Tuesday, January 23, 2024

 After a long hiatus, I am back -- focused and ready to continue tackling the difficult and fascinating questions in the philosophy of physics and consciousness.  I am also investing in my YouTube channel, focusing on clear explanations of difficult concepts to the lay audience.  Here's my most recent video, showing a logical proof why computers will never be conscious, mind uploading is impossible, and more than one instance of a person's conscious state can't exist at more than one place or time in the universe.

Wednesday, October 12, 2022

Is Time Travel Possible? Part 4

In three previous posts, I discussed whether time travel is possible.

This paper, just posted on the ArXiv, argues that time travel into the past is logically incompatible with quantum  mechanics.  Here is the abstract:

Because closed timelike curves are consistent with general relativity, many have asserted that time travel into the past is physically possible if not technologically infeasible. However, the possibility of time travel into the past rests on the unstated and false assumption that zero change to the past implies zero change to the present. I show that this assumption is logically inconsistent; as such, it renders time travel into the past both unscientific and pseudoscientific.

Saturday, April 16, 2022

YOLO, Midlife Crisis, and Meaning

Either consciousness is eternal or it’s not.  If it’s not, then there will be a point in time at which the only remaining/lasting legacy of our existence, our decisions and choices, our pleasure and pain, will be nothing more than the distribution of atoms, in one way versus another, throughout a cold, lifeless, quiet universe.  How could that matter?  If there is no one to whom it could matter, then it truly is meaningless.


I am having an existential crisis.  I’m 44, so you might just say that this a midlife crisis, and maybe it is.  Not to minimize a midlife crisis, but I also think I’m in a fundamentally different situation from most people my age.  I am financially independent and don’t have children, so already I have significantly more time than most to wonder about purpose and meaning.  Add to that the fact I’ve spent the last few years thinking deeply about some of the hardest and deepest problems in philosophy and physics. 

It’s very hard to ruminate on deep questions about the universe without also contemplating the nature of existence itself.  For example, I’ve spent a lot of time over the past few years contemplating whether the physical world is deterministic or reversible, whether quantum mechanics implies the creation of new information, whether we have free will and how free will might relate to quantum mechanics, whether a conscious state is entirely determined by the physical state within a local volume (like within a skull), whether consciousness can be physically duplicated or instantiated on a computer, and so forth.  It’s hard to do these things nearly full time, without the distractions of children and debt, at an age that many would regard as midlife, without also staring down the barrel of my own mortality.

There are times when I envy my friends who have children and jobs and debt and never-ending to-do lists.  These constant distractions are, in some sense, a luxury that allows people to divert their attention away from the ticking clock.  But I stare at it.  And it’s terrifying, particularly when I mindlessly accept this overarching and pervasive societal message: you have to live meaningfully but you have very limited time in which to do it.  Life matters, but you only have a few years.  That irritating acronym “YOLO” (You Only Live Once) may not come up much in polite conversation anymore, but its message is everywhere.  Change the world.  Leave a legacy.  Do what matters.  And do it now because you’re running out of time. 

No wonder the world is anxious.  I, for one, am experiencing incredible anxiety and insecurity about how I should spend my time.  After all, now that I know my time on Earth is (at best) half over, and that what I’m capable of will likely decay with time, it’s hard not to freak out about how to live most meaningfully in the time I have left.

But the YOLO message is actually a contradiction, and all contradictions are false.  Let’s break the message into Premises 1 and 2:

1)     You have limited time; consciousness permanently ends at death.[1]  (Logically, it could end at some time other than physical death, which wouldn’t affect the following argument.  But most people who believe Premise 1 believe that the human brain is entirely responsible for creating consciousness[2], in which case death of one’s brain would bring about an end to his consciousness.)

2)     What you do matters; how you spend your time matters.

I will argue that these two premises are contradictory.  Either or both are wrong.

Certainly most people want to believe Premise 2; I don’t know anyone who wants to believe that life is pointless.  Many people who believe Premise 1 and want to believe (or do believe) Premise 2 give this line of reasoning: “Sure, the things I do on Earth won’t matter to me after I’m dead, since I won’t exist anymore.  But they still matter to other people, and that’s what gave them meaning while I was alive.” 

In other words: “My life matters because it matters to others.”  This is the notion of legacy that people like to leave, such as through descendants, lasting impacts on the world, and so forth.  The problem is that there’s a circularity to the logic (and circular arguments are not valid).  The life of A has meaning, even after A is dead, because of his impact on B.  But why does B’s life matter?  Well, it matters, of course, because of B’s impact on C.  And C’s life matters because of her impact on D, and so on down the line.  But what if D’s life in fact does not matter?  Then neither can A’s, B’s, or C’s, because their meaning all depended on the meaning of D’s.

If one’s life is only meaningful to the extent of one’s impact on others, and if the lives of those others are only meaningful to the extent of their impact on still others, and on and on, then meaning is a metaphysical Ponzi scheme.  If true, the meaning of life would depend on an eternally unbroken chain of consciousness – that is, there must always be something conscious in the universe that is impacted by the previous lives of other conscious beings to justify the meaning in their lives.

The problem here is that physicists (who overwhelmingly believe Premise 1) would nearly unanimously agree that at some point in time the very last conscious being will die – i.e., that there cannot always be consciousness in the universe because the universe will not remain hospitable to life indefinitely.  Specifically, even if the “Big Crunch” or the “Big Rip” don’t kill off everything, the eventual heat death of the universe will.

So if no one’s life has meaning in and of itself – if any given person’s life matters only to the extent of his impact on others – then all life is indeed meaningless.  I’m certainly not saying that one’s children, or the process of leaving one’s legacy, can’t be deeply meaningful to a person.  I’m simply saying that that can’t be the entire source of life’s meaning, otherwise no life could have meaning at all.  If life does have meaning, it must have meaning at least to some extent for its own sake.  A person who believes in Premise 1 and really wants to believe Premise 2 cannot make them compatible simply by claiming that “My life matters because it matters to others.”  That won’t work.

In many ways, I’ve said something far simpler.  If the net result of all of our lives and decisions is just the scattering of dust in a cold, lifeless universe, then what’s the point of it all?  (Cue Kansas’ Dust in the Wind…)  In other words, if Premise 1 is true, then there is no meaning to life and nothing matters.  You cannot bootstrap meaning in your own life by mattering to others, because, if Premise 1 is true, there is also no meaning to their lives.  It doesn’t matter that you matter to others who don’t matter. 

Here’s my point.  Either what I do matters or it doesn’t.  Premise 1 implies that it doesn’t, which is in direct contradiction with Premise 2.  They cannot both be true. 

So if Premise 2 is true then Premise 1 is false.  If what I do matters, then my consciousness will not permanently end at death (or at all), in which case I have plenty of time to do what matters.  But if Premise 2 is false – if what I do doesn’t matter – then why the fuck am I so worried about running out of time?

As it turns out, I believe that my consciousness is eternal, but I have been very much acting as if everything I want to do or experience must be done in the short term.  That’s irrational.

I have not tried to be precise in this post with my language or argumentation.  What it means for something to “matter” or “be meaningful” is subjective, and I certainly don’t claim that this line of reasoning proves the existence of an afterlife or eternal consciousness.  I believe I have, in papers and previous posts, proven some important and very relevant facts, such as that a conscious state cannot be copied or instantiated on a computer and that a conscious state cannot be entirely determined by the information in a local volume (such as a brain), among other things.  For instance, if the information that physically produces a conscious state is not (and cannot be) contained entirely in the brain, then already there is good reason to doubt the zealotry of scientists who claim, with arrogant certainty, that brain death permanently erases consciousness.  They don’t know.  Nevertheless, though I believe my consciousness is eternal, my goal here is not to prove it, if such a proof were even possible.

Rather, my goal here is to point out that the YOLO dogma is bullshit.  We are told from every angle that we must amount to something, we must live fully and meaningfully, we must make a difference and leave a legacy – AND that we only have one lifetime in which to do it before the lights go out for eternity.  But that makes no sense.  Those messages are contradictory.  Because if my lights go out for eternity, then what I did on Earth certainly won’t matter to me, and if your lights go out for eternity, then they won’t matter to you either.  If there’s no me to regret having failed to make a difference and leave a legacy, then why put in all the effort to make a difference and leave a legacy?  Why worry about not having enough time to do everything I want to do?  Either I have plenty of time (because my consciousness survives death) or, when I die, I’ll no longer be conscious and capable of regretting.  If death is an eternal lack of existence, then any impact I leave on other people will necessarily be lost, enduring legacies are impossible, and nothing I do matters.  But if death is not an eternal lack of existence, then I’m not running out of time to live meaningfully!

That’s not entirely the end of the story.  First, I still don’t know whether or not what I do matters (or how much it matters).  Eternal consciousness does not tell me much about how much meaning my life and decisions have, just that meaning is possible.  For example, maybe free will is an illusion, in which case I cannot do anything meaningful because I cannot choose to do anything at all.  I think much more likely is that some of what I do is meaningful, but I vastly overinflate the importance of most of it.

Second, even if (as I believe) consciousness is eternal, physical death certainly happens and at that time I don’t know what I’ll perceive or experience, but it’s unlikely I will experience consciousness through a human body on Earth.  There probably are a lot of opportunities that will be foreclosed at that time, so if I want to make a positive difference on Earth, then I should do it now, while I’m here.  I also have no idea whether I will be able to continue my relationships with people in the afterlife, so I would want to enjoy those relationships now while I can.

[1] Note: Most people who believe Premise 1 don’t believe in God, and most who believe in God don’t believe Premise 1.

[2] I have shown this is false, but no one listens to me anyway.

Monday, April 11, 2022

Is Time Travel Possible? Part 3

In the last post, I continued a discussion of time travel and pointed out that any time travel into the past would result in facts/events that become permanently embedded in the information structure of the universe.  I’ve had several more insights since then about the (im)possibility of time travel.  Here they are in random order:

Traveling into the Future

The examples I gave were all about changing the “rate” of travel into the future, such as with drugs and relativistic time dilation.  I should be clear that it is impossible to “skip” to some time in the future, such as with Doc Brown’s Delorean time machine.  You can’t instantaneously skip to the future because determinism is false, so you have to just wait it out for random/quantum events to actually occur.  In other words, there is no way for the universe to know what the year 3000 looks like until it actually goes through the (time-consuming! haha) effort of evolving through that time.  Lots and lots of new information has to be created through, for example, quantum events, and the universe doesn’t “know” what quantum possibilities exist in the year 3000 until the events of the year 2999 have happened, and the quantum possibilities in the year 2999 depend on the events of the year 2998, and so forth.  Of course, you can consciously skip over time (by sleeping, etc.), but there will still be evidence in the universe of time passage, including in your own body.

Traveling into the Past

The explanations/arguments I have in my last post about why time travel to the past is impossible relate to how changes will affect the present (or the future of the past), which is problematic for reasons I’ll explain more momentarily.  But someone might say, “I don’t need to actually travel to the past if that’s impossible.  A simulation would be fine.”  Obviously, simulations are possible to some degree.  Setting aside that a realistic simulation would itself require enormous computational resources, any simulation would be limited to a tiny selection of information.  There is no way to recreate or accurately simulate the past because the vast majority of the information in the universe (correlations/entanglements) is fundamentally inaccessible.  No matter how many experiments you do, the vast majority of the information in the universe cannot, even in principle, be known.  (The reason for this is related to quantum no-cloning.)

Free Will

I made the mistake in the last post of confusing the analysis with questions about free will.  After all, imagine I don’t have free will and instead whether I travel into the past in my time machine depends on some quantum event 10 minutes from now.  The same problems arise because, at this moment, there is no fact about whether or not I’ll travel into the past, which means that whether or not I traveled into the past cannot already be embedded in the information structure of the universe.

Bad Terminology

The word “change,” particularly when philosophers and physicists discuss time travel, is misused, abused, and confused.  The word “change” itself is always relative to a particular time.  “Yesterday, I changed my mind.”  “Last week, I changed my haircut.”  “At time t0, the particle’s trajectory changed.”  But people who discuss time travel often say things like, “Tomorrow, I will change what happened last year,” or “At time t2, what happened at time t1 was changed.”  This is nonsense.  When was the event changed?  At t1 or t2??  Use of the word “change” in this fashion is inherently contradictory.

Quantum Indeterminism

But here is a much bigger problem with the notion of time travel into the past: it assumes that the universe evolves deterministically.  We tend to think that if we were to change the past just a little bit, then the present would also change only a little bit.  Or maybe it would change a lot (thanks to chaos) but we assume that the changes to the present depend only on the changes to the past.  Similarly, if we could manage not to change the past at all, then the present would remain exactly the same.  This is a natural assumption – in fact, it was one that I hadn’t even questioned until today – but it is wrong.

That’s because the information content of the universe is increasing over time, thanks to the indeterminism caused by random/quantum events.  So if we were able to travel to the past, then the past we would experience would have less information and there is no guarantee that it would evolve into the present that we actually currently experience.  In fact, it would almost certainly NOT evolve into the actual present.  And if you changed something about the past – anything at all – then different quantum information must be created because different quantum possibilities would arise. 

Here's a simplified example showing the problem, in which I am somehow able to travel back to time t0.  Assume:

·       A speck of dust went in direction A at time t0 but if I time travel to t0 then it will/would/did[1] go in direction B. 

·       In a nearby quantum experiment, an object in quantum superposition state x|X> + y|Y> could be detected at location X or Y (where x and y are complex amplitudes).  Then, at time t1, it was actually detected at location X.

Problems & Questions:

·       Is there any good reason to think that, if I time travel to t0, the object will still be detected at X at t1? 

o   No, because if there was, then that means the object was already in state |X>, not a superposition.

o   Similarly, the outcomes of every possible quantum event in the universe could be different.  There is essentially zero chance that the universe I currently experience, or anything like it, will evolve if the embedded information of events since time t0 is erased. 

·       If I am right about universal entanglement (i.e., the speck and object are already well correlated), wouldn’t the changing of direction A to B also affect the amplitudes x and y, even if only slightly?

o   If so, then there certainly would be no reason to think that the object would be detected at X at t1.  Further, correlation information with objects and fields throughout the universe would instantaneously (nonlocally) change.  Event outcomes would change, new possibilities would arise while others would be foreclosed, and so on. 

·       What does locality have to do with this?  Locality doesn’t matter regarding universal entanglement, because quantum correlations are nonlocal.  But even setting that aside, if the speck goes in direction B instead of A, then new (quantum) possibilities for creations of photons are created, and those new photons could affect the experiment, which clearly would affect the amplitudes x and y.

[1] This is more linguistic evidence of the contradiction inherent in backward time travel.  If I time travel to time t0, do I say that the speck of dust will go in direction B?  Would go?  Did go?  WHEN did I change its direction?  Now or at time t0?

Saturday, April 9, 2022

Is Time Travel Possible? Part 2

In the last post, I introduced the notion of time travel into the past and pointed out that the potential for a temporal paradox would prevent time travel that allowed changes to the past.  Is it possible, as some have argued, to affect, influence, or participate in the past without changing the past?


First, there is the Butterfly Effect.  You can get the general idea by watching the movie of the same name, but the idea is simple: small events get amplified over time into much larger consequences.  The Butterfly Effect is actually a mathematical result of chaos theory, whereby nonlinear relationships cause effects that seem to be entirely unpredictable over a long enough time period.  Perhaps one of the most fascinating papers I ever read was this one (“Gargantuan chaotic gravitational three-body systems and their irreversibility to the Planck length”), in which the authors show that even the largest objects in the universe are fundamentally unpredictable over a long enough time period.  They analyze initial conditions that are precise down to the Planck length, because that length, according to physicists, is the smallest length that has any possible physical meaning in the universe.  So if precision down to the Planck length can’t predict the motions of supermassive black holes, then nothing can.

The Butterfly Effect is a problem for time travel into the past, and here’s why.  You certainly can’t go back in time to kill one of your ancestors, as that would prevent your own existence and cause a temporal paradox.  But what if instead you did something seemingly inconsequential, but that event caused another event that caused another event… that prevented your own existence.  In the first installment of Back to the Future, Marty pushes his future father out of the road so that he didn’t get hit by a car, but of course that caused a chain of events that led to Marty’s future mother falling for him instead of his future father.

But what about something more subtle?  Say you want to go back in time just to observe your ancestors – you have no intention of killing or even interacting with them.  Say you’re just walking around and watching from a distance.  A malarial mosquito starts buzzing around you and you swat it away.  If you hadn’t been there, it would have just fed on some nearby rat.  But your swatting at it stimulates it; angry and hungry, it flies in the direction of your ancestors and stings one.  Maybe that ancestor dies from malaria before he reproduces.  Maybe he just gets sick and misses his opportunity to mate with the woman who would have been your ancestor.  Or maybe nothing serious happens at all.  Maybe he was already in the middle of having sex with her when he gets stung by the mosquito, and the brief moment he spends smacking the mosquito is enough to change which sperm cell ultimately fertilizes her egg.  That alone would be enough to change the identity of their descendants, and it would (arguably) prevent YOU from existing.

Physicists and philosophers who argue that time travel into the past is possible usually understand the Butterfly Effect and might retort something like, “Time travel into the past is possible as long as you don’t do anything, even something small that could chaotically amplify, that would change the facts of the present.”


As I discuss in this paper, information in the universe is embedded in the relationships, correlations, and entanglements among and between all the particles and fields throughout the universe.  Whether the amount of information in the universe is constant or changing is a fundamental question.  Those who believe in the assumption of U (that quantum wave states always and universally evolve linearly) believe that the information content of the universe is fixed.  As you know if you read this blog or my papers (such as this or this), I have shown that U is false and that the information content of the universe is increasing – specifically, collapse of the quantum wave state upon irreversible correlation events is what produces new information.  Nevertheless, whether the information content of the universe is constant or changing, we can all agree that at the present moment there is a certain amount of information in the universe (and it is a staggeringly large amount).

In other words, there are a huge number of “facts of the present,” and every one of them correlates to some event in the past.  Every single event in the past gets embedded as a fact in the information structure of the universe.  (Those who believe in U would say that every event in the past is already embedded in the information structure of the universe, both past and future, in a block universe.)

What does this all mean?  Imagine a speck of dust a million years ago that might have flown in direction A or direction B depending, for example, on some quantum event.  There is necessarily some physical experiment that can be done today whose outcome would depend on whether that speck of dust went in direction A or B, whether or not anyone could figure out which experiment to perform.  In other words, that seemingly insignificant event from a million years ago is currently embedded in the information structure of the universe, which means that there is some feature of the universe today that depends on that event.  That feature may (or may not) be small, depending on the extent to which it was chaotically amplified, and it may not even be possible to know what kind of experiment would differentiate that feature.  But the point is that the happening of that event a million years ago MUST be physically embedded in the universe today.  (If it wasn’t, then it didn’t happen – which is the point I’ve been trying to make over and over regarding measurement and physical irreversibility.)

So here’s the problem.  If it was possible for you to travel back in time, even if you tried your best not to change the past, your mere physical presence would be a fact that gets embedded in the future information structure of the universe.  The universe, after all, is constantly measuring you and everything about you.  Trillions of air molecules are bouncing off your body every second.  Infrared radiation (heat) is constantly being emitted and absorbed by your skin.  Even your own minuscule gravitational field causes changes around you that constantly provide evidence of your presence.

Even if all you wanted to do was watch your ancestors in the past, and you were somehow able to send back just your eyeballs (or, even better, a tiny camera[1]), your eyes work by focusing and absorbing photons on your retina, and the FACTS about whether or not particular photons in the past got absorbed get permanently embedded in the universe.  At a bare minimum, in order to see something, you have to change the trajectory and/or presence of photons.  These would constitute events, the occurrence of which would forever affect the universe.

Therefore, there is no way to send anything physical to the past, no matter how small or seemingly insignificant – and thus there is no way to directly observe the past – without affecting the present.  If, for example, it was possible to travel to the past in some kind of super-insulated vehicle that managed to affect nothing in the past, except that it allowed a few photons to be absorbed by your retinas so that you could see something, the facts about the absorption of those few photons would be embedded in the universe in a way that would reveal measurably distinct outcomes at any time after, including the present. 

So here’s what I’ve pointed out so far:

·       You can’t travel back in time to kill an ancestor because this would cause a temporal paradox.

·       You can’t travel back in time to do much at all because, thanks to the Butterfly Effect, almost anything you do will get chaotically amplified over time in a way that somehow prevents your existence or your ability or willingness to travel back in time.

·       Nevertheless, if you did travel back in time (or sent something back in time), even if you did nothing, your very presence in the past would get embedded as permanent facts of the universe.  The way the universe is right now (or 10 minutes ago or 10 years ago) depends on whether you travel back in time.

The last point is key.

Imagine that there is some conscious experience you had 10 minutes ago (at time T) that depended, at least to some small extent, on whether you time traveled to the past.  Assume for simplicity that you observed something as blue – i.e., there is a fact, embedded in the universe, about your conscious experience of seeing blue at time T.  However, if you travel into the past, your mere existence gets embedded in the universe in such a way that, at time T, you instead experience seeing red.  So what did you actually experience?

Some may reply that this is just a case of memory erasure or modification, but in these papers (here and here) I argue that retroactive modification of conscious states is impossible.  I’m not saying someone can’t misremember something.  I’m saying that conscious states are history dependent, that the facts of past conscious states get embedded in future states much as past physical events get embedded in future states of the universe.  If there is indeed a fact about what I am consciously experiencing now, then it evidences facts about previous conscious states.  It is therefore impossible for me to experience my present conscious state (which, indeed, I am experiencing) if my past conscious states are not facts and can instead be retroactively changed.  And because time travel into the past implies the ability to retroactively change my own past conscious states, then it would imply that there is no fact about my current conscious state (and also no fact about the current physical state of the universe).  This is false.

Therefore, time travel into the past in any form, even just to observe it, is impossible.

One nagging objection to the above analysis is this: it all hinges on one’s ability to choose whether to travel into the past.  If there’s no free will, then it is either true or false that I’ve already traveled into the past.  But if I have, then that fact is already embedded in the present universe and there is nothing I can do to prevent myself from traveling into the past.  Of course, I believe that we have free will, but it remains one of the problems I am currently working on.

I’ve said enough for this post.  What other objections or thoughts do you have?

[1] But then where would the image information be sent?  On that note, if you were able to send back just your eyeballs to the past without the rest of your body and brain to consciously experience it, then what’s the point?

Wednesday, April 6, 2022

Is Time Travel Possible? Part 1

Traveling into the Future

We can certainly travel into the future, which is what we are doing right now at the rate of 1 minute per minute.  We can even change that rate.  With drugs or alcohol, we can change the perceived rate in which we travel into the future.  But we can also change the actual rate, relative to others, thanks to Einstein’s relativity.  This is called relativistic time dilation and it is caused by two physical phenomena. 

The first is relative motion (characterized by Einstein’s “special” relativity).  Imagine you and a friend are wearing really accurate and precise watches.  If your friend is moving toward you or away from you at some speed, her watch will tick slower than yours.  However, the effect in our normal everyday world is very, very tiny.  Take the person’s speed, divide it by the speed of light, then square that result, and that’s the approximate effect.

For example, let’s say your friend is jogging toward you at 3 meters per second (which is almost 7 miles per hour).  Divide that by the speed of light (about 300,000,000 meters per second), and then square that result, and you get 10-16, or 0.0000000000000001.  So for every second your watch ticks, your friend’s watch will tick 100 billionths of a billionth of a second slower.  Imperceptible.  However, if your friend could approach the speed of light relative to you, then her watch would slow down significantly relative to yours.  In fact, if she zoomed away at near the speed of light for long enough, then turned around and returned to Earth, she might have aged only a few months while you aged a few years or longer.

The second phenomenon is gravity (characterized by Einstein’s “general” relativity).  Time slows inside a gravitational potential well.  For example, time ticks slightly slower on the surface of the earth than in outer space, which is why GPS satellites must account for this tiny effect to stay accurate.  The stronger the gravity field, the stronger the time dilation.  A great example was in the movie Interstellar in which the crew lands on a planet closely orbiting a supermassive black hole, leaving one crew member in the mother ship much further from the black hole.  The crew on the planet felt time passing normally, but when they returned to the mother ship, they realized that their hour on the planet had corresponded to many years passing for the crew member on the mother ship.

These things are certainly interesting to think about, but the technology does not exist – and may not ever exist – that will allow humans to experience significant time dilation.  Yes, time dilation is real and we have to account for it in GPS satellites, nuclear reactors, and particle accelerators, but will we ever have the technology to accelerate a human to near the speed of light (relative to another human)?  Probably not.  So while possible in principle, time travel into the future through use of relativistic effects will probably remain science fiction.

Traveling into the Past

The far more interesting question, and the one that most people wonder regarding time travel, is whether it’s possible to travel into the past.  We all want to be Marty McFly (or Doc Brown, depending on your personality), with the freedom to see the past, experience the past, and even change the past.

Lots of reputable physicists, like Nobel Prize winner Kip Thorne (author of Black Holes & Time Warps), claim that the laws of physics allow for time travel into the past, and even suggest fascinating ways of doing so, like stretching open and passing through wormholes.  (Note: wormholes are theoretical predictions and there is exactly zero empirical evidence that they exist.  Even if they could exist, there is no good reason to believe that we could ever create them at will, engineer them to specifications, choose their endpoints in spacetime, stretch them open, or pass matter through them.  They are, purely and simply, science fiction.)

The obvious problem with time travel into the past is the risk of a temporal paradox.  For example, what would happen if you killed your ancestors?  If you did, then you couldn’t be born, which means you couldn’t go back in time to kill your ancestors.  Doesn’t that problem by itself rule out time travel into the past? 

Some say that there’s no problem as long as there’s no possibility of actually changing the past, a concept called the self-consistency principle.  For example, maybe you can travel to the past, but as soon as you try to kill one of your ancestors, some mystical force prevents you from following through.  This statement, which comes from the Stanford Encyclopedia of Philosophy, highlights the point: “We should note the crucial distinction between changing the past and participating in (aka affecting or influencing) the past.” 

So what do you think?  Is it possible to affect, influence, or participate in the past without changing the past?

If time travel into the past was possible, what kinds of things could you do without causing a temporal paradox?

In the next post, I’ll dig into this a little more.