How can I tell I've travelled 20 million years into the future?

Question

Story starts out about 100 years from now.

While in orbit around Earth preparing for an interstellar mission, our spacecraft encounters some hand-wavey spacetime anomaly such that we're still in orbit around Earth, but approximately 20 million years into the future.

How would we be able to tell how far into the future we have traveled?

For the purposes of the story, H. sapiens has long since gone extinct on Earth (within a million years of the present day), and it's not obvious if they've left any descendants, or if they traveled to the stars. No other obviously intelligent or technologically advanced species has risen since then. The state of things is roughly the same as it was 20 million years before the present, way before the first humans showed up.

Some things that I've thought about:

• Stellar motion (constellations won't look anything like they did before, and we should have a fairly accurate catalog for navigation purposes);
• Continental drift (after 20 million years some motion should be evident, although since this meant to be an interstellar mission, we probably wouldn't have detailed maps of the Earth's surface to compare against);
• Earth-Moon distance (Moon is receding at a rate of a few cm a year, should be measurable after 20 million years);

Since this ship was designed for interstellar travel, assume we have plenty of $\Delta{V}$ to tool around the solar system and landers to explore the surface.

What man-made structures (if any) would survive for 20 million years on Earth? If we find what used to be Yucca Mountain (or a similar site), could we somehow find and use the radioactive material that was stored there to estimate how long since it was buried?

What shape would the Apollo landing sites be in (or any other man-made structures on the Moon) after 20 million years?

What other things should I look at that would provide a somewhat reliable estimate?

Edit

To add some clarity and address some questions:

1. For the purposes of the story, the estimate only has to be accurate to within a couple of million years - the exact number of years doesn't really matter. And they have to be able to determine they've moved into the future, rather than the past.

2. The effect of the hand-wavey anomaly puts the ship in orbit around the Earth where the Earth will be 20-ish million years from now (magically preserving angular momentum), meaning they've travelled through space as well as time; however, from their perspective, they never really moved (although for some cheap initial drama, I may have it deposit them in a higher or lower orbit, or in a different inclination). And, for the purposes of the story, how they got there really doesn't matter.

3. This was supposed to be an interstellar exploration mission using a hypothetical reaction drive with an insanely and likely impossibly high $I_{SP}$ allowing the ship to reach substantial fractions of $c$, such that the trip to the original target would have taken on the order of a decade within the ship's reference frame (centuries within the Earth's reference frame - the crew know what they're signing up for). Thus, they have plenty of $\Delta{V}$ to tool around the solar system (if they choose to do so), although high $I_{SP}$ usually correlates to low thrust, so it may take a while to get anywhere.

4. While there are planetary scientists on board, the purpose of the trip is to analyze bodies in another star system, not Earth. They will have some Earth reference data, but not highly detailed maps of the surface.

Answer 1

With proper excavation and radiometric dating, everything is possible.

Our current civilization is leaving a huge level of non-natural isotopes, some of these imbalances will be carried into hundreds of millions of years into the future. For example, the astronauts can use Uranium–lead dating:

The method relies on two separate decay chains, the uranium series from 238U to 206Pb, with a half-life of 4.47 billion years and the actinium series from 235U to 207Pb, with a half-life of 710 million years.

The real problem would be, as I see it, that your astronauts would have no idea what the "civilization baseline" level is, meaning, they would have no idea what isotopic composition was by the end of human civilization. It could be the same level as when they left, or 100 times higher because of a nuclear war or global contamination. Without knowledge of the baseline, accurate calculations would be very tough thing to do.

That means that they would have to perform excavation (or find exposed sedimentary rocks), just like geologists do today. They will inevitably come to a sediment layer that corresponds to human civilization, and from there they should be able to make more accurate estimate.

• Earth-Moon distance that you have already mentioned should be another good timing method;
• Individual stellar motion will be unpredictable for 20 million years into the future. But Sun's position with respect to the galaxy (and other galaxies) should be predictable.
• Continental drift would be a good rough estimate, but it will lack any precision.
• All human buildings will be buried at this point. Some will be preserved as they get buried. Your astronauts however will have to search for these buildings and do excavation.
• Apollo landing sites will get degraded, but still going to be recognizable at close inspection.

Answer 2

If you know, that you are in the solar system there might be another option you can reasonably construct. Your humanity did build an interstellar spaceship, so it is reasonable to assume that they also launched a load of other stuff into space in their time. Everything in a close to earth probably already decayed in orbit and is gone, but stuff sent further out might not be.

So I assume that in order to look for targets for interstellar travel, humanity build some space telescopes and in order to minimise interference from the sun and all the stuff in the inner solar system, they put them somewhere quiet, far out of the way, for example one might be in a really wide orbit around Pluto or something similar. Since solar power isn't good enough out there, they fitted it with an radioisotope thermoelectric generator (RTG). Since standardised parts are always good, your ship might also have the same model of RTG with identical fuel composition, maybe as an auxiliary power system. They might even have given you the tools to analyse the detailed composition of nuclear fuel, maybe for your main reactor.

Using this you should be able to get a rather precise time estimate, let's say $\pm 1 \%$ by comparing the composition of your RTG with the million years old one. The initial isotopes will probably be long gone, however some of the decay products will have half-lifes long enough to allow meaningful radiometric dating.

Answer 3

Alexander's answer is correct.

A different answer relies on the evolutionary clock. Despeciation is occurring at unprecedented rates at present time. We have begun the 6th mass extinction. This has the rather nice effect of setting the clock for the future.

Your explorers can look at the phylogenetic relatedness of life at 20 million years from now, and calculate (using the fidelity of DNA polymerase and reproductive rates) an estimate of time that has passed since H sapiens was busy killing everything.

Answer 4

Based on your premise, I assume that astronauts are not well-prepared and willing time travellers.

In this case, the easiest way to determine time difference would be to compare current star positions with star maps in the spaceship navigation system. You should be able to calculate the time difference with at least 1000 years precision. Considering Bernard's Star moves very fast (relatively to other stars), you can get it down to a century if you spend a bit more time on charting the new sky.

Most likely, there will be no man-made structures on Earth if the humanity disappeared without reaching technological levels much higher than current. In 20 million years all surface structures will be completely destroyed by natural forces. Something (some glass and plastic) might survive by luck but it probably will be buried deep underground.

Moon will be your best bet for finding the remnants of human civilisation. The rubbish we've left there will likely survive for 100 million years more.

Answer 5

First you need some degree of desired accuracy and a statement of the Units of Measurement, since you didn't say where you took off from and used the word approximately (and 20 million) in your question I will give approximation methods and the (currently) known accurate method.

Use of UT1 avoids considering where you launched from and an assumption that you wanted your answer in 'local time', continental drift caused by rotation of the core and crusts makes your starting location inaccurate after a long period of time.

For an approximate time @Alexander's answer is accurate to 1 or 2%, so 200K years. The precision and accuracy are fine, only affected by the accuracy, precision and duration of measurement available (which likely won't be very favorable). Simply bring a small rock with you and measure U vs. Pb then shield it and remeasure after your travel giving a base amount to compare with an expedition to the planets or Moon to obtain enough samples to make a plot. The answer should be accurate to a few decimal point, in Gyrs (billions of years). Use of other Elements and their decay ratios will improve the accuracy of the answer (assuming sufficiently accurate equipment and enough time to average your measurements).

Next method, measure the diameter and brightness of our Sun. Fairly easy, accurate and combined with the above method each makes the other better. Not particularly precise without accurate equipment. Combined with measurements of the sizes of other stars will improve the result.

The easiest and fastest approximation is to measure the distance from the Earth to the Moon (assuming no large meteor impacts changed the orbitb. The Moon moves away 22mm per year so 220 km per 10 million years; if it's 440 km further away then you're 20 million years in the future (this can be cross verified by doing the same calculations with other planets).

Before giving the 'accurate' method know that it relies on current science and mathematics - 20 million years from now we might have learned something new.

The "accurate" method is explained in many places including the book The Science of Time 2016: Time in Astronomy & Society, Past, Present and Future where the location of thousands of objects in space has been accurately determined and cataloged into the International Celestial Reference Frame (ICRF2) which is used to establish UT1 (Universal Time) based on their location (which varies little, in a calculatable manner, in a measly 20 million years).

Note: You're going to want a Doctorate in Mathematics and Astrophysics to read that book but there's Software available (See: https://www.iers.org/IERS/EN/Science/EarthRotation/EarthRotation.html) and plenty of information available by using Internet Search Engines.

For more information on the ICRF2 see: http://adsabs.harvard.edu/abs/2015AJ....150...58F .

I don't want to delve into Astrophysics and the associated Mathematics as I assume you don't want millisecond accuracy. In the interests of such precision I welcome edits that improve my answer as such would be greatly appreciated by our community.

I will come back and see if I can improve this answer.

From the way you've asked the question combined with your ability to pilot a spacecraft you could just look out the window, see the Sun, and guess somewhat accurately within a million years or so (if that's good enough).

Answer 6

If your story starts off 100 years from now, it's possible that by the time it begins, some more Voyager-like probes have been sent off into space, maybe some with transmitters a lot more powerful than Voyager / Voyager II.

Your protagonists pick up a weak signal from one of them, but the signal comes from where it will be in 20 million years!

Answer 7

You wouldn't need detailed maps of the Earth to know that a long time had passed. Assuming current motion remains about the same as now, you'd see:

1. The West Coast of North America looks different due to California west of the San Andreas sliding north. Vancouver Island has slammed into the mainland.
2. The Mediterranean is much narrower, with Italy starting to jam into North Africa, and probably isolated from the Atlantic due to the closure of Gibraltar. Parts of it very likely dried out.
3. The Red Sea has vanished as Africa and Arabia have re-joined.
4. Australia has moved northward, collecting New Guinea and parts of Indonesia along the way and is sideswiping the Phillipines.