Before what?

I'm guessing your answer is, "Before the time traveller set out on his journey." If so, my argument is that "before" is meaningless in this context.

I think the problem is, we've been conditioned to think of time as moving, or people moving along time. This is of course an illusion, brought about by the fact that each "frame" of time (to borrow the film analogy) is a development of each previous frame.

Instead of saying, "I was born in 1963 etc," it's better to say, "At 1963 the infant me is born; at 1967 the child me has an accident on a swing; at 1993 the 30 year old me is getting married, and so on.

In keeping with this timeline approach, we can say, at 1800 the 30 year old time traveller arrives; at 1975 the time traveller is born, at 2005 the 30 year old time traveller sets off for 1800. As I see it, there's no point on this timeline that can meaningfully be called "before".

Without freewill? I don't buy that. "Constrained" inasmuch as his/her actions will be consistent with what is known to have happened, but that still leaves a lot of scope.

It is of course easy to envisage scenarios where one's presence or actions are bound to disrupt causality. But, as I've said, for the situation to come about, a long chain of by-no-means-guaranteed events must be successfully completed. And we know that the chain will break somewhere because we know the actual outcome.

If time travel turned out to be that easy, we'd probably discover that the Man U team had their own time machine and are using it to preserve the status quo.

Are you talking about time dilation here?

I take it you're not talking about "moving into the future at a rate of one second per second" because that's clearly not time travel. It's as meaningless as saying St Paul's Cathedral is moving through London at a rate of one metre per metre.

If these theories are correct, then obviously all this musing is academic. My worry, though, is that scientists are looking for ways of ruling out time travel simply because they think time travel will give rise to these paradoxes - in which case, they are in the position of pre-Keplerian astronomers who felt compelled to fudge orbital data because they thought planetary orbits had to be circular.

I think he is talking about moving into the future, since he mentions it is obligatory. I wouldn't say it is so clear that it is not time travel.

How would you measure time travel then? Against what standard?

Part of my earlier argument was that time doesn't move, nor do we move along it. My newborn self is at 1963, my 20 year old self is at 1983, and so on. No travel is involved - any more than St Paul's is travelling through London. (This is not something I came up with - I first encountered it in a book by Paul Davies in the early 1980s, and I don't think it was original to him.)

Personal time against world time. If you're travelling at seven tenths of the speed of light, you could say that for every second you experience, the folks back home are experiencing two seconds.

But if those folks strapped a huge rocket to the earth and set off after you until you were both at rest relative to each other, then would that mean that you were no longer travelling through time, and that your ability to travel through time depends on what other people are doing?

Yes; that is one way of measuring time travel in the forward direction. If you want to visit 2 Million AD, you can set off in a Nearly-As-Fast-As-Light ship to Andromeda, turn round before you get there and come back; or you could buzz around near the event horizon of a supermassive black hole (which would also cause time dilation), or you could wait on Earth (perhaps in frozen sleep); in any event, you will get to 2 million AD(but you won't get back).

Yep; that's relativity...


As far as Man U going back in time as well, is concerned- yes, you see that if you have unlimited time travel, causality goes right out the window. The world would be shoulder to shoulder time tourists.

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Yes. Of course, both you and the folks back home (where "back home" is much closer, and much less stationary, than you probably thought when you first set out) are travelling in time relative to the inhabitants of some other world that hasn't had a blimmin' great rocket strapped to it.

And if you strapped rockets to every single object in the universe then you'd be synchronised with the universe once more, assuming they're all moving at the same velocity.

Causality doesn't go out the window at all, it just leaves us in the same situation I described early on - the time travellers are still as much a part of the era they're visiting as anyone else living at that time; they would still need to know all the events that contributed to an outcome in order to change that outcome; and the results of each time traveller's action would still be subject to a myriad of external influences - more than ever, in fact, as they would include those of other time travellers.

It's analagous to those polygon things where the number of faces minus the number of edges plus the number of vertices equals some constant. At first sight, you think you only have to draw a random line between two other lines to upset the equation - only to find that no matter how many random lines you throw in, the equation still stands.

Granted, it seems somewhat unlikely that there are time tourists everywhere, which suggests time travel will never be unlimited. But that doesn't mean it's not possible at all.

This would hold if the time travellers wanted some specific outcome, but what about a time traveller who just wants to mess things up? He could set off a bomb at some point in history, like the Constitutional Convention, killing dozens of people important to history.

Once we allow time travel, it's hard to see how people could go back in time but somehow be prevented from making a change, no matter what they tried to do or how they tried to do it.

That's one way. Poul Anderson used that in his novel "The Corridors of Time." Two waring factions in the future, both with time travel. A character suggests trying to prevent an enemy attack before the fact, and the reply is "A superior enemy force would show up to stop us."

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That's a specific outcome. We know a bomb did not go off during the Constitutional Convention (I assume - I don't know much US history!), therefore an attempt to set off a bomb at that point would fail.

The chain of attempts still applies. You have to obtain the bomb, you have to ensure it doesn't go off prematurely, you have to gain access to a time machine, you have to arrive at the right location, you have to have the opportunity to light the fuse, you have to avoid time travellers who hunt bomb-carrying time travellers for sport, and so on. In other words, a whole bunch of circumstances. You cannot know all the circumstances, but you can know the outcome: the bombing attempt failed.

As I see it, it's not so much that time travellers couldn't make a change; rather, they didn't.

I believe I understand what you mean by didn't.

A time traveller could go back and look through the window while the Constitution was being signed, but if he tried to toss in a hand grenade, something would happen to stop him, because something did happen to stop him.

The situation could be simplified. For example, in In L. Sprague deCamp's story "A Gun for Dinosaur", a man tries to go back in time to shoot someone who didn't get shot. As soon as he tries to do anything that didn't already happen, he gets thrown back to the present. His gun doesn't jam, a dinosaur doesn't eat him.

In another story, deCamp explains why the same time machine can't bring back information from the future. The machine can go to the future, but if it tries to come back with information that would create a difference, the same force prevents the machine from returning.

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To allow this kind of time travel which does not change history it is neccessary to place severe limits on the freedom of action of the travellers; otherwise every new wave of time travellers would change history to suit themselves.

Time tourism is only one of the problems you would encounter if time travel was possible and easy; worse would be the time refugees, immortal emigrants from the end of time (crunch, rip or heat death, whatever).

They come back to our era (and before) and live their lives into the deep future until the physical conditions of the universe become too uncomfortable and then they come back again. If time travel does not have severe restrictions on its utility the universe instantly fills up with immortal time travellers who already know every detail of history. In fact this sort of mass emigration actually increases the mass of the universe and changes its physical characteristics.

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there arent waves of time travelers. they allready were there the first time. there is only 1 version of history, not repeats with different characters and diferent events.

the absense of time travelers today would be proof that one can not travel back in time.

I knew what you were talking about. But look at your example. Your newborn self is at 1963 in a hospital, your 20 year old self is at a pub. They don't move either. They're stationary.

You can't then say that there was no travel involved in getting from the hospital to the pub. Similarly for the time dimension.
Sure, but that's all relative. If you take two folks back home, then isn't one of them experiencing one second for every one second--just what you thought eburacum45 meant, but said could not be true?

[quote="A Thousand Pardons"]I knew what you were talking about. But look at your example. Your newborn self is at 1963 in a hospital, your 20 year old self is at a pub. They don't move either. They're stationary.

You can't then say that there was no travel involved in getting from the hospital to the pub. Similarly for the time dimension.
[quote]

There is travel, but not through time. It took me twenty years to travel from a maternity ward in Nottingham to a pub in Portsmouth. (Needless to say this was a very indirect route.)

Yes. Time travel (in the context of time dilation when moving at near-light velocities, for instance) is generally expressed in terms relative to a clock or person who has not been accelerated to near-light velocities.

I didn't say it could not be true, I said it did not count as travel. It sometimes feels as if it's travel, sure - especially when our youthful years seem ever further away. But if you think something is travelling, you should be able to say what it's travelling relative to, and at what rate. If we divide out the units of the expression "one second per second" we get 1s/s = 1. I don't know what meaning you'd give that, but to me it sounds like not moving at all.

The trouble with this scenario is, we'd have to identify a mechanism that gets time travellers "spat out" in this way. There might be such a mechanism, but it sounds a bit artificial to me.

(It also reminds me of the Morphail effect from Michael Moorcock's Dancers at the End of Time series.)

That's almost indistinguishable from saying time travel into the past is impossible!

But all these effects are necessary, if we are to suppose a form of time travel which does not change history.
For example, if you cant change the score of the Chelsea v Manchester match, you simply send a bomb. Or, more peacefully, but just as effectively, perhaps,
a sofa...
(HHGTTG)
To prevent simple random acts of surrealism we must imagine some kind of cosmic censorship principle with respect to time travel, one which allows acts of no consequence but which prevents sofas.

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So, 1 second per 2 seconds sounds like time travel to you, but 1 second per 1 second does not? I don't see much difference.

I've already addressed the sending-back-a-bomb scenario.

It's a ratio.

One means your self at 2015 is only five years older than your self at 2005, whereas everybody else is ten years older.

The other means your self at 2015 is as much older as your self at 2005 as everybody else.

I can see how you could argue that that's not actually "travel", it's merely a "stretching" of your timeline. But the upshot is that you can hope to see future years that you'd otherwise not expect to live to see. Which is what most people mean by time travel.

The "absense" of timetravelers today could be explained by them not wanting to reveal themselves. If you said you were a timetraveler you'd be commited unless you could prove it. Much easier to not say anything.
If you were a spy in Russia back in the Cold War would you go walking around saying you were a spy? They could even have cloak suits where we can't see them. Or timetravelers could be small robots that look like houseflies, gnats, ticks, lizards etc. If they are here we don't have to know about it. Like aliens. This is one of the reasons I think alien abduction stories are so stupid, in addition to being a skeptic that demands proof. If aliens have the technology to get here from hundreds, thousands, millions or billions of lightyears away they would have tech so advanced that we couldn't detect them. We wouldn't know they are here. They would also wipe memories if need be, and their tools would be much more advanced than dentist picks. :roll: Active optical camo is already being worked on, stealth for RADAR isn't hard etc. What kind of tech will we have in millions of years, which is the amount of time aliens would have on us.

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I can easily see such a mechanism as being a basic part of whatever principles govern travelling in time. And it's simpler than identifying a mechanism that organizes events so as to prevent a time traveller from changing anything.

For example, I can see something like "You must understand that while in the past, you will be in an unstable state, like a needle balanced on its tip. If you try to change anything significant, the needle falls over."

Rather than "the time equations clearly show that if you try to shoot John Wilkes Booth, every person in the area will suddenly trip and fall on you."

No, it means that you can't travel to a part of the past where it is too easy to introduce a significant change. You can go back millions of years and leave footprints and dead dinosaurs, but you couldn't go back to 1963 and throw Lee Harvey Oswald out the window.

Someone going to 2100 and simply looking out a window is liable to see something of importance. "Hey! Where is everybody?"

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it's what we do know that ain't so.

Time travel works if you eliminate the concept of free will. If our quantum particles are the balls in an interdimensional billiards game, then there are no paradoxes. Someone goes back in time and fails to kill JWB because he went back in time and failed to kill JWB. It had to happen because it happened. No more, no less.

Determinism writ very large. :P

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We've done the free will thing.

But time would somehow have to "know" that some acts would change history (and so it has to return the traveller to a place where he can do no harm) whereas other acts wouldn't.

I'm not suggesting there's a "mechanism that organises events". I'm saying that past outcomes are the result of a myriad of circumstances leading up to that outcome - including, in some cases, the contribution of a time traveller.

If you've been physically delivered into the past, you're there. Why should your presence be unstable?

Put another way, what if someone from 1864 travels a year into the future, learns of JWB's assassination of Prez Lincoln, then returns to the present. What if he tries to kill JWB? Where will he be spat to?

It's more a case of, "Don't try to shoot JWB prior to the assassination. We know that the assassination succeeded, therefore we know that if there were any attempts to avert it, they obviously failed. That includes any attempt you might have made."

Again, time would have to know which is which.

What you see is what you get - that's the essence of time travel! Returning to the present, you can say with some authority that at some point in 2100 there will be no people visible from a certain window.

The cosmic censorship principle that you seem to be proposing doesn't sound like it would work to me; it appears to be -
you can only travel back in time if you don't change anything.

This cannot be managed on a voluntary basis- the only way to allow inconsequential time travel is for you to go back to perform a set of actions that already exists.
This sort of time travel would only allow a vanishingly small number of time travel events.

Otherwise very time you go back you change an almost infinite number of tiny things- you absorb certain oxygen molecules and certain photons, simply by going back in time you change the mass of the universe; and the effects of chaos theory mean that the universe is changed forever just by the fact of you being there.

if time travel is possible then every person from the future, billions of years of humans, robots and evolved chimpanzees will want to travel back in time to visit certain events; the assassination of Lincoln for instance...
they will all want to go back not once, but as often as they can afford the fare as well; Instantly every historical event has quadrillions of people attempting to observe the action.


The only way to have time travel which does not change the past is to avoid all time travel at all. It is impossible to have such information paradoxes; if relativity is correct then this rules out faster than light travel as well.

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This is starting to go round in circles. I feel too ill to discuss it much longer.

On the contrary there would be only a few "forbidden" events.

You are speaking as if your arrival in the past is an addition to established history. As if there was an 1800 (say) in which you did not make an appearance; in which you did not absorb those photons or those oxygen molecules. Yes, you'd affect the world around you, but those effects are as much a part of history as the effects of anyone else around in 1800.

I accept, though, that time travel into the past means the mass of the universe will fluctuate.

If travel to absolutely any point in space and time was readily available, then it's difficult to imagine how this problem (christened the Golgotha effect by SF writer Garry Kilworth) can be avoided. Leaving aside the fact that there are only so many seats in Ford's Theatre, of course...

But from what I can gather from the physics involved in time travel, it's always going to be difficult to do. There might even be a natural limit to the number of possible time journeys in a given region.

And it might even be that time travel is completely impossible.

But the point of my argument is, if time travel into the past is, in some circumstances, possible, don't dismiss it just because of the supposed paradoxes and arguments about free will. Most of these can be addressed quite readily; others might be addressable in due course.

They're both ratios. Of the same quantities.
Right.
Actually, I'm not aguing that--your argument is that one is not time travel, the other one is. That seems inconsistent to me.
I've never heard that before.

What about seeing future years that I had not been certain of living to see? Why isn't that "time travel"?

Okay, imagine you bought a plant. You measure it one month and it's two metres high. You measure it the following month and its height is still two metres. The growth over that period is 2m/2m, which is to say 1. In other words, no growth has taken place.

OTOH, if when you next measure the plant, the height is 4m, then the growth is 4m/2m, which is to say it has doubled in height. Growth has occurred.

Referring to my own timeline again, my newborn self is at 1963, and my 20 year old self is at 1983. There's no time travel involved. However, if my 20 year old self was located anywhere other than 1983, that would be time travel.

I think I'll let you sort that one out for yourself. I'm too ill to debate any further.

What is it you are using in your ratio, in the timeline examples?

If you are using the other people, then that would be akin to using other plants for comparison. Since the other plants would have grown too, the ratio in the second example would be 4m/4m--no growth. So, that's inconsistent, again.