have been a number of experiments by Rhine, and they gave statistical indications that either these effects exist, or something very improbable had happend (or somebody used a few tricks unknown to the experimenter).
Most experiments were done by guessing cards or pictures someone else turned around in a different place. The number of hits was very much higher than can be explained statistically. In a number of cases not the current, but the next card that would be turned around in a few seconds, was predicted with much too many hits.

Whether his is real I do not know, but I do not a priory declare it impossible. I did a few of these experiments on myself, and got the same statistical flukes. I can never prove I did not cheat, but for myself I know I didn't.

IMHO, an excellent article.

The people who have already commented know this stuff better than I do, so I'll leave making any more suggestions.
Just one typo, where you mention dimensions near the beginning, you say 'forth' which should be 'fourth' with a 'u'.

Good entry and well done,

Kenrick

Well done my friend!
Your wonderful article is bound for the Edited Guide!
First it gets sent to the Sub-Eds to go over it with a fine-toothed comb to get rid of any remaining typos and then it's off to the Front Page. When it gets there, you'll recieve a nice email from Mark Moxon saying well done in some fancy and prosaic manner .
Please be patient though, as it takes some time to go through the system.
Have a on me,
Nice one,

Kenrick

Editorial Note: This thread has been moved out of the Peer Review forum because this entry has now been recommended for the Edited Guide.

If they have not been along already, the Scout who recommended your entry will post here soon, to let you know what happens next. Meanwhile you can find out what will happen to your entry here: http://www.bbc.co.uk/h2g2/guide/SubEditors-Process

Congratulations!

Oops, posted this in the wrong place...
I do think this Entry would benefit from a couple of references to classic time travel sf stories. I'll kick off with a few...
The Time Machine - H.G. Wells. Duh!
"-All You Zombies-" Robert Heinlein. Twelve pages of utter brilliance and a story that makes you go "wow!". The most tightly wound time-travel paradox story ever written. Anyone with any interest in time travel paradox should read this story. I've summed up the plot on my own space, so click the link above if you want to know what happens in the story. Warning - this will spoil your enjoyment of it, but it may be hard to find or you may be impatient.
"The Man Who Folded Himself" David Gerrold.
"The End of Eternity" Isaac Asimov
"The Fox and The Forest" Ray Bradbury.