4 ms·

Something like FHSS (https://en.wikipedia.org/wiki/Frequency-hopping_spread_spectrum https://en.wikipedia.org/wiki/Frequency-hopping_spread_spect...), not only

Something like FHSS (https://en.wikipedia.org/wiki/Frequency-hopping_spread_spectrum https://en.wikipedia.org/wiki/Frequency-hopping_spread_spect...), not only a low level of signal is spread throughout a board frequency range, the central frequency is also changing many times per seconds, synchronization and decoding is only possible if the CSPRNG key is known. The signal of interplanetary communication is also highly directional. I guess an alien civilization has much better ideas to avoid detection.

garmaine 8y ago
There's a difference between successful real-time decoding, and just determining from a full-spectrum recording whether it is the case that a signal exists, even if its behavior is unpredictable.
- pps43 8y ago
  The point of spread-spectrum signal is that it's indistinguishable from noise unless you know the key. With frequency-hopping you still have spikes in the spectrum around the frequencies you hop between, but with most modern systems you no longer have anything distinctive to find.
  - garmaine 8y ago
    It's not indistinguishable from noise, if it is powerful enough to be received at all. If you filter based on the background noise threshold, you will discover a signal that quickly jumps frequencies in a pattern that is unpredictable without knowing the secret key. If frequency hopping is all that is used, you could even recombine the pieces to put the original message back together again. But in any case it will be very plainly obvious that the signal exists. A radio with a hardware limitation of only tuning to a specific frequency at a time would be unable to pick up on it, and would only hear transient blips as the signal enters and leaves the channel, which is what people mean when they say spread spectrum "looks like noise". You could defeat this analysis by actually broadcasting noise on all frequencies, so the transmission frequency is indistinguishable, assuming the encryption used is also indistinguishable from a random oracle. But now you have a noisy point source in the sky, even if you can't decipher the contents of the message. Succinctly put, obfuscation techniques like frequency hopping spread spectrum are tactical techniques that try to hide transmissions from actual radios in widespread use, not hide the existence of the radio signal in some physical sense. FHSS radio transmissions look like noise sources, not silence. Which is basically what SETI is looking for -- sources of radio noise unexplained by our understanding of astronomy.
    - pps43 8y ago
      It is possible for the signal to be below the background noise. For example, GPS works this way. Sonars on submarines do the same thing, they send out a noise-like signal that can be detected only if you know what to correlate it with. There's always some noise to hide the signal in.
      - garmaine 8y ago
        This isn’t true. If the signal was below the noise threshold, and no mechanism provided for detecting noise separately (as twisted pair provides, for example) it would be literally impossible to separate signal from noise. That’s what the noise threshold means. GPS is not below the level of background noise. Not is sonar.
mikx007 8y ago
what if there exist ultimate limit to knowledge itself and laws of universe that can be discovered? it would imply that any civilization that reaches that point will have nothing to gain from communication with other civilizations.
- zaarn 8y ago
  How would you know that you have reached the limit of knowledge?
  - chimprich 8y ago
    You probably wouldn't be able to for sure but if you had no more unexplained phenomena around then you'd probably have a reasonable idea. Of course that only applies to the laws of physics. You'd still have an effectively infinite universe of maths, geography, biology, etc. to explore.
    - zaarn 8y ago
      You would still not know. Maybe there is a branch of physics that you're simply biologically not equipped to even perceive, comprehend or understand in the slightest. Maybe not. The point is that you can't know. Not too long back, people thought gravity as entirely newtonian and couldn't imagine that gravity could behave like waves. Or that particles can exist as both particle and wave. IMO the limit of knowledge is simply the limit of information you can cram into a region of space without it collapsing into a blackhole because of that.
  - kamaal 8y ago
    No matter how hard you try, you cannot teach a mouse to learn french. That is why.
    - zaarn 8y ago
      That's more a reason why knowledge is unlimited; a mouse is incapable of understanding french, but knowledge of french does and can exist, so the mouse doesn't know everything. There is a biological but not inherent limit.

garmaine 8y ago

There's a difference between successful real-time decoding, and just determining from a full-spectrum recording whether it is the case that a signal exists, even if its behavior is unpredictable.

pps43 8y ago
The point of spread-spectrum signal is that it's indistinguishable from noise unless you know the key. With frequency-hopping you still have spikes in the spectrum around the frequencies you hop between, but with most modern systems you no longer have anything distinctive to find.
- garmaine 8y ago
  It's not indistinguishable from noise, if it is powerful enough to be received at all. If you filter based on the background noise threshold, you will discover a signal that quickly jumps frequencies in a pattern that is unpredictable without knowing the secret key. If frequency hopping is all that is used, you could even recombine the pieces to put the original message back together again. But in any case it will be very plainly obvious that the signal exists. A radio with a hardware limitation of only tuning to a specific frequency at a time would be unable to pick up on it, and would only hear transient blips as the signal enters and leaves the channel, which is what people mean when they say spread spectrum "looks like noise". You could defeat this analysis by actually broadcasting noise on all frequencies, so the transmission frequency is indistinguishable, assuming the encryption used is also indistinguishable from a random oracle. But now you have a noisy point source in the sky, even if you can't decipher the contents of the message. Succinctly put, obfuscation techniques like frequency hopping spread spectrum are tactical techniques that try to hide transmissions from actual radios in widespread use, not hide the existence of the radio signal in some physical sense. FHSS radio transmissions look like noise sources, not silence. Which is basically what SETI is looking for -- sources of radio noise unexplained by our understanding of astronomy.
  - pps43 8y ago
    It is possible for the signal to be below the background noise. For example, GPS works this way. Sonars on submarines do the same thing, they send out a noise-like signal that can be detected only if you know what to correlate it with. There's always some noise to hide the signal in.
    - garmaine 8y ago
      This isn’t true. If the signal was below the noise threshold, and no mechanism provided for detecting noise separately (as twisted pair provides, for example) it would be literally impossible to separate signal from noise. That’s what the noise threshold means. GPS is not below the level of background noise. Not is sonar.

mikx007 8y ago

what if there exist ultimate limit to knowledge itself and laws of universe that can be discovered? it would imply that any civilization that reaches that point will have nothing to gain from communication with other civilizations.

zaarn 8y ago
How would you know that you have reached the limit of knowledge?
- chimprich 8y ago
  You probably wouldn't be able to for sure but if you had no more unexplained phenomena around then you'd probably have a reasonable idea. Of course that only applies to the laws of physics. You'd still have an effectively infinite universe of maths, geography, biology, etc. to explore.
  - zaarn 8y ago
    You would still not know. Maybe there is a branch of physics that you're simply biologically not equipped to even perceive, comprehend or understand in the slightest. Maybe not. The point is that you can't know. Not too long back, people thought gravity as entirely newtonian and couldn't imagine that gravity could behave like waves. Or that particles can exist as both particle and wave. IMO the limit of knowledge is simply the limit of information you can cram into a region of space without it collapsing into a blackhole because of that.
- kamaal 8y ago
  No matter how hard you try, you cannot teach a mouse to learn french. That is why.
  - zaarn 8y ago
    That's more a reason why knowledge is unlimited; a mouse is incapable of understanding french, but knowledge of french does and can exist, so the mouse doesn't know everything. There is a biological but not inherent limit.