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In the movie Primer (2004), there are two main characters who travel back 24 hours in time using their time machine. When they arrive, both they and their past selves exist in this same town at the same time. Unbenownst to them however, one of the characters named Aaron has forgotten that he had his cell phone in his pocket when using the time machine. Both his cell phone and its molecular duplicate in the alternate Aaron's possession exist at the same time; likely both being in relatively similar vicinities (same town, less than ~20 miles apart).

After arriving back in the past, Aaron receives a phone call from his wife. Before time traveling, Aaron received this phone call previously while we was inside of a nearby hotel. However now he is receiving the call while outside, possibly implying that his double who is inside the hotel did not receive the phone call.

Both characters panic and discuss the ramifcations of what just happened. They are unsure of how cell towers work when it comes to having two identical receivers (phones).

In reality, what would/could happen in that situation? If you were to clone a cell phone, would both people holding the cell phone receive a ring? Would both people be able to pick up and speak? If one person picks up, does the ringing stop for the other? Does it depend on where both people are located (ex. closer person to the tower)? Is it possible to simulate this in real life (ex. duplicating the SIM card)? Why or why not and how, for any/all of those questions?

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    $\begingroup$ Fraud detection system kicks in, IMEI and SIM are blacklisted, both phones are deregistered from the network and go into emergency calls only mode. Neither phone can receive calls. (Phone cloning is a real thing, and carriers don't like being defrauded of their just profits.) $\endgroup$
    – AlexP
    Jan 27, 2020 at 22:24
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    $\begingroup$ Setting this up in 2004 makes it a little murky. What particular cellular technology was used there? Was a SIM card used at all? $\endgroup$
    – Alexander
    Jan 27, 2020 at 23:19
  • $\begingroup$ Not sure this is suitable for this stack. It's a question more related to an established world, so Science Fiction & Fantasy is an option. Although, since it's a technical question about how real mobile technology works (or worked about 15 years ago), then perhaps another stack is even more appropriate...but I'm not sure which one. Do we have anything for mobile stuff? I can only find Network Engineering but it I'm not sure they deal with networks by mobile carriers. $\endgroup$
    – VLAZ
    Jan 28, 2020 at 9:19
  • $\begingroup$ @VLAZ Unfortunately Sci Fi & Fantasty deemed it off topic. Physics was also discussed as not a viable place for the discussion either. Network Engineering I believe only handles internet related networking, not sure. I believe if anything, the Security board is probably the most technically correct place. However I don't see why World Building is unsuitable. $\endgroup$
    – Hatefiend
    Jan 28, 2020 at 9:34
  • $\begingroup$ I mostly see it as unsuitable because it's an established world...and it also happens to be our world. I love Primer but the core of the question is "If we have two absolutely identical SIM cards, what happens?" and that's not really something that aids in creating a fictional world. I'm also worried that WB might not have the correct experts to answer this. Although, I do agree that it doesn't really seem we have a "correct" place to ask this question as I'm not aware of any stack that deals with mobile networks/carriers/related. $\endgroup$
    – VLAZ
    Jan 28, 2020 at 9:40

2 Answers 2

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Disclaimer: I'm not a cellular network engineer, nor was I in 2004. I have a rudimentary knowledge of network design, and enough knowledge of programming to be dangerous.

Now, let's make some assumptions:

  1. I'm going to call the wife's phone 'sender' and her cell tower 'tower 1'
  2. The original phone is 'receiver 1' and the clone is 'receiver 2.' They're both connected to tower 2, and receiver 2 is closer to the point of origin.
  3. Receivers 1 and 2 have the same hardware and software information.

I'm going to discuss protocol from the networking side. Basically, cellular networks work by keeping track of the phones in a specific tower's area and handing over control when they leave the area. That's why it's cellular, because there's 'cells' of influence.

Now, you're a cell tower and receiver 2 enters your sphere of influence. What do you do?

Well, since receiver 1 is listed already, fraud protection might kick in, as @AlexP said. The two devices have identical IMEI and SIM information, but different location signatures after all. Somewhat. While your cell tower can determine your approximate location through analysis of the signal's direction and strength, the technology isn't all that accurate. I don't know if it was even extant in 2004, but I suspect it was, this being after the events of September 2001. If the two devices are close enough, the discrepancy could simply be ignored as signal noise.

If we assume that fraud protection doesn't kick in, and that the two phones stay close enough together for the cell tower to be unable to determine that they're different devices, then what happens when the call comes in?

First, the tower would send the receivers a network packet telling them they've got a call. At some point, the receivers need to respond. I don't know the true internals of the 2004 era cell network, but I assume that they handshake immediately. The cell tower receives two ACK messages. Now the cell tower needs to act on this. It can act in one of two ways: Either ignore the second ACK packet, or clue in to the fact that there's a phony phone and boot them both off the network until the situation can be figured out by a human employee.

I'm going to assume the former. The cell tower, having received confirmation, may or may not reply to the receivers with another ACK, then tells the sender that they've connected. The sender will begin hearing the beep beep beep of the waiting noise.

Now receiver 2 picks up the phone. A 'ready to connect' packet gets sent to the cell tower. The cell tower either handshakes it, or simply begins the call connection. In either case, I'd imagine that receiver 1 is left ringing, as it has not been told that the call has been canceled and has not prepared itself to receive the call.

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  • $\begingroup$ Thank you for such a detailed response. One part I don't understand though is when the cell tower sends a packet that they've got a call. Both phones will receive that packet I believe, then both would send back a response attempting to handshake. How can the tower somehow handshake with one without the other being involved, despite them being in similar geographical areas with duplicate hardware? I'm not sure how accurate radio waves can be. Is it similar to computers where they will both do a key exchange? Could both be on the phone call at once? $\endgroup$
    – Hatefiend
    Jan 31, 2020 at 23:23
  • $\begingroup$ In this scenario I'm assuming that the tower doesn't send a response to acknowledgement packets, and that it moves on to the next procedure in the call coding once it receives the ACK. Ergo, receiving a stray ACK from the number is erroneous. $\endgroup$
    – Jakob Lovern
    Feb 1, 2020 at 4:43
  • $\begingroup$ A lot of my scenario is built on supposition and what-if because I don't actually know what would happen. I didn't even know what a cell tower was in '04. $\endgroup$
    – Jakob Lovern
    Feb 1, 2020 at 4:45
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A plothole. Those are not the same phones, those are not the same numbers, those are not the same people. One is 24 hours older. Same with the phone. They share similarities but one of them have integral clock that is 24 hours in the future. And not by setting the time forward. The amount of 0 and 1's to make program change it's date have passsed.
The phone "of the future" just don't log into the system. This is somewhat similar to "2k bug". The problem was that the date fill eff programs, bios, systems up. And everyone panicked and no one really even considered "Hey, let's just set the date to 1972 and use just some overhaul to show 2000 and in the meantime try to fix the problem".

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    $\begingroup$ Many phones sync time from cell towers. $\endgroup$
    – Monty Wild ♦
    Jan 30, 2020 at 10:14
  • $\begingroup$ @MontyWild That was the reason "hand set time to 1972" not an option. Machines check their times with each other. I assume that the phone would either only check hours and not date of if checked date it would <syntaxerror>. $\endgroup$
    – SZCZERZO KŁY
    Jan 30, 2020 at 10:18
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    $\begingroup$ Ugh, no. Phones take the time from the network; the network broadcasts the current time for anybody who is listening. And in computer systems time is not represented in human-readable broken-down format; that's for human consumption only. The Y2K scare was related to really really old software (used mostly in the financial sector) which performed date and time calculations based on human-readable representations. $\endgroup$
    – AlexP
    Jan 30, 2020 at 11:22
  • $\begingroup$ @AlexP To be more specific Y2K was about how dates were stored as the last two digits of the year for dates, which caused difficulties once the dates reached above 99. Years were stored in 32 bit WORD format which only allows for 2^4 combinations. In other words values from 0 to 63, rolling over to 0 at 64. $\endgroup$
    – Hatefiend
    Jan 31, 2020 at 23:31

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