Time travel

[Mr Q]

Originally posted by Stevo
But why? Is it impossible in a physical sense (is it impossible due to the contingent nature of our universe), or is it impossible in a purely logical, conceptual sense.

Hey, I'm only trying to outline possibilities. Not why particular possibilities might be true or otherwise! I would postulate that in this case time travel is non-directional - ie it just "is" and that there is only one potential way to move in time - the stock standard way we all know.

Originally posted by Stevo
It would seem that in this case [that you can travel in time but not change anything that would upset the timeline] that in every case where causality works backwards, it is necessarily involved in a loop. Not sure though...

Actually it doesn't quite require a loop, it can easily be explained by the concept that everything is predetermined. If so, it was required that you travel in time because you already had. It also says that if you had a cheese sandwich for lunch you were always going to have a cheese sandwich for lunch. However since we are unaware of this predetermination we are able to believe in our own free will.

Or of course there could just be a lot of loops...

Originally posted by Stevo
These two are the really tricky ones. I hinted above at the problem with (4). Suppose I go back in time and delete the tapes of Rocca striking Lade. Rocca is cleared, plays in the GF, and Collingwood win. Now, this is different to what has happened. So, (4) proposes that a new alternate universe is created to accomodate for the new chain of causality. However, for the original universe to continue, its own chain of causality must be unchanged, and this would require that I did not delete the tapes, that Rocca was suspended, and Collingwood lost the GF.

How do I explain this concept? I guess what I'm saying is that you come from universe "A" where Rocca was suspended. You then go back in time and erase those tapes (not that that would have won the GF for Collingwood ), and Rocca plays. You hve thus created universe A^. A^ differs from A in that at one particular point the tapes of Rocca being a dirty bastard are erased. However in your personal timeline, you come from A + X where X is the time difference from the point of divergence to where you travelled back in time. What would be interesting to know would be whether if you return to your normal time whether you would reappear at A+X or A^+X.

Originally posted by Stevo
Case (3) results in a loop. If I can shoot my grandfather, then I cease to exist which means in the future I don't exist to build a time machine to go back in time to shoot my grandfather.

Case (3) is the most problematic. Now assuming that you travel back in time to point A-X, and then kill your grandfather at point A, creating alternate timeline A^ as per above. In A^ you cannot come to be, so you are erased from future history if you like. However at a point prior to A, you do exist in both the A and A^ timelines as a visitor from the future. So perhaps for the period of time between A-X where you reappear in the past and A you exist as the potential for you to exist still well exists. So the version of you prior to time point A is a potential you should path A be followed, not A^. So what exists in that timeframe is a potential, although it seems and acts real. I guess what I'm saying in this case is that up until the point at which you kill your grandfather you are real (albeit in a future sense), so you would not cease to exist (potentially) until that point, so a future you travelling to the past could exist right up until the point you killed your grandfather.

Does any of that make any sense at all?

 

[MightyFighting]

Enough with the grandfather killing!!!

At its most basic level the Universe is literally random. i.e. not just random as in we cannot predict what will happen, but random as in events occur without a cause.

Consider this:
There was a point in time before your father was conceived. Then the Universe's metaphorical dice were rolled many times, resulting in the birth of you father and of you and everything else that has happened between that point in time and now.
Now imagine that you go back in time, to that point, and effect the world in some tiny way. Shurely the metaphorical dice must be rolled again and there is only a negligible chance that they will come up with the same metaphorical numbers. i.e. you will not exist and never will have existed.

The question remains: 'Will you remain in the past, or will you cease to be there also?' To me the idea that you would disappear is absurd.
Imagine a particle floats into a time machine, goes a splitsecond back in time and bumps into its former self, causing its former self to not enter the time machine. Does it then disappear from that reality? Why would it? It does not know where it has come from and neither does the Universe. i.e. if a particle (or person) can be transported backwards through time, then they can remain there regardless of what they do.

 

[Stevo]

Originally posted by ScouseCat
Like what happened in Back To The Future, if you went back in time and altered what originally happened, that would then be reality and the present we now know will cease to exist. Instead, an alternate present would exist following on from the past events which have now happened, (including what had been altered) and the only way the present could be restored, is if you went back to that same point in the past and made sure certain events happened as per what originally occured, bearing in mind you would be the only person who knows how certain events will unfold.

So what happens to the original present? Is it destroyed? Or does branching occur?

Both result in absurdities, I think.

Firstly, if the present is destroyed: Then the time from which I travelled from no longer exists. So in travelling back in time I make it impossible to travel back in time.

Secondly, if branching occurs: Suppose event X originally occurred in the past, in the original universe A. I go back in time, in universe A, and alter event X, so that it now becomes event Y. The branching theory says that with this alteration of event X to event Y, universe B is created, and branches off from universe A. However, if universe A is still to exist, then, necessarily, event X must occur. And if universe B is to exist, then, necessarily, event Y must occur. So, simultaneously, two incompatible events, X and Y, occur. I have changed the event, yet it also exists in its original unchanged form.

A solution: Time is already branched before the event is changed, whether in anticipation, or because it is the nature of time simpliciter. The basic point is that the branching must occur before the event is changed, since this allows us to posit the existence of two incompatible events. If the branching occurs as a result of the changed event, then we have already had two incompatible events occur.

 

[steve_rat]

sorry if this has been mentioned coz i can't be bothered reading through all those posts on time travel but i reckon we will travel in the future but not back coz you just go faster than the speed of light or something like that or the opposite and you can make time go fast.. but does that mean our bodies will decay at that speed..

sorry i have no scientific knowledge, just trying to remember what people have told me

steve rat

 

[Stevo]

Originally posted by daddy_4_eyes
This idea about something 'stopping' me from killing my grandfather is overly simplistic, which suggests that either time travel cannot be achieved, or it is not linear (one of Cyclops theories).

If moving back in time is in fact moving to another dimension, then technically, we're not moving back in time. Therefore past events cannot be changed, and we can conclude that we "live in the moment".

This is all crap that came out of my (relatively) uneducated head. I still believe time travel does not exist. What has happened, has happened. What will happen, hasn't been written yet.

Firstly, in your first paragraph, there's the case that time is one-dimensional, and that time travel occurs. While loops can not be explicable from the outside. - Take the time traveller who visits himself forty years earlier and tells him how to build a time machine, and then forty years later when the time machine is built the time traveller goes back in time to visit himself - they are not inherently contradictory. They are merely strange. It is like circular reasoning. It is regarded as fallacious because it is self-referential, because it only appeals to itself for truth. That doesn't mean that one can not arrive at a correct conclusion by circular reasoning, merely that the premises don't support the conclusion (since the premises are, essentially, the conclusion).

I agree that we can not change the past, since I think past events can be taken as atemporal, just like numbers, sets, classes, functions, etc. However, this does not mean time travel is impossible. Since, if I go back in time, then my appearing in some past time will necessarily cause me, some time in the future, to hop into my time machine and travel into the past. Again, this results in a loop, inexplicable from the outside, but not inherently contradictory.

 

[Stevo]

Originally posted by steve_rat
sorry if this has been mentioned coz i can't be bothered reading through all those posts on time travel but i reckon we will travel in the future but not back coz you just go faster than the speed of light or something like that or the opposite and you can make time go fast.. but does that mean our bodies will decay at that speed..

sorry i have no scientific knowledge, just trying to remember what people have told me

steve rat

The issue of whether mass and information can travel faster than the speed of light is unsolved. But it is generally regarded that the speed of light is the limiting speed of the universe. Nothing can go faster than light. In fact, anything with mass can not travel as fast as light.

In any case, travelling close to the speed of light results in time dilation, but this is distinct from time travel in its philosophical sense. This is a consequence of special relativity.

Basically, if an astronaut travels through space close to the speed of light, when the astronaut returns home his family will have aged considerably more than the astronaut has.

 

[Stevo]

Originally posted by Rodion
Suppose time travel were possible. Then people would travel back in time and their mere presence would cause changes even if they tried very hard not to do much of anything. Their gravity and respiration would cause very minute alterations. So we have a new future which is very, very slightly different to the original. People would travel back in time and cause more changes. And over more and more repetitions, then changes will add up until the world is left in a state in which time travel is never invented. Then all the changes stop.

Conclusion: Time travel will never happen.

Why do the changes result in a world in which time travel is never invented?

In fact, that seems inconsistent. If time travel is occurring, then time travel has been invented. That time travellers are appearing in the past necessarily causes the time travellers in the future to hop into their time machine and travel to the past.

 

[Stevo]

Originally posted by Thrawn
Not true. There are probably laws of physics we haven't even discovered yet, let alone have the capacity to understand them. As our minds evolve and as science discovers new things, we are bound to maniulpate time in, say, 1000 years time where we will probably have the technology and the knowledge of new physical laws to make time travel possible. Even today, scientists are experimenting with time travel, and are on the verge of transporting sub-atomic particles (and they've also done experiments on time dilation - which were conclusive and proved that such dialation exists). We only need the necessary knowledge of quantum physics to go further - we are only starting this area of knowledge. Think back two hundred years ago... people back then would laugh at the idea of going to the moon and thought it would be impossible. Go back even further, and you would've been considered a heretic. Imagine what we could do if we had all the knowledge of quantum physics, and discovered and applied new laws? In this case, why does time travel have to be impossible? If we have the necessary technology and knowledge, we can do it. Our minds cannot comprehend such things unless we discover how it works, and how to apply the knowledge to make it work (just like with most things in life you learn).

Exactly. GR allows for time travel, although GR and QM are irreconcilable. Perhaps when a theory is propounded that unites the two we will see whether time travel is physically possible.

However, the physical sense of possible is different from the philosophical sense of possible, and so whether time travel is possible in our universe has no bearing on whether time travel is possible logically.

 

[Stevo]

Originally posted by Mr Q
Hey, I'm only trying to outline possibilities. Not why particular possibilities might be true or otherwise! I would postulate that in this case time travel is non-directional - ie it just "is" and that there is only one potential way to move in time - the stock standard way we all know.

But again, do you mean this in a physical sense, that the only way we can move in time in this universe is the "stock standard" way? Or do you mean it in a philosophical/logical sense, that, conceptually, time necessarily has a certain structure which dictates that time travel is logically impossible?

Actually it doesn't quite require a loop, it can easily be explained by the concept that everything is predetermined. If so, it was required that you travel in time because you already had. It also says that if you had a cheese sandwich for lunch you were always going to have a cheese sandwich for lunch. However since we are unaware of this predetermination we are able to believe in our own free will.

Or of course there could just be a lot of loops...

Fatalism and causal loops are different. Fatalism basically rests upon the assumption that something in the future will happen, regardless of what happens prior to it. In other words, there is no question of causality. Causal loops are different - they still involve causal connections. The person hopping into their time machine isn't predetermined by them appearing in the past, it is caused by them appearing in the past.

How do I explain this concept? I guess what I'm saying is that you come from universe "A" where Rocca was suspended. You then go back in time and erase those tapes (not that that would have won the GF for Collingwood ), and Rocca plays. You hve thus created universe A^. A^ differs from A in that at one particular point the tapes of Rocca being a dirty bastard are erased. However in your personal timeline, you come from A + X where X is the time difference from the point of divergence to where you travelled back in time. What would be interesting to know would be whether if you return to your normal time whether you would reappear at A+X or A^+X.

But, again, branching does seem to involve an impossibility.

As I posted above: "Suppose event X originally occurred in the past, in the original universe A. I go back in time, in universe A, and alter event X, so that it now becomes event Y. The branching theory says that with this alteration of event X to event Y, universe B is created, and branches off from universe A. However, if universe A is still to exist, then, necessarily, event X must occur. And if universe B is to exist, then, necessarily, event Y must occur. So, simultaneously, two incompatible events, X and Y, occur. I have changed the event, yet it also exists in its original unchanged form.

A solution: Time is already branched before the event is changed, whether in anticipation, or because it is the nature of time simpliciter. The basic point is that the branching must occur before the event is changed, since this allows us to posit the existence of two incompatible events. If the branching occurs as a result of the changed event, then we have already had two incompatible events occur."

Case (3) is the most problematic. Now assuming that you travel back in time to point A-X, and then kill your grandfather at point A, creating alternate timeline A^ as per above. In A^ you cannot come to be, so you are erased from future history if you like. However at a point prior to A, you do exist in both the A and A^ timelines as a visitor from the future. So perhaps for the period of time between A-X where you reappear in the past and A you exist as the potential for you to exist still well exists. So the version of you prior to time point A is a potential you should path A be followed, not A^. So what exists in that timeframe is a potential, although it seems and acts real. I guess what I'm saying in this case is that up until the point at which you kill your grandfather you are real (albeit in a future sense), so you would not cease to exist (potentially) until that point, so a future you travelling to the past could exist right up until the point you killed your grandfather.

Does any of that make any sense at all?

The last is very problematic. What distinguishes merely potential "you" from real "you"? How many potential "yous" are there? One for each possible act? Does that mean there are infinite "yous".

Basically, the point is that the identity criteria for these potential "yous" is extremely problematic. If am merely a potential, how can I participate in causal interactions?

I don't know... None of that makes much sense to me at all, really.

 

[Au_Blue#24]

Originally posted by Stevo
The issue of whether mass and information can travel faster than the speed of light is unsolved. But it is generally regarded that the speed of light is the limiting speed of the universe. Nothing can go faster than light. In fact, anything with mass can not travel as fast as light.

In any case, travelling close to the speed of light results in time dilation, but this is distinct from time travel in its philosophical sense. This is a consequence of special relativity.

Basically, if an astronaut travels through space close to the speed of light, when the astronaut returns home his family will have aged considerably more than the astronaut has.

Stevo - you obviously have an excelent handle on this subject. Above you say that anything with mass cannot travel as fast as light. On the surface this seems true. Im reaching back into my uni days where i did physics some time ago so i may be inaccurate, but doesnt the duallity of light principle state that light is made up of both em waves plus particle (photons) - hence it has mass. The bending of light around black holes which have enormous gravitation forces also proove this ??

 

[dr nick]

Originally posted by Cyclops
If you posit a Universe is a linear string of moments motoring along on causality (a happens & makes b happen which makes c happen) then there is a paradox with time travel, because to kill your ancestors would remove your cause. The linear string wouild break or at least radically alter, so probably it isn't possible in this kind of Universe.

If the Universe is a random agglomeration of moments which our intellects mistake as a sequence, then such an "event" is possible. Our existence which seems to have a cause and effects is simply a ripple in disorganized matter, an illusion so any change is just another illusion.

Another possiblity is that time isn't a continuum but just another dimension, albeit one we have not yet learned to move in (other tha forwards). Think of time as a road-we are like a child stuck in a car driving down the road, seeing trees and houses go past. Trying to make sense of the parade of images we evolve a simple idea of causality-we saw the tree first, then the house. Maybe the tree caused the house?

If we could control the car, we could reverse, go left or right, have a look at the trees and houses and get a better understanding. Blowing up a garage a few blocks back doesn't stop or journey from ever beginning.

If the events in time exist as dimensional points, we might be able to alter points across the 4 dimensions without neccesarily harming ourselves. Do we exist because of an event spatially and temporally removed from us in time? Are we intellects existing in the moment and able to control events across time and space?

I favour the "string of causal moments" theory because it gives me less headaches-unfortunately it also makes time travel unlikely.

whoa dude, no need for the weed after reading that.

 

[CharlieG]

It's time for me to confess. I was born in the year 2357. I came back here because I wanted to enjoy the Swans' 2004-5-6 premierships... but time travel is still sort of inexact, and I ended up in 2002. So, I'm sitting it out.

By the way, the Federal Republic of Oklahoma is the next target in the war on terror, if you're interested.

 

[Stevo]

Originally posted by Au_Blue#24
Stevo - you obviously have an excelent handle on this subject. Above you say that anything with mass cannot travel as fast as light. On the surface this seems true. Im reaching back into my uni days where i did physics some time ago so i may be inaccurate, but doesnt the duallity of light principle state that light is made up of both em waves plus particle (photons) - hence it has mass. The bending of light around black holes which have enormous gravitation forces also proove this ??

Photons have momentum but not mass.

I think, but I'm not sure, that the bending of light is a result of the curvature of space-time, which is itself a consequence of the prescence of mass.

 

[MightyFighting]

Originally posted by Stevo
I think, but I'm not sure, that the bending of light is a result of the curvature of space-time, which is itself a consequence of the prescence of mass.

I can verify that. Gravity is a fictitious force and the acceleration of particles toward large masses is solely due to the curvature of space caused by the presence of very large masses. That is why massless particles are affected by gravity but (presumably) cannot cause it.

 

[MightyFighting]

Originally posted by Au_Blue#24
doesnt the duallity of light principle state that light is made up of both em waves plus particle (photons) - hence it has mass.

Light is Photons.
They are small packets of electromagnetic wave. Hence they are waves but also have a finite size (like particles).

 

[Stevo]

Originally posted by MightyFighting
Enough with the grandfather killing!!!

At its most basic level the Universe is literally random. i.e. not just random as in we cannot predict what will happen, but random as in events occur without a cause.

Now, it is true that at a molecular level we can not discern causes to an ultimate degree of accuracy (Heisenberg), and thus we need to use probability density functions.

However, does this mean that events don't have a cause, or that we simply can't discern them?

 

[MightyFighting]

Originally posted by Stevo
Now, it is true that at a molecular level we can not discern causes to an ultimate degree of accuracy (Heisenberg), and thus we need to use probability density functions.

However, does this mean that events don't have a cause, or that we simply can't discern them?

According to Heisenberg, it's because they have no cause. How could the double-slit defraction experiment work otherwise?


For example: If anything could affect the rate of decay of Carbon-13 it would not be an accurate clock.

 

[Stevo]

Originally posted by MightyFighting
According to Heisenberg, it's because they have no cause. How could the double-slit defraction experiment work otherwise?


For example: If anything could affect the rate of decay of Carbon-13 it would not be an accurate clock.

I agree, but perhaps there are things which participate in causal interactions of which we are not aware. Of course, this goes against Ockham's razor.

 

[dr nick]

if photons of light have no mass (and here i was thinking momentum was mass times velocity), why can photons of light hit a reflective surface and cause it to move ala a solar sail?
i thought they just had no rest mass.

 

[Stevo]

Originally posted by nicko18
if photons of light have no mass (and here i was thinking momentum was mass times velocity), why can photons of light hit a reflective surface and cause it to move ala a solar sail?
i thought they just had no rest mass.

Well, the energy of a photon is given by E = hv. Planck's constant is in units of Joules seconds, which means it is in units equivalent to momentum X distance. So E is in units of momentum X distance / time. Or momentum X velocity, which is also given by Einstein's relation E = pc.

As for solar sails, I just read an article on it recently, but can't recall the details, unfortunately. But I think most of it was in response to those claiming that solar sails violated thermodynamics.

 

[CharlieG]

Does the word "nerd" mean anything to you people?

I can't get my head around all this physics/astronomy/philosophy stuff. As far as science goes, I don't care how the world works, as long as it keeps working.

I'll keep an eye on this thread... perhaps I'll understand something...

 

[Stevo]

Originally posted by CharlieG
Does the word "nerd" mean anything to you people?

Feel free to use it in my direction if it helps to alleviate your insecurity.

 

[CharlieG]

Originally posted by Stevo
Feel free to use it in my direction if it helps to alleviate your insecurity.

Perhaps you chose to study philosophy, of all things, because you could avoid having to actually answer the questions!

 

[Stevo]

Originally posted by CharlieG
Perhaps you chose to study philosophy, of all things, because you could avoid having to actually answer the questions!

Well, perhaps that's true! No doubt, I enjoy discussing such things, and while there may not be an absolutely right or wrong answer, there are certainly good and bad answers. The search for finding an insightful response (I shouldn't use the word answer) to some of our most pressing questions is never frivolous, IMO, despite philosophy being often viewed as a rather esoteric and "cappuccino" subject.

 

[CharlieG]

Originally posted by Stevo
Well, perhaps that's true! No doubt, I enjoy discussing such things, and while there may not be an absolutely right or wrong answer, there are certainly good and bad answers. The search for finding an insightful response (I shouldn't use the word answer) to some of our most pressing questions is never frivolous, IMO, despite philosophy being often viewed as a rather esoteric and "cappuccino" subject.

Is it true about that philosophy exam at one of the unis where the question was "Why?" and the answer that got perfect marks was "Why not?"