Craig Callender & Tim Maudlin: Time Travel, Time’s Arrow, and The Block Universe | RP #115

Craig Callender is Professor of Philosophy and Co-Director of the Institute for Practical Ethics at UC San Diego. Tim Maudlin is Professor of Philosophy at NYU and Founder and Director of the John Bell Institute for the Foundations of Physics. Craig and Tim are leading philosophers of science and physics. Craig also appeared on episode 73, in which he and Robinson discussed pseudoscience and conspiracy theories. Tim was a guest on episode 46, which covered laws of nature, space, and free will, and episode 67 with David Albert, which was all about the foundations of quantum mechanics. In this episode, Craig, Tim, and Robinson delve into the philosophy of time, touching on the reality of the past, present, and future, the direction of time, its relationship to relativity and quantum mechanics, and time travel. Craig and Tim have both written on time. Check out Craig’s book What Makes Time Special? (Oxford University Press, 2017) and Tim’s book Philosophy of Physics, Volume 1: Space and Time (Princeton, 2012). If you’re interested in the foundations of physics—which you absolutely should be—then please check out the John Bell Institute, which is devoted to providing a home for research and education in this important area. At this early stage any donations are immensely helpful.

OUTLINE
00:00 In This Episode…
01:12 Introduction
04:43 The A- and B-Series of Time
21:20 Presentism, Possibilism, and Eternalism
42:03 Foliations in Time
57:39 Foliations of Time in Quantum Theory
01:03:30 Superluminal Signaling
01:11:56 The Direction of Time
01:35:24 Philosophy and Time Travel
02:03:07 The John Bell Institute

Robinson Erhardt researches symbolic logic and the foundations of mathematics at Stanford University. Join him in conversations with philosophers, scientists, weightlifters, artists, and everyone in-between.

[honeyj]

I love that Tim maudlin is doing a lot of podcasts. Thanks professor!

[shaunlanighan]

Very good. Has both cleared things up for me and introduced new topics.

[johnrichardson]

The present is real. The past was real. The future will be real. There. Settled it for ya'll!

[Robinson]

Their books are great! Go get 'What makes time special?' and 'Philosophy of physics: space and time'!

[mf_hume]

Craig’s comments at 14:02 are really enlightening. In my opinion it’s a pretty damning criticism of a discipline and it’s methods when it can carry on for decades ‘investigating’ pseudo-problems like this…

[patdevlin]

Tim is just a bit (a lot) too patronising! Too certain about things he knows nothing or let's say little about! I'm watching and listening and watching three people speaking drivel! Tim picks up on words like ALREADY and GROW as if he doesn't realise the limited scope of words to explain concepts. Not impressed.

[oliverjamito]

Little LIGHT came with a Candy as an offering to comfort the COMFORTER! Remembering not to flood my Footstool!

[brainpain]

Great content but I couldn't watch it. Too many adds. You might make more money turning down the adds and getting more viewers. This stuff is too heady to be interrupted every 3 to 5 minutes.

[StephenPaulKing]

The trick to superluminal signaling is that the ability to verify the results of measurements requires signaling that obeys relativity. The correlations are not communicable via light signals. There is a distinction between the signals that verify the results of the measurements and the correlations themselves.

[StephenPaulKing]

What do we measure with Clocks? Rates of Change. The change in one system relative to the other in an interaction, for example.

[DanielAnchondo]

I enjoyed this podcast. I'm glad foliations were discussed. I wonder how it may fit in with QFT or Bohemian Mechanics. The time of flight thing he talked about is pretty cool too that conceptually it is an interpretation of quantum mechanics which may answer certain questions that can't be explained by the other interpretations.

I sorta felt left wanting about the A and B series discussion. I think it's more than that some A and B people can see compatibility between the 2 views but rather what makes them distinct.

On the A view, there's something privileged about the present that the past and future aren't. There's tensed facts, the past has happened and the future has yet to come, temporal becoming is a real thing things go through. Time as we experience it isn't an illusion of our subconscious. Just because the past can't be changed, or what will happen will happen is a tautology, I think one would need more than the fact that light cones or a Minkowski space time is a useful way of understanding relativity to accept the B-view space time block.

A nice book I read that introduced the A and B series to me was Time and Eternity.

[SpecialistQKD]

The way the universe works is b comes after a unless it comes before a in a changed timeline, then the universe will adjust and turn a into b and vise versa 👈

[kilogods]

Great vid but you’re tripping w the amount of ads you put….

[doodle]

Time must be defined in relation to the size of now the observer experiences. For example, a newborn and a very elderly person may have very large, even vast sized perception of ‘now’, vs. a 40 year old with 4 kids, 2 jobs, and aging in parents may perceive a tiny, tiny ‘now’ as time flies by…. As if the amount of neurological physical change inside the brain - organized growth (newborn) vs. growing disorder (very elderly) vs. neural stasis of a middle ager forces.a dilation -or- contraction (dilation at the ends of the life cycle, contraction mid cycle) of the perception of ‘now’. Perhaps pre-post life/death - ‘now’ is ‘eternal - always- all time’. This is a theory I developed in college. Please reference me if you quote this.

[-tarificpromo-]

A pilot that flys only toward the east, still follows the block even though the pilot never experiences the future.

[rubix]

Alan Watts would've been a great addition to this panel. If only Time weren't an arrow...

[robdin]

I love that there is more than 1 guest. Robinson is a great interviewer but sometimes it feels like the answers to his questions are standard answers to such questions. Still good questions but not really a challenge for the guest. When the guests start to ask questions to each other you see that the guests have to think a bit more about the answer.

Great episode!

[johnnymann]

How do you influence the past without changing the past?

[Robinson]

Thanks for getting these two absolute legends of the current literature of time in here! I identify as an amateur philosopher dropping my few cents over here

23:35 Tim doing a lot of mental gymnastics. "There are facts, there are facts about the past, therefore there is a past." But the point is, is it manifest? Can you travel there? Indeed, mathematically or in your imagination presentism and eternalism should portray the same manifest reality, but which is more accurate concerning the word 'is'? You can travel left and right in space, therefore space 'is'. But you can't travel back into the past. Therefore the past 'isn't'.

It all determines on what you put as a 'first coordinate' (or origin) in your theory or ontology, and for me that is our conciousness or existence in this moment. And not the mathematics and predictability conjured up, no matter how truthfull, by our minds.

Therefore presentism is more relevant than eternalism in my opinion



I guess my next point is imagination has to be taken into account: both presentism and eternalism are visually imagined by our brain in the visual cortex. Which one is more 'real' though? Which one 'is'? Then you need to make a division between imagination and the external world, and as such we get into Kantian area..


I feel it's fair to say eternalism is like the Everettian interpretation of reality, taking the unitary wavefunction seriously, while presentism is like Bohmian mechanics that takes reality's side seriously.


33:00 the timelike area of the spacetime diagram (the light cone) supervenes on the spacelike area according to entanglement! Where there is time, there is also space in this diagram! Timelike doesn't equate to time, but to time + space! Therefore time is not different than a mathematical algorithm on the space dimension, which sounds like an indication for presentism.

Proof of this is if you hypothetically decrease the lightspeed, the current timelike area will become spacelike. And for other reasons this is true: the null lines moves through space, thus the intermediate timelike area must also be in space.


Also I think again time travel is an important distinction between what can be counted as reality. If we can travel to the past, making the past the future, then eternalism makes sense. If however you can't travel to the past, a.k.a. make your past the future, then presentism makes sense. It means time is mostly current, the present, which can oscillate backwards and forwards with its internal energy but is restricted to a future move due to statistical mechanics. Otherwise the past could be the future according to this logic (Huw Price).


1:17:00 David Wallace speaks of recurrent systems and irreversible systems with attractors, in deterministic classical mechanics. This is a better physical explanation then "causes precede effects", because this presumes human categorization of causes and effects (David Hume).

In my opinion in classical mechanics the Hamiltonian is conserved, which means energy is conserved. Yet time is still deterministic with due to the first law of Newton, which is due to the conservation of energy.

Therefore I feel there is something like a cosmic Hamiltonian, of potential and kinetic energy, starting with the big bang to now, in which potential energy gets turned into kinetic energy according and equalling the second law of thermodynamica, increasing entropy and with that causing the arrow of time. So spacetime has a local Hamiltonian which is conserved and called classical mechanics; and on top of that there is a global Hamiltonian of the expanding universe which causes a forward time direction due to entropy. So basically my thesis is that the potential for (literally!) thermodynamics is caused by the expansion of the spacetime of the universe...maybe similar to prof. Richard Muller's idea from Berkeley? Haven't read his book yet

[SandipChitale]

Finally I am so glad to hear what Tim said about spacetime. Spacetime is not a 4D object where D standing for space like dimensions. Spacetime should be thought of as 3SD+1TD composite object where SD stands for space like dimensions which use the unit of distance and no intrinsic direction and TD stands for time like dimensions with a unit of time and intrinsic directional property and moving like a infinite, exorably moving treadmill belt undergirding the reality (only a vague metaphor). The rate of the time dimension is a relative notion and makes sense if we have clocks, which could be thought of as some cyclical subpart of the universe. If there were no cyclical subpart of the universe we will not be able to meaningfully talk about the rate of flow of time. In other words we can only measure the rate using some cyclic subpart if the universe. Without it, time will be reduced to a concept of change only. And if there is no change of state at all - time effectively seizes to exist. I think scientists know this but I blame their sloppy language/talk of spacetime as a 4D object to mislead people into conjuring up 4 spatial dimensional thingy.

But one part I disagree with Tim is about is about the notion of global now - that global now is not from the perspective of a specific point in space, but from the point of view of each point's own perspective - in other words the proper time of each point itself. Think of the statement - the universe is thought of as being 13.7 years old no matter which spatial location you are in the universe. Why is that? Or think of the hypersurface of the end of every point's world line (worm). And if at the big bang the universe was at a point then long distances do no cause the issue of time needed to travel those distances. The light cones at that point are very very short (in height) with narrow base (spatial extent) and effectively touch each other for their full extent (from tip to the base) and then we follow the proper time of each point to get the global now. Hope this makes sense.

The light cone structure tells us that the "now" that we visually perceive (fastest) is the very surface of the past light cone from our vantage point and not the global now. The auditory perception cone is a much narrower past sound cone which is obviously in the bulk of the past light cone.

Also people should appreciate the fact that in terms of the day to day units we use for distance and time, the surfaces of the past and future light cones almost touch each other.

Think this (crudely):





i.e. the future and past light cone surfaces almost touching each other (for all practical, perceivable purposes) and only starting to sperate out vast distances away from our vantage point.

Don't think this:

. .
. .
. .
. .
*
. .
. .
. .
. .

Agree with Tim that time intrinsically has a direction which is hinted at by the light cone structure and the fact that spacetime follows Minkowski hyperbolic geometry/metric (s^2 = t ^2 - x^2 - y^2 -z^2) i.e. the minus sign (i.e. the difference in SD and TD I talked about above) and not Euclidean geometry (d^2=x^2+y^2+z^2) and.