That Time (Only) Traveller is Already Dead!

Written by: Ray Duell

Gravity is the problem and the cause of my issue with a lot of time travel stories (and if you’ve listened to the ‘Science Behind Science Fiction’ section of any time travel IPs we’ve covered in the podcast, you’ve experienced my rant). Everything in the universe is moving relative to everything else, due to the effect of the gravitational pull of… well… everything. Either mostly toward the nearest large (or dense) object, or in orbit of said object.

But why orbit, you might ask. Well, orbiting another object is basically falling toward it, but because of the physical law of the conservation of angular momentum, continuously missing. The gravity of the Earth pulls on the moon, causing it to orbit, the gravity of the sun pulls on the Earth, causing it to orbit, the gravity of the cluster of black holes at the centre of the Milky Way causes our sun to orbit, and a gravity anomaly in the local supercluster that’s pulling thousands of galaxies towards it.

It doesn’t matter how big or small an object is (quantum realm aside), it is all affected by the gravitational pull of other objects ‘nearby’. Which means everything moves over time, and for those who haven’t looked into it, the velocities of the movement can be quite surprising.

But before we break it down, have a think about how fast you ‘feel’ like you are moving? Got it? OK, now consider this.

Earth’s Rotation

The Earth (the ball of mud and fire on which we live) rotates, which is why we get a day/night cycle. That means it is constantly spinning. Now, depending on where on the Earth’s surface you are, you will actually be rotating at different speeds. Fastest near the equator, and slower the further toward the poles you go. But rather than making it more complicated, we’ll take a point right on the equator, because then we can use the Earth’s circumference for the calculation. The speed is calculated based on the Earth’s circumference and a 24-hour period:

v_spin ≈ 0.46 km/s (0.286 miles/s)

So your body, on the equator, covers a kilometre in just over 2 seconds, and a mile in just over 3. That’s kinda fast.  But compared to what, you might ask? Well, in this case, the centre of the Earth, but this is the only step in this calculation that uses the Earth as a reference point. So, in Back to the Future, when the Delorean travelled forward in time 1 minute, due to the rotation of the Earth, the Delorean would have appeared 27.6 km (17.6 miles) away from its starting point, because the starting point would have moved.

Sounds concerning, right?

But wait, there’s more.

Earth’s Orbit

Remember that whole ‘gravity making thing’s orbit’ point I brought up earlier? Well, this is where that starts to take effect. Not only is the Earth spinning, such that a point on the surface moves around the surface of the planet, but the Earth is also orbiting the Sun at a fair clip. On its elliptical orbit around the Sun, the average orbital velocity of the Earth (because it isn’t always stable) is:

v_orbit ≈ 29.78 km/s (18.5 miles/s)

So, going back to our example of the Delorian doing its 1-minute jump forward in time, Earth would have moved 1,786.8 km (1,110.7 miles) away from the time jump’s timespace extraction point, relative to the centre of the Sun. This could leave the time traveller in the upper reaches of low Earth orbit, embedded in the lower half of the Earth’s mantle, or anywhere in between.

Not a great outcome for Einstein’s (Doc Brown’s dog’s) survivability, seeing as the Delorean isn’t pressurised.

But wait, there’s more…

The Solar System’s Galactic Orbit

Gravity doesn’t stop with moving the planets of our solar system around the Sun. Our Sun, like all the other stars in our Milky Way galaxy, orbits the centre of our galaxy (theorised to be the domain of supermassive black holes, which themselves exert huge gravitational forces). So our Sun, and thus ourselves, orbit around the centre of the galaxy at a significantly higher velocity than we have been dealing with so far:

v_solar ≈ 230 km/s (142.92 miles/s)

Going back to poor Einie in the Delorean, after a 1 minute time jump, if we just consider the Solar System’s galactic orbit, the Twin Pines Mall car park on the surface of the Earth would be 13,800 km (8575 miles) away from our Delorean, which is Medium Earth Orbit if the Earth was moving directly away from that surface point at the time of transit, to a few hundred kilometers above the ground if the Earth was moving in the exact opposite direction.

All pretty disastrous for Einie, and the recovery of a terrestrial time travel vehicle.

But… (you know what’s coming next, right?) wait. There’s still more!

Galactic Motion (CMB Frame)

Even something as big as an entire galaxy isn’t immune to the motion effects of gravity. The Milky Way itself is moving towards a gravitational anomaly in the Centaurus/Norma constellations called The Great Attractor (an accurate, if uninspired title), which acts as the gravitation centre of the Laniakea Supercluster (a collection of galaxies). We know little about it because, compared to our location in the Milky Way, it resides behind and is hidden by our own galaxy’s dust (the Zone of Avoidance).

Now, for the rotation of the Earth, we used the centre of the Earth as the reference point, for the orbit of the Earth, we used the centre of the Sun, for the orbit of the Solar System, we used the centre of the galaxy, but what do we use as the reference point for the movement of the entire galaxy? Well, that’s where it gets a little more complicated.

Physicists have chosen a phenomenon called the Cosmic Microwave Background (CMB) as the most universal frame of reference for movement through space. The CMB is the faint, leftover heat and light from the Big Bang, filling the entire universe as a faint glow of microwave radiation. As the CMB is fairly constant across spacetime, varying only slightly due to the motion of objects, such as the solar system, through it, it can be used as a reference for our speed through the universe.

So, the Milky Way’s movement towards the Great Attractor, relative to the CMB, when combining the Sun’s local motion with the galaxy’s bulk motion, the Solar System’s velocity relative to the CMB is:

v_cmb ≈ 369.8 km/s (229.78 miles/s)

So, not counting Earth’s orbit or rotation, that 1-minute time jump would have the Delorean and Einie appearing roughly 22,188km (13,787 miles) away from the starting point in the Twin Pines Mall car park, relative to the CMB.

Doomed would be an understatement, but the Delorean and poor Einie would be giving Elon’s Tesla, on its way to Mars, a run for its money.

So, What Is The Final Answer?

It is difficult to emphatically state the speed of a point on the Earth relative to the CMB because of all the vectors involved. The spin of the Earth, the orbit of the Earth around the Sun, the orbit of the Sun around the galactic centre, and the movement of the galaxy itself. Depending on the position on the Earth, the spin, two orbits and a directional movement, the closest we can estimate is somewhere in the range of 360 to 390 km/s (223.7 – 242.3 miles/s), or an average of approximately 1.35 million km/hr, or 838,851 miles/hr.

So, considering all possible movement vectors, this could put our unfortunate terrestrial time-travel vehicle anywhere from low Earth orbit, with the planet having moved entirely through that point in space in that 1-minute period, to close to geostationary orbit if the planet was moving away the entire time.

And that is just one minute.

Travelling from 1985 to 1955, a period of 30 years, that point the Delorean disappeared from in 1985 would be 354.78 BILLION kilometres (220.45 BILLION miles) in that point on the Earth’s travel future. The Delorean would appear in deep space, waiting 30 years for Earth to meet it.

Can You Feel It?

But wait-a-minute, you may be thinking: I don’t feel like I’m moving at the blistering speeds mentioned above. And you’d be totally right. None of us can.

The reason for this is that you can’t feel the movement on these scales. You can only feel a change in inertia. You can feel acceleration and deceleration, but you can’t feel constant inertia. You could see the movement if everything around you were moving differently or stationary compared to the CMB, but the Earth and everything on it move together at the above-mentioned speeds. You can’t feel the speed, and you can’t see it because everything else is moving relatively the same. Just like you can’t feel a jet moving at hundreds of kilometres or miles per hour through the sky (unless it changes course or there is turbulence), but you can feel the acceleration as it speeds up to take off, or feel the breaking and flaps as it slows down at the end of the flight.

You can feel change, but not a constant.

The freaky thing is, you’ve been travelling at these insane speeds through the cosmos all your life, and the atoms that make up your body will continue to do so after you are dead, if they stay on the Earth. And none of this is a problem in the slightest, until you take yourself out of the spacetime continuum. By time-travelling. The universe will careen on, ever moving, never in the same place twice. But when you come back into it, at the same point where you left… Well, you likely have a problem.

So, Are All Time Travel Stories Pointless?

The simple answer is, all time-only travel stories are ridiculous. That point on the Earth that you leave from, going either forward or backward in time, has never, ever and will never, ever be in that position again (it’s a big universe after all). If your time-travel apparatus has no space travel ability, then you are doomed.

But timespace travel craft, such as the TARDIS, or a spaceship that can also time-travel, is definitely viable, although the latter would have to travel a significant distance to return to the planet they left, depending on how much time has elapsed.

And that, as they say, is how the timespace cookie crumbles.

This has been a public service announcement to improve the survivability of (space)time travellers. Always remember your towel, don’t panic, and have a good day.

Note

This article blends scientific research, speculative scenarios, and examples from science fiction to engage readers in an immersive exploration of the possibilities of Science Fiction. While scientific findings provide a foundation, the imaginative elements of science fiction allow us to contemplate extraordinary possibilities.

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