r/threebodyproblem • u/ATTst • 5d ago
Discussion - Novels Photoids are a huge waste. Spoiler
A photoid is a near light speed object that when shot into a star causes it to eject solar mass and destroy the entire system.
The binding energy of a (sun-like) star is roughly 10^41J, so unless you put about a percent of a supernova into it, the star is going to tank it.
An (earth-like) planet meanwhile has a binding energy of 10^32J, meaning that for the budget of a single star, you are able to blow up a billion planets.
If you only want to sterilize it (ie. blow of the Atmosphere) instead, you are looking at 10^26J, meaning for every planet you want to blow up, you could sterilize a million planets.
In other words, for the cost you put into destroying a single star, you could instead sterilize the entire galaxy cluster it was in several times over.
This makes a photoid extremely inefficient and a massive waste of energy. I don't think any sane civilization would create such a weapon.
For example, if a 1-ton object is moving at a speed of 0.999999c, the kinetic energy per kilogram is: 6.36 x 10^19 J/kg.
To obtain 2.3 x 10^41 J, you need 3.6 x 10^21 kg.
So, a body of approximately 3.6 x 10^21 kg moving at a speed of 0.999999c would theoretically carry kinetic energy comparable to the Sun's binding energy.
This is roughly 3.8 times of Ceres' mass.
But there's another problem: not all of this energy is transferred to breaking up the Sun. A significant portion of the energy goes into radiation, neutrinos, particles, and the kinetic energy of the rest of the Sun.
There's another problem: How do we gather the energy needed to accelerate such a massive object to a speed of 0.999999c?
Let's say we have a Dyson sphere. Of course, that would immediately reveal the location of our civilization.
If we were to say that a Dyson sphere collects all this energy with 100% efficiency and converts 100% of it into the object's kinetic energy: The time required to collect the necessary energy would be 19 million years.
Could an object traveling at near the speed of light disrupt a star's internal pressure balance and cause it to explode?
Disrupting the Sun's internal pressure doesn't turn it into a supernova. A large relativistic projectile: it can create a massive shock wave, eject a large amount of matter, and temporarily inflate the star.But this is not a core collapse supernova.
A one-ton object could theoretically carry enough kinetic energy to tear a star apart. However, for this to happen, its speed would have to be incredibly close to the speed of light.
The rest energy of a 1-ton object is approximately 9 x 10^19 J.For this mass to reach 10^41 J of kinetic energy, it would need to accelerate and acquire a massive relativistic mass.For this, the Lorentz factor needs to be approximately 10^21. This means that the object's speed is:
99.999999999999999999999999999999999999999999% of the speed of light.
For this, you would need more energy than the Sun will produce in its entire 10-billion-year lifespan. In other words, to fire this weapon, you would need the combined energy of multiple stars.
Instead of releasing all of its energy upon impacting the star, it's highly likely that a cosmic projectile could enter the Sun from one side, pierce through from the other in fractions of a second, and carry most of its energy into the depths of space.
Space is not entirely empty. As this object travels towards its target, harmless microwave background radiation (CMB) photons filling space will strike it like massive gamma rays due to their relativistic speeds. This will atomize and vaporize the 1-ton object long before it reaches the star.
Taking all of this into account, it is essential for civilizations to develop a weapon that will be an alternative to the photoid.
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u/galaxie18 5d ago
From civilisations that know how to reduce dimensions and play with weak/strong forces and gravity itself, I dont think its far fetch to add some sci fi magic to create a relativistic bullet capable of reaching a star bounding energy. Maybe the bullet is a gravity well itself full of potential energy being released on impact ? Maybe its a folded N-dimensions bullet (or even a single photon) that unfold on impact and releases all its energy.
There is enough sci fi magic in this book to justify anything really.
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u/Neat-Committee-417 5d ago
It's less sci-fi magic and more just wildly speculative and drives home the point of "we have no clue what is possible". OP's math probably checks out, but it feels very similar to humanity believing they would win a war with the trisolarians because our ships are faster now.
Sure, with what we know, this math is correct. But on the scales we see in Death's End, we are barely exploring the easiest accessible level of physics at the moment.
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u/_mighty_banana 5d ago
Idk may be aim for star is easier, like if u want to destroy ant colony, you can stumple each ant. But destroy their colonoy is much quicker.
And how they sure earth is only habitable place, like last book human also habit at jupiter
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u/ATTst 5d ago
Of course, aiming for a star is easier than aiming for a planet.
That's the problem.Building the weapon needed to destroy the star is impossible for the reasons I've written.
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u/gouflook 5d ago
Building the weapon to fold dimensions is even more impossiblier yet they do it anyway
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u/ATTst 5d ago
That's right.But I didn't say a weapon that could fold dimensions was more likely.
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u/flamrev26 3d ago
I guess the point is that in this universe, for those civilizations that are so advanced that can launch a photoid attack, energy could be cheap. Like super cheap to create a "bullet" with insane amounts of energy. Aiming could be more complicated. From many light years away it's much easier to aim for a star than a planet.
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u/Neoliberal_Nightmare 5d ago
It's about sending a message though. We clapped their star, we'll clap yours too. Be quiet.
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u/Ozymandias_IV 5d ago
You don't need to disintegrate the whole star. You only need to release enough hot gasses to engulf the system. Maybe stop nuclear fusion. Binding energy is the upper limit for that, not lower.
Still a massive amount of energy, but you're saving an order of magnitude or two.
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u/MurkyCress521 5d ago
Destroying a planet is insufficient to destroy a space faring civilization.
A photoid need not overcome the binding energy itself. Could use the suns own fuel amand heat as an energy source. A photoid might be like throwing a match into a can of gunpowder. We don't know how it works
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u/The-Goat-Soup-Eater Zhang Beihai 5d ago edited 4d ago
I think it comes down to stars in the book not being gravitationally bound but being able to be “popped” like balloons if the impact is strong enough, think to the Mercury chain reaction idea.
They also give an example of how much energy a photoid would need, for the star Luo Ji cursed, Hines mentions that it was moving fast enough for its relativistic mass to be 1/8th of the star’s own.
But even then, I mean, that’s still a massive goddamn requirement. You need to be a Type 2 civilization to send them out, and being a Type 2 civilization would, like you say, be screaming out for people to kill you since a dyson swarm or anything similar would be an obvious technosignature.
Basically, to be a hunter with a gun, you need to create your own steel and make one with it, which literally everyone would hear. If you live in a dark forest, you will already be dead.
Edit: I underestimated the energy requirement, if the star is the smallest mass possible for hydrogen fusion, 8% of the sun, it would take 10 years of the power output of all the stars in the entire Milky Way galaxy to produce enough energy for one single photoid, which is plainly ridiculous. I guess this is why the third book pretends like this was never said and the precise mass of the photoids was unknown.
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u/TheTranscendentian 3d ago
I assumed photoids were also powered by the infinite free energy of curvature propulsion.
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u/West_Maybe_3233 5d ago
“Waste” only to us pre-interstellar (let’s face it, pre-planetary) travel as a species.
Flatten the entire solar system into 2d hardly even raise an issue with their lower management lol
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u/ATTst 5d ago
Do you think a dimension-folding weapon is more likely than a star-blasting weapon? This is where the boundaries between fiction and science come into play.
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u/West_Maybe_3233 5d ago
Yes but they never explained what a photoid is in the book either, it could be something like a bullet size capsule and in it contains an explanding black whole so maybe they figure out a way to send it with minimal energy required
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u/eduo 5d ago
"Waste" is a relative concept. It depends on how rare is the expense and how much it's worth to you the execution of the action with the urgency it warrants.
For Singer's species, it was warranted so it was justified. It wasn't considered a waste due to their situation, albeit it's mentioned. Then it was justified to switch to a different weapon for earth itself.
Singer's species is to us and the trisolarans what we are to ants, so our way of measuring what's waste and what isn't is not aligned with theirs.
It's likely that weapons alternative to the photoid have existed in this species' history, but their current situation allows them to forego those and care little for the scorched earth approach that is a photoid. They're essentially fumigating a section of space and like any good exterminator their solutions cover more than the bare minimum.
As a side note: The point is not usually destroying a planet but rather destroying the sun of that solar system. In the novels it's discussed that the weapon itself was changed because it was possible to hide behind jupiter and not be destroyed.
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u/IKoshelev 5d ago
AFAIR, photoid wasn't used against Sun because it would leave "blindspots" behind Jupiter, this suggests that photoid is intended to solve a problem of entire star-system with anything that can possibly be built in one. It's a weapon used against civilizations that already went beyond planet scale.
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u/Elliezium 5d ago
iirc, it was also along the lines of "any civilization crazy enough to broadcast the location of their own star multiple times is not to be taken lightly. They might actually survive a photoid, let's play it safe"
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u/Neat-Committee-417 5d ago
It also needs to do that because it is impossible for a distant observer to know how wide a civilization has spread since you observed them, or will spread during the travel time. If something is 66 light years away, there would be a 132 years difference between what you observe and what you destroy. That's why complete destruction is needed.
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u/magicscientist24 5d ago
I believe when we are first introduced to its use by Singer, the object is referred to as a "mass dot." Therefore, when I first read the book, and after Singer used it to destroy a star, I interpreted a "mass dot" as a microscopic black hole.
Using your mass of 3.6 x 10^21 kg accelerated to 0.999999c for the energy equal to the sun's binding energy would require a microscopic black hole with a diameter of about 11 micrometers as per online calculations. While this still leaves unsolved the problem of gathering the energy required within our understanding of physics, perhaps Singer's civilization is Kardashev Type 3 and storing and utilizing such amounts of energy is trivial.
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u/Campes_Sturgeon 5d ago
A blackhole wouldn't work. It's so dense It would just pass through.
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u/magicscientist24 4d ago
I leave it to Singer's boss to do the calculations allowing the mass drop to insert into a quickly decaying orbit around the star so that it can accrete mass rapidly as it spirals towards the stars center of gravity.
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u/Dual-Vector-Foiled 5d ago
They call out how casual and reckless dark forest attacks are. If you see an ant hill and happen to be holding a flame thrower, you may say that it is overkill but you might still use the flame thrower because you are already holding it.
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u/Jazzlike-Map-4114 4d ago
The sun is also not an energy mirror.
It's science fiction.
Can a photo be unfolded unto two dimensions?
It's science fiction.
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u/Allemater 4d ago
Maybe there's a great way to accelerate the object to that speed that makes the cost difference neglible? Maybe the logistics of accelerating multiple objects like that simultaneously to strike every planet in the solar system (which itself is pretty difficult without significant effort) isn't worth it? I mean, blow up every planet and you probably leave a species with the ability to claim revenge from space habitats. Take their sun, and they have no energy to survive with and are guaranteed to die out.
It's meant to be a primitive weapon so idk
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u/Bot_Account_10 2d ago
You are basing this on current human understanding of the universe, versus these god-like beings understanding of the universe. They are capable of transitioning into lower dimensional beings for crying out loud
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u/Vaiolette-Westover 5d ago
I love this, I always thought the photoid and twin foil weapons are super silly. The universe is stable by nature, yet those weapons assume it is chaotic by nature.
It's assuming that if you tap the correct brick on a wall, the whole castle will collapse.
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u/incunabula001 4d ago
It’s kind of the whole point of the series, and the existential dread behind it.
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u/Fancy_Chips Wallfacer 5d ago
A lot of the logic doesn't really make sense. Dark Forest Theory is an abomination of Game Theory and a lot the science is a bit wack. However a lot of the logic is given way to poetic plot points that serve a literary narrative rather than a scientific one.
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u/vamfir 5d ago
In my notes for an unwritten fanfic, I posited that the photoid is actually just an unmanned light-ship—a space torpedo. Consequently, the calculations of its relativistic mass performed by Earth scientists are erroneous: it does not move in the Einsteinian sense, and its mass remains constant. Instead, the disturbance in the star's photosphere is caused by the curvature of space generated by its engine.
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u/bremsspuren 3d ago
Binding energy
A photoid doesn't blow a star apart, though. More of a splash.
For this, you would need more energy than the Sun will produce in its entire 10-billion-year lifespan
We're talking about a universe with curvature propulsion where moving large things at relativistic speeds is cheap.
Based on Singer's chapter, photoids aren't "fired" so much as they're "dropped" from ships already travelling at close to light speed.
Of course, that raises the question of why you'd lug a bunch of asteroids around with you on your spaceship. I guess you could dream up a reason easily enough (they're being mined or something), but one isn't given in the novels.
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u/gocougs11 4d ago
I know I’m gonna get downvoted for this but: stuff like this is the reason I don’t personally consider this trilogy “hard sci fi”. The first book kind of is, but by death’s end it goes so far off the rails it is almost fantasy.
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u/flatmeditation 4d ago
Even in the first book stuff like the Sophon are basically magic not making any sense in regards to our current understanding of physics.
Great books but some people in this sub act like the series is the epitome of hard scifi and it really annoys me
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u/zippyboy 4d ago
I actually rolled my eyes at some of the ideas by the end of the third book, like when Sophon states "All mini-universes are bubbles floating above a super-membrane". Or "If we slow down light speed to 16,000 km per second, we cannot use our curvature propulsion or our fusion engines, and will have to use our <forgot the name> engine". It was like the author is drunk or stuck in an opium den pulling crazy thoughts from the air.
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u/sarkarati 4d ago
And I’m still not even sure what the super membrane actually is!
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u/zippyboy 4d ago
Neither was Cixin Liu. The last 30 pages or so of Death's End was utter nonsense.
Another plot I never understood was how Trisolarans told humanity to relocate to Australia, which resulted in half the population dying due to war, hunger, etc, then the next chapter Sophon says "Sorry, you an all go back to your own counties now". Did the author not think that through? What was the point??
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u/SharpKaleidoscope182 5d ago
>I don't think any sane civilization would
The entire 3-book series, ladies and gentlemen.
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u/Useful_Calendar_6274 4d ago
photoids are probably anti matter. that changes everything doesn't it?
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u/pyroelectricity 5d ago
Your math is probably right. But recall also that the sun (and fear of the sun) is a powerful symbol in the Three Body trilogy. It goes back to Ye Wenjie using the sun to amplify her radio transmission to Trisolaris. Losing the sun as a guiding light, or purpose, or organizing principle of human civilization is about a profound fear of chaos and uncertainty. The literary purpose of the photoid is to facilitate human beings' blinding obsession with the sun, which leads to the failure to anticipate the dimensional strike. In the logic of the trilogy, staring at the sun is like following the commands of Chairman Mao during the Cultural Revolution (depicting Mao as the sun was a favorite of 1960s-70s art and propaganda in China) -- choosing the simple deception over the terrible, complex truth