The Doctor in Star Trek: Voyager is a hologram. Voyager's Sick Bay is loaded with holo-projectors.

The episode "Living Witness" shows that the Doctor regularly makes backup copies of himself. After he gets his Mobile Emitter he is able to transfer his program completely to the device.

In a medical emergency with multiple patients, why doesn't the Doctor ever duplicate himself?

If it's a matter of the ship not having enough processing power, why not transfer his program to the Mobile Emitter and then launch his backup copy from the ship's emitters in Sickbay?

In the episode "Concerning Flight", the ship's main computer core is stolen. This cripples weapons, navigation, transporters, and propulsion yet the Doctor continues on in Sickbay unaware as if nothing has happened.

This suggests that the Doctor requires less processing power to sustain his program than the weapon targeting systems, which makes answers suggesting limited processing power seem less likely to be correct.

  • Probably the same reason people can't be cloned by the transporter (except Riker and Evil Riker). There would be no peril for people and holographic doctors who could simply be replicated if lost.
    – Chad Levy
    Commented Nov 19, 2011 at 4:24
  • 8
    In the episode Lifesigns the Doctor created a holographic body for a dying Vidiian while also loading the memories and synaptic patterns for them into the computer. This would seem to negate any supposed technological limitations in either the computer memory or the holo-emitters in-so far as not being able to run multiple "artificial" intelligences or complex holo-matrixes (yes, in this case I am considering any biological intelligence copied into a computer to be artifical).
    – Xantec
    Commented Nov 19, 2011 at 12:53
  • 4
    My question would be: If you copied the Doctor, would you then have two Doctors who could potentially become two different "people"? If so, killing the copy would be immoral.
    – Jeremy
    Commented Nov 26, 2011 at 4:05
  • 1
    EXEs can't run from a ZIP file. Just because there are backups doesn't mean those backups can be executed. Commented May 4, 2016 at 15:02
  • 1
    @JeremyHeiler: You can bypass the morality by simply having all instances of the Doctor access and store their experiences in the same repository. Then, since all running instances of the Doctor will essentially have the same experiences and moralities, you can bypass the moral dilemma without concern. In fact, I'd leave it up to the Doctor's primary program to decide which copies to deactivate, so now the choice is entirely up to the Doctor.
    – Ellesedil
    Commented May 19, 2016 at 17:32

15 Answers 15


In "Basics: Part 1" when Voyager fights multiple Kazon factions, they created multiple holographic fighter ships to distribute the shots from the Kazon ships so that Voyager came under less fire. When they first initialise these holographic ships, the Doctor appears in space stating:

TORRES [OC]: Doctor, are you there? Are you all right?

EMH: I told you we should have run one last systems check.

Indicating that these holographic ships run under the same computer program as the Doctor. This proves that it is, in fact, possible for there to be multiple copies of a hologram running simultaneously. I assume that this never happens in the show for the same reason that transporters cannot create copies, or replicators cannot be used for cloning, yet they can be used to create organs.

There are tons of contradictions like this in the Star Trek series. Stuff like that is inevitable with science fiction, although Star Trek does a very good job at being scientifically accurate as often as possible (where the narrative allows anyways). In one episode of Star Trek (TNG I believe) they used a transporter to get rid of a virus in one of the crew's body using a backup from the transporter logs. If this was truly possible, don't you think they would have done this more often? Such as healing injured crew members, or even bringing them back from the dead?


Despite Living Witness, Voyager does not have enough memory to make a backup copy of the Doctor's program (see the note at the bottom of this Wikipedia article). If he were to create two Doctors using the mobile emitter and Voyager's own memory then both copies would be stuck where they were, since there would be nowhere that either of them could be transferred.

  • 16
    Considering that Voyager has nearly the complete Federation Database with it, plus multiple holographic programs with many characters (though none of them so complex, but the doctor is not that complex either, at least not in the beginning, he's just backed by a vast medical database) I consider this a excuse. Maybe in-universe correct, but a very unbelievable excuse.
    – Bobby
    Commented Oct 14, 2012 at 10:42
  • 2
    What's the opposite of Deus Ex Machina? Cause it seems like this is it ;) Commented Aug 11, 2015 at 16:02
  • "Stuck in the machine"
    – Kalamane
    Commented Nov 2, 2015 at 18:59
  • 3
    @WayneWerner: Excusatio ex stercore bovis? Commented May 19, 2016 at 14:39
  • 3
    Tom: Captain, there isn't enough memory to have two doctors! Janeway: Harry replicate & install enough computer memory for another doctor instance. Harry: Captain, the bandwith considerations, the paralellisations, it's just impossible! Janeway: (raised eyebrow™) Really? We're in a state of the art space ship and that's your best excuse? Mr Tuvok relieve Mr Kim of his command and get another doctor up and running? Commented May 26, 2017 at 16:15

I thought that it was Digital Restriction Management at work. It's probably much more pervasive in Voyager's time than even today and it wouldn't surprise me if even medical units and the military/Starfleet would not be exempt from this.

  • 2
    Can you offer any evidence of this or was this just intended to be a funny answer?
    – Valorum
    Commented Nov 6, 2018 at 18:19

According to Star Trek technical manuals I've found, and other resources from various places, the Sick Bay systems are completely separate physically from the rest of the ship. It''s got its own power supply, it's got its own replicators, it's even got it's own Emergency Power. It is for all intents and purposes a complete separate system. The only reason the Doctor is at all aware of anything that goes on on the ship is because in Episode 01x02, Janeway has the Doctor's program linked to all logs for everything that happens ship wide. That way he can be made aware of any emergencies that could come up. So therefore the separate system is networked with the rest of the ship.

Also, Sick Bay's systems are mainly only capable of storing one maybe two EMH's at a time. Any other holograms are taken from the main computer's memory, or the holodeck's memory storage. Assuming the doctor needed a secondary back up, he could instruct the computer to transfer a copy of his program into the holodeck, and use It as a secondary Med bay. The only problem with that is that in emergency situations upon all starships, the first systems to be deactivated to reroute power is the Holodeck. Which is done by the computer. This can be seen in TNG Episode 3x06 when the power on the ship is waning and the computer shuts down the holodeck because of it was Standard Operating Procedure.

It must also be taken into consideration that the EMH by his own admission has the sum of almost all the Medical Knowlege known to Starfleet. In order to be able to serve a star ship in the event that something like him would be necessary, and considering the types of encounters most exploration ships have, this would be absolutely necessary.

The size of all of this information would be tremendous, not to mention the computer in Sick Bay most likely draws upon His programmed knowledge when asked a question by the standard medical officer who would normally be there. It would be a waste of space to have both his knowledge, and a copy of this knowledge in the system at the same time.

And since Sick Bay is as stated before a separate system designed to be able to sustain itself in case of a ship wide power failure, or computer failure, we must assume that the amount of space within the computer must be finite.

  • 5
    The only problem with that is that in emergency situations upon all starships, the first systems to be deactivated to reroute power is the Holodeck. - Not in this case, the power systems on Voyager's holodeck were incompatible with the rest of the ship.
    – Izkata
    Commented Oct 13, 2012 at 20:25
  • This could be the best answer. Can you provide some citations? Commented Jun 29, 2015 at 14:37

The Doctors program is "transferred" - cut and pasted - into various places throughout the series, and never copy and pasted. This is, IMO, a mistaken approach, but at least it is consistent. It would suggest that copy an pasting the Doctor would not work, or it would have been done in many situations.

I understand that his memories would not be consistent if he made copies, but they certainly do some re-modelling of his memory on occasions. And I am not sure why missing a few memories is such an issue - one version would have some memories, isn't that enough?

  • This also, in my mind, raises the issue of why not using the transporter in a similar fashion. Need more engineers? Store La Forge's pattern and copy away. Need a bunch of redshirts for a suicide attack? Find the best and "clone" away.
    – erdiede
    Commented Nov 19, 2011 at 22:31
  • @erdiede See DS9 4x10 Our Man Bashir for why that would not work for humans: So much memory is required to store a person's neural patterns, an extremely volatile type of memory is used.
    – Izkata
    Commented Nov 22, 2011 at 4:20
  • @Izkata Still not sure that the real answer is that the writers don't know what RAID 1 is.
    – erdiede
    Commented Nov 22, 2011 at 5:44
  • In Living Witness (VOY, Episode 4x23) there is a back-up version of the doctor.
    – liftarn
    Commented Jun 30, 2014 at 13:42

Take your pick:

  • Voyager runs on limited resources. The holographic doctor program is supposedly a very big program, very demanding in memory (and presumably processing power too).
  • Holograms are usually confined to the holodeck. Having holo-emiters in sickbay was novel technology, might not support more than a single hologram (although this has been contradicted later on. Neelix had holographic lungs for a while in an episode (but even then it was "straining the resources").
  • The writers just didn't think that far ahead of the curve.
  • 3
    Don’t forget about the hologram of Denara Pel, which co-existed in sickbay with the doctor.
    – Timwi
    Commented Dec 8, 2011 at 1:07
  • 4
    @Timwi Also supporting your comment, in Voyager 5x8 Nothing Human A holographic copy of Crell Moset exists with the Doctor in sickbay as a consultant.
    – Kalamane
    Commented Jan 13, 2012 at 15:07
  • 3
    And now that I remember, Barclay showed up as a hologram to fix the Doctor, I think?
    – MPelletier
    Commented Jan 13, 2012 at 16:32
  • And Kim was tasked with creating a second EMH for a while (before Crell Moset, IIRC), so computer usage wasn't a constraint
    – Izkata
    Commented Oct 13, 2012 at 20:22
  • @Izkata IIRC at that time the EMH had been transferred off the ship. Maybe it was the episode in which they encountered the Hirogen sensor network?
    – user
    Commented Jan 9, 2013 at 9:05

The question becomes one of ethics.

Let's say the Doctor could duplicate himself to the mobile emitter, and one to the ship. When it becomes time to 'unduplicate' himself, which one do you suppose gets deleted?

If they retain both, the issue is the Doctor's experiences become fragmented. The mobile emitter version would gain knowledge from away missions, and the medical bay version would be contained there forever (also implicitly unethical), but whatever knowledge he gained cannot be shared easily with the one on the away mission, because which parts do you include? Which parts do you cut out? Maybe there were a series of mundane events of minor breakthroughs before he makes one big breakthrough?

This in turn burdens the systems of voyager. It has to maintain two EMHs (which have a history of going wrong), has to leave space for what is essentially duplicate knowledge, and has duplicate processing demands, in what is a power conservation situation where knowledge cannot be consistently shared.

Indeed, the Doctor hasn't been avoiding 'duplicating' himself, he just does it in a different way: he's been training Tom Paris as another medic to help out.

  • This is a solvable technical problem. You assume that the experiences from the various Doctor's could not be stored in a single repository. If every copy of the Doctor shares the same store of experiences, then there's no additional systems burden (and no moral dilemma for "killing" off copies). In fact, that's what plenty of software systems do today: take the data from multiple instances, and store it all in one place.
    – Ellesedil
    Commented May 19, 2016 at 17:29
  • Note that playing around with the Doctor's memories has been seen to cause problems (S05E11 - Latent Image). And, of course, the Captain has chosen to treat him as a human, rather than as equipment, on multiple occasions - he might have problems with the idea.
    – RDFozz
    Commented Nov 5, 2018 at 23:37

First for context - For some reason this is one thing that was always being changed. At first the doc was so complex they couldn't have a backup or replace him and so it was vital to keep him from being damaged, then they like magic make Crell Maset into nearly as complex of a program, then somehow they manage to make a backup of the doctor only its lost and finally when the doctor is stolen all the sudden they cant make a backup or a new doc let alone a Crell Moset like hologram to be a stopgap.

Second to answer - At least two times more then one hologram was active in sickbay the love interest of the doc and Crell Moset one was an actual humanoid working through the comp. And the other almost as complex as the doc, so it's possible for him to do so but the issue with if he is too complex or not was retconned and retconned so many times it's hard to say anymore.


It is quite common for a program to take up more memory when running than the program takes. As there are two types of memory as most people think of it hard disk space and RAM. Most browsers, for example, would take up maybe a couple of 100mb of disk space but can easily take more than a gig of RAM when running. It's not a perfect example as a browser can have multiple tabs windows open.

As someone who works in IT it seems to me that running a program as complex as the doctor would take up an enormous amount of RAM.

So the doctor's program might take up a relatively small amount of memory (even less if compressed but could be extremely resource intensive while running requiring a large amount of RAM.)

  • Thus, explaining how there could be a backup copy of the Doctor (as noted elsewhere, "Libing Witness"), but they couldn't run two copies of the Doctor at the same time.
    – RDFozz
    Commented Nov 5, 2018 at 23:25

At the beginning of the series, there was a limit in the resources available to the ship. Holograms are complex, the EMH especially so, and having a duplicate was probably too resource intensive when it was just as easy for him to supervise a number of trained assistants.

It wasn't until later on when they found out ways to be able to expand the doctors programming that they could figure out other resources for the buffers.

  • 1
    This really doesn't address the question.
    – Tango
    Commented Jan 13, 2012 at 7:40

It is never mentioned, but one problem with running multiple instances is concurrency: maybe Holodoc is not thread-safe. I.e., there are crucial data structures somewhere else that are shared between all EMHs, or something.

  • 1
    Speculative answers should be backed up with at least some sort of canon source.
    – Valorum
    Commented Apr 25, 2014 at 23:02

A key point not noticed so far: By Voyager's time, computing has long ago evolved into to quantum computing - i.e all major computer hardware exploits features of Quantum Mechanics - superposition, entanglement, etc - to perform computing tasks as effectively as possible (e.g quantum computers can perform searches vastly faster than classical computers because they can use superposition to inspect all search candidates simultaneously). Because of this, computing tasks - such as is needed to generate moving holograms - all run up against a fundamental limitation:

THE NO-CLONING THEOREM: Given a single particle in a specific quantum state, there is no physically possible process that can both leave that particle in its original state and produce a second particle in the exact same state.

The Doctor's hologram is a large assemblage of projected subatomic particles (and not just light, because it is 'solid') which are undoubtedly highly entangled, and so the entire hologram can be viewed as a large particle in a specific, very complicated, quantum state. Projecting a second instance of the Doctor would then require replicating that particle - which is not physically possible.

  • Plenty of the technical manuals talk about optical computers and faster-than-light acceleration. Do you actually have any quotable reference for your quantum computing assertion?
    – T.J.L.
    Commented May 4, 2016 at 13:54
  • Also, if this were true then the doctor would have been lost when the backup that was used in the Living Witness episode was created.
    – Kalamane
    Commented May 12, 2016 at 0:13

No other answer seems to mention this, so I'll add it.

Throughout most of the run, the Doctor is treated (as much as possible) the same as any other member of the crew. As critical as he is, he's allowed to continue to expand his program, even though it's been shown to be dangerous (e.g., S03E18, Darkling, where he develops a split personality).

In addition to singing and holophotography, he's allowed to:

  • Create a holographic family (S03E22, Real Life)
  • Daydream (S06E04, Tinker Tenor Doctor Spy)
  • Participate in away missions (note that his holo-emitter is 29th century technology, and as far as we know cannot be replicated, so he risks both that technology and his program in these cases).

He's even granted permission to remain behind on a planet, leaving Voyager without his services (S06E13, Virtuoso), and to travel to the Alpha Quadrant, using a communication link that may not be entirely reliable, risking the permanent loss of his services, to treat his creator (S06E24, Life Line)

We've been reminded, on occasion, that he is a program, such as when the crew of the Equinox removed his ethical subroutines, and he was perfectly happy to experiment on Seven of Nine, even if it would permanently injure her (S05E26 - S06E01, Equinox).

We've also seen that monkeying around with his memories can cause problems (S05E11, Latent Image, where a patient died because of a triaging choice, but he blamed himself and became unable to function). And, the ultimate solution, in that story? Treat him like a biological crewmember, not a program; help him work through the issue until he can get back to work, where he'll probably be an even better doctor for the experience.

So, when he's in the Alpha Quadrant for a month, he's not replaced by a backup. As others have noted, that would leave them in the position of having two programs with unique experiences, and with potential difficulty in merging the programs together again. And, since he has been accepted as a member of the crew, it would be unthinkable to simply delete one copy.

Would a backup copy (assuming they exist, as indicated by "Living Witness") of the Doctor have been used if he had actually left the ship in "Virtuoso"? We simply don't know. He decided not to leave before such an eventuality would have been necessary. I will note that, apparently, he would have taken the one-of-a-kind mobile emitter with him if he left.

TL;DR - The fact that the Doctor is accepted and treated as a member of the crew, not merely a holoprogram, may ethically limit the uses for his program as much as it's limited by the technology on board.


It is my opinion that this would be quite possible to copy the doctor and re-integrate his experiences after running double or even more instances, he is just a program with a database, but it would be too problematic to film the same actor in the same shot repeatedly and because of that they didn't do it.

You have to remember there are hundreds of EMH mark 1 running around scrubbing plasma conduits after the program was replaced by Mk II, many in the same facility. For example, in "Author, Author".

Granted they do not share the same database, but they are running multiple instances of the same program. (i.e the first day the Mk I's were started at the facility they were exactly the same, diverging from that point on).

  • Again, I note that the Voyager's EMH had expanded his program over the years; I don't know by how much, but we're told in Life Line that a small subset of his "extra" routines take up at least 12 "megaquads".
    – RDFozz
    Commented Nov 6, 2018 at 18:22
  • And, of course, that the facility mentioned isn't Voyager. That facility's ability to run multiple former EMH programs (possibly without the presumably huge medical database that still forms part of the Voyager's EMH's program.
    – RDFozz
    Commented Nov 6, 2018 at 18:29
  • people seem very preoccupied by the "huge medical database" taking up too much space... the entire collective size of every book written by every human throughout human existence if compiled into an efficient database would barely reach petabyte size. In Star Trek times you could probably fit a petabyte on a thumb drive.
    – Cyber Trec
    Commented Nov 6, 2018 at 18:34
  • I'm pretty sure that the people mentioning that are taking their cue from Voyager itself. Logically, your point is well taken, of course, it's just that the show has stated the medical DB is very large, in their terms.
    – RDFozz
    Commented Nov 6, 2018 at 20:43

It is just a story telling device. Since the doctor is a computer program youcan just copy him (hot or cold backup). If the RAM usage (as mentioned earlier) is a concern, you don't need to use RAM for a stored copy, only for a running version.

It is probably theoretically possible to use some sort of synchronisation to keep a sickbay and emitter version of the doctor in sync so that memories and experiences are copied over. Same would be to copy a version of the cooy as a data stream back to the alpha quadrant to treat dr Zimmerman.

The thing is, it makes it all a bit boring. If there is no risk of loss, there is not much of a story. So not being able to copy a holoprogram is imho just a plot device and not a pseudo-technical limitation.

As for copying people using the transporter: i am pretty sure there are moral/ethical reasons to not allow this. On a metaphysical level you could argue that you can copy the body and chemical processes, but not the soul of a person (if you believe in that kind of thing). But again... if you could revert to a backup of yourself as last stored in the teleporter, how boring would that make a scifi series? Bit like "go back to the last save point" in a game :)

  • 1
    Do you happen to have an in-universe answer?
    – Mithical
    Commented May 26, 2017 at 13:45

Not the answer you're looking for? Browse other questions tagged or ask your own question.