I want Hyperspace but I can't have it

It seems that there are people who classify sci-fi into two distinct camps: hard and soft. One of the main characteristics of hard sci-fi is that nobody, no matter how far advanced or high alien, can break that universal speed limit, the speed of light. This is a major downer.

The problem is that the speed of light, although locally really zippy (seven and a half times round the Earth in a second) is actually extremely, utterly, terribly, slow for space travel, in a narrative-ruining way.

It sounds fast but space is mindblowingly large. The moon (astronauts? to the moon? haha) is 1.28 light seconds away. Mars, our neigbouring planet is a shocking 3 light minutes away at its closest (and 22 munutes when furthest away). This is already a problem. You can't use a game controller to drive a mars rover because at best there is six minutes between pressing forward, and seeing the rover move forward on your screen.

Of course to actually travel to the nearest star takes light 4.5 years. Any reasonable journey to any interesting destination takes light years between locations. People of course do not travel at the speed of light, we have to accelerate up to near the speed of light, accelerate down again and all this at tolerable g-forces. Let's say your destination is a guessed 25ly away, and we have technology that can keep us alive and accelerating at 3g. The trip does not sound so bad - surprisingly, you can get up to 99% of light speed in under a year, and under a year to get back down. If you accelerate all the way to the midpoint of the journey and decelerate all the way down, the journey time as measured from your destination is comparable to the 25 years light year distance, and the time on board that you would experience, thanks to relativistic shortening of the distance, is less than 3 years. Result!

Let's call that a typical journey, and let's assume that any advanced civilisation can overcome the energy and safety requirements for such a journey. Well, it's about 3 years onboard for the travellers, and 25 years elapsed back home. For science fiction, this is a problem. "Captain, Omega Three is under attack, we must rush to their aid!" Unfortunately in hard sci-fi or you know, real life - your news of the attack was already 25 years old, and you only arrived to save the day, another 25 years later. It is unlikely that you can save the citizens of Omega Three as you missed the action by more than 50 years. This also makes for tricky writing, hence the preponderance of "soft" science fiction, where we can teleport or hyper-space travel our way around.

Jump!

Hyper-space basically uses some sort of future technology to create a shortcut, allowing near-instantaneous space travel. Wormholes, warp drives, teleporters. We don't have this technology, but maybe an advanced civilisatoin would? In exactly the same way as they could solve the hard sci-fi problems of amazing engines, shields, life support and table top gaming to relieve the boredom ... they would certainly solve the problem of hyper-space travel, unlocking one of these faster-than-light technologies and also allowing for the pleasing narrative capability to actually save the long-suffering people of Omega Three.

I'm sure it's a tough physics problem to solve but I would normally assume that someone would eventually solve it.

Limits of the limit

Why does this limit exist? Little "c", the speed of light, that we cannot exceed.

From very well-understood relativictic physics, we know that it takes infinite energy to accelerate something with mass up to the speed of light. Right, a very practical limit - we do not have infinite energy - this is a very practical problem. There are many other practical problems including the fact that a dust particle striking your ship at close to light speed will detonate like a nuclear bomb and so on. More engineering problems.

Unfortunately it's time to get to the crux of the matter now - the problem with faster-than-light travel is more fundamental than engineering. If we are correct about special relativity - I mean really basic modern physics - then travelling faster than the speed of light implies backwards travel in time and breaks causality.

I'm sorry. You may not have known this. I studied physics at University and really this never came up, we were too busy trying to get integrals right, or doping semi-conductors with various materials to see how it would affect their resistivity. I'm sorry therefore to ruin this for you - but as I understand it today, if you could go faster than light by any means at all - any means - that would neccessarily imply that you could travel back in time and cause all sorts of havoc such as preventing yourself from taking the trip that you have just returned from.

Backwards to the Future

Travelling back in time is already awful in fiction. What if your parents never met, because your time travel to 1985 caused them to not catch that fateful bus journey together, and now you are fading our of existence! This is rubbish, the real universe does not work this way. The only really wishy-washy way that physicists can stomach backwards time-travel is to haul out the super-permissive many-worlds theory. With this theory in hand, backwards time-travel is fine, because when you arrived in the past your arrival simply started up a new universe where the future is now unset. You can trip up your dad so he missed that bus, and you will be fine because you were born in a universe where your parents did meet up after all, even if they do not meet up in this one.

I am not a fan of the many-worlds theory despite that fact that it's quite mainstream and almost required reading for explaining quantum mechanics. It has always seemed very expensive, branching off universes at every turn.

In any case - even with the many-worlds theory, you would presumably end up rescuing the citizens of Omega Three in your new universe, where you teleported right there, but you did not save them in the more realistic universe where you did not arrive till 25 years after getting the 25 years-old news of the attack.

The problem

I'm going to start from the beginning and build this up slowly.

The speed of light is constant. That's not a metaphor — it's a measured fact. No matter how fast you're moving, light always travels at exactly the same speed relative to you. If you run toward a beam of light, it doesn't go faster. If you run away, it doesn't go slower. It's always the same.

That seems simple, but it forces something strange to happen.

Imagine a train going half the speed of light. On the train, someone shines a flashlight at the front wall. From their perspective, the light hits the wall after a normal delay. Nothing unusual.

From your perspective on the ground, watching the train race by, something different happens. The light is still travelling at the same speed (it never changes). But the front wall of the train is moving away from the light — the light has to catch up to it. So from your perspective, the light takes longer to reach the wall.

We have a contradiction. On the train, the light reached the wall quickly. On the ground, it took longer. Both observations are correct. The only way to resolve this is: time itself runs at different rates for different observers. This is time dilation. It's real — GPS satellites adjust for it every day.

If time runs at different rates for different observers, then the order of events can also differ. Two events that happen at the same time for you can happen one after the other for someone moving past you. This is called the relativity of simultaneity. It follows directly from the speed of light being constant — full stop, no further assumptions needed.

Now bring in the FTL ship.

You leave Earth at noon and travel faster than light to a star ten light-years away. From your perspective, you arrive an hour later.

But "leaving Earth" and "arriving at the star" are two events. For an observer moving past Earth at a constant speed — call her Carol — the timing of these two events can flip. Carol sees your arrival at the star happen before your departure from Earth. Not because Carol is seeing delayed light — she's doing the maths, correcting for light travel time, just like astronomers do. In Carol's mathematically correct, physically valid frame of reference, you arrived before you left.

This is not an illusion. Carol's view is as valid as yours. Both follow from the same fact: the speed of light is constant for everyone.

Now here's the kicker. If Carol sees your arrival before your departure, and you can interact with Carol while you're at the star — even just send a signal — then Carol receives information from you before you ever left Earth. She could warn you not to go. You could get a message from your future self, telling you to stop the trip you've already completed. That's backwards time travel.

No clever engine design avoids this. Warp drive, wormhole, hyperspace, teleportation — it doesn't matter how you go faster than light. The problem is that the constancy of the speed of light forces different observers to disagree on the order of cause and effect for anything that moves FTL. And when cause can follow effect, causality is dead. You cannot have FTL without breaking it.

The sad ending

That is basic, non-contested physics. No matter how technologically advanced or brave your space crew is, you cannot travel faster than light because it breaks basic common sense about the world.

I'm sorry - have a good evening.