Artistic rendition of an interstellar spacecraft traveling near the speed of light. Credit: Made with ChatGPT
Someday, in the not-too-distant future, humans may send robotic probes to explore nearby star systems. These robot explorers will likely take the form of lightsails and wafercraft (a la Breakthrough Starshot) that will rely on directed energy (lasers) to accelerate to relativistic speeds – aka. a fraction of the speed of light. With that kind of velocity, lightsails and wafercraft could make the journey across interstellar space in a matter of decades instead of centuries (or longer!) Given time, these missions could serve as pathfinders for more ambitious exploration programs involving astronauts.
Of course, any talk of interstellar travel must consider the massive technical challenges this entails. In a recent paper, a team of engineers and astrophysicists considered the effects that relativistic space travel will have on communications. Their results showed that during the cruise phase of the mission (where a spacecraft is traveling close to the speed of light), communications become problematic for one-way and two-way transmissions. This will pose significant challenges for crewed missions but will leave robotic missions largely unaffected.
By popular request, Isaac Arthur and I have teamed up again to bring you a vision of the future of human space exploration. This time, we bring you practical construction tips from a pair of Type 2 Civilization engineers.
To make this collaboration even better, we’ve teamed up with two artists, Kevin Gill and Sergio Botero. They’re going to help create some special art, just for this episode, to help show what some of these megaprojects might look like.
The Big Bang. The discovery that the Universe has been expanding for billions of years is one of the biggest revelations in the history of science. In a single moment, the entire Universe popped into existence, and has been expanding ever since.
We know this because of multiple lines of evidence: the cosmic microwave background radiation, the ratio of elements in the Universe, etc. But the most compelling one is just the simple fact that everything is expanding away from everything else. Which means, that if you run the clock backwards, the Universe was once an extremely hot dense region
A billion years after the big bang, hydrogen atoms were mysteriously torn apart into a soup of ions. Credit: NASA/ESA/A. Felid (STScI)).
Let’s go backwards in time, billions of years. The closer you get to the Big Bang, the closer everything was, and the hotter it was. When you reach about 380,000 years after the Big Bang, the entire Universe was so hot that all matter was ionized, with atomic nuclei and electrons buzzing around each other.
Keep going backwards, and the entire Universe was the temperature and density of a star, which fused together the primordial helium and other elements that we see to this day.
Continue to the beginning of time, and there was a point where everything was so hot that atoms themselves couldn’t hold together, breaking into their constituent protons and neutrons. Further back still and even atoms break apart into quarks. And before that, it’s just a big question mark. An infinitely dense Universe cosmologists called the singularity.
When you look out into the Universe in all directions, you see the cosmic microwave background radiation. That’s that point when the Universe cooled down so that light could travel freely through space.
And the temperature of this radiation is almost exactly the same in all directions that you look. There are tiny tiny variations, detectable only by the most sensitive instruments.
Cosmic microwave background seen by Planck. Credit: ESA
When two things are the same temperature, like a spoon in your coffee, it means that those two things have had an opportunity to interact. The coffee transferred heat to the spoon, and now their temperatures have equalized.
When we see this in opposite sides of the Universe, that means that at some point, in the ancient past, those two regions were touching. That spot where the light left 13.8 billion years ago on your left, was once directly touching that spot on your right that also emitted its light 13.8 billion years ago.
This is a great theory, but there’s a problem: The Universe never had time for those opposite regions to touch. For the Universe to have the uniform temperature we see today, it would have needed to spend enough time mixing together. But it didn’t have enough time, in fact, the Universe didn’t have any time to exchange temperature.
Imagine you dipped that spoon into the coffee and then pulled it out moments later before the heat could transfer, and yet the coffee and spoon are exactly the same temperature. What’s going on?
Alan H. Guth. Credit: Betsy Devine (CC BY-SA 3.0)
To address this problem, the cosmologist Alan Guth proposed the idea of cosmic inflation in 1980. That moments after the Big Bang, the entire Universe expanded dramatically.
And by “moments”, I mean that the inflationary period started when the Universe was only 10^-36 seconds old, and ended when the Universe was 10^-32 seconds old.
And by “expanded dramatically”, I mean that it got 10^26 times larger. That’s a 1 followed by 26 zeroes.
Before inflation, the observable Universe was smaller than an atom. After inflation, it was about 0.88 millimeters. Today, those regions have been stretched 93 billion light-years apart.
This concept of inflation was further developed by cosmologists Andrei Linde, Paul Steinhardt, Andy Albrecht and others.
Inflation resolved some of the shortcomings of the Big Bang Theory.
The first is known as the flatness problem. The most sensitive satellites we have today measure the Universe as flat. Not like a piece-of-paper-flat, but flat in the sense that parallel lines will remain parallel forever as they travel through the Universe. Under the original Big Bang cosmology, you would expect the curvature of the Universe to grow with time.
The horizon problem in Big Bang cosmology. How is it that distant parts of the universe possess such similar physical properties? Credit: Addison Wesley.
The second is the horizon problem. And this is the problem I mentioned above, that two regions of the Universe shouldn’t have been able to see each other and interact long enough to be the same temperature.
The third is the monopole problem. According to the original Big Bang theory, there should be a vast number of heavy, stable “monopoles”, or a magnetic particle with only a single pole. Inflation diluted the number of monopoles in the Universe so don’t detect them today.
Although the cosmic microwave background radiation appears mostly even across the sky, there could still be evidence of that inflationary period baked into it.
The Big Bang and primordial gravitational waves. Credit: bicepkeck.org
In order to do this, astronomers have been focusing on searching for primordial gravitational waves. These are different from the gravitational waves generated through the collision of massive objects. Primordial gravitational waves are the echoes from that inflationary period which should be theoretically detectable through the polarization, or orientation, of light in the cosmic microwave background radiation.
A collaboration of scientists used an instrument known as the Background Imaging of Cosmic Extragalactic Polarization (or BICEP2) to search for this polarization, and in 2014, they announced that maybe, just maybe, they had detected it, proving the theory of cosmic inflation was correct.
Unfortunately, another team working with the space-based Planck telescope posted evidence that the fluctuations they saw could be fully explained by intervening dust in the Milky Way.
Planck’s view of its nine frequencies. Credit: ESA and the Planck Collaboration
The problem is that BICEP2 and Planck are designed to search for different frequencies. In order to really get to the bottom of this question, more searches need to be done, scanning a series of overlapping frequencies. And that’s in the works now.
BICEP2 and Planck and the newly developed South Pole Telescope as well as some observatories in Chile are all scanning the skies at different frequencies at the same time. Distortion from various types of foreground objects, like dust or radiation should be brighter or dimmer in the different frequencies, while the light from the cosmic microwave background radiation should remain constant throughout.
There are more telescopes, searching more wavelengths of light, searching more of the sky. We could know the answer to this question with more certainty shortly.
One of the most interesting implications of cosmic inflation, if proven, is that our Universe is actually just one in a vast multiverse. While the Universe was undergoing that dramatic expansion, it could have created bubbles of spacetime that spawned other universes, with different laws of physics.
Artist concept of the multiverse. Credit: Florida State University
In fact, the father of inflation, Alan Guth, said, “It’s hard to build models of inflation that don’t lead to a multiverse.”
And so, if inflation does eventually get confirmed, then we’ll have a whole multiverse to search for in the cosmic microwave background radiation.
The Big Bang was one of the greatest theories in the history of science. Although it did have a few problems, cosmic inflation was developed to address them. Although there have been a few false starts, astronomers are now performing a sensitive enough search that they might find evidence of this amazing inflationary period. And then it’ll be Nobel Prizes all around.
Using data provided by the Very Large Telescope in Chile, the ESO has been able to discern the "fingerprints" of the early Universe. Credit: ESO
Back in November, a team of researchers from the Swinburne University of Technology and the University of Cambridge published some very interesting findings about a galaxy located about 8 billion light years away. Using the La Silla Observatory’s Very Large Telescope (VLT), they examined the light coming from the supermassive black hole (SMBH) at its center.
In so doing, they were able to determine that the electromagnetic energy coming from this distant galaxy was the same as what we observe here in the Milky Way. This showed that a fundamental force of the Universe (electromagnetism) is constant over time. And on Monday, Dec. 4th, the ESO followed-up on this historic find by releasing the color spectrum readings of this distant galaxy – known as HE 0940-1050.
To recap, most large galaxies in the Universe have SMBHs at their center. These huge black holes are known for consuming the matter that orbits all around them, expelling tremendous amounts of radio, microwave, infrared, optical, ultra-violet (UV), X-ray and gamma ray energy in the process. Because of this, they are some of the brightest objects in the known Universe, and are visible even from billions of light years away.
Artist’s interpretation of ULAS J1120+0641, a very distant quasar. Credit: ESO/M. Kornmesser
But because of their distance, the energy which they emit has to pass through the intergalactic medium, where it comes into contact with incredible amount of matter. While most of this consists of hydrogen and helium, there are trace amounts of other elements as well. These absorb much of the light that travels between distant galaxies and us, and the absorption lines this creates can tell us of lot about the kinds of elements that are out there.
At the same time, studying the absorption lines produced by light passing through space can tell us how much light was removed from the original quasar spectrum. Using the Ultraviolet and Visual Echelle Spectrograph (UVES) instrument aboard the VLT, the Swinburne and Cambridge team were able to do just that, thus sneaking a peak at the “fingerprints of the early Universe“.
What they found was that the energy coming from HE 0940-1050 was very similar to that observed in the Milky Way galaxy. Basically, they obtained proof that electromagnetic energy is consistent over time, something which was previously a mystery to scientists. As they state in their study, which was published in the Monthly Notices of the Royal Astronomical Society:
“The Standard Model of particle physics is incomplete because it cannot explain the values of fundamental constants, or predict their dependence on parameters such as time and space. Therefore, without a theory that is able to properly explain these numbers, their constancy can only be probed by measuring them in different places, times and conditions. Furthermore, many theories which attempt to unify gravity with the other three forces of nature invoke fundamental constants that are varying.“
A laser beam launched from the Very Large Telescope (VLT) at the ESO’s La Silla Observatory in Chile. Credit: ESO
Since it is 8 billion light years away, and its strong intervening metal-absorption-line system, probing the electromagnetic spectrum being put out by HE 0940-1050 central quasar – not to mention the ability to correct for all the light that was absorbed by the intervening intergalactic medium – provided a unique opportunity to precisely measure how this fundamental force can vary over a very long period of time.
On top of that, the spectral information they obtained happened to be of the highest quality ever observed from a quasar. As they further indicated in their study:
“The largest systematic error in all (but one) previous similar measurements, including the large samples, was long-range distortions in the wavelength calibration. These would add a ?2 ppm systematic error to our measurement and up to ?10 ppm to other measurements using Mg and Fe transitions.”
However, the team corrected for this by comparing the UVES spectra to well-calibrated spectra obtained from the High Accuracy Radial velocity Planet Searcher (HARPS) – which is also located at the at the La Silla Observatory. By combining these readings, they were left with a residual systematic uncertainty of just 0.59 ppm, the lowest margin of error from any spectrographic survey to date.
High Accuracy Radial velocity Planet Searcher at the ESO La Silla 3.6m telescope. Credit: ESO
This is exciting news, and for more reasons that one. On the one hand, precise measurements of distant galaxies allow us to test some of the most tricky aspects of our current cosmological models. On the other, determining that electromagnetism behaves in a consistent way over time is a major find, largely because it is responsible for such much of what goes on in our daily lives.
But perhaps most importantly of all, understanding how a fundamental force like electromagnetism behaves across time and space is intrinsic to finding out how it – as well as weak and strong nuclear force – unifies with gravity. This too has been a preoccupation of scientists, who are still at a loss when it comes to explaining how the laws governing particles interactions (i.e. quantum theory) unify with explanations of how gravity works (i.e general relativity).
By finding measurements of how these forces operate that are not varying could help in creating a working Grand Unifying Theory (GUT). One step closer to truly understanding how the Universe works!
Sometimes I figure out the weak spot in my articles based on the emails and comments they receive.
One popular article we did was all about Stephen Hawking’s realization that black holes must evaporate over vast periods of time. We talked about the mechanism, and mentioned how there are these virtual particles that pop in and out of existence.
Normally these particles self annihilate, but at the edge of a black hole’s event horizon, one particle falls in, while another is free to wander the cosmos. Since you can’t create particles from nothing, the black hole needs to sacrifice a little bit of itself to buy this newly formed particle’s freedom.
But my short article wasn’t enough to clarify exactly what virtual particles are. Clearly, you all wanted more information. What are they? How are they detected? What does this mean for black holes?
In situations like this, when I know the actual Physics Police are watching, I like to call in a ringer. Once again, I’m going to go back and talk to my good friend, and actual working astrophysicist, Dr. Paul Matt Sutter. He has written papers on subjects like the Bayesian Analysis of Cosmic Dawn and MHD Simulations of Magnetic Outflows. He really knows his stuff.
Fraser Cain:
Hey Paul, first question: What are virtual particles?
Paul Matt Sutter:
Alright. No pressure, Fraser. Okay, okay.
To get the concept of virtual particles you actually have to take a step back and think about the field, especially the electromagnetic field. In our current view of how the universe works all of space and time is filled up with this kind of background field. And this field can wibble and wabble around, and sometimes these wibbles and wabbles are like waves that propagate forward, and we call these waves photons or electromagnetic radiation, but sometimes it can just sit there and you know bloop bloop bloop, just you know pop fizzle in and out, or up and down, and kind of boil a little all on its own.
In fact all the time space is kind of wibbling/wabbling around this field even in a vacuum. A vacuum isn’t the absence of everything. The vacuum is just where this field is in its lowest energy state. But even though it’s in that lowest energy state, even though maybe on average there is nothing there. There’s nothing stopping it from just bloop bloop bloop you know bubbling around.
Credit: NASA, ESA, Q.D. Wang (University of Massachusetts, Amherst), and S. Stolovy (Caltech)
So actually the vacuum is kind of boiling with these fields. In particular the electromagnetic field which is what we are talking about right now.
And we know that photons, that light, can turn into particle, anti-particle pairs. It can turn into say an electron and a positron. It can just do this. It can happen to normal photons, and it can happen to these kind of temporary wibbly wobbly photons.
So sometimes a photon or sometimes the electromagnetic field can propagate from one place to another, and we call it a photon. And that photon can split off into a positron and an electron, and other times it can just wibble wobble kind of in place and then wibble wobble POP POP. It pops into a positron and an electron and then they crash into each other or whatever, and they just simmer back down. So, wibble wobble, pop pop, fizz fizz is kind of what’s going on in the vacuum all they time, and that’s the name we give these virtual particles are just the normal kind of background fuzz or background static to the vacuum.
Fraser:
Okay. So how do we see evidence for virtual particles?
Paul:
Yeah, great question. We know that the vacuum has an energy associated with it. We know that these virtual particles are always fizzing in and out of existence for a few reasons.
One is the transition of the electron in different states of the atom. If you excite the atom the electron pops up to a higher energy state. There is kind of no reason for that electron to pop back down to a lower energy state. It’s already there. It’s actually a stable state. There is no reason for it to leave unless there is little wibble wobbles in the electromagnetic field and it can giggle around that electron and knock it out of that higher energy state and send it crashing down into a lower state
Another thing is called the Lamb Shift, and this is when the wibbly wobbly electromagnetic field or the virtual particles interact again with electrons in say a hydrogen atom. It can gently nudge them around, and this shift effects some states of the electron and not other states. And there are actually states that you would say have the exact same say energy properties, they are just kind of identical, but because the Lamb Shift, because of this wibbly wobbly electromagnetic field interacts with one of those states and not the other, it actually subtly changes the energy levels of those states even though you’d expect them to be completely the same.
And another piece of evidence is in photon photon scattering usually two photons just, phweeet, fly by each other. They are electrically neutral, so they have no reason to interact, but sometimes the photons can wibble wobble into say electron/positron pairs, and that electron/positron pair can interact with the other photons. So sometimes they bounce off each other. It’s super rare because you have to wait for the wibble wobble to happen at just the right time, but it can happen.
Credit: NASA/Dana Berry/SkyWorks Digital
Fraser:
So how do they interact with black holes?
Paul:
Alright, this is the heart of the matter. What do all of these virtual particles or wibbly wobbly electromagnetic fields have to do with black holes, and specifically Hawking radiation? But check this out. Hawkings original formulation of this idea that black holes can radiate and lose mass actually has nothing to do with virtual particles. Or it doesn’t speak directly about virtual particle pairs, and in fact no other formulations or more modern conceptions of this process talk about virtual particle pairs.
Instead, they talk more about the field itself and specifically what’s happening to the field before the black hole is there, what’s happening to it as the black hole forms, and then what happens to the field after it’s formed. And it kind of asks a question: What happens to these wibbly wobbly bits of the field, these like transient kind of boiling nature of the vacuum of the electromagnetic field? What happens to it as that black hole is forming?
Well what happens is that some of the wibbly wobbly bits just get caught near the black hole, near the event horizon as it is forming, and they spend a long time there, and eventually they do escape. So it takes awhile, but when they escape because of the intense curvature there, the intense curvature of space-time, they can get boosted or promoted. So instead of being temporarily wibbly wobbly’s, in the field they get boosted to become “real” particles or “real” photons. So it’s really like an interaction of the formation of the black hole itself with the wibbly wobbly background field, that eventually escapes because it’s not quite trapped by the black hole.
Eventually it escapes and gets turned into real particles, and you can calculate like what happens with say the expected number of particles near the event horizon of the black hole. The answer is the negative number, which means the black hole is losing mass and spitting out particles.
Now this popular conception of virtual particle pairs popping into existence and one getting caught inside the event horizon. That’s is not exactly tied to the mathematics of Hawking radiation but it’s not exactly wrong either. Remember the wibbly wobbly’s in the electromagnetic field are related to these pairs of particles and anti-particles that are constantly popping in and out of existence. They kind of go hand in hand. So by talking about wibbly wobbly’s in the field you’re also kind of talking about the production of virtual particles. And it’s not exactly the math, but you know close enough.
An artist’s conception of a supermassive black hole’s jets. Image Credit: NASA / Dana Berry / SkyWorks Digital
Fraser:
Okay, and finally, Paul. I need you to just randomly blow the minds of the viewers. Something about virtual particles that is just amazing!
Paul:
Alright. So you want to bend people’s minds? All right. I was saving this for the last. Something juicy, just for you, Fraser.
Check this out, it’s one other big piece of evidence we have for the existence of these background fluctuations and the existence of virtual particles, and that’s something we call the Casimir Effect, or Casimir Force.
You take two neutral metal plates, and what happens is this field that permeates all of space-time is inside the plates and it’s outside the plates. Inside the plates, you can only have certain wavelengths of modes. Almost like the inside of a trumpet can only have certain modes that make sound. The ends of the wavelengths must connect to the plates, because that’s what metal plates do to electromagnetic fields.
Outside the plates you can have any wavelength you want. It doesn’t matter.
So it means outside the plates you have an infinite number of possible wavelengths of modes. Every kind of possible kind of fluctuation, wibble wabble in the electromagnetic field is there, but inside the plates it’s only certain wavelengths that can fit inside the plates.
Now, outside there’s an infinite number of modes. Inside, there is still an infinite number of modes, just slightly fewer infinite number of modes. And you can take the infinity on the outside, and subtract the infinite infinity on the inside, and actually get a finite number, and what you end up with is a pressure or a force that brings the plates together. And we have actually measured this. This is a real thing, and yes, I am not kidding around, you can take infinity minus a different infinity, and get a finite number. It’s possible. One example is the Euler Mascheroni Constant. I dare you to look it up!
So there you go, now I hope you understand what these virtual particles are, how they’re detected, and how they contribute to the evaporation of a black hole.
And if you haven’t already, make sure you click here and go to his channel. You’ll find dozens of videos answering equally mind-bending questions. In fact, send your questions and he might just make a video and answer them.
"Color" radio image of galactic cluster Abell 2256. Credit: Owen et al., NRAO/AUI/NSF.
Even though it’s said that the average human eye can discern from seven to ten million different values and hues of colors, in reality our eyes are sensitive to only a very small section of the entire electromagnetic spectrum, corresponding to wavelengths in the range of 400 to 700 nanometers. Above and below those ranges lie enormously diverse segments of the EM spectrum, from minuscule yet powerful gamma rays to incredibly long, low-frequency radio waves.
Astronomers observe the Universe in all wavelengths because many objects and phenomena can only be detected in EM ranges other than visible light (which itself can easily be blocked by clouds of dense gas and dust.) But if we could see in radio waves the same way we do in visible light waves – that is with longer wavelengths being perceived as “red” and shorter wavelengths seen as “violet,” with all the blues, greens, and yellows in between – our world would look quite different… especially the night sky, which would be filled with fantastic shapes like those seen above!
View of the VLA in New Mexico. Image courtesy of NRAO/AUI.
Created from observations made at the Very Large Array in New Mexico, the image above shows a cluster of over 500 colliding galaxies located 800 million light-years away called Abell 2256. An intriguing target of study across the entire electromagnetic spectrum, here Abell 2256 (A2256 for short) has had its radio emissions mapped to the corresponding colors our eyes can see.
Within an area about the same width as the full Moon a space battle between magical cosmic creatures seems to be taking place! (In reality A2256 spans about 4 million light-years.)
See a visible-light image of A2256 by amateur astronomer Rick Johnson here.
The VLA radio observations will help researchers determine what’s happening within A2256, where multiple groups of galaxy clusters are interacting.
“The image reveals details of the interactions between the two merging clusters and suggests that previously unexpected physical processes are at work in such encounters,” said Frazer Owen of the National Radio Astronomy Observatory (NRAO).
Radio image of the night sky. (Credit: Max Planck Institute for Radio Astronomy, generated by Glyn Haslam.)
The Parkes Radio Antenna. Credit: R. Hollow, CSIRO
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Radio waves are electromagnetic waves, or electromagnetic radiation, with wavelengths of about a centimeter or longer (the boundary is rather fuzzy; microwaves and terahertz radiation are sometimes considered to be radio waves; these have wavelengths as short as a tenth of a millimeter or so). In other words, radio waves are electromagnetic radiation at the lowest energy end of the electromagnetic spectrum.
Radio waves were predicted two decades or so before they were generated and detected; in fact, the historical story is one of the great triumphs of modern science.
Many years – centuries even – of work on electrical and magnetic phenomena, by many scientists, culminated in the work of James Clerk Maxwell. In 1865 he published a set of equations which describe everything known about electricity and magnetism (electromagnetism) up till that time (the next major advance was the work of Planck and Einstein – among others – some four decades or so later, involving the discovery of photons, or quantized electromagnetic radiation). Maxwell’s equations, as they are now called, predicted that there should be a kind of wave of interacting electrical and magnetic fields, which is self-propagating, and which travels at the speed of light.
In 1887, Heinrich Hertz created radio waves in his lab, and detected them after they’d travelled a short distance … exactly as Maxwell had predicted! It wasn’t long before practical applications of this discovery were developed, leading to satellite TV, cell phones, GPS, radar, wireless home networks, and much, much, more.
For Universe Today readers, the discovery of radio waves lead to radio astronomy. Interestingly, theory again preceded observation … several scientists – Planck among them – predicted that the Sun should emit radio waves (be a source of radio waves), but the Sun’s radio emission was not detected until 1942 (by Hey, in England), nearly a decade after celestial radio waves were detected and studied, by Jansky (and Reber, among others).
Light spectrum can mean the visible spectrum, the range of wavelengths of electromagnetic radiation which our eyes are sensitive to … or it can mean a plot (or chart or graph) of the intensity of light vs its wavelength (or, sometimes, its frequency). More possible ambiguity: ‘light’ … which can refer to what we see, or to the part of the electromagnetic spectrum that optical telescopes (especially the ones down here on the ground) work in (and sometimes, just occasionally, it means the whole of the electromagnetic spectrum, or any electromagnetic radiation). Good news: the context makes it clear!
The realization that visible light is made up of colors is most often attributed to Isaac Newton (though a strong case can be made that it was known well before him), who used a prism to create a spectrum (rainbow of colors) from a beam of white light, and another to recombine them back into white light. And what’s it called when you spread light into a spectrum, for the purpose of studying it (in astronomy, chemistry, …)? Spectroscopy. And is there a different word if it’s infrared, ultraviolet, x-rays, … which are spread into a spectrum (rather than visible light)? Nope, it’s still spectroscopy.
Visible light ranges from about 380 nanometers (nm) to about 750 nm (or, as is still common in astronomy, ~3800 angstroms (Å) to ~7500 Å); the window in the Earth’s atmosphere which allows us to do astronomy from down here on its surface (and lets the light of the Sun through, so we can see!) is a bit wider than the visible spectrum; it goes from about 300 nm to about 1100 nm (or 1.1 µ).
To an astronomer, a light spectrum has two main components, the continuum and the lines (sometimes bands as well). The lines are discrete wavelengths (well, they do have some ‘width’, hence ‘narrow lines’ and ‘broad lines’), either emission or absorption, and correspond to a particular atomic transition (an electron jumps between one allowed energy level in an atom, or ion, and another; bands are the same thing, except for molecules … and the allowed states are either vibrational or rotational). And the continuum? Well, it’s the part that isn’t lines! It varies smoothly, and generally slowly, across the spectrum.
Spectroscopy – analysis of the light spectrum – is one of the most powerful tools astronomers use to work out what’s going on, and what it’s like, way out there where the light from the sky originates. Do you know why? If not, then these two NASA webpages will help! Visible Light Waves , and Electromagnetic Spectrum.
