4 ms·
I learned what Dyson sphere is as by product of reading this by Bruce Scheiner, really great read: " One of the consequences of the second law of thermodyna
by lossolo 10y ago
I learned what Dyson sphere is as by product of reading this by Bruce Scheiner, really great read:
" One of the consequences of the second law of thermodynamics is that a certain amount of energy is necessary to represent information. To record a single bit by changing the state of a system requires an amount of energy no less than kT, where T is the absolute temperature of the system and k is the Boltzman constant. (Stick with me; the physics lesson is almost over.)
Given that k = 1.38×10-16 erg/°Kelvin, and that the ambient temperature of the universe is 3.2°Kelvin, an ideal computer running at 3.2°K would consume 4.4×10-16 ergs every time it set or cleared a bit. To run a computer any colder than the cosmic background radiation would require extra energy to run a heat pump.
Now, the annual energy output of our sun is about 1.21×1041 ergs. This is enough to power about 2.7×1056 single bit changes on our ideal computer; enough state changes to put a 187-bit counter through all its values. If we built a Dyson sphere around the sun and captured all its energy for 32 years, without any loss, we could power a computer to count up to 2192. Of course, it wouldn’t have the energy left over to perform any useful calculations with this counter.
But that’s just one star, and a measly one at that. A typical supernova releases something like 1051 ergs. (About a hundred times as much energy would be released in the form of neutrinos, but let them go for now.) If all of this energy could be channeled into a single orgy of computation, a 219-bit counter could be cycled through all of its states.
These numbers have nothing to do with the technology of the devices; they are the maximums that thermodynamics will allow. And they strongly imply that brute-force attacks against 256-bit keys will be unfeasible until computers are built from something other than matter and occupy something other than space."
- Franciscouzo 10y agoHow would counting with Gray code [1] change the math? [1] https://en.wikipedia.org/wiki/Gray_code https://en.wikipedia.org/wiki/Gray_code
- tromp 10y agoIt changes things by a factor of 2, since a Gray code increment flips exactly one bit, while a regular binary increment flips 2 bits on average. That's ignoring all the logic required to figure out exactly which bits to flip...
- _sigma 10y agoLooks like some ^ were eaten in that. This has the same quote, makes much more sense. http://security.stackexchange.com/a/25392 http://security.stackexchange.com/a/25392
- dkbrk 10y agoYour linked version also doesn't mess up the units: 3.2°Kelvin is incorrect, it's 3.2 K. Kelvin is never used with the degree prefix. I could understand that in copying the quote one might have not noticed the superscripts were messed up, except it is evident deliberate alterations were made. Very strange.
- dpark 10y ago> Your linked version also doesn't mess up the units: 3.2°Kelvin is incorrect, it's 3.2 K. Kelvin is never used with the degree prefix. This is pointless pedantry. Yes, the official unit is just "Kelvin" now. No, it's not impressive to recognize this misuse. This is little better than pointing out that someone wrote "too" instead of "two". Historically, it was also called degrees Kelvin, so it's not as if there's a lack of precedent for this, even if it's no longer the preferred usage. > I could understand that in copying the quote one might have not noticed the superscripts were messed up, except it is evident deliberate alterations were made. Very strange. It's not evident at all. I don't know why you would assume that some random StackExchange answer is the definitive version. Schneier's original version uses °K, per Scheier's blog: https://www.schneier.com/blog/archives/2009/09/the_doghouse_cr.html https://www.schneier.com/blog/archives/2009/09/the_doghouse_... As does the original answer on StackExchange, actually (look at the revision 4 diff): http://security.stackexchange.com/posts/25392/revisions http://security.stackexchange.com/posts/25392/revisions
- eps 10y ago> No, it's not impressive to recognize this misuse. Actually, it is. It's a geek pedantry, the best kind there is :)
- mcv 10y ago
- zump 10y agoHuh. I don't get it. A star powering a 219-bit counter? What?
- detaro 10y agoThe argument is that if even just counting through all values of a perfectly energy-efficient 192-bit counter requires the power of an entire star for many years, it's obvious that you'll never have to worry about someone brute-forcing through all keys of something encrypted with 192 bit strength (or better). And even tapping a supernova only gets you 219 bits.
- kilovoltaire 10y agoTo cycle through all 2^219 possible values in a 219-bit counter would take a huge amount of energy.
- rossy 10y agoIt's about the feasibility of breaking 256-bit encryption keys with a naive brute force attack. The simplest component of a computer that performs this attack would be a 256-bit counter that iterates through all possible key values, however Schneier is saying that according to our current understanding of physics, even if you built the most efficient computer possible and powered it with the energy output of a supernova, it would only have enough energy to iterate through all the values of a 219-bit counter.
- ams6110 10y agoOn average you only need to iterate half the values.
- AngrySkillzz 10y agoYeah, that's still 255 bits worth.
- deleted 10y ago[deleted]
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- smegel 10y agoThe connection between elementary physics and "information" is unproven pop-sci that desperate physicists have come up with in order to stay cool and relevant, in the absence of any real breakthroughs in physics for close to a century. Throw it in the same rotten bag as string theory and super-symmetry.
- alanbernstein 10y agohttps://en.wikipedia.org/wiki/Limits_to_computation#Physical_limits https://en.wikipedia.org/wiki/Limits_to_computation#Physical... Are you claiming that everything here is bullshit? Wanna back that up?
- JonnieCache 10y agoI think thermodynamics is pretty solid bro.
- tzs 10y agoThat calculation is for irreversible computing. Build your computer out of reversible gates and you can compute using much lower energy (although I believe it will be slower, and need more bits).