Okay, So…

Lets assume you’re a would-be time travel catapulting through the universe like you usually do. Maybe you’re on a mission to remove the Borg from your timeline, you just happened upon a TARDIS or something, your father gave you a Delorian and you found out it was equipped with a trans-flux capacitor, or you just found a device that’s capable of going back in time without all the hullabaloo of time-travel paradoxes that can really mess things up.

Anyways… you want to travel back in time with your device. You’re not going anywhere in space, you just want to go backwards in time a day and collect some lottery winnings. You’re going from your own backyard to your own backyard.

Or are you?

The problem is, time travel is also space travel.

(And no, not because space and time are really the same spacetime.)

Wait… What?

See, the problem relies in the fact that the Earth is both spinning around its axis and rotating around the Earth. As time passes, the Earth moves around the sun in orbit.

If you time travel from December back to July without changing your location in space, you’re going to end up in the dead of space with Earth about 300 million km away.

But Wait! There’s More!

You may think that “Oh, we’ll I’ll just travel from “July 8, 2009 12:34:56 PM” to “July 8, 2109 12:34:56 PM” and not have to deal with space travel because the Earth would have orbited back to where I was.

But no. According to this Wikipedia article:

The Sun’s orbit[s] around the Galaxy [in a] roughly elliptical [orbit] with the addition of perturbations due to the galactic spiral arms and non-uniform mass distributions. In addition, the Sun oscillates up and down relative to the galactic plane approximately 2.7 times per orbit.

…in other words, the Sun is moving around the galaxy, and the Earth is following it. The Sun takes about

225–250 million years to complete one orbit of the galaxy. This means that 20–25 orbits have occurred during the lifetime of the Sun and 1/1250 of a revolution since the origin of humans. (If you like more stats, that places the orbital speed of the Sun at approximately 220 km/s. At this speed, it takes around 1,400 years for the Solar System to travel a distance of 1 lightyear, or 8 days to travel 1 AU.)

…But what I really mean is that unless your going from “July 8, 2009 12:34:56 PM” to “July 8, 22500009 12:34:56″ you’re not going to get nearly back to where you once where, and furthermore — that’s assuming the galactic orbit is extremely regular, which of course — it’s not.

Without space travel, there’s no way your time traveling back to the exact same spot you were at.

So What Do We Do?

Well if you’re in a TARDIS, you’re safe. Doctor Who’s time travel vehicle safely travels space in addition to time. So while he can time travel from Point A to the same Point A in a future (or past) time, he actually also space traveled the needed distance as well.

In Clifford Simak’s 1950s short story “Mastodonia“, the protagonists don’t take into affect the orbits of the Earth and the sun, but they do take into account the idea that, when time traveling back millions of years, the topography of the Earth changes. Therefore, they keep their time machine on a helicopter so that they don’t risk time traveling to a point that is underground in the past.

Another interesting application of the movement of the Earth in fiction is that in 2000 AD‘s comic titled “Strontium Dog”, the character Johnny Alpha uses “Time Bombs” with explode causing anything caught in the blast radius to time travel a few seconds into the future. The Earth has moved away relatively, causing the victim to reappear in space.

Relativity’s Answer

A solution for those who want to time travel without space traveling may also appear in the concept of theory of relativity, which rejects the idea that there can be any “absolute point” in spacetime, and therefore:

There is no universal truth about the spatial distance between events which occurred at different times and thus no objective truth about which point in space at one time is at the “same position” that the Earth was at another time. This is because of a feature known as “diffeomorphism invariance”.

Bonus Note #1: A Study of the Time Bomb

How far into the future would Johnny Alpha’s “Time Bombs” have to send their victim into the future in order for the victim to be hopelessly away from the Earth’s atmosphere?

In order to calculate this, we first need to find the amount of distance the Earth drifts per second. The Earth is orbiting the sun at a velocity of 107,300 km per hour.

In this calculation, here are some things we have to ignore because they’re very hard to calculate and don’t affect the final answer too much:

• The Earth is orbiting in, roughly, a circle, so the distance the Earth would be from the hapless, stationary victim, would be equal to the arclength of the orbit it covered, not it’s absolute distance covered.
• The Sun is orbiting around the galaxy.
• The Earth’s orbit and rotation has a bit of a wobble.
• Gravitational affects of other stellar bodies.

So, ignoring all this, the victim would find himself or herself an additional 1788km away for each second he or she is “flung” into the future.

The breathable atmosphere of Earth is the troposphere and stratosphere, which extend roughly 50km upward, varying depending on where you are on Earth (poles versus equator), and with some variation due to weather.

Therefore, flinging the victim .003 seconds into the future would result in a change of 60km, popping the victim into the mesophere where he would not be able to breathe and would, more importantly, be affected by the gravitational pull of Earth and would burn up in the atmosphere upon re-entry.

Flinging the victim 0.4 seconds into the future would result in a change of about 700km, putting the victim into orbit around the Earth. …An orbit without oxygen, of course.

If you wanted to get the victim to smack into the Moon, however, it would take a “time-fling” of 3min, 34sec and a decent amount of luck.

Distance to scale between the Earth and the Moon. 1 pixel = 600km. Sizes not to scale.

But Wait! There's More!

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