Straight Dope Message Board

[Wallaby]

One major (but always unmentioned) problem with time travel as shown in Sci-Fi is that it doesn't take into account spatial transposition. Example, if you only want to travel back in time one day, you also need to transpose yourself in space about 1.5 million miles - which is how far the earth travels in its orbit around the sun in 24 hours.

And the whole earth is spinning, the entire galaxy is rotating at some enormous speed (I feel a song coming on!). It would require ........... I don't know. Red Dwarf briefly addressed this, by finding a Time Machine, and going back to the time of the Renaissance - and enjoyed 'The heady atmosphere of 15th century deep space!'.

Has any writer made a serious stab at adressing how this problem could be solved?

[DavidwithanR]

Time travel is such a ridiculous topic in real-life terms, that ALL attempts to justify/solve it are necessarily half-hearted foolishness. Taking just a certain bit of it very seriously, only accidentally emphasizes how un-serious the writer really is. Serious writers who "do" time travel, know enough to just do the perfunctory hand-waving and get on with the story.

[DavidwithanR]

Missed edit window: Unless they intend to make some part of it into a plot point, of course.

[Helena]

I remember reading one story that used the ongoing expansion of the universe--everything was smaller in the past and bigger in the future.

[Gatopescado]

Pfft! I time travel damn near every day!

I drink a 12-pack of beer on Tuesday, and next thing I know, its Noon on Wednesday. No machine or spatial correction needed.

Except that one time I woke up in the little ditch in front of the house.

[Lucas Jackson]

Originally Posted by Helena

I remember reading one story that used the ongoing expansion of the universe--everything was smaller in the past and bigger in the future.

Ooooooo...

[Alessan]

Originally Posted by Wallaby

Has any writer made a serious stab at adressing how this problem could be solved?

Well, Doctor Who solved this by having the TARDIS capable of appearing at any point in time and space. It's up to you to decide how "serious" that solution is.

[Snowboarder Bo]

Originally Posted by Gatopescado

Pfft! I time travel damn near every day!

I drink a 12-pack of beer on Tuesday, and next thing I know, its Noon on Wednesday. No machine or spatial correction needed.

Except that one time I woke up in the little ditch in front of the house.

Perhaps you'd be interested in a Time Travel Face-bag™?

[Jonathan Chance]

Originally Posted by Wallaby

One major (but always unmentioned) problem with time travel as shown in Sci-Fi is that it doesn't take into account spatial transposition. Example, if you only want to travel back in time one day, you also need to transpose yourself in space about 1.5 million miles - which is how far the earth travels in its orbit around the sun in 24 hours.

And the whole earth is spinning, the entire galaxy is rotating at some enormous speed (I feel a song coming on!). It would require ........... I don't know. Red Dwarf briefly addressed this, by finding a Time Machine, and going back to the time of the Renaissance - and enjoyed 'The heady atmosphere of 15th century deep space!'.

Has any writer made a serious stab at adressing how this problem could be solved?

One of Spider Robinson's Callahan books had this as a minor plot point. The time traveler ended up in space because she failed to account for it.

[Shoeless]

It's been years since I read it and probably have some of the plot details wrong, but in Gregory Benford's "Timescape" I recall there are scientists in the future trying to send a message back to the past on a tachyon beam and they had to account for the position of past Earth at the time in order to aim the beam correctly.

[gdave]

Yeah, but your position relative to what? Spatial position isn't absolute.

If your time travel device puts you where the Earth *was* 1000 years ago, what is that relative to? The Sun? The center of the galaxy? R'lyeh?

[Alessan]

Originally Posted by gdave

Yeah, but your position relative to what? Spatial position isn't absolute.

If your time travel device puts you where the Earth *was* 1000 years ago, what is that relative to? The Sun? The center of the galaxy? R'lyeh?

Relative to where you are now?

[Just Asking Questions]

I thought we just had a thread about this. If only I could time travel....

Why is it that big a deal? The earth moves around the sun, and WE don't get left behind. Who is to say time travel doesn't follow along?

If you have to calculate the changing positions of Earth when you time travel, then a time machine is also a star ship. I want to travel .00000001 seconds into the future, but end up on Vulcan.

[RealityChuck]

Time and space are fused, so if you're traveling in time, you're traveling in space. It's trivial to synch the space you're currently in the one in the future or past.

[gdave]

Originally Posted by Alessan

Relative to where you are now?

But where is that? Again, there's no such thing as absolute spatial position. For ordinary, everyday purposes, I talk and think about my position relative to my visible surroundings. I am currently, for all ordinary intents and purposes stationary. I'm also stationary relative to my common center of gravity with the Earth. I'm not stationary relative to the Sun, or to the center of the Milky Way, or to a star in the Andromeda Galaxy. So which frame of reference do I use?

If my chair were a time machine, and I used it to travel back in time one day, why would that put me in the position Earth was in relative to the Sun one day ago? Why wouldn't I remain the same position relative to my desk? Or relative to my common center of gravity with the Earth? If my position did "move", what would it be moving relative to?

[E-DUB]

The OP was not looking to ignite a discussion of the topic here (interesting though that may be), but wishing to know if any writer has addressed it. Some specifically have, but most just handwave it, or say that the Earths fields drag you along it.

I do recall a comic book where a Kryptonian scientist experimented with time travel. His destination was a hundred years in the future. (It did not go well.)

[Andy L]

Originally Posted by Helena

I remember reading one story that used the ongoing expansion of the universe--everything was smaller in the past and bigger in the future.

"Time Locker" by Lewis Padgett?

[manson1972]

Originally Posted by gdave

If my chair were a time machine, and I used it to travel back in time one day, why would that put me in the position Earth was in relative to the Sun one day ago?

Because that's how time machines work.

[Czarcasm]

Originally Posted by RealityChuck

Time and space are fused, so if you're traveling in time, you're traveling in space. It's trivial to synch the space you're currently in the one in the future or past.

(bolding mine)They are?

[KneadToKnow]

I decided a long time ago that arguments about this topic miss the obvious: travelling through space is a ridiculously fundamental task compared to travelling through time. It's like arguing "everyone makes a big fuss over the pitching skill of an ace Major League pitcher but no one ever points out that his brain autonomously keeps his metabolism on track!"

[Czarcasm]

Originally Posted by RealityChuck

Time and space are fused, so if you're traveling in time, you're traveling in space. It's trivial to synch the space you're currently in the one in the future or past.

If they are fused in that way, would it work the other way around? Instead of instantaneous time travel, let us say there is instantaneous space travel. If you do not stay locked to your position in space when you time travel, would you stay locked to your position in time of you teleport to a distant galaxy?

[Andy L]

Originally Posted by Czarcasm

If they are fused in that way, would it work the other way around? Instead of instantaneous time travel, let us say there is instantaneous space travel. If you do not stay locked to your position in space when you time travel, would you stay locked to your position in time of you teleport to a distant galaxy?

No you wouldn't. Teleporting "instantaneously" to a distant galaxy, where (for convenience) instantaneousness is measured in your current reference frame would mean teleporting to the distant past or the distant future as measured by someone moving at walking speed towards or away from you when you teleported.

[Ignotus]

Originally Posted by RealityChuck

Time and space are fused, so if you're traveling in time, you're traveling in space. It's trivial to synch the space you're currently in the one in the future or past.

If time travel is teleportation through spacetime, you would have to deal with the problem of matter already present (or not present!) at the destination. You might find that local topography has changed over thousands or millions of years, enough to make you end up trapped inside a rock, or falling to your death into a rift which wasn't there at the start!

I believe this problem was actually mentioned briefly by H. G. Wells in The Time Machine; but his time traveler didn't come up with any solution, and had to rely on pure dumb luck.

Actually, I believe even collision with the air would have killed him, by forcing all sorts of chemical reactions and rupturing of cell membranes, DNA molecules and whatnot. Clearly, space is the place to do this sort of things, if you absolutely must!

[Little Nemo]

Originally Posted by gdave

Yeah, but your position relative to what? Spatial position isn't absolute.

If your time travel device puts you where the Earth *was* 1000 years ago, what is that relative to? The Sun? The center of the galaxy? R'lyeh?

Originally Posted by Alessan

Relative to where you are now?

He's making a valid point. What's the fixed point you're using to determine what's moving?

[DPRK]

There are no problems with time travel. If there are (were? might be?), you just go back in time to fix them so there was never any problem in the first place.

[Ignotus]

Originally Posted by DPRK

There are no problems with time travel. If there are (were? might be?), you just go back in time to fix them so there was never any problem in the first place.

See, that's the problem! Whenever a time travel device is invented, there's always these guys from the future who use it to go back in time and make sure it stays uninvented, simply out of safety concerns!

[Jasmine]

Time is a whole separate dimension from length, width, and height. I think the idea is that, spatially, the device isn't moving at all. It arrives in exactly the same geographical location from which it left. The only movement was in time, not space.

Thing is, a million years ago, that location could be under 1,000 feet of water because conditions were very different at that particular point in time.

[Czarcasm]

You are in a car travelling north at 60 mph and you time travel forward 1 hour-Where are you?

[silenus]

Certainly not on the 405 at rush hour, that's for sure!

[Just Asking Questions]

Originally Posted by Czarcasm

You are in a car travelling north at 60 mph and you time travel forward 1 hour-Where are you?

In my view of TT, exactly where you left. Like sending Einstein one minute into The Future at the Twin Pines Mall.

If you go to all the trouble to invent a time machine, and forget to compensate for the movement of the earth, (or it can't be compensated for), then you've just invented the world's most useless machine. Not much call to time travel to deep space. Even if you used it as a space ship rather than a time ship, it's a one-way trip. Your movement in the mean time would prohibit you from ever returning home again.

[Ignotus]

Originally Posted by Just Asking Questions

If you go to all the trouble to invent a time machine, and forget to compensate for the movement of the earth, (or it can't be compensated for), then you've just invented the world's most useless machine.

Hey, you could always use it to get rid of toxic or radioactive waste!

[Lemur866]

There are perfectly reasonable methods of traveling into the future. Time travel is only a problem when you travel into the past. The methods for traveling into the future are by traveling at a significant fraction of the speed of light. So to travel 4 years into the future, you just head for Alpha Centauri at 99.99999% the speed of light, and when you get there a few hours will have passed for you, but 4 years for everyone back on Earth. For another 4 years, just travel back to Earth. Now you're 8 years in the future! Congratulations.

The methods for traveling into the future don't involve magically teleporting into the future, they involve changing the speed at which time passes for various different observers. If you could travel 4 years into the future while staying on Earth, you wouldn't disappear and then magically appear 4 years later. You'd stay right where you are, but time would stand still for you. It would be more like a stasis field where time passed differently rather than a conventional time mechanism.

[Czarcasm]

Originally Posted by Just Asking Questions

In my view of TT, exactly where you left.

Going north at 60 mph without a car?

[Ignotus]

Originally Posted by Czarcasm

Going north at 60 mph without a car?

Another interesting point for writers of time travel/teleportation fiction to address. Is momentum conserved?

[Just Asking Questions]

Originally Posted by Ignotus

Hey, you could always use it to get rid of toxic or radioactive waste!

Smart!

I like it.

[Just Asking Questions]

Originally Posted by Czarcasm

Going north at 60 mph without a car?

If you mean, you, and only you time traveled, without a machine, in a speeding car, then yes. You'll be two feet above the ground, moving north at 60 mph. Hope you wore a helmet and leather pants.

But if you're going to posit it that way, you'll not only be car-less moving north at 60 mph two feet above the ground, but you'll be naked.

But, as Ignotus asked, is momentum conserved? I would suggest that momentum is compensated by the machine just as spatial movement must be. What good would a time machine be if you ended up popping into the past moving 1000 mph in the opposite direction?

If the proposed time machine can't compensate for the earth and sun moving, and can't compensate for momentum and inertia, then the first time someone uses it is the last. They either end up in space somewhere, or dead from slamming into something as soon as they arrive.

Maybe that's why there are no time travelers here. They're all dead somewhere in space, or embedded in a mountain somewhere.

Hmm...good short story idea. Space travelers find interstellar space is littered with time machines carrying dead time travelers.

[Pantastic]

Larry Niven wrote an essay in which he either originated or popularized the concept that if time travel to the past that changes history is possible, history will settle on a pattern where time travel is never invented. If it's possible to go back and change the past, then people will keep going back to 'fix' problems or just to investigate the past. This means that from the perspective of someone living in the world, history will keep changing in a constant blur, though they'll never notice it because it will keep having always been that way. "Eventually" (from the outside perspective, not from someone inside history) one of those changes will happen to create a world where time travel never gets invented. Once that happens no one can/will/did go back and change history, so it ends up that's the only history that will actually stick.

[terentii]

Time and space are no longer thought of as separate and absolute. There is only space-time. If you travel backward in space-time, you'd be retracing the path you followed in both space and time to get to the present. Since you didn't collide with anything on your way here, you wouldn't have to worry about matter getting in your path on the way back.

You also couldn't travel elsewhere in the Universe. So Doug and Tony couldn't possibly have materialized to meet OTT on Alpha Centauri.

What I want to know is: How far back can you go? To your own birth? Your conception? If you went back farther, would your atoms disperse to retrace their own individual paths through space-time?

[Folacin]

Originally Posted by Ignotus

Another interesting point for writers of time travel/teleportation fiction to address. Is momentum conserved?

Niven(?) had a story (possibly a series) with teleportation where it was conserved. Going up or down in altitude heated/cooled you. There was some mechanism (multi-hops? don't remember) to scrub off the momentum from moving around the glovbe (I recall some sort of chamber submerged in water that sloshed?).

[Horatius]

Originally Posted by Just Asking Questions

If you go to all the trouble to invent a time machine, and forget to compensate for the movement of the earth, (or it can't be compensated for), then you've just invented the world's most useless machine.

No, what I've invented is the cheapest way to launch spacecraft ever developed. Time travel (forward or back) however many minutes/hours it takes to get to the approximate altitude you want to be in, then use on-board rockets to stabilize your orbit. Circularizing the orbit is one of the easiest parts of space travel.

Not much call to time travel to deep space. Even if you used it as a space ship rather than a time ship, it's a one-way trip. Your movement in the mean time would prohibit you from ever returning home again.

Only if you travel too far, and forget to bring fuel with you. Where you end up is just a function of the movement of the Earth within the Solar System, the movement of the Solar System within the Galaxy, the overall movement of the Galaxy, and the duration of your time jump. Add up all the vectors and plot your arrival time and position as needed. Doesn't seem to hard.


And if you do it by going backwards in time, you know if you succeeded or not before you even launch!

[Lemur866]

But of course all this misses the point. There is no preferred reference frame.

So suppose your motion with respect to the Earth's spin is not conserved.

And your motion with respect to the Earth's orbit around the Sun is not conserved.

And your motion with respect to the Sun's motion through the Galaxy is not conserved.

And your motion with respect to the Milky Way's motion through the observable universe is not conserved.

And this means...nothing. Because we can say that the Earth is screaming through the Universe at the blistering speed of 99.9999999% of the speed of light, it's just that the galaxy and the local cluster and the rest of the observable universe are all co-moving with the Earth.

So when you teleport forward in time and find yourself 100 billion light years away, now you've found your preferred reference frame. And now we can use Einstein's spinning motion in his grave as a perpetual motion machine for free energy.

[Czarcasm]

I'm beginning to believe that time travel will be the Last Solved Problem.

[Darren Garrison]

Originally Posted by Czarcasm

If they are fused in that way, would it work the other way around? Instead of instantaneous time travel, let us say there is instantaneous space travel. If you do not stay locked to your position in space when you time travel, would you stay locked to your position in time of you teleport to a distant galaxy?

FTL travel is time travel.

[DPRK]

Note that in any actual travel (not facilitated by wizards), whether you call it space travel, time travel, or space-time travel, you follow a particular path, which could be written down in terms of coordinates. Whether or not there are any planets there where you arrive and how fast you are going when you get there depends on the path you take, and can be predicted given that information.

[Ignotus]

Originally Posted by Just Asking Questions

Smart!

I like it.

Of course, this presupposes that manufacturing and operating the time machine doesn't produce equal or greater amounts of waste...

[DSYoungEsq]

Originally Posted by gdave

Yeah, but your position relative to what? Spatial position isn't absolute.

If your time travel device puts you where the Earth *was* 1000 years ago, what is that relative to? The Sun? The center of the galaxy? R'lyeh?

Originally Posted by Alessan

Relative to where you are now?

Did someone fail to understand relativity?

[Lemur866]

Originally Posted by DPRK

Note that in any actual travel (not facilitated by wizards), whether you call it space travel, time travel, or space-time travel, you follow a particular path, which could be written down in terms of coordinates. Whether or not there are any planets there where you arrive and how fast you are going when you get there depends on the path you take, and can be predicted given that information.

And this is the actual answer to the question.

Fictional time machines typically work by having you disappear from one space-time coordinate and re-appear in another space-time coordinate.

But that's not how real things work. Instead things move from one space-time coordinate to another space-time coordinate.

Yeah, yeah, quantum teleportation. Except this only physically happens at scales that we scientists call "really really small". If you've figured out a way to make this effect happen at macroscopic scales, then we're in a land of wind and ghosts and there's no telling what would "really happen", because nothing we know about how the universe works will apply any more. Up is down, left is right, next Tuesday is 100 miles away, 1+1=blue, and the speed of light is ice cream.

[Ignotus]

Originally Posted by Horatius

No, what I've invented is the cheapest way to launch spacecraft ever developed. Time travel (forward or back) however many minutes/hours it takes to get to the approximate altitude you want to be in, then use on-board rockets to stabilize your orbit. Circularizing the orbit is one of the easiest parts of space travel.





Only if you travel too far, and forget to bring fuel with you. Where you end up is just a function of the movement of the Earth within the Solar System, the movement of the Solar System within the Galaxy, the overall movement of the Galaxy, and the duration of your time jump. Add up all the vectors and plot your arrival time and position as needed. Doesn't seem to hard.


And if you do it by going backwards in time, you know if you succeeded or not before you even launch!

I believe the point is that if you have no initial idea of the point of destination, it would be very hard to collect data as to how it is related to the settings of your spacetime transportator. You could send out a million probes, all equipped with radio beacons and systems to power them, and still not hear from a single one of them in millennia..!

[Enola Straight]

https://boards.straightdope.com/sdmb...ghlight=street

[Grestarian]

Originally Posted by Czarcasm

Going north at 60 mph without a car?

Uhh...yeah... I kinda did that a couple years back on Park as it approaches 7th.
I was on (well, okay, OFF) the motorcycle at the time.
Were you there? Did you notice me wave (well, okay, flail) as I passed you?
Sorry about the half-hour delay -- it was the mini-van driver's fault!

--G!