So you’ve heard about this “quantum entanglement” stuff and how Einstein was apparently worried it might break the speed of light.
It doesn’t and he wasn’t.
Here is the simple version: Take a piece of paper. Rip it apart once. Check the pieces that you got. Maybe scribble something on it. Mix the two pieces behind your back. Give one of them to a friend without looking. Send the friend to the end of the universe. If he refuses, find a real friend. Wait a few billion years. Open your hand. In that instant, no matter how far away your friend is, you will know what piece he has in his hands.
The complicated version: Quantum is weird but some stuff is actually easy to understand. There are just a few ways to create entangled particles. All of them have something in common: The pieces must add up exactly to what you put in. This isn’t magic or some badly written Star *beep* episode. Imagine you put in a piece of paper (or a photon – a blip of light). You can’t have more paper (or more light) after splitting it. In the case of the photon: If you add energy, you get more light but no entanglement.
So the entangled photon pairs are always half of the original in terms of energy (which roughly translates to “half as bright”). And they always go in exactly opposite directions (see conservation of momentum). Things like that. Which means you know everything about the two particles except one thing: You don’t know which is which unless you look.
If you keep one of them around and sent the other away, and then at some later point look at what you kept, you know exactly and instantly what the other must look like, no matter how far away it is now.
But you can’t change the far particle anymore. Same as you can’t add text to the paper which your friend at the edge of the universe is holding. So you can’t use this to beam information.
Sorry.
Be grateful for having friends like that. A pity that you sent him away.
Posted by digulla 
Dark Views 
Dark Forest is a Fairy Tale
16. December, 2021The Dark Forest, an idea developed by Liu Cixin for his Remembrance of Earth’s Past series (also known for the first book “The Three-Body Problem” is just a fairy tale: Interesting to think about, there is a morale but it’s not based on reality.
Proof: We are still here.
The Dark Forest fairy tale is a solution to the Fermi paradox: If there are billions of planets like earth out there, where is everyone? The Dark Forest claims that every civilization that is foolish enough to expose itself gets gets wiped out.
Fact: We have exposed ourselves for millions of years now. Out planet has sent signals “lots of biological life here” for about 500 million years to anyone who cares.
Assuming that the Milky Way has a size of 100’000 light years, this means every civilization out there know about Earth for at least 499.9 million years. If they were out to kill us, we would be long gone by now. Why wait until we can send rockets to space if they are so afraid of any competition?
How would they know about us? We can already detect planets in other star systems (the current count at the writing of this article is 4604, see http://www.openexoplanetcatalogue.com/). In a few years, we’ll be able to tell all the planets close to us which can carry life, for values of close around 100 light years. A decade later, I expect that to work for any star system 1’000 light years around us. In a 100 years, I’ll expect scientists to come up with a trick to scan every star in our galaxy. An easy (if slow) way would be to send probes up and down out of the disk to get a better overview. Conclusion: We know a way to see every star system in the galaxy today. It’s only going to get better.
Some people worry that the technical signals we send could trigger an attack but those signals get lost in the background noise fairly quickly (much less that 100 light years). This is not the case for the most prominent signal: The amount of oxygen in Earth’s atmosphere. If you’re close to the plane of the ecliptic (i.e. when you look at the sun, the Earth will pass between you and the sun), you can see the Oxygen line in the star’s spectrum for thousands of light years. Everyone else has to wait until Earth moves in front of some background object.
There is no useful way to hide this signal. We could burn the oxygen, making Earth inhospitable. Or we could cover the planet with a rock layer; also not great unless you can live from a rock and salt water diet.
For an economical argument: When Ruanda invaded the Democratic Republic of Congo to get control of Coltan mining, they made roughtly $240 million/yr from selling the ore. China makes that much money by selling smart phones and electronics to other states every day (source: Home Deus by Yuval Harari). My take: killing other civilizations is a form of economical suicide.
Conclusion: The Dark Forest is an interesting thought experiment. As a solution for the Fermi paradox, I find it implausible.
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