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IIRC this argument was originated by Larry Niven, though that may just be where I read it first.

The idea is this. Either time travel is possible, or it is not. If it is not, then we will never invent time travel.

If it is, then eventually time travel will be invented, and people will inevitably travel into the past.

When any given person travels into the past, he or she will either change the the future, or they will not. If they do not change the future then it will continue to be one in which time travel is invented, and people will continue to travel into the past.

If they do change the future, then the new future will either be one in which time travel is still invented, at some point, or it will be one in which time travel is never invented.

If the new future is one in which time travel is still invented, at some point, then people will continue to travel into the past and the future will continue to be changed, with each new future being one in which time travel is either invented, or it is not.

Sooner or later a time traveller will change the future to one in which time travel is never invented. At that point there will be no opportunity to ever change the past again, and so this future will remain. Thus, the only timeline that can ever be stable is one in which time travel is never created.

As such, it is inevitable that this is our timeline. And thus either time travel is an impossibility and always will be, or time travel is possible, but it will never, ever be invented.


Make sense?

Interesting train of thought although I wonder what exactly a time traveller has to do to ensure that time travel never will be invented (if we suppose it will be possible at some point) short of destroying all of humanity?

Now I know...Sci Fi has a huge hard on for the "Butterfly Effect" but I always wondered.....how big an impact can a time traveller really be? How long before everything evens out again in the long run? Go back in time and kill Hitler. Would that prevent WW2? Most likely not. Would it change how it played out? Certainly. But 100 - 200 years later...how different would the world look?

You sent a time traveller back to ancient Rome with a macbook and a couple of powerbanks to examine Caesar gallic wars. Even if the Madman tries to alter history and teach the romans modern tech......how successfull can he really be? Suppose he even manages to teach a couple of people modern concepts.....how likely are these to survive in the chaos of history?

My point is....it seems history has a certain "momentum" which is needed to really affect meaningful change. It seems ancient greeks where perfectly aware of steam engines and had some steam engine toys. It might well be that a time traveller tought them the concept of it. But in the end they only had a couple of toys crafted, it was a curiosity and it would take far more factors to come togehter to get to the industrial revolution, non of which were present in ancient greece.

Maybe our timeline is much more robost and immune to these things that time travellers really don't matter all that much and hence...we don't notice.

Maybe we are already living in a timeline where time travel is possible. Why don't we encounter time travellers? Maybe we do...who could tell? Maybe in the future they already know all big mysterys from our past (how were the pyramids built, what killed the dinosaurs etc.) we just are not there yet.

Or maybe time travel is possible...but only to the past with no way of bringing a person back? That is why nobody is doing it. Who wants to be stuck in a - comparatively - horrible past with no way back?

So many possibilies....

I think time travel is impossible but, that's because i can't grasp the idea of time being a tangible object to travel through.

There is no concept in physics that prevents time travel from being possible. There are even theories (for example one concerning rotating black holes) which allow it to work. They are, at least to us at the moment, all completely fatal.

Time travel of a sort is possible - you can essentially travel into the future by just accelerating to near light speed. (Just, he says, as if that's a thing we can do). Or as Ian says, go close to a black hole. In fact if you fell into a black hole you would see the universe fast forwarding as you approached it, and you'd see the end of the universe as you reached the event horizon. Of course you'd also be contending with your ship and yourself being ripped apart.

Travel into the past, not so easy.

Actually Kerr black holes, or rotating black holes. Have some very interesting properties. They often don't have an event horizon. Instead they create a wormhole to a white hole. The stuff that falls in the "black hole" comes out of the "white hole". The white hole can be at any other time / location or different even universe.