It’s About Time – For a Change: Part Two

Introduction: The real nature of what we call ‘time’ is one of the Big Questions. It’s been debated ever since humans evolved thought and language. That puzzlement remains true to the average person today, although ‘time’ has become the professional subject or province of philosophers, metaphysicists, and theoretical physicists, not to forget many a science fiction author! Here’s my two cents worth – ‘time’ is an illusion!

Continued from yesterday’s blog…

Travel through Time? The question remains despite my earlier negation; can you revisit and experience a past event? Can there be an instant (or not so instant) replay? Take the example of the now dispersed ink in the bowl of water. If all the ink bits (particles) were to exactly retrace their movements (that’s just so highly improbable that you’d wait longer than the age of our Universe to actually see it), they would eventually come together as an ink drop. If they now retrace those retraced movements (ditto on the statistical improbability) you get back to the exact same configuration of dispersed ink bits in the bowl of water. You will have witnessed an instant (or not so instant) replay of a past event. You would have in a sense travelled back in time to ‘record’ an event that had already happened. Of course you would have ‘time’ travelled with respect to that specific event and only that event.

Ah, that word ‘record’. Of course you could have filmed the original ink drop to dispersed ink event then watched the film at a later date, but that’s cheating a bit, don’t you think?

Assuming for a moment that time is actually something tangible and that travel through it is possible (that’s in agreement with Einstein’s General Theory of Relativity). Einstein’s Relativity aside, I’d maintain that travel backwards in time is probably nonsense.

If you go back in time with a view to either preventing something from happening or inaugurating something and you succeed, then when you return to your own present time the original motive to go back in time in the first place has ceased to exist. That’s because in your now altered present, there’s nothing motivating you to go back in time and so you don’t, but if you don’t then you couldn’t have changed the past in the first place. That actually suggests that your actions have split the Universe and generated two timelines (or universes), one in which you go back to fix something, and one in which you don’t because there’s nothing that needs fixing.

If you don’t succeed, if you can’t tamper with the past* then it’s all an exercise in futility and so there’s no point being a damn fool about it. If at first you don’t succeed, give up!

If you go back in time just to observe (as historian, scientist or even tourist), your very presence in the past has introduced a change that didn’t previously exist, and any change has a ripple effect which will change, even if only slightly, your own present, in you’re your before-the-fact time travelling present was different to your after-the-fact time travelling present, which could, as we’ll see, generate a paradox. You may not care about the alteration, but other people may not be so happy and laid back with your inadvertent meddling.

But wait a second. Those other people probably wouldn’t know or be aware that anything had altered. Having reset the clock when you went back in time, the ripples would have become part and parcel of their world view, so only you, upon your return (having bypassed all the rippling) would notice the change.

But what if you go back in time only to materialize in front of a speeding train and are killed. Of course that doesn’t affect your ancestors so presumably they still meet and marry and breed and ultimately your born – again – only to go back in time and get hit by that train!  

Or, you materialize back in time and so startling someone of that era into failing to notice that train and gets killed. Now say that someone was your father-to-be; your father before he met your mother. Now you have two universes – timelines – again. One timeline is where you went back in time and presumably returned; one in which you were never born.

That’s one of many variations on the ‘grandfather paradox’ which in general has as a plot where you go back in time and somehow prevent your own birth by say, killing your grandfather before he met your grandmother, or in a less gruesome way, prevented them from ever meeting. When I was a teenager I remember writing a short story (never published) where a group of scientists travelled back in time some four billion years to just after the Earth formed and solidified and cooled. One of the scientists was a pipe smoker, and after finishing his smoke, tapped out the pipe ashes into a small puddle of water. Of course that puddle was the very puddle that was to have given rise to that first proto-cell, but the pipe ash polluted the puddle and so that origin of life event never happened, and so back in what would have been the 20^th Century, the sun shone down on a sterile planet! The only trouble with the story was that four billion years ago, there was no oxygen in Earth’s atmosphere so one couldn’t smoke. Still, it was the mother of all grandfather paradoxes!   

Here’s another time travelling curve ball coming your way. You have a set of coordinates with respect to Planet Earth – latitude, longitude, and altitude. But you also have a set of coordinates with respect to the Moon – lunar latitude, longitude and altitude. You have a set of coordinates with respect to the Sun (solar latitude, longitude and altitude). Ditto Mars, and ditto the nearest star and ditto the centre of the Milky Way Galaxy, etc. In fact, although they change from moment because of relative motions of all the bodies concerned (that also applies to you and Planet Earth since you move around), you have a set of coordinates with respect to every bit of matter in the Universe. The question is, when you time travel, what set of coordinates do you take with you? Where exactly do you end up? It’s taken for granted in works of fiction that its Earth’s coordinates, but is it necessarily so? What if you retain your exact position (relative to where exactly is a mystery) but travel in time. Then presumably when you materialize else-when, the Earth will have moved far away, and there you are flailing around in empty space, breathing a deep vacuum!

So we see that while time travel stories are a staple of the sci-fi authors’ bag-of-tricks – they stir up those little grey cells – there doesn’t appear to be much chance of time travel in any physical reality we know of. Time travel is only a reality in the imagination. We in fact have a version of the Fermi Paradox here. While the Fermi Paradox referred to aliens that should be knocking on our collective doors (“where is everybody?”), if time travel were possible, as astrophysicist Stephen Hawking has so absolutely pointed out, then where are all those gawking time travelling tourists and historians from our future?

*Akin to astrophysicist Stephen Hawking’s Chronology Protection Conjecture which states that there is as yet some undiscovered principle in physics which will make it impossible to travel back in time and thus make the Universe safe for historians.

Further recommended readings about time and time travel:

Carroll, Sean; From Eternity to Here: The Quest for the Ultimate Theory of Time; Dutton, New York; 2010:

Davies, Paul; About Time: Einstein’s Unfinished Revolution; Penguin Books, London; 1995:

Gott, J. Richard; Time Travel in Einstein’s Universe: The Physical Possibilities of Travel Through Time; Phoenix, London; 2002:

Hawking, Stephen & Penrose, Roger; The Nature of Space and Time; Princeton University Press, Princeton, New Jersey; 1996:

Hawking, Stephen W. et al.; The Future of Spacetime; W.W. Norton and Company, N.Y.; 2002:

Le Poidevin, Robin; Travels in Four Dimensions: The Enigmas of Space and Time; Oxford University Press, Oxford; 2003:

Lockwood, Michael; The Labyrinth of Time: Introducing the Universe; Oxford University Press, Oxford; 2005:

Mahid, Shahn (Editor); On Space and Time; Cambridge University Press, Cambridge; 2008:

Randles, Jenny; Breaking the Time Barrier: The Race to Build the First Time Machine; Paraview Pocket Books, New York; 2005:

Toomey, David; The New Time Travelers: A Journey to the Frontiers of Physics; W.W. Norton & Company, New York; 2007:

Reality: Really Real, Pseudo-Real, or Unreal? Part Six

Reality – It’s all relative; it’s reversible; it’s symmetric; it’s personal; it kicks back when you kick it; it’s conceptual; it’s theoretical; it’s actual; it’s abstract; it’s bio-friendly; it’s unforgiving; it’s emotional; and ultimately the reality of life, the Universe and everything resides in your mind.

Continued from yesterday’s blog…

Is Your Reality Certainty or A Fluke?

If there is one thing you are absolutely certain of it’s that you exist, and I won’t dispute that (even of you are a figment of my imagination or exist only as the creation of some kind of computer software). Seriously, you exist as a three dimensional, flesh-and-blood, absolutely unique entity inside a three dimensional Universe. However, is your reality something wildly improbable or 100% guaranteed?

On the surface, you are an absolute fluke. Think of those millions of sperm that could have won that race to your mother’s egg, but didn’t. Or that winning sperm could have fertilized some other of your mother’s eggs. Either case, you’re a total nothing in anyone’s philosophy! Now think of all the males and females available at any given time and the odds are against your parents having met, courted, married, etc. Potentially, millions of other males could have married your mother; and potentially millions of other females could have married your father. And of course your mother and father are flukes because a similar argument holds for them via your grandparents, and further backwards for them to your great grandparents, etc, etc. Go back with those sorts of odds a hundred, a thousand, a million, a trillion, generations; back through your mammalian, reptilian, amphibian, etc. ancestry; back through your family tree of invertebrate ancestors (untold generations worth), right down to your unicellular ancestors. What are the odds, all those chance meetings be they sperm and egg, or male and female, of organisms that survived long enough through to a reproductive age (and relatively few, apart from humans do), that you would someday grace the world? Zillions upon zillions to one! You’re an absolute fluke! Or are you?

There appear to be ultimately three sets of either/or conditions which ultimately govern that. The first is, is there one universe, our Universe, or many universes (a Multiverse)? Secondly, is there absolute causality resulting in a deterministic clockwork Universe where once the initial set of relationships were set into play, things evolved with 100% certainty from that point on? Lastly, is there one, and only one possible physics, or could our physics have been different, or are different in another universe?

But first some definitions are in order.  

Universal Physics – One and only one kind of physics is possible, throughout all universes. If you believe in God, She didn’t have any choice in the matter!

Variable Physics – Each universe could have its own individual brand of physics, and the physics that rule our Universe could have turned out differently.

Determinism Rules – There’s 100% causality; there’s absolute certainty.

Chance Rules – Represents less than 100% causality; free will if you will.

Multiverse – Lots of separate and apart universes, perhaps even an infinite number of them.

Universe – The one and only Universe; our Universe. However, if our Universe is near infinite in space and time and stuff, then for all practical purposes it’s the same as the Multiverse since there’s enough space and time and stuff for nearly all possibilities to take place within.

Combining the various possibilities suggests how fluky you really are! But first, let’s deal with the supernatural, instead of natural scenario. If God exists and wanted you to exist, assuming there is a God of course, then you’re Probability One – no contest. You’re no fluke. Seeing however as how there is no evidence for any God (or gods), we’d better get back to all things natural and plough on.

Scenario 1: Universal Physics [+] Determinism [+] Multiverse = the exact same set of outcomes in all universes is predicted. If you exist in one universe, you exist in all universes. You’re not a fluke. But if you don’t, you don’t, full stop. However, you probably don’t since you’re existence would be equivalent to tossing a coin several billion times in a row all billion coming up heads. The assumption here is that the initial set of conditions was identical in each and every single universe within the Multiverse. If not, then they won’t be identical further on down the track. If that assumption is incorrect, you could exist in one or more universes; not exist in one or more other universes.

Scenario 2: Universal Physics [+] Determinism [+] Universe = if you exist, then you were a certainty from the get-go. If you don’t, that too was a certainty. On balance, a betting person would probably bet against your existence ever happening because only one exact set of circumstances gives the universe you, while billions of other sets of circumstances don’t.

Scenario 3: Universal Physics [+] Chance [+] Multiverse = your existence in one universe has nothing to do with your existence in any other universe. You’re a fluke in whatever universe(s) you exist in. But, the saving grace is that you do probably exist in at least one universe.

Scenario 4: Universal Physics [+] Chance [+] Universe = If you exist, you’re a fluke. The probability is extremely high given the odds against your existence such that nobody would have been willing to wager anything on your eventual behalf.

Scenario 5: Variable Physics [+] Determinism [+] Multiverse = assuming one universe has our kind of physics, then you’re existence is a 100% certainty, or 100% against. In all other universes, since the physics are different, nothing remotely like you, or anyone else, will exist. If you exist, you’re not just a fluke, but a super-fluke!

Scenario 6:  Variable Physics [+] Determinism [+] Universe = if there’s only one universe, then there can only be one physics, so assuming that that universe is our Universe, the outcome is the same as Scenario 2.

Scenario 7: Variable Physics [+] Chance [+] Multiverse = in the Multiverse, nearly all possibilities are realised (except in Scenario 1), so in at least one universe, you exist. You’re not a fluke due to the vast number of universes that could contain you. But you’re still a fluke in that you don’t exist in the near infinite other possible universes.

Scenario 8: Variable Physics [+] Chance [+] Universe = again, one universe must equal one physics. Since things however are based on a throw of the dice, if you exist, you’re a fluke.

In short, the betting odds towards your existence are to count on there being a Multiverse. If you’re dealt one hand (one universe), the odds against a royal flush are extreme. Being dealt a near infinite number of hands (the Multiverse) doesn’t ensure a royal flush, but it damn well improves the odds. So, if you exist and there’s one universe, our Universe, you’re a real fluke. In the Multiverse, you might be a fluke in any one universe, but there are so many universes that in that broader context, you’re not a fluke.

Do you want to improve the odds in favor of your existence? There’s a variation that could apply to the Multiverse theme called the Many Worlds interpretation of all things quantum. In the Many Worlds scenario, absolutely all possibilities are realized within any given ‘moment’ within the timeline. In terms of eggs and sperm, at any given moment, the universe splits into as many universes such that all sperm available fertilize all eggs available, and that applies regardless of species (so all possible males that can meet and mate with all possible females do so – or vice versa); all possible seeds that could have been are, and produce viable plants somewhere. 

You might note the subtle combining of both the Multiverse and the Many Worlds concepts. Each universe within the Multiverse has the additional complication (or added attraction) of having to jump through the Many Worlds hoops.

Universes within the Multiverse can have differing timelines. Thus Cleopatra, assuming a Cleopatra, in another universe might have been born into a 21^st Century Egypt, or Australia for that matter (assuming these nations exist or existed in this other universe). Likewise, in another, that other, universe, you may have been born a slave (or as Royalty) in an ancient Egypt, again, assuming such a nation existed. So, who knows, in some other universe (modern or ancient) you and (modern or ancient) Cleopatra (or substitute any modern or historical hunk male of your choice if you’re female) have this thing going! I’m not that likely going to be, in any universe, the one to mate with Cleopatra, any more than you’re (if female) are likely to get your act together with Marc Anthony. But, unlikely becomes Probability One when you merge the Multiverse with Many Worlds. The universe that contains you and Marc Anthony, or you (or me) and Cleopatra, will split when it come to that fork in the road. Many Worlds universe #1 split #1 fork – no pairings; Many Worlds universe #1 split #2 fork – you (we) live happy ever after! That’s because all possibilities are realized; have to be realized.

That’s a real headache inducer! So, to repeat, take a near (or actual) infinity of universes, each with its own unique timeline (which will not of necessity mirror our own). Then, multiply all those existences by all the possible forks in the road encountered by each of those universes since all paths are taken. Anything not forbidden is compulsory. You’re wildest dreams may not come true in this, our Universe, and on this particular pathway you’re experiencing, but somewhere you did, or will – but ditto that your worst nightmares, so perhaps be thankful for what you got in the here and now.

Now let’s reduce that headache somewhat by unmerging again the Multiverse and the Many Worlds ideas. Say in one alternate timeline universe Cleopatra and I get our act together (‘we do’). In another alternate timeline universe we don’t (‘we do not’). In both universes, the Many Worlds forks would now have us respectfully ‘we do not’ and ‘we do’ instead. But that’s already happened in the two universes, so do we now have two universes where ‘we do’ and two universes where ‘we do not’? Rather than accept that, perhaps the Multiverse and the Many Worlds concepts are ultimately, for all practical purposes, the same thing.

Ah, but the ‘we do’ in the first alternate universe took place in an alternate Australia thus the Many Worlds fork had us ‘we do not’ also in an alternate Australia. The original ‘we do not”  in the second alternate universe too place in that alternate ancient Egypt, so that Many Worlds fork now has us as a ‘we do’, also in an alternate ancient Egypt. So the two ‘we do’ universes and the two ‘we do not’ universes aren’t the same after all.

But then again, all those four possibilities could also be catered for in four actual separate and apart universes that are part of the Multiverse, so again there’s no need to bring into consideration the Many Worlds interpretation of all things quantum! Back to square one! And this sort of adding on further qualifiers, isn’t going to ultimately alter the apparently near equivalence of the Multiverse and the Many Worlds.

I’m not however entirely sure this apparent equivalence will sit well with quantum physicists, because I’m not entirely sure this is what quantum physicists mean by the phrase ‘Many Worlds’ (indeed, lots of quantum physicists deny any such interpretation at all exists – it’s too big an ask for them). However, it seems to deal with the issue of That Cat! In two separate physical universes you have Schrodinger’s Cat (in the box) experiment. Identical cats; identical setups; identical observers (and they can be identical because the fundamental bits that make them all up are identical – all electrons (neutrons, protons, etc.) are 100% clones of each other – absolutely identical). In one unverse, the observer observes the cat alive; the other, well it’s the demise of the feline. Neither universe has to split into two. All possibilities have been exhausted without resorting to the requirement of a Many Worlds either/or split. The only real difference I can see between the Multiverse and the Many Worlds interpretation is that with Many Worlds, the outcome (all possibilities realized) is certainty; with the Multiverse it’s only probable or possible.

In conclusion, you may think our planet is an example of total biodiversity, but imagine what things would be like if all the theoretical possibilities had come to pass (the Many Worlds and/or the Multiverse)!!! How many Einstein’s were never born - on the other hand, how many Hitler’s? Then throw in all the plants and animals that never were but could have been, and you could come up with multi-trillions upon trillions of alternative Earths! Ouch!

Perhaps it’s easier to accept that there’s just one Universe (our Universe) and no such a thing therefore as a Multiverse, and no such an animal as the Many Worlds interpretation of all things quantum. You’re therefore a total fluke, and so just enjoy the company of the rest of us total flukes! That’s unless of course our Universe is near infinite enough in extent, all on its lonesome, to allow for a duplicate copy (or three or four) of you – recall those elementary particles that make you, you, can be identically assembled elsewhere to accomplish exactly that.

To be continued…

Electron-ness: Why Are All Electrons Identical? Part Two

INTRODUCTION: Individual members of the fundamental or elementary particles are absolutely identical with one another. In a police identification line-up, you couldn’t tell them apart. Why? Rather than go into an exhaustive review of the entire particle zoo, I’ll just hit on the electron and its antimatter mirror twin, the positron.

Continued from yesterday’s blog…

There is no violation of physical laws at the micro level in travelling through time (apart from going forward at a rate of one second per second which we do whether we like it or not), no exact causality mechanism has been proposed to explain how and why an elementary particle shifts gear into time reverse (or forward again). But, this is interesting, this time travel bit, so let’s explore it in some greater detail.

I’ll poor water on this fire at the outset by stating that time is but an illusion. Time doesn’t exist; therefore time travel** isn’t possible. Time is but a label and has no more reality than the label ‘Wednesday’. We just arbitrarily call every 7^th ‘day’ Wednesday, but you can no more hold Wednesday in your hands and you can time. Time has no independent reality. You can’t assign any physical properties to the concept. I mean time isn’t a solid, it isn’t a liquid, and it isn’t a gas. Time has no size, weight, colour, texture, density, it doesn’t vibrate or have a wavelength, ditto no odour or flavour, it has no temperature or pressure, it doesn’t consist of any known combination of known forces and/or elementary particles, it corresponds to no known element or compound. You can’t pour time into a bottle and store it; you can’t confine time in a force field or in a prison cell or trap it on a piece of sticky fly paper. Therefore, if time has no substance, one can not actually travel through time. Our sole electron (or positron) isn’t going anywhere – in time at least.

What we perceive as ‘time’ is nothing but change – change in our environment; in our natural world; in our mechanical devices; and even in ourselves and associated companions (animal and human). Repeat – time is but an illusion. Change is real since it involves forces and particles; energy and matter, the sorts of things that when you kick them, they kick back. We measure ‘time’ by the rate things change; rate of change is what we call ‘time’. Repeat - it’s the change (in something) that is real. 

Now on the macro scale, that is scales we interact with on a day-to-day basis, change appears to all intents and purposes to go one way (usually from a state or high order to a state of disorder) and so we view time as flowing from past to present to future – in one direction; order to disorder – past to present to future. But change in just one direction (order to disorder – past to present to future) is ultimately a function of numbers and probability. The simple illustration is to introduce a drop of ink into a bowl of water. That’s a high order situation. Now left to itself, there will be a change. The ink will disperse throughout the bowl of water. That’s a state of disorder. If a disordered uniform mix of ink and water separated all by itself into a drop of pure ink and a bowl of pure water (high order), that too is change, but we would interpret or view such an event as going backwards in time. If you viewed such a happening on film, you’d immediately conclude the film was being run backwards.

There’s a far greater probability for individual ink particles to spread out throughout a large volume than to come together in a small space. There’s lots of pathways to spread out; far fewer pathways to come together. But at the micro level, the level at which those individual ink particles do their thing, they don’t care where in the bowl of water they are. They are just as ‘happy’ to be all together as a drop of ink, as dispersed and diluted. If they do come together as a drop of ink from a dispersed/diluted state, that’s statistically unlikely, but such an event violates no laws of physics. It would be going from a state of maximum disorder to a highly ordered state; or, from an apparent future to present to past ordering. Such a change would appear for all practical purposes as apparent time travel – going backwards in time.

The catch – there’s always a catch – is while all those ink particles are defying statistical probability and undergoing apparent time-reversal, the rest of the cosmos is acting in a statistically normal way – going forward in ‘time’. So, perhaps we have a Universe where for 99.9999% of the time, 99.9999% of events within the Universe march to the beat of the standard past – present – future ordering of things. That is, in terms of change happening in a statistically probable way. While now and again tiny pockets of the Universe reverse direction, they do so at least just temporarily.  One can only defy statistically probability for only so long. So the ink particles come back together again as a drop of ink within a bowl of water – then what? They no doubt reverse direction again and proceed normally.

An analogy might be that while some individuals are winners while playing the slot machines (high order), the club still rakes in the profits from the vastly greater majority of (disordered) losers, and that no doubt the few highly ordered winners will eventually descend into a state of disorder and contribute ultimately to the club’s profit margin! It’s more statistically likely for a winner to become a loser than for a loser to become a winner.

Back to electrons, on average, any given electron has a very high probability of participating in a changing set of circumstances consistent with statistically probability. That is, the electron is moving forward in ‘time’. But if in those rare (loser to winner) occasions the changing set of circumstances goes against the grain of statistically probability, then we would view that electron as moving backwards in ‘time’. But there ultimately is no backwards or forwards in time, just change which statistically goes or moves in one general direction (order to disorder), but which can now and again, and briefly, go the reverse direction (disorder to order).

To belabour the point, what we call the past is change which has already happened; the present is change which is happening; the future is change that will happen.

The question remains, can you revisit and experience a past event? Can there be an instant (or not so instant) replay? Take the example of the now dispersed ink in the bowl of water. If all the ink bits (particles) were to exactly retrace their movements (that’s just so highly improbable that you’d wait longer than the age of our Universe to actually see it), they would eventually come together as an ink drop. If they now retrace those retraced movements (ditto on the statistical improbability) you get back to the exact same configuration of dispersed ink bits in the bowl of water. You will have witnessed an instant (or not so instant) replay of a past event. You would have in a sense travelled back in time to record an event that had already happened. Of course you would have ‘time’ travelled with respect to that specific event and only that event.

Ah, that word ‘record’. Of course you could have filmed the original ink drop to dispersed ink event then watched the film at a later date, but that’s cheating a bit, don’t you think?

Okay, all that was an aside – back to the original question, why are all electrons identical? Or not, as the case may be.

To be continued…

**Assuming for a moment that time is actually something tangible and that travel through it is possible (that’s in agreement with Einstein’s General Theory of Relativity). That aside, I’d maintain that travel backwards in time is probably nonsense.

If you go back in time with a view to either preventing something from happening or inaugurating something and you succeed, then when you return to your own present time the original motive to go back in time in the first place has ceased to exist. That’s because in your now altered present, there’s nothing motivating you to go back in time and so you don’t, but if you don’t then you couldn’t have changed the past in the first place. That actually suggests that your actions have split the Universe and generated two timelines (or universes), one in which you go back to fix something, and one in which you don’t because there’s nothing that needs fixing.

If you don’t succeed, if you can’t tamper with the past (akin to Stephen Hawking’s Chronology Protection Conjecture) then it’s all an exercise in futility and so there’s no point being a damn fool about it. If at first you don’t succeed, give up!

If you go back in time just to observe (as historian, scientist or even tourist), your very presence in the past has introduced a change that didn’t previously exist, and any change has a ripple effect which will change, even if only slightly, your own present, in you’re your before-the-fact time travelling present was different to your after-the-fact time travelling present, which could, as we’ll see, generate a paradox. You may not care about the alteration, but other people may not be so happy and laid back with your inadvertent meddling.

But wait a second. Those other people probably wouldn’t know or be aware that anything had altered. Having reset the clock when you went back in time, the ripples would have become part and parcel of their world view, so only you, upon your return (having bypassed all the rippling) would notice the change.

But what if you go back in time only to materialize in front of a speeding train and are killed. Of course that doesn’t affect your ancestors so presumably they still meet and marry and breed and ultimately your born – again – only to go back in time and get hit by that train!  

Or, you materialize back in time and so startling someone of that era into failing to notice that train and gets killed. Now say that someone was your father-to-be; your father before he met your mother. Now you have two universes – timelines – again. One timeline is where you went back in time and presumably returned; one in which you were never born.

Here’s another time travelling curve ball coming your way. You have a set of coordinates with respect to Planet Earth – latitude, longitude, and altitude. But you also have a set of coordinates with respect to the Moon – lunar latitude, longitude and altitude. You have a set of coordinates with respect to the Sun (solar latitude, longitude and altitude). Ditto Mars, and ditto the nearest star and ditto the centre of the Milky Way Galaxy, etc. In fact, although they change from moment because of relative motions of all the bodies concerned (that also applies to you and Planet Earth since you move around), you have a set of coordinates with respect to every bit of matter in the Universe. The question is, when you time travel, what set of coordinates do you take with you? Where exactly do you end up? It’s taken for granted in works of fiction that its Earth’s coordinates, but is it necessarily so? What if you retain your exact position (relative to where exactly is a mystery) but travel in time. Then presumably when you materialize else-when, the Earth will have moved far away, and there you are flailing around in empty space, breathing a deep vacuum!

So we see that while time travel stories are a staple of the sci-fi authors’ bag-of-tricks – they stir up those little grey cells – there doesn’t appear to be much chance of time travel in any physical reality we know of. Time travel is only a reality in the imagination. We in fact have a version of the Fermi Paradox here. While that referred to aliens knocking on our collective doors, if time travel were possible, then where are all those time travelling tourists from our future?