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

You wake up and have your breakfast; read the paper; pop on your slippers and then get into your time machine. You go backwards 30 minutes...

You are wearing one pair of slippers but you've arrived at a time when your slippers are also still in the corner being chewed by the dog.

You have effectively created a pair of slippers from nothing (along with a time machine, a newspaper, your ingested Corn Flakes and yourself). Doesn't that break some sort of matter/energy can't be created/destroyed rule?

*scratches head*

No, because it's the *same* pair of slippers, just existing at the same time in two different spaces.

Actually, I don't believe that time travel is possible in the way we think it *should* work. That is, I think that if (backwards) time travel were possible, then we could not travel backwards in time and see our grandparents etc, since they *would no longer be there*. They have travelled forward in time to the present day, and would no longer be in that 'past' time.

If this all sounds a bit weird, it's because words like 'no-longer', are referring to an extra dimension relative to which travel is possible (compare with travelling through space. This only makes sense if there is an extra dimension - time - to travel relative to. Likewise with time travel - travel through time presumes a fifth dimension to travel relative to.)

I think this'll go in as a journal entry...

This would appear to be some strange, new definition of "same" with which I am unfamiliar.

One thing doesn't exist at the same time in two places. And don't go all quantum on this point - we're talking about actual lumps of rubber and nylon here, not some sort of potential slipper wave.

Anyway, if you put both pairs of slippers on some bathroom scales they'd show double the weight of one pair of slippers so you've still created matter.

Aaaarrrggh Mo

Philosophers in this field go on at this point about 'personal' time-lines. In the slippers time-line, they start off on the kitchen floor, get put on your feet, and half an hour later they are back in the kitchen again, standing next to themselves... One set of slippers, in two places, at one time - but at *two* times in the slippers personal timeline.

But you are right - this is a big failing in the common perception of time travel. Hence my offered alternative, which is perhaps better stated by analogy. I am in the kitchen, have a slice of toast and cup of tea, go back in time to when I'd put the bread in the toaster. This is a circle in time - I'm back where I started. So do I meet myself there making toast? No.

Analogy: I am driving along the M25, pass the Heathrow turn off. 3 hours later I am still on the M25 and I pass the same turn off. Do I see myself there? Of course not - my 'old' self is no longer there. The same with time - since leaving that point in time, I have travelled forward in time, and then back in time and then forward again. Hence I am not there to meet myself in the Kitchen. But then the kitchen wont be there either, having travelled forward in time with me, but then carrying on forward as I make the step backwards in time...

Headache. Time for a coffee!

If people move forward in time (according to the argument that you cannot meet yourself in the past), all other matter must do the same. So, if you travel back in time even the tiniest fraction of a second, you would find yourself in a black, empty void. I see a problem developing here.

Okay, I think I see the reason for the confusion here.

As I understand it, you're not -creating- matter, you're just transferring it.

If we agree for the moment that time is the fourth dimension, then someone could conceivably create a device that allows one to move from one point to another in that dimension. We can already move in the third -- that's called "walking." Time travel would just be an extension of that movement.

Exactly! Although it could well be more complicated than that...

People and things have spatial extent, so why not temporal extent? And perhaps things have different temporal extents. So you perhaps could travel back a fraction of a second, and see a temporally shifted part of yourself. And travel back a couple of years and see perhaps objects from now with a large temporal extent, and many other weird and wonderful things that started their temporal journeys further back down the track than we did....

The problem developing is only a problem from the classical interpretation of time travel, not to this version. Would explain phenomena such as ghosts as well - temporally shifted entities whose consciousness (which would have to have no temporal extent) is located a few seconds before ours, but whose physical form overlaps into our time-frame.

Something to think about anyway! But not too hard...

OK, Piece of the Universe, do you mean that, for conversation of energy purposes, you are totalling energy across all 4 dimensions as opposed to across 3 dimensions at any one time? Are you saying, in other words, that it would be permissible to "borrow" some matter/energy from one time frame and "lend" it to another so long as the total doesn't change?

Mo

if time is a series of events percieved by the mind, and free-will controls what events the mind chooses to see, then if you traveled back in time you would be rewinding what you have already seen. all you could do is watch it over again, because you control your free will in your own second by second time, and what you are seeing has already happened and cant be changed. what ever you try to do would be happening in present time, so you might look like a huge idiot to everyone else and not know it....
just a thought...

We are constantly travelling backwards and forwards in time but we don't notice. When time is "reversed" the laws of physics still apply but things will move in "reverse" as well. Chemical reactions happen the other way round and stars seem to absorb light. However long we "go back" for has to be played out again the same way. As everything is reversed we forget as we go back and repeat as we go forward again. If we pass a point that we have "never" passed before then it happens for the "first" time but may be repeated.

It is only our perception of these events that interprets "time" as a journey through "space-time" at a constant "time-velocity".

This is the only scenario that fullfils the restrictions on conservation of energy(/matter) without crossing interdimensional boundries and ignoring the consicuences (and spelling).

Result; we can travel back and forward through time but our memory is time dependant so le don't gain new information from the experience.

At least, that's what I told my physics lecturer after 3/4 of a bottle of Tequila...

To quote Doc Brown, Moadib, "You're not thinking four-dimensionally."

The only conservation of matter/energy we know about is in what we call a forward direction. There would be no "borrowing," only a displacement. You still exist in the same universe, just on a different dimensional plane. You would want to use however much energy it takes to get to the fourth dimension, find where you want to be on the timeline, and touch down again in the third -- although how much that might be is an as-yet unknown factor. As I understand it, there would be no subtraction or addition of energy, if we assume that the matter/energy levels are constant throughout the whole of time.

To call it displacement is cheating. If I move a bottle of Tequila from one side of the room to the other I DON'T end up with 2 bottles. Why should time be any different? I did exagerate when I said that it was the only scenario that fitted the rules, but the others include cancelling out the previous selves and that's gonna hurt, trust me.

You're thinking of time from the point of view of memorable experiences. there's nothing in the laws of physics that say anyone has to remember what happened. Time reversal is literal (and probably sober) It's a pure applicable physical principle nothing more nothing less. It doesn't exist for narative purposes, no romance, no regrets. just simple laws.

ok so you can go back in time but you wouldnt remember it, so if it happened to you, you wouldnt know it, nor would anyone else, so you cant prove or disprove that it happens.
also, why is everyone talking about traveling back in time? there is always the prospect of going to the future, buying a sports almanac and coming back so that you might be a huge millionaire. and you cant accidentally screw up the future like you might the past....

Going to the future is easy. Take one bottle of tequila, drink, arrive 8 hours into the future. Simple. The problem wwith travelling into the future is that you don't know if you'd have died in the mean time. Your atoms might be needed for something importamt. If you just take them back for your future self then things are going to go a bit pete tong if you ask me.

Coming in late but landing while running...

Another problem with the tequila method is not only do you not know if you are still alive, but you also don't know where you are, where your clothes went, or who that is in bed with you.

Seriously though, on the subject of E=mc^2 and al that, an interesting proposition; when sending something either forward or backward through time, you might end up actually creating energy rather than using it. It's very simple: Say you send your slippers forward in time. The machine flashes lights, makes noises, puffs smoke, whatever, and bang, your slippers are gone to wherever you sent them. Suddenly, in the present time, we exist in a universe minus one pair of good slippers. Uh-oh. What happened to E=mc^2? We just made matter vanish off the face of space-time, because the slippers haven't come back yet relative to us. But wait, Einstien comes to the rescue and we're all blown to pieces from the resulting explosion, because making the slippers vanish released an amount of energy equal to their mass at the moment they vanished. Presumably, when the slippers show up again, an equal amount of energy will be absorbed.

This leads to some interesting conclusions: For instance, would we have to be there to pump that energy back out of the universe to make the slippers appear at the set time? And sending things back in time has its own problems which are even more complicated by this theory.

if we time travel all the time and keep forgeting it, how come nothing hapens to us, like, say a simple stain on your slipers from, say, a urinating dog?

If you move a bottle of tequila from one side of the bar to the other, the air molecules that were at that side of the bar move to where the tequila bottles once were. For time travel to be conservative, when you backslide, past-time air would forwardslide, and since we're talking mass and not volume, when you leave, there would be a whoosh, and when you came back there would be a sucking in. So don't time-travel to near loose-leaf paper, or you'll experience what I call 'Temporal Paper-Cut Syndrome.'

LoL