This is MIRI; the mid infrared camera on board the
James Webb Telescope. Watch what happens when light enters the camera. A complex series of mirrors and filters
direct and split the light up into different wavelengths which are then resized and directed onto the sensors. With this camera, Webb can see extremely distant stars
and galaxies that are completely invisible to the human eye. This is only possible by cooling the camera down to just
6 degrees above absolute zero. But why does it need to be so cold? In this video we're going to look at how Webb
captures infrared light, and how a simple sound like this is used to cool its camera all the
way down to a mind-boggling temperature. We'll also be giving away this Lego ISS model, so stick around
to the end of the video to see how you could win. All of the cameras on board James Webb detect
infrared light, which is invisible to the human eye. As light travels through space its wavelength
is constantly being stretched. If something is far enough away, the light will be stretched so
much that it is no longer visible by the time it reaches us This means there is a physical limit
on how far we can see into space. Since MIRI's sensors are made from Arsenic and silicon,
it can detect this super stretched infrared light and see beyond that limit. These sensors work like regular camera sensors
by converting photons of light into an electrical signal but in order to detect the faint signals of infrared light
the sensors on MIRI have to be extremely sensitive. Increasing the sensitivity, however, introduces a lot of noise. Whenever we point a camera at something,
it's sensor isn't just detecting what we want it to see. There is so much more light bouncing around us
that our eyes simply can't detect. This can trick the pixels in the sensor into registering
random levels of light, creating a layer of noise in the image. If the thing you are looking at has a bright enough signal,
it will stand out much more compared to the noise. However, if the thing you are trying
to image is faint like the infrared light from a Galaxy you'll need to increase the sensitivity of
the sensor which in turn will drastically increase the noise. Since Webb is detecting infrared light,
the problem gets even worse. Every object in our Universe emits heat energy Some of which is in the form of light. Most objects aren't quite hot enough to emit visible light but they do emit a lot of
infrared light. The hotter the object, the more infrared light it will emit which is essentially how thermal imaging cameras work. Because of this, James Webb itself would emit so much infrared light that its sensors would be completely drowned out and so in order to limit the amount
of infrared light produced by the telescope its cameras need to be kept at a temperature of
negative 234 degrees - that is extremely cool. Just like Wondrium, the sponsor of today's video. Wondrium is a learning platform where you can explore a variety of subjects including some of my
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description below for your Wonderium free trial. In order for MIRI to operate at negative 267 degrees it's located behind the massive, five-layer sun shield. This alone reflects so much heat, and cools
Webb's cameras down to around negative 234 degrees. But, since MIRI is much more sensitive
than the other cameras it faces an even bigger problem -
dark current. This is where the atoms inside the sensor itself vibrate and mistakenly register a photon of light creating more noise. Since temperature is just a measure of how fast an atom
vibrates, lowering the temperature will lower the vibration and therefore reduce the amount of dark current. Even at - 234 degrees, the vibration of
these atoms would be too much for MIRI's sensor and so they have to be cooled all the way down
to negative 267 degrees just 6 degrees above absolute zero.
At this temperature the atoms in the sensor are almost completely still, drastically reducing the noise and allowing the faint infrared signals to shine through. But how did NASA take these
sensors all the way down to negative 267? The cooling process all starts here, at the bottom of
the telescope with a device called a pulse tube. Inside these tubes are two Pistons which move
back and forth to compress the helium into the pulse tube. Since this movement would create a lot
of unwanted vibration, these Pistons have to move in the exact opposite direction with precision
timing to cancel out each other's movement. These pistons move very quickly, just like a speaker to
create a low frequency sound wave with a frequency of 30 htz. This sound wave travels down the
tube and gets compressed once it reaches the end creating an area of higher pressure, then as it
bounces back in the opposite direction, it expands creating an area of low pressure before being
compressed once again. If the next wave is sent out exactly as the previous one returns, it means the frequency perfectly matches that of the tube. This creates a standing wave where the areas of
high and low pressure remain in the same place. By tripling the frequency, it causes the waves
to combine and create a stationary wave with multiple areas of high and low pressure. Since
temperature and pressure are related this also leads to areas of high and low temperature. This alone wouldn't change the overall temperature because the hot and cold parts simply cancel each
other out, but if there was a way to extract the hot parts then the temperature would start to
drop, and so three heat exchangers made of thin metal sheets are placed at the points where
the hot and cold gas meet, these allow the gas to pass through whilst also absorbing some of its
temperature, causing a heat gradient to form. The heat from the hot side is pulled out and sent to
the radiators via a heat exchanger then the cold temperatures on the other side are also drawn
out. This causes the temperature to drop from 27 degrees all the way down to negative 256 degrees
at the final heat exchanger. But how is this used to cool the sensors located at the opposite end of the telescope? Next to the pulse tube is another set of
pistons that compress helium in a completely separate line of tubing. This line passes through the cold parts of the
pulse tube's heat exchangers, cooling the helium all the way down to around negative 256 degrees. But still the helium isn't quite cold enough. From there, the helium goes on a long journey winding
its way up through 10 meters of thin tubing until it reaches the cold head assembly. Inside, there
is another heat exchanger and a tiny hole less than one millimeter in diameter. The helium is
pushed through the hole and undergoes something called The Jewel Thompson effect. As the gas moves
through the hole, it gets compressed before quickly expanding on the other side. This rapid expansion
causes the pressure to drop cooling the gas very quickly. This is why blowing air through our mouth
is colder if we make a smaller hole with our lips. On James Webb, this cools the helium all the
way down to just negative 267 degrees. From here, the helium flows onto copper plates attached to the back of MIRI's sensors, cooling them down to the required temperature. After that, the helium has
done its job and it flows back down the telescope where the entire process begins again. When NASA began designing James Webb,
no cryocooler with this level of cooling existed and so, engineers had to really
push the boundaries of physics all wilst conforming to the limits of fitting it
into a space telescope. And now, time for something really special. The winner of the previous giveaway
is Daniel Weinman. Congratulations! But, as always we'll be giving away another awesome space prize
in the next video. To win this amazing Lego space station model, sign up at the link below and
leave a comment saying how long you think James Webb will operate for. Thank you very much
for watching and I'll see you in the next video.
