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I was once in a roleplaying campaign where time machines had this odd quality. None of us knew why; we abstracted that as stuff the characters knew. But is there any (pseudo)scientific justification for this?

A time machine is limited in how far back in time it could go. (It might have a similar limit in the future.) Note this isn't a temporal range limit. It's not "can't jump back more than 100 years at once." It's "can't get me to a time before 1919, regardless of when I jump from." And no, these aren't time machines that stay at their starting point and launch you to another time; they're your much-more-common-in-fiction time machine that's a vehicle, leaving no physical object behind. And it's not that there's some barrier in time affecting all time travel: a newer, more advanced time machine could go back to an earlier year. Rather than being like an aircraft's range limit, it's like an aircraft's service ceiling.

edit: No, the time limit wasn't the date the time machine was made; it was always something far in the past.

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    $\begingroup$ Ad: Go back in time to see the bang, this model is running on next gen 2048-bit processors... battery sold separately. $\endgroup$ – user6760 Nov 28 '19 at 7:06
  • $\begingroup$ Just a thought would it help if there were 2 time dimensions. One we all know and love and another that is normally inaccessible to us. Sort of implied in films like H G Wells The Time Machine where there are two time frames, the variable speed and possibly rapidly passing time of the outside world and the normal time frame of the time traveller himself? $\endgroup$ – Slarty Nov 28 '19 at 9:29
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    $\begingroup$ DRM? You paid extra. $\endgroup$ – user253751 Nov 28 '19 at 15:17
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    $\begingroup$ "Time travel, by its very nature, was invented in all periods" - Douglas Adams. $\endgroup$ – Mindwin Nov 28 '19 at 17:50
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    $\begingroup$ I'd highly recommend watching the movie Primer. Your edit makes Primer not quite a perfect fit, but its an amazing piece of prior art! $\endgroup$ – Cort Ammon Nov 30 '19 at 0:45

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The time-machine's dependent on its reference core, a thing from the past that can be dated back to a certain period or even point-in-time (e.g. a painting, book, pebble).

The more historically/culturally significant a thing is, the better it works for time travel. While a pebble or rock can bring you back millions of years, the machine will easily drift off the target time by tens of thousands of years due to the insignificance of the chosen reference core.

Instead e.g. a painting by Leonardo da Vinci would be as perfect a reference core as they come and allow pinpoint accuracy.


The historical/cultural significance of a reference object is determined by the interactions of sentient/feeling/whatever beings that have been had with it.

Thus, e.g. the Mona Lisa will allow travelling to almost any date and moment in time since it's creation. While said pebble might have huge gaps between the possible targets (e.g. a day where it was purposely thrown at something, a day where it was used in a sandcastle, a day where it became part of the wall of a house, etc).

A good example for a temporally limited object might e.g. be a plow or shovel. There's a stretch in time where it was regularly interacted with - perfect for travelling. Then there's a stretch of time where it stood abandoned in a shed or similar - no interaction, no travelling.

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    $\begingroup$ This makes sense, but the only thing I don't like about it is you have to assume solipsistic consciousness-driven quantum woo-woo. $\endgroup$ – Spencer Nov 28 '19 at 15:48
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    $\begingroup$ @Spencer aka pseudoscience. Honestly It's timetravelling. So it's already quantum woo-woo innit? $\endgroup$ – dot_Sp0T Nov 28 '19 at 15:53
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    $\begingroup$ Rather than just culturally significant, perhaps it just needs to be unique enough to be identified by the 'time scanner' and not mistaken for a similar object. And also similar enough to its current state. If an object is damaged or warn down it may no longer be identifiable as the same object in the past or future. More advanced time scanners can distinguish more similar objects and also correct for small amount of wear and tear. $\endgroup$ – Richard Garside Nov 28 '19 at 16:12
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    $\begingroup$ Or perhaps only certain elements can be detected through time, so the object needs to have some gold in it, or your preferred (probably rare) element of choice. $\endgroup$ – Richard Garside Nov 28 '19 at 16:14
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    $\begingroup$ @RichardGarside A more scientific line of attack might be if we imagine scrubbing through the history of the anchor object looking for clues as to when you’re actually targeting. So for a painting you might spot people marching around with selfie sticks - dating it as being in that narrow time period where selfie sticks had become a thing but museums hadn’t caught up to banning them yet. Whereas with a random rock we probably only know enough to date geological events - though amusingly you could probably date 1945 pretty accurately, which explains why everyone tries to kill Hitler $\endgroup$ – Pingcode Nov 28 '19 at 21:25
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There are a number of science theories about time travel that suggest that a time machine can only take you back in time to the moment that machine was constructed and no further. This would explain why we don’t see time travelers all the time today.

These time machines also solve the problem of Earth drift. Rather than transporting you through time, the machine moves you (its cargo) to earlier or later versions of itself. Since those earlier instances moved with Earth, so do you.

So to answer your question directly: you can go back in time further than me because your machine was built earlier.

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  • $\begingroup$ And the similar limit in the future, is that you can't go ahead of where it will break down or be destroyed. $\endgroup$ – nigel222 Nov 28 '19 at 15:15
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    $\begingroup$ Specifically, theories of time travel via traversable wormholes work this way--one would have to accelerate one of the two ends of the wormhole to sufficiently close to the speed of light to create a time difference between them that would allow time travel, and one couldn't go back further than when this happened. I talked about physicists used the theoretical possibility of wormholes to explore time travel paradoxes and their possible resolution in this answer on the scifi stack exchange. $\endgroup$ – Hypnosifl Nov 28 '19 at 15:27
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    $\begingroup$ This answer does the exact opposite of the suggested: ..a newer, more advanced time machine could go back to an earlier year.. $\endgroup$ – dot_Sp0T Nov 28 '19 at 15:32
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    $\begingroup$ @dot_Sp0T I read that as a “for example” to explain the “no time walls” restriction, not a restriction on answers generally. $\endgroup$ – SRM Nov 28 '19 at 16:14
  • $\begingroup$ @dot_Sp0T Perhaps a "temporal pivot" then - a Time Machine can go back only as far before the Pivot Point as the Time Machine (or, at least, some key, hard to replace, component) was created after it. So, if the Pivot was in 1500AD, a 2000AD time machine can only go back to 1000AD, but a 2500AD machine can go back to 500AD... And a really expensive time machine might be able to "latch onto" an earlier Pivot, such as in 400BC $\endgroup$ – Chronocidal Nov 28 '19 at 21:40
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The problem is time density. The substance of time is harder to travel through the earlier you go, needing more powerful and efficient machines.

But it's not linear, increases in density are sudden and discrete events, which means that it's really easy to go to 1901 but the first machines couldn't go further.

The next and more efficient versions had the ability to go through 1901 but got blocked at the next spike in time density, around 1454.

enter image description here

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    $\begingroup$ This is dangerously close to violating the "no time walls" restriction of the question, but I like the approach nonetheless. Maybe time density could be tied to the number of low-probability events. A time traveler who goes back in time creates more and more potential paradoxes... if time is easily stitched back together despite the changes the time traveller makes, that is low-density. If the time traveller can't even breathe in a particular time without skewing history completely, that's high-density. $\endgroup$ – SRM Nov 29 '19 at 17:58
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    $\begingroup$ I know it's not far from a time wall but what OP didn't want was absolute barriers that better machine couldn't get through either, here it isn't a problem. And in my head I was tying time density to "matter density", as in the expansion of the universe and all that. So it's not that the time get more and more dense the further you travel, it's that it gets less and less dense as the universe expand, in an absolute way. $\endgroup$ – Echox Nov 29 '19 at 20:20
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The Earth isn't stationary

The Earth is spinning while rotating around the sun rotating around the galaxy in an expanding universe. Continents are drifting and even an earthquake can alter the tilt of the planet.

Travelling in time is the easy bit. Working out where the Earth is and the exact spot you want to be is the hard bit. Each newer time machine has better computers than can calculate the destination more accurately further back in time with less margin of error.

A slight error in calculations means either you are inside a solid object or even drifting in space. Older time machines don't have the calculating power to travel further back in time safely.

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    $\begingroup$ in this case, what if, I travel 100 years back in time and then use that moment as a reference point to calculate the position of the earth for another 100 years in the past, This breaks the concept of this question. $\endgroup$ – V.Aggarwal Nov 28 '19 at 7:02
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    $\begingroup$ @V.Aggarwal not necessarily. There might not be fuel in the past. Only a machine that can make the jump in one go can do it if the fuel is per-jump instead of per-second-traveled. $\endgroup$ – SRM Nov 28 '19 at 16:20
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    $\begingroup$ @V.Aggarwal: Calculating the past positions of the Earth depends not only on the current position of the Earth, but also on the positions of objects that Earth interacts with gravitationally. If you go back a hundred years, you don't have the ability to gather all the data you would need to calibrate the machine to go back further. $\endgroup$ – ruakh Nov 30 '19 at 2:49
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Our observable present is carried along through time like a stick floating on a river. The only thing that stops us leaving the stick and travelling with a different speed and/or direction is our inability to swim - the "water" of time has little friction and we can't (so far?) make a machine that can push against it to propel us away from the present.

Time travel was made possible by a force that we can interact with, a bit like a string running the length of the river. This force is named the "Thread" for this reason. The Thread pervades space and doesn't interact noticeably with most matter, but a time machine can grip it to pull itself and its contents backwards or forwards in time.

The Thread is thought to exist throughout history but its characteristics gradually change over time, making it harder for a time machine to interact with. The further back or forward you go from the present, the more energy required to grip the Thread, and beyond a certain point it is impossible. Better made (newer or more expensive) time machines can grip more efficiently, so they can get further away.

There has been limited research into the possibility of re-calibrating a time machine when it's near its limit, so that it can pull itself further from that point, but calibration requires complex measurements and calculations using advanced instruments that can't be made small enough to fit into current time machines.

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  • $\begingroup$ Great first answer. Welcome to world building $\endgroup$ – Trevor Nov 28 '19 at 17:52
  • $\begingroup$ Thanks, @Trevor. $\endgroup$ – Pastychomper thanks Monica Dec 2 '19 at 13:11
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My timemachine can only go back as far as it was created (see other answers).

Your time machine works on a different premise - it's on a spaceship which punches through to a universe where time runs backwards. You then go into stasis for as long as you want to go back, revives you and then you punch through back to your original universe. It's much more risky due to the stresses and dangers of inter-universe space travel, but technically you can go as far back as the reliability of your spaceship and stasis machine will allow.

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  • $\begingroup$ This “I use a fundamentally different approach to time travel than you” solution to this question can be used with lots of variations. The other answers tried to give each machine different jump distances. This gives each class of machine a different distance. Not as versatile, but a good approach that may work for many stories. $\endgroup$ – SRM Nov 28 '19 at 16:25
  • $\begingroup$ Alternatively, you use the risky version to create time machines further back in the past, and then create safer time machines to effectively create a vantage point from which more time machines can be built. $\endgroup$ – wizzwizz4 Nov 30 '19 at 12:17
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It has been designed with this restriction

From the user point of view, you enter your DeLorean, set the desired datetime in a display, and it moves you there. Easy enough, right? But in order to do that, behind the curtain the time machine will need to know the current datetime and location, determine the destination location (by default it will be the place where the "things" around you would have been at that time, a fair end-user expectation but computationally non-trivial), convert those human coordinates into the actual units with which it will work, map a route in the spacetime from the current location to the final one, and determine the needed motor impulses to "jump" there.

A time machine with a quite limited temporal range will be easier to build than one with a larger one. It's not just that you will need to load a larger spacetime map on its memory (which someone must have somehow calculated before!), but it will also need much more extensively testing (don't let me started with the endless number of regulations they must follow!).

At the same time, very few users will need to jump back more than 50-100 years, so it makes sense to segmentate the market with different products that differ in their available range. You build many low-range time machines that are cheaper to produce, and a few extremely expensive ones for those (typically companies) that need going that extra mile century. If the average consumer would want to go on holidays to an out-of-range date they would simply use a commercial flight to travel then.

Thus we will be dealing with many time machines in the market with a limited time range (with slight differences e.g. a commodity time machine from a luxury brand may allow a time-span 5 years larger than their competitors) in which they may be used (according to the manufacturer).

Of course, the fact that the manual says "do not use outside the tested years" doesn't mean that the users wold do that, no matter it being extremely dangerous to the occupants. "Family dead in Mercedes-Benz time machine accident" is very bad for PR, no matter if they went out of their way to use it way outside the operational specifications. Plus government regulations.

So the manufacturer build the time machines to cover a time-range (let's say, fifty years) plus a safety margin, then lock it so you can't use it to jump outside the stated range. That's why it doesn't matter that you are already in the past when you try to go earlier.

A shady mechanical may be willing to remove that limiter from your time machine, but operating it more than a few more extra years would require a full revamp of the motor and its associated computer system. Obviously, that is completely unsupported, although military time machines may allow to bypass the limiter. There are some known cases where a low-range model of time machine is actually the same as an higher model but with a different limiter. These are popular among tuners, and relatively safe once they are loaded the pirated maps from the higher model. However, in some cases they would be tagged as the lower model precisely because in QA they failed to operate properly for a bigger range.

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Understanding the details and maths of time-travel is hard, but let's look at it in some layman terms.

Time while passing through at its constant pace on surface of earth generates some extraordinary particles, we call then time-tachyons, These particles don't exist in the current world but in a type of zone that has no concept of time on its own. Our machines use these time-tachyons, to navigate itself through time.

Now, here is the trick, as a time-tachyon travels through time in that zone, it creates a bubble of encryption onto itself, our machines capture these time-tachyons of a specific time, solves the encryption surrounding it, once solved, it opens up a time-wrap portal and travels to the time, at which that particle was originated.

So, what's the problem, The issue lies in the solving of these encryptions, it requires huge amount of computation done simultaneously on the device that is about to travel through that portal (no distributed computing allowed), and with the limited technology of today, we can solve only till a 100 years back in past or in future, no matter when we capture these particles, because these particles themselves, don't exist in physical world, they exist, where there is no-time (The machine can quite easily access this no-time zone quite easily).

So, as our technology advances and cost reduces, we can have more and more on-board computing power and can increase our range.

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  • $\begingroup$ Why would it require more advanced hardware to decrypt 1000 year old tachyons than 100 year old ones? $\endgroup$ – zovits Nov 28 '19 at 14:40
  • $\begingroup$ @zovitssupportsGoFundMonica Tachyons lose information over time, so their decryption also involves guessing the missing bits, where each one doubles the effort. $\endgroup$ – maaartinus Dec 1 '19 at 5:34
  • $\begingroup$ @maaartinus That directly contradicts what OP wrote: "because these particles themselves, don't exist in physical world, they exist, where there is no-time" $\endgroup$ – zovits Dec 1 '19 at 7:54
  • $\begingroup$ @zovitssupportsGoFundMonica I see, I presented an obsolete theory. They don't lose information, they just appear like this as they're less understood because of the distant "time, at which that particle was originated". Maybe. $\endgroup$ – maaartinus Dec 1 '19 at 16:50
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Something about the process of time travel means that the machine is "attuned" to "its present". (Maybe the machine has to create some sort of not-entirely-stable "field" in order to displace in time, which has to be maintained continuously, or everything that has been displaced will "snap back" to the present.) As a result, time machines can only travel a certain "temporal distance" from "the present", but as the technology improves, that "distance" increases.

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A new quantum discovery: The Heizenberg Uncertainty Time Principle

The Heizenberg Uncertainty principle is the mathematical trade-off between where a particle is and how much momentum it has. The particles position is uncertain and exists in a 'broad range' depending on its velocity and mass.

Just as a particle has an uncertain location based on its momentum, your Time Machine has it's temporal equivalent: It exists in a Quantum state of uncertainty in time.

In fact, that is how the Time Machine works. By having uncertain temporal momentum, it can exist within a certain range of times. Much like a vibrating particle, the Time Machine 'vibrates' in time.

Activating and stepping into one would allow you and the Time Machine to exist in many different temporal states, at which you could exit the Machine at any given time. However, it remains in this temporal state and in order for you to use it again, it continually vibrates within this time range. Deactivating it would collapse its 'wave function' and coalesce it back into its original time, as now it has a much more certain temporal momentum.

Thus you can only time travel within a certain range of when the Time Machine is activated.

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Your first generation time machines were built on the principle of quantum teleportation (spooky action at a distance). As researchers had theorized as early as the 21st century, the property of entanglement and quantum teleportation was a kind-of wormhole. Benico Flores, in his famous proof of 2057, demonstrated mathematically that quantum teleportation happens across both space AND time.

Those first generation machines - built on bundles of entangled pairs "stretched" across time by keeping those bundles preserved against quantum decoherence had hard limits : they could go no further into the past then their moment of creation, and no further into the future than the engineering limits of keeping the threads of entangled particle pairs coherent - and also the social limits of second generation time travel societies that didn't want to maintain the bundles. Mostly, this technology covered data-carrying chronographs and chronograms, although a few bulk matter devices existed.

Although many improvements labelled themselves as second, third, and greater generation time travel, including a notable mention of the Mintz Deutch-Politzer screwdrive to lay bundles prior to initial power-on, the first truly second generation time travel device was LeRoyce Mettals strong force skimmers. Professor Mettals recognized that the holographic properties Leonard Susskind had theorized nearly a hundred years earlier described "event horizons" around every subatomic particle. In addition to making bulk matter chronodynamics obvious, from an engineering point of view, since neutrons and protons are both naturally generated, were generated a very long time ago, and will "live" a long time to come, the hard limits to future and past travel really opened up to all but the most extreme cases.

Then there's third generation u-balls that work within that high temperature region where everything unifies and you can skip across births and deaths of individual universes. Hold on. What time code did you post this question from?

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At a certain point in history, something happened which made time travel possible. This moment is the fulcrum around which time travel occurs.

Handwavium is the core component of a time machine. All existing handwavium was created as a result of the fulcrum event. Handwavium naturally accumulates a temporal charge over time, but must be shielded during the time machine construction process to avoid unfortunate side effects during time travel. There is also a critical mass of handwavium, accumulation of more than this having similar catastrophic consequences.

Thus a time machine can only contain a limited amount of handwavium, and the temporal charge of this is higher the later the time machine was constructed. As a result, the later the construction, the further back in time it can travel.

Possibly, elements of handwavium are temporally connected to when they were original created, like a string gradually unspooling. So a week after the fulcrum, you can travel up to 2 weeks into the past; after a month, 2 months; after ten years, you can travel back to ten years before the event

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There was a story I read, I believe it was in the first volume of "Isaac Asimov: The Complete Stories", which was a collection of his short stories.

The Ugly Little Boy by Isaac Asimov.

In the short story, time had charge or momentum. So the further an object was out of its own moment, the more energy required to keep the object stable. So in the story, some prehistoric life form, a Neanderthal child, had been grabbed from the past and was kept in a special sealed area to isolate the time charge. The caretaker protected the Neanderthal. The story stressed the danger of being out of time. So the caretaker did something which broke the rules and put everyone at risk, so the organization was forced to send the child back.

It could also be included that the more significant something is (could be or will), then the more charge it carries. Send back people with advanced knowledge (that's usable in the past) and well, spontaneous human combustion does exist.

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  • $\begingroup$ @SRM-ReinstateMonica thanks for updating my answer and filling out the details $\endgroup$ – Fering Nov 29 '19 at 19:25
  • $\begingroup$ I just visited your link to wiki and what I read was very different. Its the same story but the one I read wasnt 260 pages. I know for a fact that the one I read was in that short stories collection. $\endgroup$ – Fering Nov 29 '19 at 19:27
  • $\begingroup$ Look close at the Wiki ... there was both a short story AND a novel, both of the same name. $\endgroup$ – SRM Nov 29 '19 at 19:27
  • $\begingroup$ You started so good but then lost me with the significance thing. It implies the universe carries human values on events, which there is no reason it should, in which case all events should carry the same significance to the laws of physics. $\endgroup$ – Muuski Nov 29 '19 at 21:16
  • $\begingroup$ @Muuski When I talked about significance I meant on things that would impact the timeline. A pebble itself is unlikely to cause a disruption, but put that same pebble in say the cogs of the first printing machine during an important demonstration, its considerably more important. $\endgroup$ – Fering Dec 1 '19 at 2:03
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How far back you can go depends on the earliest identifiable fork in the time stream you can nail down

Almost all models of time travel involve some kind of forking in the time stream. Each branch can form the definition of an epoch time, where $t = 0$. Because of how the time equations work or time travel, movement in $t > 0$ requires evaluating the $ln(t)$. Trying to move to $t<0$ causes an undefined value which is liable to put the traveler someplace they really don't want to be.

Choosing a branching event/decision must be done with care. Nearer events, with better resolution yield safer travel. Setting t={5 minutes ago} is easy because the resolution is absurdly high. Setting t={assassination of Caesar} is a lot harder since if you guess wrong, $t$ may end up less than zero and you go "somewhere else".

This sets up a nice exploration mechanic where explorers pick some reasonably well know event then choose some date afterwards to try to pin down $t$ to a higher resolution. Reliance on branches of the time stream rather than objects in the time stream means you have a much stronger point of reference. Anywhere after your epoch time can be explored. Pushing your definition of epoch time backwards requires care and exploration.

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Well, clearly such a machine would have some sort of reference to the time it was built. And that property would be central to the core element of the machine that enables time travel. So, maybe it's some kind of an exotic matter that is required to travel through time.

The exotic matter would have its technological imprint of the time at which it was created, like a timestamp. That imprint, be it a configuration of internal oscillations of particles or something else:

  • Is not an optional quality of the time machine, but rather an essential technological part of the implementation that makes time travel possible at all.
  • Serves as a point of reference for the machine and allows it to calculate the "time distance" from that exact reference point during any travel.
  • Serves as a beacon or an internal compass. Without it time navigation is not possible because the warp wave does not form, or dissipates together with the machine and its contents along the time continuum resulting in the loss of the machine.
  • Can only be attained by the exotic matter during the device manufactoring process and can vary in grade, depending on the cost and technological level.

The time travel invloves supplying energy to the exotic matter, and the structure of the exotic matter allows for roughly 1:1 relation between supplied energy and travel distance up until a certain threshold posed by the quality of the exotic matter.

Travelling beyond the distance allowed by the exotic matter imprint requires an increasingly higher energy supply and quickly degrades the matter itself because of the overload within the material matrix, much like a capacitor can be burned or a crystal in a laser can be destroyed due to excess energy throughput.

Think of it as a process similar to overclocking a CPU.

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It's obvious when you understand temporal mechanics. You better get an A in quantum mechanics first though.

When a time machine is first turned on, it must begin by surveying the temporal landscape. This initial state is represented by a quantum matrix within the machine. This means it must obey the no-cloning limitation preventing this state from ever being duplicated for another time machine or backed-up in any way. This also means that turning off the machine meaning losing the baseline that defines the starting point for time travel. You can turn the machine off, but this resets it to a later baseline which is undesirable because you lose the oldest part of the time window for that machine. Of course, this also means that time machines are designed for redundancy, hot-swap of components, etc. But key components that determine the overall temporal power of the machine cannot be hot-swapped. Time machines must continue to survey the temporal landscape, but if they do not also maintain the initial state, it limits their time baseline to the current time.

Early time machine designs did not maintain their initial temporal state, thus limiting their travel to a fixed distance in the past, not a fixed point in the past.

Time travel requires more advanced quantum computers to travel further distances, so the more advanced time machine can travel further into the past from its baseline. A sufficient technical advance more than overcomes the later time reference baseline, so more advanced machines can typically travel further into the past.

You can upgrade a machine, but this requires replacing the quantum core, so that this resets the machine baseline. Still can be worthwhile if the upgrade adds more capability than is lost when the baseline is lost.

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  • $\begingroup$ But why can't you just travel back in time, take less time than you travelled to reset the temporal core, then repeat? $\endgroup$ – wizzwizz4 Nov 30 '19 at 12:20
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    $\begingroup$ @wizzwizz4 Look at the 3rd derivative of the complex time vector, and you will see that attempting a core reset outside the native time stream will be fatal due to temporal shear. $\endgroup$ – Gary Walker Dec 1 '19 at 15:10
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It's not actually a more advanced time machine.

In order to successfully navigate time time machines need two reference objects which must be built into the machine, they're not replaceable parts. These objects must have been obtained from two different points in time and provide a calibration scale only between those two points.

Note that I say successfully. You can attempt to set your time machine for a point outside the calibrated zone, it's just your arrival will be unpredictable. You're not going to emerge in a planet because that would take energy you don't have but the odds are very high you emerge in deep space and at a somewhat unpredictable time (and the farther beyond the reference object you go the greater the error.) You can of course return and try again--but it's going to take many, many tries before you arrive someplace you can build reference objects. This makes reference objects for more distant times valuable items (think of drugs--the pills are cheap, the R&D to make them can reach a billion dollars.)

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  • $\begingroup$ Most likely, you won't have a second chance... $\endgroup$ – Ángel Nov 30 '19 at 23:37
  • $\begingroup$ @Ángel Why? The machine can still return home normally and the exploration machines will be sealed to handle emergence in space. When you emerge you look around, can you make a safe landing? If so, land, otherwise hit the recall and try again. The only thing that would be dangerous would be emerging too close to the ground to make a proper landing and I would figure a proximity sensor would immediately kick you back. $\endgroup$ – Loren Pechtel Dec 1 '19 at 3:01
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I'd argue with a reasoning on two bases:

  1. Time travelling is only partially understood. Yes, there are theories, working time machines, and we even have experimental proof. But all theories start to break down if you try set the time interval too big.

This part is quite realistic - yes, we have theories, and they do explain the parts of reality they model really well, but we have struggled for a unifying, working theory probably since the beginning of religion, and probably will struggle for it for a long time to come.
As a bonus, this allows you to keep some mysteries about time travel for later use.

  1. There are discrete parameters of time travel which are dependent on the "absolute" time (think of them like the emission line spectra of elements).

Just like travelling through space, travelling through time is harsh and dangerous - so dangerous that we need active fields counter-acting the negative effects during time travel. However, the active fields only block a very limited spectrum of these negative effects, which can be calculated out of the discrete parameters.

So when travelling back through time, there comes a (more or less) hard point after which your current shield system gets useless and your time machine will break.
However, after you went back in time, gathered the necessary data and used your incomplete time-travelling theories, you can build a new time machine that uses the old and a new shielding system, and therefore can travel further back in time.

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Time machines experience an effect known as temporal drag. The further you move through time the stronger the effect becomes, until eventually it completely balances out the ability of the machine to move. Interestingly, the effect is relative to the subjective "now" the machine comes from, so the effect causes drag when moving "away" from subjective "now" but acts like a rubber-band to drag the machine back to "now" when the time field collapses. The "drag" effect also also means that fuel (i.e. eludium-Q32, tempronium, "Captain! The di-lithium crystals!", whatevah) must be consumed at all times (!) that the machine is away from subjective "now", which could lead to some interesting story ideas. :-)

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The issue is that there's not just one timeline, but there are many. And with many, I don't mean two, or three, not even a million. Even for time jumps less than a year, the number of significantly different timelines is larger than the number of atoms in the universe. And even worse, while 21st century scientists still thought only the future timelines diverge, in reality the same is true also for the past; information about the past is objectively lost, and then there are many timelines converging to the same future.

Yes, technically there's nothing stopping you from going to an arbitrary time. But the point is: You want to come back into your own time, or more precisely, into a time that differs as little as possible from the time you left, considering the changes you did to the past. The better those algorithms work, the farther you can go in the timeline without getting lost in the web of time.

The art of building time machines is not in the time travel itself; as soon as you understood the quantum chronology theory, that's easy; a third-semester student of the time travel university could do it. The art of building time machines is navigation. It's the art of remaining close to one time line.

Of course, time machine manufacturers keep their navigation algorithms a well-guarded secret. All that's known is that the ability of travelling depends not only on the accuracy of the time drag sensors and chronon field generators, but also on the computing capacity of the time machine's navigation computer and, crucially, on the quality and range of the installed temporal maps. There are also some rumours that some companies installed secret timeline beacons that only their own time machines can access. Of course that's strictly forbidden, and therefore no company would openly admit to it, but since it is very hard to find such beacons if you don't know exactly where and when to search, it is not unlikely that they really exist.

That's why different brands have different time travel ability. And for economical reasons, there are also time machines with different travel capabilities from the same company. After all, even if you can build time machines that travel wider time intervals, if you compete with cheap time machines that can only go up to a century back, you'll build a line of cheap machines to compete with them, in addition to your more expensive time machines that can go back till the middle ages.

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End of rope

The time machine cannot go back in time by the vehicle alone, it needs a rope tied to the current time, act as:

  • an energy cable, time travelling need much more energy than what could possibly stored. It also take a lot of energy just to keep the machine at the time point. And there is limitation on how long can the cable reach.
  • a trail, there are zillions of timelines and you need to come through a lot to get to the destination. The trail is used to keep track of the timelines, without it you can go back to the wrong timeline and will stuck in alternative history forever.
  • a trace, like @dot_Sp0T proposed, a great array of historical items or information span from the current time to the destination, each must be created/unique at that time period. You are going one step at a time, finding the timeline matches with the item/information. Without it you simply can not know if you are going back to the history or just going to parallel timelines.

This could introduce plot points like broken rope, sabotage in current time, lost trail, invalid trace,...

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You know how some cell phone providers have better network coverage because they have put up more 3G/4G/5G towers around the countryside? It's just like that with time-machine service providers.

There are specialist exploratory time-machines. Those things are large, inefficient, slightly dangerous, very inaccurate (and the perceived time for the trip can sometimes be weeks!) Large crews that carefully use these to go backwards in time to deeply bury carefully calibrated time-transmitters - a process that can take a week of effort.

Once in place, a time-transmitter can be used by small, efficient consumer time-machines to jump to any point while the time-transmitter is operating.

There are only two problems.

  1. Limited Life. The transmitters only operate for about 24 months before they degrade, so the exploratory crews have a lot of work to do, burying time-transmitters. (As you might imagine, they choose the most popular times for consumer travel first, with a bias on recent times, because travelling further back takes more perceived travel time.)

  2. Capitalism. Competition means that your consumer time-machine can only connect to time-transmitters installed by your provider server at the time of the point-of-sale. Every provider has their own format for time-transmissions, so if you choose a company with less coverage, too bad. Another time-machine (perhaps a newer one, or one from a better supplier) will have better coverage. Of course, some providers focus on particular eras, so you can visit more accurately during the Rennaissance, but less accurately during the Mesozoic, and other companies might have the opposite. Choose well.

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Your time machine has a Y2K bug.

the time circuit display of Doc Brown's DeLorean

On Doc Brown's DeLorean there appears to be no way to enter a year before 1BC or after 9999AD. And even 1BC is a stretch, because Doc Brown has no QA department and I wouldn't want to be the first person to try putting in 0000 as the date and seeing whether it works or just explodes.

Maybe on the inside your time machine stores the date as a string yyMMdd, like the old computer systems that had the Y2K bug, and only has a fixed 100 year range because of that. Or maybe it uses 32 bit Unix dates internally, so it only understands dates from the evening of 13 December 1901 to the morning of 19 January 2038.

https://en.m.wikipedia.org/wiki/Year_2038_problem

There are such a wide variety of ways that real computer systems have hecked this up, that essentially any date could plausibly be the limit of your poorly programmed time machine.

https://en.m.wikipedia.org/wiki/Time_formatting_and_storage_bugs

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Better engines need better fuels

Using current tech, even a tiny nuclear reactor requires a ginormous hydroelectric dam to perform the enrichment processes you need to make the reactor work. I'd imagine the facilities you need to make the fuel your time machine requires something at least as impractical to attach to your tiny timeship. Your older time ships use low quality fuels that were only produced in bulk during a few particular decades. Go too far back, and the mega structures needed to fuel your ship do not yet exist meaning you can only go as far back as your fuel source allows. To far forward and the fuel you need is no longer produced... Try putting Plutonium-240 in an Handwavium-114 reactor or vise-versa and all you get is a city sized smoldering crater...

The older fuels have lower power densities; so, a fission powered time ship might have a range of just a few weeks, fusion reactors a few years, anti-matter might get you a couple of decades, then far future tech might be able to get you several centuries or even millennia before their fuel runs out.

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Time machines charge up over time, but primitive ones only hold a certain charge. So, a machine built in the year 2030 could send someone back 1 year before its activation for each year it is on, so in 2050 you could go back to 2010, but its time boundaries would max out at 100 years in 2130, allowing a maximum time travel to 1930. A newer time machine from 2100 has a capacity in millennia, so although it takes will have a lower range than the older machine for a while, by 2200 it will be able to go back a greater distance in time than the older machine, and by 2270 will be able to go back to before the older one could go back to.

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Processing Requirements

Simply put, the computer isn't smart enough to know that far. Not without an upgrade.

See, while the time machine is able to travel to various different times, the computer intelligence was made at a very specific point in time. And in order for time machines to work, these computers effectively estimate every instance of true events that occurred in the past from when they were first activated/calibrated.

Similarly, there are an infinite number of digits to Pi, and the limitation on how many digits you can go is mostly based on how many you can process.

Well, in this case, your time machine is capable of going to 1919 (100 years), only because it doesn't have the capacity to memorize that 101st year. And because of how time-travel computations work, it needs to memorize every possible instance from its normal current time to the targeted past.

This could lead into a plot-point, where someone figures out they can reboot the system to start learning from a new reference point (allowing you to go 100 years back from your current time), but then you'd lose any ability to jump to your normal time. And while this is known to be theoretically possible, nobody has been that crazy to try (as you'd be stranded, possibly without a working time machine, with no resources from your time).

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