Re: TimeTravel

Dan Fabulich (
Thu, 22 Oct 1998 12:14:02 -0400

Michael Wiik wrote:
>Randall Randall writes:
>> OK, I can follow you so far: if you return to the past of
>> yourself, you were always part of that past. However, if
>> we introduce the idea of "Many Worlds", another possibility
>> arises: that we may be able to return to a past exactly
>> similar to our own, but with the addition of ourselves (the
>> time travelers). The only difficulty I can see is that
>> there appears to be no way to get back to the "present" you
>> left.
>Since it can't be tested, how would this be different from a
disintergration device?

Actually, it CAN be tested, interestingly enough, using reversible nano-electronics.

On a relevant note, tests have been done to see if one can travel to other histories, (ie testing for exact linearity in the waveform,) and it seems that we can't. Take a look at Q36-Q39 in the excellent "Many Worlds FAQ" at

Interestingly, the author of that FAQ makes more or less the same argument that I do to show that the waveform is linear:

Q39 Is linearity exact?

Linearity (of the wavefunction) has been verified to hold true to better than 1 part in 10^27 [W]. If slight non-linear effects were ever discovered then the possibility of communication with, or travel to, the other worlds would be opened up. The existence of parallel Everettworlds can be used to argue that physics must be *exactly* linear, that non-linear effects will never be detected. (See "Is physics linear" for more about linearity.)

The argument for exactness uses a version of the weak anthropic principle and proceeds thus: the exploitation of slight non-linear quantum effects could permit communication with and travel to the other Everett-worlds. A sufficiently advanced "early" civilisation [F] might colonise uninhabited other worlds, presumably in an exponentially spreading fashion. Since the course of evolution is dictated by random quantum events (mutations, genetic recombination) and environmental effects (asteroidal induced mass extinctions, etc) it seems inevitable that in a minority, although still a great many, of these parallel worlds life on Earth has already evolved sapient-level intelligence and developed an advanced technology millions or even billions of years ago. Such early arrivals, under the usual Darwinian pressure to expand, would spread across the parallel time tracks, if they had the ability, displacing their less-evolved quantum neighbours.

The fossil record indicates that evolution, in our ancestral lineage, has proceeded at varying rates at different times. Periods of rapid development in complexity (eg the Cambrian explosion of 530 millions years ago or the quadrupling of brain size during the recent Ice Ages) are interspersed with long periods of much slower development. This indicates that we are not in the fast lane of evolution, where all the lucky breaks turned out just right for the early development of intelligence and technology. Ergo none of the more advanced civilisations that exist in other worlds have ever been able to cross from one quantum world to another and interrupt our long, slow biological evolution.

The simplest explanation is that physics is sufficiently linear to prevent travel between Everett worlds. If technology is only bounded by physical law (the Feinberg principle [F]) then linearity would have to be exact.

[F] Gerald Feinberg. _Physics and Life Prolongation_ Physics Today Vol

     19 #11 45 (1966). "A good approximation for such [technological]
     predictions is to assume that everything will be accomplished that
     does not violate known fundamental laws of science as well as  many
     things that do violate these laws."

[W]  Steven Weinberg _Testing Quantum Mechanics_ Annals of Physics Vol
     194 #2 336-386 (1989) and _Dreams of a Final Theory_ (1992)

Anyway, a lot of people have criticized my argument re: time travel, so I'll summarize here.

Michael Lorrey said:
>Yet you do not know if time travel will be acheived in the future or not.

You're right, I don't. That's why I said "almost certainly not," rather than "I'm absolutely certain that it will never happen." My argument was simply that if someone develops time travel in the future, then someone could return to our past or our present. Considering how many people there are likely to be in the future, I think that someone (in fact, a great many someones) almost certainly would visit some point in our past. Since it appears that no one from the future has appeared in our past, this indicates a strong likelihood that it can't be done.

I could very well be wrong, however. There are a number of circumstances under which my argument might break down:

  1. No one who discovers time travel will ever want to travel into our past.

Michael Scarazzo said:
>This reasoning is flawed for a couple of reasons. You are saying that
>because we do not see something, it is not there. I think that you
>are also assuming that those beings in the future who do develop full
>time travel will want to journey to their own past to share the
>knowledge that time travel will be possible in the future. As beings
>of finite existence, I believe that time has meaning for us, but
>perhaps there is no such dimension or measurement at all for those who
>have infinite existence, as transhumans or post-humans will have.

While this isn't strictly impossible, I find it profoundly unlikely. For this to hold, whenever *anyone* discovers time travel, no matter how many people do so, they must all uniformly decide not to travel any further back than today. While I might believe that one or two could show such restraint, it seems inconceivable that all intelligent life for the remainer of the lifespan of the universe will make the same choice.

2) My other assumption is wrong: someone from our future has appeared in our past, but for some reason or other very few people knows anything about it.

Michael Scarazzo said:
>This is assuming much about a technological/biological future that is
>very difficult to predict and will become virtually impossible to
>predict as the singularity approaches and passes. Isn't that saying
>something along the lines of since we've never seen something it must
>not be. There is no other life in the universe because they have
>never bothered to come tell us that they are there.

This also seems unlikely. After all, if one person can travel into our past, then two can. Considering that our population looks like it's going to keep growing for a long time, what would prevent LOTS of people from travelling into our past? Or rather, once they did, how could they possibly keep it a secret from most of the world? And why would they? Why not tell the world?

Again, it isn't strictly impossible that no one will ever decide to come back in time and tell the world, but it's awfully unlikely.

3) For some reason, time travel is limited in some way: perhaps it can't be used to travel further back than the date of the invention, for example.

Again, this is another point which I can't honestly claim cannot be true. With my current understanding of physics, I can't currently imagine what would prevent time travel beyond a certain date, but then, I'm not out to invent a time machine. :)

However, I will make the argument that even this is unlikely. First, note that if I had an engine which could move me around faster-than-light (FTL) then I could travel back to any arbitrary point in time. This point has been pretty well hashed out; I think the FTL FAQ out there on the net discusses it in a way that makes sense to those who don't have much physics. Alternately, consider the scenario Haradon presents:

Zeb Haradon said:
>In "Hyperspace" by Michio Kaku, a scenerio is sketched out which seems like
>it would work. You make a worm hole - he descibred how to do this, it
>involves something with charging two sets of plates and relies on something
>called the Casmir Effect, which has been observed in the lab on a small
>scale, but he didn't go into much more detail about it - anyway, you make a
>worm hole using the Casmir Effect, and you have the two ends of a wormhole
>right next to eachother. You then put one on a ship, and accelerate it to
>near-light speed, and fly it around for a while, then after 20 years or so,
>return it to its original position. While this one end of a worm hole was
>going very fast, time was compressed for it - say it was going fast enough
>that only 10 seconds passed for it while 20 years passed for the other end
>of the worm hole which did not travel. Now, you have a wormhole which has
>one end at point A in space, and its other end only a mile or so away, but
>one end is twenty years ahead of the other one. You pass through one end and
>come out the other end twenty years into the future. Pass the other way, and
>go twenty years into the past.

FTL movement => arbitrary backwards time travel.

Now suppose, for example, that your flavor of time machine uses special limiting "gateways." You build the gateway today, wait a week, walk into the gateway, and you've just travelled one week back in time.

It would appear that the inventor's ability to time travel is limited: If I build the gateway tomorrow, I can't travel back to today, though I can travel back to tomorrow from any arbitrary point in the future.

Unfortunately for this argument, I can couple this simple device with an engine that moves me around at relativistic speeds (subluminally) and get a faster-than-light engine. Indeed, any trip can be made in an arbitrarily short amount of time simply by travelling back in time during the trip. So, for example, if I wanted to get somewhere a light-week away, I could set up my gateway, fly there in a week and a day, then gate back to a week ago. I find myself having arrived at my destination only one day after departure. A photon I might have emitted from my origin would arrive six days later. Since FTL travel allows arbitrary time travel, a limitation of this kind is no real limitation at all.

In short, for this argument to work, there would have to be some bigger physical principle about time travel which we have not yet discovered which imposes serious limits on the dates to which one may travel.

Note that strong determinism won't do it. We'd still get time travellers who may even have a profound effect on the age to which they travel; all that determinism would signify is that there would be no chance that they would do anything else.

Again, while I can't claim to be certain about this, I would definitely put money against the possibility that we will be able to move an entire person backwards in time, even under strong limitations.

Brian D Williams said:
>This assumes we live in the past, not the present. If we are (as I
>believe) living in the present, the future hasn't happened yet.
>But it was fun to reason out. "Is it time for the tea party yet?
>I'll get my hat... Where's Alice?"

I'm well aware that the future hasn't happened yet. However, if backwards time travel is in the future, then by definition you can use it to reach the future's past. Go far enough back and that's the present. Even further back and you've got the past.

Anyway, if you happen to be a determinist (as I am), then this argument isn't even relevant. The future hasn't happened yet, but it will, and if it's got time travel, then it will very likely alter its past.


"Decay is inherent in all compounded things. Strive unceasingly."