Time travel and the universe.

[JarodRussell]

^Yes, I was trying to straighten out Fordsvt's replies as to who he was responding to.

And yes, in my original question I was ASSUMING it would create more timelines.

(You do know that CAPITALIZING words in forum etiquette is akin to yelling? please try using italics if you want to emphasize a word. Capitalization comes off as insulting to your audience)

You mean like captitalizing in SCRIPTWRITING? It's a fair method of EMPHASIZING important points. Whoever created that presumptuous notion that it would be insulting was an IDIOT (haha). Another example of the Internet and its inhabitants taking themselves WAY TOO SERIOUSLY.

 

[STR]

^Yeah, capitalizing a single word, or a short phrase, is acceptable to emphasize that word or term. No different than using bold or italics, and really you're only option if the method of communication doesn't allow rich text (like SMS texts or older forums or boards that didn't update their software).

 

[Vurok]

Time travel is impossible as it contradicts reality. The temporal paradox is like dividing by zero.

 

[sojourner]

^Now THAT'S a great addition to this thread. It really illustrates a constructive answer to the initial question and it's incorrect to boot.

 

[YellowSubmarine]

OK, this may be more of a philosophical question, but I thought it might be interesting.

Using the explanation of time travel where changes to the timeline cause new branching timelines, what happens to the whole "energy cannot be created nor destroyed" thing? wouldn't this imply that each new branching timeline shortens the lifetime of the universe as a whole? IF taken to the extreme and viewed as branches being created with every decision point, wouldn't random number generators (which have become much more ubiquitous with the information age) be increasing the rate of new branches being created and thus increasing the rate at which the universe's age shortens?

Not sure I used the right terms for this, so hopefully you guys will get what I am trying to say.

The law of conservation of energy states that the energy of the universe as a closed system is constant. In other words, all physical processes that take place in the universe that we are aware of only convert energy, they do not create it or destroy it, and we use this property to model and predict these processes better.

If something decides to branch the universe at some point, each branch corresponds to a different universe. Inside those two universes the law of conservation of energy isn't violated because the processes that happen within them never created or destroyed any energy for any observer within the universe.

Time travel on the other hand seems to violate the law of conservation of energy but that can be simply explained if you just stop looking at the universe as a closed system. The time traveller came from the outside of the universe. Also, if there are no different timelines involved, the energy within the whole spacetime would remain constant and I even think that the universe would always end up with the same amount of energy that it started with. If there are multiple timelines things get more complicated as the energy within a timeline isn't preserved.

This isn't a problem since the law of preservation of energy only applies to known processes and their cousins. Time travel might be a completely different and unrelated beast so the violation might turn out to be irrelevant.

If there are time travellers from the future, I bet they were never heard from again because they didn't account for the movement of things in space.
Travel one single second back in time, and Earth has moved 27 kilometers, the solar system 200 km, our galaxy 500 km, and so on. If you go back a thousand years, you might end up anywhere.

Movement in relation to what? All motion is relative, you need a fixed point to measure it.

My assumption is that if time travel is possible (and I deeply believe it is absurd), if you remain stationary in relation to an axis that is moving inertially you'd end up at the same point in relation to this axis when you arrive at your chosen time. And you should chose your point and speed when starting your time journey well so that you still arrive in the solar system at the other end.

One problem with that assumption is that if it is true, after time travel all your particles would end up in different parts of the galaxy. So I'd go for means of time travel that do not have any problem with motion: worm holes (where the destination is known), superlight speeds (where your motion through time is a side-effect of your motion through space), or time machines that exist at both ends of the journey (like in “Primer”).

 

[JarodRussell]

If there are time travellers from the future, I bet they were never heard from again because they didn't account for the movement of things in space.
Travel one single second back in time, and Earth has moved 27 kilometers, the solar system 200 km, our galaxy 500 km, and so on. If you go back a thousand years, you might end up anywhere.

Movement in relation to what? All motion is relative, you need a fixed point to measure it.

Movement in relation to its prior position, of course.

 

[YellowSubmarine]

Movement in relation to its prior position, of course.

There is no such thing. There are no positions in the universe. Being stationary and moving with a constant speed are the same thing. If you disregard the gravitational pull of the galaxy centre, the solar system is stationary. It doesn't move. So, if you also don't move, you'd end up together again. Now, you just need to account for the motion towards the centre of the galaxy which will you lead you away, and everything is fine.

 

[ProtoAvatar]

OK, this may be more of a philosophical question, but I thought it might be interesting.

Using the explanation of time travel where changes to the timeline cause new branching timelines, what happens to the whole "energy cannot be created nor destroyed" thing? wouldn't this imply that each new branching timeline shortens the lifetime of the universe as a whole? IF taken to the extreme and viewed as branches being created with every decision point, wouldn't random number generators (which have become much more ubiquitous with the information age) be increasing the rate of new branches being created and thus increasing the rate at which the universe's age shortens?

Not sure I used the right terms for this, so hopefully you guys will get what I am trying to say.

The law of conservation of energy states that the energy of the universe as a closed system is constant.

Yes, the first law of thermodynamics.
Are there explanations to the following inconsistency with regards to it?:

As was recently found out, the universe is expanding - which means NOT that galaxies move away from each other, as shrapnel in an explosion, but that space itself expands, it becomes larger, its volume grows.

Also, as Heisenberg's uncertainty dictates, space is filled with virtual particles that permeate it.

At a given time, these virtual particles occupying space have a given energy.
As space expands, virtual particles, occupying this larger volume, will have a larger total energy - which violates conservation of energy.

 

[ProtoAvatar]

Movement in relation to its prior position, of course.

There is no such thing. There are no positions in the universe. Being stationary and moving with a constant speed are the same thing. If you disregard the gravitational pull of the galaxy centre, the solar system is stationary. It doesn't move. So, if you also don't move, you'd end up together again. Now, you just need to account for the motion towards the centre of the galaxy which will you lead you away, and everything is fine.

Of course there are stationary positions in the universe, YellowSubmarine:

In flat space, an object in a 'stationary position' is defined as either stationary or moving with a constant speed.
Any object, while accelerating, is moving away from - or towards - this stationary position.

In a gravity field, it's even simpler.
The only object in a 'stationary position' is an object in free fall.
All other objects are accelerating aka moving away or toward this stationary position.

As for observers from outside the gravity field: if you're in a gravity field, they - and their perspective - are irrelevant, theoretical abstractions.

 

[JarodRussell]

Movement in relation to its prior position, of course.

There is no such thing. There are no positions in the universe. Being stationary and moving with a constant speed are the same thing. If you disregard the gravitational pull of the galaxy centre, the solar system is stationary. It doesn't move. So, if you also don't move, you'd end up together again. Now, you just need to account for the motion towards the centre of the galaxy which will you lead you away, and everything is fine.

I have absolutely no idea what you are trying to tell me here.

http://illuminations.nctm.org/Lessons/MarsOrbit/2MarsOrbit-MarsEarth.jpg

Are we in agreement that this image illustrates Earth's and Mars' orbit around the sun, and that Earth's position in the solar system changes, because it travels through space at a certain speed (which is average 29 km/s)? The same way, our solar system orbits the galactic center at a speed of about 220 km/s. Earth itself is rotating, meaning that you, while standing on the equatorial line, move with about 1600 km/h.

 

[Christopher]

Movement in relation to its prior position, of course.

There is no such thing. There are no positions in the universe. Being stationary and moving with a constant speed are the same thing. If you disregard the gravitational pull of the galaxy centre, the solar system is stationary. It doesn't move. So, if you also don't move, you'd end up together again. Now, you just need to account for the motion towards the centre of the galaxy which will you lead you away, and everything is fine.

See, the flaw in your argument is that you can't disregard that gravitational pull. The Earth's orbit around the Sun and the Sun's orbit around the galactic center (and the galaxy's motion as well) are examples of accelerated motion. The statement that a stationary reference frame is equivalent to a moving reference frame only applies if both frames are unaccelerated. An accelerated reference frame cannot be treated as equivalent to a stationary, unaccelerated one.

In other words, if something moving through time kept the "momentum" of the planet it was on when it started, it would follow a straight-line path on a tangent to the planet's motion through space. It wouldn't curve with it unless it were subjected to an acceleration.

Of course, the fictional conceit of time-travelling while standing still is just that, fictional. All motion through time is also motion through space, and vice-versa. A time warp would be a region of spacetime that's so heavily distorted that the light cones have bent more than 45 degrees and it's possible for a traveller within that distorted spacetime to travel in a spatial direction that corresponds to a negative temporal direction in external space. So you have to move through space to go back in time anyway. The catch is, you can't necessarily guarantee that said motion through space will cause you to exit anywhere near where you want to be. Not to mention the severe gravitational forces that would be required to create such a time warp. You'd pretty much have to be in a spaceship to travel through time, not on a planet's surface.

The one possible alternative is quantum time travel, a recently proposed idea involving postselection and quantum entanglement. Since entanglement is nonlocal, it could potentially allow some form of time travel that would automatically send you to the same physical location as whatever you were entangled with, no matter where it is/was/will be relative to your starting position in space.

 

[YellowSubmarine]

See, the flaw in your argument is that you can't disregard that gravitational pull. The Earth's orbit around the Sun and the Sun's orbit around the galactic center (and the galaxy's motion as well) are examples of accelerated motion. The statement that a stationary reference frame is equivalent to a moving reference frame only applies if both frames are unaccelerated. An accelerated reference frame cannot be treated as equivalent to a stationary, unaccelerated one.

I have calculated that myself, and the deviation from the equivalent unaccelerated reference frame was minimal, and you could (with minimal resources) change your speed to intercept Earth's real trajectory. If it was possible, that is.

Are we in agreement that this image illustrates Earth's and Mars' orbit around the sun, and that Earth's position in the solar system changes, because it travels through space at a certain speed (which is average 29 km/s)? The same way, our solar system orbits the galactic center at a speed of about 220 km/s. Earth itself is rotating, meaning that you, while standing on the equatorial line, move with about 1600 km/h.

My point is that when you're calculating speed you need a frame of reference that you consider stationary. If your frame of reference is fixed on Earth the speed of Earth would be exactly 0 km/h. Of course, such a frame of reference sucks, because it is rotating and accelerating, so people like to use frames of reference that aren't (called inertial frames of reference). They also suck, because you can't find one that says that the Earth is stationary. However, you can find one that says the Earth remains almost stationary for the next few days. Which should be enough if you're travelling two days back to cheat on the lottery. That is, if the discussion made any sense, and as Christopher pointed out, it doesn't.

 

[Christopher]

I have calculated that myself, and the deviation from the equivalent unaccelerated reference frame was minimal, and you could (with minimal resources) change your speed to intercept Earth's real trajectory. If it was possible, that is.

Well, then you're basically agreeing with me: that you wouldn't just automatically arrive in the same point on the Earth's surface that you left, a la H. G. Wells' time machine, but would have to have the means to aim and propel yourself through space in order to "catch up" with the Earth. In other words, you'd need either a spaceship or a steerable wormhole.

Then again, if you're travelling through a wormhole, it isn't an issue; you'd come out at the other end, wherever that may have been in the past. In theory, if you, say, created two ends of a wormhole in your lab, sent one travelling through space at 0.98c for 25 years, turned it around and brought it back at the same speed for another 25 years, then reinstalled the moving end in your lab, then you could step through that mouth of the wormhole and come out the other end in your lab 40 years earlier (since it moved for 50 years and was time-dilated by a factor of 5, so that only 10 years passed for it, and since the two mouths are stationary relative to their internal reference frame, they occupy the same point in time relative to each other). So that's a way you could move back in time and still be in the same place without having to go in a spaceship. Except for the spaceship you would've needed to time-dilate the wormhole mouth in the first place.

 

[Silvercrest]

Time travel is impossible as it contradicts reality. The temporal paradox is like dividing by zero.

^Now THAT'S a great addition to this thread. It really illustrates a constructive answer to the initial question and it's incorrect to boot.

Hey, give him a break. The Vulcan Science Directorate has determined that time travel is ... unfair.

 

[JarodRussell]

My point is that when you're calculating speed you need a frame of reference that you consider stationary. If your frame of reference is fixed on Earth the speed of Earth would be exactly 0 km/h. Of course, such a frame of reference sucks, because it is rotating and accelerating, so people like to use frames of reference that aren't (called inertial frames of reference). They also suck, because you can't find one that says that the Earth is stationary. However, you can find one that says the Earth remains almost stationary for the next few days. Which should be enough if you're travelling two days back to cheat on the lottery. That is, if the discussion made any sense, and as Christopher pointed out, it doesn't.

Yeah but you can combine all reference frames, can't you. Have a look at this video to see what I mean:
http://www.mogi-vice.com/Scaricamento/Luna%20eliogeocentrica.zip
It illustrates our Moon's orbit around the Sun. In an Earth centered frame, Moon's orbit is a circle, in the Sun centered frame it becomes a different path.

And the same way you can plot the path of the Moon in the galaxy, and in the Local Group, and eventually in the whole universe. And I doubt that when you plot it for the whole universe that the line suddenly becomes a single point that remains at the exact same position over time.

 

[Lindley]

In theory, if you, say, created two ends of a wormhole in your lab, sent one travelling through space at 0.98c for 25 years, turned it around and brought it back at the same speed for another 25 years, then reinstalled the moving end in your lab, then you could step through that mouth of the wormhole and come out the other end in your lab 40 years earlier (since it moved for 50 years and was time-dilated by a factor of 5, so that only 10 years passed for it, and since the two mouths are stationary relative to their internal reference frame, they occupy the same point in time relative to each other). So that's a way you could move back in time and still be in the same place without having to go in a spaceship. Except for the spaceship you would've needed to time-dilate the wormhole mouth in the first place.

I've read that concept before. The Temporal Protection Conjecture implies that any such wormhole would destabilize and collapse as soon as the two ends got close enough together that it would be possible to traverse it, then return to the first end and actually be at an earlier time.

 

[YellowSubmarine]

And the same way you can plot the path of the Moon in the galaxy, and in the Local Group, and eventually in the whole universe. And I doubt that when you plot it for the whole universe that the line suddenly becomes a single point that remains at the exact same position over time.

And you'd plot the path in reference to which part of the universe?

 

[JarodRussell]

And the same way you can plot the path of the Moon in the galaxy, and in the Local Group, and eventually in the whole universe. And I doubt that when you plot it for the whole universe that the line suddenly becomes a single point that remains at the exact same position over time.

And you'd plot the path in reference to which part of the universe?

I don't know. But the point is that there definately is a motion path through the universe, isn't it, no matter how you look at it?

 

[Lindley]

Yes....however, a choice of origin and axis is necessary in order to specify meaningful coordinates.

One could set the starting point as the origin, and the initial orientation of the traveler could define the axis. This would allow you to specify mathematically the path.

However, in practice, navigating such a path would be difficult because the coordinate system is not in reference to any external points, and thus it is difficult to tell whether you're getting off-course or not.

 

[JarodRussell]

Yes....however, a choice of origin and axis is necessary in order to specify meaningful coordinates.

One could set the starting point as the origin, and the initial orientation of the traveler could define the axis. This would allow you to specify mathematically the path.

However, in practice, navigating such a path would be difficult because the coordinate system is not in reference to any external points, and thus it is difficult to tell whether you're getting off-course or not.

See, isn't that exactly the point where mathematical concepts and reality diverge? A frame of reference is just a mathematical construct that helps humans to understand observations better. It's like the silly age old debate of centrifugal vs. centripetal force. There is a force, no matter how you name it and no matter what frame of reference you use. And in the universe, there is motion, no matter how you see it. Objects move. One second ago we all were at a different point in the universe than we are now.

That you can't calculate it because you don't know what frame of reference you need to choose is one of the issues why I think time travel would simply go wrong.