Thanks, that was actually what EY said in his quote, which I put just below my model - that we should change the bit each time. I somehow missed it ("send back a '0' if a '1' is recorded as having been received, or vice versa—unless some goal state is achieved").

As I stated in the epistemic status, this article is just a preliminary write-up. I hope more knowledgeable people will write much better models of x-risks from time machines and will be able to point out where avturchin was wrong and explain what the real situation is.

I am going to post about biouploading soon – where the uploading is happened into (or via) a distributed net of my own biological neurons. This combines good things about uploading – immortality, ability to be copied, easy to repair, and good things about being biological human – preserving infinite complexity, exact sameness of a person, guarantee that the bioupload will have human qualia and any other important hidden things which we can miss.  

Thanks! Fantastic read. It occurred to me that sending code or AI back in time, rather than a person, is more likely since sending data to the past could be done serially and probably requires less energy than sending a physical body.

Some loops could be organized by sending a short list of instructions to the past to an appropriate actor – whether human or AI.

Additionally, some loops might not require sending any data at all: Roko's Basilisk is an example of such acausal data transmission to the past. Could there be an outer loop for Roko's Basilisk? For example, a precommitment not to be acausally blackmailed.

Also (though I'm not certain about this), loops like you described require that the non-cancellation principle is false – meaning that events which have happened can be turned into non-existence. To prevent this, we would need to travel to the past and compensate for any undesirable changes, thus creating loops. This assumption motivated the character in Timecrimes to try to recreate all events exactly as they happened.

However, if the non-cancellation principle is false, we face a much more serious risk than nested loops (which are annoying, but most people would live normal lives, especially those who aren't looped and would continue through loops unaffected). The risk is that a one-time time machine could send a small probe into the remote past and prevent humanity from appearing at all.

We can also hypothesize that an explosion of nested loops and time machines might be initiated by aliens somewhere in the multiverse – perhaps in the remote future or another galaxy. Moreover, what we observe as UAPs might be absurd artifacts of this time machine explosion.

The main claim of the article does not depend on the exact mechanism of time travel, which I have chosen not to discuss in detail. The claim is that we should devote some thought to possible existential risks related to time travel.

The argument about presentism is that the past does not ontologically exist, so "travel" into it is impossible. Even if one travels to what appears to be the past, it would not have any causal effects along the timeline.

I was referring to something like eternal return—where all of existence happens again and again, but without new memories being formed. The only effect of such a loop is anthropic—it has a higher measure than a non-looped timeline. This implies that we are more likely to exist in such a loop and in a universe where this is possible.

I would add that there are a series of planetary system-wide risks that appear only for civilizations traveling within their solar systems but do not affect other solar systems. These include artificial giant planet explosions via initiating nuclear fusion in their helium and lithium deposits, destabilization of the Oort cloud, and the use of asteroids as weapons.

More generally speaking, any spacecraft is a potential weapon, and the higher its speed, the more dangerous it becomes. Near light-speed starships are perfect weapons. Even a small piece of matter traveling at very high velocities (not necessarily light speed, but above 100 km per second, as I recall) will induce large nuclear reactions at the impact site. Such nuclear reactions may not produce much additional energy but will cause significant radioactive contamination. They could destroy planets and any large structures.

Additionally, space colonization will likely develop alongside weapon miniaturization, which could ultimately result in space-based grey goo with some level of intelligence. Stanisław Lem's last novel, "Fiasco," seems to address this concept.

"Explain as gwern ELI5"

This means that straightforward comparison of flops-per-USD between home computer GPU cards and data center flops-per-USD is incorrect. If someone already has a GPU card, they already have a computer and house where this computer stays "for free." But if someone needs to scale, they have to pay for housing and mainframes.

Such comparisons of old 2010s GPUs with more modern ones are used to show the slow rate of hardware advances, but they don't take into account the hidden costs of owning older GPUs.