RE: Is Time Travel is Possible?

["Nicholas Day" <apyxnd@nottingham.ac.uk>]

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A couple of things about what Bernie has said:

Think you’re being a bit unfair on K&N.  Presentism is the denial of the 4 dimension – it is eternalists/4 dimensionalists (which you could guess from the name I s’pose) who accept that there is a 4th dimension.  ‘Presentists deny the analogy between time and the spatial dimensions’ – they think time is some fundamentally different sort of thing – so I don’t think you can be right to say that the K&N’s  argument that we ARE travelling through time as we speak is based on ‘the equivalence of the 4th dimension to the other three dimensions’.

The presentist is going to say that movement through time and movement through space are fundamentally different sorts of things, and you’re gonna be making some kind of equivocation fallacy if you try and use 'movement' in the same way for both.  The presentist is going to agree with you ‘that movement can only take place in space’ only as far as you mean ‘spatial-movement’, as opposed to ‘time-movement’.  Time-movement being something about the flow of time, the way in which some object can move from existing then to existing now.

>>‘"Presentists" would argue that the origin of the fourth dimension is the present
>> and therefore we are not time travelling. The issue is what do we mean by
>> travelling. If we were travelling through time by one day per day, then can we 
>> stop travelling ? If the answer is no, then we are only travelling in the same way 
>>we should say we are travelling at 26,000 miles per second relative to the centre >>of the universe’

I don’t agree here.  First, I don’t think presentists would argue that about the origin of the 4th dimension for two reasons: 1) they don’t accept the existence of a 4th dimension and 2) often presentism is a way of maintaining a the intuitive belief in the flow of time, something that we seem to lose in eternalism/4th dimensionalism.  So I don’t think many would want to deny that we are (time-)travelling at one day per day.  Secondly, not sure how far you can take these analogies to spatial movement, again, because it is the sort of analogy that the presentist does not accept.

>>Annoyingly, at times the time travel thought experiment is used as evidence for the existence of time travel as well as the past. Indeed this is a clear case of begging the question and therefore an invalid argument.

K&N don’t use the thought experiment for the existence of the past – that would just be plain crazy given presentism and the whole point of the paper (I mean, prove me wrong – but I like to give these academics at least a mildly charitable interpretation).  Secondly, the thought experiment is just to show that if time travel is compatible with 4dism, it is also compatible with presentism, because they can tell an equally coherent story.  I don’t think it is fair to say that they are using it as evidence that backwards time travel actually occurs (if that is what you mean by ‘the existence of time travel’). (and, for that matter, if they did, though I accept that would be pretty crazy, it wouldn’t be question-begging)

>> They then seem to degenerate into a conclusion which borders on gibberish
>> e.g. "Perhaps presentist time-travel is not real time travel" As presentists are 
>>defined by the authors as believing that time travel is impossible, this statement 
>> illustrates the authors severe confusion.

Again, prove me wrong on this one, but I don’t think they do define presentists as believing that time travel is impossible.  My first reason for thinking that is just that if you call your paper ‘presentists should believe in time-travel’ and then define ‘presentists’ as believing that time travel is impossible, then you have done something very silly – and I just don’t think they are quite that silly.  Secondly, I’m not sure I understand which bit you are referring to when you say that define them in that way.  On page 334, they state the *common view* as the view that time-travel is not possible if presentism is true, and then they say they will argue against the common view.  Hmm – am I misunderstanding you? 

>>So although the paper does state the presentist position with some accuracy and clarity at the outset, it fails to provide any sound argument for the existence of a past to which one could travel.

Well, they certainly aren’t trying to provide an argument to show that there is an existing past to which one could travel – that would be to argue against presentism.  Instead, they are arguing that even if presentism were true, and thus there was no past to travel to, you can still make as much sense of time travel as the eternalist/4dimensionalist.

So, basically, I’m just generally disagreeing with your view that this paper is complete rubbish.  Perhaps because of a na*ve hope that academic philosophers aren’t as silly as would seem to be the case if what you said is true.  But feel free to crush my hopes if these guys really are that silly and I’ve misunderstood what you were saying :-).


>>> "Bernie Doeser" <bernie@doeser.org> 12/08/05 9:08 AM >>>
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Firstly I'd like to thank Alice for drawing our attention to Keller and
Nelson's paper (see http://people.bu.edu/stk/Papers/Timetravel.pdf ).
Although the first couple of pages are very readable and articulate well the
argument that the past and future do not exist, significant flaws creep in
as soon as some examples of time travel are discussed.

Firstly it is stated that time travel does exist. We are all travelling from
our past to our present. This argument is based upon the equivalence of the
fourth dimension to the other three dimensions. This assumes that all
dimensions are the same - if one can move in the x dimension, then one can
also move in the t dimension. Physicists tend to specify 3 "spatial"
dimensions to distinguish them from the other 10 or 11 non-spatial
dimensions (there is not yet widespread agreement on the existence of the
14th dimension). Movement can only take place in space, i.e. along the three
spatial dimensions.

In addition movement is always defined in Cartesian terms by movement
relative to the origin, which is arbitrarily fixed. If we locate the origin
at the centre of the universe, then I am travelling several thousand miles
per second. If we locate the origin on the top of Big Ben then I am
stationary. Now the origin, as it relates to the fourth dimension may either
be fixed, say at January 1st 2000, or it may move at one day per day, such
that the origin is synchronised with the present day. "Presentists" would
argue that the origin of the fourth dimension is the present and therefore
we are not time travelling. The issue is what do we mean by travelling. If
we were travelling through time by one day per day, then can we stop
travelling ? If the answer is no, then we are only travelling in the same
way that we should say we are travelling at 26,000 miles per second relative
to the centre of the universe (this might pose some problems defending speed
traffic offences).

The paper gives an example of an old woman travelling back in time to talk
to her younger self  which presents a whole series of paradoxes that are not
adequately addressed in the paper, and are indeed not problematical for
presentists for whom time travel is impossible. Annoyingly, at times the
time travel thought experiment is used as evidence for the existence of time
travel as well as the past. Indeed this is a clear case of begging the
question and therefore an invalid argument.

The arguments on causality are surreal. Let me quote the following challenge
to presentists "How can it be true that the Sydney Olympics are more
expensive than were the Melbourne Olympics, when there are no existing
Melbourne Olympics for the Sydney Olympics to be more expensive than?" Now
the Melbourne Olympics took place in 1956 and the Sydney Olympics took place
in 2000. What the authors seem to have done here is to extrapolate from a
position that I cannot travel to the past, because it no longer exists, to a
position where the past does not exist and therefore I cannot say anything
about it at all. This is not what presentists say. They say that past
existed, and we remember it and have a record of it, and what happened in
the past influenced the present, but the past is gone, and it does not now
exist, only our memories of it exist.

Keller and Nelson, then explore various esoteric problems, for example can
two instances of a person exist at the same time, if so, who is the real
person and what problems does this present for the concept of identity. They
then seem to degenerate into a conclusion which borders on gibberish e.g.
"Perhaps presentist time-travel is not real time travel" As presentists are
defined by the authors as believing that time travel is impossible, this
statement illustrates the authors severe confusion.

So although the paper does state the presentist position with some accuracy
and clarity at the outset, it fails to provide any sound argument for the
existence of a past to which one could travel.


By the way, I did not know that I was a "Presentist", and would feel
uncomfortable being characterised by something which did not exist and which
I did not believe in, as I would then fall into a large number of other
groupings, as would, I suspect, all of us.

Bernie Doeser
Sandiway, Cheshire, UK.

-----Original Message-----
From: owner-bups-dis@purplepancake.com
[mailto:owner-bups-dis@purplepancake.com]On Behalf Of Alice Evans
Sent: 07 December 2005 12:38
To: BUPS-DIS@bups.org
Subject: RE: Is Time Travel is Possible?

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Very funny, i utterly agree
So, do u think time travel is compatible with presentism then?
if so, do u adopt the endurantist/ perdurantist model?

hey, check out,

Keller and Nelson article, "Presentists should believe in time-travel"
(Australasian Journal of Philosophy, Vol. 79, No. 3, September, 2001). pp.
333-345.

in addition, contrary to hawking's retorts, i suggest the reason we dont
have time-travelling tourists nowadays is perhaps because the invention has
not only not been invented but perhaps our present/past arent particular
popular temporal locations. But yeh, i agree, the lack of evidence fails to
undermine the time travel argument

lurve you too...

Alice

>>> "James Alexander Cunningham" <0203734C@student.gla.ac.uk> 12/07/05 11:13
AM >>>
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BUPS-DIS@bups.org


Better late than never, here are my thoughts. Firstly, time travel, in the
sense of sending of a spaceship, having it return eighty Earth years later
to find that only five years have passed for the crew of the ship, happens.
Granted, not on such a dramatic scale, but the effect has been observed. A
few years ago, on the Royal Society Christmas Lectures an atomic clock was
placed on an airliner and flown around for a week or so. When it returned it
was something like 0.0000002 seconds out of sync with a similar clock that
remained on Earth. If this is considered to be time travel then we are all
travelling through time constantly.
The argument that Bernie gives for supposing that if time travel were
possible it would occur in the natural world is very suspect. Allow me to
re-iterate the argument with my comments:

-If time travel were possible it would occur in the natural world.
Firstly, it may. One explanation for anti-matter is that it is matter moving
backwards in time. Secondly not everything that is possible is observed in
the natural world (the synthisization of certain elements for example, or
even certain chemicals). Thirdly, it presupposes that everything that is
possible (at least in some sense) has happened. Any argument following this
line would need further explanation.
-Time travel in the natural world would be observable.
Not necessarily. This would suppose that either a) Time travelling objects
only moved to within the observable universe, within recorded history
(including fossil records, archaeological data, written sources etc.) or b)
That the observer is omniscient. Denial of both poses no problem for the
supporter of time travel.
-There is no evidence that time travel has occurred in the actual world.
-Ergo, time travel is not possible.
In fact all this argument proves is that there have been no recorded
examples of time travel within the observed universe. To claim from this
that time travel is impossible requires the claim that everything that is
non-actual is impossible (which you may wish to argue for, but most would
shy away from it).

Bernie then goes on to deny that the past exists, claiming that it cannot
exist because we cannot interact with it. The example given is that I cannot
eat a cheese sandwich in 1837. This is very true, but equally I cannot eat a
cheese sandwich on Sirius B. The reason is very simple. I cannot eat a
cheese sandwich on Sirius B because I am not there. Equally I cannot eat a
cheese sandwich in 1837 because I am not there (or then) either. The fact
that I (or Bernie) am not present does not mean that either Sirius B or 1837
do not exist. Just a couple of thoughts. People who are interested in this
sort of thing might like to check out Time, Change, Cause and Effect in
issue 1 of the British Journal of Undergraduate Philosophy, a superb article
that I really cannot praise highly enough (if only because I wrote it).

When I get a moment I'll knock out a few of my own ideas on this subject but
I'm busy busy, and tend to change my mind on a daily basis anyway.

Love you

James




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