How do we know that the moon is moving away? | BBC

You may be one of those who, when they hear news, ask themselves, "How will they know that?"

We were told that the moon is moving away from the Earth. And it's not that we doubt it, but how do you know it?

Scientists Adam Rutherford and Hannah Fry, who investigate the BBC's public doubts, found the answer to our question in a somewhat complicated way. do not worry, we will find out, but on the way we will discover amazing things.

It all started with a question from listener Eddie Griffith:

"The other day I was watching James Bond's film Goldfinger He's bragging that his laser is powerful enough to project its light onto the moon, is it really possible and how powerful should this laser be? "

For those who do not remember the plot of this film, Auric Goldfinger is a devilish megalomaniac obsessed with l? Gold and plans to blow up a bomb inside Fort Knox – where much of the US gold reserves are stored since 1937 – so their own gold reserves are more valuable.

James Bond learns the plan, but he is kidnapped and taken to Goldfinger's secret lair, where they extend and attach him to a gold table.

After being informed of the power of his laser, Goldfinger lights the burning beam and shifts ] between Bond's legs in the direction of his … crown jewels .

But let's go back to the question, that it was not exactly if a laser could burn the intimate parts of the famous spy, but if we could see the light of its ray on the surface of the Moon if it is projected in his direction

How powerful should it be for to reach the Moon?

"I had very little time to prepare the answer since I was asked the question, so this is only an estimate," warns the physicist Andrew Pontzen.

"I think if you want to create a point of light on the surface of the moon, you would need a laser with a power of 200 megawatts."

"A watt is the measure used in the light bulbs, to have a perspective, today a good energy saving light bulb is 10 watts, so this laser has to be 20 million times stronger. "[19659002] And will we have such powerful lasers? Of course, and more!

One of the most powerful in the world is found in Didcot, UK, and is called Super Intense Gemini Laser. What would happen if you pointed your laser at the moon?

"It's probably too powerful," says physicist Ceri Brenner

"His power is about half petawatt ". laser line "alto =" 50 "ancho =" 976 "src =" https://c.files.bbci.co.uk/58C3/production/_102632722_linea-laser.jpg "/>

Explanation of urgency : A petawatt (PW) are 10 high at a power of 15 watts or, what is the same, 1,000 terawatts or 1,000 billion (1,000,000,000,000,000) watts A Petawatt laser generates this power for an interval of a few femtoseconds .

Explanation of Emergency II : A femtosecond is the unit of time equivalent to one billionth part of a second, it is deci r, that in one second there has a billion billion femtoseconds.They correspond to so many seconds to 100 million years (that is to say 3,153,600,000,000,000 seconds)

Back to this laser of 200 megawatts … but once you add the extra effects to compensate for things like diffraction in the Earth's atmosphere that spreads light, you might need a gigawatt .

It is basically equivalent to the power of a nuclear generator. And probably, the laser should be about 15 meters in diameter.

But after they left our mouths open talking about petawatts and femtoseconds, it does not seem so bad anymore.

The man who launches lightning at Luna

Trying to finally fulfill the mission of solving the doubt of the listener, Rutherford and Fry have discovered a physicist who actually pulls laser pulses on the Moon .

He calls Tom Murphy, of the University of California at San Diego, USA, and is engaged in what they call "laser measurement experiments."

Murphy uses a gigawatt laser located at the top of a New Mexico mountain and shoots impulses toward the mirrors left on the moon by astronauts from three missions of the program . Apollo and two Soviet "lunar".

Note that when the light reaches our natural satellite, it is so weak that you need telescopes to see it … and that – unlike Goldfinger – he does not use his laser for criminal purposes but scientists.

"Indeed, we use it for scientific purposes as to try to prove if the general theory of relativity correctly describes the lunar orbit ," Murphy confirms .

For this they have to measure "It's a simple idea: we send short pulses of light traveling to the reflectors on the Moon, these pulses return to Earth and we simply measure how long it is. round trip took, and we use the speed of light to convert it into distance. "

And so it is known that the Moon is moving away

"The Moon migrates slowly, as a result of the acceleration of It moves on average at about 3.8 centimeters a year from the Earth."

But there is there any reason to worry because we do not want it to go …

"No, the effect diminishes as it away, so this one is not a mechanism that will let him escape ", says the physicist.