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Original post by nmi Probably you may say, effect causes cause. Why shouldn't the cause be earlier in time than the effect.

I don't say that. It's the interconnection that causes the paradox,
not the events themselves - they are perfectly constistent.

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Have you heard of the guy that made microwaves go through a whole where they shouldn't be able to pass through, but they did and they even traveled through time. The music starts to play before you hit the play button...

First, this effect (it's called tunneling, btw.) cannot be viewed
as information transfer in a physical sense, since the information is crippled during the process (e.g. music in -> static out).

Second, this 'story' seems to be made up (not by you, of course) otherwise it would have gotten enormous feedback from the scientific community, which it didn't.

To summarise some of the major ideas of time travel and to avoid paradoxes (that have not been observed):

1) Many worlds theory
- paradoxes are avoided by introducing an infinitve number of parallel universes (ie. 'Sliders' series)

2) Immutable past approach
- Nature itself prevents time travellers from altering history by either preventing certain actions that change future events (e.g. H.G. Wells 'The Time Machine') or 'restoring the state' after the traveller returns

3) Time is an illusion philosophy
- Each component of the universe negotiates it's 'current' state with 'future' and past states; A paradox is avoided negotiating the 'current' action (which may happen in the past, too) with the 'future' state and avoiding contradictionary situations (infinite effort required to keep things consistent) by not allowing such events.

Either approach to time travel is contra-intuitive and does not match common-sense (hence they are viewed as thought experiments by most serious physicists), which does not render them impossible.

Some examples for paradoxes can even be found in nature itself.
One of the most simple is the invisibility paradoxon:
One particle (e.g. an electron) is invisible.
Many particles (for example an electron cloud) are invisible, too.
Nevertheless some number of particles is visible (us, for example or a table), while there is no number x for which we can say every number of particles that is greater than x is visible.

Another unsolved mysterium is the 'border' between quantum world and macro world. The laws of quantum physics apply until a certain dimension is reached (e.g. amount of energy) from where on classical physics applies. For example you can shoot an electron through a barrier of some kind, which it normally cannot cross. Now try that with a tennis ball and a conrete wall [smile]. Theory says, there is a (infinitively small) chance for that ball to 'tunnel' through the concrete wall.

Another observable situation is, that sub-atomic particles seem to 'decide' how they behave with regards to the observer, e.g. an electron can appear to be a wave (interference experiments) or a particle depending on the observer.

Even more spooky is the fact that entangled particles seem to know each others state no matter how distant they are.

So to sum things up, in order to create a consistent time travel plot you need to pick one of the presented approaches and resolve paradoxes using the tools provided by the chosen theory.

The Dr. Alpha example would be solved as follows:

Idea for theory 1)
- The reprogrammed android creates a world where Dr. Alpha didn't build andriods. The android therefore travelled back into a different timeline. Problem solved. In this theory you cannot travel back into your own timeline, but into a parallel universe where you are actually present at that date.

Idea for theory 2)
- Dr. Alpha decides not to build the android. Instead a hacker already downloaded the necessary information from his PC and builds the first android, crediting Dr. Alpha for his work to avoid legal trouble. Dr. Alpha becomes involved in constructing androids anyway.

Idea for theory 3)
- Though Dr. Alpha is convinced not to build the first andriod, the reprogrammed andriods existence finally forces android tech to be established anyway. Because of Dr. Alpha's publications another engineer starts development, again crediting Dr. Alpha.

The interesting thing with idea 2) and 3) is that all changes are decoupled from the original cause of the paradox - e.g. Dr. Alpha himself. The engineer's and hackers memory wouldn't have to be altered at all as long as they haven't had contact with Dr. Alpha. For them, the android wave was started by Dr. Alpha, regardless if he build the first andriod or not. Dr. Alpha's memory would change, but since humans are in not able to perceive time directly - he wouldn't even notice.
As theory 3) says that time doesn't exist anyway, this wouldn't be a problem.

Sory for the long post, but the topic is way to interesting [smile].

Cheers,
Pat

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Original post by Anonymous Poster Little broblem of dumb programer. Time machines weight = 10 kg. Time machines programing. 10 Start 20 Wait 5 minutes 30 Time Travel 5 minutes to past. 40 Goto 10 How meny time machines there would be after 6 minutes from start of first on? Change to mass of universum? /Aki

Interesting problem.

The logic consequence would be that no energy may travel through time, but just information.

According to quantum physics that would mean changing the interpretation of the wave functions in the universe. The people that don't travel through time don't change their wave function interpretation, but just Dr. Alphas android.

Go to your nearest library and pick a copy of

"The End Of Eternity" - Isaac Asimov

Luck!
Guimo

...right beside "I robot"...

I guess I should rephrase my position. I don't see any contradiction in the Doraemon example. There is a seemly confusion if you state that the causes of all event must originate from the past and cause and effect cannot form a loop. But I did not state that.

In the Doraemon example, time travel cannot change the past. If you travel in the past and did something and go back to the future, you will only find that your pervious knowledge of the history was incorrect.

For example, if I go back and kill my grandfather and return to the future, nothing would change. After I went back to the future, I called my grandfather and he is still alive. WHY?

One possible episode:


1954
This is a piece of history unknown to you. An alien had abducted your grandfather. The alien is running around in your grandfather's body.

1955
The future you went back in time and killed your grandfather. After the murder you returned to 2004. Another piece of history unknown to you: You have murdered the alien and successfully rescued your grandfather.


2004
You decided to go back in time and kill your grandfather. You went back. You seemingly pwned him. You returned. You call your grandfather. He is alive. 'What The Hell?!' you said.

Hell no Estok [smile]! Your second example is sound. No cause-effect problems, no time warp. Everything is sound. It's not about the time thing. Not even about when each event takes place.
The Doraemon example simply lacks a cause for the initial event.
The event of the stolen coin was causes by the event of the stolen coin! That's the whole point. Entropy reaches zero, which is impossible. Why? Entropy is about choices and possibilities. To defeat entropy you need energy. Lots of. So in the Doraemon example, he has no choice but to travel back and steal the coin so that he can be accused for doing it in the first place. Number of possibilities to keep the universe in a sane state? One! Travelling back in time and stealing the coin.

The difference to your second example lies within the coupling of events. The guy didn't travel back because he killed his grandpa - he just did it. No direct coupling of events as in the Doraemon story.

I hope you finally understand the paradox of the story [wink].

you fell into your own trap it seems.


You kills the alien; therefore

Your grandfather lived; therefore

you were eventually born; therefore

you exist and one day you want to kill your grandfather; therefore

you kills the alien; ...


I don't see how this loop is different from the gold coin loop.

ok, i have been away a while, but now i have another one to break your brains on.

if time would be a whole, then people in the future would already be here. Now is the problem(or rather 2), we havent seen them yet. if time-travel WOULD be possible they would have to be here already. you can tell me everything you want, but when people from ages, millenias are even further come back, e have to see at least 1 sometime. if time would be a whole, then it doesnt matter they are to far in the future.

ok that was prob 1. now number 2
IF people have come back, there will be loads of em. in 21000 one comes, in 2200, but cause time seems to be infinete, i can write numbers all day. so, if there are so many people, they are changing the whole history... 1 makes a difference, but if there are more, it could be a huge inpact on time. also another prob. in the somewhat more rich countrys everyone has a passport. how do time travels get one? and dont tell they only arrive in poor countries

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Original post by Estok you fell into your own trap it seems.

Seems so [smile]. I missed the part of the grandfather not being able to raise birth to your father if you didn't went back in time.
Otherwise, yes - another impossible situation, which nature avoids to become reality (without having multiple-worlds).

You could win your very own time machine!

How to enter:
1. on a piece of paper write down the date and time you want to receive your time machine
2. wait until that moment
3. if you are the lucky winner, a time machine will arrive. Congratulation!

After receiving the time machine:
1. Go ahead to have fun with it, it is yours!
2. After having fun with it, be sure to send it back to the date and time you have written on the piece of paper. You are allowed to replicate the time machine and keep the original or the copy.