The scene is where the astronauts are talking to ground control about HAL's erratic behavior.

The astronauts are somewhere near Jupiter and ground control is on Earth so the minimum communication transit time is 35 minutes. If they were true to science, the communication would near to impossible, so did Kubrick cheat and use instantaneous communication?

  • 16
    The book makes it very clear there is a delay between messages.
    – Darren
    Sep 19, 2021 at 10:02
  • @Darren FtL, being completely fictional doesn't imply instantaneous. Sep 20, 2021 at 15:02
  • 2
    @AzorAhai-him- Sorry, I have no idea what your point is or how it's releveant to either the OP's question or my comment.
    – Darren
    Sep 21, 2021 at 10:32
  • 1
    @AzorAhai-him- have you either read the book or seen the film? They are about about as realistic as science fiction gets. There is no such thing as ftl comms or travel (by humans, at least). As I said, the book makes it VERY clear there is a significant delay in comms due to the speed of light nature of the communications medium.
    – Darren
    Sep 21, 2021 at 13:08
  • 1
    @AzorAhai-him- I didn't specifically mention the speed of light nature of the comms, no, but I pointed out the long delays which is what OP wasn't aware of. If you have seen/read the source material why are you even picking this fight about the possiblity of ftl comms when you know there isn't any?
    – Darren
    Sep 21, 2021 at 13:20

4 Answers 4


No cheating. They are not interacting just receiving messages.

Edit 1 Another recorded message. Birthday greetings for Frank. (And thanks to the youtube uploaders.)

Edit 2 The news interview Keith Morrison describes in his comment to this answer - which he points out the BBC reporter mentions the time lag.

Edit 3 regarding real deep space communication. The Voyager probes are outside of the solar system now and are sending a 23 watt signal. A cellphone uses 3 watts. So I think a manned spaceship would certainly put power into the communications. Hopefully the AI doesn't sabotage it. See https://science.howstuffworks.com/question431.htm

  • 38
    Note also at the beginning of the mission sequence, the news anchor introduces the interviews he conducted with Poole, Bowman, and HAL by pointing out that the delays caused by the distance between answer and response were edited out. A perfectionist like Kubrick wouldn't have missed light speed delay. Sep 19, 2021 at 4:16
  • @KeithMorrison Thanks I added your comment's video. Sep 19, 2021 at 5:47
  • How does the Voyager radio intensity compare to other sky objects'?
    – Mark C
    Sep 19, 2021 at 23:44
  • @KeithMorrison, True, but also, a perfectionist like Kubrick would not have had characters say anything about the speed of light delay if they all were presumed to be expert in the subject. That's the whole point of the TV interview in the film. By presuming a non-expert, fictional TV audience, Kubrick gave the characters an excuse to say things for the benefit of us, the movie audience, that they never would have needed to say to each other. Sep 21, 2021 at 12:32

In the book it is very specific that there is an ever increasing time delay between Earth and the Discovery, it is used as a plot device to emphasise the isolation that the crew feel. As the book goes on and Bowman is left as the sole survivor he muses on the fact that not only will no help be coming but that he will be long dead before mission control even knows it.

  • Can you provide any quotes to back this up?
    – Valorum
    Sep 19, 2021 at 12:00
  • No, I no longer have a copy of the book.
    – user144665
    Sep 19, 2021 at 12:06
  • @Valorum Page 134: "It would be two hours before any reply could reach him. And it was difficult to imagine what answer Earth could possibly send, except a tactfully sympathetic, 'Good-Bye.'" He doesn't expect to be dead in two hours, though. The reply from Earth is actually Heywood Floyd explaining the real purpose of the mission, which in the movie was prerecorded and played somewhat implausibly on a monitor in HAL's brain.
    – benrg
    Sep 21, 2021 at 5:13
  • @benrg, I always assumed that the message was presumed (by Dr. Floyd) to be so momentous that he wanted all of the crew to hear it at the same time, no matter where on the ship they happened to be when it was triggered. Therefore, he would have it played simultaneously on every monitor. Sep 21, 2021 at 13:05

It's made very clear in both the book and the movie that there is a long time lag for radio communications with Discovery. For example, in the birthday scene in the movie, the parents' transmission is one way and refers to Frank Poole's previous transmission. This all makes sense, because Clarke was writing hard SF with as much scientific accuracy as possible. Clarke himself was one of the originators of the idea of a communications satellite, so it's not as though he'd be ignorant of things like the fact that radio signals propagate at c.

However, later in the movie, after Dave Bowman passes through the star gate, it's equally clear that he is zooming around faster than the speed of light. The implication is that this is something like a wormhole, which is perfectly consistent with relativity. (The speed limit c is a local one.) IIRC the book is vague and hand-wavy about this sort of thing, but mentions something like an idea that the 1:4:9 ratio of the spatial dimensions of the monolith is actually an infinite sequence extending into infinitely many dimensions. Basically this is an example of Clarke's laws.

  • 1
    +1 for the handy reminder about Clarke's laws. :) Sep 20, 2021 at 8:11
  • I think when Bowman passes through the monolith he's traveling through a wormhole, so it's a shortcut between distant locations. Whether this is equivalent to FTL travel is not clear to me. Sep 20, 2021 at 16:07
  • Note: due to time dilation, it is possible for someone to travel hundreds of light years, without them perceiving that time (or aging), if they are travelling near the speed of light. For example, light travels from the sun to earth in about 8 minutes from OUR perspective, but from the perspective of the light, NO TIME PASSES. If you were travelling with the light, you would see the universe squash up, until there was NO DISTANCE between the sun and the earth. Sep 20, 2021 at 21:59

The "3000" sequel story also makes clear that the monoliths are capable to send and receive messages over a distance of 500 light years, but only at the speed of light. The result is significant action in the year 3,000.


Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

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