15 ms·

How many photons are received per bit transmitted from Voyager 1?

1027 points by williamsmj 2y ago

deleted 2y ago
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notorandit 2y ago
It's really nice! Both the question and the answer!
jsjohnst 2y ago
Didn’t realize the math would be that straightforward. Is there something the author isn’t taking into account or is that a decent plausible range?
- krylon 2y ago
  I was surprised, too. Knowing little about physics, this was a pleasant surprise, however.
- rcxdude 2y ago
  It looks like a pretty reasonable order of magnitude estimate to me. Energy arguments tend to be quite neat for that because if the efficiency is at all reasonable they constrain things well with fairly simple calculations. The antenna directionality is also reasonably well understood and characterised. The exact noise level discussed later on is probably where things get a bit more uncertain (but aren't directly needed to answer the question).
- magicalhippo 2y ago
  One thing that seems missing to me is that while the probe might send 160 bits/sec of useful data, those bits are not sent directly as such[1]: The TMU encodes the high rate data stream with a convolutional code having constraint length of 7 and a symbol rate equal to twice the bit rate (k=7, r=1/2). So the effective symbol rate is 320 baud[2], and thus a factor of two should be included in the calculations from what I can gather. Note that the error correction was changed after Jupiter to use Reed-Solomon[3] (255,223) to lower the effective bit error rate, so effectively I guess the data rate is more like 140 bps. [1]: https://web.archive.org/web/20130215195832/http://descanso.jpl.nasa.gov/DPSummary/Descanso4--Voyager_new.pdf https://web.archive.org/web/20130215195832/http://descanso.j... [2]: https://destevez.net/2021/09/decoding-voyager-1/ https://destevez.net/2021/09/decoding-voyager-1/ [3]: https://destevez.net/2021/12/voyager-1-and-reed-solomon/ https://destevez.net/2021/12/voyager-1-and-reed-solomon/
  - ooterness 2y ago
    For the "photons per bit" question, the useful throughput of 140 bps is the only thing that matters. For the "how many photons are needed" question, I agree that 320 baud (i.e., the effective analog bandwidth of 320 Hz) should have been used for the Shannon-Hartley calculations.
lordnacho 2y ago
I love these kinds of questions. So what does that conclusion mean about when the probe will be so far away that we are below the Shannon limit? And can we beat the Shannon limit somehow, eg collect for longer, put the dish outside the atmosphere, and so on?
- fsmv 2y ago
  Seems like we can just build a bigger receiving dish
  - rcxdude 2y ago
    There's not a lot of appetite for that, though. the 70m receiver is already one of the largest ever built, and for most use cases it's looking better to use an array of smaller ones. Which works fine for receiving but not so much for transmitting (while there are multiple sites in the world capable of receiving from voyager 2, there's only one dish which can actually transmit to it) (I recall seeing a video on that dish, and the director seemed confident there was enough noise margin left that voyager's power would fail before they lost contact with it)
- Asraelite 2y ago
  I guess we could just program Voyager 1 to lower the data bitrate, adding more redundancy / error correction. I think there's no real limit to how far it could get if we keep doing that.
- ethbr1 2y ago
  Not my field, but assuming transmitting hardware (including beam forming) is constant and that atmosphere can mostly be ignored (see comments about it usually being a non-impact in the transmission frequencies), two approaches would suggest: 1. Increase the effective receiving dish size, to capture more of the signal. Essentially, this would be effective in direct proportion to beam spread (the more beam spread, the bigger dish you can use to capture signal). In practice, this would use multiple geographically-displaced dishes to construct a virtually-larger dish, to allow for better noise-cancellation magic (and at lower cost than one huge dish). I believe the deep space network (DSN) already does this? Edit: It certainly has arrayed antennae [0], though not sure how many are Voyager-tasked. 2. Increase the resilience of the signal, via encoding. The math is talking about bits and photons, but not encoded information. By trading lower bit-efficiency for increased error tolerance (i.e. including redundant information) we can extract a coherent signal even accounting for losses. Someone please point out if I'm wrong, but afaik the Shannon–Hartley limit speaks to "lower" in the physical stack than error coding. I.e. one can layer arbitrary error coding on top of it to push limits (at the expense of rate)? If the above understanding is correct, is there a way to calculate maximum signal distance assuming a theoretically maximally efficient error coding (is that a thing?) ? Or is that distance effectively infinite, assuming you're willing to accept an increasingly slow bit receiving rate? [0] https://en.m.wikipedia.org/wiki/NASA_Deep_Space_Network#Antennas https://en.m.wikipedia.org/wiki/NASA_Deep_Space_Network#Ante...
amirhirsch 2y ago
TLDR; 4e22 photons per second 2.6e22 per bit. For comparison, ~2e26 photons will be received through your iris in your life
- KeplerBoy 2y ago
  Does this number account for all photons or just those in the rather narrow optical band? Then again RF photons just don't fit through the pupils and will get backscattered, i guess.
- amelius 2y ago
  How many of them come from Voyager 1?
  - amirhirsch 2y ago
    Someone’s asking the hard questions! According to the oracle, 1 in 4 people will experience one photon from voyager in their lifetime.
- rcxdude 2y ago
  That's how many are sent by Voyager. Only about 1500 or 400 photons per bit are actually received by the radio dish (depending on which frequency is being used).
  - croemer 2y ago
    Maybe OP sends photons from their eyes in addition to receiving them
pcdoodle 2y ago
I am confused. I thought photons were just visible light but I guess these little buggers are everywhere. Also very surprised voyager is using 2.3ghz, that's crazy saturated on earth due to wifi. How these engineers make this all work, is magic to me.
- mpreda 2y ago
  Electromagnetic radiation includes visible light, radio spectrum, X-rays, etc. Photons.
- nilamo 2y ago
  > Also very surprised voyager is using 2.3ghz, that's crazy saturated on earth due to wifi Wifi didn't exist when Voyager was launched...
  - thsksbd 2y ago
    But the band was free for use, wasn't it? (Obviously not crowded)
    - anotherhue 2y ago
      Available for microwaves since 1947, intentional emission came later in the 80s.
    - ianburrell 2y ago
      The ISM band is 2.4-2.5 GHz. Voyager at 2.3Ghz is outside the band.
- drmpeg 2y ago
  WiFi is at 2.4 GHz. LTE band 30, satellite radio (XM/Sirius) and aeronautical telemetry all exist between the deep space downlink at 2290 to 2300 MHz and WiFi at 2400 MHz.
- noneeeed 2y ago
  Nope. It's one of those things that can take a bit to get used to, but everything on the electromagnetic spectrum is just light in the general sense. The only difference between radio-waves, x-rays, infra-red and (human) visible light is the frequency/wavelength. If the frequency is high enough then the waves of light can be detected by things as small as cells in the back of your eye, or the pixels in a camera sensor. If it is too low then you need much larger detectors. Other animals have detectors for different frequencies/wavelengths, allowing them to see either infra-red (mosquitos) or ultraviolet (bees, butterflies etc). What we call "visible light" is just the particular range that our eyes can detect (about 400 to 800THz). If we were the size of a planet, and our eye cells were the size of a radio-telescope dish we would be able to "see" in those wavelengths. In fact, when we see images taken by radio telescopes, those have been essentially pitch-shifted up to something we can see, like the reverse of what we do when listening for bat clicks (where the pitch is downshifted to our hearing range). The wikipedia article has a nice little diagram putting the wavelengths into perspective. https://en.wikipedia.org/wiki/Electromagnetic_spectrum https://en.wikipedia.org/wiki/Electromagnetic_spectrum
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moffkalast 2y ago
You know, I never really thought of lower wavelengths than light as being carried by photons, but I suppose it's all EM. Antennas are technically just really red light bulbs.
- gjstein 2y ago
  This is true enough, though remember that material properties change dramatically when you start moving through wavelengths by orders of magnitude. Silicon is transparent in the mid-infrared, which is what makes silicon photonics possible [1] [1] https://en.wikipedia.org/wiki/Silicon_photonics https://en.wikipedia.org/wiki/Silicon_photonics
bandrami 2y ago
It's crazy to me how many theoretical limits Shannon predicted way before the hardware was there.
- ziofill 2y ago
  That’s because his results are about pure information (and in the limit for infinite string lengths), so sooner or later some hardware will hit onto those limits or tend to them.
  - bandrami 2y ago
    Agreed, but: he's still understudied. I think in retrospect any 21st-century math course has to include Shannon, and they don't all, yet.
    - sebzim4500 2y ago
      On some level he's a victim of his own success. He invents information theory in the same paper that proves the most interesting results, so who else will work on it?
      - aidenn0 2y ago
        Even the practical work was done surprisingly early. I have a book on error correcting codes from the 1950s and it's missing very little (Most notably trellis codes and LDPC; the former being invented in the '70s and the latter in 1963).
        sebzim4500 2y ago
        There's been much more progress on compression though (Arithmetic Coding, ANS, etc.)
      - moffkalast 2y ago
        Shannon: "I'm about to make and end this field's whole career."
Strilanc 2y ago
Wasn't expecting my question to hit top of HN. I guess I'll give some context for why I asked it. I work in quantum error correction, and was trying to collect interesting and quantitative examples of repetition codes being used implicitly in classical systems. Stuff like DRAM storing a 0 or 1 via the presence or absence of 40K electrons [1], undersea cables sending X photons per bit (don't know that one yet), some kind of number for a transistor switching (haven't even decided on the number for that one yet), etc. A key reason quantum computing is so hard is that by default repetition makes things worse instead of better, because every repetition is another chance for an unintended measurement. So protecting a qubit tends to require special physical properties, like the energy gap of a superconductor, or complex error correction strategies like surface codes. A surface code can easily use 1000 physical qubits to store 1 logical qubit [2], and I wanted to contrast that with the sizes of implicit repetition codes used in classical computing. 1: https://web.mit.edu/rec/www/dramfaq/DRAMFAQ.html https://web.mit.edu/rec/www/dramfaq/DRAMFAQ.html 2: https://arxiv.org/abs/1208.0928 https://arxiv.org/abs/1208.0928
- nico 2y ago
  Very cool. It’s interesting to realize that at some level, every system is a quantum system if you “zoom in” enough
  - Ringz 2y ago
    I would spontaneously respond that you are right and at the same time have no problem if someone explains to me that it is not so.
  - empyrrhicist 2y ago
    I think the point is the model though - if a system's behavior can be modeled/described classically, it's a bit silly to to call it a "quantum" system in the same way that it's reductive to say Biology is just applied particle physics. Sure, but that's not a very useful level of abstraction.
    - jessriedel 2y ago
cycomanic 2y ago
Actually the limit predicted by Shannon can be significantly beaten, because Shannon assumes gaussian noise, but if we use photon counting receivers we need to use a poisson distribution. This is the Gordon-Holevo limit. To beat Shannon you need PPM formats and photon counters (single photon detectors). One can do significantly better than the numbers from voyager in the article using optics even without photon cpunting. Our group has shown 1 photon/bit at 10 Gbit/s [1] but others have shown even higher sensitivity (albeit at much lower data rates). [1] https://www.nature.com/articles/s41377-020-00389-2 https://www.nature.com/articles/s41377-020-00389-2
- nico 2y ago
  Interesting. Is that related to compressed sensing? I wonder if compress sensing could be used for something like the Voyager signals It seems there might be multiple ways to go beyond Shannon’s limit, depending on what you are trying to do
  - cycomanic 2y ago
    I don't think compressed sensing is really extracting more information than Shannon, it simply exploits the fact that the signal we are interested in is sparse so we don't need to sample "everything". But this is somewhat outside my area of expertise so my understanding could be wrong.
    - nico 2y ago
      Maybe I’m mixing Shannon’s limit with the sampling rate imposed by the Nyquist-Shannon Sampling theorem > Around 2004, Emmanuel Candès, Justin Romberg, Terence Tao, and David Donoho proved that given knowledge about a signal's sparsity, the signal may be reconstructed with even fewer samples than the sampling theorem requires.[4][5] This idea is the basis of compressed sensing … > However, if further restrictions are imposed on the signal, then the Nyquist criterion may no longer be a necessary condition. A non-trivial example of exploiting extra assumptions about the signal is given by the recent field of compressed sensing, which allows for full reconstruction with a sub-Nyquist sampling rate. Specifically, this applies to signals that are sparse (or compressible) in some domain From: https://en.m.wikipedia.org/wiki/Nyquist%E2%80%93Shannon_sampling_theorem https://en.m.wikipedia.org/wiki/Nyquist%E2%80%93Shannon_samp...
ziofill 2y ago
What a lovely question. The estimate is 10-100 photons/bit (minimum). If you’re curious about how many bits a single photon can carry, in controlled settings (tabletop quantum optics) a single photon can carry log(n) bits where n is the size of the state space of the photon, which theoretically is infinite and in practice it can reach into the hundreds/thousands.
- layer8 2y ago
  No, the estimate is around 750 or 200 photons/bit received, depending on the transmission frequency. The answer to the question is B, not C. Your numbers are the estimated minimum needed, not the actual amount received, which is what the question was asking.
- mnw21cam 2y ago
  Visible light is different, because each photon has a lot more energy than in the 2.3GHz range. Your average decent consumer-level camera has a sensor that can nominally just about detect single photons some proportion of the time (as in, some of them bounce off instead of being detected) though it can't technically count them. The graininess on digital camera images is more from the Poisson noise of the incoming photons than it is from the applied noise of the sensor itself.
HarHarVeryFunny 2y ago
The fact that we can communicate with Voyager, and in both directions, blows my mind. It's completely counter-intuitive. At least for Voyager->earth we can use giant radio telescopes to detect the faint signal, but how do we manage to focus on those few hundreds of photons per bit coming from a pinpoint source a light day away?! In the earth->Voyager direction it seems even less intuitive - sure we can broadcast a powerful signal, but it's being received by a 12' wide antenna 15 billion miles away. WTF? I guess radio communications in general is magic, a bit like (in nature of counter-intuition) quantum entanglement of particles arbitrarily far apart. It seems there is something deeply wrong about our mental models of space and time.
- philipwhiuk 2y ago
  Per the article we lose 99.9% of the photons sent. The antenna is as directional as possible but with a similar dish you could pick up the signal on the Moon at this point I expect (probably a smaller dish given the lack of atmospheric noise) Equally we're broadcasting to the area of space Voyager's in. We're not able to to target it to the dish - 12' isn't the DSOC accuracy - it's the size required to pick up enough data given the signal diffusion.
cycomanic 2y ago
For anyone who is interested in the ultimate limits to communications the seminal paper by Jim Gordon is quite easy to understand even without a physics degree (unlike the Holevo paper IMO). He was incredibly good at writing in an accessible manner (apart from probably being the person who most deserved a Nobel prize but didn't get it). https://doi.org/10.1109%2FJRPROC.1962.288169 https://doi.org/10.1109%2FJRPROC.1962.288169
- prof-dr-ir 2y ago
  > probably being the person who most deserved a Nobel prize but didn't get it You probably want to read up a bit on the remarkable life of Lise Meitner.
bebopalula 2y ago
[flagged]
- magnoliakobus 2y ago
  Bad bot!
superposeur 2y ago
The overwhelming loss in this calculation is from the antenna’s radiated energy spreading out over a larger and larger area (despite the directional “gain” factor). I’m wondering: would a probe launched today instead employ a laser to communicate? This would seem to offer many orders of magnitude improvement in the directionality of the signal.
- deelowe 2y ago
  I imagine it'd certainly employ some type of beamforming at the least.
  - wongarsu 2y ago
    Assuming you don't need fast steering, is a 3.7m transmitter array doing beamforming really better than a 3.7m dish transmitting at the same power? My intuition would have been that you are better off using a fairly standard transceiver and spending your engineering budget either increasing power or getting a bigger dish (either by launching on a wider rocket or with a folding design). Lasers might interesting for the downlink, but receiving a laser signal on the probe sounds difficult (earth is pretty bright).
    - outworlder 2y ago
      There's some value in getting rid of mechanical devices (or reducing the need to rotate the entire spacecraft).
    - cycomanic 2y ago
      Diffraction scales inversely proportional to wavelength so you gain significantly by going to optics, i.e. you can use a much smaller aperature in optics.
    - CamperBob2 2y ago
      The array doing beamforming can be spread out much farther. If the DSN were being built now, I'd think its antennas would look more like the Square Kilometer Array.
hammock 2y ago
>Voyager sends 160 bits/second This makes me wonder, are the bits = the power turned on for exactly 1/320th sec, every 1/160th sec? Or is the power on/power off ratio something different? Does it vary by protocol? What are the pros and cons?
- danbruc 2y ago
  Without looking up what kind of encoding and modulation they are using, I would assume that they are sending a continuous sine wave at the carrier frequency that has the bits - probably after encoding the raw data bits with some error correction code - modulated onto it by changing frequency, amplitude, phase, or a combination of them depending on the value of each bit or group of bits.
ks2048 2y ago
Nice question. Does anyone know what exactly data is being sent? What kind of compression it is using? etc
mooktakim 2y ago
Why didn't they send out new relays as Voyager travelled out.
- philipwhiuk 2y ago
  Voyager's path required multiple flybys. You can't just send something on the same path later on - everything has moved. Plus, no budget for relays.
somat 2y ago
An interesting thing about photons (which may not be true, I just enjoy this stuff amateurishly, that is, without the effort or rigor to actually understand it.) is that they might not exist. the em field is not quantized, or at least is not quantized at the level of photons. A "photon" only exists where the em field interacts with matter, where the electrons that create the disturbance can only pulse in discrete levels. https://www.youtube.com/watch?v=ExhSqq1jysg https://www.youtube.com/watch?v=ExhSqq1jysg Not that this changes anything, we can only detect or create light with matter. but it does make me curious about single photon experiments and what they are actually measuring.
- leetrout 2y ago
  Thanks for the link. I never conceptualized photons outside of the visual spectrum so the headline made me take a step back and get nerd sniped in the process. I stumbled upon this before seeing your comment: https://physics.stackexchange.com/questions/90646/what-is-the-relation-between-electromagnetic-wave-and-photon https://physics.stackexchange.com/questions/90646/what-is-th...
- golergka 2y ago
  Isn't that simply the principle of particle-wave duality? When particle/wave in field X interacts with field X, it behaves like a wave, but interactions with other fields are quantised.
- mr_toad 2y ago
  Existence is a difficult concept for something moving at the speed of light. Light follows a null geodesic through space-time with zero (space-time) length and no proper time. Past, future, and causality have no meaning to a photon. We think of photons travelling through space because our symmetry is broken, we have mass, and we and experience time and space. Observers like us will see light follow the same world-line from its source to its target. It cannot interact with anything else, and some might say it was only ever emitted so that it could interact with its target. So from a certain point of view the “existence” of a photon is entirely bound up in its interaction with source and target, and it’s not really useful to speak of it in other terms. The quantised interaction is the photon.
croemer 2y ago
That's why I love Physics and was enamoured with it in my late teens.
wwarner 2y ago
23 watts.
tb0ne 2y ago
Super interesting! But I feel like there is a bit of a conclusion missing for me. So 1500 Photons hit the receiver per bit send, but this is obviously way to few to keep processing the signal and it will just be drowned out by noise? Where do we go from here? Does voyager repeat its signal gazillions of times so we can average out the noise on our end? Where can I find more information on what is done with these few photons?
- SonOfLilit 2y ago
  No, those 1500 photons are enough and we basically read the signal from them, from my reading of the comments here.
- djha-skin 2y ago
  Look up Solomon Reed erasure coding. Voyager uses it, so does qrp Ham radio operators.
  - ballooney 2y ago
    [Reed-Solomon is the more conventional formulation if you do end up going looking]
PeterCorless 2y ago
I just wanted to chime in with a reminder that though Voyager 1 is speeding away from Sol at a constant velocity because of the Earth's revolution around the sun it can be up to ±1 AU closer or further away, depending on the time of the year. This article is for Voyager 2, but the issue is the same. For a brief moment every year we actually get closer to Voyager 1, then we pivot away in our revolution around the sun and the distance between Earth and Voyager 1 or 2 increases sharply. So distance, when plotted over time, looks like a wobbly line. https://earthsky.org/space/voyager-spacecraft-getting-closer-to-earth/ https://earthsky.org/space/voyager-spacecraft-getting-closer...
- olddustytrail 2y ago
  Surely 2AU depending on whether we're on the same side of the sun or we're on the dark side.
rocho 2y ago
Wow, I never thought about how Voyager communicates with Earth. But now I wonder: if Voyager just sends photons towards the Earth, at the receiving end how are we recognizing which photons are coming from Voyager and how is the "signal" decoded?
- RachelF 2y ago
  Two main reasons for recognizing the photons: They have a specific frequency, 8.3GHz in this example. It's like tuning an FM radio to a station. The photons are coming from a specific direction. As to how they are decoded, you'll need to understand some modulation techniques.
RachelF 2y ago
What is equally impressive is the number of photons received from radar imaging of asteroids. They are closer, but the radar equation received power is inversely proportional to range to the fourth power, not range squared as with Voyager. Anything proportional to 1/R^4 degrades very quickly.
aaron695 2y ago
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omgJustTest 2y ago
25 ph is like massive SNR! Let's talk about 30ps timing and 3 photons & you have insight into my daily work!
ggm 2y ago
Could we e.g. decrease the bit rate, and in effect send less information but for longer? Eg send a continuous wave with low signal beyond its phase, and measure at a rate of (digital) bits per month?
asdfman123 2y ago
I was trying to answer a question about the Voyager space probe and got our of my depth, BUT I found this great PDF in the process. Covers a lot of technical details at the level a person without much specific training can understand. https://voyager.gsfc.nasa.gov/Library/DeepCommo_Chapter3--141029.pdf https://voyager.gsfc.nasa.gov/Library/DeepCommo_Chapter3--14...
basil-rash 2y ago
For the spectrum in question: FCC’s “US258 G117”, 8.4-8.45GHz reserved for “Space Research (deep space)(space-to-earth)”. Pretty neat. https://www.ecfr.gov/current/title-47/chapter-I/subchapter-A/part-2/subpart-B/section-2.106 https://www.ecfr.gov/current/title-47/chapter-I/subchapter-A...
throwaway81523 2y ago
That SE post doesn't mention the code rate of whatever ECC code Voyager uses.
foxbeneficial 2y ago
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bryangreen 2y ago
Sincere question: how does a radio signal manifest as a photon?

1027 points by williamsmj 2y ago

deleted 2y ago

[deleted]

notorandit 2y ago

It's really nice! Both the question and the answer!

jsjohnst 2y ago

Didn’t realize the math would be that straightforward. Is there something the author isn’t taking into account or is that a decent plausible range?

krylon 2y ago
I was surprised, too. Knowing little about physics, this was a pleasant surprise, however.
rcxdude 2y ago
It looks like a pretty reasonable order of magnitude estimate to me. Energy arguments tend to be quite neat for that because if the efficiency is at all reasonable they constrain things well with fairly simple calculations. The antenna directionality is also reasonably well understood and characterised. The exact noise level discussed later on is probably where things get a bit more uncertain (but aren't directly needed to answer the question).
magicalhippo 2y ago
One thing that seems missing to me is that while the probe might send 160 bits/sec of useful data, those bits are not sent directly as such[1]: The TMU encodes the high rate data stream with a convolutional code having constraint length of 7 and a symbol rate equal to twice the bit rate (k=7, r=1/2). So the effective symbol rate is 320 baud[2], and thus a factor of two should be included in the calculations from what I can gather. Note that the error correction was changed after Jupiter to use Reed-Solomon[3] (255,223) to lower the effective bit error rate, so effectively I guess the data rate is more like 140 bps. [1]: https://web.archive.org/web/20130215195832/http://descanso.jpl.nasa.gov/DPSummary/Descanso4--Voyager_new.pdf https://web.archive.org/web/20130215195832/http://descanso.j... [2]: https://destevez.net/2021/09/decoding-voyager-1/ https://destevez.net/2021/09/decoding-voyager-1/ [3]: https://destevez.net/2021/12/voyager-1-and-reed-solomon/ https://destevez.net/2021/12/voyager-1-and-reed-solomon/
- ooterness 2y ago
  For the "photons per bit" question, the useful throughput of 140 bps is the only thing that matters. For the "how many photons are needed" question, I agree that 320 baud (i.e., the effective analog bandwidth of 320 Hz) should have been used for the Shannon-Hartley calculations.

lordnacho 2y ago

I love these kinds of questions. So what does that conclusion mean about when the probe will be so far away that we are below the Shannon limit? And can we beat the Shannon limit somehow, eg collect for longer, put the dish outside the atmosphere, and so on?

fsmv 2y ago
Seems like we can just build a bigger receiving dish
- rcxdude 2y ago
  There's not a lot of appetite for that, though. the 70m receiver is already one of the largest ever built, and for most use cases it's looking better to use an array of smaller ones. Which works fine for receiving but not so much for transmitting (while there are multiple sites in the world capable of receiving from voyager 2, there's only one dish which can actually transmit to it) (I recall seeing a video on that dish, and the director seemed confident there was enough noise margin left that voyager's power would fail before they lost contact with it)
Asraelite 2y ago
I guess we could just program Voyager 1 to lower the data bitrate, adding more redundancy / error correction. I think there's no real limit to how far it could get if we keep doing that.
ethbr1 2y ago
Not my field, but assuming transmitting hardware (including beam forming) is constant and that atmosphere can mostly be ignored (see comments about it usually being a non-impact in the transmission frequencies), two approaches would suggest: 1. Increase the effective receiving dish size, to capture more of the signal. Essentially, this would be effective in direct proportion to beam spread (the more beam spread, the bigger dish you can use to capture signal). In practice, this would use multiple geographically-displaced dishes to construct a virtually-larger dish, to allow for better noise-cancellation magic (and at lower cost than one huge dish). I believe the deep space network (DSN) already does this? Edit: It certainly has arrayed antennae [0], though not sure how many are Voyager-tasked. 2. Increase the resilience of the signal, via encoding. The math is talking about bits and photons, but not encoded information. By trading lower bit-efficiency for increased error tolerance (i.e. including redundant information) we can extract a coherent signal even accounting for losses. Someone please point out if I'm wrong, but afaik the Shannon–Hartley limit speaks to "lower" in the physical stack than error coding. I.e. one can layer arbitrary error coding on top of it to push limits (at the expense of rate)? If the above understanding is correct, is there a way to calculate maximum signal distance assuming a theoretically maximally efficient error coding (is that a thing?) ? Or is that distance effectively infinite, assuming you're willing to accept an increasingly slow bit receiving rate? [0] https://en.m.wikipedia.org/wiki/NASA_Deep_Space_Network#Antennas https://en.m.wikipedia.org/wiki/NASA_Deep_Space_Network#Ante...

amirhirsch 2y ago

TLDR; 4e22 photons per second 2.6e22 per bit. For comparison, ~2e26 photons will be received through your iris in your life

KeplerBoy 2y ago
Does this number account for all photons or just those in the rather narrow optical band? Then again RF photons just don't fit through the pupils and will get backscattered, i guess.
amelius 2y ago
How many of them come from Voyager 1?
- amirhirsch 2y ago
  Someone’s asking the hard questions! According to the oracle, 1 in 4 people will experience one photon from voyager in their lifetime.
rcxdude 2y ago
That's how many are sent by Voyager. Only about 1500 or 400 photons per bit are actually received by the radio dish (depending on which frequency is being used).
- croemer 2y ago
  Maybe OP sends photons from their eyes in addition to receiving them

pcdoodle 2y ago

I am confused. I thought photons were just visible light but I guess these little buggers are everywhere. Also very surprised voyager is using 2.3ghz, that's crazy saturated on earth due to wifi. How these engineers make this all work, is magic to me.

mpreda 2y ago
Electromagnetic radiation includes visible light, radio spectrum, X-rays, etc. Photons.
nilamo 2y ago
> Also very surprised voyager is using 2.3ghz, that's crazy saturated on earth due to wifi Wifi didn't exist when Voyager was launched...
- thsksbd 2y ago
  But the band was free for use, wasn't it? (Obviously not crowded)
  - anotherhue 2y ago
    Available for microwaves since 1947, intentional emission came later in the 80s.
  - ianburrell 2y ago
    The ISM band is 2.4-2.5 GHz. Voyager at 2.3Ghz is outside the band.
drmpeg 2y ago
WiFi is at 2.4 GHz. LTE band 30, satellite radio (XM/Sirius) and aeronautical telemetry all exist between the deep space downlink at 2290 to 2300 MHz and WiFi at 2400 MHz.
noneeeed 2y ago
Nope. It's one of those things that can take a bit to get used to, but everything on the electromagnetic spectrum is just light in the general sense. The only difference between radio-waves, x-rays, infra-red and (human) visible light is the frequency/wavelength. If the frequency is high enough then the waves of light can be detected by things as small as cells in the back of your eye, or the pixels in a camera sensor. If it is too low then you need much larger detectors. Other animals have detectors for different frequencies/wavelengths, allowing them to see either infra-red (mosquitos) or ultraviolet (bees, butterflies etc). What we call "visible light" is just the particular range that our eyes can detect (about 400 to 800THz). If we were the size of a planet, and our eye cells were the size of a radio-telescope dish we would be able to "see" in those wavelengths. In fact, when we see images taken by radio telescopes, those have been essentially pitch-shifted up to something we can see, like the reverse of what we do when listening for bat clicks (where the pitch is downshifted to our hearing range). The wikipedia article has a nice little diagram putting the wavelengths into perspective. https://en.wikipedia.org/wiki/Electromagnetic_spectrum https://en.wikipedia.org/wiki/Electromagnetic_spectrum

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moffkalast 2y ago

You know, I never really thought of lower wavelengths than light as being carried by photons, but I suppose it's all EM. Antennas are technically just really red light bulbs.

gjstein 2y ago
This is true enough, though remember that material properties change dramatically when you start moving through wavelengths by orders of magnitude. Silicon is transparent in the mid-infrared, which is what makes silicon photonics possible [1] [1] https://en.wikipedia.org/wiki/Silicon_photonics https://en.wikipedia.org/wiki/Silicon_photonics

bandrami 2y ago

It's crazy to me how many theoretical limits Shannon predicted way before the hardware was there.

ziofill 2y ago
That’s because his results are about pure information (and in the limit for infinite string lengths), so sooner or later some hardware will hit onto those limits or tend to them.
- bandrami 2y ago
  Agreed, but: he's still understudied. I think in retrospect any 21st-century math course has to include Shannon, and they don't all, yet.
  - sebzim4500 2y ago
    On some level he's a victim of his own success. He invents information theory in the same paper that proves the most interesting results, so who else will work on it?
    - aidenn0 2y ago
      Even the practical work was done surprisingly early. I have a book on error correcting codes from the 1950s and it's missing very little (Most notably trellis codes and LDPC; the former being invented in the '70s and the latter in 1963).
      - sebzim4500 2y ago
        There's been much more progress on compression though (Arithmetic Coding, ANS, etc.)
    - moffkalast 2y ago
      Shannon: "I'm about to make and end this field's whole career."

Strilanc 2y ago

Wasn't expecting my question to hit top of HN. I guess I'll give some context for why I asked it. I work in quantum error correction, and was trying to collect interesting and quantitative examples of repetition codes being used implicitly in classical systems. Stuff like DRAM storing a 0 or 1 via the presence or absence of 40K electrons [1], undersea cables sending X photons per bit (don't know that one yet), some kind of number for a transistor switching (haven't even decided on the number for that one yet), etc. A key reason quantum computing is so hard is that by default repetition makes things worse instead of better, because every repetition is another chance for an unintended measurement. So protecting a qubit tends to require special physical properties, like the energy gap of a superconductor, or complex error correction strategies like surface codes. A surface code can easily use 1000 physical qubits to store 1 logical qubit [2], and I wanted to contrast that with the sizes of implicit repetition codes used in classical computing. 1: https://web.mit.edu/rec/www/dramfaq/DRAMFAQ.html https://web.mit.edu/rec/www/dramfaq/DRAMFAQ.html 2: https://arxiv.org/abs/1208.0928 https://arxiv.org/abs/1208.0928

nico 2y ago
Very cool. It’s interesting to realize that at some level, every system is a quantum system if you “zoom in” enough
- Ringz 2y ago
  I would spontaneously respond that you are right and at the same time have no problem if someone explains to me that it is not so.
- empyrrhicist 2y ago
  I think the point is the model though - if a system's behavior can be modeled/described classically, it's a bit silly to to call it a "quantum" system in the same way that it's reductive to say Biology is just applied particle physics. Sure, but that's not a very useful level of abstraction.
  - jessriedel 2y ago

cycomanic 2y ago

Actually the limit predicted by Shannon can be significantly beaten, because Shannon assumes gaussian noise, but if we use photon counting receivers we need to use a poisson distribution. This is the Gordon-Holevo limit. To beat Shannon you need PPM formats and photon counters (single photon detectors). One can do significantly better than the numbers from voyager in the article using optics even without photon cpunting. Our group has shown 1 photon/bit at 10 Gbit/s [1] but others have shown even higher sensitivity (albeit at much lower data rates). [1] https://www.nature.com/articles/s41377-020-00389-2 https://www.nature.com/articles/s41377-020-00389-2

nico 2y ago
Interesting. Is that related to compressed sensing? I wonder if compress sensing could be used for something like the Voyager signals It seems there might be multiple ways to go beyond Shannon’s limit, depending on what you are trying to do
- cycomanic 2y ago
  I don't think compressed sensing is really extracting more information than Shannon, it simply exploits the fact that the signal we are interested in is sparse so we don't need to sample "everything". But this is somewhat outside my area of expertise so my understanding could be wrong.
  - nico 2y ago
    Maybe I’m mixing Shannon’s limit with the sampling rate imposed by the Nyquist-Shannon Sampling theorem > Around 2004, Emmanuel Candès, Justin Romberg, Terence Tao, and David Donoho proved that given knowledge about a signal's sparsity, the signal may be reconstructed with even fewer samples than the sampling theorem requires.[4][5] This idea is the basis of compressed sensing … > However, if further restrictions are imposed on the signal, then the Nyquist criterion may no longer be a necessary condition. A non-trivial example of exploiting extra assumptions about the signal is given by the recent field of compressed sensing, which allows for full reconstruction with a sub-Nyquist sampling rate. Specifically, this applies to signals that are sparse (or compressible) in some domain From: https://en.m.wikipedia.org/wiki/Nyquist%E2%80%93Shannon_sampling_theorem https://en.m.wikipedia.org/wiki/Nyquist%E2%80%93Shannon_samp...

ziofill 2y ago

What a lovely question. The estimate is 10-100 photons/bit (minimum). If you’re curious about how many bits a single photon can carry, in controlled settings (tabletop quantum optics) a single photon can carry log(n) bits where n is the size of the state space of the photon, which theoretically is infinite and in practice it can reach into the hundreds/thousands.

layer8 2y ago
No, the estimate is around 750 or 200 photons/bit received, depending on the transmission frequency. The answer to the question is B, not C. Your numbers are the estimated minimum needed, not the actual amount received, which is what the question was asking.
mnw21cam 2y ago
Visible light is different, because each photon has a lot more energy than in the 2.3GHz range. Your average decent consumer-level camera has a sensor that can nominally just about detect single photons some proportion of the time (as in, some of them bounce off instead of being detected) though it can't technically count them. The graininess on digital camera images is more from the Poisson noise of the incoming photons than it is from the applied noise of the sensor itself.

HarHarVeryFunny 2y ago

The fact that we can communicate with Voyager, and in both directions, blows my mind. It's completely counter-intuitive. At least for Voyager->earth we can use giant radio telescopes to detect the faint signal, but how do we manage to focus on those few hundreds of photons per bit coming from a pinpoint source a light day away?! In the earth->Voyager direction it seems even less intuitive - sure we can broadcast a powerful signal, but it's being received by a 12' wide antenna 15 billion miles away. WTF? I guess radio communications in general is magic, a bit like (in nature of counter-intuition) quantum entanglement of particles arbitrarily far apart. It seems there is something deeply wrong about our mental models of space and time.

philipwhiuk 2y ago
Per the article we lose 99.9% of the photons sent. The antenna is as directional as possible but with a similar dish you could pick up the signal on the Moon at this point I expect (probably a smaller dish given the lack of atmospheric noise) Equally we're broadcasting to the area of space Voyager's in. We're not able to to target it to the dish - 12' isn't the DSOC accuracy - it's the size required to pick up enough data given the signal diffusion.

cycomanic 2y ago

For anyone who is interested in the ultimate limits to communications the seminal paper by Jim Gordon is quite easy to understand even without a physics degree (unlike the Holevo paper IMO). He was incredibly good at writing in an accessible manner (apart from probably being the person who most deserved a Nobel prize but didn't get it). https://doi.org/10.1109%2FJRPROC.1962.288169 https://doi.org/10.1109%2FJRPROC.1962.288169

prof-dr-ir 2y ago
> probably being the person who most deserved a Nobel prize but didn't get it You probably want to read up a bit on the remarkable life of Lise Meitner.

bebopalula 2y ago

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magnoliakobus 2y ago
Bad bot!

superposeur 2y ago

The overwhelming loss in this calculation is from the antenna’s radiated energy spreading out over a larger and larger area (despite the directional “gain” factor). I’m wondering: would a probe launched today instead employ a laser to communicate? This would seem to offer many orders of magnitude improvement in the directionality of the signal.

deelowe 2y ago
I imagine it'd certainly employ some type of beamforming at the least.
- wongarsu 2y ago
  Assuming you don't need fast steering, is a 3.7m transmitter array doing beamforming really better than a 3.7m dish transmitting at the same power? My intuition would have been that you are better off using a fairly standard transceiver and spending your engineering budget either increasing power or getting a bigger dish (either by launching on a wider rocket or with a folding design). Lasers might interesting for the downlink, but receiving a laser signal on the probe sounds difficult (earth is pretty bright).
  - outworlder 2y ago
    There's some value in getting rid of mechanical devices (or reducing the need to rotate the entire spacecraft).
  - cycomanic 2y ago
    Diffraction scales inversely proportional to wavelength so you gain significantly by going to optics, i.e. you can use a much smaller aperature in optics.
  - CamperBob2 2y ago
    The array doing beamforming can be spread out much farther. If the DSN were being built now, I'd think its antennas would look more like the Square Kilometer Array.

hammock 2y ago

>Voyager sends 160 bits/second This makes me wonder, are the bits = the power turned on for exactly 1/320th sec, every 1/160th sec? Or is the power on/power off ratio something different? Does it vary by protocol? What are the pros and cons?

danbruc 2y ago
Without looking up what kind of encoding and modulation they are using, I would assume that they are sending a continuous sine wave at the carrier frequency that has the bits - probably after encoding the raw data bits with some error correction code - modulated onto it by changing frequency, amplitude, phase, or a combination of them depending on the value of each bit or group of bits.

ks2048 2y ago

Nice question. Does anyone know what exactly data is being sent? What kind of compression it is using? etc

mooktakim 2y ago

Why didn't they send out new relays as Voyager travelled out.

philipwhiuk 2y ago
Voyager's path required multiple flybys. You can't just send something on the same path later on - everything has moved. Plus, no budget for relays.

somat 2y ago

An interesting thing about photons (which may not be true, I just enjoy this stuff amateurishly, that is, without the effort or rigor to actually understand it.) is that they might not exist. the em field is not quantized, or at least is not quantized at the level of photons. A "photon" only exists where the em field interacts with matter, where the electrons that create the disturbance can only pulse in discrete levels. https://www.youtube.com/watch?v=ExhSqq1jysg https://www.youtube.com/watch?v=ExhSqq1jysg Not that this changes anything, we can only detect or create light with matter. but it does make me curious about single photon experiments and what they are actually measuring.

leetrout 2y ago
Thanks for the link. I never conceptualized photons outside of the visual spectrum so the headline made me take a step back and get nerd sniped in the process. I stumbled upon this before seeing your comment: https://physics.stackexchange.com/questions/90646/what-is-the-relation-between-electromagnetic-wave-and-photon https://physics.stackexchange.com/questions/90646/what-is-th...
golergka 2y ago
Isn't that simply the principle of particle-wave duality? When particle/wave in field X interacts with field X, it behaves like a wave, but interactions with other fields are quantised.
mr_toad 2y ago
Existence is a difficult concept for something moving at the speed of light. Light follows a null geodesic through space-time with zero (space-time) length and no proper time. Past, future, and causality have no meaning to a photon. We think of photons travelling through space because our symmetry is broken, we have mass, and we and experience time and space. Observers like us will see light follow the same world-line from its source to its target. It cannot interact with anything else, and some might say it was only ever emitted so that it could interact with its target. So from a certain point of view the “existence” of a photon is entirely bound up in its interaction with source and target, and it’s not really useful to speak of it in other terms. The quantised interaction is the photon.

croemer 2y ago

That's why I love Physics and was enamoured with it in my late teens.

wwarner 2y ago

23 watts.

tb0ne 2y ago

Super interesting! But I feel like there is a bit of a conclusion missing for me. So 1500 Photons hit the receiver per bit send, but this is obviously way to few to keep processing the signal and it will just be drowned out by noise? Where do we go from here? Does voyager repeat its signal gazillions of times so we can average out the noise on our end? Where can I find more information on what is done with these few photons?

SonOfLilit 2y ago
No, those 1500 photons are enough and we basically read the signal from them, from my reading of the comments here.
djha-skin 2y ago
Look up Solomon Reed erasure coding. Voyager uses it, so does qrp Ham radio operators.
- ballooney 2y ago
  [Reed-Solomon is the more conventional formulation if you do end up going looking]

PeterCorless 2y ago

I just wanted to chime in with a reminder that though Voyager 1 is speeding away from Sol at a constant velocity because of the Earth's revolution around the sun it can be up to ±1 AU closer or further away, depending on the time of the year. This article is for Voyager 2, but the issue is the same. For a brief moment every year we actually get closer to Voyager 1, then we pivot away in our revolution around the sun and the distance between Earth and Voyager 1 or 2 increases sharply. So distance, when plotted over time, looks like a wobbly line. https://earthsky.org/space/voyager-spacecraft-getting-closer-to-earth/ https://earthsky.org/space/voyager-spacecraft-getting-closer...

olddustytrail 2y ago
Surely 2AU depending on whether we're on the same side of the sun or we're on the dark side.

rocho 2y ago

Wow, I never thought about how Voyager communicates with Earth. But now I wonder: if Voyager just sends photons towards the Earth, at the receiving end how are we recognizing which photons are coming from Voyager and how is the "signal" decoded?

RachelF 2y ago
Two main reasons for recognizing the photons: They have a specific frequency, 8.3GHz in this example. It's like tuning an FM radio to a station. The photons are coming from a specific direction. As to how they are decoded, you'll need to understand some modulation techniques.

RachelF 2y ago

What is equally impressive is the number of photons received from radar imaging of asteroids. They are closer, but the radar equation received power is inversely proportional to range to the fourth power, not range squared as with Voyager. Anything proportional to 1/R^4 degrades very quickly.

aaron695 2y ago

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omgJustTest 2y ago

25 ph is like massive SNR! Let's talk about 30ps timing and 3 photons & you have insight into my daily work!

ggm 2y ago

Could we e.g. decrease the bit rate, and in effect send less information but for longer? Eg send a continuous wave with low signal beyond its phase, and measure at a rate of (digital) bits per month?

asdfman123 2y ago

I was trying to answer a question about the Voyager space probe and got our of my depth, BUT I found this great PDF in the process. Covers a lot of technical details at the level a person without much specific training can understand. https://voyager.gsfc.nasa.gov/Library/DeepCommo_Chapter3--141029.pdf https://voyager.gsfc.nasa.gov/Library/DeepCommo_Chapter3--14...

basil-rash 2y ago

For the spectrum in question: FCC’s “US258 G117”, 8.4-8.45GHz reserved for “Space Research (deep space)(space-to-earth)”. Pretty neat. https://www.ecfr.gov/current/title-47/chapter-I/subchapter-A/part-2/subpart-B/section-2.106 https://www.ecfr.gov/current/title-47/chapter-I/subchapter-A...

throwaway81523 2y ago

That SE post doesn't mention the code rate of whatever ECC code Voyager uses.

foxbeneficial 2y ago

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bryangreen 2y ago

Sincere question: how does a radio signal manifest as a photon?