Hacker News Reader

> The faster you go, the less distance there is in front and behind you. At the speed of light, the entire width of the universe shrinks to zero.

Since you could get to anywhere in the universe almost instantly by travelling sufficiently close to the speed of light, why do many science fiction writers and science fiction movies want to introduce faster than light travel? It doesn’t seem necessary; just tell the readers that the spaceship was travelling at 0.99999c to get to the other size of the galaxy in a week or at 0.99999999c to get there in a minute. (I didn’t do the calculation, but you can imagine what I mean.)

I’m going to guess that it’s easier to move the story along by saying “faster than light” than it is to explain the above. Most people would think the author made a goof by saying that something travelled 100,000 light years in less than 100,000 years if travelling at less than the speed of light. (Talking from the traveller’s perspective of course.)

From an outside point of view the ship takes 53K years to cross our galaxy, from inside it looks like an instant. Time flows at different speeds.

Star Wars would have been a very different story if the Rebels had arrived at the Death Star 10k years after everyone aboard had died.

OTOH, by the time the Death Star arrives at Yavin, the Rebel civilisation will have collapsed a couple times and won’t even remember why they even got there in the first place.

Ursula Le Guin's Hainish Cycle explores how an interstellar society would function if people could travel this way: an instant for the traveler, but decades back home. Despite glossing over all the technobabble that other scifi finds so fascinating, these books, more than any others I've read, show just how great the distances are between stars.

Funny thing is that this difference of passed time that remains after the spaceship slows down has more to do with acceletation and GR than speed and SR.

The fact that you can describe this effect from the point of SR (by swuinting a bit) means that you can get GR time dilatation from SR time dilatation with no additional input.

The moment that we knew that the acceleration is the same as the gravity and that the speed of light in vacuum is constant we only had to think long and hard to see that gravity causes time to flow differently in different places.

> Since you could get to anywhere in the universe almost instantly by travelling sufficiently close to the speed of light

But the universe is billions of light years across - if someone is travelling at just under the speed of light, wouldn't that mean it would take them billions of years (as experienced on the ship) to traverse the universe?

> why do many science fiction writers and science fiction movies want to introduce faster than light travel?

My thought on this, from a purely sci-fi perspective, is that accelerating close to light speed would use a phenomenonal amount of energy, and take a long time to do. Furthermore, time is experienced differently for those on the ship and those outside it. So with these points in mind, some kind of "hack" that allows you to fold spacetime and jump several lightyears instantaneously (as experienced by everyone) makes sense.

> wouldn't that mean it would take them billions of years (as experienced on the ship) to traverse the universe?

No, because time passes more slowly in the ship frame. This means it’s billions of years for your friends back home, but it could be hours for you.

I get that time passes slower for those outwith the ship, but I'm struggling to wrap my head around how long it would take those on the ship to travel large distances at close to light speed - whenever I think on this topic, my head almost bursts with awe and wonder!

Let's consider a ship, travelling from Earth, and say it somehow accelerates almost instantly to just below light speed, and travels for a distance of 1 light year. How much time would pass for those still on Earth, and for those on the ship?

1y for Earth and 0 time for the ship.

If it's zero time for the ship, what I'd they wanted to travel a different distance?

It's not really zero, but the closer you get to c, the closer to zero the duration experienced by the astronauts will be.

> why do many science fiction writers and science fiction movies want to introduce faster than light travel?

Because this way you keep the human timescales for all characters. It allows you to tell stories more familiar to your audience while introducing elements that we wouldn’t be able to introduce without this plot device.

If you embark on a relativistic trip to Kepler-186f, when you return you will find a completely changed planet. The only way this would work is if every character spends most of the time at relativistic speeds reuniting from time to time as the whole universe ages around them.

That’s a nice premise, BTW.

Most writers did not take science classes in college; they got a liberal arts degree and not a physics degree. On the other hand there are a few writers who do a better job of things.

Vernor Vinge does a great job in A Deepness In The Sky of creating a human space–fairing society that has to deal with some degree of relativity. Their ships only manage a 0.3× the speed of light, but they also use suspended animation to hibernate for most of their voyages. A trip where you age a year might mean that hundreds of years pass on the planet you’re going to. One character founded a vast trading coalition that was enormously successful in spite of this and other handicaps.

Greg Egan wrote a book called The Clockwork Rocket that is set in a universe where the space–time metric is different. This has huge effects on all aspects of physics from relativity to quantum mechanics, and he worked out all of those implications before writing the story. You can learn a lot about our universe just by contemplating all of the ways that this fictional Riemannian universe is different from our own Lorentzian universe. For example, in the Riemannian universe there is no fixed upper bound on speeds, light travels at different speeds based on its color, and thermodynamics is backwards so plants gain energy not by absorbing light but by emitting it.

Is this the foundation for the "holographic" description of the universe? Photons, traveling at the speed of light, effectively don't see one of the dimensions, so can be described using "N-1" dimensions. (Edit: Or does zero rest mass make them a special case?)

Can someone also confirm whether the following thinking is valid? The expansion of space means that the edge of the observable universe appears to be receding at the speed of light. As such, it has "N-1" dimensions and that is where the idea of a holographic boundary comes from? The same reasoning applies to the "N-1" dimensions of the event horizon in a black hole?

No, that comes out of quantum mechanics rather than relativity.

What made it click for me was that the speed of light isn't just the speed of light, but actually the speed of information/causality.

So if you're traveling at the speed of causality, everything you "hit" or go past happens instantly, because you're traveling "with" causality/information itself.

With a ship going the speed of light you could travel from galaxy to galaxy instantaneously. Zero distance. Though time on the other hand... If you travel to a star 4 light years away and then back, what would be no time for you, everyone on Earth would be 8 years older.

It ends up being worse than that once you plug in general relativity. You're only using special relativity, but that doesn't hold when acceleration is involved (as would be required to go from 0 to 0.99999999c).

It’s SR pretty much fine for accelerations of fairly small masses (like a spaceship that’s much smaller than, say, a planet)?

"It's a common misconception that special relativity cannot handle accelerating objects or accelerating reference frames. Sometimes it's claimed that general relativity is required for these situations, the reason being given that special relativity only applies to inertial frames. This is not true. Special relativity treats accelerating frames differently from inertial frames, but can still deal with accelerating frames. And accelerating objects can be dealt with without even calling upon accelerating frames.

This idea that special relativity cannot handle acceleration or accelerated frames often comes up in the context of the twin paradox, when people claim that it can only be resolved in general relativity because of the acceleration present. Their claim is wrong."

https://math.ucr.edu/home/baez/physics/Relativity/SR/acceler...

Interesting, but the point remains that the answer is different from the 8 years mentioned given that you do have to accelerate up to near-c (at gees the human body can survive in) and then decelerate from it. You can't just instantly go up to near-c and then back down to zero. Whether it's special/general relativity that's necessary for this calculation, it's a materially different result.

Travelling in a ship at 0.95c, what is the danger of hitting anything on the way? Is it possible to dodge anything at all?

Probably quite risky. You have little reaction time, every maneuver is 10× more expensive because your effective mass has gone up so much, and even collisions with small objects will release fantastic amounts of energy.

You don’t perceive the mass from inside the ship as its engines and attitude control are not relativistic in relation to you. You’ll see the object approaching at a relativistic speed, however, and it will release a very non Newtonian amount of energy if it hits.

You will definitely notice that it takes 10× more propellant and 10× more time to complete a maneuver.

I don't think you will - need to do the math though - but my impression is that the distorted time and space from the ship's frame of reference will account for the observed energy expended.