Do you believe time travel will ever be possible for humans?

If so, how the heck would it work?

Would it be possible only to travel forward? If you traveled back, what would happen to your future time? Would two separate realities form? If so, could you ever go back to your original present or would you be stuck in the new bifurcation you created by going back?

And of course the weird situation of having TWO yous (if you went back to when you were 8 or 16 or whenever you wouldn't turn into your former self, you would be there WITH your former self). Wouldn't this break the law of "matter cannot be created or destroyed), after all the, new matter could constantly be pumped backwards, creating a new reality (or multiple new realities) with way more matter than it would otherwise have.

Do any serious scientists actually concern themselves w/ these questions?

The idea of thinking about it I just find fascinating. Basically because I know so much more now than I did then. Also, cause I've lost or lost touch w/ so many people over the years. Also, cause it would just be cool to experience.

I doubt it will be possible in my lifetime (perhaps not ever, especially if society cannot handle the challenges of the next century or two), but it's fun to think about.

Anyway, this thread is to talk about time travel in a scientific (as well as a "it would be cool if") manner. :)

We travel forward all the time. ;)

But, given that there have been small differences in atomic clocks between one on the ground and one on a spacecraft would show that it is possible to "travel" to the future, albeit, for any noticeable effect, it would take technology greater than our own.

I don't believe it's really possible to travel backwards and the possibilities arising from going back there (which you mentioned) give me a severe brain cramp.

I also had a thought that since we can see the past merely by watching the light hitting us now from distant places, that one could "travel" back if only you could move faster than that light. But then again, just because you can move faster than the images can doesn't mean that the matter will be where it used to be. And then I got another brain cramp.

Such things are for minds far greater than my feeble own.

Theoretical ftl particles have time reversed world lines. If you look at a Feynmen diagram of creation of a particle/antiparticle pair that annihilate shortly after, you can look at it also as a single particle that travels forward in time and then backward in time along a slightly differnet path.

Given how hard it is to move through time at a slightly reduced rate (relativistic velocity needed) how hard would it be to move from one point in time to another without passing through all the moments inbetween? We can't teleport in space yet.

I think the only way for you to travel backwards through time would involve you setting up an entrance to a wormhole.. anchoring it in a specific place in time.. and then holding on to the other end for a couple hundred/thousand years (or however long you wanted to), which would allow you to travel back to the time when you first created the wormhole.

Of course this would require amazing amounts of energy.. and a very advanced understanding of physics.

but I don't think it'd be possible to just travel to wherever you wanted to, as it is often portrayed in movies.

Actually I have thought quite a bit about time travel, and I think that it would indeed be possible, but that our present views on the subject are rather romanticised compared to the true possibility.

First step is: Why does everyone discuss time travel before scrying? Scrying would be SEEING through time, and personally I like to have a windshield on my car before I go 60 mph down the highway...

Really, one must start with being able to see where they are going, otherwise you run into massive complications. Hence the first possibility which must be worked toward is scrying, not time travel.

As for time travel, I do not think that all of those paradox dilema are even valid problems. You are just adding another direction to your options for travel, and as such, you can still only travel to a place (well, now place/time) that exists. Hence, you will only encounter the same objects in the new time you move to if those objects happen to be 4-dimensional entities which also extend in the same direction as your travel.


The standard method for thinking about 4-dimensional travel is to consider a being who can only see 2 dimensions suddenly gaining a 3rd dimension of travel.

So, let's say that there are beings living on page 26 of a book, and that the pages of this book when closed have no gaps between them. One of these beings discovers how to travel in the third dimension without being able to see in the third dimension and decides to try the "Down" setting first. He meets quickly with Page 24 in a catastrophic colision and is dead without ever managing to leave Page 26.

Now, if the book happens to be open to Page 26, and he happens to have learned how to look in the 3rd dimension, or just randomly selects to move "Up" first, he manages to break free into the 3rd dimension.

To parallel time more appropriately, let us assume that the book is falling. This allows it to travel in the third dimension, but always in 1 direction. Having just moved himself off of the page, our Flatlander now finds himself floating suspended above the book. There is nobody and nothing there with him. He sees just empty space.

Of course, if someone were reading this open book while it was falling, and they had their finger on the page to indicate where they were at, then he would view a strange circle moving left to right, then disappearing only to appear at the left, but slightly further from him, and travel again to the right.


So, bringing it back to Time travel. Assume that the world happens to be a 4-Dimensional object, and that you manage to move to a new 4th dimensional co-ordinate that is still near the world (but safe to move onto). You now find yourself on the earth, but unless people are ALSO 4-dimensional objects, there will not be any people. Well, unless there is a completely seperate set of people existing on the earth at a slight 4-Dimensional displacement from ourselves that is.

Were the world not a 4-dimensional object, then there is really no telling what you would manage to find around yourself when you suddenly shift yourself in the 4th dimension slightly.


Since people are constantly traveling forward in time, it would be like if you were with a line of people holding hands and walking across a field. Let's say that every couple of steps you move 1 person down the line to the left while everyone else maintains their same position in the line.

Now, you wanted to go back and see that person you were standing next to 12 steps ago, so you discover the ability to walk backwards. You figure that walking backwards 12 steps will put you back in contact, and so you begin your travel.

Problem is, you just left the line, which is still traveling forward. And nobody was following the line. So when you move back 12 steps and again begin walking forward, you find yourself covering familiar ground (which exists in that expanse of the 3rd dimension), but you are now alone (because your line of people do not exist that far into the third dimension).

I alwasy assumed that if travel back in time were possible, they would have gone back from when it was invented to now and told us.

So my question is, if time travel was possible, why haven't we seen anybody from the future yet?
Possibilities are:
a. time travel is widespread in the future but you can only time-travel to another "reality", not your own. (whatever that means)
b. time-travel is possible but so difficult that very few travels by very few people occur, and using them to visit us does not make sense
c. time travel is widespread but people for some reason decided to not visit us, which could be slightly insulting or very wise.

Also, is FTL travel related to time-travel?

C doesn't seem likely; I mean, look at all the threads here where they ask what time period you would visit if you had a tank or a machine gun or something, and how many people responded that they would try to take over the world with them, and then tell me that idea will never occur to anyone in the future.

C doesn't seem likely; I mean, look at all the threads here where they ask what time period you would visit if you had a tank or a machine gun or something, and how many people responded that they would try to take over the world with them, and then tell me that idea will never occur to anyone in the future.

I totally agree. So we can probably rule out the "time travel becomes a widespread technology that anyone can use to travel back to their own reality".

My general opinion on this is like this and I've heard nothing but semantics games as arguments against my views:

Time travel is not possible. This means you cannot travel TO OUR OWN PAST. Traveling in another dimension in a different time there, somewhere where the past has been identical, assuming it would be possible, is STILL NOT TIME TRAVEL. It's an imitation of time travel. It's like saying you can draw a beautiful black and white painting while in fact you are only able to photocopy a great one. Yes, the end result looks the same, but it's not the same thing. Lastly, I believe traveling to other dimensions and things like that are not possible either with our understanding of physics (would break quite a few basic laws of physics, but those will anyway change as our understanding of physics gets greater).

Thus, my personal conclusion is that it can never be proved to be impossible, but it's as possible and likely as discovering a new continent where there is no friction and inertia.

Or maybe you can simply observe the past, not actually go there.

I've never heard anything that suggests to me that Time Travel might ever become a reality.

Or maybe you can simply observe the past, not actually go there.

True. :D That seems perfectly possible, and it doesn't need any total revolution of physics either. ;)

So my question is, if time travel was possible, why haven't we seen anybody from the future yet?
Possibilities are:
a. time travel is widespread in the future but you can only time-travel to another "reality", not your own. (whatever that means)
b. time-travel is possible but so difficult that very few travels by very few people occur, and using them to visit us does not make sense
c. time travel is widespread but people for some reason decided to not visit us, which could be slightly insulting or very wise.

Also, is FTL travel related to time-travel?FTL travel is certainly related to time-travel.

As for the possibilities, here's another:
d. time travel requires opening a "door" at either end and so one can't travel back to a time earlier than when (and where) the first "door" is opened.

Current physics, as I understand it, suggests that:
Time travel into our future is certainly possible.
Time travel into our past, however, is/will/can only be the case if our universe exists on/in/as a closed timelike curve which loops around, in which case we've got predestination on our hands in a nasty way, and we don't so much "invent" time travel as we inevitably play out the construction of a time machine and the travel into the past, "over and over".

That reminds me of an odd link between time travel and religious debates.

If I go into the past, e.g. to 1920, is the Second World War doomed to happen? Can I prevent it? Have I robbed people of free will by knowing what's going to happen "before" it happens? Does a hypothetical God's omniscience work the same way? Does omniscience, or any knowledge of a decision before it happens, really contradict free will?

C doesn't seem likely; I mean, look at all the threads here where they ask what time period you would visit if you had a tank or a machine gun or something, and how many people responded that they would try to take over the world with them, and then tell me that idea will never occur to anyone in the future.

Maybe they did? But there were "unforeseen circumstances" along the way. Such as America's late entry not coming late enough.

And how many lunatics have claimed to be from the future, only to be institutionalised :p

Personally I'd apply the anthropic principle to this -- if the past were altered by some time travel, we wouldn't find it odd, because it would be our past.

FTL travel is certainly related to time-travel.

Current physics, as I understand it, suggests that:
Time travel into our future is certainly possible.

I'm linking two things from your post that you didn't link, but that's related to my question.

Is FTL linked to travel in the future, or travel in the past?

Basically I want to know if the FTL they have in pretty much every sci-fi material is a theoretical impossibility (we will NEVER be able to go from point A to point B faster than light) or a technological one (we currently don't know how to go from A to B FTL).

Look at wormholes and theories about space-time folding and you will see that it is technically possible to arrive at a location before light does, without traveling any faster than light (just taking a different path).

EDIT: Mildly upset nobody has any comments on my earlier post :( I was hoping someone might find a logical failure with it at the least so I could return to the thought process :)

I alwasy assumed that if travel back in time were possible, they would have gone back from when it was invented to now and told us.

Maybe they have; ever heard of someone being "ahead of their time"?:lol:

Look at wormholes and theories about space-time folding and you will see that it is technically possible to arrive at a location before light does, without traveling any faster than light (just taking a different path).

But wouldn't wormholes and/or folding, by allowing information to arrive at their destinations faster than light, create some paradoxes?

I cannot think of what paradox is possible through this method. You are merely arriving before certain aspects of the information will manage to. Nothing is being changed in particular, so no Paradox comes readily to mind. It would be roughly equivalent to splitting a laser, bouncing part of it off a mirror, and then recombining the two at some later point. There is a shift, but no paradox.

Travelling forwards is 100% possible (faster then normal speed), going back in time is a possibility which may be proved next year.

Or maybe you can simply observe the past, not actually go there.
That would still be pretty cool.

Travelling forwards is 100% possible (faster then normal speed), going back in time is a possibility which may be proved next year.
What's happening next year?

Time travel is not a possibility for humans until we are able to manipulate wormholes.

But then what is to prevent all of the unholy paradoxes such as the Grandfather Paradox?

I cannot think of what paradox is possible through this method. You are merely arriving before certain aspects of the information will manage to. Nothing is being changed in particular, so no Paradox comes readily to mind. It would be roughly equivalent to splitting a laser, bouncing part of it off a mirror, and then recombining the two at some later point. There is a shift, but no paradox.

Wormholes are full of paradoxes. Here's a paradox for you:

- you have the entrance to a wormhole exactly below the exit, let's say 3 meters below it.
- you put a cat between the two, so above the entrance, below the exit.
- the cat is going to fall in the wormhole, exit through the top, and fall again, ad infinitum
- result? Perpetuum Mobile. Law of conservation of energy, anyone?

That would still be pretty cool.


What's happening next year?

Next year the new particle accelerator is opening next year at CERN which has the possibility to prove that it is possible to create wormholes and that they can (or cannot) go back in time.

It is only a theory at the moment so we'll just have to wait;)

EDIT: Mildly upset nobody has any comments on my earlier post :( I was hoping someone might find a logical failure with it at the least so I could return to the thought process :)I was wondering if anyone else would comment actually. It seemed to me that you were trying to treat time as the dimension in which multiple universes exist - i.e. your page change sounded like flipping between alternative realities. In fact a page change would mean stepping forward or backward in time.

I think it's easier to imagine stepping through alternate versions of our 3D world than to imagine stepping through the same version of our 3D world via a 4th dimension. In other words, I think that it's more instructive to view time as the first dimension, rather than the 4th (or perhaps the zeroth dimension).

I think that makes it more obvious to see that time might be special, and may not be traversible in the same way that x, y or z are.

But then what is to prevent all of the unholy paradoxes such as the Grandfather Paradox?
Who said there needs to be one....?

Consider: you lead a normal life until the age of 35. Then you invent a time machine, go back in time and kill your grandfather.

So, now your father can't be born and you can't exist, right?

Wrong. Examine your personal timeline as you stand there over your grandfather's corpse. You were born, you grew up, you SAW your father strolling around the house and singing "Happy Birthday" to you once a year. You probably met your grandfather too. All this happened BEFORE you killed your grandfather--from your viewpoint. You can't kill your grandfather before you were born because you had already been born.

It's like travelling south along the California coast from San Francisco. After three to four hours, you'll pass through San Luis Obispo. Another three hours or so, you'll reach Los Angeles.

But what if you start in San Diego and go north instead? Then you pass through Los Angeles before you pass through San Luis Obispo. The sequence of events is different depending on your direction of travel.


The reason time travel is so confusing is because everybody tries to view it from some absolute reference frame--which, according to Einstein, doesn't exist.

We experience the fourth dimension any time we see something move. Talking about alternate realities invokes a fifth dimension.

I was wondering if anyone else would comment actually. It seemed to me that you were trying to treat time as the dimension in which multiple universes exist - i.e. your page change sounded like flipping between alternative realities. In fact a page change would mean stepping forward or backward in time.

Well the concept of alternate dimensions has always been laughable from a scientific view, because it is so undefined. Do you mean the second instead of the third? Do you mean inverting the Kalibi-Yao folds to exist in the 5-11 Dimensions instead of the lower energy 1-4?

The chance of an "Alternative reality" would rely upon the earth itself being a 4 dimensional object. I didn't go into it completely, but the chances of that are slim since we can accurately account for the forces involved with changing the earth (ie - digging) based on our 3-Dimensional reference frame. If by digging a hole I was actually lifting 2 lbs of earth across the span of a few milenium, it is doubtful I could heft my shovel. And if we were only removing the portion of it from that particular 3-Dimensional point, then as soon as we ourselves moved forward in the 4-th Dimension, then the hole would be filled in and our shovel empty.

But overall, the chances that humans are 4-Dimensional objects are slim to none (No senses or ability to move in the 4-th Dimension would imply no need for survival instincts/reactions in that direction, which would imply we are the ONLY thing which is 4-th Dimensional. It is more plausible that we are not, then that the entire universe is not). The reason you cannot go back in time and kill your grandfather is that your grandfather no longer exists back in that time. He exists in his grave, as bones or dust, or at your Grandma's House waiting to have you call up and report on the success of your cute little "Time Machine" idea.

Wormholes are full of paradoxes. Here's a paradox for you:

- you have the entrance to a wormhole exactly below the exit, let's say 3 meters below it.
- you put a cat between the two, so above the entrance, below the exit.
- the cat is going to fall in the wormhole, exit through the top, and fall again, ad infinitum
- result? Perpetuum Mobile. Law of conservation of energy, anyone?

Why would there be a gravitational pull toward the Wormhole underneath the cat? Gravitation is a sign of differences in Potential Energy. The amount of energy in a wormhole quite likely outweighs the amount of energy equivalent to the Earth's Mass. Hence the largest Gravitational pull would be due to the Wormholes, not due to the Earth. Since the Cat is between the wormholes, it would be attracted to the one of them with the lowest potential, and would most probably wind up stuck inside of the wormhole because to move away from that point would result in increasing the potential energy, which will not happen naturally.

Why would there be a gravitational pull toward the Wormhole underneath the cat? Gravitation is a sign of differences in Potential Energy. The amount of energy in a wormhole quite likely outweighs the amount of energy equivalent to the Earth's Mass. Hence the largest Gravitational pull would be due to the Wormholes, not due to the Earth. Since the Cat is between the wormholes, it would be attracted to the one of them with the lowest potential, and would most probably wind up stuck inside of the wormhole because to move away from that point would result in increasing the potential energy, which will not happen naturally.

So do those wormholes also attract objects inside them? And then do they push them away at the end?

Well the concept of alternate dimensions has always been laughable from a scientific view, because it is so undefined. Do you mean the second instead of the third? Do you mean inverting the Kalibi-Yao folds to exist in the 5-11 Dimensions instead of the lower energy 1-4?I see no need to get specific. It's an extra dimension.
The chance of an "Alternative reality" would rely upon the earth itself being a 4 dimensional object... Any three dimensional object that exists from one moment to another is at least four-dimensional since it's existence is extending into a fourth dimension (time). The world, humans, the whole shebang.
If by digging a hole I was actually lifting 2 lbs of earth across the span of a few milenium, it is doubtful I could heft my shovel. Proof please. :D

I see no need to get specific. It's an extra dimension.

But what do you MEAN by dimension in this case? Most people who speak of "Traveling to another dimension" actually are discussing passing into an alternate reality or a seperate universe. Simply shifting over slightly in a direction which people are not aware of would be motion in a new dimension, but anything which exists across that shift would have to exist as an object with measureable existence in that dimension.

Any three dimensional object that exists from one moment to another is at least four-dimensional since it's existence is extending into a fourth dimension (time). The world, humans, the whole shebang.

So any 1 dimensional object which is falling is actually a 2 dimensional object? Motion within a dimension does not imply a measureable existence in that dimension.

Proof please. :D

Simple to prove: A single sheet of paper is lighter than a stack of papers. Hence a 2 dimensional object is lighter than a 3 dimensional object. The same can be stated for any extra dimension of existence.

But what do you MEAN by dimension in this case? Most people who speak of "Traveling to another dimension" actually are discussing passing into an alternate reality or a seperate universe. Simply shifting over slightly in a direction which people are not aware of would be motion in a new dimension, but anything which exists across that shift would have to exist as an object with measureable existence in that dimension.Yes, and?
So any 1 dimensional object which is falling is actually a 2 dimensional object? Motion within a dimension does not imply a measureable existence in that dimension.If you could show us proof that anything was moving in time we wouldn't need this thread. Existence from one moment to the next is not motion in time.
Simple to prove: A single sheet of paper is lighter than a stack of papers. Hence a 2 dimensional object is lighter than a 3 dimensional object. The same can be stated for any extra dimension of existence.Well, apart from the fact that sheets of paper are 3 dimensional already, as i'm sure you are aware;

How many dimensions do the fundamental particles from which those sheets are comprised occupy? (How wide is a quark if you like.)

Why would you assume that I have to apply the force necessary to move every 'moment' of that shovelful of Earth? I move it, then it carries on existing in the new position. I might ask why when I move it it doesn't move five minutes ago as well.

Existence from one moment to the next is not motion in time.

Which is why you have to consider what we CAN measure about something if it does have some extent into the 4th dimension.

I move it, then it carries on existing in the new position.

You are right here stating precisely what I am stating actually, as I read it. If you move the object and it carries on existing in that new position, either that object is moving along with you in the 4th dimension, or you have moved it across all of the dimensions.

The primary basis for my arguement I believe would be that in order for travel through time to exist, it must share the same properties as the spatial dimensions. Meaning that motion is possible, and were one to know how to "twist" something, it could be rotated in the direction of time (or counter to that direction).

From there, I then assume that if we were to exist in any macro-scale manner within the dimension of time, that we would have developed senses and reflexes within that direction as a defense mechanism, unless there was never any threat which required it. Just as eyes of a predator focus forward for depth of view and eyes of prey focus on the sideways for width of view. And our necks allow us to look up and down, whereas if there was never any threat or benefit above us, we would likely not have developed the ability to look up.

Who said there needs to be one....?

Consider: you lead a normal life until the age of 35. Then you invent a time machine, go back in time and kill your grandfather.

So, now your father can't be born and you can't exist, right?

Wrong. Examine your personal timeline as you stand there over your grandfather's corpse. You were born, you grew up, you SAW your father strolling around the house and singing "Happy Birthday" to you once a year. You probably met your grandfather too. All this happened BEFORE you killed your grandfather--from your viewpoint. You can't kill your grandfather before you were born because you had already been born.That's one way of looking at it. But whatever wormhole took you back in time does not erase your past. In order for you to go back in time, you must enter the wormhole. If you change what goes into the wormhole, in way that would result in a paradox. If you change anything about the wormhole, then that would result in a paradox.

It's like travelling south along the California coast from San Francisco. After three to four hours, you'll pass through San Luis Obispo. Another three hours or so, you'll reach Los Angeles.

But what if you start in San Diego and go north instead? Then you pass through Los Angeles before you pass through San Luis Obispo. The sequence of events is different depending on your direction of travel.I don't see the connection. We're talking about wormholes, not traveling backwards in time.

The reason time travel is so confusing is because everybody tries to view it from some absolute reference frame--which, according to Einstein, doesn't exist.I don't see how this is related either.

Or maybe you can simply observe the past, not actually go there.

Except you can't observe something without affecting it. Observer Affect and Heisenberg Uncertainty Principle and all that.



I don't think 'time travel' is possible or that time is like a spacial dimension but it makes good Sci-fi.

Except you can't observe something without affecting it. Observer Affect and Heisenberg Uncertainty Principle and all that.



I don't think 'time travel' is possible or that time is like a spacial dimension but it makes good Sci-fi.

It is possible to go forwards in time, but going backwards is the problem..

I think that, if you actually were there even for a single millisecond, the world would be split. There would now be two different dimensions, one where you came from, and the other one you just time-traveled too. Even being there for a millisecond changes the world in some way or another, whether having one breath of carbon dioxide to having one more influenza germ you were carrying, the world will change.

I don't see the connection. We're talking about wormholes, not traveling backwards in time.
CivGeneral asked a question about time travel. I answered it. Well, tried to, anyway.

Except you can't observe something without affecting it. Observer Affect and Heisenberg Uncertainty Principle and all that.

You could observe it without being there though. Imagine that in the future, we discover that everything that ever happened in the world can be "read" from certain sources. It's very unlikely, but at least it's not contradicting any of those - it would be just like looking at a recording.

Oh and yes, I agree with the second part of your post (the one I didn't quote). :)

Backwards in Time:
Reverse the flow of all the atoms in the universe in their exact pathes.

Forwards in Time:
The exact opposite. The exact path thing is for traveling to the past.

It's not perfect, as I've never really sat down to think it all the way through.

You could observe it without being there though. Imagine that in the future, we discover that everything that ever happened in the world can be "read" from certain sources. It's very unlikely, but at least it's not contradicting any of those - it would be just like looking at a recording.

Oh and yes, I agree with the second part of your post (the one I didn't quote). :)

Then whatever made the recording would cause minute changes that would have larger affects in the long term.

The real problem in time travel is who invents it:

A person from the future comes and gives me the blueprints of a time machine. I build it.

I realise I'm the person from the future and go to the past to give the blueprints to myself.

Who did invent that machine?

The person who visited you from the future--he invented it.

So what if that person turns out to be an alternate you? This is not a problem--maybe he's simply from an alternate future where he invented a time machine without anyone else's help. After that point, if you want to go and start up a causality loop, go right ahead. :)

Then whatever made the recording would cause minute changes that would have larger affects in the long term.

:confused: I don't get what you mean. You DON'T turn back to make those things, I meant the possibility of them being already there!

Let's take a (pretty stupid, I admit) example to understand what I mean: let's IMAGINE that in the future we discover that every movement of any object together with its properties (like size, color, etc) remained stored in... let's say, any metal. And after that, we learn how to read that information. Then we would be able to OBSERVE what happened in the past without affecting it AT ALL. It's extremely unlikely, but it's not breaking any principle or law of physics, at least!!

:confused: I don't get what you mean. You DON'T turn back to make those things, I meant the possibility of them being already there!

Let's take a (pretty stupid, I admit) example to understand what I mean: let's IMAGINE that in the future we discover that every movement of any object together with its properties (like size, color, etc) remained stored in... let's say, any metal. And after that, we learn how to read that information. Then we would be able to OBSERVE what happened in the past without affecting it AT ALL. It's extremely unlikely, but it's not breaking any principle or law of physics, at least!!Yes, but if we did that we would quickly realize how small we are in comparison to the whole infinity of the creation, and die of shock! ;)

Yes, but if we did that we would quickly realize how small we are in comparison to the whole infinity of the creation, and die of shock! ;)

:D Probably!

Let's take a (pretty stupid, I admit) example to understand what I mean: let's IMAGINE that in the future we discover that every movement of any object together with its properties (like size, color, etc) remained stored in... let's say, any metal. And after that, we learn how to read that information. Then we would be able to OBSERVE what happened in the past without affecting it AT ALL. It's extremely unlikely, but it's not breaking any principle or law of physics, at least!!
I've got a surprise for you--this example may in fact violate a law of physics.....

The law I'm referring to is the limit on knowledge. I think I found this in Scientific American, I forget where--but somebody, somewhere, proved that there is a mathematical limit on the amount of information that can be known in any one place. Go beyond that limit, and the storage medium (regardless of what it is) will collapse into a singularity. Curiously, that limit is not dependent on the volume the stored knowledge takes up--but on the surface area of the space it occupies!

This limit is absolutely gigantic. Actually bigger than that. But the amount of knowledge represented by every single event that has ever happened, anywhere, throughout the existence of the Cosmos? That's also absolutely gigantic (actually bigger than that). I have no idea how big, but if the storage space needed is bigger than The Limit, then your example isn't possible.

Fun to imagine, but kinda not practical. :)

Yes, but if we did that we would quickly realize how small we are in comparison to the whole infinity of the creation, and die of shock! ;)
universe, not creation
:D Probably!
or just lose our minds, á la The Restaurant at the End of the Universe

Since there appears to be some mention of observer effects and Heisenberg here, I thought I'd opine on it.
The HUP says that the product of "delta-X" (the uncertainty of a particle's position at a given moment) and "delta-P" (the uncertainty of a particle's momentum at the same moment) is larger than or equal to a certain constant. (About 10 to the minus 35 joule-seconds.)

In practice, it only applies on ridiculously tiny scales. If you try to locate a particle by bouncing low-energy photons off it, you won't push the particle much (delta-P is low) but you'll get fuzzy "pictures" back (delta-X is high). Whereas if you try to use high-energy photons, you'll get sharper readings of the particle's position, but it'll also be imparted more energy and thus move more. If you try to locate a person by bouncing photons off him, your effect will be negligible relative to e.g. sunlight. :p

So, if you have a recording of what happened in the past, then making the recording affected the past, and viewing the record affects the record. (Technically, the record is affected first by something hitting it, and then the result of that gets to you.) The HUP also says that you can't get a perfect recording of what happened in the past.

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As for the grandfather paradox and whatnot: I'm firmly of the opinion that the timeline exists in a non-paradoxical state due to this being an optimal position in timeline phase space.

It's easy to go from that to the idea that the only time trips that happen are the thoroughly non-paradoxical ones, meaning that they were probably cleared and planned in advance, which neatly explains why they haven't happened yet. It also solves any problem of "who invented it" - the inventor will probably invent it before the full plans for a trip backwards in time can be made, and so it's not possible to send the plans for a time machine back in time.

I have always hated time travel as a plot mechanism for science fiction, precisely because of all the arguments presented here.

My own personal argument against time travel is the space travel involved. If I want to go back in time one year, the Earth is millions of miles away from where it is now. How did I get there as well as then?

The same way as you follow the Earth one year forwards in time - its gravity hauls you along part of the time, its mass pushes you forwards the rest?

:confused: I don't get what you mean. You DON'T turn back to make those things, I meant the possibility of them being already there!

Let's take a (pretty stupid, I admit) example to understand what I mean: let's IMAGINE that in the future we discover that every movement of any object together with its properties (like size, color, etc) remained stored in... let's say, any metal. And after that, we learn how to read that information. Then we would be able to OBSERVE what happened in the past without affecting it AT ALL. It's extremely unlikely, but it's not breaking any principle or law of physics, at least!!

The process responsible of 'storing' that information in the metal would affect & change the process that was being recorded.

The process responsible of 'storing' that information in the metal would affect & change the process that was being recorded.

True. But we're already assuming that it's always been there... ;)

I have always hated time travel as a plot mechanism for science fiction, precisely because of all the arguments presented here.

I tend to agree. However, there is one movie I've seen that really seems to make it work: Primer (http://www.netflix.com/Movie/Primer/60034782?lnkctr=srchrd-sr&strkid=968767934_0_0). It was a very small release - I only saw it on a friends' dvd. Netflix does carry it, though. It really is an excellent treatment of the subject.

The same way as you follow the Earth one year forwards in time - its gravity hauls you along part of the time, its mass pushes you forwards the rest?

Really? That seems weird. Probably because time travel has that notion of being instant - I step in the machine, I push a button, and bam I'm 6 months earlier.
So I would tend to think like Veritass, that you moved in the when, but not in the where.

True. But we're already assuming that it's always been there... ;)

I don't see what you're getting at.. It's impossible to observe an event without affecting it.

He isn't saying you observe the event, he is saying that you observe a residual trace of the event.

For example: Looking out your telescope at the nearest star. You are seeing the light from that star 9 years ago. Thus you are observing the past, but you are not interacting with it at all.


So were we to find that there was some resonance in the metal that notifies us of what has happened around it, we would just be examining that resonance. For crappy example: Lifting a fingerprint off of it. That tells you that in the past someone put their finger on the metal, but it does not require you to go and ask that person to let you see their finger.

For example: Looking out your telescope at the nearest star. You are seeing the light from that star 9 years ago. Thus you are observing the past, but you are not interacting with it at all.

You are though; you're observing photons that have interacted with the star.

What exactly are you trying to prove? That you can play semantics?

You are though; you're observing photons that have interacted with the star.

Yeah and you are observing them now, while they interacted with the star years ago. So you are not affecting their past interaction with the star at all.

No, I'm just saying that you can't observe something without affecting it.. it's impossible! It's not semantics :)

Someone correct me if I'm wrong, but it's one of the basics of quantum physics: An observer always affects the object he/she observes.

In this example, you are observing a star that has been affected by photons you've used to observe it.

No, I'm just saying that you can't observe something without affecting it.. it's impossible! It's not semantics :)

Someone correct me if I'm wrong, but it's one of the basics of quantum physics: An observer always affects the object he/she observes.

In this example, you are observing a star that has been affected by photons you've used to observe it.

I thought that was true on the quantum level, not on the physical world level.

I thought that was true on the quantum level, not on the physical world level.

Well.. how do you observe things? With the help of very small particles, right?

The photons already affected that star a long time ago. They did that independant of you happening to observe them now, and do not care one way or another if you happen to be looking in your telescope tonight.

It would be correct to state that you cannot observe anything without it being affected by something, or having affected something (which of course would in return affect it...).

It is NOT correct to state that you cannot observe something without affecting it yourself.


And yes, the level at which you affect something by observing it is typically only relevant at the quantum scale, where being hit by a photon is something worth writing home about.

There is another problem with time travelling: Some scientists state that a time machine would produce an infinite amount of energy in the moment you activate it, simply because one single particle goes back in time through the machine and takes the usual way back to the future. It enters the machine again, meeting itself. Two particles go back in time... four particles enter the machine... eight...
You'd need a particle that doesn't interact with itself like a neutrino or a photon, Still I would probably not use a time machine when there's a chance that it destroys the universe. :lol:

Ooh, I like that one. I'll take a swing at it.

When the particle passes through the time machine and travels into the past, that means we have interacted with it--thereby altering its position and velocity. There's no reason to expect it to travel just the right path to hit the time machine again in the future--in fact, after a finite number of "passes" we can expect it with statistical certainty to miss the future time machine entirely and terminate the loop.

universe, not creation

or just lose our minds, á la The Restaurant at the End of the UniverseIt actually "whole infinity of the creation". Though it uses "the whole infinite Universe" elsewhere to refer to the same thing. I had actually bothered to look it up before posting. Yes, I suppose an annihilated brain could qualify as insanity.

Has anyone mentioned that according to modern physics time travel whilst not impossible is hard to reconcile with any scientific theory atm? Sorry no time to read the whole thread? As I said on another thread, I frequent a physics forum and the best way to get banned then permanently banned is to advocate time travel in any "rigorously" scientific way.

The real problem in time travel is who invents it:

A person from the future comes and gives me the blueprints of a time machine. I build it.

I realise I'm the person from the future and go to the past to give the blueprints to myself.

Who did invent that machine?You did, or nobody did. The problem is with the word "invent", not with time travel, or so I think.

Just to be even more specific there is not a single recorded experiment in physics were time travel has been observed from the quark level to the classical universe. Thus it's hypothetical at best, and crackpottery at worst. According to the laws of SR, any particle travelling at or slower than the speed of light cannot travel faster than the speed of light. You could if you wanted to hypothesise say that this also suggests any anti particle (bearing in mind the photon is its own anti particle) travelling faster than the speed of light cannot "travel" slower than the speed of light, and that such particles or tachyons would travel backwards in time, this does not breach causality even if these particles exist and that is highly controversial.

Time travel is pure science fiction atm, impossible? No, theoretically impossible as science stands, yes.

I just call that "impossible" ;)

I put 10 dollars in a time travel savings and I have had creepy people driving by my house ever since. Coincidence or movie potential?