Hacker News Board Hacker News, browsed like a classic message board
Live data from Hacker News

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

physics.stackexchange.com

Post reply on HN
Print view View original

81–90 of 205 posts

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

#81

Post by superposeur »

Out_of_Characte wrote: post #72
superposeur wrote: post #65

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.

There's several projects for laser-based communication and research. It would also make it really difficult to aim since you can miss your target now. https://www.jpl.nasa.gov/news/nasas-deep-space-optical-comm-... https://en.m.wikipedia.org/wiki/Laser_Interferometer_Space_A...

Interesting about the JPL program and I’m amazed this prototype was only launched last year! Apparently the answer to comms laser use is “not yet but soon”.

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

#82

Post by superposeur »

londons_explore wrote: post #71
superposeur wrote: post #65

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.

The main challenge is the earth to probe comms for distant probes, since the earth is often very close (in an angular sense) to the sun from the probes perspective, and the sun gives out a lot of black body radiation. However, due to the shape of the black body radiation curve, the sun gives out relatively less microwave radiation than it does visible light, which might outweigh the advantages of more directionality…

Ok what about using a maser instead of a laser?

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

#83

Post by sebzim4500 »

londons_explore wrote: post #71
superposeur wrote: post #65

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.

The main challenge is the earth to probe comms for distant probes, since the earth is often very close (in an angular sense) to the sun from the probes perspective, and the sun gives out a lot of black body radiation. However, due to the shape of the black body radiation curve, the sun gives out relatively less microwave radiation than it does visible light, which might outweigh the advantages of more directionality…

Presumably though it would be useful to have a high bandwidth link back to earth even if we had to use conventional microwave transmitters to send data back.

We want to download high resolution images/spectrographs whereas we only want to upload code/instructions.

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

#84

Post by sebzim4500 »

bandrami wrote: post #47
ziofill wrote: post #37

Earlier quoted context omitted.

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.

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.

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?

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

#85

Post by ooterness »

magicalhippo wrote: post #35
jsjohnst wrote: post #4

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?

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…

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.

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

#86

Post by aidenn0 »

sebzim4500 wrote: post #84
bandrami wrote: post #47

Earlier quoted context omitted.

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.

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?

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).

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

#87

Post by Kerbonut »

Strilanc wrote: post #67
grog454 wrote: post #50

Earlier quoted context omitted.

> 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.

It's because unintended measurement is a type of error in a quantum computer. Like, if an electron passing near your qubit would get pushed left if your qubit was 0 and right if was 1, then you will see errors when electrons pass by. Repeating the 0 or 1 a thousand times just means there's 1000x more places that electrons passing by would cause a problem. That kind of redundancy makes that kind of error mechanism wor…

If there's interference, could you do something like when using 7 repetition for each bit, take whatever 5 of 7 is, e.g. 1111100 is 1 and 1100000 is 0.

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

#89

Post by stracer »

cycomanic wrote: post #43

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 cpunt…

Is there some fundamental limit to the number of bits per photon that can be communicated via EM radiation? I think it does not exist, because photons aren't all equal, we can use very high frequency and X-ray quantum can probably carry much more information than RF quantum.

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

#90

Post by Sanzig »

ramraj07 wrote: post #51
cycomanic wrote: post #43

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 cpunt…

Can’t you calculate the CRLB for any given distribution if you wanted? That’s what my lab did for microscopy anyway. Saying you’re beating the Shannon limit is like saying you’re beating the second law of thermodynamics to me.. but I could be wrong.

You are correct. People often say "Shannon limit" (the general case) when they are really referring to the "Shannon-Hartley Limit" (the simplified case of an additive white Gaussian noise channel).

For example, MIMO appears to "break" the Shannon-Hartley limit because it does exceed the theoretical AWGN capacity for a simple channel. However, when you apply Shannon's theory to reformulate the problem for the case of a multipath channel with defined mutual coupling, you find that there is a higher limit you are still bounded by.

Post reply on HN