Albert Einstein's theory of general relativity suggests that time travel to the past is possible via rotating wormholes and/or black holes. The actual technical practicality of actually carrying out such journeys need not concern us since this essay is in the realm of the thought experiment. Now Stephen Hawking says time travel to the past is not possible because he proposes that there is such a thing as a yet undiscovered Chronology Protection Conjecture that prevents this and thus makes the world safe for historians. I've come up with a unified theory of time travel into the past that incorporates Einstein's general theory of relativity; Hawking's Chronology Protection Conjecture, along with other assorted bits like parallel universes that are thrown into the mix.
Time travel is a staple in sci-fi stories, novels, films and TV series. And, time travel is possible - in theory. We all know about journeying to the future which we do at the rate of one second per second whether we like it or not. Apart from that, if one travels at close to light speeds relative to your place of origin then you can travel to the distant future (with respect to that place of origin) without aging an equivalent number of years (the twin paradox). Travel to the past is apparently allowed too, via the weird physics inherent in rotating worm holes and maybe Black Holes which is where Einstein's general theory of relativity comes into play. The problem there is that relativity theory predicts worm holes, if they exist at all, will exist for nanoseconds and be very tiny to boot, and thus not very useful in the foreseeable future for the purposes of time travel. Because we don't know exactly what the inside of a Black Hole is, and where it leads, if anywhere, current thinking suggests that jumping into Black Holes are a more useful means for committing suicide than for traveling to the past, but the jury is still out on that one.
Anyway, the fun bit about time travel is the various paradoxes that arise, the most famous one being the grandfather paradox. That is, what if you travel back in time and kill your grandfather before he sired your father (or mother). If you did that it means that you could never have been born, but if you were never born you couldn't go back in time to kill your ancestor. This is the sort of stuff sci-fi authors (and philosophers) love - ditto physicists! My favorite time travel paradox however is the one where you get something for nothing. Say you have this edition of "Hamlet", and you want Shakespeare to autograph it. So back you go in time to Shakespeare's era. You knock on his door, but the housekeeper says he's out for the day but if you leave the book he'll autograph it and you can come by and collect it next morning. When Shakespeare comes home, he sees the book, reads it, and is so impressed he spends the night making a copy. You come back the next morning, collect your now autographed edition of "Hamlet", and return to the present day with your now very valuable book. The question now becomes, where did the original "Hamlet" come from? You didn't write it; but Shakespeare didn't either as he plagiarized your copy which he then passed it off as his own work.
Another favorite is you meeting yourself. Say you're 50 and not all that well off. You get the brilliant idea to travel back in time and convince your younger self to invest in some stocks you know will pay off big time later on down the track. And so it comes to pass that your younger self so invests, and becomes filthy rich, only, in leading such a high life, dies of a heart attack at the age of 45! Or you always regretted not proposing to the love of your life when you were young, and thus go back and convince your younger self to muster up the courage and do so. He does, but as they fly off on their honeymoon, the plane crashes with no survivors. Sometimes you don't know when you're well off.
Or if you can travel back in time, then of course others can to. Naturally there's going to be lots of people interested in particular events, maybe even at the time, seemingly trivial events (yet which turn out in the long run to have had major impact(s)). And so you might have any number of people going back to particular historical focal points, each with their own particular agenda (most of which will be mutually exclusive), and ultimately causing havoc. I mean if person one goes back and influences an event producing a new outcome, then person two might go back and has a go at that result and things get altered again, which will then prompt person three to go back and influence things more to his liking, etc. In other words, history would never be fixed, rather always be fluid. The world is not safe for historians. Since we believe that history (or the past) is fixed, then that what's written on your history book page today will not alter overnight. Thus, you have probably concluded that time travel cannot happen, will not happen, and has not happened, however much you yourself might wish to go back in time yourself and change something. (Don't we all really wish some past something, personal and trivial, or perhaps something of major significance could be changed and you'd be that instrument of change?)
Its paradoxes and situations such as the above that prompted Stephen Hawking to postulate that there is as yet an undiscovered law or principle of physics which prohibits time travel to the past - he calls it his 'Chronology Projection Conjecture'. Since we have never seen, according to Hawking, to the best of our knowledge at least, any time travelers - tourists or historians - from our future, he's probably right.
So, putting it all together, here's my theory of time travel: my unified theory of time travel, at least to the past.
Relativity theory has passed every experimental test thrown at it, so the theory isn't in much doubt and one can have a high degree of confidence in what it predicts, even if that prediction is currently beyond any experimental test. Relativity theory allows for time travel into the past, but, IMHO, only to parallel universes (otherwise known as alternative or mirror or shadow universes) where no paradoxes can happen.
Why only parallel universes? The ways and means by which you can use relativity theory to time travel backwards involves rotating Black Holes or wormholes. There are serious reasons behind the speculation that what's on the other side of a Black Hole and/or wormhole is another universe. So, therefore it's relativity's time travel allowance, but probably to another universe. The Black Hole or wormhole 'exit' isn't within our Universe.
Whatever you do in that parallel universe is predetermined. It's fate. It's destiny - all because causality rules. Therefore, there are no unexpected ripple effects other than what was destined to happen. You were meant to be there and do what you do. Therefore, there will be no paradoxes arising.
Astrophysicist Stephen Hawking has proposed his Chronology Protection Conjecture that prohibits time travel to the past within your own universe because of the possible paradoxes that could arise. Why can't you go back in time in your own universe? That would mean that at a specific time and place you both were not (originally) and were (as a result of going back) present. That's a paradox. And if you were to travel back in time to a set of time and space coordinates you were actually originally at, then there would be two copies of you occupying the same space at the same time - also a paradox.
But take the grandfather paradox. If you go back in time and kill your grandfather, but your grandfather in a parallel universe, then you don't prevent your existence, just the eventual existence of yourself, your other self, in that parallel universe. In the case of Shakespeare and "Hamlet", you gave your copy to a parallel universe Shakespeare. In your original (our) Universe, Shakespeare is still the legitimate author.
Once you time travel from your universe A, to parallel universe B, you can't return again to universe A because of Hawking's Chronology Protection Conjecture - paradoxes could arise. However, you could go from parallel universe B to parallel universe C, but, hence never return to either universe A or B - Hawking's Chronology Protection Conjecture again.
Perhaps some people you've seen or known or heard about might be time travelers from a parallel universe's future. If they then time travel to another parallel universe, then that might account for some missing persons' cases!
In short, we can time travel to other parallel universes but not to our own; entities from other parallel universes can visit our Universe. No paradoxes need arise. Both Einstein (relativity) and Hawking (Chronology Protection Conjecture) are satisfied and happy campers.
Is that right? No, it's wrong!
There's still one very nasty loose end here. What's to prevent those from a parallel universe meddling and altering our time stream? It's not enough for them to have a Prime Directive against that - we all know Prime Directives are meant to be broken! So, it looks like Hawking's Chronology Protection Conjecture must apply to those visitors from parallel universes to our Universe as well. I mean what difference does it make to your existence whether you travel back in time within your own universe and kill your mother before you were conceived, or some serial killer escaping from a parallel universe to our Universe who kills your mother before you were conceived - even though in the latter case there's no paradox, you still wouldn't have been conceived of here in anyone's philosophy!
OK, so relativity allows time travel back in time, but only to parallel universes. The Hawking Chronology Protection Conjecture not only prevents time travel paradoxes in general, but it also prevents parallel universe time travelers meddling and altering our timeline; ditto we humans time traveling to someone else's parallel universe. But how would the Hawking Chronology Protection Conjecture actually accomplish this? My best guess is that parallel universes aren't in phase - they aren't polarized or synchronized in-phase like a laser beam, or the light that passes through your polarized sunglasses - otherwise we'd have some rather hard evidence of them; certainly way more than we do now.
So, if we go to parallel universe B or those from parallel universe B visits us, we'll, or they'd be respectively out of phase with respect to the universe they are now in. Translated, they, or we, could look, but not touch for all practical purposes. I say for all practical purposes as now and again what's out of phase (high probability - the usual state of affairs) will sync into phase (that's rare). But the in-phase times are so few and far between, and last for such a brief duration that it's unlikely to result in any inadvertent or deliberate timeline alterations. That's my rendering of the Hawking Chronology Protection Conjecture - he could well have other ways and means in mind.
So another way of putting this is that time travelers would be spectral or ghostlike in their host universe, and maybe that's where our traditions of ghosts and other things that go bump in the night come from! This is much like the parallel universe ghost or shadow photons that are conjectured to explain some highly mysterious aspects or phenomena contained within the famous quantum double slit* experiment. Now an obvious question is how do all the parallel universe ghost photons get into our physics labs where double slit experiments are carried out? I mean there are no local macro Black Hole or wormhole exits present - are there? Yes in fact there are! Not a macro wormhole, but a micro wormhole - actually wormholes. Theoretically, micro wormholes should exist all around you. It's just that they are at quantum levels - incredible tiny; way subatomic in size. And they exist for just nanoseconds before collapsing. They are just part of the quantum foam** reality at super microscopic levels, a reality at the level where all things exhibit the quantum jitters or quantum fluctuations. Thus, every second of every day, everywhere, there are little quantum gateways - quantum sized wormholes connections between universes which quantum sized particles - like photons - can traverse! From the standpoint of the double slit experiment, it doesn't matter whether the parallel universe's ghost photons came from the past, future or present - just as long as they are, indeed, present!
Now you may think it would be easy to detect these ghostly photons. Just put a photon detector in a totally dark and sealed room. Well, not quite so easy. Some photons can pass through 'solid' matter. X-Ray photons anyone? Radio wave photons pass through the walls of your home. If you look at a bright light, you'll still see light even if you close your eyes. So, your photon detector in your dark and sealed room could easily detect our local variety.
The ghostly bits aside, parallel universe time travelers (or even ordinary time travelers from within our Universe assuming Hawking is wrong)) might explain the sometimes uncanny, often incredible look-a-likes that we all seem to have. A long shot to be sure, but something interesting to ponder.
There's still one more problem on the horizon. Just because a macro Black Hole or wormhole plunks you into a parallel universe (and of course you've got to be able to survive the trip itself which might be problematical), doesn't mean you're going to be with spitting distance of your ultimate destination(s) - say a parallel Earth(s). So, time travelers might also need more conventional transport - like Flying Saucers (okay, forget the saucers - like spaceships with fins and rocket motors). But then what's really there to distinguish a visiting time traveler from a parallel universe from say a run-of-the-mill extraterrestrial from within our own Universe? Maybe you could just put out the welcome mat for both options!
One final thought. Could there be a Clayton's time travel? - Time travel without traveling in time? At the risk of making Einstein turn over in his grave; I'm going to propose a universal NOW across all universes. Now I know that NOW, when it comes to observers, is a relative thing. An observer in Martian orbit sees Mars' NOW somewhat before you on Planet Earth sees the same Mars' NOW because the speed of light is finite. And relative motions and velocities complicate what is NOW. But, I propose (a thought experiment remember) to instantaneously freeze-frame the entire collection of universes' NOW. Everyone and everything everywhere comes to an instant standstill. Right! We now have a universal NOW that we can study at our leisure (the freeze doesn't apply to you and me - we're outside the space-time continuum).
Let's focus on that subset of all parallel universes - all parallel Earths and time travel between them. Now there's no reason to assume that all parallel Earths are identical in all aspects. Indeed, some parallel universes may not even contain a parallel Earth! There maybe some parallel Earths identical or so close to identical to our Planet Earth as makes no odds - abodes you'd feel right at home in. Other Earths would differ in various ways, some minor, some major. Still others might be really weird and alien, as in having evolved a dinosaur society, civilization and technology. There was no parallel asteroid impact 65 million years ago; thus no human beings around the traps 65 millions later.
Your subset of parallel Earths would show near infinite diversity in infinite combinations. I say 'near' because you can only stretch the term 'Earth' or 'Earth-like' so far and no farther, before it's not Earth or Earth-like. A 100% oceanic world is not Earth. If a parallel 'Earth' has Venus-like temperatures, it is not Earth-like. If it has a density approaching that of a neutron star, it is not Earth-like. If it has no life on it, even though in all other respects it is a near carbon copy of our Earth, it is not Earth-like.
Time travel is a staple in sci-fi stories, novels, films and TV series. And, time travel is possible - in theory. We all know about journeying to the future which we do at the rate of one second per second whether we like it or not. Apart from that, if one travels at close to light speeds relative to your place of origin then you can travel to the distant future (with respect to that place of origin) without aging an equivalent number of years (the twin paradox). Travel to the past is apparently allowed too, via the weird physics inherent in rotating worm holes and maybe Black Holes which is where Einstein's general theory of relativity comes into play. The problem there is that relativity theory predicts worm holes, if they exist at all, will exist for nanoseconds and be very tiny to boot, and thus not very useful in the foreseeable future for the purposes of time travel. Because we don't know exactly what the inside of a Black Hole is, and where it leads, if anywhere, current thinking suggests that jumping into Black Holes are a more useful means for committing suicide than for traveling to the past, but the jury is still out on that one.
Anyway, the fun bit about time travel is the various paradoxes that arise, the most famous one being the grandfather paradox. That is, what if you travel back in time and kill your grandfather before he sired your father (or mother). If you did that it means that you could never have been born, but if you were never born you couldn't go back in time to kill your ancestor. This is the sort of stuff sci-fi authors (and philosophers) love - ditto physicists! My favorite time travel paradox however is the one where you get something for nothing. Say you have this edition of "Hamlet", and you want Shakespeare to autograph it. So back you go in time to Shakespeare's era. You knock on his door, but the housekeeper says he's out for the day but if you leave the book he'll autograph it and you can come by and collect it next morning. When Shakespeare comes home, he sees the book, reads it, and is so impressed he spends the night making a copy. You come back the next morning, collect your now autographed edition of "Hamlet", and return to the present day with your now very valuable book. The question now becomes, where did the original "Hamlet" come from? You didn't write it; but Shakespeare didn't either as he plagiarized your copy which he then passed it off as his own work.
Another favorite is you meeting yourself. Say you're 50 and not all that well off. You get the brilliant idea to travel back in time and convince your younger self to invest in some stocks you know will pay off big time later on down the track. And so it comes to pass that your younger self so invests, and becomes filthy rich, only, in leading such a high life, dies of a heart attack at the age of 45! Or you always regretted not proposing to the love of your life when you were young, and thus go back and convince your younger self to muster up the courage and do so. He does, but as they fly off on their honeymoon, the plane crashes with no survivors. Sometimes you don't know when you're well off.
Or if you can travel back in time, then of course others can to. Naturally there's going to be lots of people interested in particular events, maybe even at the time, seemingly trivial events (yet which turn out in the long run to have had major impact(s)). And so you might have any number of people going back to particular historical focal points, each with their own particular agenda (most of which will be mutually exclusive), and ultimately causing havoc. I mean if person one goes back and influences an event producing a new outcome, then person two might go back and has a go at that result and things get altered again, which will then prompt person three to go back and influence things more to his liking, etc. In other words, history would never be fixed, rather always be fluid. The world is not safe for historians. Since we believe that history (or the past) is fixed, then that what's written on your history book page today will not alter overnight. Thus, you have probably concluded that time travel cannot happen, will not happen, and has not happened, however much you yourself might wish to go back in time yourself and change something. (Don't we all really wish some past something, personal and trivial, or perhaps something of major significance could be changed and you'd be that instrument of change?)
Its paradoxes and situations such as the above that prompted Stephen Hawking to postulate that there is as yet an undiscovered law or principle of physics which prohibits time travel to the past - he calls it his 'Chronology Projection Conjecture'. Since we have never seen, according to Hawking, to the best of our knowledge at least, any time travelers - tourists or historians - from our future, he's probably right.
So, putting it all together, here's my theory of time travel: my unified theory of time travel, at least to the past.
Relativity theory has passed every experimental test thrown at it, so the theory isn't in much doubt and one can have a high degree of confidence in what it predicts, even if that prediction is currently beyond any experimental test. Relativity theory allows for time travel into the past, but, IMHO, only to parallel universes (otherwise known as alternative or mirror or shadow universes) where no paradoxes can happen.
Why only parallel universes? The ways and means by which you can use relativity theory to time travel backwards involves rotating Black Holes or wormholes. There are serious reasons behind the speculation that what's on the other side of a Black Hole and/or wormhole is another universe. So, therefore it's relativity's time travel allowance, but probably to another universe. The Black Hole or wormhole 'exit' isn't within our Universe.
Whatever you do in that parallel universe is predetermined. It's fate. It's destiny - all because causality rules. Therefore, there are no unexpected ripple effects other than what was destined to happen. You were meant to be there and do what you do. Therefore, there will be no paradoxes arising.
Astrophysicist Stephen Hawking has proposed his Chronology Protection Conjecture that prohibits time travel to the past within your own universe because of the possible paradoxes that could arise. Why can't you go back in time in your own universe? That would mean that at a specific time and place you both were not (originally) and were (as a result of going back) present. That's a paradox. And if you were to travel back in time to a set of time and space coordinates you were actually originally at, then there would be two copies of you occupying the same space at the same time - also a paradox.
But take the grandfather paradox. If you go back in time and kill your grandfather, but your grandfather in a parallel universe, then you don't prevent your existence, just the eventual existence of yourself, your other self, in that parallel universe. In the case of Shakespeare and "Hamlet", you gave your copy to a parallel universe Shakespeare. In your original (our) Universe, Shakespeare is still the legitimate author.
Once you time travel from your universe A, to parallel universe B, you can't return again to universe A because of Hawking's Chronology Protection Conjecture - paradoxes could arise. However, you could go from parallel universe B to parallel universe C, but, hence never return to either universe A or B - Hawking's Chronology Protection Conjecture again.
Perhaps some people you've seen or known or heard about might be time travelers from a parallel universe's future. If they then time travel to another parallel universe, then that might account for some missing persons' cases!
In short, we can time travel to other parallel universes but not to our own; entities from other parallel universes can visit our Universe. No paradoxes need arise. Both Einstein (relativity) and Hawking (Chronology Protection Conjecture) are satisfied and happy campers.
Is that right? No, it's wrong!
There's still one very nasty loose end here. What's to prevent those from a parallel universe meddling and altering our time stream? It's not enough for them to have a Prime Directive against that - we all know Prime Directives are meant to be broken! So, it looks like Hawking's Chronology Protection Conjecture must apply to those visitors from parallel universes to our Universe as well. I mean what difference does it make to your existence whether you travel back in time within your own universe and kill your mother before you were conceived, or some serial killer escaping from a parallel universe to our Universe who kills your mother before you were conceived - even though in the latter case there's no paradox, you still wouldn't have been conceived of here in anyone's philosophy!
OK, so relativity allows time travel back in time, but only to parallel universes. The Hawking Chronology Protection Conjecture not only prevents time travel paradoxes in general, but it also prevents parallel universe time travelers meddling and altering our timeline; ditto we humans time traveling to someone else's parallel universe. But how would the Hawking Chronology Protection Conjecture actually accomplish this? My best guess is that parallel universes aren't in phase - they aren't polarized or synchronized in-phase like a laser beam, or the light that passes through your polarized sunglasses - otherwise we'd have some rather hard evidence of them; certainly way more than we do now.
So, if we go to parallel universe B or those from parallel universe B visits us, we'll, or they'd be respectively out of phase with respect to the universe they are now in. Translated, they, or we, could look, but not touch for all practical purposes. I say for all practical purposes as now and again what's out of phase (high probability - the usual state of affairs) will sync into phase (that's rare). But the in-phase times are so few and far between, and last for such a brief duration that it's unlikely to result in any inadvertent or deliberate timeline alterations. That's my rendering of the Hawking Chronology Protection Conjecture - he could well have other ways and means in mind.
So another way of putting this is that time travelers would be spectral or ghostlike in their host universe, and maybe that's where our traditions of ghosts and other things that go bump in the night come from! This is much like the parallel universe ghost or shadow photons that are conjectured to explain some highly mysterious aspects or phenomena contained within the famous quantum double slit* experiment. Now an obvious question is how do all the parallel universe ghost photons get into our physics labs where double slit experiments are carried out? I mean there are no local macro Black Hole or wormhole exits present - are there? Yes in fact there are! Not a macro wormhole, but a micro wormhole - actually wormholes. Theoretically, micro wormholes should exist all around you. It's just that they are at quantum levels - incredible tiny; way subatomic in size. And they exist for just nanoseconds before collapsing. They are just part of the quantum foam** reality at super microscopic levels, a reality at the level where all things exhibit the quantum jitters or quantum fluctuations. Thus, every second of every day, everywhere, there are little quantum gateways - quantum sized wormholes connections between universes which quantum sized particles - like photons - can traverse! From the standpoint of the double slit experiment, it doesn't matter whether the parallel universe's ghost photons came from the past, future or present - just as long as they are, indeed, present!
Now you may think it would be easy to detect these ghostly photons. Just put a photon detector in a totally dark and sealed room. Well, not quite so easy. Some photons can pass through 'solid' matter. X-Ray photons anyone? Radio wave photons pass through the walls of your home. If you look at a bright light, you'll still see light even if you close your eyes. So, your photon detector in your dark and sealed room could easily detect our local variety.
The ghostly bits aside, parallel universe time travelers (or even ordinary time travelers from within our Universe assuming Hawking is wrong)) might explain the sometimes uncanny, often incredible look-a-likes that we all seem to have. A long shot to be sure, but something interesting to ponder.
There's still one more problem on the horizon. Just because a macro Black Hole or wormhole plunks you into a parallel universe (and of course you've got to be able to survive the trip itself which might be problematical), doesn't mean you're going to be with spitting distance of your ultimate destination(s) - say a parallel Earth(s). So, time travelers might also need more conventional transport - like Flying Saucers (okay, forget the saucers - like spaceships with fins and rocket motors). But then what's really there to distinguish a visiting time traveler from a parallel universe from say a run-of-the-mill extraterrestrial from within our own Universe? Maybe you could just put out the welcome mat for both options!
One final thought. Could there be a Clayton's time travel? - Time travel without traveling in time? At the risk of making Einstein turn over in his grave; I'm going to propose a universal NOW across all universes. Now I know that NOW, when it comes to observers, is a relative thing. An observer in Martian orbit sees Mars' NOW somewhat before you on Planet Earth sees the same Mars' NOW because the speed of light is finite. And relative motions and velocities complicate what is NOW. But, I propose (a thought experiment remember) to instantaneously freeze-frame the entire collection of universes' NOW. Everyone and everything everywhere comes to an instant standstill. Right! We now have a universal NOW that we can study at our leisure (the freeze doesn't apply to you and me - we're outside the space-time continuum).
Let's focus on that subset of all parallel universes - all parallel Earths and time travel between them. Now there's no reason to assume that all parallel Earths are identical in all aspects. Indeed, some parallel universes may not even contain a parallel Earth! There maybe some parallel Earths identical or so close to identical to our Planet Earth as makes no odds - abodes you'd feel right at home in. Other Earths would differ in various ways, some minor, some major. Still others might be really weird and alien, as in having evolved a dinosaur society, civilization and technology. There was no parallel asteroid impact 65 million years ago; thus no human beings around the traps 65 millions later.
Your subset of parallel Earths would show near infinite diversity in infinite combinations. I say 'near' because you can only stretch the term 'Earth' or 'Earth-like' so far and no farther, before it's not Earth or Earth-like. A 100% oceanic world is not Earth. If a parallel 'Earth' has Venus-like temperatures, it is not Earth-like. If it has a density approaching that of a neutron star, it is not Earth-like. If it has no life on it, even though in all other respects it is a near carbon copy of our Earth, it is not Earth-like.
