Not out of reach if you get very close to light speed. Time would advance very slowly for you, so counterintuitively it is possible to travel 5000ly in your life time.
Although for everyone else at least 5000 years will pass, so better say goodbye to family and friend.
Hm, not sure if that is really less depressing...
Also light isn't slow. A photon instantly travels to the end of time and yet it still takes a few minutes from the surface of the sun to us. Or about 100000 years from the center of the sun to its surface.
Yeah if you have a body that can tolerate sudden jumps between reference frames you could pretty much explore the entire galaxy trivially, so long as you don't mind that few places will stay the same long enough to visit twice.
You wouldn't need a sudden jump. If you had a rocket that accelerated at a pleasant 1G forever, you could reach and stop at the center of the milky way in about 20 (your time) years, and you could reach and stop at the Andromeda galaxy in about 28 years. Play around with some of the online space travel relativity calculators--it's wild!
Of course building and fueling such a rocket is what's totally out of reach.
No, it would direct the photons out the back of the space craft, producing a beam intense enough to vaporize any solids or liquids close by. This beam would ideally be able to provide perhaps 1G or so of continuous acceleration.
With a traditional rocket, I believe you'd need decreasing energy to maintain the same acceleration as the flight progressed, since you are carrying along with you and burning the fuel, and so the total mass (payload + fuel) that needs to be accelerated is constantly decreasing.
Of course there's the pesky problem that for every N kg of mass you want to accelerate at 1G for that kind of a trip, you're probably going to need somewhere on the order of N billion kg of fuel to burn.
We're already on that starship. Our engine is about 8 lightminutes away. All we need is to figure out how to steer this thing - and how to not wreck it while en route.
I would prefer the concept of people building an artificial planet/asteroid/spaceship for a starship, instead of messing with our star system. But luckily that debate is some years away and currently we cannot even figure out, how to deal with some increased CO2 levels.
How would that feel as a traveler? Does all motion slow down to a crawl, all sub-atomic particles just "freeze" and essentially your thoughts and body aging too? So it would seem like you got there in an instant?
For sure you're not just sitting there watching people get born, live and die in second and shrugging your shoulders.
You’d feel nothing out of the ordinary whatsoever. The starscape outside the ship would look strange though, shrinking into a small, blueshifted patch of sky straight ahead, while stars behind you would redshift out of the visible range. Everything moving at very low speeds relative to you would indeed appear to happen really fast.
There's about one particle of dust per million cubic metres. c is about 300 million metres/second. So even at 0.5c that's still a lot of particle collisions per second, each having significant kinetic energy.
Basically it would be like flying through explosive sandpaper. Each dust particle would be reduced to plasma, which creates problems of its own.
If you're accelerating there's also the Unruh Effect, which will raise the perceived temperature. By a lot.
There's no way to make this work with any kind of engineering we know about today.
The Unruh effect is theoretical, and no evidence at all has ever been found that it's real. It literally exists as nothing more than a hypothetical mathematical model, that also happens to be debated by others who know enough to effectively debate it, and disagree.
Mostly that it's plentiful, ablative, expendable, plus good radiation shielding (yeah, cosmic ray protons are really going to mess things up at relativistic speeds too unless there's enough mass to stop them).
It's one of those cases where you have very small numbers multiplied by very large ones. The actual risk is hard to intuit because there are so many orders of magnitude involved in both directions.
In any case it's probably a moot concern as long as we are living under the twin tyrannies of Newtons Third Law and the Rocket Equation. Building a rocket that can accelerate constantly and noticeably for weeks, months, or even years on end in order to accelerate up to a velocity where Relativity starts to matter requires an absurdly large rocket. Like converting the mass of Jupiter into rocket fuel to make it to the next habitable solar system in a couple of centuries level of craziness.
But it's also very big, and GP doesn't even specify how far of a trip they're asking about nor how small a meteorite.
"Extremely" and "very" don't cut it here. This is beyond the human ability to guess. You'd actually do at least some back-of-the-napkin math to give a real answer, and with a far enough trip, the answer may well become "Almost 100%".
If the light behind you redshifts out of the visible spectrum, would the light in front of you blueshift into dangerous territory? X-rays, gamma rays, etc?
Yes, and this provides a nice intuition about the relation of wavelength to energy. But x and γ wavelengths are several oom shorter than visible light, so you'd have to be traveling at very close to c to experience that amount of Doppler shift.
It depends on acceleration though. If acceleration and deceleration take long enough, it could take an entire generation to get up to a fast enough speed that relativistic effects make any difference, and another generation to slow down enough to interact with anything you might see.
Plus if you're traveling at near light speed, running into any matter at all would be pretty devastating for whatever craft you're in.
Edit: someone further down claimed that the math says that accelerating at 1G would get you to 0.1c in a month, so that's actually not that bad all in all. I still maintain that hitting any matter at those speeds might be unpleasant.
But unless you have a way of slowing down again you'll never see anything of your destination, just the briefest of flares of light as you sail past. And if you do have a way that involves anything like physics that we recognise, you've brought along a huge rest mass that then got accelerated to near light speed. Probably your civilization needs to be approaching Kardashev Level 2 to pull this off.
One thing I've always wondered is what fraction of c is actually realistically achievable with current technologies? (Maybe with scenarios for manned/unmanned spacecraft.)
Like are we at 0.1% or 0.01% or more orders of magnitude off?
The best speed for interstellar travel with technologies that current theory says should be within our reach can be achieved with a vehicle with a light sail pushed by a giant laser, that is powered by solar power. There is even a way to brake it when it reaches the target star. I forget what the predicted velocity was though.
This technology is basically the same as one that the Moties developed in the story, The Mote in God's Eye.
It slows down by releasing a large light sail in front of it, designed to reflect light back to a much smaller light sail behind it. The laser then pushes the large sail away, and as the sail goes it pushes the smaller sail (and ship) back. This leaves the ship at moderate speed relative to the new star, and a large sail traveling very, very quickly beyond it.
We do not yet have this technology. But we can show that it is plausible.
First set gamma as being 1/sqrt(1-v^2/c^2), with "c" being the speed of light. The factor for time dilation and distance contraction in special relativity is gamma and 1/gamma respectively.
That means that when you get to speeds equal to c, your time runs infinitely slower and the distances are infinitely shorter. So if your clock is infinitely slower, so every travel at "c" speeds means that no time passes for you. And if your distances are infinitely shorter, all travels at "c" speeds cover any distance as immediate. So you could reach every point of the universe as if it was immediately closer and in no time at all.
So in the frame of reference of the photon, the moment it is created it has already reached its destination, be it wherever it is on the universe.
Of course we can never reach "c" as beings with mass, but we can get closer to that. So for example if you get to 99.99999999999999% of the speed of light, you could travel a distance of 54,794,520 ly and only one year would pass to you, while 54,794,520 years would pass on earth.
Follow up question from someone who's mostly forgotten his university physics.
Do photons actually exist, in the traditional sense of physical matter.
Or are they just a convenient short hand to describe the transfer of energy via waves in the fabric or space time, if they dont experience the universe when passing through it but only when interacting with matter and matters "dents" in space-time.
As a non-physicist, my understanding is that they actually exist, but can't be thought of as flying around like ping pong balls. I think it's one of those things that comes down to interpretation though, where the math is very clear but how you think of what it "means" lies beyond science.
A very layman’s explanation I read a long time ago. Imagine you are in a car driving in a big open desert. You can drive in any direction you want. You can drive East or South or West or North, or some degree between them. But the more you travel East, the less you can travel North or South. The more you travel North, the less you can travel East or West. If you drive completely North, you aren’t traveling at all in the East/West direction.
Spacetime is like that, except North/South is space, and East/West is time. The more you travel through the spatial dimension (the faster you go) the less you travel through the time dimension (the less time passes for you). Photons are traveling completely through the spatial dimension (North/South) and so aren’t moving through the time (East/West) dimension at all, so from a photon’s perspective (if they had perception) no time ever passes so they would zip around “instantly” and never “experience” anything.
It is also why the speed of light is the fastest you can go. Once you are going 100% North, you cannot be going any more North.
That doesn’t make sense - if you were traveling at the speed of light, it would take you 5000 years to travel 5000ly - longer if you were just ‘very close’ to C. Time wouldn’t advance slowly for you, it wouldn’t advance perceptively different at all - you’d still live every second of those 5000 years.
I dont think you are right. Light for example doesnt perceive time at all. From the photons point of view it never aged even a microsecond while it traveled lightyears. Time is relative too so from our POV 1 year passed when a photon traveled 1 ly, but for the photon no time passed.
You two are talking about different meanings of "time".
Traveling 5,000 LY at 0.5 c will cause you the spaceship pilot to age 20,000 years. It's non-relativistic, inside that inertial frame. Clock second hands still sweep slow but noticeable circles.
Meanwhile, everyone outside of the spaceship is happening FAST, by your observations. You'll see stars turn red and go supernova.
Doesn't this depend on the initial distance to the destination? I'm thinking you have to be going ~140M light-years for cosmic expansion to exceed 1%c, and Proxima Centauri is only ~4 light-years away
There's nothing about 0.1c or even 0.999c travel that's detrimental to meatbags. They would both feel exactly the same to the traveler. If your (for now) imaginary rocket could accelerate at a constant, gentle 1G, you could reach 0.1c in about a month (traveler's time), and you could reach 0.999c in about 44 months. Building and fueling such a rocket is the hard part.
It depends on how you define the bounds of the Solar System, but eg. a flight from Pluto at its most distant to the same distance on the opposite side of the sun that hits .1C at peak needs ~5G for the entire duration. And it seems quite wasteful to bother getting up to speed before immediately reversing the acceleration.
If you're travelling between points in the Oort Cloud, 1G should be more than sufficient to hit .1c on the trip.
My point was that the GP talked about flight times assuming instantaneous acceleration and deceleration. Also, 1G of acceleration sustained over a month is more or less impossible for meatbag-sized spacecraft, especially if you need to also accelerate all the fuel you’ll need to decelerate. The rocket equation is simply way too brutal. Something like nuclear pulse propulsion might come close. Or antimatter propulsion if we’ll ever be able to create and store entire moles worth of antimatter.
Assuming our models of the universe are correct, and faster than light travel is impossible. There are very strong reasons to believe this, but perhaps we can cheat by stretching and compressing space around us.
It makes me wonder what kind of "life" could perform interstellar travel? I used to imagine a spaceship being alive, with people inside being analogous to "cells" in a multicellular organism.
Perhaps this is really how AI achieves consciousness?
To make a generation ship work you have to build a self-contained ecology that is stable and self-repairing, inside mechanical and software systems that are fault tolerant and either extremely redundant or self-repairing, run by a political and social system that is also fault-tolerant and self-repairing.
More to the point the ship needs to be absolutely self sufficient, it can't even use solar power and has no access to outside mass whatsoever. But if you have a ship like this you could build an orbital habitat using the same technology, and it would be much much easier to build since it doesn't have to accelerate, can use solar power, and has access to the rest of the resources of a solar system.
If you have all of this why would you go to the enormous extra effort to move the habitat to a different solar system? Even if your civilization is so old that the star is a dim brown dwarf that's still plenty of energy for day to day life.
There's a CRPG I've been meaning to play where this is basically the plot; there was a generation ship, it was heading towards some planet or another, but the social and political structure on the ship broke down at some point and now there's no one actually in charge, the ship is getting run down, and they probably blew past their destination a hundred years ago if they were even still on course at all.
I remember someone pointing out that a generation ship could be problematic because you have one generation who decides to launch this expedition but will never see the end, multiple generations who didn't choose this life and won't get to see the benefits, and then one generation who actually gets to the planet but might not even want to be there. Without some kind of cryogenic sleep or relativistic speeds the whole thing might fall apart just because most of the people involved "didn't sign up for this" but they have to toil away anyway for someone else to benefit from it.
What of the "just so" attitude of a child growing up? Everything is taken at face value, there is no comparison, only stories (unless you have a catalogue of 30EB of 8K earth footage or something to that effect for them to fawn over). They don't have the reference frame for other situations for a while, perhaps long enough to not be able to see things differently?
This makes me think of multi-generational migrations north out of Africa. There's only so much that can be passed orally losslessly. Eventually the group in north siberia after 20K years doesn't see living any other way.
> I remember someone pointing out that a generation ship could be problematic because you have one generation who decides to launch this expedition but will never see the end, multiple generations who didn't choose this life and won't get to see the benefits, and then one generation who actually gets to the planet but might not even want to be there.
That isn't really different from the way things are now. We are, in fact, traveling through the galaxy for many generations and none of us signed up for it. We just happen to be on a largeish ship and have no destination.
Well, to be frank, we currently have such a ship, but we're doing quite a lot to disrupt its capability of sustaining human life.
Of course, even if we stopped doing that, we'd need to figure out how to visit another place if our ship is passing close by. That also seems to pose a problem: both Voyagers are barely out of the exhaust fumes of our ship's motor, and getting so far took ~40 years.
It's a wonderfully entertaining book and for that reason I loved it, but Andy Weir really, really glosses over and hand-waves away all kinds of other difficulties for so quickly and easily building a ship that can travel at nearly the speed of light.
He basically just has it work because the fuel difficulties are solved and bam, the main character can zip around nearby start systems at close to perfect C on a ship built with little more than our current 21st century technology. Fun, but not even in the most basic way an attempt at presenting any science seriously.
What makes it more amusing is that for many other parts of the main drama, he puts a lot of effort into making the descriptions and scenarios seem as realistic and science-rich as you could like. I suspect a lot of entertaining word salad there too though.
Not really, because it was a single person and the spaceship was a vehicle. The other race was still in a vehicle.
Think more about the difference between single celled organisms versus a multicellular organism: IE, the spaceship itself is alive and has a lifespan where interstellar travel is a fraction of its life. The people inside are like cells inside our body.
Yes, agreed. I find it a little depressing. An unimaginably huge universe, tantalisingly there, but completely out of reach.