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Is the invention of a faster computer (let's say a quantum computer) or the lack of advancements in computing power (let's say it takes a decade to make any progress greater than incremental increases on currently technology) capable of affecting the amount of time it takes to halve the block reward?

In other words, does the block reward get halved every 4 years, regardless of how much the hardware available is or isn't progressing?

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The block halving takes place every 210,000 blocks. The difficulty retargeting mechanism makes it so that 210,000 blocks take approximately 4 years, but this is not exact. If the hashrate is rapidly growing, it will take a little less than that.

For example, if the total network hashrate doubles every year, it will take about 1 month less. If it doubles every week (increases by a factor of 4.5 quadrillion every year), it will take only 2 years.

  • so the answer is yes, it will get halved approximately every four years, regardless of hardware advancements or lack thereof. do i have this right? – Gaia Oct 27 '13 at 5:16
  • @Gaia: Correct. – Meni Rosenfeld Oct 27 '13 at 5:28
  • hmm, just saw your edit: "if it doubles every week, it will take only 4 years". So the 4yrs can be affected only down, not up? – Gaia Oct 27 '13 at 5:35
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    @Gaia: 2 years, not 4. Anyway, if hashrate is going up it will take less time, if hashrate is going down it will take more time. – Meni Rosenfeld Oct 27 '13 at 7:52
  • Yes, I meant 2 not 4 yrs. So is the answer now "no, the halving period does vary according to advancements (or lack thereof) in hardware" ? – Gaia Oct 27 '13 at 16:46
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The short answer is yes, provided the difficulty factor feedback loop (adjusted after 2016 (14*24*6) block awards which nominally maps to every two weeks for whatever next generation hardware gets used) remains stable, which it should from basic control theory 101.

The reason has everything to do with mathematics and the convergence of the geometric series to limit the number of Bitcoins to 21M.

Go to http://www.basic-mathematics.com/geometric-sequence-calculator.html. Enter 10.5M (number of Bitcoins mined during the first four years 50*4*365*24*6) into the first field, 0.5 into the second field (number of Bitcoins awarded gets scaled back every 4 years), 8 into the third field for n (roughly the number of 4 year time constants between 2009 & 2040), and select Sn and hit calculate. As n approaches infinity, the geometric series converges to 21M.

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