Reversible Turing Machine: Difference between revisions
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A '''Reversible Turing Machine''' (RTM) is a [[Turing machine]] for which the computation can always be run backwards from any step back to the previous configuration (and so forth all the way to the start of the computation). This property (called logical reversibility) has theoretical implications for the limits of computation. Specifically, non-reversible computation cannot scale beyond some limit due to the inherent energy cost whereas reversible computations may be able to | A '''Reversible Turing Machine''' (RTM) is a [[Turing machine]] for which the computation can always be run backwards from any step back to the previous configuration (and so forth all the way to the start of the computation). This property (called logical reversibility) has theoretical implications for the limits of computation. Specifically, non-reversible computation cannot scale beyond some limit due to the inherent energy cost whereas reversible computations may be able to. | ||
== History == | == History == | ||
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Bruce Smith called this "microscopic reversibility"<ref>https://scottaaronson.blog/?p=4916#comment-1851339</ref> | Bruce Smith called this "microscopic reversibility"<ref>https://scottaaronson.blog/?p=4916#comment-1851339</ref> | ||
Furthermore, let BB<sub>rev</sub>(n,m) be the maximum step function restricted to only reversible Turing machines. In other words, BB<sub>rev</sub>(n,m) is the maximum runtime across all halting n-state, m-symbol TMs. | |||
== Busy Beaver Champions == | == Busy Beaver Champions == | ||
{| class="wikitable" | {| class="wikitable" | ||
|+ | |+ | ||
!Domain | !Domain | ||
!Max Steps | !Max Steps (BB<sub>rev</sub>) | ||
!TNF Size | !TNF Size | ||
!Champion | !Champion | ||
Latest revision as of 15:37, 21 September 2025
A Reversible Turing Machine (RTM) is a Turing machine for which the computation can always be run backwards from any step back to the previous configuration (and so forth all the way to the start of the computation). This property (called logical reversibility) has theoretical implications for the limits of computation. Specifically, non-reversible computation cannot scale beyond some limit due to the inherent energy cost whereas reversible computations may be able to.
History
Charles Bennett described Reversible Turing Machines in a 1973 paper in which he proves that any standard TM can be simulated by a 3-tape quadruple RTM.[1] He states that they can also be simulated by a 1-tape quadruple RTM, but with quadratic slowdown. It seems likely that a standard TM can also be simulated by a 1-tape quintuple RTM (the type considered in the rest of this article), however, that was not explicitly discussed in Bennett's paper.
Definition
For 1-tape quintuple TMs, it is reversible if and only if:
For all states, all transitions to that state:
- Must move in the same direction
- Must write different symbols
Bruce Smith called this "microscopic reversibility"[2]
Furthermore, let BBrev(n,m) be the maximum step function restricted to only reversible Turing machines. In other words, BBrev(n,m) is the maximum runtime across all halting n-state, m-symbol TMs.
Busy Beaver Champions
| Domain | Max Steps (BBrev) | TNF Size | Champion | Reference |
|---|---|---|---|---|
| BB(2) | 6 | 21 | 0RB1RZ_1LA1RB (bbch)
|
Shawn Ligocki on Discord |
| BB(3) | 17 | 356 | 0RB1RZ_0LC1RA_1RB1LC (bbch)
|
Shawn Ligocki on Discord |
| BB(4) | 48 | 9,853 | 1RB0LD_0LC0RB_1LA1LD_1LC1RZ (bbch)
|
Matthew House and Shawn Ligocki on Discord |
| BB(5) | 388 | 359,852 | 1RB0RD_1RC0RB_1RD1RZ_1LE1LA_0LE0LA (bbch)
|
Shawn Ligocki and Matthew House on Discord |
| BB(6) | ≥537,556 | ≥9,931,603 | 1RB1LD_1LC1RE_0LD0LC_0RE0RF_0RA1RZ_1RF1RA (bbch)
|
Shawn Ligocki on Discord |
| BB(7) | ≥542,487,066 | 1RB1LD_0LC0LD_1LC1LA_0LA1RE_0RF0RE_0RG1RF_0RB1RZ (bbch)
|
(Early result from enumeration) |
See Also
- Discussion on Discord on 1 July 2025: https://discord.com/channels/960643023006490684/1243312334907375676/1389756466482647142
References
- ↑ C. H. Bennett, "Logical reversibility of computation", IBM Journal of Research and Development, vol. 17, no. 6, pp. 525–532, 1973
- ↑ https://scottaaronson.blog/?p=4916#comment-1851339