Is time travel possible?

James asked:

Is time travel possible?

Answer by Craig Skinner

In brief:

* relativistic time travel into the future is well-established.

* time travel into the past is logically possible but may not be physically possible.

* time travel into the past may be possible only as far as the point when the time machine was invented but no farther back.

Space is so accommodating: three dimensions to travel in, in a direction and speed of our choosing. Time is stingy: only one dimension, one-way travel along it, and at one speed, crawling into the future at one hour per hour.

Could we time travel more freely?

First, we (probably) couldn’t travel to the future or to the past if these don’t exist. For those who think only the present is real (‘presentists’), there is no future or past to go to or from (the no-destination problem). Similarly, for those who think the past and present are real, but not the future (the Growing Block view of time), there is no future to go to: also we can’t expect to get travellers from a nonexistent future, and this is true for all past times, so there is never a departure point for travel into the past. In short (leaving aside some dodgy arguments to the contrary) we need the Static Block View in which all times co-exist, linked by earlier-than and later-than relations (‘eternalism’, the B-series view). This matches the familiar spacetime manifold view of modern science. This view will be assumed in what follows (but I am not denying that, for us, there is such a time as the present).

It’s a well-established feature of Special Relativity that measure of elapsed time depends on the frame of reference. An astronaut accelerating away from Earth to reach almost light speed, returning twenty years older, could arrive back to find Earth a million years older. However, she can’t go back in time to join her long-dead loved ones. This ‘sliding’ rather than ‘jumping’ time travel, due to ‘time dilatation’, has been empirically confirmed using atomic clocks and measurement of lifetimes of short-lived particles moving at high speed.

What about travel to the past?

I’ll deal with logical puzzles, then with the physics.

Some say time travel yields logical contradiction and is thus impossible. Two paradoxes illustrate this.

1. The grandfather paradox.

My grandfather died peacefully in bed aged 85 years. But I travel back to when he was a lad and kill him. So, I can’t exist. But I do exist. Contradiction. I don’t think there is a problem here. The fallacy is thinking that there could be more than one VERSION of the past ie that I could go back and CHANGE the past. But the past can’t be changed. I could go back and AFFECT the past, but if so I was there (as a traveller from the future) when the past events happened all those years ago – the actions of any and all travellers from the future are already built in to the past. So, since my grandfather survived to reproduce, I didn’t kill him. Therefore if I travel to the past intending to kill him, I won’t succeed. Of course if I try repeatedly, there will be a series of flukes and coincidences that beggar belief (the gun jams on my first attempt, next try I fire but miss, next I was given blanks by mistake, fourth attempt I kill the wrong person, and so on). But this is just what we must expect in the unusual circumstances of time travel.

I’ll just mention parallel- and branching- universes. I could travel to the past of a universe just like ours up to the time of my arrival but having a different future in that I kill the lad who would have been my grandfather, so that I never get born in that universe, but am there as a visitor to do the killing (so no paradox). Alternatively, I go back in our own universe, do manage to kill my grandfather, and this causes the universe to branch in two (one where I don’t have a future existence, and the other where I came from). However, I feel we can deal adequately with the puzzles of time travel without invoking branching/parallel universes.

2. The free knowledge problem.

Looking for a sock in a drawer, I find a notebook giving detailed instructions on how to build a time machine. I labour for twenty years, build it, then travel back twenty years to leave the instructions in the drawer.

I am a physicist, fed up with failed attempts to find a theory of quantum gravity. I travel two hundred years into the future, look up the accepted quantum gravity theory (aha!), write it all down, travel back, submit it for publication and it becomes the (Nobel-prize winning) accepted theory.

In these strange-loop scenarios, we can’t say where the knowledge came from in the first place. Strange indeed, but knowledge is knowledge, do we have to know it’s origin. I can imagine a distant future when we know how to make universes with specified sets of laws of nature, one of us then travels back 14 billion years and sets off the Big Bang which began our universe.

As for the physics, the possibilities include a high speed rotating cylinder, and worm-holes. But the cylinder would need to be more massive than all the matter in all the galaxies in the known universe, not to mention the colossal speed of rotation. And the worm holes would be picosecond-lasting and with diameters less than proton-sized unless enlarged and stabilized by unthinkable quantities of exotic matter. So don’t hold your breath. Importantly, most physicists think time travel could go no farther back than the point when the time machine was first built. Which explains why, so far, no visitors from the future have been received. For if, in the future, travel to ANY past time were possible, tourists would have ‘must see’ trips. Christ’s crucifixion, say, might be enduringly popular. In which case contemporary accounts of the event would have recorded the mysterious presence of huge numbers of oddly dressed strangers in the crowd, and no such accounts exist.


What’s the most interesting philosophical argument you have come across?

Salik asked:

What is the most interesting philosophical argument the panel has come across?

Answer by Helier Robinson

For me it is the arguments in response to the question: are the empirical objects that we perceive around us real objects, or are they only images of real objects? There are good arguments for each side.

On the side of us perceiving real objects we have:

1. Real objects are outside of us, and so are empirical objects, so empirical objects are real.

2. Real objects are public, and so are empirical objects, so empirical objects are real.

3. Real objects are material, and so are empirical objects, so empirical objects are real.

4. Real objects continue to exist when unperceived, and so are empirical objects, so empirical objects are real.

On the side of perceiving only images of real objects we have:

5. Empirical objects are composed of sensations, such as coloured shapes, tactile qualities such as various degrees of hard and soft, hot and cold, rough and smooth, penetrable (e.g. marshmallow) and impenetrable (e.g. cast iron), heavy and light, etc., as well as sounds, smells and taste. That is all that empirical objects are: structures of sensations. And sensation are manufactured in our brains, out of afferent neural impulses that come from the sense organs. So empirical objects are images of real objects, in our brains, not the real objects themselves.

6. All empirical objects are illusory to some degree. Imagine a straight road lined with telephone poles: as it goes into the distance the road gets narrower, the poles get shorter, and the poles get closer together. In other words, visual space shrinks with distance from the perceiver, in all three dimensions. But real space does not shrink with distance, so visual space is illusory. The only explanation of illusions is that they are misrepresentations of real objects, not the real objects themselves, because illusions are unreal. But misrepresentations are images.

These two positions can be reconciled, but I will leave it to you to work out how. The starting point is the fact that you own body is an empirical object.


Answer by Geoffrey Klempner

I would like to put in a bid for Wittgenstein’s considerations on the possibility of a private language, which occupy a considerable portion of his later work Philosophical Investigations.

This was a major impetus for me when I wrote my D.Phil thesis, ‘The Metaphysics of Meaning’ (Oxford 1982) and indeed right up to the present time.

What is the private language argument? Although, arguably, there is no simple statement that collects together all the various strands of dialectic, Wittgenstein himself makes a stab at capturing the central move:

‘Always get rid of the idea of the private object in this way: assume that it constantly changes, but that you do not notice the change because your memory constantly deceives you’ (PI, p.207).

Although the quote talks about memory, as Wittgenstein makes clear elsewhere the issue isn’t to do with scepticism about memory as such, but rather the idea of ‘recognizing’ an entity of a certain kind, where no criterion can be given for what counts as ‘same’ or ‘different’, where, in effect, whatever you say or think will be ‘right’.

(The hypothesis concerning a private object is stated explicitly in PI para 258, which you will often see quoted, inaccurately, as ‘the’ private language argument.)

I wrote in my D.Phil thesis that the private language argument first presents itself as a wall, blocking the path of one’s thoughts. You look for a way past the wall, over it, or under it. But then you reach the wall, only to find yourself facing the other way.

The idea of a private inner object, a discrete component of my mental life, that I can make true judgements about, judgements which in principle cannot be wrong because my mind is in direct contact with the object itself, is an illusion. It’s like attaching a target to your arrow, and then claiming that in shooting the arrow you have ‘succeeded in hitting the target’. There is no ‘success’ or ‘failure’, nothing to ‘think’ or ‘judge’.

With that seemingly simple argument, a whole philosophical tradition falls, or so it has been claimed. I actually don’t think any philosopher (Descartes included, someone who has often been branded as believing in ‘private objects’) ever literally believed this, but the point is that until Wittgenstein no-one had thought to pose the question about our inner life in this way.

Do we not have an inner life? Yes, of course. I would go further and state that no-one, in principle, can know ‘what it is to be I’ because my unique attunement with reality is the result of the way my brain and nervous system interact with the world outside, an attunement which cannot be captured in language or judgements, because it concerns my very being as an agent in the world.


Descartes on the insufficiency of sense experience to determine what is real

Latorshia asked:

Why doesn’t Descartes simply determine what’s real by looking around him and use his sense experience?

Answer by Helier Robinson

Your assumption is that everything empirical (i.e. known through the senses) is real; but this is not so, because some empirical data are illusory and thereby unreal. Illusions are contradictions within sense experience (e.g. the half immersed spoon in a glass of water, which is bent to the sight and straight to the touch) or between sense experience and well established belief (e.g. the railway lines appearing to meet in the distance) and there are no contradictions in reality. The business of finding out what in the empirical world is real is empirical science (as opposed to theoretical science, which tries to explain empirical reality by describing its underlying causes) and empirical science is still not yet complete.

Empirical reality may be defined as everything empirical that is not illusory, as opposed to theoretical reality which is every that exists independently of being perceived. It is widely believed that these two definitions are equivalent but in my opinion they are not, because of the argument from illusion. This is the argument that since illusions are unreal they are misrepresentations of reality rather than reality itself. In other words, illusions are false images of reality. But there is no clear distinction, in perception, between illusions and non-illusions, so non-illusions must be images of reality also. That is, illusions and non-illusions are all made of the same ‘stuff’, namely, sense data, which are sensations provided by the sense organs. The argument from illusion is widely disregarded because its conclusion is so opposed to common sense, but it has never been satisfactorily refuted.


Answer by Craig Skinner

Because he thinks REASON comes before OBSERVATION as the route to secure knowledge. He is a rationalist, as opposed to empiricist, philosopher. Of course all agree that reason and observation both contribute, the distinction between rationalists and empiricists is only rough and ready. and one of emphasis.

As a rationalist Descartes must reason his way to the view that he can trust his senses, not start off by taking sense experience as a reliable basis for knowledge. He notes that sense experience suggests, for example, a flat Earth, unmoving Earth, small sun and tiny stars – all incorrect.

Descartes is disenchanted with medieval philosophy based on Aristotle (who was pretty much an empiricist), and says he wishes to start from the beginning and build a solid foundation of knowledge for the sciences. His method is the famous Method of Doubt or methodological scepticism: he will only accept as knowledge what is so clear and distinct to his mind that it can’t be doubted, proceeding from there by reason.

He begins by doubting the evidence of his senses including the existence of the external world and his own body (at any time, he might be dreaming, or an evil demon could be deceiving him) Then he doubts even truths of reason (an evil demon could affect his mind tricking him into thinking 2+2=4 when it’s really nothing of the kind).

After all this doubting, is there anything left he can rely on? Yes, the fact that he is doubting means he is thinking, so he must exist (I think therefore I am, ‘cogito ergo sum’ in Latin).

Can he move on from there ? Is there anything else which is clear and distinct ? I suppose most of us would go straight from the Cogito to saying we had a clear and distinct idea of an external world including our own bodies, gained through sense experience. But, for Descartes, such a jump is unjustified. He says he has a clear and distinct idea of God as an omnipotent, omniscient, infinite, necessary being, and then argues as follows:

I. God therefore exists

2. God is no deceiver, so

3. I can (mostly) trust my God-given senses and reason.

4. My senses yield a clear and distinct idea of an external world including my own body.

5. These exist.

This argument from the Cogito back to the everyday world via God as a bridge is full of holes – do we all have this alleged clear and distinct idea of God (I don’t); even if we do, this doesn’t mean there is any God answering to this idea: even if there were a God, why shouldn’t he be a deceiver?

So he is no nearer to establishing the existence of the external world with CERTAINTY than he was when he started. But, as he says himself in the Synopsis of the Meditations, ‘no sane person has ever seriously doubted these things’. And of course it’s well known to the beginner in philosophy that the existence of the external world can’t be proved – solipsism, idealism, brains in vats, world-including-all-of-us as a simulation, all logically possible. But nobody really believes these things. One or two current philosophers argue that since there could be many simulations run by advanced intelligences but only one real world, it’s statistically more likely that we are virtual beings in an elaborate simulation rather than real flesh and blood humans in the real world, but I doubt they really believe it. If you are unsure, make sure you lead an entertaining (virtual) life so the higher intelligences stay interested and don’t switch off the simulation.

A final. important, philosophical point. Can we trust reason more than our senses ? The rationalist traditionally argues that reason yields a priori, analytic, necessary truths; observation yields second-class, a posteriori, synthetic, contingent truths. I wont go into the modern debate as to whether the analytic/synthetic distinction exists or whether the a priori is a coherent notion. But why should we trust our reason more than our senses?. Both can go wrong. Neither can be justified in any absolute sense – our senses can deceive us sometimes, and, logically speaking, could deceive us always. Likewise with our reason – we can only justify reason by reason, a circular argument. Ultimately we just accept our senses and our reason as generally reliable guides to the world. We can say, with Descartes, that they are God-given and so reliable, or, post-Darwin, that they are part of evolved human nature, promoting survival by being reliable guides to the world. My general view is that the senses and reason are on a par. As the Scottish philosopher, Thomas Reid, observed in 1764:

‘Why, sir, should I believe the faculty of reason more than that of perception? – they come both out of the same shop, and were made by the same artist: if he puts one piece of false ware into my hands, what should hinder him from putting another?’

So, in the end, Descartes does determine what’s real by looking around him, like everybody else, but with a deeper understanding than he had when he started, and we all do well if we manage the same.


Will there come a day when philosophy no longer exists?

Christopher asked:

Do you think that there will come a day when philosophy will no longer exist? Many of the questions/ problems that philosophy attempts to answer/ resolve have been asked for centuries and I’m guessing that some of those questions have no real definite answer and have been looked at from every possible side. Also, the world is only so big, therefore, eventually won’t we know everything there is to know about everything? Hypothetical questions may always exist, but how much room is there in that for philosophy?

Answer by Shaun Williamson

No there will never come a day when philosophy no longer exists. There will never be a day when we know everything about everything. The world is very big, much bigger than you seem to think.

Now you say ‘I’m guessing that…’ and this is just the problem. I don’t want to guess, I want to know the truth and if there are no answers to philosophical problems then I want to know precisely why there are no answers to philosophical problems. For some of us guessing just isn’t good enough.


Answer by Geoffrey Klempner

I remember once hearing from a fellow graduate student while I was at Oxford in the late 70s that David Wiggins (who later went on to become Wykeham Professor of Logic at Oxford from 1993 to 2000) had expressed the view that philosophy was in its last stages, and that in the foreseeable future the rigorous methods of logic would solve all the major problems of philosophy.

There are not a few philosophers around today who believe that proposition. It is at least believable, given that a philosopher as eminent as the young Wittgenstein in Tractatus Logico-Philosophicus held something like this view — although, arguably, in his case there remained the ‘mystical’, or that concerning which nothing can be said (all that can be said are the ‘propositions of natural science’).

Without doubt, logic has risen to giddying heights since the revolution brought about by Frege, Wittgenstein and Russell. Problems have been solved, or at least rendered with a clarity that previous generations had never dreamed of. But is philosophy nearly over?

I leave aside not altogether dissimilar views expressed by philosophers from the continental tradition. Someone better qualified than I can answer that.

The answer to your question is I don’t know. If philosophy is nearly over, however, then whatever comes next will still have a plate full of unanswered questions. What methods we might use to solve these problems, if they are not the methods of logic, is beyond me. Or maybe ‘logic’ will become something totally different too, something we can barely conceive or imagine at the present time.


David Hume’s view on personal identity

Jackson asked:

What is Hume’s view on personal identity?

Answer by Craig Skinner

This can be summed up in three short quotes. I will give these, and say a little about each.

1. ‘The essence of the mind…equally unknown to us with that of external bodies’ (‘A Treatise of Human Nature’, 1739, Introduction, para. 8).

2. ‘When I enter most intimately into what I call myself I always stumble on some particular perception or other….and never can observe anything but the perception’ (Treatise, 1.4.6. para. 3).

3. ‘My hopes vanish when I come to explain the principles that unite our successive perceptions…..all our distinct perceptions are distinct existences, and that the mind never perceives any real connexion among distinct existences’ (Treatise, Appendix, para 20/21).

Two preliminary points. First, Hume uses ‘person’, ‘mind’ and ‘self’ interchangeably. Secondly, by ‘perception’, Hume means what we now call ‘experience’.

Now, to the quotes.

1. Here, Hume expresses his trademark epistemological scepticism. He feels that, speaking philosophically, we know a deal less than we often think we do. For example, as regards the existence of a stand-alone external world as the explanation of our successive sensations, he ‘feigns no hypothesis’, and again, whereas we commonly suppose that we observe a connection between cause and effect, what we actually observe is only regularity or ‘constant conjunction’. In reality, the true basis for our succession of sensations, and the connexion between cause and effect (if any), are unknown to us.

For Hume, the only knowledge we can rely on in philosophy is empirical knowledge, and, for him, this is derived from impression-based ideas. And so, coming to the self, what are the ideas we can rely on ?. The second quote deals with this.

2. On ‘looking inwardly’ I am aware only of a bundle of perceptions (experiences). An accurate view, I feel. It is sometimes called the ‘bundle theory of the self’. But some have gone too far, claiming Hume held there is only a bundle, nothing else, no ‘self’ which unites the bundle. This ‘no ownership’ view, or ‘illusory self’ view (which I think is incoherent) was first misattributed to Hume by his contemporary Thomas Reid, and still runs. To be fair, Hume contributes to this: when considering our everyday natural assumption of an enduring self, he describes this as a ‘fiction’ of the mind. But Hume clearly thought that every experience was experienced ie an experience requires an experiencer or subject of experience. It is just that, on Hume’s view, we have no knowledge of the nature of this subject of experience or self. Clearly, the self might be an enduring entity, the same from day to day, its persistence depending either on a persistent immaterial substance (soul), or on continuity of consciousness/memory (Lockean view). Or the self might be a momentary entity, replaced next instant by a new momentary self, so that I am a series of transient selves, as Buddhism holds. But Hume offers no view on this, holding only that we don’t know.

3. Why do Hume’s ‘hopes vanish’? It is because he can’t, using only his impression-certified notion of legitimate knowledge, assume anything about the self beyond the evident fact of bundles of experience, and the logical fact that an experience entails a subject of experience (but there could. logically, be a separate subject for every single experience, there is no evident connexion between them). This approach, he realizes, is fine for, say, causation: he need assume nothing other than constant conjunction. But as regards the mind, he has throughout his philosophical writings assumed a structure to the mind, continuity of memory for instance which obviously does connect our successive bundles of experience. Who for instance is this ‘I’ of quote 2 which does the entering and stumbling. In short, Hume has gone beyond what his own philosophical lights allow him to do. I feel he may have realized something of the self-referential features and puzzles of self and consciousness.

In conclusion Hume’s view on personal identity is in keeping with his sceptical approach, although he ultimately realizes he has in fact gone further than this strict approach allows, and that indeed he must do so.

Even today, argument rages as to what Hume’s view on personal identity really was. I have mentioned the Reid-inspired ‘no-ownership’ view of self, often attributed (wrongly I believe) to Hume. And there are views other than those I have expressed as to why Hume said his ‘hopes vanish’.

Best read Hume yourself and make up your own mind.