Is time travel to the past impossible?

[cousinbasil]

If you misunderstood, I apologize. However, you're looking for trivial ways to call it a logical contradiction but I can easily dodge them by clarifying what I meant or if necessary, by redefining an example. Give me a non-trivial demonstration of why a universe could never be composed of only banana peels which involves a logical contradiction.

Burden of proof is on you.

You seem to be dodging the issue of what a logical contradiction is. I get the picture of banana peels and nothing else, in all sorts of combinations. You say there are no trees and no monkeys. Are you saying trees and monkeys have never existed?

Again, it's not a *logical* contradiction because the very definition of a BPU doesn't contradict itself. It's simply a situation that could not possibly arrive no matter how improbable a series of situations might be prior to the BPU.

But you are not saying why this would be impossible. The burden of proof is really on you to demonstrate that no series of events could lead up to a BPU as you describe it. What laws of physics does it violate? Isn't that what this discussion centers on, after all?

In other words, a universe consisting of a single proton. Possible or impossible?

[eyekwah]

I get the picture of banana peels and nothing else, in all sorts of combinations. You say there are no trees and no monkeys. Are you saying trees and monkeys have never existed?

Maybe, maybe not. It's irrelevant because I say it's impossible in either case.

But you are not saying why this would be impossible. The burden of proof is really on you to demonstrate that no series of events could lead up to a BPU as you describe it. What laws of physics does it violate? Isn't that what this discussion centers on, after all?

I think possibilities rely heavily upon empirical evidence. Quantum theory seems to strongly support that the probability of something happening changes according to whether or not it was observed. That can only lead me to assume that there are universes which reflect actual possibilities. In other words, if I sit down in my favorite restaurant and am torn between ordering the steak or ordering the hamburger, there exist universes for both situations to reflect either choice I could make: one universe for what happened if I ordered the steak and another for what happened if I ordered the hamburger. Obviously it doesn't stop there, because there would be universes for what happened if I decided to be spontaneous and order a third item altogether and another still for what happened if I ripped up the menu and tried to pee on it.

My point is, there does *not* exist a universe branching from the restaurant scenario which involves an entire universe made entirely of banana peels being created... I would argue that no possible decision or series of decisions could ever lead to such a universe. If you claim a BPU does not exist because there's a logical contradiction, I'd like to hear it. If you claim the BPU does exist, then you need to explain why my argument is wrong.

You tell me, could a universe of one proton really exist? Could I create such a universe by my decision at the restaurant on what I'd like to eat?

[cousinbasil]

My point is, there does *not* exist a universe branching from the restaurant scenario which involves an entire universe made entirely of banana peels being created... I would argue that no possible decision or series of decisions could ever lead to such a universe.

But we didn't agree that the BPU had to branch out from any particular point. The error in your thinking is evident in that you are assuming it branches out from you in a restaurant, or any other point. If we assume an initial highly ordered state such as postulated to have existed prior to a Big Bang type of inception, it only has to be more highly ordered than our BPU for it to be possible to become that BPU.

If you claim a BPU does not exist because there's a logical contradiction, I'd like to hear it. If you claim the BPU does exist, then you need to explain why my argument is wrong.

Now that I understand your BPU does not consist of only banana peels - that each peel is like the ones we slip on, made of cells, etc, I don't say it is impossible at all, since there is no logical contradiction. It seems vastly less likely than our own, but how can you demonstrate it is not possible, if there is no evident logical contradiction? Look - a particular peel in our BPU might be thinking, what - a universe where there is a guy in a restaurant ordering a steak? Like that could ever happen!

You tell me, could a universe of one proton really exist? Could I create such a universe by my decision at the restaurant on what I'd like to eat?

Is that how we determine which hypothetical universes are possible and which are not?

[eyekwah]

Is that how we determine which hypothetical universes are possible and which are not?

Precisely, yes. There's a line far off in the horizon which defines the difference between an improbable universe and an impossible one. The problem is, it's so far off, it all looks the same. Some would say the line doesn't even exist, yet I assure you that it does. Otherwise a universe would exist where there are beings that could traverse universes and destroy them. Seeing how we're still here, perhaps not *every* universe is possible, just perhaps, a good many...

Look - a particular peel in our BPU might be thinking, what - a universe where there is a guy in a restaurant ordering a steak? Like that could ever happen!

That's an excellent point! In fact, I would argue that if such a universe truly existed, ours would not because the subset of universes allowed to stem from one that creates the BPU would be a radically different subset of universes allowed to stem from one that created ours. However seeing that for all intents and purposes, we exist, the BPU therefore does not.

[cousinbasil]

Precisely, yes. There's a line far off in the horizon which defines the difference between an improbable universe and an impossible one. The problem is, it's so far off, it all looks the same. Some would say the line doesn't even exist, yet I assure you that it does.

Interesting. Your line of thinking is one I resort to when I hear people getting in a tizzy about the so-called "butterfly effect." You are arguing the causal web cannot be universally continuous, and I agree with that. But I thought the laws of physics were what we were talking about. I am saying the broken teacup reassembling is on the near side of your postulated line if we use as the sole criterion does it violate the laws of physics as we know them?

In other words, it is one thing to assert as you do the existence of the line dividing the impossible from the merely highly unlikely. It is another to come up with a method of determining on which side a hypothetical event would lie. We seem to agree that if the event (or state) would imply a logical inconsistency, it is on the far side of the line.

There is for me always an element of faith that we are able to express Laws of Nature in any manner. These laws take on an idealized symbolic aspect and are manipulated in a Platonic space and then miraculously reconnect with the physical world, and miracles tend to bolster one's faith.

It is these very laws which we are talking about. Your example of ordering something in a restaurant leading to or not leading to a BPU is needlessly vague. Any local event centered on your (evidently all-meat) menu has associated with it, like all events, a light cone. No event inside this light cone (which is a hyper-cone in 4-D space time) can possibly cause or affect anything in the "elsewhere" outside of the light cone - which is the majority of space time. Therefore it is impossible that your choice of dinner can cause the entire universe to to go all BP at any future point, no matter how distant.

That's an excellent point! In fact, I would argue that if such a universe truly existed, ours would not because the subset of universes allowed to stem from one that creates the BPU would be a radically different subset of universes allowed to stem from one that created ours.

How many is the "many" in the "many-worlds" view of reality? It has been surmised that the intial expansion phase of the earliest universe following a postulated Big Bang had space itself growing spectacularly faster than any limitations imposed by the finite speed of light in a vacuum. Light (and therefore matter) is limited in how fast it can traverse spce, but space itself was not travelling "through" space, obviously.

Therefore, how do you know that somewhere out there, there is not a BPU causally isolated from our own? Radically different, yes, but how do you know?

[eyekwah]

It is these very laws which we are talking about. Your example of ordering something in a restaurant leading to or not leading to a BPU is needlessly vague. Any local event centered on your (evidently all-meat) menu has associated with it, like all events, a light cone. No event inside this light cone (which is a hyper-cone in 4-D space time) can possibly cause or affect anything in the "elsewhere" outside of the light cone - which is the majority of space time. Therefore it is impossible that your choice of dinner can cause the entire universe to to go all BP at any future point, no matter how distant.

Yes, no event can cause the entire universe to go BP at any future point. The same can be said at any single point in the life of a universe, not from its birth nor towards its end. The cone of possible universes may be vast, but it doesn't include any conceivable universe. That was my point simply.

You are arguing the causal web cannot be universally continuous, and I agree with that. But I thought the laws of physics were what we were talking about. I am saying the broken teacup reassembling is on the near side of your postulated line if we use as the sole criterion does it violate the laws of physics as we know them?

I have to apologize since it wasn't my original intention to go off on a tangent discussing BPUs and ordering meat off a menu at a restaurant, and it wasn't meant to prove that a reassembling teacup breaks the laws of physics. For what concerns that, if the laws of physics say that a pool ball moving 1 meter per second hits another, several things can be said about the result:
1) Energy throughout the system will not increase or decrease, but rather convert to different forms.
2) 2nd ball will proceed in a 45 degree offset towards the pocket and a velocity slightly less than 1 meter per second.
3) The 1st ball will continue in a direction that will be the product between the vector of its original direction and the vector of the push from the 2nd ball at a speed indicative of the energy lost in the collision and lost due to friction with the table during its travel thus far.

No where will you find that the laws of physics say that the 2nd ball will turn into a penguin or that it will explode. If anything, there is a very small possibility that the 2nd ball fractures and falls apart on the table. However, energy is never gained from the resulting reaction. Likewise, the energy lost from burning a piece of paper or a tea cup losing its potential energy due to a fall cannot be reversed, not even statistically, because of entropy. If what you say were possible, then it would be possible to make engines that work at 100% fuel efficiency. Of course that level is impossible to achieve because something is *always* lost. Even if that weren't true, if you claim no laws of physics are violated if "played in reverse," energy would *always* be gained.

Therefore, how do you know that somewhere out there, there is not a BPU causally isolated from our own? Radically different, yes, but how do you know?

"How do you know there *is* one?" is a more appropriate question.

[cousinbasil]

cousinbasil: Therefore, how do you know that somewhere out there, there is not a BPU causally isolated from our own? Radically different, yes, but how do you know?

eyelwah: "How do you know there *is* one?" is a more appropriate question.

The whole point of this exchange, though, is that this alternative question is not a more appropriate one. Because I do not know there is one - I have never asserted there is one, much less that I know anything about one. It is, after all, your hypothetical construct. You, on the other hand, assert there is NOT one because there cannot be. Therefore, I ask you how you know there cannot be. I am not asserting that I know anything about a BPU - merely that the laws of physics do not proscribe one.

My point is fairly innocuous, as I see it. You are claiming there is a dividing line between what is impossible and what is merely highly improbable. I am willing to concede that, since it makes a kind of intuitive sense. But how does one go about placing a hypothetical state or event on one side of the line or another? As far as I can tell, we have only agreed that logical contradictions place things on the far (impossible) side.

You want me to take the example of the billiard ball turning into a penguin, concede that is impossible, then you will say it is no different from a BPU which you claim is also impossible, even though it lacks logical contradictions.

Not so fast, my hawk-eyed friend. Do we agree that wormholes cannot exist, that the pool table cannot be connected to the Antarctic by a miniature, local breach of space-time, and we aren't witnessing an exceedingly rare but possible exchange of matter between the two ends of the wormhole?

If you are saying this cannot happen, and that a penguin weighs more than a cue ball and matter cannot be gained or lost in this manner, then I assert you are correct - but then you have given an example of a logical contradiction. IOW, it cannot be simultaneously true that matter can be created ex nihilio and that it cannot be so created.

The original discussion was on the arrow of time. I am saying that reversing the time variable with its additive inverse in existing physical equations does not result in a mathematical inconsistency. This is just a fact. The arrow of time is a meta-phenomenon - something observational not generated or mandated by the laws of physics.

[eyekwah]

Perhaps I haven't been clear up until now. If that's the case, I apologize.

I believe what lies in the realm of possibility is any subset of every possible universe which could have led to that realm of possibility. In other words, there exists a universe where Richard Simmons was elected president in 2008 because albeit improbable, a universe existed prior to his election that also existed. Likewise, there also exists a universe where Britney Spears is president and another still where a penguin was named president. They are all existing universes because the universe which spawned it existed as well.

This doesn't make any universe possible, although it makes a good many possible. I gave the example of the BPU to illustrate an example of a universe which cannot exist because just being able to imagine a universe doesn't mean it exists. It would have had to have been the offspring of other universes, however improbable, and I suppose it ultimately comes down to a matter of opinion but I don't think a BPU could ever be the product of possible universes.

For what concerns, time flow and physics, I'll give you another example which I hope will make it more clear. If an atomic bomb goes off, could you imagine the opposite happening? Heat forming from nowhere and assembling in a mushroom cloud singularity getting continually smaller until it turns into an atomic bomb? That cannot happen! It would take far more energy to make the energy in an atomic bomb shrink to the size of an atomic bomb prior to the explosion (ignoring the reassembly of the bomb itself). The fact that it requires far more energy to do this by definition means the "system" is much larger than the initial explosion, and hence it isn't the same thing as a mushroom cloud magically shrinking onto itself.

A more practical example of the same concept is refreezing the melted water from an ice cube. In order to do it, you'd need a freezer, which means a freezer and energy in the form of electricity would have to be involved, which is *not* a natural occurrence but a forced one.

[mental vagrant]

The "time" talked about in the theory of relativity is not the same as the time I'm talking about. The former is strictly limited within the context of the theory itself, and must be understood in that context. The time I am talking about is simply the appearance of change.

You are missing the actual point of relativity, then. Another quote from Green's book suffices to corroborate this crucial point: "Time and space are in the eye of the beholder." The appearance of change remains the same for a person within a moving reference frame; however, to a person at rest with respect to the moving reference frame, all changes within the moving frame appear to slow down.

It should be noted here that things do not appear to change - things actually do change. How fast they seem to change gives us our experience of time.

But time and space are intimately connected, such that travel through space slows down travel through time.

Say you had two identical accurate watches, worn by two different people. Let's put them on a space station, although this doesn't really matter. If one of them boards a spacecraft and takes off for a ride that has him moving at great speed relative to the station, then eventually returns, his watch will show less time has elapsed than the watch of the person who remained on the station.

In other words, time is the appearance of change on the face of the watch. If one watch appearers to change less, then less time has actually elapsed for the person wearing it. There is no other thing that is time for him and his watch but how things change. To him, it will seem as if the watch worn by the person who remained on the station has somehow sped up, but that his own watch is accurate.

Now let's say you are the person that remained on the station, and I took off on the ship. Assume I was gone for fifty years. All this time I was moving at great speed relative to you, either receding from you or approaching you on the way back. When I finally return to the space station, you will be old and gray, and I will not. Our watches have calendar functions: yours says fifty years have gone by, while mine says one year has elapsed. Again, the changes in the watches appear to be different. But this time, we also can notice the changes in ourselves.

Obviously, you felt 50 years go by. It's not as if I felt 50 years go by and my body just did not age - I only felt one year go by. Time itself is the appearance of change - how fast they appear to change is relative to the observer; these appearances will be different if one observer is moving with respect to the other. Moving with respect to means traversing space as well as time, while the reference fram has only traversed time.

Thus, it does not matter how you define time. Time and sapce are different, yet by their very nature connected in the manner I have described.

Time and space are mutual functions of one-another. 15 years as an ant, oh god when will it end?

Also note, just because measurements are derived from human creation doesn't invalidate their existences. An expression of how your percieve your environment, which is as are you, from it you learn't, it shall be as you make our self.

[cousinbasil]

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[Pincho Paxton]

Time doesn't exist. A force exists that is causal for motion. The force is a flow, and scalar force. So Time is really a bunch of particles popping out of a hole, and has nothing to do with Past, Present or Future. Past, Present, and Future are the brain turning causal movement into memories. Memories are energy translations from cause to effect.

Time doesn't really exist.

[chikoka]

This:

Time doesn't exist.

then:

Time is really a bunch of particles popping out of a hole,

[Pincho Paxton]

This:

Time doesn't exist.

then:

Time is really a bunch of particles popping out of a hole,

Mankind uses two versions of time.. Past, present, and future, and atomic clocks. Atomic clocks have particles popping out of the nucleus. They exist. I use the word 'time' for them to keep things simple. Then Time Displacement doesn't need to be re-written.

[Aniihya]

In my opinion, nothing is impossible. However by physics, travelling backwards in time isnt possible but travelling forwards is. Though I doubt that travelling backwards in time is impossible because physics has yet to completely understand time. I would think that the concept reaches a near impossibility.

[mental vagrant]

If you accelerate a localized portion of space, then you can be in the future and go back to the current as if it were the past :P

I'm joking of course..

[Diebert van Rhijn]

We are all dwelling into the past already, trying to approach the present to no avail.

The thing we know is always the past, the future only as predictable as shadows thrown by what's behind.

Not to mention the folly of the now: just admit being later than you think.

[mental vagrant]

We are all dwelling into the past already, trying to approach the present to no avail.

The thing we know is always the past, the future only as predictable as shadows thrown by what's behind.

Not to mention the folly of the now: just admit being later than you think.

True, we are a sneeze from the past.