# How did the watch do what it did?

In Interstellar, after young Murph places the watch on the bookshelf in her bedroom, her father - in the same bedroom but from another dimension - moved the second hand and recorded data in a Morse-signal message on it. Many years later, the second hand still signals the message, grown-up Murph finds the watch and takes it with her to NASA and there she translates the message back into the data needed.

If we assume the watch had a long lasting battery, then I still see some problems with this plot:

• From what we know from the explanation in the movie itself is that the only thing capable to travel from them in the 4th or 5th dimension to present Earth is gravity. Gravity is a vertical force on Earth. It pulled dust from the air to the floor in a barcode-like pattern. So how can it adjust the horizontal position of the second hand of the watch? Like a magnet?
• And how is a multiple of those movements of the second hand recorded on the watch? How does it play the message for all these years, again and again. Not only in Murph's bedroom, but still at NASA too?

Normally I would answer this myself by saying that it is film, it is science fiction, everything is possible, and there is no need to question things we cannot explain. But this movie certainly has the intention of being truthful to science, to explain what is going on and this particular director has a reputation of telling stories that in one way or another always has plausible explanations for plot-hole-like aspects.

• I think you are missing the point of where Coop actually was when he was doing this. Where he was at transcends all time. He was in the same place with access to any time. Because of this he was able to make the second hand do what he needed it to do through all time. Secondly, gravity is whatever direction it is being applied. How was he able to get the books to move if it were? If he can manipulate gravity, it can be applied in any direction. Not an answer, really, just some thoughts. – Pᴀᴜʟsᴛᴇʀ2 Nov 9 '14 at 11:32
• "Gravity is a vertical force" - Uh, you know that gravity is only vertical to us here on earth, pretty much because of itself, gravity actually defines what is vertical. – Napoleon Wilson Nov 13 '14 at 22:00
• +1 - After rethinking this, I agree that this is the only part in the movie I can't yet make sense out of. He was only manipulating the gravity inside Murph's room (and I guess also only in young Murph's time). I'm not sure how the watch could then store those gravity waves. I have to recheck Thorne's book for this and hopefully he gave an answer I missed or misunderstood. (But of course the points about "vertical" are still irrelevant, they're arbitrary gravity waves afterall.) – Napoleon Wilson Nov 25 '14 at 16:14
• I think it's reasonable to expect that the beings who created the Tesseract looped the message for him. They set up the Tesseract specifically to translate his movements into a form of communication. After he recorded his message on the watch, they could've just looped it. Alternatively, he could've just recorded it three times for good measure. That's assuming that it did loop. It's also possible that it ran through once, and Murph just missed a bit at the beginning and had to piece things together in spite of some missing data. – Liesmith Dec 16 '14 at 19:56

For the first question about how gravity could move the watch sideways, apparently the idea was that gravitational waves were being transmitted into the past. In Chapter 30 of The Science of Interstellar by Kip Thorne, the physicist who was the scientific consultant for the movie, it's said that Cooper is scooped out of the black hole by the tesseract, which is a four-dimensional cube in a higher spatial dimension known as "the bulk"--the tesseract seems to be a piece of advanced technology created by the beings living in that dimension, which they used to allow Cooper to transmit the "quantum data" about the black hole singularity to Murph in the past. When he's inside the tesseract, the book says that he can see the "world tubes" of various objects (their paths through spacetime, a sort of 4D tube where each cross-section is the 3D object at a particular instant in time, analogous to the world line of a point particle), and that:

when he pushes on a book's world tube, or on the world tube of the watch's second hand, he generates a gravitational signal (a gravitational wave in the bulk) that spirals into and through the tesseract's bulk interior ... The signal travels forward in local, bulk time, but backward in bedroom time, arriving before it started out. It is this gravitational signal that pushes the book out of the bookcase and twitches the watch's second hand.

Gravitational waves can travel in any direction, so for example we might detect them coming at us in a "sideways" direction if a major source of gravitational waves (like two black holes colliding) had occurred in that direction relative to us. And as explained in this answer on physics stack exchange, they stretch and compress things in a plane that's perpendicular to their direction of motion, so gravitational waves coming in a vertical direction could actually stretch and compress objects in a horizontal plane.

For the second question about how the watch was able to continually play the message, I'll repost my answer from here:

It seems to be a function of the tesseract. From Ch. 30 of The Science of Interstellar:

By the time Cooper has received the quantum data form TARS, he has mastered this means of communication. In the movie we see him pushing with his fingers on the world tube of the watch's second hand. His pushes produce a backwards-in-time gravitational force, which makes the second-hand twitch in a Morse-encoded pattern that carries the quantum data. The tesseract stores the twitching pattern in the bulk so it repeats over and over again. When forty-year-old Murph returns to her bedroom three decades later, she finds the second hand still twitching, repeating over and over again the encoded quantum data that Cooper has struggled so hard to send her.

• Hmm, I had read that answer over on SciFi.SE but am still not entirely sure how the tesseract "stores the twitching pattern in the bulk", especially since the tesseract was only anchored to our brane in Murph's bedroom and she seems to take that watch to NASA (or even just out of its original place in the shelf). But I also admit that this world-line part and the way the gravity waves move backward in time was not entirely clear to me from Thorne's explanations after the first read. But I guess it's as near as we're going to get to an answer. – Napoleon Wilson Nov 26 '14 at 17:25
• @Napoleon Wilson - Given that the tesseract is a piece of advanced technology, it seems natural to assume it was capable of "storing" a sequence of data just as a computer might, and then using that data to repeatedly send the same pattern of gravitational waves to the watch. And as I said in a comment on the other answer, the fact that it was "anchored" on the bedroom doesn't give any reason to think it couldn't transmit gravity waves elsewhere, just as a radio transmitter sends radio waves far from its own location. – Hypnosifl Nov 26 '14 at 17:33
• consider the "thing" that Cooper moves to generate gravitational waves as "strings" - once they start vibrating, they continue to vibrate in the same pattern - I'm not sure how much this is correct mathematically - but a vibration in the higher order would produce a pattern in the lower dimensions – coolharsh55 Mar 23 '16 at 17:38
• @Hypnosifl to elaborate on your point: The tesseract device was constructed specifically to aid Cooper's relay of information to Murph. It's not completely inconceivable that it would fulfill its function ;b – Skek Tek Dec 4 '18 at 18:05

I think we don't have to rely on the battery of the watch, because if you had noticed the watch was not working, when a grown up Murph took it up. It was not updating time. It was Cooper who was making the second needle move. On how the watch generated data, I think Cooper was using the second needle to transmit data in Morse code.

Morse code is a method of transmitting text information as a series of on-off tones, lights, or clicks that can be directly understood by a skilled listener or observer without special equipment.

On how he was able to use gravity to move the needle, we also notice that gravity is used to disperse sand on the floor and make patterns out of it. So pretty much the same way with clock's needle. But how exactly he could use gravity for making movements along the x-axis is beyond me. May be someone with a deep knowledge of physics could explain whether this is possible. Anyway, we might ask how did he get hold of gravity all? But since according to the story, he could, it doesn't seem so hard to assume that indeed he could make a movement along the x-axis, using gravity.

• I don't see why everyone's fixating on the axis. Gravity isn't bound to one direction in space. Heck, travel a few countries along the globe and even the Earth's gravity is entirely perpendicular to what it was at your point of origin. I guess you're all assuming that Coop manipulated Earth's gravity rather than sending unrelated gravity waves... but there's no evidence of such a restriction. – Lightness Races in Orbit Nov 22 '14 at 17:25
• There is no battery in such a watch. – Cape Code Nov 15 '16 at 15:15

The 'room' where Cooper is in is essentially a dimension that contains every instance of her daughter's life spent in the room. Time in this dimension is non-linear. This allows Cooper to just walk around and find an instance in time that he chooses.

The watch did not store any information. He was moving the hand at the same time Murphy (the grown up version) was looking at it.

The next part contains some speculation.

The dimension that Cooper is in allows you to affect the gravity of every single thing in the room. In the movie it is likened to the threads attached to a puppet; you can influence it by plucking at it. Yes, I agree that this is quite a stretch from the point of view of Physics, but that is how I see it. There is an instance where they show this in the movie; to move the hand Cooper is pushing against a vertical stream of light which I think is a physical representation of the gravity that is acting on the watch hand.

• Grown up Murph takes the watch with her to NASA. There she spends (presumably) several days registering the needle's signals and writing down the morse code. All this time was Cooper moving his hand? – NGLN Nov 11 '14 at 9:19
• Yes... quite a stretch isn't it? Bordering on ridiculous.. but according to the movie that's how it is done. – bobbyalex Nov 11 '14 at 9:20
• Actually you have a very good point: how was Cooper able to move the watch outside the room? – bobbyalex Nov 11 '14 at 9:22