B.

PS: A comment subscription feature for the posts here would be handy. I tend to forget about comments I’ve made if I don’t get replies by email.

Stevie– Unfortunately not; by the time we knew about Viola’s book, the die had been cast, as it were.

However, I think that the scenario becomes less far-fetched when you consider individual particles, which travel as waves unless measured. Since we are uncertain about the exact position of a particle at any given moment, we are unable to determine whether it crosses through the gate in the same exact position each time. Also, we are uncertain about the precise time that the particle crosses through the gate. In fact, at some point on the particle’s world line, the probability distribution of its location in spacetime would span both sides of the gate. Does that concept conflict with any known laws of physics or would that be permitted?

Also, closed timelike curves can exist without containing particles, thus also eliminating the issues that you discussed.

Just for fun, could we have an example that is even more odd than your scenario? Could the stranger that you describe, whose life story you described as “a one-day loop repeated ad infinitum,” actually walk through the gate facing forwards on one day, then walk through the gate facing backwards the next day (in his world line), then restart the loop by walking through forwards again such that he lines up exactly with his past self? In this way, we create a two-day loop, occurring within a single day, perhaps playing out in two parallel universes. I understand only a little about how “spin” works in quantum physics, but isn’t this the concept of how some particles must rotate twice to return to their original position?

Regardless, if closed timelike curves were to exist, we can be certain that the conditions would not permit the evolution of conscious beings, so paradoxes in terms of free will not occur.

http://blogs.discovermagazine.com/cosmicvariance/2010/01/12/from-eternity-to-here-book-club/

Thanks. I’m not sure what you mean with “effective” here. I of course agree that we’re not able to predict the future. The actual question is whether you think you can change it, or whether you think you just think you can change it. It boils down to whether or not you believe time-evolution to be entirely deterministic or not. By the chapter I’m currently reading you haven’t yet said “quantum,” so I’m curious. Best,

B.

I don’t believe in a strong Kantian type of free will, in which agents are laws unto themselves over and above the laws of nature. But I do believe in an effective kind of free will that simply results from our inability to accurately predict the future on the basis of incomplete information. That’s what becomes problematic in the presence of time travel.

Anu– As Bee says, there is a well-known fact in special relativity, that if you can (reliably and controlably) travel faster than the speed of light, then you can travel backwards in time. You could imagine more complicated theories in which there was still an upper speed limit, which just wasn’t equal to the speed of light, and which prevented time travel.

My idea was more around observation…assuming that not me or someone else but may be some bots/machines/ willing people overtake the speed of light to observe it from a point ahead on its straight/curved path. I don’t know what happens if we try to signal back…but if one observes without interfering, wouldn’t one (the bot/ machine) end up ‘seeing’ some fragment of the past….and then once it has recorded it, bring it back for us to observe and analyse?

It may not be fun if it is just one or two years kind of distance…but if it is possible for one year, then it can be for 50 yrs or 100 yrs …can it then record what happened in 1910 and bring it back for us to observe….though the people who observe it may not be us, but those of a few generations later.

All this assumes that we are just observing. The signalling/interfering may happen if you reflect back at the same speed as you travel (that is faster than light). So the underlying assumption is that one travels a few times faster than light. Actually, even a second faster would make it theoretically possible.

On a lighter note, I really won’t mind a ‘Hi” from a future-me to the past-me

This is a really interesting discussion. I stumbled upon this discussion today and am totally totally enthralled- the time mystery and the questions around time and space have always been questions for me, unanswered – and they beauty lies in them being mysteries i guess. In hope of some answer somewhere, can’t stop to wonder at the inexplicability of our presence in this space, and hence the related question of time – the dimension which we can notice . Have ordered my copy of your book just right now. Hope to get it soon – keenly looking forward.

Meanwhile, there is a pet idea that I hold regarding time travel. Its less on interference, and more on observation. Rather amateur (if you may). The idea being – If and when it is possible to travel faster than light (say thrice the speed of light) – then why don’t we/ our machines go on some other planet with that speed – and then camp there and observe earth – the photons/view that they observe will represent the past. They can thus see what happened in the past (without interfering with it) – the farther they go and the faster they go, the more of the past (in terms of the common understanding of time’s direction) they can observe. Derived from the fact that when we look at the sun, we look at sun as it was 8 mins back. When we look at something which is say 3 light years away – we reach there in 1 and then try to see what earth looked like…then it would be three years back, for us 2 years back.

Well, this again assumed a lot of things – levels of entropy being one, and then that its sort of a straight line…i hope i am not intruding into this really technical discussion with this amateur idea, but I really look forward to see some dash of clarity on this huge grey misty inexplicable horizon of universe that we have. Happy even if the idea gets nullified.

On another note, thanks for this forum…really glad to hear so many people talk about the questions that many others in this world don’t even stop to ponder about.

Can you have CTC’s then between regions of spacetime that have the same entropy? Could you argue the other way round that them having CTC’s implies they have the same entropy?

I totally disagree with Joe, the part about free will entirely belongs there. If you think readers will ask a question while reading what you wrote, better address that question. Btw, do you believe we really have a free will? Best,

B.

Joe– You might be right. Kip Thorne had similar concerns, so if you both have that reaction I must be doing something wrong. But I suspect it’s a communication failure on my part, rather than a problem with the topic. I did talk about the problem of determining what would happen in the future if there were closed timelike curves, which reduces to the issue with Cauchy surfaces, but that’s definitely not the major point I’m trying to get across. Which is: we are bugged at a visceral level by time travel, and the reason why is because we lose the clean division between “past” (already happened, set in stone) and “future” (yet to occur, and therefore something we can still hope to influence through our choices). And the reason why we ordinarily can rely on that distinction, of course, is because of entropy and the arrow of time.

So I think it’s a useful illustration of one of the main themes of the book. (Also, if you’re going to write a book about time, people are going to have questions about time travel.) But I’m happy to admit that it might have been better placed somewhere else in the book, or that I let the discussion of determinism cloud up the main thrust of the chapter.

In any event, Chapter Seven is where the important stuff really starts.

But isn’t the relation between entropy and the arrow of time more complicated when we’re talking about a non-isolated system? A macroscopic object following a closed timelike curve might have its entropy lowered by an interaction with the external environment on some segment of its worldline (so that its entropy could self-consistently be increasing over the rest of its worldline), that doesn’t seem equivalent to saying the arrow of time was actually going backwards in a small region of spacetime containing that segment (or a portion of that segment).

I started your book recently and am enjoying it. You present some standard material in a fresh way, as well as topics not so often covered. However, I think that chapter 6 is out of place. It distracts from the otherwise tightly reasoned argument: the reader does not need to be confronted with hypothetical questions about free will, which depend in turn on limited results about mathematical physics without Cauchy surfaces (in contrast to most everything else in the early chapters, which is well-vetted in the scientific literature).

Thank you