How long an interstellar journey will take

[@ImprobableMatter]

A lot of good comments so far. I am putting a like particularly on ones I would like to address in future. If this one is popular enough, I will probably do a longer form, unscripted (possibly live) video to answer them.

[@pinkypink5161]

Why do you think generation ships are impractical? They're far more practical than hoping you don't get taken out my a grain of sand. I guess to each his own. I'd rather take the slow and steady route myself.

[@ImprobableMatter]

@@pinkypink5161 I guess it comes down to what field will make more progress: big, "hard" tech which could avoid or destroy dust, or social tech which would be required to make sure that generations born and dying on the ship would remain talented and motivated.

[@pinkypink5161]

@@ImprobableMatter we would have to develop new physics for your way. We know how we can detect a speck of dust at thousands of kilometers. But doing it requires energies that would be large percentages of your total energy budget. It's literally as tyrannical as the rocket equation itself. You've gotta emit enough infrared energy that the particles manage a slight increase in temperature over the background so it can be detected by a sensor. The less you're capable of heating up the particles Infront of you the more power hungry your sensor becomes. However, in the past ten thousand years we have yet to lose any technology. Empires come and go but science trudges onward. Therefore I don't see that changing when keeping their tech operational is the one thing keeping them alive. The key, I think, is to not allow it to turn into a human version of the Rattopia experiments. I think there needs to be far more space than the minimum requirements. There should be a forest large enough to be alone in it. There should be work that everyone must do to maintain the function of the ship. Minimal automation. People without purpose are a dangerous thing.

[@pinkypink5161]

@@ImprobableMatter I hope I'm not being a nuisance. There's a lot of tricky things that would have to be done for this to work without major advances in biology as well. Currently for this to work the women sent could be no older than 22. Five years of travel time plus one year for each baby and a year of rest puts them at the end of their quality eggs. The men needed for such a trip would need to be late thirties early forties to have the knowledge needed for such an endeavor. One way past this is send already created and "finished" families where both parents have a depth of knowledge for the mission and their children will be the ones to make the first born in a new star system. The next issue is no matter how long the journey takes they have to be able to make anything they could possibly need upon arrival or in situ. Which means any new engine development on earth should be able to be built in route to help speed up a ship that now has a legacy drive. If Artemis and Starship go as planned we could be sending our first interstellar ship by 2200. Built in lunar orbit. Sourced from the moon. It'll only go 1% of c but if we don't have the kind of tech you're talking about by then we can assume it's probably at least 500 years out.

[@ImprobableMatter]

I agree with your points in general. There would definitely need to be developments in the "softer" sciences - the biology and physiology of human spaceflight, dealing with depression and so on. Without speculating on future developments, I would assume something like this based on modern technology: a series of slow robotic ships will chart the target exoplanet, deliver supplies and establish rudimentary industrial facilities (steel, concrete and so on). A crew of mostly women in their 20s with frozen sperm (this can last decades) take 10 years to get to Proxima Centauri. The crew starts having kids in the deceleration phase and shortly after arrival. That means the astronaut generation are in their 60s when the first Centauri-born have reached adulthood.

[@Barwasser]

As a German the introduction of BAT as an universal unit of time hurts. (though its well deserved)

[@narfwhals7843]

Sad Stuttgart 21 noises.

[@Barwasser]

@@narfwhals7843 we don't know when it will be finished so it can't be a unit yet

[@semorgh2854]

Your Countrymen have done lot of good stuff with the Plasma Physics that Iranian Nuclear Scientists have given them.

[@randomperson9187]

Get rid of some industry killing regulation. You'll get that BAT down from 12 to 5.

[@philipwells2793]

@@Barwasser Take it to be a definition of too long.

[@wktodd]

Oligarchs accidentally falling out of windows, get you a sub 8⁠-⁠)

[@xiphosura413]

"You could, in principle at least, accelerate a huge spacecraft to half the speed of light by firing out just a single atom with an eye-wateringly huge amount of momentum." I like the idea of this 'Oh-My-God particle' drive!

[@randomaster138]

I love the image of a rocket dramatically powering up it's drives and preparing to launch at incredible speeds, only for it to fire out a singular incredibly high momentum particle and fly forwards.

[@planets9102]

Imagine the g's you'd be pulling tho.

[@dynamicworlds1]

I feel like that's even more terrifying because any real world attempts to dump that much energy into a single particle would probably (thought just the inefficiencies alone) turn said particle into what amounts to the nucleation point for a kugelblitz....which means your drive is spitting out a relativistic black hole....which then explodes. "Oh-My-God particle" indeed!

[@RawDoggin_78]

@@planets9102 OG

[@floridaman318]

​@@dynamicworlds1 metal af

[@patr5902]

While I appreciate the approach be sure to keep in mind that space is not empty. The velocities you are working with would require extensive shielding out front. Not a deal breaker but might require an upper limit on velocity to keep shielding mass within reason.

[@TheSateef]

very good point and i actually think that could be a deal breaker. even a tiny spec of dust at those speeds would feel like been hit with a H bomb.

[@Ryrzard]

@@TheSateef Interstellar space is fairly clean of debris. It is unlikely you will hit anything. A bigger problem is radiation. When you travel at velocities close to c, the light you encounter shifts in wavelength. For example, infrared light head on at 0.95c would shift into uv. That's not too bad but higher velocities make this much worse. Edit: Turns out I was wrong and the interstellar space is actually full of dust.

[@dynamicworlds1]

Yeah, turns out that "vacuum" is relative to your velocity. Even if you have some kind of regenerating shielding at high relativistic speeds you're going to start loosing energy to what amounts to air resistance as you collide with interstellar dust and plasma and that'll put a practical cap on the speed you can get to. Does help with the deceleration phase at least, especially if you deploy a (disposable) solar sail that is strong enough to temporarily withstand the relativistic bombardment to supplement the reaction mass for that stage of the journey.

[@Ailtir]

We only need to accelerate the dust off to the side, out of our way as we travel. We don't need to collide and accelerate it to the same speed as us. A very very long tapering 'field' in front of the ship forcing debris sideways would work.

[@trikrein]

We are talking about using currently available physics. The only “field” we have to accomplish this would be the electro-magnetic field. Your concept would work well for charged particles. Unfortunately most of what is out there are neutral hydrogen atoms & tiny dust particles. The only way to move these is to impact them. This shield could be angled like you suggest or ablative. Either way it would add significant mass that would increase as velocity increases.

[@superakman14]

Personally, my favorite interstellar rocket concept is the nuclear salt water rocket. I like the idea of a continuous controlled nuclear explosion moving you forward.

[@dynamicworlds1]

Much slower but much easier to build and fuel.

[@innacrisis6991]

not a great interstellar drive, but a wonderful main torch for an interplanetary ship

[@antaresmc4407]

Not a great design in my opinion. It has a lot of hype for the sheer kerbality of the concept, but has serious issues in design, safety and dubious engineering, all for a oerformance that honestly isn't that great: The whole deal with a nuclear fuel premixed witth a moderator is terrifying, and the ability for the flame to climb up the tubes is pretty high, leaks and boiloff can lead to tank ignitions too, not to mention that'll be a nightmare to cool and thus have a high weight and low thrust to weight ratio... And all for what? 100km/s is an Isp that can be attained wityh much more polished concepts (gas and plasma core NTR, minimag) and better thermals (which translatte to better thrust), more safety and less pollution if used in low orbits (dont, lol) It would make a lot more sense to separate the fuel from the propellant and channel it all in a magnetic nozzle...

[@Katniss218]

@@antaresmc4407 gas core NTR can't really do more than 10k s Isp, that's an order of magnitude less

[@antaresmc4407]

@@Katniss218... 10ks≈100km/s... Heck, "10000 seconds" are the last two words of the title of the NSWR's paper, it's not an order of magnitude higher. As for the other designs mentioned, they do be able to reach an order of magnitude more (optimistically)...

[@sachafriderich3063]

this channel realy looks starts looking like early Isaac Arthurs, just with a slightly less excited guy. Loving it.

[@shawns0762]

I made a couple videos on this subject a few years ago. The key is a fission rocket that consumes only uranium or plutonium. All proposed fission rocket concepts incorporate hydrogen or xenon in their process which automatically makes them useless for interstellar travel because you can't bring hundreds of tons of that with you. 1 kg of uranium has the same energy as 120,000 tons of coal and plutonium has more than that, you would not need a lot of it for the trip. Uranium and plutonium are jittery atoms that are on the verge of fissioning all by themselves, there should be a way to get them to fission in a linear fashion. What's needed is a controlled, time released nuclear explosion. This might be as simple as having negatively charged atoms coming into contact with positively charged atoms. Or perhaps with laser energy or electromagnetic forces.

[@riderpaul]

That is an interesting concept since the fission products are probably ions as they leave the nucleus. I imagine a bunch of thin sheets orthogonal to the direction of thrust with a large positive change behind them. The thin sheets would keep the ions from neutralizing as they exit the source material. A large negative charge near the exit would also pull the ions in the direction of thrust. Hollow tipped electrodes at the exit would take the electrons from the foil and neutralize the ions upon exiting. Any interstellar ship would probably have several meters of water surrounding the vessel as shielding from cosmic rays and micro impacts. Hydrogen wouldn't be an issue, but it probably wouldn't be needed in this model. Love it. What is your KZbin channel?

[@shawns0762]

@riderpaul I had a video made on the subject "Best method for interstellar travel" you can click on my icon. It has diagrams of potential fission rocket concepts. I plan to make a new video soon.

[@chengong388]

anti-matter powered interstellar spaceship is certainly not new even in popular media. They did an awesome job with it in that movie Avatar. It's a large ship with both a laser sail for leaving and arriving at Earth, and its own anti-matter engines for braking into and accelerating from the remote solar system.

[@ImprobableMatter]

That's true. I never said that antimatter power had never been considered, just that most sci-fi hadn't done the sums about how long a relativistic trip would take.

[@LemonsRage]

I love how you started off the video with how long a trip would take and then went deeper into explaining how to accomplish that aswell!

[@eewls]

Best summary on KZbin so far. You are asking (and andswering all important questions) Great job!

[@Siddingsby]

Minature Blackholes are infinitely safer than Anti-matter. If the containment around your Anti-matter fails you, and everything within a few hundred thousand kilometres, is going to be instantly destroyed. If the containment around your Blackhole fails then all that happens is the Blackhole exits your Spaceship (causing a tiny amount of stress damage to whatever hull it passes through). Sure, you're now stuck without a power source in the void of interstellar space - possibly at relativistic speeds, but at least you're still alive.

[@en31337]

Seems like we’re having lots of issues delivering ‘biology’ to a distant areas, with all the cons like avg mass of 70kg, tolerance to acceleration around 1g and TTL of 80 years and insane amounts of ‘food energy’ in terms of mass needed to take care of. Delivering DNA and ‘soul objects’ might be a better option to consider and put mental energy into.

[@captaincruise8796]

I see two problems with the approach to 1-year 1G acceleration, those are dust and particle collisions (as pointed out by other comments) and heat buildup since your engine won’t be completely efficient and some of its energy will end up heating your ship. That much burn would I think vaporize your ship eventually? I think the practical limit on our conceptual horizon is about 0.1-0.2C, which would work for trips to nearby systems with some sort of extended life expectancy. You’d need a large rotating chamber for the majority of the trip as well since 0G for years will not be in any way healthy for the pioneers. Ultimately, we need a ship that people can more or less permanently inhabit that can also make half-century trips to nearby stars when it wants.

[@gueratom]

1:56 I'm pretty sure this is the cockpit view from a Eurofighter Typhoon.

[@randomperson9187]

What a fantastic video. Explained so well that even a laymen can understand.

[@robbie8142]

It must have been cause I did! 👻🚀

[@diego1694]

What would happen if there was a failure containing this amount of anti-matter? Not even during the journey, but here on Earth at some point before launch. And how can we expect to contain antimatter when we can barely contain a normal matter plasma for a few milliseconds in a fusion reactor. With that in mind, is this really any less science fiction than warp drives?

[@ImprobableMatter]

Really good point. It would almost certainly be assembled far from the Earth itself for this reason. The antimatter would probably also not be put into a single ship either, but sent out in multiple secondary ships, canisters and so on.

[@syntaxed2]

It would be catastrophic...for that reason I suspect we will not be storing antimatter directly but rather some inert normally charged matter and only make the conversion at the last stage of a propulsion process .

[@JoeOvercoat]

@@ImprobableMatter That is a detail that would have to be entered into the numbers or perhaps expressed as an assumption that we’re starting at the edge of the solar system, or some such thing. Likewise you’re going have to put in some fuel losses: recommend call 1.8 = 2.0 for purposes of losses, for example.

[@ciCCapROSTi]

"With that in mind, is this really any less science fiction than warp drives?" Yes. Warp drives only exist in math. Anti-matter actually exist. The challenges are engineering related, not "find negative pressure" related.

[@diego1694]

@@ciCCapROSTi I don't really see the difference, this is not a matter of "we know how to do it, just put enough money into it and it will be done". We have no idea how to do it, these engineering challenges have no solution. Both exist only in math.

[@andriyshevchenko6689]

You’ve made some great points. In theory you would never need generational ships at all, even to travel to Andromeda. Just crank up the speed to something like 0.999999999c and the people inside might feel like they’re taking a short ocean cruise. Of course civilization could collapse and be reborn on Earth in that time. You could basically have a social class of space seamen who have a normal age but have lived thousands of years in Earth terms from traveling back and forth. Fascinating stuff

[@randomaster138]

Cool worlds did a video on this I believe. They had a similar concept where they discussed the possibility of people being able to reach the edges of explorable space in a human lifetime with constantly accelerating ships.

[@countedcat477]

That's basically what happens in the 2nd ender's game book

[@planets9102]

The andromeda part is probably impossible, space isn't quite a vaccuum and at those speeds, even the intergalactic medium will cause significant drag.

[@dl5244]

[Edit to correct ship time] It would take a ship accelerating at 1g just over 22,000yrs earth time or 10yrs ship time to reach that extreme speed (nine 9's of c), and the same time to slow down. During those 44 millennia/20yrs the ship will have just left our galaxy but only covered less than 1% of the distance. The remaining ~2.5 million light years coasting will take about 2.5 million years earth time but only ~1 year of ship time

[@andriyshevchenko6689]

@@dl5244 nice of you to do the math, guess we gotta add 3-4 more nines there and maybe go a bit faster than 1g. Though I’m sure if these future engineers can solve the 0.999999c part, they can also solve centrifugal “gravity” for the coasting part of the trip lol

[@haydenmaines5905]

I seem to remember reading a paper discussing particles at high speed, with a theoretical limit of around 25% of c before particle impacts turn into essentially unlockable radiation

[@angryonion9550]

What about fission fragment rocket engines? You still have to solve the problem of controlling the plasma but the potential isp and thrust could be enough to get us to our stellar neighbors.

[@riderpaul]

Another problem is ½mv². As the acceleration voltage on the ions increases the power needed increases with the square of the voltage. At 1g of constant acceleration/deceleration Mars becomes a weekend trip, but the power required to achieve 1g of constant acceleration is off the charts. At a far more reasonable .05g the trip time during the Mars transfer window would be a month or two. Humans have not spent a lot of time outside of Earth's protective magnetosphere. Time spent in space becomes a lot worse once you leave low earth orbit. Interstellar travel would require at least 10 meters of ice to shield against the cosmic rays. Even a 60 day transfer to Mars would be a crapshoot without proper shielding in my opinion.

[@entropyachieved750]

Great to see another video. Really found those ones on fusion interesting and your style of teaching suits me. Hello form Australia

[@ozne_2358]

Impact with interstellar dust at substantial fractions of the speeds of light is a challenge. The kinetic energy is magnified by relativistic effects. But that's not all : once I did some simple calculations whose details now escape me but it seemed possible that impact with a speckle of ice might have enough energy to fuse some of the hydrogen atoms in it, causing even more damage. I could be wrong though.

[@ImprobableMatter]

Very good point. The spacecraft would have to detect and either avoid or vaporize anything from a grain upwards, then have a magnetic field to deflect ionized particles and the probably a physical shield. Hydrogen itself will not fuse in any significant amounts - don't get me wrong, the collision itself would be bad, but fusion would not add much energy to it, as explained in this video: kzbin.info/www/bejne/aHXdfIuEZtWWrbs

[@hamjudo]

@@ImprobableMatter To shield my entirety hypothetical science fiction near light speed vehicles, I lead with a widely spaced stack of spinning shield disks. Particles will hit the first layer and turn into showers of particles. The collision will absorb some energy and spread the resulting shower across a larger area for the next layer. Include enough layers such that an insignificant quantity of particles penetrate the last layer. The disks are spun for a few reasons. A spinning disk only needs to be supported in the center. If the disks spin at different speeds, the damaged sections won't remain lined up. The disks would be made out of graphene or some more suitable material. The spinning disks would be mounted to a corkscrew-shaped hollow rod, a unicorn horn, on the leading side of the ship. The corkscrew is necessary so that no triplet of lightly shielded motors and bearings are in a straight line. If the rod was straight, a single collision event could destroy a significant fraction of the shield.

[@Ryrzard]

I picked some values from google and it seems that you'd be plowing through a constant steam of dust on your way. Assuming that about 1% of interstellar medium is dust, the dust particles are 1 nm in diameter, have density of ~1 kg/L and there are between 10^-3 and 10^6 proton masses of stuff per cm^3 in interstellar space then the result is that there are between 0.1 and 100 million solid particles per m^3. If the space craft is 2 meters in diameter then at 0.95 c it would be colliding with 9*10^7 to 9*10^16 solid particles every second at Earth's time or ~3*10^7 to 3*10^16 particles per second your local time. If I'm calculating it correctly that's bombardment of solid particulate at a power between 1 W and 1 GW so a physical shield would probably be ablated too fast without another method. Also, the path through space could be slightly bent to avoid regions with high density. This would make navigation more complicated and sending refueling tanks ahead with railguns would no longer work.

[@ImprobableMatter]

@@Ryrzard Good analysis. I assumed that lasers or other directed energy beams would be used to partially ionize the dust, if it isn't already from the surrounding plasma and radiation. The charged dust would then be deflected by a magnetic field - this is produced by the round thing at the end of the spike in my cartoon depiction of a spaceship. Whether the drag from this deflection is enough to kill the whole idea, I'm not sure.

[@Ryrzard]

@@ImprobableMatter I have done some back of the napkin calculations and 10 ton ship accelerating at 1 G with 0.95 c exhaust velocity already consumes 20 TW of power. So the drag is probably not an issue. The power consumption itself is worrying me. Though, I guess a civilisation that is already traveling between stars could spare power to power interstellar ships remotely. On the topic of remote energy, once remote colonies at a neighbouring star system is established, you could construct similar laser/microwave/radiowave arrays there so you could eliminate on-board energy generation entirely since you can both accelerate and brake remotely. If antimatter was able to be produced and stored safely in vast quantities and its energy harnessed, then this becomes trivial in some regard. I calculated a while ago, after watching Cool Worlds video about traveling to the edge of the observable universe, that you "only" need a ship with a mass ratio of 1 part ship to 1000 parts antimatter/matter fuel to accelerate at a constant rate of 1 G all the way to the edge within a human lifespan.

[@samwilson9568]

This is great! I love these understandable real science based discussions of space and rocket technology. Thanks for making this

[@GadgetUK164]

Fantastic video! The other obvious challenges are oxygen, water, food and what happens when atoms or debris hit the front of the ship @ almost speed of light? I think fuel is the least of the problems! What we need is someway to distort spacetime in front of the ship, deflecting it to the rear. I think I've watched too much Star Trek...

[@ImprobableMatter]

The astronauts would need to pack some of the fuel for power on board. Before the trip, efficient methods for using that power to recycle water and generate food (think some sort of nutrient paste) would have to be developed. In terms of debris in the way, the spacecraft would have to detect and either avoid or vaporize anything from a grain upwards, then have a magnetic field to deflect ionized particles and the probably a physical shield to stop radiation.

[@Haydy5040]

Would it be possible to make proton-boron in a thermonuclear device? I don't think the secondary would make the U-238 tamper or whatever undergo fission because the neutrons from the fusion wouldn't be there, but perhaps that might reduce fallout if you are building an environmentally destructive canal perhaps? My area is ancient dead plants not this stuff but what do you think? (I was thinking about project orion this isn't totally unrelated I promise)

[@ImprobableMatter]

I think once you commit to a thermonuclear bomb, you commit to making it very destructive and producing fallout. With its lower cross section, p-B would not improve the yield enough to be worth it. You might as well use a slightly more powerful fission bomb, or go for the usual Lithium-Deuteride secondary.

[@Haydy5040]

Figured it probably wouldn't be worth it considering how abysmal the numbers for proton-boron look compared to the traditional secondaries.

[@Aurora-nv3br]

Great video! My question to you is , what are your thoughts on the more speculative alcubierre drive? There has been some new research on it and it seems to be theoretically possible but essentially impossible to build in real life.

[@ImprobableMatter]

It's definitely interesting and I hope the theorists make more progress about it. Until then any speculation on how it will work practically is like asking a Victorian what material we should use to build a stealth fighter.

[@LunarLaker]

@@ImprobableMatter as a victorian, I've heard Vegemite has a suitable absorption spectra for reducing RCS

[@gabagabago0l]

It's impossible to build irl and the proposal of traveling faster than light is ludicrous as well. Speeds faster than light don't exist and it would break casuality.

[@HIRVIism]

Lots of people in the comments seem to be missing the point and are focusing on engineering challenges with our current tech :) Great video, thanks.

[@alexandredevert4935]

Ok, it's the best special relativity class I ever had. Thank you.

[@MushieDetails]

I want to work on graphene production in hopes of making super conducting magnets more widespread for application just like this and for them space elevators. Grid storage and increased solar panel efficiencies, I want to see the stars one day ✨

[@The_fusion_physics_guy]

i don't think graphene is used in superconductors, to my knowledge, but i support your aspirations!

[@smileyface6583]

@@The_fusion_physics_guy grapheme on its own isn’t a superconductor, but MIT recently discovered that, when twisted or stacked at some angles, it can make for a pretty robust superconductor.

[@MushieDetails]

@@The_fusion_physics_guy i was also thinking in terms of making superconducting magnets more efficient and easier to build. if you have a supercapicitor, graphene, that can quickly recharge and discharge without the need for additional coolant, It would make rail guns viable on nuclear powered aircraft carriers. Go DARPA! It's main application would be replacing silica for semiconductor manufacturing; making computers more efficient, grid scale energy storage for renewables and also it could replace silica in solar panels; again raising efficiency. It would be the end of the profitability of fossil fuels. I dont know why I took so long too respond, just popped back in my head today lol

[@prepper7311]

i love that he used Alice and Bob in his example.

[@ImprobableMatter]

It's actually the third video where they've been doing physics-related activities.

[@Kombivar]

Awesome video! I'd like to know your thoughts on "Project Hail Merry" by Andy Weir - which you have definitely read, judging by mentioning of 82 Eridani as the second object to visit after Alpha Centauri at 6:13 : :)

[@ImprobableMatter]

I'm afraid I haven't - I saw 82 Eridani as listed on Wikipedia and it was also a star identifiable in the image I used of the southern hemisphere for Alpha Centauri.

[@dl5244]

@@ImprobableMatter "Project Hail Mary" has some great ideas for a fuel source and such an amazing story! I also recommend "We are Legion (We are Bob)" aka "The Bobiverse Series" by Dennis E. Taylor dealing with travel to our stellar neighbors via von Neumann probes seeded with human consciousness.

[@WobblycogsUk]

I find this topic fascinating because we have have reached a point where we can see interstellar travel is possible but it's totally impractical. Considering the amount of energy required to get the craft going at a decent faction of the speed of light I don't see it happening any time soon. Even if we could generate anti-matter reasonably cheaply it feels like it would be better to use it to power a larger and slower ship. You've got to consider the fact that where ever you are going you're going to want to set up advanced manufacturing and that's going to require millions of people at least (this is why a self sufficient Mars colony isn't going to happen any time soon). I have to wonder as well if humans, as we know them today, will ever travel to the stars. If we haven't wiped ourselves out, I suspect that by the time we have access to the energy levels needed to get to the stars we won't look anything like we do today (I plan on being a giant robot spider).

[@eliyasne9695]

There is an auxiliary problem to deal with when you accelarate at 1g with a near light speed exhaust: heat management. I would presume that with that much power consumption any tiny leakage would be able to vaporise the ship.

[@ImprobableMatter]

I am not sure I get your point - why would it matter if you are at light speed (relative to the Earth)? As I mention in the video, a modern rocket can burn and accelerate at 1g happily. Remember that every object is stationary relative to itself - if you are onboard a Saturn V rocket in empty space, the physics work exactly the same no matter how fast it is going relative to the Earth.

[@eliyasne9695]

@@ImprobableMatter By "near light speed exhaust" I ment to say that it would be an engine with an exhaust velocity that is a substancial fraction of the speed of light. The comparison to existing rockets that can easily accelerate at 1g is flawed, as they do so with a far larger amount of reaction mass. The power of a rocket engine is at least the kinetic energy it pours into its exhaust. For an exhaust velocity v, a mass flow rate of m' and a thrust of F: F=m' v Therefore m'=F/v Power: P=½m' v² =½Fv Since the thrust needed to accelrate at 1g has nothing to do with the exhaust velocity, we can conclude that the power is proportional to the exhaust velocity.

[@ImprobableMatter]

Ah, I see what you mean. As I mentioned in the video, we already have technologies which can shoot out atoms at the 1GeV of energy (close to the speed of light), so conceivably thrusters could be build like that. So, it's definitely an issue, but one that can probably be overcome.

[@eliyasne9695]

@@ImprobableMatter The problem I'm talking about is not creating the thrust, it's staying alive near what would probably be nuclear bombs worth of energy used per second.

[@salmonjoseph9970]

Holy shit the best video on the topic!

[@o-hogameplay185]

1:37 oligarchs "accidentally" falling out of a window is such a good joke

[@YourCapyFrenBigly_3DPipes1999]

Sometimes your rival catches up to you...

[@johnredford942]

How about the old ramjet concept? Gather interstellar hydrogen gas on the way and fuse it to accelerate? On a related note, I would think the exhaust from such a craft would ionize a lot of the interstellar medium, perhaps enough to be visible to astronomers. If we saw these fast-moving contrails, could they be a sign of alien tech?

[@dynamicworlds1]

Short answer: For acceleration? No. The math works out that it creates too much drag for what you get out of it. For deceleration? Drag adds to your deceleration, so possibly.

[@cookiecookiespark7485]

What is the piece of music in the last few seconds of the video?

[@ImprobableMatter]

"Badinerie" by Johann Sebastian Bach.

[@cookiecookiespark7485]

OMG this is what I heard many times back in elementary school, loved so much but never knew which piece. Twenty years later by a stroke of luck I come across your video and finally be able to figure out. Thank you...

[@planets9102]

It seems to me we can go a lot lower tech. Why not ditch the rocket equation entirely? You instead send out a self-replicating probe, even something like a nuclear salt water rocket could accelerate a 1000ton probe to 0.1c if the starting ship were the mass of a super tanker (with something like 5-6 stages). When the probe arrives it builds an enourmous solar array and a giant pushing laser. When it's done it sends a signal back. Voyager has been out in space for 45 years, a probe specifically designed to hibernate until it arrives could surely do better. You then use such a pushing laser to accelerate a crewed ship to a high fraction of the speed of light (you'd also use this to push the probe up to speed). The pushing laser at the destination then slows you down when you arrive. Aside from course corrections you wouldn't need any fuel at all for this, thus sidestepping the rocket equation. You could also use the energy of said solar array to power the industry of your colony once you arrive. I'd say this approach seems less challenging technologically then anti matter rockets, given that those require the ability to effectively reflect gamma rays, something that might just be impossible period. Self replication is already seen in nature so we know that's possible and given that humans can make lasers and solar panels I see no reason why robots couldn't do the same. Also this channel is highly underrated!

[@henryefry]

What about using a solar sail and sol based lasers to aid in the acceleration phase of the trip so the craft would only have to carry the mass of fuel needed to decelerate. The sending fuel ahead could also be using this technique, launching it from a rail gun then deploying a sail to get an additional kick from the laser array. The idea being that the full array would be complete when the human space craft was scheduled to depart but only partially complete when the fuel leaves. Providing an opportunity to test real world acceleration and identify issues before the people leave

[@ImprobableMatter]

Definitely possible. Lasers are hard to focus over such large distances, but maybe radio waves etc. From the point of view of acceleration, it's basically like a photon rocket: you receive a photon from Earth and either reflect or re-emit it back, getting an impulse of twice the original photon momentum. The wavelength of the photons doesn't matter as long as you get the right amount of energy (i.e. 1 Joule of light carries as much momentum as 1 Joule of radio).

[@smileyface6583]

@@ImprobableMatter there’s a concept known as stellasers, which uses the sun’s corona itself as its lasing medium, and can be used as highway equivalents between the stars for very long ships with very huge, but light, thin and reflective solar sails. the sails don’t even have to be reflective for this idea to still work, it’s just that it would take longer to reach target speed. you could just replace the mirror of the sails with solar panels for power generation, or something like that. the stellasers themselves are just two highly reflective mirrors pointing at each other, over large distances between each other, with the sun’s corona between the two, orbiting the sun, and then they get “charged up” naturally by the sun.

[@ImprobableMatter]

@@smileyface6583 How do you maintain a population inversion in the corona?

[@smileyface6583]

@@ImprobableMatter because the two sails are orbiting the sun, and because the sun is constantly releasing new exited atoms in the form of solar storms of different kinds, in normal-state atoms are constantly being replaced with exited atoms, and the energy level or wavelength of the lasers can vary depending on what state of orbit of the sun the mirrors are in. remember, the mirrors themselves are INSIDE the sun’s corona, so it’s an almost self-replenishing thing. the parallel lasers stay in loop between the mirrors, with the non-parallel ones eventually leaving the loop. you can then use another mirror, or another set of mirrors, and point them to the star system you want to go to. they can range from meters to thousands of kilometers, in size.

[@ImprobableMatter]

@@smileyface6583 This is a neat idea, and I actually study coronal physics now. I will run some numbers, but as a hunch I imagine that it is too tenuous and too equilibrated (though there are non-equilibrium situations) for lasing. Are you happy with the notion that if the lasing atoms are expelled from the surface of the star in thermodynamic equilibrium, or equilibrate by collisions or radiation, that lasing is impossible?

[@andytroo]

13:50 - antimatter - can't get anywhere near 100% - gamma rays are not easily deflected, except at glancing angles, and they are emitted in all directions, including up/down. Secondly, there There is also the problem of cooling - having a literal TW of gamma rays impinging on the back of your spacecraft (implied from mass + acceleration numbers) will melt anything in short order, and the heat problems are much worse if you are talking about antimatter electrical power generation for your particle accelerator, at less than 50% efficiency ...

[@ImprobableMatter]

The rocket equation does not care about the wavelength of the photons, so in principle it would still work if you absorbed one gamma ray and re-emitted it as a bajillion thermal photons. I agree about the practicalities. Maybe a workable solution in the far future would be to ablate away matter at huge energies (so, more like the ion based example I gave).

[@dl5244]

There's definitely engineering hurdles to overcome to create controlled and directed anti matter reactions... two "half kilogram" slugs of each just touching will be no where near 100% efficiency no matter how that is gamed

[@wilurbean]

Each pv cell generates a few volts. Many little ones in a row in a linear accelerators

[@morkovija]

another banger of a video, have nothing to add to the subject, but I'm just hoping for breakthroughs in trans-uranium element synthesis and getting to the island of stability for those juicy juicy goodies

[@LeChat084]

Ok for BAT or DAT, but what about ICT (ITER Construction Time) ? 42 years ?

[@wilux2469]

One thing sometimes brought up regarding interstellar travel is using emf induced for the breaking part of the journey since you are moving way faster than interstellar medium. Could that theoretically be more efficient than the antimatter engine since the induced force doesn't depend on length, not volume?

[@ImprobableMatter]

For deceleration, yes, potentially. If some sort of EM-based method could be used to decelerate at the destination, it would of course save a lot of the fuel required.

[@theOrionsarms]

Deceleration don't depends on mass or volume, but on mass over the square of kinetic energy, so if you have 1 kg of stationary environment, and you hit with 1 km/s you can get 1Mj of dragging energy from that , but if the velocity is ten times higher (10 km/s) you can gain 100 MJ of energy from that, and that means that when you you only count for deceleration you can have a huge gain when your velocity is big and have a good enough deceleration environment.

[@orionemoray3990]

Really like your videos man, keep it up!

[@Profesor449]

Why *exactly* is it difficult do deflect gamma rays (visible light can be deflected with reasonable efficiency and so can be radio waves I believe, what is the problem behind the short waveleghts? My first idea is they *really* want to penetrate things, but my physics knowledge can't explain why it is like that) And is there (atleast in theory) an idea for such mirror or whatever would it be called?

[@ImprobableMatter]

Gamma rays have more energy than the ionization energy of any material. This means that electrons in any solid material would much more likely absorb a gamma ray and be stripped from their host atom than reflect them, unlike visible rays. In fact, this is true for any ray more energetic than ultraviolet. If you want to see a video about actual space missions which also talks about this, see the one linked at the end: kzbin.info/www/bejne/iYvNm6qffZqefpo P.S. I will cover this in more detail if I do a follow-up video.

[@johnduncan5117]

My personal view is that long distance space travel will become a thing in a distant future, but it will be artificial intelligence (which by then will have replaced us) doing it. Time doesn't matter to a machine so long as it can keep functioning.

[@stekra3159]

Space is big. Really big. You just won't believe how vastly hugely mind-bogglingly big it is. I mean, you may think it's a long way down the road to the chemist, but that's just peanuts to space.

[@markdavid7013]

The "starship" would have to have some kind electromagnetic shielding around it. Colliding with anything at relativistic speeds is a"not a good thing"

[@ImprobableMatter]

Absolutely. I tried to depict some sort of electromagnet on my cartoon spaceship as a ball on the end of a long nose. Some sort of shielding, detection radar, energy beams, small-scale maneuvering, etc.

[@punditgi]

Excellent video! 🎉😊

[@raphaelbernard7954]

If you are as most people believe are moving through vacuum. What are the forces that your craft is pushing against to achieve forward or any motion?

[@matthiasknutzen6061]

10:25 How is there no limit to momentum? Surely the momentum it would have att the speed of light is?

[@ImprobableMatter]

If you look up the relativistic formula for momentum, for a massive particle (not photons) it goes to infinity at the speed of light. This is why it is impossible for anything else to reach the speed of light.

[@matthiasknutzen6061]

@@ImprobableMatter Ahaa thanks

[@XtreeM_FaiL]

@@ImprobableMatter Should we say nothing can reach the speed of light? Photon can't reach the speed of light because it never accelerate, it just goes the speed of light.

[@ahmedabdolghani8879]

If, theoretically speaking, we managed to achieve a self sufficient fusion reactor, is there a possible way to use it as an engine for space ships?

[@ImprobableMatter]

Slightly better than a fission reactor.

[@JeffBilkins]

I can't even imagine what kind of infrastructure or safety certification you'd need to develop and fuel a spaceship with tons of anti-matter.

[@killman369547]

Hmm, it would all need to be done under as complete a vacuum as is possible. Every part of the fuel system the anti-matter has to move through will need to be magnetically confined to stop the anti-matter from touching anything. Those magnets will need to be supplied constantly with power and that power could never be allowed to be interrupted under any circumstances otherwise.... Kaboom. Anything and anyone within a few dozen kilometers of the ship is going to have not just a bad day, but they're last day.

[@gijoe41688]

is there a way for the spaceship to comunicate while traveling at close to C ?

[@ImprobableMatter]

Yes, they can still send radio messages. The messages would be red shifted relative to the Earth, but that just means that a frequency of, say, 100MHz would be 50MHz at the Earth. It would also take a few years before a reply would be received, depending on how far they are.

[@vk3dgn]

How would we know it'd be worth the journey? Even a robotic/AI expedition to scout a destination wouldn't be able to return useful/any data - the signals would be lost either due to distance or buried with the emissions of a star. There's also the matter of navigation - I expect a mid-course correction or two would be needed. I should write SF.

[@LemonsRage]

Or in a futureristic scenario humans get somehow frozen so that we aren't squishy anymore, accelerated by a magnetic railgun at almost the speed of light and then unfrozen just when the ship flips to start the slowing down burn at 1g. This would ofc make the always one way since a stationary railgun in space would be needed inorder to return back to eath (or where ever we came from) but also be pretty close to how mass effects way of transportation was.

[@GianmarioScotti]

How do you solve the problem of the ship hitting interstellar dust, and becoming a colander?

[@pizzacrusher4632]

haha, informative AND funny! I like it :)

[@HansLemurson]

Where do you draw the line between "Extremely High Specific Impulse Thruster" and "Death Ray"?

[@jaysnehpandey7089]

Well along with completing trips in human lifetime, it'll be better if we also focus on expanding the human lifespan too at this point

[@RandomPerson-vf3ld]

Is it correct that if we someday solve the fuel issue and can accelerate at 1g all the way to the halfway point and then turn and burn all the way to the destination, then is it possible to get anywhere in the universe in roughly 11 years as perceived by the astronauts? The math is beyond me but seems in the ballpark. Also, if wormholes work but crush anything you send through them, could they be used to send the antimatter and matter? Like fuel lines. No need for any weight of fuel on the ship.

[@ImprobableMatter]

For the first point, yes, but it would take enormous amounts of energy to do so. I consider wormholes beyond plausibly achievable unless some major discovery is made.

[@stekra3159]

An extremely energy dense power surse. Do you think Fusion is and appropriate power surse for such a space ship?

[@ImprobableMatter]

Fission or fusion could certainly also work. They release less energy for every kilogram of mass, which means you would need much more fuel (by thousands of times). On the other hand, these are technologies that we are likely to have fully mastered within the next hundred years and the way to store the fuel is understood and quite simple. If my suggestion of launching fuel canisters or similar could be realized, then it would probably be quite attractive also.

[@Muonium1]

It's so easy to forget the manifold problems with interstellar flight at our current level of technological sophistication. For instance even with a perfect power source, fuel, and engine, one cannot simply accelerate indefinitely close to the speed of light. At some point, you will hit the Greisen-Zatsepin-Kuzmin limit, where the cosmic microwave background radiation will be blue-shifted into gamma rays from your point of view and you will simply be roasted alive by the intense radiation.

[@ImprobableMatter]

Agreed, though even with the Eridani trip described here the red-shift factor is about 6. Not nearly enough to shift millimeter waves into nanometers.

[@jasonspitzer1503]

One day you’re cruising along at 0.7c when your ship hits a tiny rock. Game over.

[@YourCapyFrenBigly_3DPipes1999]

Shields????

[@miklov]

Fascinating. Do we know anything about the ablating power of interstellar space when nearing the speed of light? I imagine each particle would be quite bad to run into, question is, how many of them would be run into?

[@avhuf]

The one important factor you haven't included in your calculations, is that for the fraction of light speed you're aiming for, the mass of the "ship" would vastly outweigh the "100000" ton limit due to the need for extensive and massive shielding against the many small particles the ship will encounter during the journey. And that's hoping it doesn't encounter any object larger than grain sized.

[@loklan1]

Interesting video, great to have some numbers for torch drive type scenarios. But torch drives are a long way off, I don't know if you've defeated the "you'll be overtaken" argument so much as described the ships that will be doing the overtaking.

[@iankrasnow5383]

I have thought before about how time dilation would affect astronauts with 1g of acceleration. Instead of accelerating for exactly one year and then decelerating for 1 year, you could accelerate for exactly half the trip and then decelerate for the other half, never spending any time coasting, and never needing to experience weightlessness except for the brief moment where the ship starts thrusting in the other direction. This is a lot more wasteful, but this is so far off in the realm of science fiction anyway, why not be optimistic about fuel density and cost? At 1g, you could actually reach any point in the known universe within 45 years, including stopping time. If you're trying to get 9 billion ly away from Earth in a space ship accelerating at 1g for 22 years and then at -1g for the next 22 years, wouldn't that mean you only need enough fuel to accelerate for 44 years, or enough or 9 billion years? That's a pretty significant difference.

[@ImprobableMatter]

You're right that the "years of fuel" requirement is in the frame of the ship, not the Earth or the galaxy. Still an eye-watering amount of energy for that trip, though!

[@qwertyuuytrewq825]

What is a bigger challenge - propulsion system that allows to reach speed of light or surviving radiation and impacts on such speed

[@timothy705]

Enormous solar powered lasers to accelerate, thermonuclear bombs to decelerate. After the first initial colonization mission ships don’t have to carry much propellant since most of the deltaV cold come from lasers at the origin planetary system & the destination system. That would result in travel times on par with a human lifespan to alpha Centauri though. If we ever develop practical artificial fusion power generation then interstellar space could be dotted with fusion powered pusher stations wherever there’s a deuterium rich celestial body that could also keep corridors of interstellar space clear of debris to increase the practical top speed of ships.

[@dsdy1205]

Honestly, given the way progress in biotech and brain research is going, it's quite possible that the most ideal interstellar transport method is a honking huge radio antenna mounted on a slow-boat with in-built factories for colony assembly, human body growing and brain downloading when it arrives.

[@Paul-rs4gd]

I don't think brain downloading will ever be a thing (see Geoff Hinton's views on this). However I think slow-boats with robots could make the trip and grow humans from frozen embryos, or other bio-technology, when they arrive. The robots would have to care for, and educate the first generation. It's pretty unethical to do that to human beings, but it could be done.

[@TimJBenham]

6:21 If you can simply pull things out of the air then you would always accelerate as much as you can, which is for half the journey. No need to limit yourself to 1 g either. Humans can endure more and machines much much more. With enough acceleration you can travel anywhere in as small a proper time as you wish.

[@XtreeM_FaiL]

Not much more.

[@TimJBenham]

@@XtreeM_FaiL Electronics can survive well over 10000 m/s^2. Artillery shells are instrumented these days. Humans in hydrostatic equilibrium can withstand several times terrestrial gravity.

[@XtreeM_FaiL]

@@TimJBenham Only very short period. Constant +15-20% will be very hard and uncomfortable and 2G start to be leathal.

[@ciCCapROSTi]

What about photon drives? Shooting out photons is theoretically the purest form of generating impulse. Answered at 15:30

[@NoahSpurrier]

I wonder what the highest G force a human could live in for a long duration. Would 1.5 G be too much? It would be uncomfortable at first, but your muscles would strengthen over a a week or so. But what would other long term effects would the travelers feel under such a force?

[@a.v.gavrilov]

Если ракеты, несущие топливо с собой - неправильный путь для того чтобы получить желаемое, так может не надо рассматривать их? Есть концепции лучевой передачи энергии и импульса, которые позволяют достичь желаемого. Есть концепция магнитоплазменного паруса, который позволяет тормозить об плазму межзвёздной среды, что решается например проблему торможения (хотя вариант с излучением также позволяет это сделать по методу Форварда).

[@PastaAivo]

I don't know if it would really count as interstellar travel anymore, but I'd probably focus on developing and sending a fully self-replicating construction robot to wherever we want to go, and use it to build all the necessary facilities in the destination, including a cloning lab. If equipped it with enough varied genetic information, we could technically colonize the destination with first-generation artificial humans (frozen embryos and such might work and be much more achievable too). Dubious for sure, but this is more of an engineering problem rather than a "new physics" one.

[@plopsmcgee9672]

How far would a spaceship travel with 40 years of acceleration followed immediately by 40 years of deceleration? I know that from the perspective of the astronauts their acceleration wouldn't change, however, since I also know that the distance to their destination would appear to contract I don't know how to do the relativistic calculations for that trip.

[@plopsmcgee9672]

I should specify that those 80 years are from the perspective of the astronauts. From the perspective of an outside observer the trip would no doubt take much longer than 80 years.

[@ImprobableMatter]

40 years *in the astronauts' frame* gets to absurdly large numbers if they're accelerating all the way. We're talking further than the observable universe kind of distance. After 100 years on Earth, 5 have passed on the spaceship, but after 1000 years on Earth, only 7 have passed on the spaceship and it gets crazier and crazier from there due to the asymptotic nature of it all. The energy and fuel requirements get crazy too. 40 years+40 years in Earth's frame is more sensible: 78 light years, 8.5 years for the astronauts (i.e. 4.25 for each leg).

[@eddiebravofan6005]

What about using ping pong spacecraft. Image 2 spacecrafts shooting particles to each other and accelerating directly opposite direction.

[@griffith500tvr]

Is gravity a force?

[@igorigor5342]

Why won't use nuclear explosions to propel rocket? It would give to much acceleration?

[@DG-mk7kd]

Massive orbital space colonies (o'neil cylinders) seem feasible, those are essentially stationary generation ships, and will be necessary at any star because of the scacity of even vauguly habitable worlds. Being in the deep void for millennia could be disturbing, but taking a century to get to the neighboring star doesn't seem so rough (you have to be somewhere). This requires maybe 0.1c which should be pretty doable with beamed energy, redirecting a small fraction of sunlight would do it. I think humanity will steadily diffuse out through the galaxy long before a high speed ship could ever find a habitable world

[@Schona110]

Could you do a video on Thorium nuclear power generation?

[@mrcleanisin]

An imaginary world where a magic sword stretches from a planet to its moon and is within 1 inch of the moon's surface, how long would it take to touch the surface of the moon when it is pushed from planet 's end at the rate of 1 inch per second?

[@YourCapyFrenBigly_3DPipes1999]

"shorter than many prison sentences" 😂 why did this make me laugh tho?

[@kevinpond8422]

I’d be curious what kind of energy we’re talking about say in terms of our current yearly planetary energy usage. Or in terms of total solar output.

[@ImprobableMatter]

Excellent point. I would like to cover this in more detail in a follow-up video. It would probably take a century (!) of current human power production to make such a trip. On the other hand, it would take a realistic solar array of size 1000km by 1000km in space a year to do so (so, not even the size of the moon in the sky). This is eye-watering for our current civilization, but if we are well established in space around our Solar system in the next thousand years, it's not so outlandish.

[@ck88777]

I wish you had mentioned nuclear pulse propulsion

[@m.streicher8286]

Don't mind me, just binging the entire channel again oh and a+ defenestration joke

[@robind506]

And to think, this is all "back of the napkin" sort of calculation

[@Rrgr5]

I'm a little bit late here, but my first idea was something like a centrifugal mass driver using an orbital ring, if one day we reach the resolve to build an orbital ring, it could use something like the current maglev tech to give the ship the initial push together with the gravitational field over the ring and let the intertia do some of the trick, but your idea of a "space train", like using autonomous vehicles to create a line of transmission to thrust the ship along the way is a different and interesting approach, I said train but would be more like a funicular, since the deceleration at the end, if made well, could replenish the energy used during the acceleration, the only problem would be the habitation inside the ship and in the other solar system, if doable, eventually other orbital rings could be made in the new system and a transit system could be eventually be created, problem is the hell of a long journey, if the humans in the future outlives us and have no problems with time expanding, well, they won't see that as problematic.

[@METT-TC]

If you are at all interested in this topic, read Project Hail Mary by Andy Weir. Go into it as blind as you can, but take my word for it that it is an absolutely delightful book

[@Khannea]

So what is we build a launch facility, mining operation and AI automated factory the size of the New York along as many rogue asteroids, moons, plutoids and planets we can find along a narrow route towards alpha centauri. Lets say one major such link in a chain every few billion kilometers, assuming enough such waypoints are available. The sequential project of building such waystations would be an exponential endeavour, but even then we might be able to send gargantuan automated factories to build gjant autonated factories. That would assume possibly thousands upon thousands of such waypoints. Each erects a massdriver and keeps addinf more and more massdriver coil length. Such waystations might take a decade to construct individually, the full length centuries? What it means is they would act as fuel station cannons hurtling fuel tankers at extreme G to any passing vessels headed to alpha centauri. They might even be used to clear route of interspersed interstellar debris. Is this a realistic scenario?

[@Velereonics]

We might soon be able to withstand more than 1 g or less through medications. Zoledronic acid and prolea are... Tentatively effective at jncreasing bone density, for instance

[@pikatheminecrafter]

I'd like to think that you could put a sort of magnetic funnel on the bow of the ship. It would suck up tenuous interplanetary/interstellar gasses, which can then be fired from an ion engine. This would only gain more efficiency with speed. Energy would have to be imparted to it somehow, but I imagine fusion reactors could be involved. Ions are sucked up, the lightest ones (hydrogen, deuterium, tritium, and trialphium) get fused into heavier elements generating power, and then some of this power is put back into kicking the fusion waste product out. Something like that. However, the concern then is, does that violate thermodynamics?

[@asusmctablet9180]

Carl Sagan talked about this decades ago, it was one of the proposed models for an interstellar ship back in the 1970s.

[@Velereonics]

It's sort of wild how simple it is to go 95% speed of light like that's not even hard we could do that if we just had enough fuel and very reliable engineering because thatd be a lot of stress on some

[@williamsutter2152]

Another way such a spacecraft could be propelled is through Hawking radiation, if we can somehow find a way of efficiently collapsing relatively small amounts of matter into black holes.

[@valerielhw]

Though a lot of what you stated was over-my-head _(literally and figuratively),_ I have always thought that the idea of a "generational starship" was ridiculous. First, as you correctly stated, future technology would almost certainly overtake them. Second, space is deadly. They could run into a number of unexpected hazards over such a long period of time. Or their supplies could somehow be ruined. Third, the idea that people would be willing to live their entire lives in space without ever seeing either the Earth OR their future destination is highly presumptuous. They might well decide to alter their destination. or return to Earth. Four, they may be unable or unwilling to have enough children to keep the mission going.

[@cattunger]

Super late to this vid... but I think if we wanted to do interstellar travel with this method, on top of coming up with newer fuel types, you would also want to launch from either orbit or the moon, whichever seems more efficient to send fuel to & assemble the new craft at (Most likely just in orbit, due to the soil on the moon basically being super fine silica).

[@stephenfleming8030]

You've factored in time dilation into the journey of the astronauts, but have you also factored in the additional relativistic mass of the spaceship as it accelerates? Beyond a certain point (I believe 54% C) that additional mass becomes a problem. Where the Lorentz factor affords your travelling astronauts some saved time, the same coefficient is increasing the mass of the vehicle and decreasing the relative efficiency of the propulsion system, making it increasingly difficult to maintain a constant acceleration of 1G. Maybe I missed something in your calculations, but it's not at all clear to me that this has been accounted for. Great video though. Much food for thought.

[@ImprobableMatter]

This is a good point. Actually, if you do the calculations in the rest frame of the spaceship, the mass does not increase because it is stationary relative to itself. The engines are simply putting out enough thrust (the propellant is leaving it at relativistic speeds of course) to maintain 1g in its own rest frame for the length of time (again in the rest frame) to get to the required speed. The photon rocket equation is simpler and also takes account of the mass correctly.

[@stephenfleming8030]

@@ImprobableMatter Hmmm...I hadn't thought of this before. As our intrepid astronauts are unaware of the time dilation and thus their own increase in relativistic mass, so too is the propulsion system of the vehicle itself. However, the Lorentz factor not only increases the mass of the vehicle, but the spacial dimensions which affect the relative velocity per second, and thus the value of 1 second... Much to think about here...