# time travel



## SevenStars (Feb 26, 2014)

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


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## Brian G Turner (Feb 27, 2014)

This might make for interesting background reading, as it mentions some of the paradoxes:
Time travel - Wikipedia, the free encyclopedia


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## chrispenycate (Feb 27, 2014)

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.


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## farntfar (Feb 27, 2014)

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.


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## Harpo (Feb 27, 2014)

*Harpo jumps into his time machine and travels back to November 2007 to answer this*

http://www.sffchronicles.co.uk/forum/42033-time-travel-could-be-possible-in-the-future.html

*returns to now, looking smug*


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## farntfar (Feb 27, 2014)

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.


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## Harpo (Feb 27, 2014)

*Harpo travels back to a more recent past and says it again, for farntfar's benefit*
http://www.sffchronicles.co.uk/forum/4165-time-travel-4.html

*and again*
http://www.sffchronicles.co.uk/forum/533393-time-travel.html#post1530318


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## farntfar (Feb 27, 2014)

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.


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## SevenStars (Feb 27, 2014)

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.


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## paranoid marvin (Feb 27, 2014)

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.


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## farntfar (Feb 27, 2014)

Ah but according to Heisenberg, you would affect it, just by watching.


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## SevenStars (Feb 27, 2014)

paranoid marvin said:


> 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!


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## ralphkern (Feb 28, 2014)

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.


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## Nick B (Feb 28, 2014)

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


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## ralphkern (Mar 1, 2014)

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.


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## Aquilonian (Mar 1, 2014)

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.


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## Aquilonian (Mar 1, 2014)

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?


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## ralphkern (Mar 4, 2014)

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.


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## Nick B (Mar 4, 2014)

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...


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## Harpo (Mar 5, 2014)

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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## Ice fyre (Mar 6, 2014)

Aquilonian said:


> 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?


 
I think your talking about a book by Stephen Fry I think called Making History, I read it sometime back, was quite good.


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## chrispenycate (Mar 7, 2014)

@Quellist.

Einstein's special theory of relativity tells us that there is no absolute frame of reference, that any frame of reference with any velocity is equally valid. This is essential to explain away the constancy of the speed of light in the Michaelson/Morley experiments. So if, in Heinlein's 'Time for the Stars', we have one identical twin on Earth (incidentally in a gravity field) and the other is accelerated to a smidgin under the speed of light, each frame of reference is valid, and while the Earth-living brother sees his twin getting older slower, as the information of his life as the delay of the information reaching him gets steadily greater. However, his brother's viewpoint from the ship sees the Earth speeding away from him at the same smidgin below c, so the down to Earth brother is ageing slower than him. Assuming the telepathic link holds, neither sees the other twittering around like an amphetamised canary; each sees the other's thoughts like a tortoise swimming in molasses. No paradox; Heinlein has introduced an element, instantaneous telepathy, to explain a situation within which it cannot exist, as relativity specifically rejects any information travelling faster than light, and quantum entangled particles might well fall foul of this limitation.

Should the spacecraft do a handbrake turn round a neutron star and come racing back to Earth the time expansion would be inverted, each would detect the other as mentally chipmunked, and when the ship plunged through the Solar System (I can't see any way of stopping it off hand) they'd be more or less the same age. More or less, not precisely? Later. But it does put a bit of a hitch in Mazer Rackham's relativistic preservation in Ender's Game. 

OK so far? 'Cause I'm moving into General Relativity, and I've never done the maths for that.

"But…" I hear you say, "there are atomic clocks in orbit round Earth, and they've been shown to run slow."

Indeed. And that's where 'more or less' comes into play. What is the difference between the inertial field the stay-at-home brother experiences, and that of his travelling brother? Acceleration, particularly the bit round the neutron star. Which is the case with the clock, too. What, you can't see why a clock which has been travelling for years at the same speed has been continuously accelerating? Well, acceleration is change of velocity, not speed, and while speed is a scalar… Umm, what? Well, all right. Changing direction, as Isaac Newton would agree, requires as much energy as changing speed, and produces a force, this latter being (apparently) indistinguishable from gravity. In the case of the atomic clocks, this force exactly compensates for gravity, a definition of 'orbit'. So, everything, gravity or acceleration, is slowing the passage of time, ultimate asymptote, stasis, with no means of speeding it up or (very definitely) reversing it. Relativity, like cryonic hibernation, offers us a way to arrive faster (and younger) into the future, no more. When Superman circles the globe, 'faster than light, faster than time itself' he not only produces serious tidal waves, radio interference and cracks in causality, he flies out of the Einstinian universe into a Marvelverse, where the laws of physics are not the same.


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## Nick B (Mar 7, 2014)

Inertial forces and gravity wells are not the same physical force though, inertial force is simply a moving bodies desire to continue traveling in the same direction, indeed causing an effect similar to gravity, but it is not a gravity well, there is no bending of space involved.
A ship that is in orbit, as long as it is actually within the gravity well of an object is in fact traveling in a dead straight line, it is just that the dead straight line happens to be a gravity well which means it will in fact travel in a 'dead straight line' around the mass causing the gravity well due to the warping of space.

I am a fairly simple armchair physicist and by no means anything approaching an expert, so bear with me, it sometimes takes me a while to get my head around something to the point where I can explain it back.

The thing I was questioning, is that if Person A were to (somehow) slow his subjective time while standing next to person B, would person A disappear subjectively to Person B since they are existing in a time that person A has not yet reached... OR dos person B simply see person A within his timeframe, which would be the future subjectively for person A?
This would be some cause for concern as it would mean that person A's future (and therefore everyone and everything's future) was in fact predetermined since what person A will do in his future, person B has already seen (albeit only a tiny bit in A's future).

I think it was a story from my childhood in 2000AD (I am old enough to have had the first issue bought for me by my mum) where someone was hit by a 'time bomb' which put the person a split second into the past, just before the time bomb hit them, so eternally stuck within that split second. It has stuck with me all these years, I know it is a different kettle of fish due to constantly being hit by a time bomb, but that is what drives this chain of thought.


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## ralphkern (Mar 9, 2014)

I think you'll find it is the DVverse Chris


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## Harpo (Mar 10, 2014)

ralphkern said:


> I think you'll find it is the DVverse Chris


 He said "*a* Marvelverse", not "*the* Marvelverse" - other Marvelverses are available on request


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## chrispenycate (Mar 10, 2014)

Harpo said:


> He said "*a* Marvelverse", not "*the* Marvelverse" - other Marvelverses are available on request


Which doesn't change the fact I got it wrong, my expertise in comics not being as great as that in - say - nuclear physics, and I was on a rant and didn't bother to check and should be ashamed of myself. All right, DCverse; it's how Stateside federal officials see the world.

But it is inertial forces that are slowing those caesium clocks in orbit; the gravity pulls harder on those on the planet's surface.

What these time travellers are experiencing is variable duration; they are not displaced in time. All the instants the temporarily accelerated one experiences are there for the slower one, merely lasting longer. If one of them were travelling backward in time relative to the other, against entropy's arrow, they'd still each be existing in every subdivision of time, detectable, touchable, possibly wiser not to kiss. If you want separation, you're going to need another dimension - the one spacetime is curved in will do if you haven't any others to spare


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## Mirannan (Mar 10, 2014)

Pah. Marvelverse includes DCverse, and vice versa. Evidence? Well, AFAIK some work or other in both publishers' stable explicitly states that their universe includes all others (including the real world) and of course there have been rather a lot of crossovers over the years. I have, somewhere, a comic in which Spiderman, Superman, and the Hulk figure. The inevitable fight between the Hulk and Supes is quite fun. Quote, from some of the description text: "As fists that can shatter mountains slam into the body of the man who can move planets..."


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## mosaix (Mar 11, 2014)

I'm still unsure about the whole concept of time anyway.

When we say that time speeds up or slows down under certain conditions aren't we just saying that the devices that we have created to measure the concept of time speed up or slow down?

Atomic clocks work on the principle of measuring the signals emitted by an electron. I don't find it strange that these signals would vary from one to another if each was experiencing a different gravitational field. Nor would I find it odd if two different alarm clocks, each relying on a spring, gave different results under different gravitations circumstances.

Likewise, I wouldn't find it strange if cells in the human body aged differently from one body to the next under different gravitations circumstances.


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## Mirannan (Mar 11, 2014)

Mosaix - No, we are not saying that. Other time-dependent processes, such as the wavelength of light, alter as well. In any case, the processes inside an atom wouldn't be affected much by gravity except under extreme circumstances - the force of gravity is extremely weak compared to EM forces, which is what matters here.

Alarms clocks would be a bit less robust, granted. And I would be surprised if the speed of the aging process wasn't affected by gravity, but it would be for other reasons - differences in the amount of stress put on the heart, for example.


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## mosaix (Mar 11, 2014)

Mirannan said:


> Mosaix - No, we are not saying that. Other time-dependent processes, such as the wavelength of light, alter as well.



Don't you mean time-measuring processes, Mirranan? 



> In any case, the processes inside an atom wouldn't be affected much by gravity except under extreme circumstances - the force of gravity is extremely weak compared to EM forces, which is what matters here.



It doesn't have to be affected much. The timing differences between an atomic clock on earth and one in orbit are very small.

Surely, the only way we know that 'time' is affected by gravity is by the affect on gravity of those things that we choose to use to measure the passage of time - and that seems to me to be some kind of circular argument.


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## Nick B (Mar 11, 2014)

Well, theory predicts these things too. The great thing about theories of course is that _they are theories_,  not facts.  All theories are eventually superseded, that is the beauty,  we continue to learn and grow.  I think I stated somewhere else, being  proved wrong is good, changing your mind is good.  It means you have  learned and grown. You can do neither of you are always right.

I think the main issue is that we in fact do _not_  understand time at all, for all our vaunted scientific advancement and  knowledge we are just monkeys with simple technology, barely out of the  caves and trees.  In the last hundred or so years we have advanced form  horses and carts, swords and bows, to space travel and nuclear power.   If that is the last _hundred_ years, imagine what the next _ten thousand_ will bring...

Assuming  we put aside our petty differences and actually try to advance as a  single civilisation rather than the petty neo-feudal crao we live with  today.


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## Harpo (Mar 12, 2014)

Assumption:  One day time travel will be invented.

Allowing for that, and following my theory  of how it'll work, time travel will instantly be perfected forever (because travellers from the distant future will bring us portable gadgets that never fail etc etc, all problems having been solved during the meanwhile)

The hard part is convincing 7.2 billion people (or however many of us there'll be when it happens) that it's safe, that judgement day will never come, and that (insert name here) is going to be one of the most famous people in history. ("I claim this planet in the name of His Excellency Pope Beiber" etc)


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## Nick B (Mar 13, 2014)

Harpo said:


> I claim this planet in the name of His Excellency Pope Beiber"



I firmly believe it would be better that the Earth burn in a ball of nuclear fire than this ever come to pass.


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## Harpo (Mar 13, 2014)

In that case, we have the beginnings of a nice little short story!


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## farntfar (Mar 13, 2014)

So let me get this right.
Harpo came up with the theory that time travel was invented simultaneously at all moments in time some time in or around 2006, and also in 2012 and 2013.
Foolishly, or perhaps diabolically cleverly, he published this theory, on the Chrons,which allowed Douglas Adams to read the theory several years after his own death, and send his past (and living) self a letter so that he could use it 1982 in Life the Universe and Everything. 
Thus he proved the existence of both time travel and ghost reading; (Ghost writing having been established some years earlier), and also brought the average delivery times of a GPO delivered letter down to merely one or two days, in one fell swoop.
It only remains for a later and wilier Harpo (if such a creature could be imagined) to send a similar letter to his as yet unborn self  in order to break a few more records.


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