time travel

SevenStars

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I've been thinking about time travel lately and cannot help but wonder this, if you go back in time and meet younger versions of people you know in your current present, can you theoretically change history through your interaction with them? If, as theoretical physics seems to suggest, it's more a case of alternative realities allowing time travel to take place, does this mean that if you meet the same people, history cannot be changed as it's in a different reality?
Reading Einstein's theory of relativity, looking at quantum physics and getting a little confused here :)
 
Einstein's never going to give you time travel; the paradigms he used for his equations outlawed it. He can speed up or slow down duration, never stop or reverse it. Not that that, in itself, makes it impossible; there are a number of things later discovered in quantum mechanics and uncertainty he disapproved, too, (God doesn't play dice with the world), but that dislike hasn't stopped tunnel diodes working.

…matter may neither be created nor destroyed
The conservation laws, though they might have been mildly warped by the union of matter and energy into one larger field (which has at present no name mattergy, belike?) hasn't really dented them too much. Momentum, mass/energy, spin; all these quantities should remain constant in a properly organised universe. Which is why the Casimir effect or Fred Hoyle's steady state universe annoy me; they suggest we can only observe one facet of a cosmos extending in several extra dimensions, and our 'it always works like this' reduces to 'well, it's done it so far'.

A victorian gentleman in plus fours and weskit stepping out of a boxwood and silver time machine (which probably has crystal chandeliers) and waving a malacca cane at a dinosaur introduces extra mass into the universe – temporarily, we hope. Unless he, and his machine get stepped on. Extra momentum; the planet on which he lands is several light years from the one he left, in any reasonable inertial framework, and moving at many kilometres per second relative to it, despite the fact that, in a few billion rotations, it is going to become it. Actually, Wells' apparatus performs quite well in this respect, as it actually moves in time, so its mass exists 'between the frames' in all intermediate periods, rather than just snapping into existence as most modern time travelling vehicles do. It also helps in finding the planet at the other end without enormous n-body calculations. Tiny effects, but unbalancing, and if he patents his machine and lots of explorers and missionaries spread out, they won't need to step on a butterfly to destabilise the world. However, this situation is self correcting. If time travel is possible, and history can be changed, each voyage into the past will create a new universe, where a time machine will be invented, and more changes made, continuous mutation, snakes and spiders, until, in one attempt, no time machine is invented. Full stop, stability; this becomes the timeline, and we fly to the moon instead.

Or we could ensure that an equivalent mass made the inverse trip, so as we walked out of an elegant regency hall a heavy oak dining table replaced us in the lab.

Finally we have the 'static plenum' theories, where nothing can change, free will is an illusion, and everything we do we have done, the entire universe from cosmegg to heat death, if we were not so rigidly three dimensional, can be seen as a single, crystalline structure with time travellers encased in it like insects in amber. Rudy Rucker has done a couple of attempts to describe this in terms comprehensible to people who still believe in banks; I particularly remember his cat carrot. No paradoxes to paradoctor; Heinlein's 'All yo zombies' would slot neatly in here.

Mucking about with causality is a dangerous game.
 
I always enjoyed Douglas Adams' idea that
"time travel was, by its very nature, discovered simultaneously at all points in history".

A delightfully abstract and Mandlebrotian view.
 
If I'd been on the site to see your comment in 2007, Harpo, I would have quoted you, and not that nodody, DNA. :))

Oh! If only I could go back in time and correct my crass mistake.
 
Blimey, Harpo!
You're like one of those characters in Doctor Who or something, who just shows up in the background of paintings throughout the ages.
I'm not gonna make you cross, 'cause you'll already have slapped me for it.
 
Thank you for all the replies!
I'm finding this topic more than a little fascinating. Chrispy you have given me hope that I have some understanding of these concepts. The cause effect problem appears insurmountable

At the minute, I'm seeing more possibilities in Professor Kaku's alternative reality theory where he likens time to a river that can split into two forks. It doesn't seem half as exciting as zooming back and forth between past, present and future though. :)
 
In my opinion if time travel were possible, you would simply be an oberver. As you didn't exist then , you would be able to interact neither with objects nor people, and they would not be able to sense you.
 
Ah but according to Heisenberg, you would affect it, just by watching.:D
 
In my opinion if time travel were possible, you would simply be an oberver. As you didn't exist then , you would be able to interact neither with objects nor people, and they would not be able to sense you.

Surely there is an argument here that you would exist because you are there at that moment, unless it was a consciousness based form of time travel.............
I'm just theorizing that if the matter that is you is present then you are "real". Now I'm pondering what is "real"
I think I'm getting travel sick and confused! :) ;)
 
An interesting thought is captured in the realms of quantum entanglement.

The basic theory states that if you have two particles that are entangled then they are forever linked, regardless of distance.

Now lets say, you chuck one of these particles onto a space ship and send it in a big looping course at near light speed (lets ignore acceleration and the actual spacecraft engineering for the moment), returning it to Earth in (for arguments sake) 1000 years objectively. Yet subjectively, due to time dilation for the spacecraft only 6 months have gone by, then they should still be entangled.

The question is, where does the datum lie?

For the particle that remained, 6 months after launch, can it communicate with its entangled partner 1000 years in the future that has now returned to Earth? e.g. move one particle a little to the left, it's entangled partner moves to the left and that means 'yes' to the right means 'no' etc.

I did actually start a WIP on this called 'The Oracle' but beyond the basic concept I didn't get very far and spent far too much time scratching my head looking at pages of fairly incomprehensible wiki pages.
 
Nice one ralphkern, I actually had the same idea for instantaneous communication, entangled particles that simply relate to binary language, the only delay in communication over any distance at all would be the time to translate from binary into your chosen language.
Armchair science is amazing :)
 
As far as I could tell there were two problems(beyond engineering):

Dechoerance: Essentially the information was garbled in real life experiments. However some work was done on rectification. That is still in progress.

Datum: No one could say whether the datum would lie with the origin in the past, the future probe, both or neither. That seems to be experimentally based.
 
Pontificators used to say that backwards time travel couldn't happen because "it would change history." But there's no reason why history shouldn't be changing all the time. We would never know that it had changed, because the moment the past changed all memory and legacy of it would vanish and we would only remember the new history.

As for the "grandfather Paradox", (what happens if you go back in time and kill your grandad before your dad was conceived) various solutions spring to mind.

a) Your widowed grandmother remarries and in due course gives birth to your (new) dad. You look different, various things change, but Fate has a sort of "homeostasis" whereby other things alter slightly to minimise the disruption. A year after your trip a lot of things would look different, ten years later not so much, a hundred years and you'd need a detailed analysis to spot the difference.

b) Your gun jams or you have the wrong address, because seriously changing the past is simply not possible, ie homeostasis is restored instantly.

c) You shoot your grandad, who then wakes up from a nightmare in which he was killed by someone with a curious resemblance to himself.

My own suspicion is killing grandad would get easier and easier the further back in time you went, but would make less and less difference to the present. Killing Alexander the Great could be very difficult though, because today's world would be seriously different if he hadn't lived, probably more than for any other individual. Maybe that's why he led such a charmed life, recklessly courageous, leading from the front, several times severely wounded, even going off on his own sometimes despite having a huge army behind him, yet he survived and carried on conquering till his armies refused to follow any further.
 
Slightly off topic, but there's a story where someone goes back in time and kills Hitler before he comes to power, then returns feeling very self-satisfied to their laboratory in an American university, only to find the whole place draped in Nazi banners and portraits of some other Nazi who has turned out to be as bad as Hitler but more competent politically and militarily, and was thus far more successful.

Does anyone know the author and title of this story because I've forgotten?
 
To misquote John Connor, maybe the future is set. E.g if time travel becomes possible in the future, maybe any 'alterations' are already built into the existing time line. You can;t kill your grandfather as that's not part of the grand design.

The trouble is proving any evidence of time travel experimentally (or via artifacts obviously).

I suspect if it does happen then we would simply remember the 'new' history. Or perhaps the multiverse idea. However I am no temporal physicist or whatever the necessary discipline would be.
 
The big point really is that time as we experience it is something we actually don't understand.
This is going to be hard to put into words so bear with me;

We 'know' theoretically that velocity slows time subjectively and gravity speeds time up subjectively. The key word is subjectively.

A thought experiment may be needed here. Imagine you have a man in a spaceship orbiting a high mass object at a reasonably fast velocity, say 10,000 kilometers per second at a distance of maybe 20,000 kilometers. Now imagine another person is stood upon the high mass object.
Both of the people are affected by the gravity, the person on the object more-so, and the person in the spaceship is experiencing velocity.

Both are experiencing time distortion, one by gravity, one by velocity, one faster, one slower. The key word here may be experiencing.

Imagine they are in communication. Has one disappeared into the past and the other into the future? If they existed at different times, they could not communicate, one would always be in the past of the other. So they must still exist in the same time-frame.
How do we know this? Because a similar experiment has been done, simply using todays aircraft and atomic clocks, one on the ground, one on the aircraft. Fly around the world at high speed and compare the clocks.
Experiment showed that the clock that had been on the aircraft was measurably slower than the clock on the ground. A tiny, tiny amount, but measurable.

What happened and how to make sense of it? Obviously the clock in the aircraft experienced time at a slower rate, but is it in the past now? No, because you can look at the clock, it's still right there in the present, it is just 'younger'.

So has time travel happened? To my mind no, the object has not traveled in time, it has experienced less time. Objectively everything still happened at the same time, subjectively different amounts of time have passed.

Better minds than mine would ave to explain why though, coz I can't...
 
See what I said at the links I gave on page 1. When a time machine gets invented it'll be the sort that goes nowhere but within which we can travel forward and backward in time (in the same way that a railway line goes nowhere, but we can travel back and forth on it)
When such a machine is switched on, it'll immediately be the first active time machine, and somebody from the future will step out of it - the temporal equivalent of Neil Armstrong. Time travel will then become as common as rail travel currently is.
 

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