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

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

#111

Post by resters »

ganzuul wrote: post #108
resters wrote: post #100

Earlier quoted context omitted.

this is called the Shannon limit. To discern signal from noise, a minimum sample rate of 2x the frequency of the signal is required. A signal is something that can be turned on or off to send a bit. Higher frequencies can carry more data as you infer but the engineering challenges of designing transmitters and receivers create tradeoffs in practical systems.

In addition to wavelength EM also has several polarization modes and near/far field characteristics that can carry information.

Can individual photons be measured for polarization and phase or is there a similar limit that requires more than one photon to do so? I suppose both are relative to some previous polarization or phase?

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

#113

Post by jessriedel »

empyrrhicist wrote: post #58
nico wrote: post #46

Earlier quoted context omitted.

Very cool. It’s interesting to realize that at some level, every system is a quantum system if you “zoom in” enough

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.

If you want to understand the transition between a fundamental theory and its effective description in some limiting regime, you need to be able to describe a system in the limiting regime using the fundamental theory. It's not "silly" to talk about an atom having a gravitational field even if it's unmeasurably small (currently).

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

#114

Post by richarme »

stracer wrote: post #94
aptitude_moo wrote: post #64

Earlier quoted context omitted.

Very interesting, I studied telecommunications and I thought the Shannon limit was the absolute limit. I wonder now if this Gordon Holevo limit is applicable for "traditional" telecommunications (like 5G) as opposed to photon counting a deep space probe EDIT: This paper seems to answer my question [1] [1] https://opg.optica.org/directpdfaccess/8711ab35-bbc2-4d51-8e...

Can you please post another link? This one does not work.

I think it's this one:

https://opg.optica.org/jlt/viewmedia.cfm?uri=jlt-38-10-2741&...

Quantum Limits in Optical Communications

Konrad Banaszek, Ludwig Kunz, Michał Jachura, and Marcin Jarzyna

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

#115

Post by leetrout »

somat wrote: post #99

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 lev…

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-th...

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

#116

Post by Strilanc »

cycomanic wrote: post #95
Strilanc wrote: post #61

Earlier quoted context omitted.

I think you're picturing a different level of the network stack than I had in mind. Yes, above the physical level they will be explicitly using very sophisticated codes. But I think physically it is the case that messages are transmitted using pulses of photons, where a pulse will contain many photons and will lose ~5% of its photons per kilometer when travelling through fiber (which is why amplifiers are needed alon…

But we are classical, so I think it's wrong (or at least confusing) to talk about the many photons as repetition codes. Then we might as well start to call all classical phenomena repetition codes. Also how would you define SNR when doing this? Repetition codes have a very clearly defined meaning in communication theory, using them to mean something else is very confusing.

Yeah, I agree it's unusual to describe "increased brightness" as "bigger distance repetition code". But I think it'll be a useful analogy in context, and I'd of course explain that.

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

#117

Post by SJC_Hacker »

pcdoodle wrote: post #36
noneeeed wrote: post #29

Earlier quoted context omitted.

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 senso…

Thanks for the reply. This makes me think of the dual slit experiment. Does the universe treat everything as a wave to save CPU cycles or something? If we think of light as little balls (at our size i think that would make sense). If we were much bigger, we would think of these longer waves as balls too?

Wave-particle duality/quantum mechanics have different interpretations, but since "the math works" and there's nothing better, thats what they go with.

One way of thinking of it - everything is a wave until you make a measurement. Then you get a "collapse" (localization) of the wavefunction. Which leads to the question of why the wave function collapses - i.e. how does nature know we want to make a measurement.

Which leads to all sorts of crazy ideas like the simulation hypothesis. Which is not a scientific theory, because you can't falsify it, but even very educated people like Neil DeGrasse Tyson have remarked on it.

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

#118

Post by Sniffnoy »

grog454 wrote: post #50
Strilanc wrote: post #41

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 k…

> by default repetition makes things worse instead of better Can you elaborate on this a bit? My intuition is that, by default, statistical models benefit from larger N. But I have no experience in quantum physics.

You might be making the mistake of thinking that quantum mechanics runs on probabilities, which work in the way you are used to, when in fact it runs on amplitudes, which work quite differently.

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

#119

Post by tb0ne »

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?

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

#120

Post by outworlder »

wongarsu wrote: post #74
deelowe wrote: post #69

Earlier quoted context omitted.

I imagine it'd certainly employ some type of beamforming at the least.

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 th…

There's some value in getting rid of mechanical devices (or reducing the need to rotate the entire spacecraft).
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