As a lifelong fan of Doctor Who and BTTF, it pains me to have to be the one to point out to people that time travel is probably impossible. It particularly bugs me when I hear scientists talking about it like it’s even remotely possible.
And here’s why.
Most people tend to think of particles (like the ones that make up our bodies) as things that stand alone, like a billiard ball on a table. But that’s not what physics says particles are.
Particles don’t exist in a vacuum. They exist as quantum wave functions in Hilbert space / Spacetime (or perhaps, as Sean Carroll suggests, there’s just a single quantum wave function that makes up the entire universe).
Anyway, I find it helps to think of Spacetime as something more like woven tapestry, especially as we already use the term “fabric” of Spacetime.
Imagine each particle as a thread in an intricate tapestry from the Middle Ages (maybe like this one, “The Battle with the Sagittary and the Conference at Achilles’ Tent (from Scenes from the Story of the Trojan War)“).
The threads (particles) are woven together in a complex pattern, with each one tightly connected to the others around it. What you think of as your body is part of the tapestry. The design of the tapestry is the fabric of spacetime, and the threads can’t just be ripped out and moved elsewhere without unraveling the whole pattern. If you were to take one thread (a particle), it wouldn’t be able to reinsert itself in another part of the tapestry without disrupting the entire structure around it.
The idea of pulling a thread (a particle) from one spot and trying to place it in another part of the tapestry (spacetime) would cause a breakdown of the overall structure.
In science fiction films and TV shows (I’m looking at you, ST:TOS), teleportation is often depicted as a body or object simply popping out from one place and appearing in another. However, this portrayal overlooks the complexities involved in such a process, particularly concerning the removal of the particles present at the destination. Again imagine a tapestry – where are the threads that form the original picture going to suddenly disappear to when you try to jam in the new threads from a separate tapestry?
When an object materialises in a new location, keep in mind that the space isn’t devoid of matter. Even if only air particles are present, along with the area’s quantum wave function, these elements occupy an area of Spacetime. For a new object to appear, it would need to displace billions of existing particles and the underlying wave function.
- Where do these displaced particles go?
- Are they swapped into the location from which the object disappeared and somehow stitched together into the wave function / fabric?
- Or are they moved to another position within the existing universal framework?
Assuming the depiction of time travel in science fiction would also violate conservation of energy and momentum. According to the laws of physics (as we understand them), energy cannot just vanish. When a person or object exists in a given moment in Spacetime, they possess energy — both in terms of their mass energy (thanks to E=mc²) and other forms of energy, such as kinetic or potential energy. If they were to suddenly disappear, the total energy at that point in space would abruptly decrease. If a 70 kg person disappears, that’s an immediate loss of around 6.3 x 10¹⁸ joules of energy (based on E=mc²). The energy that makes up their mass is now gone, which violates the principle that energy in a closed system must remain constant. If they were moving, their kinetic energy would also suddenly vanish. For example, if someone or something moving at 88 mph suddenly disappeared due to time travel, the violations of the conservation of energy and momentum would be even more pronounced. No laws of physics allow for such a massive amount of energy to just disappear from the universe without leaving a trace or causing catastrophic effects.
A DeLorean moving at 88 mph (39.34 m/s), would have enormous momentum. This momentum isn’t just a property of the car; it’s part of the overall momentum of the Earth-car system. If the car suddenly vanishes due to time travel, the Earth’s surface is still moving relative to the car. The car’s momentum disappearing would violate the conservation of momentum, because there’s nothing else to “take on” that momentum.
Not to mention the impact on air displacement and pressure, surface friction and heat (eg the tyres on the road), gravitational influences, etc, or the reverse of those issues in the spot where it arrives.
Now, you might think “Ah, dummy, if the DeLorean reappeared in the same universe it left from, then the total energy of the closed system is still the same!” You can “balance the books”.
But this ignores the time factor of the energy system. If it disappeared from 1985 and reappeared in 1955, that would mean 30 years’ worth of physical interactions (e.g., gravitational forces) that the DeLorean would have had with the environment are effectively erased in that timeframe. Those forces need to be redistributed or compensated for somehow. If it jumps into the future, the missing energy and momentum would create disturbances the entire time it’s gone.
If we consider (as BTTF seems to) the Everett / Many-Worlds Interpretation (MWI), it’s an entirely different and parallel universe that the car is appearing in (and disappearing from), which does violate conservation of energy laws.
The bottom line (admittedly based on my completely amateur knowledge from a lifetime of reading about physics) is this – if someone or something suddenly disappeared due to time travel, you’d have massive violations of both energy and momentum conservation, which physics doesn’t allow, and the fabric of Spacetime would be ripped apart with enormous consequences. The same would be true in the section of the universe where the
Maybe it could be possible with some kind of incredibly advanced alien (Time Lord) technology – but the idea seems far-fetched to take seriously. As far as I can tell, no amount of talk of wormholes or quantum entanglement, etc, aren’t going to resolve these issues, apart from (perhaps) very, very small scales (eg sub-atomic particles).
Of course, the block theory of the universe also says everything that will happen in the future, has already happened.