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I've built a time machine and it works in unmanned testing... sorta. I tell it to go into the past, it disappears, and then comes back to the present. The problem is that it isn't terribly accurate. I need to know how far back it's going in order to calibrate the "Time Selector" dial.

I need a device that can determine the time across millions of years. So far I've used a radio clock hooked up to the onboard computer, but I want to send the machine back more than a few years. I have a pretty big compartment (6x6x7 feet) to fit the device. I'm still in the unmanned testing phase, so everything has to be automated. I can put sensors outside the vehicle (star detectors, antennas, etc). My time machine stays in the past on a 24 hour timer. Assume there is no multiverse (it makes testing harder). And assume that all of my testing is being performed in a desert area of the American southwest.

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    $\begingroup$ This seems to be a purely real word question. This has absolutely no worldbuilding component to it. I suggest migration to TemporalEngineering.se $\endgroup$ – EveryBitHelps Jun 26 '18 at 22:24
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    $\begingroup$ @EveryBitHelps - I suggest that migration to TemporalEngineering should only occur after the establishment of said stack. Currently this question is fine here - but it may be worth going then and making this suggestion again. $\endgroup$ – user42528 Jun 26 '18 at 22:58
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    $\begingroup$ @Ben It's the Temporal Engineering stack. Of course we can migrate it now and have it show up when the TE stack exists. $\endgroup$ – Makyen Jun 26 '18 at 23:18
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    $\begingroup$ @DonQuiKong TemporalEngineering.se already will have been established a few years ago, but it will be determined on meta that giving universal access to it would limit the sort of questions that could be asked there without changing history. Everyone has automatic access after the year it was created, and you can also configure your account to be able to access it in the past, but that requires logging into it after the year [redacted]. But for the benefit of actual residents of the past, any questions tagged with time-travel will automatically be cross-posted to TemporalEngineering.se. $\endgroup$ – Ray Jun 27 '18 at 14:41
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    $\begingroup$ I don't see this working, unless you handwave A LOT of stuff. Simply enough, the earth wasn't "here" a million years ago. It wasn't even here a couple of days ago. So your device will most likely be floating in space, and not on the earth, if it travels backwards "in place". If it doesn't, if it travels with the earth somehow, I presume just a glance at a couple of stars will set you in (roughly) where in the galaxy the earth currently is, and hence, 'when'. Then it is a matter of fine-tunning. But remember, you have to really handwave the 'moving with the earth' part. $\endgroup$ – tfrascaroli Jun 28 '18 at 9:08

31 Answers 31

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If you can deploy a good quality, wide-angle telescope and take photos of the night sky over a period of more than a few days, you can determine the time down to seconds over a wide range of history. It would probably be good in the range of a million years pastward or futureward.

The technique would be, first, to get a basic orientation by finding recognizable stars. We know what stars are bright and close enough to have significant proper motion (PM) and what stars are bright (probably better than 8th magnitude) and far enough away to have little PM. This gives us a set of bright stars whose proper motion we know well enough to have known tracks through the sky both forward and backward from today. We spend significant computer cycles to find the set of bright stars in the photos which are consistent with those motions. This gives us a date that's good to within 1000 years or so. Perhaps more importantly, it gives us a solid coordinate system, since some of the sources are extragalactic and don't move significantly in a million years.

We can cross-check this with the measured locations of the poles over the range of dates where the precession is predictable.

Given that, we then can do a fitting of the observed motion of the Moon and planets to that period. The motion of the planets should tie the date down to a few days, and the motion of the Moon ties it down to a few minutes.

Sources of error: (1) It's much easier if you can be sure of getting extragalactic point sources. Some of the LMC sources would be great if you're in the southern hemisphere. There's probably some decent sources in Andromeda, though it may not be realistic to have a telescope big enough to resolve them. Best would be if you could get down to magnitude 13 and pick up some quasars such as 3C273. (This ought not to be hard.)

(2) I don't have a solid number for the rate at which planetary motion prediction become too unreliable. Given that we don't see chaos for a hundred million years into the future, I'm estimating that position calculations will be accurate for at least a million years.

(3) The device needs to be stable over the period of observation and have a view of the night sky.

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    $\begingroup$ You have a time machine. You don't need to be able to predict 1M+ years into the past from only the data we have today. You can acquire whatever additional data points you need/desire starting from today and working backwards/forward. Doing so allows you to refine your models to account for whatever variation actually happened, rather than what we currently believe happened. $\endgroup$ – Makyen Jun 26 '18 at 23:20
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    $\begingroup$ The question says that no one knows how far the machine travels until the measurements are done. The whole point is to calibrate it. $\endgroup$ – Mark Olson Jun 26 '18 at 23:28
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    $\begingroup$ The question says "So far I've used a radio clock hooked up to the onboard computer". This implies that there's, at a minimum, of "short" and "long" settings. For me, I read this as meaning that they, at least, have a general idea of one date/time setting being shorter or longer than another, rather than just random. This would imply that they can progressively determine what the date/times are, while updating their methodology along with the additional data they've acquired. $\endgroup$ – Makyen Jun 26 '18 at 23:32
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    $\begingroup$ Note that I'm not saying that what you have here is incorrect. It just assumes that the machine is dumped in the middle of a random date/time without further information. That may have been what the OP wanted, but I view such a calibration as a multi-data-point process, which is easier to determine by building upon the data that you already have (fewer unknowns), rather than jumping to some distant point without first hitting some intervening points. $\endgroup$ – Makyen Jun 26 '18 at 23:39
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    $\begingroup$ Thank you! This is the best answer for a few reasons: 1. I don't have to leave anything behind and find it later. 2. I don't need any more dangerous materials. My wife doesn't like that she already has to wear a radiation badge when she's in the house. $\endgroup$ – Andrew Brēza Jun 27 '18 at 12:45
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Instead of having the device determine the time it is sent to, have it leave something in place, that you can find in the present to determine exactly when in the past it traveled. This is best if your location is very isolated (or underground in a stable rock formation) to minimize the chance of disturbance during the intervening million years.

The simplest I can think of is to leave some known amount of long term radioisotopes and use a detector in the present to determine the amount of decay, something similar to how carbon dating works. You should choose isotopes with appropriate half-lives to allow you to very accurately calibrate how long the sample has aged.

If you want multiple trips, you should ensure that the isotopes are well sealed in a container (fused in glass or other long term storage method) to prevent contaminating the whole area, so you can measure multiple samples.

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    $\begingroup$ This is a neat idea. This time capsule method wouldn't work for forward travel (if the OP even allows that). $\endgroup$ – EveryBitHelps Jun 26 '18 at 22:55
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    $\begingroup$ @EveryBitHelps Sure it would. You just leave the time capsule in a place that should remain undisturbed but accessible, and have the time machine retrieve it. $\endgroup$ – Andon Jun 26 '18 at 23:02
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    $\begingroup$ So, travel far enough into the past, and you have enough uranium to build a nuke, as well as determine what year it is? $\endgroup$ – HopelessN00b Jun 27 '18 at 3:14
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    $\begingroup$ I'm not sure how "very accurately" this would be. Do you have any figures? Would you be able to discern a sample that has aged 1.1 million years from one that has aged 1.2 million years? $\endgroup$ – pipe Jun 27 '18 at 8:25
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    $\begingroup$ @pipe Unless you put a sample so tiny and short-lived that every radiactive isotope has decayed way before, yes, it can be done with a precission higher than a few thousand years - remember that you can control the size and purity of the sample beforehand. That's way better than any kind of datation we can do today, and yet they have no problem in achieving precissions smaller than 100,000 years. $\endgroup$ – Rekesoft Jun 27 '18 at 10:39
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You need to know when you are sending stuff, not how much time they stayed in the past (from their perspective). So sending clocks is a no-no.

One way you could do it is by collecting air samples from the past. When it comes back, compare the concentration of atmospheric gases against those of Antarctica's ice cores. The oldest ones are 2.7 million years old.

If you need to go further back, you may wish to measure the distance from the Earth to the Moon with a radio transceiver. The Moon is getting farther away from the Earth at a steady, known pace (3.8 cm/year). You may need to wait a full lunar month to get your distances right - if you wish your machine to spend less time in the past, you can also measure the time an Earth day takes in the past. Eath days are getting steadily longer with time (2 milliseconds per century, also mentioned in the last link). You just need to measure the time between two consecutives sunrises.

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    $\begingroup$ The reason we can bounce lasers off the moon is that the Apollo crew left mirrors there. $\endgroup$ – Donald Hobson Jun 26 '18 at 23:21
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    $\begingroup$ @Ryan_L No, it isn't. Apart from the fact that moon dust is nearly black, the reflection is diffuse, you cannot get a signal even with a Terawatt laser. $\endgroup$ – Thorsten S. Jun 27 '18 at 1:33
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    $\begingroup$ @Renan Note that typical EME requires something like 4 meter (parabolic) antenna and is considered a pinnacle of HAM radio achievements - thus not trivial at all. And you have to aim your dish, which is kind of difficult automatically if you do not know the location of the moon. $\endgroup$ – Radovan Garabík Jun 27 '18 at 6:12
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    $\begingroup$ The moon has had a fixed size for +4 billion years. You can easily measure its distance just by measuring its apparent width and then doing the math. $\endgroup$ – RBarryYoung Jun 27 '18 at 17:52
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    $\begingroup$ Can you really get millisecond fidelity out of sunrises, which change day to day anyway? $\endgroup$ – Azor Ahai Jun 27 '18 at 21:03
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Assuming you can't take a star fix for whatever reason (during some of these time periods, Earth's surface could be pretty hazardous, and I'm not sure what precautions your time machine uses to avoid popping up in the middle of a rock, or a cave, or a sea of lava), your probe should still be able to take measurements of the atmosphere and magnetic field around it. Then when it gets back to your time, you'll have to compare that data to known values from Earth's past.

The good news is that we carry out the same process on fossils and rock strata all the time, in the study of stratigraphy. All the reference data you would need is well-studied. The bad news is that it's much less accurate than an astronomical approach, probably on the order of tens to hundreds of thousands of years. However, it has the advantage of allowing a much tougher probe compared to a delicate telescope.

You could use the two techniques together, if you felt the need. Stratigraphic fixes would let you safely determine a very rough timeframe, which will tell you roughly what geography to expect. For instance, during some past eras, much of North America was covered by a shallow inland sea - not exactly ideal for astronomy. Knowing when your probe will pop up allows you to choose a location you know will be suitable for the more delicate equipment. It also gives you a good feel for what the atmosphere will be like, so your telescope probe can take that into account.

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    $\begingroup$ +1 for using two techniques in conjunction $\endgroup$ – Shawn V. Wilson Jun 28 '18 at 17:14
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I'd say go back as far as you could possibly want to and leave an atomic clock (or several for redundancy).

When it wears out / breaks replace it with another. Eventually you'll have a continuous line of clocks from the dawn of time

Once you have a line of clocks right up to modern time, you can tell exactly when you are down to the microsecond.

You can leave it on land, bury it or have it floating around in space or even on the moon.

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    $\begingroup$ @Legisey You don't need to. You just have it start from 0 when the time machine drops it off. It will appear in your designated place in the future with for example 3155760000 which is about 100 years. $\endgroup$ – Sentinel Jun 27 '18 at 7:03
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    $\begingroup$ I'm afraid I don't follow this. How would you power these clocks? And how would you go back to replace it? You wouldn't really know when it failed, just how long it was active. $\endgroup$ – pipe Jun 27 '18 at 8:41
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    $\begingroup$ Agree with @pipe, the idea is sound but the implementation is tricky. Building a clock that will run for a thousand, ten thousand, or a million years isn't a trivial task, and you have no way of repairing the clock when it breaks. Suppose you send a clock back, and find in the present that it stopped after running for 900 years. You don't yet have a calibrated time machine, so there's no way to send a replacement clock to 900 years after you sent the first one. $\endgroup$ – Nuclear Wang Jun 27 '18 at 12:33
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    $\begingroup$ @NuclearWang You set your dial to whatever, travel and leave the clock. Back to current day. You turn the dial 1º, travel and register that the clock counted 10 years. So 1º is 10 years. You ask the supplier of your clock for the minimum expected life span of your clock. This x years divided by 10 is the number of degrees you turn the dial from the initial point, travel and replace the clock. You take the broken first clock back to current day so you can return it to the supplier and get a new one under warranty. $\endgroup$ – LVDV Jun 27 '18 at 13:09
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    $\begingroup$ @LVDV The idea of using a time machine to cheat on the warranty of the clocks is brilliant, but I worry they would suspect something after a few hundred. $\endgroup$ – user25818 Jun 28 '18 at 22:20
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Similar to measuring star positions but not reliant on any particular set of stars/location. Use the expansion of the universe.

This should provide you with a known rate which you could plug into your time selector using hind- or fore- casting algorithms.

You have a time travel device, I'm assuming you have something powerful and portable enough to measure or record two distant parts of the universe, to get a reading.

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  • $\begingroup$ This would be the best method. $\endgroup$ – user32463 Jun 29 '18 at 15:05
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So, here's a suggestion which should cover a wide swath of destinations. NASA recently developed a method of locating any spacecraft to within a few miles within the solar system, by using the position of known pulsars. This is great for location in space (perhaps well outside of just the solar system), but the catch is that they are, gradually, changing their frequency. The Crab Pulsar, as an example, is spinning down at a rate of 3.7e-10 Hz/s. By identifying these pulsars, comparing their relative frequencies with those in the previous temporal position, and mapping their position (given that their velocities are also well known), you could get a reasonably precise location from hard-to-miss stellar bodies.

What would be even better is to combine the data regarding their spin-down (or spin-up, if you're going into the past) with trajectory distances and perhaps additional clues such as broader star movement and geomagnetic data, minimizing error in the result by taking a weighted average of known error. This would all be without the need for an internal device, beyond simple precision sensors, and could get you pretty good precision. The only points at which it would fail would be extremes where the pulsars weren't formed yet (which is pretty exhaustingly far out), or in locations far enough from known space where the pulsars could not easily be found in the sky.

...I would suggest filling the remaining space with a comfy couch and an entertainment system for the trip.

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    $\begingroup$ Many of these pulsars won't exist if you go far into the past. The crab nebula pulsar isn't avaliable before the 11th century AD for example. $\endgroup$ – Geronimo Jun 27 '18 at 12:37
  • $\begingroup$ Well, define "many". Pulsars have a lifetime of around 10^7 years. We know of about 600 of them. Keeping a pulsar map across time would be helpful. You point out that the Crab Nebula pulsar would blink out in the eleventh century, which is true, but there are many others to follow. At the limit of an infinite expanse of time, roughly as many would blink out as would form. So, I don't think this really applies to the idea of using pulsars as landmarks (timemarks?); it amounts to a data point showing that you're before the 11th century. (Or after it burns out.) $\endgroup$ – Michael Eric Oberlin Jun 29 '18 at 2:21
  • $\begingroup$ The pulsar thing won't work for the same reason the people suggesting looking at the stars won't work: the further back in time one goes (and the original suggestions was millions of years), the very different the sky looks. Outside of the solar system, the starscape is going to be unrecognizable. Well known stars might not exist (Betelgeuse is only about 10 million years old). Stars that no longer exist will be visible. Stars will be obscured behind dust clouds or alignment with other stars. They might be in different spectral classes due to a younger age. $\endgroup$ – Keith Morrison Jun 29 '18 at 8:08
  • $\begingroup$ @KeithMorrison I'm sorry, are we talking about a time machine, or a space ship? Also, we're looking at absurdly high momentum on these stars. Their presence and position is what I'm suggesting interpolating on to determine the position in time; they aren't exactly going to waver off course on their own. What you're claiming to refute the usage of star maps or pulsars is actually precisely why they would work. $\endgroup$ – Michael Eric Oberlin Jun 29 '18 at 8:22
  • $\begingroup$ The "pulsar map to calculate the current time" only works as well as how far back the position of those pulsars can be predicted, and over a few million years it's "not very well at all". $\endgroup$ – Keith Morrison Jun 29 '18 at 20:50
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Since the earth and the sun move and rotate through space, too, I'm afraid you'd have to use a space-time machine, leaving you only with the ability to test whether you have hit the space-time coordinates you're aiming for.

One test-run would then look like this:

  • Calculate where in the universe the earth would be at the time you choose.
  • Send your space-time machine to these space-time coordinates.
  • The machine tries to find earth and measures the distance to it while triangulating its own position using known stars.
  • After returning, the data gives you your error margin.

Of course, you'd have to account for accidents like "there is molten lava outside the time machine" or "the time machine is a mile high up in the air and about to crash" or "the time machine has a vastly different velocity vector than the earth".

Or, you get around all that and assume your time machine is gravity-bound the whole time it is in transit.

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    $\begingroup$ Every basic time machine has to account for Earth's absolute motion, otherwise the machine doesn't survive the first test (or at least doesn't return from the test). Advanced machines calculate time and relative dimensions in space, for travel to any_where_ as well as any_when_. $\endgroup$ – Codes with Hammer Jun 28 '18 at 14:39
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    $\begingroup$ Slightly less risky would be to try to appear nearish to the Earth rather than on it. If you miss the surface by a 100 feet you are having a bad day, but if you are aiming for say L2 and miss by 10,000 miles you are fine. $\endgroup$ – user25818 Jun 28 '18 at 22:30
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    $\begingroup$ @CodeswithHammer Exactly my thoughts, but heeding that would destroy the narrative that's asked for. With this precondition, you'd need your space-time machine to be accurate in space and time on the first try, else you won't get it back (it'd be displaced on its way back to you, because you'd need to send it back to the same space-time coordinates it started from, but as the narrative states, the time span travelled is fuzzy and needs to be calibrated). $\endgroup$ – orithena Jun 29 '18 at 14:42
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    $\begingroup$ @orithena: Now I need to double check the technical description in US Patent Number One, archived at cheapass.com/free-games/broken-games $\endgroup$ – Codes with Hammer Jun 29 '18 at 15:05
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Genetic analysis of ground life form

First, sample the DNA of as many commun ground dweling fungis (or any eukaryotes) as you can.

If you have no access to the night sky, you could dig to find the same species (more exactly their ancestor) and measure the number of change, especially in the Non-coding DNA. This can give you some estimation of the time between your departure and "now"

This method can work while traveling in the past or travelling the future. But I am afraid it can not tell you if you travelled 1000 years in the past OR 1000 years in the future.

Safe travel to you!

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Use Radioactive Decay

Similar to Josh's answer, If you are capable of building a time machine, you should be able to build a small automated robotic device that can simply move out of the time machine, take a soil sample, then return to the time machine and perform analysis on the soil using spectroscopy.

Almost all soil around the world will have a small amount of natural uranium that you can analyze to check the ratios of U-238 and U-235. This is one method for determining the age of the earth, and it should do well to determine how far back your time machine went when comparing to modern ratios.

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  • $\begingroup$ The only information this will provide is the relative age of the sample back then. This gives you the elapsed live span of this piece of rock|earth|mineral since it was created, but no fixed point in time. $\endgroup$ – tofro Jun 29 '18 at 13:53
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    $\begingroup$ @tofro The problem was to determine how far back the time machine went. This can easily be accomplished using this method to compare the ratio at that time to the current ratios, then determine a time difference. $\endgroup$ – T James Jun 29 '18 at 15:16
  • $\begingroup$ You can calculate a time difference if you find the very same piece of material today and 10 million years (or whatever) back. But that is highly unlikely. $\endgroup$ – tofro Jun 29 '18 at 16:55
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    $\begingroup$ @tofro Although different soils will have different concentrations of uranium, they should have very similar isotopic ratios of U235 to U238, no matter where you are on the planet. The only exception is if you manage to find some soil that had previously been depleted in a natural fission reactor (Oklo reactor). $\endgroup$ – T James Jun 29 '18 at 18:47
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1) Send your time machine from the present to random times in the past when there were already radio broadcasts.

2) Measure the used fuel every time.

3) Wait until one of the radio broadcasts tells you the time for determining the exact time you have arrived in.

4) Calculate the relationship of time travelled and fuel used.

5) Once you have a working equation, test your time machine with travelling between past times to test whether the equation holds up if your point of origin isn't the present, and rework the equation if needed.

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    $\begingroup$ This would fail even for a car, where the fuel consumption depends on temperature, wind, inclination, etc. $\endgroup$ – pipe Jun 27 '18 at 12:59
  • $\begingroup$ @pipe true, who knows what the weather is like in the Time Vortex! $\endgroup$ – Michael Jun 27 '18 at 23:26
  • $\begingroup$ @pipe that would just mean you'd need to put those variables into your calculation as well (and find a way to measure them). $\endgroup$ – Real Subtle Jun 28 '18 at 6:17
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Are there any trees nearby? Dendrochronology could be used on species like bristlecone pine that live thousands of years. Beyond that, sediment layers in mountain ranges. You could potentially have a record of tree rings going back 10,000 years if the research has been patched together.

My main concern with time travel though is that you would move into the future or the past at the exact spot in spacetime, but the earth and solar system relative to that spot would be millions of miles away, or more. Maybe you have to aim the time ship in the direction of where the earth is at the destination’s time.

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    $\begingroup$ TemporalEngineering 101, the gravity well of the planet will ensure you are sucked right along with it :) But yes. It is a serious issue, most stories handwave it away or don't even mention it. There is probably something in the flux capacitors that deals with this. worldbuilding.stackexchange.com/q/21048/21839 $\endgroup$ – EveryBitHelps Jun 27 '18 at 8:24
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    $\begingroup$ Actually it's a boring besserwissery issue so it shouldn't concern anyone. $\endgroup$ – pipe Jun 27 '18 at 8:43
  • $\begingroup$ @pipe thank you for introducing me to that term for wise-ass. Not heard it before. While yes, it may be wise-assery, I do think it can have some funny unintended storylines connected to the incorrect positioning of the device. $\endgroup$ – EveryBitHelps Jun 27 '18 at 10:49
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Continental drift

I think a cheap way of doing it, yet giving a more or less precise reading, is to look at the position of the continents. They are moving slowly, and we have a rather good idea of where they were at which time, as well as their speed of travel.

You could send an high altitude balloon to get pictures and process them when the machine gets back. Try to have a system that sends the pictures from the balloon to your machine, it's easier than to retrieve the balloon.

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    $\begingroup$ it almost sounds like you're speaking from experience... but what if the balloon is then found by government agents after you just left it behind... it COULD be mistaken or at least reported to the local media as a weather balloon... and it could have landed/crashed in Roswell................. Legisey... are you a time traveller??? $\endgroup$ – Blade Wraith Jun 27 '18 at 7:24
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    $\begingroup$ @BladeWraith when I travel to an epoch where there are such things as government agents, I usually just go and look at a newspaper to find out the day I'm in. $\endgroup$ – Legisey Jun 27 '18 at 7:52
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    $\begingroup$ fair point... well made. but you haven't actually denied being a time traveller... $\endgroup$ – Blade Wraith Jun 27 '18 at 8:19
  • $\begingroup$ "I did not have temporal excursions with that device" $\endgroup$ – EveryBitHelps Jun 27 '18 at 9:09
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    $\begingroup$ Which leaves the question open on temporal incursions. $\endgroup$ – TemporalWolf Jun 27 '18 at 18:21
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Use the star map for short time, use the expansion of universe for longer time

For a few million years

Other answer deal with it pretty well. You can see the star map at night, you can compare air sample with air trapped in Antartica (works only for travel in the past) or human radio emision (work only in near future)

From 100 million years to a few billion years: red shift of distant pulsar

Even if you cannot predict the exact trajectory of star/galaxies on the very long run, you'll still be able to locate a few pulsar.

Even after several billion years, the frequency of their pulse should not change much.

And if you have a doubt, their relative position should also be similar. Most distant object will not move by several degree even in 10 billion years, so a pattern of 5 pulsar will still look the same( Worst case scenario, there are only 4 left).

So, you'll be able to spot a few pulsar. You know the wave lenth they emit nowadays. By measuring their wave length somewhere in the past/future tells you how much the universe did contract/expand.

Crunch a few numbers and it will give you the date.

Edit: This works better if you travel in the future as in a distant past, the pulsar may not exist yet.

For eternity: Cosmic microwave background

Curently, the Cosmic microwave background has a temperature of 2.72548±0.00057 K

This decrease with time as universe expands.

So a few computation will tell you the date.

Problem: to measure this for earth surface, with current technology you'd need a 10m wide instrument. And the dificulty increase when you travel in the future.

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    $\begingroup$ Editors: don't hesitate to correct my english. Right now, I'm suposed to work on accountacy, not to check my grammar on a foreign-language article about time travel ;-) $\endgroup$ – Madlozoz Jun 27 '18 at 13:26
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1) Air pollution.

I believe the composition of trapped atmospheric gases in Greenland ice cores have be used to detect minute changes in Earth's atmosphere over decades, centuries and millennia, This has been used to chart the rise and fall of various industries over the ages as they pollute the atmosphere.

So your time calibration package should include an automatic air sampler to bring back an atmospheric sample you can test in your laboratory to see how well it fits in with the record of Atmospheric composition changes in past years, decades, centuries and millennia. If the time machine goes back to a time within the range of scientific studies of atmospheric composition from ice core samples you should know the date within a few years or decades.

2) If you have some sort of automatic robotic optical and/or radio telescopes in your time calibration package, they can go outside and measure the important optical and radio sources during the daytime and the nighttime.

The Moon can always been seen when above the horizon, day or night, except when hidden by clouds. The direction to the Moon, and the phase of the Moon, can be measured very accurately. The phases of the Moon change over a period of 29.6306 days.

The planets Mercury, Venus, Mars, Jupiter, and Saturn would always be visible to the naked eye when above the horizon during nighttime, and sometimes would be among the brightest objects in the sky - Venus, Mars, and Jupiter can sometimes be seen during the day.

If the telescope can tell the difference between a planet and a star, it can identify those planets that are above the horizon. So the directions to the planets, and their phases, will show the positions of the planets relative to the Earth, at the time the observations are made.

For each body in the solar system, there is a synodic period, the period between the times when it's angle relative to Earth is the same. For Mercury the synodic period is 115.88 days, for Venus 583.9 days, for Mars 779.9 days, for Jupiter 398.9 days, and for Saturn 378.1 days.

So if the automatic telescopes are able to find the directions to three or four planets, that will show a relatively rare configuration. A length of time that three or four synodic different periods could fit in as integers and not fractions would be a very long period of time in which each synodic period was repeated many times over. And then if that is combined with the phase of the Moon with its own cycle, that becomes much rarer.

Then there are the apparent positions of the four Galilean moons of Jupiter, which would be easy to detect. They have their own synodic periods in relation to each other. That will make the day that your time machine spends in the past even more unique.

So astronomical observations should make it easy to identify the 24 hour period that your time machine spent in the past or the future, for as long as astronomical orbital simulations are accurate.

3) For a much longer time span, the Sun takes about 200,000,000 years to make a full orbit around the galactic center. The center of the galaxy is the first astronomical radio source ever discovered, so it should be easy for a radio telescope to find the direction to it, if it is above the horizon. Several external galaxies, such as the large and Small Magellanic Clouds, M31 in Andromeda, and M33 in Triangulum, are bright enough to be seen with the naked eye and should be easy for a small telescope to detect. The galaxies Centaurus A and M87 are also famous radio sources that should be easy to detect with a small radio telescope.

So it should be relatively easy to detect the directions to both the galactic center and one or more external galaxies, and thus determine the position of the solar system in it's orbit around the galactic center, and the nearest past or future eras that could be.

4) Send a Mars rover type robot out to collect rock samples (different rock samples in different eras, of course, since you don't want time paradoxes). Most minerals contain trace amounts of radioactive isotopes which decay into other isotopes at a rate that can be calculated. You can test the samples in your lab for the ratios of various isotopes, and if the samples are collected from far enough in the past you should be able to calculate the approximate date.

And there are many similar astronomical and geophysical methods that you can used to detect the date on various long, medium, or short, timescales, and to calibrate various methods against each other to improve accuracy.

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    $\begingroup$ Method (1) has been demonstrated in field experiments to show a coarse precision; see Nimoy & Doohan (1986). Then once the era has been established as industrialized, Legisey points out that the simplest solution to determining the precise date is to look at a newspaper. $\endgroup$ – Codes with Hammer Jun 28 '18 at 14:36
  • $\begingroup$ @Codes with Hammer In this case considering that the trip was only about 200 to 400 years, the coarse precision of Nimoy & Doohan (1986) was no doubt soon followed by a very precise date using my method number 2, even if that did not happen onscreen. $\endgroup$ – M. A. Golding Jun 28 '18 at 20:43
  • $\begingroup$ It wouldn't surprise me. A rough time frame determined by your method 1 makes the effort of method 2 much easier. $\endgroup$ – Codes with Hammer Jun 29 '18 at 15:00
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The radius of the Sun shrinks by 74 centimeters per year.

The fusion reaction of the Sun converts mass into energy and overtime it loses mass. You could measure the diameter of the Sun to determine the current year.

This approach has the advantage that the time machine can appear in any moment in history and calculate the current year. Assuming the rate of change in the size of the Sun is constant. It becomes a linear calculation of how much time has changed.

I doubt you'll be able to calculate the exact year, but if you have the technology to build a time machine you can surely build a device to accurately measure the diameter of the Sun.

Here is a reference to Sun shrinkage from Stanford University.

http://solar-center.stanford.edu/FAQ/Qshrink.html

Also, if you take a photograph of the Moon's surface. This image defines a timestamp in history. Comparing two images of the Moon's surface will tell you which image came first. As new impacts overlap old impacts. The more rapidly the surface of the Moon changes the further back in time you've gone. This isn't going to help you over small distances, but if you don't know how far your time machine has travelled, then taking an image of the Moon would be a quick way of knowing if it was a long distance.

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  • $\begingroup$ Oh yeah, couldn’t you measure the distance of the moon to a few milimeters and calculate with this. The moon is slowly moving away from the Earth $\endgroup$ – user22106 Jun 27 '18 at 15:33
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    $\begingroup$ Actually, the link you provide says that if the sun was shrinking due to gravity it would shrink by 74cm per year, but observation has shown that this is not happening. $\endgroup$ – Michael Jun 27 '18 at 23:25
  • $\begingroup$ And, measuring the radius of the sun to sufficient accuracy would be much more difficult than measuring the position of other celestial objects. $\endgroup$ – leftaroundabout Jun 29 '18 at 11:09
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I want to extrapolate on an earlier answer and comment:

Use an atomic clock.

Set the "Time Selector" dial to whereever you want to randomly send the machine, then have the automated systems deposit the atomic clock in the past. Recover the time machine. Turn the dial by, say, 1/100 of a degree forward (or adjust the dial however you can adjust it while measuring the change). Go back and read the time on the clock.

If the clock is old and rusted by the second jump, change the dial so it's only 1/1000 of a degree forward and try again. Keep using smaller calibrations until you get an usable reading from the clock. Now you have two data points; preferrably far apart in time.

Then, keep sending back the time machine to random times between your two endpoints. If you keep gathering samples, you will eventually have enough data points to approximate an equation to describe how much time a turn of the dial represents (is it linear, or is it some other strange model?). Find where this equation intersects with the present day, and this will allow you to find the numerical year that your time machine landed; given a good enough approximation, you could theoretically pinpoint your calibration to the nanosecond.

This method has the advantage of not needing to use measurements from uncertain sources (no sky readings, no DNA samples of external organisms, etc.), but only if you can overcome the obstacle of landing in the same place over a large amount of time (preferably the same desert).

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  1. Measure the luminosity and the size of the sun, and thereby derive its age (and in turn, via comparison with current-day values for the same measurements, find a temporal position).

  2. Similar to the answer that mentions measuring the rate of expansion for the universe, you can measure the redshift of gravitational waves & compare to current-day values.

  3. Again similar to the above, measure the background radiation and calculate time based on the CMB "trajectory".

For a craft of any kind, space-faring or otherwise, option 1 (out of the ones mentioned here) is probably the most feasible in respect to size of necessary equipment - and can probably easily be done with equipment you can also use for some of the other answers here, like the top-voted answer that suggests imaging the nightsky, meaning you can do several different measurement types and compare the results.

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I would recommend tracking the movement of key stars in the sky. Pick the correct ones, and you save yourself the need to track them all while still forming a comprehensive picture of the night sky that should not appear perfectly identical between two given times. This does limit the device to being used under an open sky, primarily at night: for obvious reasons, this isn't going to work inside a cave or a building. You're going to need a good telescope on your time machine as well, and probably regular inspections to check its calibration and accuracy (if it's unable to detect and repair this automatically).

Or you can just use an atomic clock, if simply having precise timekeeping is enough in the absence of outside references. Those are accurate to within a few seconds every million years, and I don't think your eventual time-travellers are going to be too concerned about being off by a couple of minutes.

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    $\begingroup$ Surely the atomic clock would only give you a highly accurate reading of how long the time travel device was visiting, not how far into the past/future it had gone. $\endgroup$ – EveryBitHelps Jun 26 '18 at 22:32
  • $\begingroup$ @EveryBitHelps There was a very good reason for that "if": it only works "if" it's as simple as winding the clock back, "if" you don't require some sort of external measurement. $\endgroup$ – Palarran Jun 27 '18 at 2:49
  • $\begingroup$ Ah. Sorry. Missed the "if" the first time I read it. I think they will definitely need some sort of external reference. Maybe if they just dropped the atomic clock (with radio beacon) outside, way in the past. Now they have an external reference :) $\endgroup$ – EveryBitHelps Jun 27 '18 at 7:37
  • $\begingroup$ @EveryBitHelps That's actually a really good trick, except for the problem of making sure it stays operational without maintenance for potentially millions of years. See: rust, running out of power, sudden natural disaster like an earthquake, a dinosaur stepping on it, etc. $\endgroup$ – Palarran Jun 27 '18 at 11:19
  • $\begingroup$ No-one said it would be easy! Lol. Try placing a solar powered version of it in space or on the moon surface. That should cut out rust, most sudden natural disasters and a dinasaur stepping on it. It obviously brings other problems... $\endgroup$ – EveryBitHelps Jun 27 '18 at 11:59
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The most accurate way of doing this would be the Stars, but many people have suggested this so its best to see their answers, but i'll offer an alternate suggestion

They'll be a device within this time machine which when more power or [insert plot device] is passed through it controls the amount of time traveled, for personal reasons i'll call this the Flux Capacitor FC for short.

If you don't know what this device is you need to rig up several sensors to every component and run the below tests for each part just to ensure you have the right component.

Once you know which part the Flux Capacitor is, then run the below tests.

Test 1: Measure the power to the FC upon embarkation very precisely with several sensors, and take star measurements. and place a radioactive isotope on the ground. return to present

Test 2: repeat test 1 to ensure that the correct sensor readings, if Stars match test from Test 1 (taking planet rotation into account of course) then you have a good basis to believe that your testing is accurate so far and you measuring the relevant FC settings is correct, and that your machine has the ability to accurately repeat a destination. in theory the radioactive isotope should read the same levels of emitted radiation as when you left it there as it would have only been x amount of time between transits

Test 2.5: in theory it would be advisable to run test 2 several more times to be 100% certain

Test 3: Reduce the FC's power slightly and embark once again, take star measurements once again. but also scan for the radio isotope, if found measure the radiation emitted from it, it should be less, on the assumption that less power into the Time Machine/FC would mean a shorter distance back in time traveled. measure how much less and this should give you an idea of how much power difference changes the amount of time traveled.

Test 4: Repeat this transit at the same power level. always double and triple check your findings with multiple identical tests using the same power levels

Test 4.5: return to previous power levels and run test to ensure that you return to the previously established moment in time

Test 5: further reduce the power into the FC and perform the same test as Test 3, again this should give you a good idea of power change

Test 6: increase the power past the initial starting point to and area your previous readings would suggest is say... 1000 years before Test 1 landed. and then place the Radioactive isotope down again and then return to the present

Test 7, return to Test 1 levels and detect the isotope from an already established moment in time, confirm settings are as predicted and adjust for any changes

Again repeat repeat repeat, these tests should give you an idea as to whether the the power requires are exponential/logarithmic as you want further back, or incremental, once you know this then its fairly simple maths to figure out, I want to go X years into the past i need Y amount of power, this allows you to know where you are going

its better to know where you're going, because then you can use Star mapping as others have suggested to confirm you are where you expect to be.

Just do us all a favour and bring a hazmat suit with you. not for you, but so you don't bring a modern illness into the past.

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Deploy a robot that travels to the past and buries a known quantity of a radioactive substance in the soil (in a properly shielded and tamper-evident container, to avoid detection and trouble with the EPA and such). In the present, dig up the container and measure how much of the substance has decayed. Depending on which element you choose, you should be able to determine when the sample was buried -and thus, how far back the machine went- to a fair degree of accuracy. You might even bury several samples of different elements, just in case strange time-travel effects might alter some of them but not others.

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  • $\begingroup$ This would be accurate if many samples were buried. The decay is based on a distribution averaging around the halflife $\endgroup$ – Sentinel Jun 28 '18 at 4:50
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Assumptions

If you want to calibrate it, I assume you already have a way to make it go further or closer, and just need to know by how much. For example, you have a dial that goes up and down, but you have no idea if one rotation is a year or a million years.

I also assume that you are using a fixed timeline (stable time loop) model for time travel, because you specifically mention how there is no multiverse, and changing the past would make paradoxes by the dozen.

First Step

First, you find an object that has been in place for, let's say, five thousand years. There are such objects on the earth, and while you might have a few legal issues bringing the time machine there if it is big, you should be able to work around it. A great example would be Stonehenge. You setup the machine to scan for a specific Stonehenge rock, or maybe the entire pattern, and do some trial and error to find a dial setting where Stonehenge exists.

If your machine has the potential to go back as far as the big bang or even before, you're in trouble regardless, because the odds of you reaching the earth are so small you might as well give up now. If your range is around the age of the earth, you will need at most 908 600 attempts, so on average 454300 attempts, to find a time period where Stonehenge exists (Stonehenge has existed for 5000 years, and the earth has existed for 4 543 000 000 years).

The trick

Now it might already seem like way too many attempts. But remember, if you can automate scanning for Stonehenge, you can definitely automate trying again by changing the dial a bit. And since your machine is a time machine, it can always come back, after scanning or even after 24h in the past, to one millisecond after it left, therefore completing all those 500 000 scans (or so) in 500 seconds, or just a few minutes.

Bonus: You can even add to your story the fact that your machine's attempts throughout the last million years are what caused people to make Stonehenge in the first place, because a strange device kept spawning around that place and acting weird, then disappearing. This can either be a fun fact or a plot point.

Next Steps

Once you've found at least a dial setting containing Stonehenge, you can check the number of attempts that have been required, and do a rough estimate of your dial's step size. Now you can do the same operation, but with an object that has existed for just a few years, but is much easier to scan for. Take the largest tree behind a house that has been in your family for a few generations. Explain grandma's irational alien sightings at the same time, and do the exact same process, with a much smaller increment on the dial. This should take about the same number of tries, but it is much less public and problematic.

Once you've found the tree, you can again roughly estimate (but with much better precision) the dial's increment size with your two measures. You can just repeat with a one month old object that doesn't move. If there is none, you can spare a month making it, just make sure to ignore the random time machines that show up during that month of waiting.

Figuring it out

You repeat until you have a few very specific measures on the dial. I suggest more than two, just in case the progression isn't linear but something weirder like square, exponential or sin. About ten good precise measures, once you found a range of dial that is easily testable like an hour, should give you all the data you need.

The potential problems

  • If your machine can also go in the future, you might "miss" the 5000 year window by a lot if your increment is a million years. If Stonehenge doesn't last that long in our future, then you might test billions of years in the future, never finding anything.

    Fix: If you don't find anything after billions of attempts, just start over with a smaller increment

  • After a certain number of attempts, your machine may not come back. Maybe it got destroyed during the scan in the past, or worse, captured. Since no one in the past had time-travel inhibiting tech, you can assume it got at least disabled completely, maybe by people, animals, climate or environment.

    Fix: Add a self-destruction mechanism with a dead man's switch. If the running code either fails or stops running at any point, wait 1 minute, then blow up. Make sure the blow up is enormous enough to annihilate everything, to avoid giving modern tech to our ancestors. It doesn't matter if you kill one of a hundred of them since it's a stable time loop. Also make sure you have the machine write a log every time it pops back in the present, so you know exactly where it was so you can resume testing from there with the next one. Have very precise plans to build it again if required.

  • Your machine doesn't compensate for earth's rotation, movement, the solar system's movement, the galaxy's velocity, etc

    Fix: Yeah you're screwed. Handwave something and compensate for earth velocity.

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Are you only looking for ways to test based on the local environment? And are you trying to avoid more handwavium than a time machine already requires?

Assuming yours is the only time machine, you could conceivably set yourself up a "time beacon". Set up a device in your lab that syncs with a similar device in your craft (quantum entanglement sounds sexy without actually meaning anything). By comparing the settings of the two devices, you could determine not only distance in miles from your point of origin, but in years.

Such devices have been used in other stories. The Legion of Super-Heroes stories has a time beacon set up in the 31st century.

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There is a much easier and safer way.. Program it to send and respond to the same signal. Send it back 100years to remain in place a few minutes and then return. Let us say you have it well calibrated to 10years but do not know exactly how well, which is why million year excursions would multiply your error and result in drift. So have the machine repeatedly send itself back 10 years 10 times. If the machine does not find itself after say 24 hours, return and assume the drift is under 10 years. Recalibrate to lengthen its concept of time and repeat. If the machine finds itself, note the delay before the first machine appears and calibrate accordingly. Repeat for 1000 years with 10 steps of one hundred. Do this then for 10000, 100000 and then one million. BONUS:The method works for future calibration too.

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If you can move it somewhere else, try to go in a place where there is some ancient monument, for instance, the most ancient and known one is Barnenenz, in France. If your time machine can find it, it means it's 4850 BC or later, if it can't find it, it's earlier.

Do some attempts in a binary search fashion and you will have your time machine calibrated to that specific date (6868 years).

I know it's not a long time span but it's a start.

To get longer time calibration , you can place your time machine in the position where a dinosaur fossil has been discovered (possibly an easy one to find), this will give you a way longer time span. If the fossil is not there it means the dinosaur hasn't died yet 😉

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Rather than using external cues, as others suggest, you could simply fit it with an odometer*.

I.e. you don't know exactly where* it's going to end up, but it could be capable of recording how far* it has travelled. You set the dial to 1 million years, it goes, and on it's return the odometer reads 2.5 million years (there and back again), you know it was out by 0.25 million (plus or minus 24 hours if the odometer records time elapsed in the old fashioned way). A few taps with the spanner later and you're in business.

This was pretty much the solution that H.G Wells used in The Time Machine, he controlled the machine with a lever, relying only on the 'odometer' to tell him how far he had travelled.

*temporally speaking, not geographically

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You take a geological sample and do an analysis of uranium isotopes in the sample, using alpha b, mass spectrometry or maybe some other technique to obtain the isotope ratio between U-238 and U-235. As the halflife of U-235 is shorter than that of U-238, this ratio is slowly increasing. - this will easily tell you how many million years you are back, maybe within 100000 as well (I have not done the math ;-) ) - if you want to compare times a few millions ore more of years back, you may even be able to use ratios involving 236-U or 233-U - with much shorter halflifes, they will give a much better resolution 10000 / 1000 years, but they can not help you to obtain an absolute point in time.

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If you consider that gravity and the magnetic field are not constant through time, the machine can just accurately measure Earth's values and do the proper calculations to exactly determine the time it ended up in. There is no need for other external measuring tools.

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Fundemental problem

And assume that all of my testing is being performed in a desert area of the American southwest.

There is a fundamental problem that most time travel designers oversee. They all seem to assume that planet earth's position is at the center of the universe.

You are traveling through space-time. Let's say you assume to keep your position in space coordinates and you travel back or forward for only a few minutes, you are going to miss earth big time.

Wikipedia:

Earth's orbital speed averages about 30 km/s (108,000 km/h; 67,000 mph), which is fast enough to cover the planet's diameter in 7 minutes and the distance to the Moon in 4 hours.

Taking into account the speed of the solar system itself (Again wikipedia):

The Solar System is traveling at an average speed of 828,000 km/h (230 km/s) or 514,000 mph (143 mi/s) within its trajectory around the galactic center.

Now, this could go on taking all movements and expansion of the universe in account, but I will stop here. Point is: you will need a pretty good coordinate calculation system before you send your time machine. This can be done, if you know exactly what time you are sending it to. But there appears the paradox: you want to send it back in time to calibrate it's "Time dail". It is therefore impossible to put it anywhere on the planet.

Problem becomes the solution

So, don't try to land in earth. Make sure that the design is capable of withstanding outer space. Send it to exactly the same space coordinates, which will put it somewhere in deep space, with the best view of the galaxy you can ever get.

Now, it can do wavelength identification of our own solar system and other known stars or even remote galaxies. And based on the absolute or relative differences, you can accurately calculate the time.

Risk calculation

Before sending back your machine, try to estimate its inaccuracy. Based on the galactic model, try to keep the probability small it ends up in another star that since traveled to the same coordinates. Changes are really small, but the calculation is not difficult vs loosing your precious machine.

More runs

Do more runs, to different time frames. Try do to confirmation measurements that the galactic model is indeed correct. Each run your "Time dail" should become more accurate. Once you have enough confidence, start tuning the coordination system.

Do the same range of trips, but closer to earth. Assess earth's position and own rotational speed. Try to create your own calibrated movement model, not based on calculation of modern science, but what is now actually measurable.

In the final test you send it in the same place in the south-western American desert. If the machine returns in one piece (eg you did not put it in earth's crust or let it plunge down from a reasonable height) your machine is ready for human travel!

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  • $\begingroup$ Welcome to Worldbuilding, Tim! If you have a moment please take the tour and visit the help center to learn more about the site. You may also find Worldbuilding Meta and The Sandbox (both of which require 5 rep to post on) useful. Have fun! $\endgroup$ – FoxElemental Jun 29 '18 at 13:15
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Turn off the space unit in your time-space machine (in order to avoid catastrophic failure, I'm assuming it is necessarily a time-space machine, but we don't need that part here).

Travel back some "moderate" unknown amount of time (couple of hundred years, or some thousand, whichever), that will place the vessel somewhere in space. Drop off a radio beacon, go back.

Receive the signal. If you don't receive anything, something went wrong (beacon destroyed, out of power, waaaaaayyyy too far into past?) - try again.

Either you need a timestamp encoded in the signal (then you need to correct for the speed of light, but luckily this is pretty constant, and well-known), or you can use Doppler's Effect (then you'd need 3 beacons minimum), whichever. You could use both, and cross-validate.

Calculate the distance between "then" and "now". Earth's speed while tumbling around Sun through Milky Way is known. There you go.

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  • $\begingroup$ I'd say that the "going back" part is gonna be a problem, because the machine needs time to drop off a beacon -- and when it jumps back the same amount, the present earth has already moved enough that the time machine wouldn't return to the same coordinates relative to earth. Also, assuming the beacons don't move after sending them back, you can't receive the signal after sending the beacons back in time -- you'd need to do it before the beacon burns up in earths athmosphere (if you send a beacon back in time from earth, but not in space, then the earth will cross the beacons' position). $\endgroup$ – orithena Jun 29 '18 at 15:00
  • $\begingroup$ @orithena: Earth crossing the path is a rather small risk. It orbits Sun, which orbits the center of the galaxy. Staying in place for the beacon is a problem, I'll give you that. Actually, the beacon should move with a speed identical to the speed of Earth (conservation of energy). However, time machines must be time-space-velocity machines. Otherwise, you could never arrive somewhere, anywhere, without catastrophic failure. You'd always arrive either in a solid object, or moving in the wrong direction very fast. Or something. So... handwave that part, magic time machine does it right. $\endgroup$ – Damon Jun 30 '18 at 17:28
  • $\begingroup$ If you send the beacon 1 hour back in time from some point on earth, and it then stands still (relative to some "universal absolute coordinate system") -- then, from the point of view of the beacon, it will be at a point in space that the starting point and therefore the earth inevitably will cross in one hour. So, that beacon will burn up in the athmosphere slightly before the point in time where you started it from. This is true for all time spans, just think it through. $\endgroup$ – orithena Jul 1 '18 at 12:37
  • $\begingroup$ Anyway, I very much agree on your remarks on time-space-velocity machines and needing to handwave a lot ... but I really would like to read a story that gets that part right in detail :) $\endgroup$ – orithena Jul 1 '18 at 12:41

protected by James Jul 2 '18 at 17:53

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