In the Star Trek universe how many Emergency Medical Hologram's can be online at any one time? For example, in Season 4 Episode 14 "Message in a Bottle" the Doctor engages the Prometheus EMH with no apparent problem, but in the same season in Episode 4 "Revulsion" when the Doctor and B'Elanna leave Voyager to meet another AI, the Doctor mentions that he is leaving Tom Paris as a nurse. Couldn't Voyager just switch on another EMH? It is, after all, just a programme.

(I mention Star Trek universe rather than just Voyager since the EMH turned up in the film First Contact.)

  • Basically a dupe of this; scifi.stackexchange.com/questions/128249/…
    – Valorum
    Commented Jan 3, 2020 at 0:30
  • 1
    Also for the record the Prometheus is a state-of-the-art ship with holoemitters in every room. I would assume that that means that the ship is capable of sustaining multiple programs.
    – Valorum
    Commented Jan 3, 2020 at 0:32
  • Also, in the two-parter "Equinox" you had the two EMHs interact on one ship, so it is doable. Commented Jan 3, 2020 at 4:07
  • Remind me - was "Message In A Bottle" before or after they invented the light-bee/mobile-transmitter?
    – komodosp
    Commented Jan 3, 2020 at 9:11
  • @colmde It must have been after since the Doctor can move freely from the Medical Quarters?
    – user66716
    Commented Jan 4, 2020 at 21:46

1 Answer 1


It would depend entirely on the computer that was running the programs

at the end of Voyager S7E20 "Author, Author" we see a mining colony where hundreds of EMH programs are active at once.

The Prometheus had two EMHs programs running because it had two EMH programs in its database, the Mark 2 it came equipped with and the Mark 1 that Voyager sent to it.

Copying an EMH is apparently very very difficult; why this is we can only guess but it's possible each EMH has a unique license code or equivalent but ultimately it kinda just comes down to bad writing.

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    It was clear that the show wanted to present the EMH as a character. So if the "bad writing" originated anywhere, it's during the show's initial premise of having a holographic character. Once the character grew, then the value of having an experienced crew member make almost unlimited copies of itself and to perform almost any function would be actual bad writing. So, restricting the EMH to avoid that makes the show more interesting. Consider Data in TNG, which a common question being "Why didn't Starfleet just build more Androids?" Because it'd be boring to tell that story.
    – Ellesedil
    Commented Jan 3, 2020 at 21:16

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