# How can I explain that a time travelling apparatus moves itself through time but appears in the same location?

For the purposes of this question, let's ignore the causality issues of time travel itself.

Suppose our hero, or an accomplice of our hero, has invented a time travelling apparatus. It works like this: a person steps into a chamber, sets some controls, and initiates a jump through time. The apparatus then transports itself and anything inside it to the designated time. It is powered by a set of handwavium-unobtainium-based power cells that provide enough power to sustain a small number of time jumps between recharge cycles.

To keep this from becoming a too-easy deus ex machina, I want this apparatus to be able to transport through time but to be forced to consistently appear in the same location on Earth at the target time. That is, if you start at Brandenburger Tor, Berlin, Germany on midnight, New Year, and go half a year plus twelve hours into the past, you end up at Brandenburger Tor, Berlin, Germany at noon in the height of summer. In other words, it somehow works in such a way that we can (and indeed are forced to) disregard the Earth's own rotation, the Earth's orbital movement around the Sun, the Sun's orbital movement in the Milky Way, and so on, but we cannot instruct it to move geographically to another location as part of the time jump.

There is no need for the apparatus to function at all further away from the surface of the Earth than some small distance, say 1 km or so (to allow for high-rise buildings).

How could I possibly explain the fact that the apparatus after transporting through time appears in the same geographical location, as explained above?

The explanation absolutely does not have to meet strict scientific criteria (we're throwing those out already with time travel in the first place), but it'd be nice if it doesn't break suspension of disbelief too badly.

The time jumps will be on the order of hundreds of years or less, so geological stability should not be a major concern and might even provide an interesting obstruction to our hero. (Thanks Pavel Janicek for mentioning this issue.)

• BTW, double thinking about it: You might want to change behavior of your aparatus: Ending on same location (even just geographical) could be really risky if travelling more than 100 000 years in time – Pavel Janicek Mar 23 '15 at 12:49
• @PavelJanicek That's a good point. However in what I have in mind, jumps will be on the order of hundreds of years or less, and generally to the past of the invention of the apparatus, so I don't think that'd be a major issue; under such circumstances it should be practical to choose sites that are stable over the necessary period of time, and it provides another slight inconvenience (having to choose an appropriate site; can't just go where the machine happens to be). – a CVn Mar 23 '15 at 12:51
• If the apparatus remains in the exact same location in space time it will be spending most of its time in space! I am assuming however that you mean for it to orbit the Earth, otherwise every 365 days it will ram through our planet (or rather our planet will run into it) and it will be destroyed. You specified that it disregards earths rotation......but my idea might make for a good story :) – JDSweetBeat Mar 23 '15 at 14:29
• What happens if you send the apparatus back 1 day. And it occupies the same space it did .. yesterday .. O.o Just curious how you want things to behave when the device would occupy the same space as something else ? – Ditto Mar 23 '15 at 15:05
• Of course, if we posit sufficiently advanced technology, it could just scan "ahead" and refuse to stop at the particular requested time if doing so would involve moving anything dense out of the way... – a CVn Mar 23 '15 at 18:42

The time travel machine could be tied to the Earth and its gravity. So as it moves forward/backwards in time it also follows the rotation of the Earth (and all other movement of the Universe). Simply put the gravity of the Earth ensures that the time machine ends up at the same place as before. The Earth basically pulls along the machine as the machine travels through time.

This might need some more thought put into it, but this is the basics of the idea that sprung into mind.

• Note that the machine would also have to keep its momentum, so that it remains on the same spot on the earth’s surface. A more serious problem would however be that nothing would prevent the time machine from sinking to Earth’s core – being attracted by gravity but not being held back by the ground. – Wrzlprmft Mar 23 '15 at 15:50
• You could calibrate it for large, immovable, dispersed landmarks as long as we're not talking about geological timescales. Some handwavium mechanism knows the distance from the machine to Uluru, Mount Narodnaya, and Aconcagua. You could even handwave over what those points are. Better yet, your machine could use the moon as a reference point. It's drift is well known and could place you in a calculated position for eons. – Clinton Pierce Mar 23 '15 at 19:08
• Calibrated by the Earth's magnetic field, perhaps. – A E Mar 23 '15 at 22:46
• @Wrzlprmft This is the mechanism used by the vehicle in HG Wells's The Time Machine. The particles in the vehicle still interact with the particles in the ground at any given moment. – Damian Yerrick Mar 23 '15 at 23:25
• I agree to locking to gravity: When releasing power, the handwavium-crystals lock themselves right onto the nearest gravitational field configuration. When that happens, NOTHING can move them from their gravitational spot, so they keep anything attached to them (like the time machine) in position as well. Only when the crystal powers down again, the machine can once more be moved. (Note: use "gravitational field configuration" to avid falling down, the configuration is 3-dimensional... because!) – Layna Mar 24 '15 at 11:06

What if the constraint is that the machine can only navigate to a time that it's been in....bear with me here, there's more to it.

Now obviously the passengers haven't been to the time in question, so clearly not all of the machine has to have been to the time in question.

Thus this constraint boils down "the machine travels through time following the time path of a core component".

Similarly the core component was something that your protagonist built, so being aware of the constraint the component would have been designed to key off something with a known timeline inserted in it.

What you now have is a time navigation engine that allows you to move relative to any item inserted into it.

So you chip off a piece of masonry from the Brandenburger Tor, insert the chunk into the navigation engine and now can move through time with the same relative location to the Brandenburger Tor for as long as that chip was part of it.

Try to move to before the chip existed and you drop out of the time stream requiring you to scavenge around for another item you can reference from.

This allows you to easily make short jumps, longer jumps require vandalising historical monuments and really long jumps become downright scary if you key off something like a cobblestone from the road that could still have been in it's current form but on a river bed thousands of years ago...

• I like that very much – Solmead Mar 24 '15 at 20:38
• I also like this. The key is that navigation through space time is relative to a known world line, rather than attempting to perform absolute navigation through a complex curved space. This allows the control system to be a simple feedback system rather than some improbably complex absolute navigation through an unknown space. Perhaps rather than just using a "chunk of masonry", one might follow a specifically designed beacon or set of beacons. – Keith Mar 25 '15 at 4:02
• The movie "Primers" uses this concept, but the artifact to which everything is relative is a cabinet which the person travels inside, so as the person travels through time the person would effectively travel through space along exactly the opposite path taken by the cabinet (though the person could only exit the cabinet at a time which had been selected earlier in an "outside" timeline; presumably if something happened to the cabinet between the time it was prepared and the time someone would have entered, the person would have been unable to enter. – supercat Aug 11 '16 at 18:23
• This is a brilliant idea worthy of a worldbuilding forum, because it not just explains the restriction, it adds so much storytelling potential that I can see half a movie in my mind already. Wonderful! – Tom Sep 30 '16 at 13:48

"You may have noticed that my time machine never leaves Berlin. That is one of my favorite features! It wasn't easy to make it work like that, but I finally figured it out." The aging professor smiled at his guests, obviously expecting their praise.

Confused, his guests gaped back at him mutely.

"I don't understand, Doc." Little Sally broke the tense silence. "Your machine can jump across the centuries, but it can't take us up to Hamburg. Why is that a good thing?"

"Wait till you've grown up a little, My Dear." He patted the child's shoulder fondly. "Someday you will learn that globe-trotting is overrated. Dorothy was right all along. There is no place like home."

• I like this. It reminds me of John Scalzi's "Old Man's War" where the only explanation of high tech was why it would not work either before or while using it. Much fun. – hildred Mar 24 '15 at 16:59
• In a flashback to Asimov's The Dead Past, the best use of a time machine that can move locations is as a instant transport craft. Set the location to be 10 seconds from now in Paris and - instant Airliner! – Oldcat Mar 24 '15 at 18:09

Very similar to Xathien's Answer, but more explicit.

After going to bed at night and waking up in the morning, you are not surprised to find yourself in the same location, even though you have travelled roughly 8 hours into the future. Sure it took you 8 hours to get there, but what do you want for nothing?

Along the same lines, when your machine travels 20 years into the past, it actually passes through every moment in between now and then, it just does it quickly. The same laws of momentum apply as you would expect had you simply built a box, waited 20 years, and played a film of it in reverse at high speed.

Perhaps there is even some previously unknown sense that all creatures have which makes them unconsciously avoid the space your machine takes in transit. A bare patch of ground where nothing grows, concrete poured beneath it but all the machinery involved just off to the side, interior room space for a while, but the building was destroyed in a fire or war, etc.

• But if that happens, wouldn't you be able to see yourself in it before you go in? And then you have to wait until the you in the machine (which is moving backwards in time) and the you outside are the exact same age to enter... It seems problematic. – sumelic Mar 24 '15 at 6:50
• Good thought, but I don't think so. The inside of the machine exists in the subjective now. You could enter at any point that the device is visible, though it would still be possible to run into yourself. While the machine occupies every moment during the travel, it does it so quickly that it isn't visible. If you put your hand through its space, maybe you'd feel a tingle. I don't know. I think you'd just not want to. – IchabodE Mar 24 '15 at 16:06
• This wouldn't seem to work, because since the travel device is moving through every moment in time, it would exist in every moment in time. Just because it is moving quickly wouldn't change that fact that is exists for all time. – beattyac Mar 24 '15 at 16:51
• @beattyac That goes back to the sleeper taking 8 hours to travel 8 hours into the future. The device does exist in every moment, but it doesn't take nearly a full moment to be there. Oh, and yes. I do watch too much Doctor Who. – IchabodE Mar 24 '15 at 17:01

From a conservation of energy point-of-view, you don't need to explain why it appears in the same location.

Any movement within Earth's gravity well requires a change in energy. Regardless of the time aspect, any geographical movement requires an energy change - firstly energy is put in to accelerate the device, and then to decelerate it again energy must be taken out.

Up or down is easy to visualise a change in energy, so consider this: Imagine the device is on the trailing edge of the Earth, and it jumps a moment into the future. An Earth-bound observer would see it disappear, and then re-appear a moment later. If it re-appeared at the exact same point in space (relative to the sun, for ease of visualisation), The Earth would have moved on and the device would appear to have gained altitude, now "trailing" above the Earth. It has therefore gained gravitational potential energy. From where did that new potential energy come? Going into orbit takes shedloads of energy, so this device can't simply be used to bypass that - there's no such thing as a free lunch!

Similarly, if it were in orbit on the leading side of Earth, and jumped momentarily into the future, it would now be closer to Earth, and would need to shed that gravitational potential energy that it just lost. That energy could be converted it to heat (yikes!), or as gained velocity, so now it's hot and suddenly hurtling through space, towards the Earth!

The only sensible solution when traveling through time then is for this device to maintain a constant position relative to the Earth's gravitional well (actually the whole universe's, which is obviously dominated by Earth's when near Earth).

If it were moving, things get slightly more complicated, but not by much: A jump into the future would cause the device to appear in the place where it would have been, had it not 'jumped'. Again from a conservation of energy (and momentum) point-of-view, the device couldn't instantaneously decelerate to a stop a the instant of jumping, so that it could reappear at the same place. If the device were travelling 100mph and jumped forward one hour, it would re-appear 100 miles away. It must be treated as if it never jumped during that hour. (Now you have a plot device to travel through walls - travel at the wall, and jump seconds into the future. Now you're through the wall!

• Interesting. Can we really be sure that conservation of energy applies when we're dealing with time travel? – sumelic Mar 24 '15 at 6:46
• Time jumping is very different from time travel. If you time traveled several seconds into the future, you would still hit the wall because you still have to pass through each moment in time as well as each three dimensional point in geographical space in your chosen path. Remote jumps are more like time teleportation, which could be an interesting concept, but it's not what the OP specified. – Aaron Mahan Mar 24 '15 at 23:24
• Another interesting point is that since time travel (by definition) occurs at some rate - say it takes 1 second to travel forward in time 1 hour. Then if you traveled at 100 mph, then relatively speaking, from the perspective of the traveler, would (s)he not experience a speed of 100 mi / sec? Or does the time travel aspect offset the relative speed by a factor of 1/3600? Or does the 1/3600 factor only apply to an outside observer? – Aaron Mahan Mar 24 '15 at 23:32

So there are two issues:

1) Orbital movement.

You can actually ignore this. We usually model the Earth as moving through space because it makes sense and the math is easier, but there's no such thing as "objective" spacetime or movement. It is perfectly valid to state that for your purposes the Time Machine is the center of the universe, and that the Earth doesn't move at all.

2) Rotation.

Unfortunately we can't ignore the Earth's rotation. So what you could do instead is force the device to only work in 1-day increments (down to Planck Time), so that when you go back you're in the same physical location. This would be a restriction your inventor places on the device though, rather than a fundamental requirement. If you wanted to you could transfer to any other longitude on your latitude, it's just that you'd often end up underground or up in the air.

• Well, the earth's movement around the sun, though it is less non-inertial than its rotation, is still non-inertial and therefore cannot be used as the "center of the universe". The same goes for the sun's movement around the galactic center, and the galactic center's (very slow) "fall" towards Andromeda. – k_g Mar 23 '15 at 22:27
• Nope, the orbital movement around the sun is the big point. According to relativity, you may treat any inertial frame as equal to any other and "the center of the universe", however, all inertial frames that intersect with your Time Machine now would very quickly get very far from the surface of the Earth. The closest you can get is a tangent of Earth's orbit, going out in a straight line as if the sun wasn't there and wasn't pulling you towards it. – Peteris Mar 24 '15 at 17:57

To expand on the existing answers, in particular Niffler’s, I suggest the machine to adhere to the following rules:

• When travelling forward through time, it keeps its existing momentum. For example, if we ignore orbital motion for a second and the time machine is on a train and travelles 30 s into the future, it arrives at almost exactly the same spot where it would have been at the same time, if it had not travelled through time. When the machine travels backward through time, the momentum gets reversed in analogy.
• When travelling through time, the machine is affected by gravity like a regular object but not by the other forces (because it would interact with other objects in a noticeable and possibly harmful manner otherwise). Thus, in the above example, the machine would actually start falling down since it is not hold back by the ground anymore (which is actually not what you want, I will address this in a minute).

The above rules sum up the main forces that apply to planets, stars and similar. So, if you built your time machine around the whole earth, it would do exactly what you wish. However, as your time machine is not that big, it would start falling through the earth as soon as it begins time travelling, go through the core with high speed and emerge on the other side, slowing down such that it reaches its maximum height just at the surface (not accounting for geographical height), and then repeats all over in an oscillatory way (see picture below, also see gravity train). Apart from that it would follow the Earth’s motion through space, as it is attracted by the Sun, the Milky Way and other celestial bodies just the same way as the earth.

Now, here are some ideas of addressing the issue of falling through the earth:

• The machine is in a geostationary orbit. Here only gravity is needed to keep it in place. Obviously, this would require all the trouble of getting into and out of a geostationary orbit.
• The machine is capable of weakening the influence it experiences by gravitation such that it can make its starting point a geostationary orbit. Note that this would require a selective weakening of the Earth’s gravitational forces, as the time machine needs to experience the same gravitational forces from the sun to follow the earths orbit. This in turn can be achieved by making this feature directional, i.e., the machine attenuates the gravitation it experiences from the direction of the Earth but keeps everything else the same (this in turn requires a continuous adjustment due to the constant change of the Earth’s and Sun’s relative position with respect to the time machine).
• The machine only stops travelling through time, when its at the top of its oscillation through the Earth’s core. It may arrive anywhere though. Alternatively, you handwave the Earth’s rotational motion away and say that the machine always oscillates between the same two points. In this case it can emerge exactly where it started (and on the other side of the planet, ignoring geographical height), but only every 84 minutes (when it’s at the height of its oscillation).
• At first I thought you were wrong about the machine ending up on opposite sides of the Earth, in traditional orbits the apogee occurs at only one point; however, inside the Earth gravity goes like $r$, not $1/r^2$, and the orbits in such a potential are, interestingly, ellipses with their centers (not foci) at the center of mass. Note that the period is the same as a surface-skimming orbit (84 minutes) so you'd end up 21 degrees more westerly after every "interior orbit." (Or after 42 minutes you'd be 160 degrees more easterly, in the opposite north/south hemisphere.) – 2012rcampion Mar 23 '15 at 23:04

One approach would be to have a device whose exterior travels linearly through time in the same fashion as anything else, but which allows items to enter at one time and leave at another. Under such a scenario, if someone enters a time machine at one location and transports himself ten years into the future, and during that time someone else moved the time machine to a different location, then the time traveler would find himself in the latter location. This approach would allow travel forward or backward in time, but only to places the machine either had been or would be.

The movie Primer used this concept and took it a little further with a time machine that only transported things back in time, and required the travel parameters be set "in advance" (if I remember correctly, transporting from midnight Jan 1 2014 to midnight Jan 1 2013 would have required that the machine be configured, prior to Jan 1 2013, so that at Jan 1 2013 it would contain whatever was in it on Jan 1 2014).

Note that either the restrictive approach from Primer or a more general approach given above would avoid some time-machine paradoxes surrounding "bootstrapping", since there's no way that time travel under such rules could be used to facilitate the construction of the first time machine (since such construction would represent the earliest time to which anyone could return).

The device latches onto the Earth magnetic field and uses quantum entanglement with nearby particles as buoys - more info - still more info.

So the device travels through time by locking onto the magnetic field of the nearby area, and by seeding the area with quantum locked particles.

As it moves through time, it follows Earth because it is pinned to the magnetic field, and burns the locked particles to do measurements regularly. Once it runs out of quantum locked particles, it has to exit time travel. Also the amount of particles required increases exponentially with the distance in the T dimension.

It could add to the story, because seeding these quantum locked particles (it seeds one particle of the pair, and keeps the other) costs a great deal of energy, so the farther the jump, the more particles it need. Also since the quantum locked particles share properties, they also travel to the past around the machine.

If the enemies can find the location of the machine, they could jeopardize the jump by undoing the locked status on the seeded particles, cutting the jump short.

This also explains why it can only jump a century or so at a time: the magnetic field shifts slowly, but do shift, and the whole battery would have to be used in order to jump farther.

• You link to “how entanglement works” but what you describe is nothing like what entanglement can acheive. How does an entangled particle act as a buoy? Why does flux pinning work while time travelling, when the normal way of the ground supporting an object doesn’t? – JDługosz Jan 18 '17 at 5:00
• So you should have linked to a tropes page, not real details that refute the answer. – JDługosz Jan 18 '17 at 15:55
• @JDługosz here is a link that I hope can satisfy you. - Now can we move along? – Mindwin Jan 19 '17 at 12:45

Sidestep the issue:

The device could be some kind of field generator - that way, it wouldn't even need to move in time, never mind space - just enter the field and exit at a different time.

There's even been actual science talking about this possibility - though you're limited to when the machine was powered on. Which could cause tension for a forward moving traveller - what if the machine broke after they'd arrived in the future. (No such worries going backwards - you know the machine was powered all the back - unless you stuff up the timeline somehow).

From what I remember about the theory for this device - it would use magnetic fields to bend light waves round into a circle with a diameter smaller than the wavelength of light. It would somehow twist space-time so you could "walk" along time within the circle. You can travel as far back or forward as the device has power.

• I really like this answer, and it makes more sense to me than the "gravity" method. But this answer is a bit vague; could you flesh it out a little more? The math is probably too complicated, but maybe just a description of the macroscopic features of this sort of field would be really cool. – sumelic Mar 24 '15 at 7:02

Question as posed at time of answer: How can I explain that a time travelling apparatus moves itself through time but appears in the same location?

Example: if you start at Brandenburger Tor, Berlin, Germany on midnight, New Year, and go half a year plus twelve hours into the future, you end up at Brandenburger Tor, Berlin, Germany at noon in the height of summer.

Simplest answer: Explain that the apparatus stands still (i.e. is as permanently fixed as the Brandenburger Tor itself).

While this simplified answer may be a little different from what the questioner intended, consideration of this rather simple method of time travel while retaining constant position with respect to a position on the Earth's surface should not be ignored from some consideration. Perhaps, at the very least, it has some component that is permanently installed like a building, and the machines simply moves into [temporal] alignment with that component that can open to reveal the traveling compartment, a little like an elevator moving through a "permanent" elevator shaft with doors on each floor, which is never in all of those spatial positions at the same moment (just like your machine isn't at all times with range simultaneously, as the Tor is).

See also T.A. Barron's book The Ancient One for another description of a tree-based time machine with some analogy to what you're looking for.

• How does this allow that "The time jumps will be on the order of hundreds of years or less"? – a CVn Mar 24 '15 at 14:34
• For The Ancient One answer, it's easily explainable by the lifespan of the tree. For the elevator example, that's just the range of possible values...just like a physical elevator stops at intervals that are maybe 10-30' apart within a shaft limited by the height and depth of the building that was built, the "time elevator" might similarly have only certain ranges between stops or maximum range. Even "permanent" buildings that humans make often have a lifespan of hundreds of years or less. – WBT Mar 24 '15 at 14:40

My first inclination is somewhat akin to Mindwin's answer, but with an explanation that might be a bit easier to grasp for the layperson:

The idea goes: Travelling through time is actually relatively easy, if you don't mind ending up anywhere in the universe when you get there. Controlling where you end up is actually the difficult thing, and due to Relativity, there's really no such thing as an absolute position, so the only way to make sure you can do that consistently is by "anchoring" your relative spatial position and orientation to surrounding (non time-travelling) masses so that you always end up in the same position and orientation to them as you started.

An obvious way to do this would be to design the device such that it is intrinsically tied to all surrounding particles, but the anchor-strength decreases with distance (perhaps an inverse-square relationship, similar to EM/gravity). Thus all particles in the entire planet would participate in the positioning, and the end result would be a consistent position/orientation always relative to the position on Earth, regardless of how the Earth has moved over time.

Arguably, you might run into trouble with this due to the substantial motion of convection currents under the Earth's surface meaning that a substantial amount of the Earth's mass doesn't really stay static over time, though (it would definitely be easier on a planet without a molten core). An alternative to avoid that would be to have it anchored to certain key masses carefully chosen by its creator(s) (for example, several spots of the Earth's mantle chosen from particularly geologically stable areas). (This could also potentially be an explanation for why the machine might need to be in a particular place on the Earth to work properly, if that's useful to the story)

If you need one, this could also be an explanation for limited temporal range: The anchored masses inevitably will move a bit over time relative to each other, so the positioning mechanism would need to account for that with some fuzziness/averaging/etc over all the associated subatomic particles. Beyond a certain time, the discrepancies might become too much to manage and it would not be able to consistently resolve a landing position due to "drift" of the anchor material within the planet.

Poor idea which jumped out right now and I have to put it down:

The time machine actually works only on time scale of our space-time continuum. Which means that while your engine bends time, it does not bend any space.

But hard thing is, that in that case you would end up somewhere else because of all the rotation of our belowed planet Earth.

So, you could explain it by loads of time-travelling cycles: The machine itself makes sure it ends up on the same place by travelling small amount of time in every step. Say, every millisecond, you travel 10 milliseconds in time. So in 100 milliseconds, you travelled one second in 100 time travel jumps.

EDIT Idea of my second though is to adjust your position in every micro-time-jump. So, you do move in space, but in all, you moved about one millimeter, so you end up on almost same place as you started. If the time machine is relatively huge, then one millimeter in space can be considered as same place

• Interesting idea, but wouldn't the accumulated space shift in the end be very near the same as it would be for one large (long?) jump? – a CVn Mar 23 '15 at 12:41
• I worded down my second thought badly. Will update also answer, but the idea is: Jump. Adjust in space and jump again. Repeat as many times as neeeded in as many very little jumps as possible – Pavel Janicek Mar 23 '15 at 12:43
• You may want to see my hopefully-clarifying-only edit as well. – a CVn Mar 23 '15 at 12:45
• If you adjust the movement of the timemachine to fit the movement of the earth, this would fix the problem. No need to assume that 1 mm is small enough that you don't need to move it (which you still will). – Clearer Mar 24 '15 at 9:22

Let me tell you a bit from the secret technicians manual of your device, but don't tell anyone, I am not supposed to do that.

Since Einstein we know that everything is tied to gather, we call it space time. Maybe from some vacation space trips you also know that behind a black holes apparent horizon time and space coordinates are swapped.

Anyways, your device basically works the following way (for simplicity we assume only one time dimension). It scans it surroundings and determines the properties of a sample set of 4 dimensional space time coordinates around it. Then it establishes a 4 dimensional sphere of the size of time distance you want to travel, intersects it with the destination direction and on the remaining circle it searches for the best matching space properties. It does that for each sample, and in the end choses the sample set with the best match and performs a space time folding into that set.

The reason here is to save energy; for each of those points the energy required grows in a complex but more than linear way. This all has multiple consequences (read: plot devices):

• For smaller time differences, the space difference can be bigger, meaning you could end up at a fairly different location that just looks alike (e.g. a chinese replication of the brandenburger tor; those things grew fairly popular after they copied a whole schwarzwald village). Check your lock accuracy settings.
• For bigger time differences the space differences should not be too big, otherwise it would fail to acquire a lock. This is why you can adjust the space reading parameter; it will require more energy (i.e. less subsequent jumps with one charge) but allows the geological properties to dominate, instead of the man made objects around you.
• In the end it imposes a limit on the time you can jump; depending on the edition you have (each year they are more energy efficient) you could jump only a couple of hundred years, or even millenia. Jumping back to the dinosaurs would probably require you to scan the whole earth (I doubt that even the newest editions have charge left to do the jump back though).

To expand somewhat on Niffler's answer: What if we had our apparatus obey more than just gravity?

Let's assume that time is a 4th dimension that our apparatus can now travel through without deconstructing itself. The earth is rotating the opposite direction, but because we still have 3-dimensional forces acting on us, we stick to the earth, conserve momentum, etc. Thus, as we move backward in time, we simply let gravity and friction carry us around in the same place.

This presents us with a few interesting problems -- what happens when we run into things that were already there? Since we're traveling 4th-dimensionally (and a little bit in the other 3 dimensions), we could say that we'll actually collide with things. The faster we travel, the harder we would hit objects. This could be taken into account in the construction of the device -- a super-strong hull that would let it plow through trees in the forest that used to be in this location, or the resiliency that would let it be deflected slightly. If we want to further enforce the no-movement rule, perhaps the device needs to be anchored deeply into bedrock that is known to exist in roughly the same place/state for the past 100 years, for safety. Otherwise we risk self-destruction.

Not to mention, we're pushing existing air out of our way wherever we go, so our device's design should account for some extreme heat. A physicist could likely come up with two dozen types of radiation you'd encounter because of this, too.

These problems could provide some interesting narrative. If you travel too fast, you may damage your craft, or maybe you've bounced around over the course of a few kilometers (and a hundred years). Maybe the navigator has time-based maps and knows when they need to slow down their 4th-dimensional travel to avoid the rapids, as it were. Perhaps when they arrive at their destination, they cause a grass wildfire because of all the heat they've picked up. Things like these could provide some realistic considerations for your hero and/or sidekick while keeping them rooted to (roughly) the same spot.

You could use the same-time principal. Essentially everything that has ever happened or will ever happen is happening NOW. Since some of us are not given to a full understanding of this, you would need to preface your description of the timeline. What we conceive of us the past, the present, and the future is only based on our "view" of the timeline right NOW. Your time machine would be able to calibrate its position in the NOW and in the NOW your want to visit. Since it is physically located where it is, it can see all of the activities of every-NOW for its location.

So if the user of the time machine moved from their NOW to another NOW, let's use the forest idea, the time machine would alter its view of the NOW to the NOW the user requested and could alert the user that the new NOW would place them inside a tree and that the user should select a safer NOW, or the machine could give a warning that the user can choose to override and force the machine to follow the NOW of the request regardless of personal/property damage.

This approach could provide some second tier direction for your characters as mentioned earlier, or provide some humorous outcomes as mentioned earlier.

However you write it, according to this principal, you are writing it NOW, I am reading it NOW, and I am discussing the reading of it with my book club NOW....Good Book by the way.

Have fun NOW!

Honestly, if the concern is people getting caught up in facts, why not take a relative approach. Say that while the machine travels through time it keeps its position relative to some ambiguous constant, such as the 'center' of the earth. Because it's relative, this negates anything that effects both the planet and the machine as far as displacement in space goes. By leaving 'center of the earth' ambiguous, you also don't over-explain, preventing prospective details which are immediately disputable.

edit: To further explain, anything situated relatively keeps it's position and state to the object it is related with in that manner. Therefore, any force that acts on both equally is negated. By binding it to an ambiguous location (such as the 'center' of Earth, as mentioned above), as opposed to a force or absolute location(s) you avoid concerns such as deterioration, natural disasters shifting mountains (as used in another example), and other external stimuli. So long as you, of course, presume that its ability to stay put relative to whatever the fixed point is is stronger than any external forces acting directly on it whilst it travels.

• This sounds quite a bit like the suggestion of anchoring the machine to the Earth's gravity, except that this answer explains it less. We like answers to add something substantive that has not already been suggested; can you perhaps edit your answer so that it does so? Thanks, and welcome to Worldbuilding. – a CVn Mar 23 '15 at 20:14
• I suppose they're similar in that they're both relative to something, thus negating outside forces that act on both equally, so in a functional sense yes. I think the perception giving by anchoring it to an ambiguous "place" (i.e. "center of earth" or some other significant landmark) may be easier to interpret then anchoring it to a force such as gravity, though. I suppose the audience that this is directed at could make a substantial difference. – Michael Mar 24 '15 at 13:10
• I'll admit that I saw this question while at work and didn't have a ton of time to elaborate, I made an account just to throw in the "relative" idea as I didn't notice it. My apologies if I've broken rules but I don't really have anything more to add, my answer is fairly simple unless "relative" and how that negates forces acting on both sides of the relationship needs explaining. I can delete if that's preferred, I'm not sure what the conventions are here. – Michael Mar 24 '15 at 13:17

Lets make a time travel machine that's limited to a time road. It can no more go cross country than a train can. You build the time "road"--it's just a platform to anyone observing it from a spacelike dimension but a timelike observer sees the road.

Note that travel is limited to the time in which the machine exists (both forward and backwards.) Off the top of my head I come up with Asimov's The End of Eternity and Robert Forward's Timemaster. (The latter is especially interesting as he was a hard SF guy--there's a couple of handwaves to set the whole thing up but otherwise he stays solidly in the science realm other than the biology of the aliens--and his biology never was up to his physics) as fictional examples of what I'm talking about.

There was also a time communicator (a road for a few particles) in the Cheela novels (Dragon's Egg/Starquake) but it was pretty much off screen.

• I don't remember time travel (of information) in the Cheela novels. Can you gice more context to jog my memory? – JDługosz Jun 16 '15 at 8:36
• @JDługosz We saw them experimenting with a one bit/channel time communications device (and the channel would be occupied between the receive and send times), an experimental rig, not a meaningful communications device. The quake hit and we saw no more of their tech beyond what they could do with it. – Loren Pechtel Jun 17 '15 at 0:36
• I'll have to read it again. I do recall they had FTL travel at the height of their civilization before the quake. – JDługosz Jun 18 '15 at 4:44

Depending on the feeling (serious and gritty vs a little ridiculous) you could use the Anthropic Principle.

Lets assume there are a (near) infinite number of universes. When you time travel, you create a new near infinite number of universes, in which the time travel machine appears in a different position in each one.

Now, lets assume that disapearing from one place and reappearing in another place involves an infinite impulse (scientific term, rate of change of force), as it is done over a literally 0 length of time. The impulse invovled in this would kill the time traveller.

So an infinite number of universes are created, but there is only one in which the time traveler survives.

You, as the author, cannot be blamed for choosing to tell the story of the universe where the time traveler survived.

And the time traveler, using the anthropic principle, should not question the fact that they are infinitely lucky to be the one time traveler duplicate to survive. If they didn't survive, they wouldn't be there to question how lucky they are.

The appartus has the exact same momentum of the earths orbit around the sun, the spin of the earth. Just like everything else on it does. But it is very weird for something to suddenly dissapear?

It might be better to use a firth dimension to "avoid" time alltogether. (If I lift my beer can from the table it will not "exist" on the table anymore. But it can "suddenly" reappear on it. The table cannot move up and down but the beer can - can move up an down.

If we move the table sideways - and the beer can move up (it will keep the tables given side momentum), and drop back in the same position. (It dosent matter wether the table was still or not). If earth is the table and your apparatus is the beer can - the apparatus have the momentum of all four dimensions, but "dissapears" in a fifth dimsenion - and come back in the excact same way - beacuse it will still have the momentum of all the other 4 dimensions.

An x cordinate does only effect the x cordinate. If y is added - the x motion remains.

I would suggest working on the theory that your machine is moving through space-time, rather than just time. Perhaps you're tied into the current gravity well, as mentioned earlier?

Another avenue to explore is that if I stand still for 5 minutes, I don't move relative to the Earth, so why should I move just because I'm moving through time more quickly? What causes me to move with the Earth when travelling at 'normal' time and why should that not change when moving more quickly?

I think you already have answered your question: 1. The time travelling machine should 'land' near enough to the Earth surface in order not to kill its temponauts. It is, give or take, 1 meter. 2. The Earth, Sun and Galaxy move constantly. Ergo, to make sure nobody is killed in the process, the time machine is built so that it actually stays at the same place. Just in case.

Warning: Discussion below is in sci-fi mode. So the machine works like this: 0. Observe the local curvature of the space-time continuum, Earth, Moon and Sun velocities, etc. 1. Make a small jump back for a day or even less. Let say, it is easier to calculate. You CAN move through space, but dare not use this option save for spatial trajectory adjustments. 3. Observe what is the difference between where you have landed and where you should have, then loop to point 0, unless you have reached your goal.

By the way, trajectory adjustments in 3 dimensions is how spacecraft works these days. You can perform the calculations, but after 10 years in space solar wind alone can move the probe too far away its trajectory. So, you adjust.

Calculations and jumps are performed in 1 MS, so a jump of 1000 years should take about 6 minutes. And you can definitely enjoy the view.

I would go along the idea that device is only bending time, thus it still abides by the other positional axis's while time traveling thus is keeps the same position on Earth as if it had just gone unused for 100 years.

To explain people not seeing it before or after, I would go with the idea that nature (through a yet to be explained mechanic) does not allow you to alter the primary timeline. Thus anytime you travel through time, a separate timeline instance is spawned specific for those involved in the time travel. On the primary timeline you would disappear and you would "pop into existence" in the instanced time line. Allowing any entity to alter the primary timeline is far too reckless and eliminates paradox's.

You might also explain that time is nullified while making this jump between timelines, thus that's what keeps the passengers from aging...

Is it a time transporter or a time teleporter?

If it's a time transporter, i.e. it can move through time like just another dimension, then your solution is simple as the capsule can just rest in one place as it moves through time. To the outside observer it will appear that time inside is very slow, standing still or moving in reverse depending on the direction you're traveling.

If it's a time teleporter, like the DeLorean in Back To The Future, i.e. it disappears from one point in time and reappears in another, then the explanation is more complex but still doable. Like all teleporters, it needs to lock on to the coordinates at the destination in order to reconstitute the capsule. Coordinates need an origin and axes, so there's no reason why you can't use something like the centre of the Earth and the magnetic poles. You can even work in some laws of physics which state it's easier to lock onto the same spacial coordinates that you left. Maybe there's a "temporal field" that persists between the source and destination.

The real question though is, do you need to explain this? I mean sure you'll get some smart-arses snorting this fact in an attempt to show off their knowledge that the Earth moves, but I'd argue it adds little to the integrity of the concept.

Remember, Time Travel isn't an exact science in real life. This is just one of possibly hundreds of equally, if not more difficult details of the overall question, which is, how the hell can you make an object travel through time?

In other words, unless you also happen to be writing a companion book on the science of how time travel works, this is just one more issue that the inventor just happened to come across, worked on, and overcame, no different from, "How did you obtain the unobtanium?", "How did you keep the tachyon pulses from losing their polarity at sub-luminal velocities?", "How did you prevent gamma rays from the sun from altering the spin of the ion pulse strings in the flux capitor?", etc.

I think a brief hand-wave is enough in any time travel story unless this is the main focus of it.

Having quickly scanned the answers, I haven't noticed the simplest one: don't explain it! I'm assuming your book will be read by average people, not people with physics PhDs. I never thought of the issues of space time until this question mentioned it. It's a basic accepted tenet of time travel that your machine stays in the same place relative to the earth, isn't it? I never noticed an explanation in H.G. Wells' The Time Machine.

Simple variation:

The time machine follows the same world line as a normal piece of matter would. If you left the TM not-turned-on for a hundred years, where does it wind up?

An explicit version that satisfies this naturally is yhe non-Wellsian idea that the exterior of the TM is still seen and felt in normal space. To an outside observer, it appears to be a stasis container. but, it can work in reverse, which is still exactly like a stasis container in that the internal state is the same at both past and future endpoints; it's just causalty that is reversed surrounding its creation.

You could get creative about how the machine+contents appears out of nothing if travelling backwards.

Or, to keep it following (only) the gravity, further stipulate that you need to operate it in orbit, not on the surface. It doesn't get dragged along by the spinning earth any more than a satellite does. While "in transit" it behaves as a microscopic black hole, as far as the outside world is concerned. Or, if you don't want to be able to detect it at all, say that it behaves as a particle of infinitecimal (but not zero) mass that has no interactions with anything other than gravity.

This doesn't have to be complicated. Since Jules Verne, moving through time rather than space has been a thing.

As a Safety Feature It's only geared mainly to move through time, not space, but it does have a mechanism in place which "scans" the area it will be before it materializes, so that you do not end up inside a wall or something--call it a safety feature.

Because moving through time is not the same as space. Set things on fire if they try Further, moving through space is a different kind of travel, and you may only have room for so much. Also, it may malfunction if you try to move it through space instead of time--it's just not what it does. It's a specialized piece of equipment. The TARDIS from Doctor Who famously does both, however, in both the new series and the old one, it's not good at actually navigating space without also navigating through time. It's actually trickier to manage and can lead to things being on fire. You can do the same with yours, but make it more restrictive.

Exploitation of a local time anomaly not present elsewhere. Also--maybe the time travel has something to do with the locality. That is, there's a particular time rift or anomaly in that geographic location which is being exploited.

I like to think of time travel kind of like the way we look at the orbits of electrons around a nucleus, and the act of trading positions with an electron in a position 3 dimensionally observable as ahead of, or behind the trading electron. Might sound lame, but that's the way my brain works :)

• Hi Steve, welcome to the site. Please try to answer the question that is asked. He specifically wants to know why if he travels in time, would he be in the same geographical location, and not somewhere in space because the earth moved in that time. – Mathaddict Dec 13 '18 at 17:31
• In other words: take your conceptualisation of time travel and apply it to the OP's query! – elemtilas Dec 13 '18 at 17:48

Since time is just one more dimension in the whole string theory tangled mess, you can just say that as you travel along the time string, you also shift in the space strings*, which keeps you in the same place physically.

Another would be that since we're in a gravity well, we're pulled along even in the time stream.

* Just a word picture, I realize that is not how string theory is believed to work.