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OK, say you have a man, who will invent a time travel machine that can send objects to do it back in time. But in order for him to do it, he needs to stumble across a chance document in a locked safe. Fortunately, a key pops through time that allows him to open the safe, which he sent to himself twenty years hence. He opens the safe and twenty years later, he develops a working time machine. Then he sends the same key he used to open the safe back through time to himself twenty years ago.

Now here is the question. How old is the key? Think about it for a moment or two.

When was the key made?

Spanky The Dolphin wrote:When was the key made?

That's actually irrelevant. He received the key from the future and thus for those twenty years, has always had it until he sent it back to himself.

Its like the Triluminary in Babylon 5.

Sinclair took it back in time 1000 years and passed it on to the Minbari, who gave it to Sinclair, who took it back in time...

It is infinitely old.

Assuming that this isn't an alternate timeline/parallel travel method, it is twenty years old, but it causes a paradox loop.

Ah, good, but how can something be infinitely old? Keys wear over time.

Spanky The Dolphin wrote:Assuming that this isn't an alternate timeline/parallel travel method, it is twenty years old, but it causes a paradox loop.

But how can it be? It was twenty years old when he first received it, so when he sends it back, it would be forty. But if it's forty years old when he sent it back... et cetera.

Maybe this is a nitpick, but wouldn't the man have to gain access to the safe to build the time machine in the first place? I mean the key can't travel back until he builds the machine, and he can't build the machine without the document in the safe, and he would have to have access the safe to start this whole paradox rolling, right?

Paradox loop.

Agreed, but not the first time through.

This is actually a trick question, because there is no answer. This is because the situation is an impossibility.

The Vulcan Science Directorate has deemed time travel impossible.

That you in the AV, Spanky? You're looking rather... sultry.

Drewcifer wrote:Maybe this is a nitpick, but wouldn't the man have to gain access to the safe to build the time machine in the first place? I mean the key can't travel back until he builds the machine, and he can't build the machine without the document in the safe, and he would have to have access the safe to start this whole paradox rolling, right?

No he wouldn't, because he sent himself back the key to open the safe before he first encountered it.

Spanky The Dolphin wrote:This is actually a trick question, because there is no answer. This is because the situation is an impossibility.

:makes sounds by clapping one hand:

Gil Hamilton wrote:

Drewcifer wrote: Maybe this is a nitpick....he would have to have access the safe to start this whole paradox rolling, right?

No he wouldn't, because he sent himself back the key to open the safe before he first encountered it.

But he can't the first time, because he doesn't have the key yet. And he never can (as long 4d spacetime is singular linear expression )

Drewcifer wrote:But he can't the first time, because he doesn't have the key yet. And he never can (as long 4d spacetime is singular linear expression )

But he can, because the he sent the key backwards in time to before he encountered the safe, thus allowing him to open it, and thus built the time machine, ergo he can send the key back in time. This is a time loop, and as such, there is no first time.

Drewcifer wrote::makes sounds by clapping one hand:

That's actually a corruption of a Zen Buddhist Koan ("riddle"). The correct form is as follows:
"In clapping both hands, a sound it heard. What is the sound of one hand?"

And the correct answer is:
*faces questioner and thrusts one hand forward*

That's all there is to it.

Gil wrote:But he can, because the he sent the key backwards in time to before he encountered the safe, thus allowing him to open it, and thus built the time machine, ergo he can send the key back in time. This is a time loop, and as such, there is no first time.

But he can't send the key back, because he can't build the time machine without it.

What would fix it for me would be if he at a later date gained access to the safe anyhow, and then built the time machine to send the key back to start the whole loop. Does that make sense?

Spanky: thanks for the info, very interesting.

edit: added quote, and kudos to Spanky.

Gil, as I said above, this is a trick question because there is no real answer whatsoever.

The situation is impossible, and that is why it is a paradox.

Drewcifer wrote:But he can't send the key back, because he can't build the time machine without it.

What would fix it for me would be if he at a later date gained access to the safe anyhow, and then built the time machine to send the key back to start the whole loop. Does that make sense?

Because it's unessecary. Because of the nature of causality, in this particular situation, at no point during the process does he not have the key. This is a counter-intuitive idea, but that's time travel logic for you.

Gil Hamilton wrote:Because it's unessecary. Because of the nature of causality, in this particular situation, at no point during the process does he not have the key. This is a counter-intuitive idea, but that's time travel logic for you.

My thinking is that before he sends the key back, he doesn't have the key, and therefore can never have the key.

I guess I need to refresh myself on casuality loops

Drewcifer wrote:My thinking is that before he sends the key back, he doesn't have the key, and therefore can never have the key.

I guess I need to refresh myself on casuality loops

The point that I think we are confused on is that in my head I'm picturing the problem as the effect causing itself in a big circle. If that is the case, there isn't a first cause. The key always existed in that span of twenty years and he always possessed it.

By the way, this thread does have a point. Right now, I'm trying to demonstrate time travel is unworkable by this very scenario. If this scenario was possible, that means you've got a key that has always existed. In that case, some perversity of the universe made the key pop into existance without anyone making it.

It's like the Heinlein story "All You Zombies", where the person in the story turns out to be his own parent, and thus, by implication, sprung into the timeline without any cause.

So either the universe is a truly perverse place where things can spring into existance and always exist thanks to time travel, or time travel is impossible. This thread was a thought exercise to get other peoples thoughts on the matter.

Gil Hamilton wrote:The point that I think we are confused on is that in my head I'm picturing the problem as the effect causing itself in a big circle. If that is the case, there isn't a first cause. The key always existed in that span of twenty years and he always possessed it.

I guess that's why these are paradoxes: he always has the key, yet never does. Like that german cat in the box that always lives and always dies.

Thinking about the age of the key is interesting. It leads me to suggest that you can't set up a paradox loop as described: it's a closed loop, ie we can say that it has an end, it doesn't go on indefinitely. The end comes about because of the fact that a key wears. Since its specified that the same key is sent back, each time it's used, a microscopic amount of wear occurs. Eventually the cumulative wear means that the key won't lift the levers/tumblers of the lock and at that point no time machine can be created and the loop is closed. I'd suggest that this means the whole thing falls, and that at no point is a time machine ever created.

The Third Man wrote:Thinking about the age of the key is interesting. It leads me to suggest that you can't set up a paradox loop as described: it's a closed loop, ie we can say that it has an end, it doesn't go on indefinitely. The end comes about because of the fact that a key wears. Since its specified that the same key is sent back, each time it's used, a microscopic amount of wear occurs. Eventually the cumulative wear means that the key won't lift the levers/tumblers of the lock and at that point no time machine can be created and the loop is closed. I'd suggest that this means the whole thing falls, and that at no point is a time machine ever created.

That occured to me to. Fascinating, isn't it?