- #61
ElectrikRipple
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Understanding the Paradox of Backward Time Travel: Why We Can't Go Back
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The discussion centers on the impossibility of backward time travel due to inherent paradoxes. When attempting to travel to a time before the initiation of travel, contradictions arise, such as existing in two places at once. The "Pretzel Time" concept, which suggests a predestined loop of time travel, is debated, with some arguing it leads to infinite duplicates of a person. Additionally, the conversation touches on the nature of time itself, questioning whether the past exists as a tangible place and proposing that time may be an artifact of universal expansion rather than a physical dimension. Ultimately, the consensus leans toward the conclusion that true backward time travel is not feasible.
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- #62
Hurkyl
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Really, it's just as astonishing that the particles (whether they're going the same way through time or not) manage to meet at the same point in space.
But I think you meant that in the sense that "only the present exists". There are a couple of ways that could be explanationed.
The first and most obvious is simply that all particles really do exist in the past, present, and future simultaneously; reality is trajectories, not particles. Causality needs to be slightly modified from the linear sense; the state of a particle can only influence events in its future or past lightcones and nothing outside (i.e. it can only influence events with a timelike separation not spacelike).
Another possibility is more in the spirit of the reverse parametrization idea I mentioned; a lot of physical equations have the lorentz factor &gamma which is dt/d&tau (&tau is proper time)... &gamma involves a square root, though, so if one desired they could take the negative square root to get &gamma, which would correspond to a negative dt/d&tau (i.e. proper time is running backwards with respect to coordinate time). This allows one to continue with the "only the now exists" philosophy, and the modification from ordinary SR is that clocks are permitted to run backwards, not just slow down. In this sense, the equations really do permit you to smoothly go from "forward in time" travel to "backwards in time" travel (but it still doesn't let you get to your "past", nor does it provide a way around the infinite energy requirements).
Hurkyl
- #63
DrChinese
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That is how I picture things too. Especially as relates to causality. There is both a future lightcone (the traditional one) and a past lightcone. These 2 together reflect causal limits. This is fully in keeping with both relativiry and QM in all respects. However, it is possible there may be issues with the 2nd law of thermodynamics.
- #64
Hurkyl
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Well, for completeness's sake, I don't particularly like the "reality is trajectories" interpretation.
Of course, ANY causal "only now exists" physical theory can be rewritten as an "all of time exists" theory, so you lose nothing by permiting the latter. It's just that aside from its utility for computation, I just don't like it.
It would be an interesting exercise to prove or disprove whether bidirectional causality and unidirectional causality are logically equivalent; I have a strong hunch that they are, but I'm not sure.
Hurkyl
Of course, ANY causal "only now exists" physical theory can be rewritten as an "all of time exists" theory, so you lose nothing by permiting the latter. It's just that aside from its utility for computation, I just don't like it.
It would be an interesting exercise to prove or disprove whether bidirectional causality and unidirectional causality are logically equivalent; I have a strong hunch that they are, but I'm not sure.
Hurkyl
- #65
Mentat
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*Slaps self on head* now I remember why I disagreed with Zefram, when he made (sort of) the same point as Hurkyl is making. It's all in the way we think of time. You see, you can think of time as being like the spatial dimensions, in the sense that if I put something in a certain place (in space), it will still be there when I look away. But this is not how I view time. As I see it, when you continue down the T dimension, everything that you "have done" (past tense) "disappears" (or ceases existing). I say this because, if it were "still happening", then it would be happening in the present, not the past. The past is compose of things that "have happened", it does not contain things that "will happen" or "are happening". This doesn't seem to fit your view of time, does it?
- #66
newton1
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no...
the pass was happened...
if that pass space no you...
i think impossible u travel back to that space
but we maybe can see the pass...with no change anything
just like watch a movie...
the pass was happened...
if that pass space no you...
i think impossible u travel back to that space
but we maybe can see the pass...with no change anything
just like watch a movie...
- #67
Messiah
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MA 00:1
Time is neither a field nor a fabric. It is nothing more nor less than a measurement - a differentiation of the relative rate of change within an element or among multiple elements.
To effectively go back in time on a Universal scale, it would be necessary to find a means to first halt all change in the Universe, and then apply sufficient (infinite) force to exactly reverse all processes in progress.
Even such an extraordinary procedure would not reverse the course of time. Whatever process was engaged to controvert time would have to CEASE at the same temporal point it began to act - or time would not truly be reversed.
Time is neither a field nor a fabric. It is nothing more nor less than a measurement - a differentiation of the relative rate of change within an element or among multiple elements.
To effectively go back in time on a Universal scale, it would be necessary to find a means to first halt all change in the Universe, and then apply sufficient (infinite) force to exactly reverse all processes in progress.
Even such an extraordinary procedure would not reverse the course of time. Whatever process was engaged to controvert time would have to CEASE at the same temporal point it began to act - or time would not truly be reversed.
- #68
Mentat
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Time is a dimension (according to Relativity), and it warps and changes along with the "fabric" of space. Your reasoning is based on time's being just the progression of events, when in fact the progression of events is just the result of movement along the Time dimension.
- #69
Alias
Again, time is an artifact of other processes.
Time is what you get when one universe replaces the previous universe. In the new universe, the minimum of action can be noted occurring from the previous universe. For example, light only travels one unit of length (planck length?) during this period of one universe replacing another.
How about a computer analogy?
It's like the refresh rate of your computer monitor. The monitor redraws the entire screen at a certain rate, say 80Hz. Every refresh allows for a new image, possibly a slightly different image in the case of a video with motion.
Time is only perceived because action is allowed. Objects do not move continuously through space passing through infinite numbers of points in between. Objects jump from one position to the next as the universe refreshes. As the universe refreshes, light (in a vacuum) travels one minimum length at a time.
So the refresh rate of the universe, and the 'resolution' or minimum possible length are what determines the speed of light.
What happens to all of the old universes that have been replaced? Maybe God keeps them in a jar. Or maybe they are all connected by a string and he wears it around his neck.
In any case, your only hope at traveling into the past is to die first and make it to heaven. Even then your going to need permission because you will surely cause a paradox. God might not be to happy about that. Good luck.
Time is what you get when one universe replaces the previous universe. In the new universe, the minimum of action can be noted occurring from the previous universe. For example, light only travels one unit of length (planck length?) during this period of one universe replacing another.
How about a computer analogy?
It's like the refresh rate of your computer monitor. The monitor redraws the entire screen at a certain rate, say 80Hz. Every refresh allows for a new image, possibly a slightly different image in the case of a video with motion.
Time is only perceived because action is allowed. Objects do not move continuously through space passing through infinite numbers of points in between. Objects jump from one position to the next as the universe refreshes. As the universe refreshes, light (in a vacuum) travels one minimum length at a time.
So the refresh rate of the universe, and the 'resolution' or minimum possible length are what determines the speed of light.
What happens to all of the old universes that have been replaced? Maybe God keeps them in a jar. Or maybe they are all connected by a string and he wears it around his neck.
In any case, your only hope at traveling into the past is to die first and make it to heaven. Even then your going to need permission because you will surely cause a paradox. God might not be to happy about that. Good luck.
- #70
Messiah
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Every polar coordinate is a dimension. X, Y and Z are axes. There are an INFINITE number of dimensions in the XYZ range. In calculus, time can be 'treated' as a dimension. The progression along the X axis versus the progression along the Y axis is differentiated and used as a 'standard'. To correlate with the Z progression, rather than use dz/(dx/dy) you can group the procedure of x against y and call the differentiation t(time) - dx/dy=dt and dz/(dx/dy)=dz/dt. It makes for a great mathematical shorthand notation, but this convenience does not create a dimension - only a functional illusion of one.
Time is a man made measurement. We measure time by events. The spin of the earth, its rotation about the sun, the pulses of a cesium atom (a standard), etc. For every cycle of 'x' there are a corresponding number of cycles of 'Y'(how else would you measure time??). Time is simply the relative measurement of change - whether it be in position or condition. If nothing changed, there would be no time. If we did not note change, we would be unaware of it.
Do you see some logical component to the definition of time which I fail to note? if so, please explain what YOU think time is.
- #71
Messiah
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ARGHHHHHHHHHHHHHHHHH! One UNIVERSE replaces the previous UNIVERSE?
Something either does or does not exist. If something exists, it is included in the set labeled 'UNIVERSE'. That which is not within the set does NOT exist.
There is only one Universe, and unless qualifiers are added to designate a sub-set, the only correct definition of the term is - "all which exists".
Theory of Reciprocity
- #72
Alias
Is it such a stretch to postulate that there may be something outside of this 'place' that I have so loosely(apparently) termed our 'universe'.
How about this. If our universe is finite, then might we say that there may be something outside of our universe?
For someone with a user name that leads me to believe you may not be a religious person (not a Christian anyway) you sure do treat the word 'universe' like it's some sort of all encompassing god.
How about this. If our universe is finite, then might we say that there may be something outside of our universe?
For someone with a user name that leads me to believe you may not be a religious person (not a Christian anyway) you sure do treat the word 'universe' like it's some sort of all encompassing god.
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- #73
Alias
You defined how time is measured very well. Whoop-dee-doo.
Tell us what 'time' is. What is the mechanism?
What is this property of the universe that allows for motion and action?
- #74
Messiah
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If you qualify the term Universe to be the known or observable universe, then, of course, there is a whole WORLD outside of that infinitesmal boundary.
The term finite means - by its very definition - defined. If the Universe were finite, there would be a defined and observable boundary to it...i.e. if you sail too far out in space, you WILL fall off the edge. Such used to be conventional wisdom among navigators.
As for religion, the Universe is the entire spectrum of existence - every variety which reciprocal symmetry can produce. The Universe would not be complete if it did not contain them all. In that respect, the Universe can be considered an entity unto itself, and every point, every element within it contributes to its completeness. You may have noticed my capitalization of the word Universe. I believe this is appropriate, for the Universe - nature itself - is, indeed, omnipotent and I am but an infinitesimal portion of that power.
While nature demands respect, she has no use for praise or worship. Nature does not desire your obedience, she already has it. The Laws of Nature cannot be broken. She cares not what your beliefs may be - the truth will still be true even if it is never acknowledged. Nature rewards those who understand her. She is not so kind to those who do not.
Notwithstanding the above, I cannot claim any semblance of belief in the conventional deities touted by organized religion - the god icons to represent good and the satan icons to represent evil. Nature is both. And if you put your human bias aside, good and evil depend upon your point of view. Just ask a bacterium if taking antibiotics is a just and righteous act.
- #75
Messiah
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There is an infinite number of mechanisms - everything which exists. Relative rate of change can be measured within an element or derived from any number you wish to include in the set under consideration.
Every element in the Universe is different from every other. It is that difference which drives change - elements morphing to accommodate the differential in their properties to achieve stability. At the boundry where one element touches another, there is 'NOTHING' between them. In my hairbrained thaory, at the point of contact, the sum of the values of the entities must = Ø in order or a law of nature is broken. They must change in condition in order to achieve a Ø balance.
Have you browsed Theory of Reciprocity??
- #76
DrChinese
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You said that every element of the universe is different than the other. According to QM, every like particle is interchangable with all like particles. I.e. they are indistinguishable.
By the way, very cool looking graphics at the link supplied.
- #77
Messiah
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Thanks, Doc.
Yeah...the old (revised)billiard ball theory. It is one of the reasons I changed major in college 30 years ago from phys to math. It just doesn't wash. The entire field of physics was looking for something which isn't there.
- #78
Alias
A "whole world" outside an infinitely small boundry. No offense, but these statements seem nonsensical.
fi·nite
Pronunciation: 'fI-"nIt
Function: adjective
Etymology: Middle English finit, from Latin finitus, past participle of finire
Date: 15th century
1 a : having definite or definable limits
This statement is potentially false. It is possible with proper curvature that the universe is finite yet unbounded. Ask the 2-D creatures that live in the 2-D skin of a balloon universe. They'll tell you all about the limited size of their universe and the fact that there are no walls around it.
That is both your definition and your opinion.
You, by definition, are not infinitesimally small.
"Nature" may be defined as the laws of the universe. But this in no way disproves the existence of things outside of this universe.
- #79
Alias
My original statement was as follows...
You defined how time is measured very well. Whoop-dee-doo.
Tell us what 'time' is. What is the mechanism?
What is this property of the universe that allows for motion and action?
Your response was...
This is not an answer. For example if I asked you to define the mechanism of gravity, you might say that gravity is what you get when you observe the force between two massive objects. While this may be helpful in measuring gravity, it does not define gravity. A better response to the question of what is the mechanism of gravity would be something like, "Gravity is caused by the warpage of space-time surrounding a massive object." That statement more effectivly describes the "mechanism" of gravity.
So, I'll try again.
What is the mechanism of time? Describe this property of the universe that allows for motion and action.
- #80
Alias
Good grief! Never mind.
- #81
Mentat
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Make up your mind, Messiah. Is time the measurement, or is it what is measured?
Carl (re-named quantumcarl) had a big problem with this too. There are two points that you (Messiah) are missing.
First point:
Either time is a dimension that warps and changes, just like space, or Relativity is flawed. There are no two ways about it, Relativity requires that time be a dimension.
Second point:
If you refer to "dimensions" as just "measurements", then you have to contend with the concept of space's being just a "measurement". Think about that, seriously. Saying that we measure space is all well and good (because it's true), but saying that space itself is just a measurment contradicts both QM and GR.
- #82
Messiah
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There are two basics under consideration -
1) Existence(s)
2) Change in condition or location of existence(s)
In order to change or be changed, something must first exist. Hence, change is - by definition - a function of existence.
Change is a process. Existence is not.
Time is the measurement of change. Change can be in condition as well as along/within the XYZ axes we call dimensions, so it is not completely dependent on the 3 polar axes. In this way, it may be considered a 'dimension' - but that is semantical hyperbole.
Indeed, space is NOT just a measurement. Space occupies volume, it has physical presence, it EXISTS every bit as much as matter exists. It is relatively inert. It seems to offer little resistance to motion. Many people consider space to be a "non-existence".
The fact that its properties are not readily visible to us is not remarkable. Ancient man used to consider air 'nothing'. We are not that far ahead of the Neanderthal.
I have a pet theory that gravity is caused by the shrinkage of space which it is in contact with or in proximity to matter.
Theory of Reciprocity
- #83
Mentat
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No, change occurs of over a certain period of time.
Which would explain why you fail to graps the physical nature of the time dimension (no offense, I'm not calling you a Neanderthal, I'm just making a point).
- #84
Messiah
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a) You have an element
b) That element changes (positionally or in condition)
c) That change (c) occurs over a certain period of time (t)
Time is the rate of change dc/dt - it is a measurement. Hey this is basic definition
My Apple Commodore brain of the 20th century is NOWHERE near as powerful as the Cray grey matter of the 30th century. Temporally, we are just a little bit more developed than the Cro's. I don't apologize for my current equipment. I plan to upgrade in the next 30 years or so.
So explain to me the 'time' dimension...and yes, I've read relativitity and string theory. Did you know Einstein was an advocate of the Steady State theory??
- #85
Mentat
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By your definition (which contradicts Relativity (just wanted to make sure that you remembered that)), I should be able to say that...
a) You have an element.
b) That element moves.
c) That movement occurs over a certain portion of space.
Space is the rate of movement - it is a measurement.
This is not what a dimension is - spatial or temporal. Change occurs over a period of time, that's true. But that means that change can be used to measure the element's travel along the time dimension - not that the time itself is just a measurement of change, as you say. In fact, if "change" equals "that which occurs over a period of time, then "time" cannot equal "the measurement of change" because that is a self-inclusive definition (and thus paradoxical).
So? He was the one who invented GR, and GR postulates that time (which he (Einstein) referred to as the fourth dimension) warps and changes, as does space.
- #86
Messiah
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That portion of space ('certain' is a definition - unstated, but we can call it 'X') is a measurement. It is a defined change in position from point a to point b.
Time is a bit more complex. It compares one change against another...i.e. for every measurement of change (object) travels along the x axis, it travels (insert number here) measures of distance along the y axis. It is a comparative measurement.
- #87
Mentat
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No, it's a portion of space, that can be measured..
Ah, now you start referring to "change" as actual movement along an axis. This is all I was getting at. The axes are dimensions. The two terms are synonymous. Time is a dimension/axis, that things move along (and that warps, due to the presence of matter). Do we now agree?
- #88
Messiah
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Change can take place without motion. A change in condition can be realized without travel in any dimension.
- #89
Mentat
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Not so, it takes an amount of time for any change to occur, and for time to pass, one has to move along the time dimension.
- #90
Messiah
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Answer me this - if there were no change in the Universe, would there still be time?
All perception would cease, as perception is a process and no processes would occur.