Early man had few clues to the mysteries of
the universe. But every night they would be reminded there was much more to the cosmos than just the Earth, Sun and Moon. Humanity has been interested in the skies
above for at least as long weâve had any sort of civilization able to record anything
and likely far longer. Even before we invented the telescope we had
a sophisticated and detailed knowledge of the motion of the stars. Of course we didnât understand the science
behind it, but donât confuse that with stupidity on our ancestorsâ part. They lacked the tools to get more details
and as soon as they had them, their view of the Universe changed enormously. Today, as we discuss some options for making
truly huge telescopes that might actually let us see planets in far off solar systems,
not just detect the bare essentials of their existence, it probably behooves us to remember
that we could end up having as big a swing in our thinking as we had going from Astrology
to Astronomy. We have discovered countless mysteries and
surprises in our relatively brief survey of the heavens and should expect many more. And since that journey of exploration properly
began with the invention of the telescope, itâs a good place to begin to see how we
can make superior ones in the future. The basic concept of a magnifying lens is
quite old, and the concept is fairly simple. Light bends when passing into transparent
materials like glass, and various geometries with appropriate symmetry will bend that light
to the same place, the focal length of that particular lens for light coming in perpendicular
to it. By taking a very large area of incoming light
and concentrating it into a smaller area, assuming you can keep it even and undistorted,
you can let the eye see things normally too dim to make out. This is the very simplest approach but hard
to scale up too much. That lens needs to be very high quality and
precisely shaped, and you canât realistically go making telescopes kilometers across on
a planet. The early versions were typically only a few
centimeters across. But itâs worth noting that up in the vast
emptiness of space you can make some pretty big lenses, potentially a million times wider
than those early ones Galileo used, a megatelescope. There was quite a variety of designs of those
first telescope of the early 1600s, but they all used the same basic principle, which we
call a Refracting Telescope. A refractor uses not one but two lenses, the
objective lens which gathers over its entire area and focuses it to a point, and the eyepiece
which makes rays of light parallel again. Of course you also have the tube housing the
lenses, which keeps them a fixed distance apart and keeps out light coming from other
directions. And while thereâs a lot of variety of lens
types used, this basic design remains quite common and can be made quite large. Indeed in 1641, only a generation after the
first ones were developed, Johannes Hevelius built one with a 46 meter or 150 foot focal
length in Danzig, which was in Poland at the time, and itâs a good reminder our ancestors
werenât hesitant to do big projects. Sadly the observatory was destroyed by a fire
in 1671, though the Kings of both Poland and France sent him money to continue his work. Galileo and Kepler rightly get a lot of credit
as fathers of modern astronomy but it is good to remember there were a lot of other folks
working to advance our knowledge at the time, and we already skipped a bunch by jumping
to the 1600s. You donât absolutely need the tube on a
telescope, at least not for the whole length. In 1675 Christian and Constantine Huygens
built a tubeless 210 foot aerial telescope, and bigger ones soon followed with Adrien
Azout proposing one 1000 feet long to observe animals on the moon. I always like to point out that intention,
since a lot of folks tend to think erroneously that life off Earth was some entirely new
idea of strictly modern times. Now you might think these behemoths let you
see very distant objects indeed, but in truth they were designed to get around a problem
called chromatic aberration. Visible light bends when passing from one
medium to another, like from air to glass or glass to water, but different wavelengths
or colors bend different amounts and split from each other. The effect is called dispersion, and itâs
why you get the rainbow effect coming out of a prism. You can reduce this problem by building a
long and skinny telescope with lenses of enormous focal length, hence the behemoths. Once Isaac Newton explained that chromatic
aberration was caused by dispersion, lensmakers began figuring how to make achromatic lensesâwhich
are not quite accurately named because they reduce the problem, they donât eliminate
it. The simplest and still most common achromatic
lens consists of two different kinds of glass that focus red and blue light to the same
focal length, and the wavelengths in between are only dispersed somewhat. The achromatic lens allowed refracting telescopes
to boom once more, and while its invention saw the end of super long monsters there were
some pretty big ones, both in length and more importantly in aperture diameter up front. That culminated with one 49 inches or 1.25
meters wide in 1900, and a slightly smaller version that is still in service today at
the Yerkes Observatory and helped pioneer adaptive optics, which weâll discuss later. Newton, meanwhile developed his own approach
to solving the chromatic aberration problem, the reflecting telescope. Mirrors reflect all wavelengths of light by
the same angle, without dispersion, so reflecting scopes focus light using a curved mirror rather
than a lens. The other advantage of reflectors is that
a large mirror is lighter and, for several reasons, easier to manufacture than a large
lens. As astronomers built ever larger scopes, the
ubiquitous design was a large parabolic mirror at the back that gathered and reflected the
light to a flat mirror that reflected it to an achromatic eyepiece on the side. This is the favorite of hobbyists even nowadays,
or at least since the 1960s when John Dobson came up with a particular large, cheap, and
easy to build design which is often referred to as a Dobsonian Telescope or simply a Light
Bucket. Another invention of the 1600s was Cassegrainâs
design that also has a mirror at the back, a parabolic one, but with a hole in it. That mirror reflects the light coming down
the tube back up to a hyperbolic mirror that shoots it back down through a hole in the
middle of the rear parabolic mirror and into an eyepiece. Remember that parabola though because it will
be important in a minute. I donât want to imply that was the end of
major improvements to telescopes, indeed we could spend several episodes just on the major
ones alone, but those basic designs serve as a good refresher as we launch into ways
to super-size them. None of these have actual maximum size limits,
in theory anyway. If you had ultra stiff materials or were operating
in zero gravity so that your components donât deform under their own weight, you can go
ridiculously large. However gravity can be handy for one trick. Weâve often discussed creating artificial
gravity by spin and weâve also mentioned that you can increase the apparent gravity
on some low-gravity place like the moon by combining its natural gravity with spin, producing
gravity at an angle rather than down. If you did this, taking a cylinder and spinning
it around on the moon, then dumped some water in it, you could watch as that water formed
a parabolic shape at the bottom. If you used a liquid that was highly reflective,
like mercury, you would have a big parabolic mirror, which as we mentioned a moment ago
is what forms the back side of a Cassegrain-style reflecting telescope. By changing the spin rate of that cylinder
you could alter the steepness of that parabola. Nothing high tech involved in that, just a
spinning tube with liquid in it, and itâs also dirt cheap. Thereâs one a bit east of Vancouver, the
LZT or Large Zenith Telescope, that is amongst our largest optical telescopes and only cost
about half a million bucks. Unfortunately one problem with them is they
can only point straight up, because that mirror is being made by gravity plus spin and you
canât change gravityâs direction, hence the name Zenith Telescope. Another problem is that of course it needs
gravity to work, so you couldnât use it in orbit, but if you put one some place like
the far side of the moon, you could have a monstrously big telescope that didnât have
Earthâs atmosphere in the way and blocked Earthâs reflections, emission, and general
noise. You could also lower the telescope down into
the moon to protect it when not in use or if you detected incoming meteors. Unlike solid-mirror telescopes, a LZT is also
somewhat self-healing, provided you donât develop a leak in the container holding the
liquid. Small dust particles are going to be smoothed
over by the liquid and small pits filled; so in some ways, it is also more robust than
a standard mirror. Weâve also talked about OâNeill cylinders
and other rotating habitats a lot on this channel, which are just monstrous big tubes
when you think about it, and if you built one into the side of a decently massive asteroid
or moon you could make a telescope like this many kilometers wide and long. Indeed you could potentially build pretty
large ones on icy dwarf planets like Pluto or floating around in the upper atmospheres
of places like Venus. But we can be tricky and clever. Just because spinning something creates apparent
gravity in one direction doesnât mean you canât spin it some more. If you take a spinning tube or bucket and
attach it to a long tether, you can spin it around on that tether and create a second
spin-gravity force perpendicular to the first, like gravity was. Now you can point it any direction you please,
though only briefly as itâs spinning, and your size is limited by how strong that cable
or cables are. But you can move it around to any direction
you like. Thereâs a couple other geometries for this,
and we could also potentially use magnetics, rather than gravity, to create that shape. And since it doesnât rely on natural gravity
you can put it anywhere, like in orbit, or way out in deep space. Speaking of orbiting space telescopes, if
youâre wondering, Hubble is a reflecting telescope with a 2.4 meter mirror, named for
Edwin Hubble who helped pioneer the notion of an expanding Universe. Its big advantage is that being up in space
it doesnât have to contend with atmospheric interference. There are dozens of ground telescope with
larger mirrors, including the Liquid LZT we just discussed at 6 meters diameter. The largest single, or monolithic mirror is
the Subaru, Japanâs 8.3 meter leviathan in Hawaii, thatâs held that record for 20
years now. If youâre curious Subaru is the Japanese
name of the star cluster many of us know as the Pleiades and also means âuniteâ, folks
tend to ask if the better known car company sponsored it or assume itâs pronounced differently. Itâs not actually the biggest reflector,
and wasnât when it was made, just the biggest with a single mirror. As I mentioned we have continued improving
telescopes beyond the basic designs from the 16 and 1700s. Thereâs 2 terms youâve probably heard
that are important to discuss. Adaptive Optics and Active Optics. Active optics is an alternative approach to
mirrors. In the past we tended to make them rather
thick to help cut down on deformation of the mirror from its own weight or wind. However, we eventually realized we could use
a thin mirror supported by actuators instead, ones able to push a little here or there under
control of a computer to keep the mirror in the right shape against sagging, wind, thermal
expansion and so on. Adaptive optics is an improvement on this,
that allows us to manipulate the mirror quickly and cause deformations to counter atmospheric
distortion. The air in the atmosphere is really no different
than glass or water, it bends light too, but it bends it depending on how dense it is or
how much moisture is present and air turbulence can make that rapidly change, indeed thatâs
what makes the stars twinkle. Needless to say you need a lot of computers
to do this well so itâs a fairly recent thing. We should probably take some time to discuss
astronomy in wavelengths besides the visual range and interferometry, but weâre going
to mostly bypass that today. Some things are invisible to normal light
or act differently in other wavelengths, but the basic physics for the telescope remains
the same. Unlike mirrors and lenses that everyone is
already very familiar with, and thus easy to explain how itâs used in astronomy, interferometry
would probably need itâs own video to do it justice. Short form: waves can interfere with each
other and two identical but out of phase ones can cancel out, if theyâre not quite identical
you can see the differences when everything identical has been removed. For radio astronomy we can use this to take
two antennas and merge their signals to look at a specific point. We often see arrays of radio dishes rather
than a single big one, and this comes down to resolution. You take a pair of radio dishes separated
from each other, and each still gathers the same amount of light or radio waves, but the
resolution improves. This is the key of astronomy, trying to brighten
an object so you can see it by squeezing a wide cross-section of light down to a small
one, and enhancing resolution so you can make out details of a distant object so it isnât
fuzzy or drowned out by noise of other sources off at different angles. So one radio dish has less resolution than
two, two has less than three, and so on, especially if they are spaced apart. Normally the resolution of a telescope is
controlled by the lens diameter divided by the wavelength. Thatâs fine for an optical telescope because
even a lens half a dozen centimeters across, like youâd pick up at a store, is a hundred
thousand times larger than the wavelength being observed. But radio waves have wavelengths of meters
or even kilometers. So while a lens as big as your hand allows
potential resolution of visual light of an arc second, if you wanted the same resolution
for radio waves in the AM radio band, tens of meters in wavelength rather than hundreds
of nanometers, youâd need a lens a hundred million times wider. Or in fact bigger than our planet. Indeed the AM band is also known as the medium
frequency band, and the extreme low frequency band has radio waves whose wavelength alone
is already bigger than the planet. Incidentally that doesn't mean you can't see
anything with a smaller dish, it just wouldnât have any resolution, like a computer monitor
showing all the same color, 1p instead of 1080p, you can still tell color or wavelength
and how bright it is. However if you want any resolution you need
that bigger lens, and we can fake that by using two dishes that are spread apart but
looking at the same thing, and combining the signals we receive. Doesnât gather much more light, just double,
but does give you that resolution. Needless to say that recombination is rather
precise and tricky and why two dishes arenât just placed on opposite sides of the planet. But weâve been getting much better at precision
and calculation as years go by and in this way we can create a fake giant lens or aperture,
for the purpose of resolution, a synthetic aperture. One of the most important innovations over
the past few decades has been use use of interferometry in astronomy. This gives us that desired resolution. Now, it is not only used in traditional radio
telescopes, but also in gravitational wave detection too. This is something that we were only recently
able to get right with LIGO, an acronym for Laser Interferometer Gravitational-Wave Observatory. Iâll spend a bit of time on this explaining
this technology as it is recent and will probably tell us a lot more about our universe as sensitivity
and gravitational wave events get detected. Itâs also a good reminder that astronomy
is not only extended to areas of the electromagnetic spectrum beyond the visible range, but can
also include gravity. As weâll see, a LIGO is going to be one
of the first true Megatelescopes in the next 20 years. We started to detect gravitational waves in
2016 and up until now, we have 6 confirmed gravitational wave detection events. The most recent of these was the August 17,
2017 detection of a collision of two neutron stars, which was neatly simultaneously detected
by optical signals by conventional telescopes. LIGO makes use of two 4 kilometer vacuum tubes
set at right angles to each other. Now, 4 kilometers is a much bigger distance
than the diameter of any telescope mirror that weâve made, but it doesnât act in
the same way. A laser beam is split and reflected off a
mirror at the end of the vacuum tube and detected again at the source. The resolution of this telescope is such that
it can detect a distance change between the two 4 kilometer corridors of less than a ten-thousandth
the charge diameter of a proton. This is the equivalent of measuring the distance
from Earth to Proxima Centauri with an accuracy smaller than the width of a human hair, very
impressive! So how does it get this kind of accuracy? The answer lies in interferometry. Light is a wave with peaks and valleys. The LIGO is designed so that the paths are
tuned so the returned light is exactly inverse for each path, so if one light beam from the
one tube arrives at the detector as a peak, the light from the other right angle tube
arrives at the same detector as a valley, cancelling each other out in destructive interference. What we read at the detector normally is zero
as the waves perfectly cancel each other out. This is the magic of an interferometer; itâs
designed to detect something thatâs interesting, otherwise we detect nothing. If space is warped by a gravitational wave,
one of the vacuum tubes is going to get slightly longer or shorter and this will cause the
light waves to move out of sync with one another so destructive interference doesnât happen
and the detector detects the light. The more out of sync the waves are, the more
light is detected. So thatâs where our incredible resolution
comes from. Now, thereâs a fair amount of noise in the
system so there are actually two LIGO detectors, one in Hanford, Washington and the other in
Livingston, Louisiana. The gravitational wave travels at the speed
of light and the two LIGO observatories are 10 milli light seconds apart, so we can do
some interesting astronomy with these in being able to tell the direction from where a signal
came from and also eliminate noise by requiring that a signal be detected by both observatories
before it is a confirmed signal. Doing gravitational astronomy is challenging
at the moment because the noise levels are almost at the same levels as the signals being
read; the arms of the LIGO are really very short for the type of detection they are trying
to do. If we want to increase the resolution, we
need to increase the arm distances. True to my heart, the European Space Agency
is going to supersize a LIGO, not on the ground here on Earth, but in space. I mentioned that each arm of the current LIGO
observatory is 4 kilometers in length, but how about supersizing each arm to 2.5 million
kilometers long? Thatâs what theyâre proposing to launch
in 2034 in a project called LISA, or Laser Interferometer Space Antenna, which should
give us much better resolution. Radio telescope arrays also use interferometry
to cancel out signals that are the same and can get better resolution as a result too. In much the same way as LIGO, the idea is
to detect waves that are actually different and travelling so one telescope picks it up
and then another, some milliseconds later. Anyway, back to optical telescopes. An arc minute is a sixtieth of a degree and
an arc-second is a sixtieth of an arc-minute, and there are 1,296,000 of them in a circle. The human eye is rather high resolution but
we top out at about an arcminute, not an arc second. Any star see from Earth is much less wide
than an arcsecond, again you can see it but only as a single pixel dot of various brightness,
not including twinkle caused by atmospheric turbulence. The sun is about 2000 arc seconds and still
appears as blob but thatâs simply due to sheer brightness, the moon has the same angular
width and you can make out some features with the naked eye. If we had an Earth-like exoplanet 10 light
years away, one about 13,000 kilometers in diameter and a distance of 95 trillion kilometers,
it has an angular diameter of just 28 microarcseconds. So if you wanted a picture of just 28x28 pixels,
one microarcsecond each, able to see blue and green light enough to make out the existence
of continents and oceans, where each pixel is 500 kilometers across, you would need a
telescope lens over a hundred kilometers across, if you wanted to bump that up to a 1080p image,
that lens needs to be the size of a continent, and that scales up linear to distance, put
it 100 light years away instead of 10 and you need a lens 10 times bigger. Thatâs why I often mention planet-sized
telescopes. If you want to be able to watch a planet in
another solar systems at nice resolutions, with a single lens telescope, thatâs the
scale you need to be considering. Needless to say itâs a lot easier to look
at one dot of distant star and see it dim slightly every 365 days, and infer from that
dimming that some planet passes between us and it every year, blocking some light, then
figure out its size by how much it blocked and how long it blocked that much at the distance
a 365 day orbit requires. Also, needless to say thatâs why it's handy
to be able to enhance resolution by spreading out two telescopes and merging their imaging,
you donât get much light this way either which can be fixed in part by long exposure
times. And we have somewhat parallel examples like
the large binocular telescope and a lot of the newer telescopes, starting with Keck,
which began using mirror segments in honeycombs rather than a single large mirror, this being
cheaper and easier to control. Taken to an extreme, NASAâs been playing
around with taking clouds of glitter, million of little flecks, and manipulate them to an
overall shape with lasers. We looked at liquid mirrors earlier and these
are more like a vapor or fog. Itâs also been suggested that a spherical
arrangement of optical telescopes in space, with their position known down to an accuracy
smaller than the wavelength of light theyâre observing, which is hundreds of nanometers
rather than hundreds of meters like with radio telescope, could permit something dubbed a
âHyper Telescopeâ, which could potentially directly image features on exoplanets. Now, all the things weâve discussed so far
could be scaled up a lot in deep space, far from any gravity perturbations or other potential
problems. Whatâs more you can do some pretty precise
manufacturing without all the gravity, air, vibrations, and so on that we have to deal
with down here. Telescopes could hypothetically be built of
planetary proportions. We also want to keep in mind that while surface
area, or aperture size, controls how much light you collect, itâs really the distance
between elements that helps with resolution and even noise cancellation. Many telescopes spread far apart looking at
the same thing from different angles can have their data put together and analyzed to produce
a very accurate picture. Telescopes bigger than planets are potentially
possible in the distant future but so is spreading them out over interplanetary or even interstellar
distances. Such distances also allow unprecedented parallax
for calculating distances with high precision. Thereâs really not much limit how big you
can go, with two obvious caveats. First you need a lot of computer power and
second you obviously can only go so big before it will collapse under its own weight. On that computer power front though, it is
worth remembering that the whole point of a telescope is to take a wide area of light
and crunch it down to a small size thatâs now bright enough for our eye to see, and
retains its original pattern with little distortion. But hypothetically you could just have a big
sheet of tiny light sensitive cells instead of a mirror, each reporting what hit it and
letting a computer do the work, fundamentally not really any different than the aperture
synthesis done with many telescopes, only scaled way up in number and down in size. Or potentially also up size. In the end all that really matters is how
many photons you can catch and if you can know what angle they came in from. Needless to say a lot of the options we discussed
a month back in Advanced Metamaterials might help with this a lot. Including options like a superlens able to
pass the diffraction limit. One can easily imagine whole arrays of planet
sized telescopes sheathed in metamaterials with a planet sized computer running them
which allow you to potentially look at distant worlds in very high resolution. The flip side of the interstellar beacons
we discussed a couple weeks back, that let you transmit messages civilizations could
pick up on the other side of the galaxy, telescopes that could pick up normal transmissions that
far away. Now you do have a mass limit to building these
things, under which theyâd collapse under their own weight, and that raises another
example. Youâve probably heard of gravitational lensing
and a black hole is particularly good at this. Weâve talked about making artificial black
holes before and one could potentially make one large and clear its area of any mass that
might cause interference falling in and radiating as it goes. Weâve even talked about living around these
things. Interestingly with gravitational lensing like
this, you donât have a focus point but a focus line. Now thatâs maybe not as handy as it sounds,
because diameter is the important thing and a galaxy for instance produces far more lensing
than a black hole and is far larger. But thinking of exotic materials for telescopes,
itâs also worth noting that dark matter, which is far more plentiful than the normal
kind, also doesnât interact with anything other than gravity so in a really high tech
civilization you could potentially use it for making a lens that operated by bending
spacetime, rather than bending light. Not likely to be practical any time soon,
or maybe ever, but same as we could get away with tubeless telescopes in the old aerial
telescopes or do away with a lens for a mirror or multiple mirrors, or even do away with
all of those in favor of light sensitive cells and a lot of computers, we donât want to
get rigidly bound to any conventional notion of a telescope. Donât rule out scenarios down the line like
a billion worlds with many telescopes all feeding their data into a giant Matrioshka
Brain, a computer powered by a whole star, and pumping out a ridiculously accurate, high
resolution map of space. Donât rule out learning to manipulate spacetime
to form a lens made of nothing but warped spacetime. We might have problems even detecting something
the size of an entire planet in neighboring solar systems these days, but one day we might
be able to image those as well as our satellites can image Earth now. I think weâll close out here. We did have to touch on more math than I usually
like to put in an episode for discussion but much of it was supplementary, so you could
see how we derived these values rather than me just saying them. We also skipped all sorts of tricks weâve
learned over the last few centuries both in terms of the technical side of making telescopes
and squeezing more data out of them and using that to paint a picture of the galaxy and
what its made up of. Telescopes just let us gather data, figuring
out what it means is a whole different story. If youâd like to learn more about that Iâd
suggest the Astronomy Course at Brilliant.org, that course walks through practical exercises
on everything from the Atomic Spectra we used to determine what stars were made of and derived
their energy from to the exoplanets orbiting them and how we detect them. It offers a very hands-on active learning
approach and will introduce you to many of the terms and concepts we see in astronomy
that are often left fairly vague in a of lot articles and discussions of these topics. âEffective learning is activeâ - Thatâs
one of their 8 principles of learning. Another of their principles that I strongly
agree with is âEffective learning sparks questionsâ. I not only answer a lot of questions here,
but also encourage folks to think of whole new ones that follow from the material and
I try to give you some of the tools to find answers on your own too. Brilliantâs a great place to expand your
toolbox, both to help you answer many of the questions you have and help you know what
to ask. So if youâd like improve your understanding
of math and science, and help support the channel while youâre at it, go to brilliant.org/IsaacArthur
and sign up for free. And also, the first 200 people that go to
that link will get 20% off the annual Premium subscription. Okay next week weâll be continuing our look
at futuristic conflicts we began with space warfare and looking more at the planetary
side of things in Planetary Invasions, and reviewing our Book of the Month, Robert Heinleinâs
Starship Troopers. The week after that itâs back to Post Scarcity
Civilizations for a look not just at Virtual Reality but how people living in such civilizations
might have their basic perception of reality altered. For alerts when those and other episodes come
out, make sure to subscribe to the channel, and if you enjoyed this episode, hit the like
button and share it with others. Until next time, thanks for watching, and
have a great week!
Isaac has rhotacism, which is the inability to pronounce or difficulty in pronouncing the sound R. If you can get past his speech impediment, he puts out weekly videos that are deep-dives about the limits of science.