地球必須發出多少電磁傳輸噪聲才能引人注目? - How much electromagnetic transmission noise does Earth have to emit to be noticeable? -开发者知识库

地球必須發出多少電磁傳輸噪聲才能引人注目? - How much electromagnetic transmission noise does Earth have to emit to be noticeable? -开发者知识库,第1张

From time to time I'll see things talking about how thanks to all of our radio communications, we're effectively broadcasting our existence to the entire universe. However, when I did some back-of-the-envelope calculations a while ago I found something interesting - by the time a radio station's broadcast reaches Alpha Centauri (the solar system closest to ours), ignoring any effects of our atmosphere, a 1 m^2 receiver would, on average, receive a single photon every seven hours. Of course that is only a single radio station, but throwing more radio stations into the mix would just make the signal less coherent and indistinguishable from random noise. When you consider the sun as well, it seems like all of the Earth's radio signals would be a drop in the bucket.

我會不時地看到事情在談論如何感謝我們所有的無線電通信,我們有效地將我們的存在傳播到整個宇宙。然而,當我不久前做了一些背后的計算時,我發現了一些有趣的東西 - 當廣播電台的廣播到達半人馬座阿爾法星(距離我們最近的太陽系)時,忽略了我們大氣的任何影響,1 m ^ 2接收器平均每七小時接收一個光子。當然,這只是一個單一的無線電台,但是在混合中投入更多的無線電台只會使信號不那么連貫並且與隨機噪聲無法區分。當你考慮太陽時,看起來所有地球的無線電信號都會成為一滴水。

How much noise would the Earth have to broadcast in order to change that? For this question, the goal is for the noise broadcast from Earth to still be noticeable by the time it reaches a distance of 500 light years (that only covers about 0.01% of the galaxy). You can assume that alien civilizations are looking for signals of intelligent life, but not that they are focused on our solar system.

為了改變這種情況,地球必須播出多少噪音?對於這個問題,我們的目標是,當它達到500光年的距離時(僅覆蓋大約0.01%的星系),來自地球的噪聲仍然可以被注意到。你可以假設外星文明正在尋找智能生命的信號,但並不是說他們專注於我們的太陽系。

1 个解决方案

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First off, this question is just the inverse of this question. So, apologizes to MichaelKjorling, I'm going to crib his answer to get some results.

首先,這個問題只是這個問題的反面。因此,向MichaelKjorling道歉,我將試着給出他的答案以獲得一些結果。

Assumptions

  • In order to be noticeable at 500 light years, we need to be able to detect us at that distance.
  • 為了在500光年處注意到,我們需要能夠在那個距離探測到我們。

What is the attenuation over 500 light years?

Free space path loss in decibels (dB) is given by

以分貝(dB)表示的自由空間路徑損耗由下式給出

$$20\log_{10}\frac{4\pi d}{\lambda}$$ where $d$ is the distance between antenna and $\lambda$ is the wavelength of the frequency in question.

$$ 20 \ log_ {10} \ frac {4 \ pi d} {\ lambda} $$其中$ d $是天線與$ \ lambda $之間的距離,是所討論頻率的波長。

In the US, the highest energy general transmission (as far as I can tell) are UHF stations in the 512-608 MHz range. This may be an important thing to revisit; emission into space is determined heavily by directional broadcasting characteristics. But we'll just use this assumption for now and say that the wavelength is 0.5 meters. Meanwhile, 500 light years is $4.7\times10^{18}$ meters.

在美國,能量最高的通用傳輸(據我所知)是512-608 MHz范圍內的UHF站。重訪這可能是一件重要的事情;通過定向廣播特性嚴重決定了向太空的發射。但我們現在只使用這個假設,並說波長為0.5米。同時,500光年是$ 4.7 \ times10 ^ {18} $米。

Plugging into the equation above, free space path loss is 401 decibels. That is a lot! This means power is $10^{40}$ lower at the receiver than at the emitter.

插入上面的等式,自由空間路徑損失是401分貝。那是很多!這意味着接收器的功率比發射器低10 ^ {40} $。

What is the most sensitive receiver that we have?

First, since radio transmission terminology is not generally well known, a brief discussion. In order to receive a signal, you need to be able to pick it out of background noise. That background noise includes both background noises from deep space, and interference from local signals. For example, if our UHF transmissions at 550 MHz happen to line up with the most common radio frequency on a distant planet looking for us, in-atmosphere recievers are going to have a tough time finding us.

首先,由於無線電傳輸術語一般不為人所知,因此進行簡短的討論。為了接收信號,您需要能夠從背景噪聲中選擇它。背景噪聲包括來自深空的背景噪聲和來自本地信號的干擾。例如,如果我們在550 MHz的UHF傳輸恰好與尋找我們的遙遠星球上最常見的無線電頻率對齊,那么大氣層內的接收器將很難找到我們。

The unit of measure for the signal here is dBm, or decibel-milliwatts. If $P$ is the power of a signal in milliwatts, then the signal strength is $$10\log_{10}(P).$$ Thus, a 1 W signal is 1000 mW or 30 dBm. The gain of an antenna is measured in terms of dBi. This measure uses the same log scale that the signal strength does, so we can simply add the measures to together to calculate total signal strength.

此處信號的測量單位是dBm或分貝 - 毫瓦。如果$ P $是以毫瓦為單位的信號功率,那么信號強度為$ 10 \ log_ {10}(P)。$$因此,1 W信號為1000 mW或30 dBm。天線的增益以dBi為單位測量。此度量使用與信號強度相同的對數刻度,因此我們可以簡單地將度量添加到一起以計算總信號強度。

The rating of a receiver is the smallest signal that it can pull out of the noise background. Following MichaelKjorling's answer, assume that an alien civilization specifically searching for us can detect the signal at -200 dBm. In addition, assume a 80 dBi antenna gain from a dish like Arecibo.

接收器的額定值是它可以從噪聲背景中拉出的最小信號。根據MichaelKjorling的回答,假設一個專門搜索我們的外星文明可以檢測到-200 dBm的信號。此外,假設像Arecibo這樣的碟形天線增益為80 dBi。

How much noise do we have to put out?

Free space path loss is -400 dB; antenna gain at a foreign civilization is 80 dB and the signal detection threshold is -200 dB. Therefore, signal emission strength must be 120 dBm at the source to make it detectable. This is equivalent to $10^{12}$ mW or 1 GW of emission power.

自由空間路徑損耗為-400 dB;外國文明的天線增益為80 dB,信號檢測門限為-200 dB。因此,信號發射強度必須在源處為120 dBm才能使其可檢測。這相當於$ 10 ^ {12} $ mW或1 GW的發射功率。

Conclusions

The Earth is almost certainly not gong to reach this power level. WBCT is the most powerful FM radio station in the US, with effective radiated power of 320kW, or about 85 dB. But this dish is not pointed into space, and so its emissions are further attenuated by the atmosphere before reaching into deep space. There is really no reason to increase the power of such a dish, becase it already has plenty of power to reach the horizon. Any direct signal's line of sight is limited by the horizon; once past the horizon the signal no longer reaches the surface and coverage moves up into the atmosphere, where it isn't really useful.

地球幾乎肯定不會達到這個功率水平。 WBCT是美國最強大的調頻廣播電台,有效輻射功率為320kW,約為85 dB。但是這道菜沒有指向太空,所以它的排放物在進入深空之前會被大氣進一步衰減。沒有理由增加這種菜的力量,因為它已經有足夠的力量到達地平線。任何直接信號的視線都受到地平線的限制;一旦超過地平線,信號就不再到達地表,覆蓋范圍向上移動到大氣層,在那里它並沒有真正有用。

Furthermore, if you start increasing the emission power of lots of different radio communications methods, then they will start interfering with each other. A 120 dB signal emerging from the earth's atmosphere will bounce of the moon and back to Earth at a much higher power than it will reach 500 light years away.

此外,如果您開始增加許多不同無線電通信方法的發射功率,那么它們將開始相互干擾。從地球大氣中出現的120 dB信號將以比距離500光年遠的更高的功率反射到月球並返回地球。

So, all in all, the Earth is unlikely to ever emit enough noise to reach a planet 500 ly away. Now, a directed signal is another story...

所以,總而言之,地球不太可能發出足夠的噪音來到達500離開的行星。現在,定向信號是另一個故事......

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