Better to light a candle than to curse the darkness. -Chinese Proverb
Every once in a while, we'll look out into the sky with a telescope, and see some spectacular glowing gas.
These nebulae typically come about from dead or dying stars, and are some of the most spectacular sight in the sky for astronomers, from amateur to professional.
But in the 1940s, an astronomer named Bart Bok observed these little dark "defects" in a few of these nebulae. It looked like something dark was simply sucking in all of the light around it, and refused to let any out.
In fact, looking even with modern technology (like the Hubble space telescope), we can see that a few of these regions are loaded with these weird globs of darkness.
(FYI, those black squares at the upper left tell you that this was taken with Hubble's old camera, WFPC2.)
These are named Bok globules after their discoverer, but for a long time, we didn't really know what they were. Of course, they must be gas and dust that blocks the light, but what was going on inside of them?
In 1947, Bok and Edith Reilly wrote what they hypothesized was going on in these globules.
Similar to an insect's cocoon, they said, "these probably represent the evolutionary stage just preceding the formation of a star." Only, for over 40 years, there was no way to look for them.
It was only in 1990, seven years after Bok's death, that they were able to confirm that, indeed, there are stars being born in these collapsing regions! Why did it take so long? Because we needed to look in infrared light to find the signatures of star birth, since visible light is unable to make it through these dark globules.
And today? Let's take a look at the famous "pillars of creation" in the Eagle Nebula.
These are clearly collapsing regions of gas and dust, but are there stars forming there? Again, visible light is no help. But if we look to our X-ray telescope, Chandra, what do we find?
Amazing! Sometimes, we get it right on our first guess, even if it isn't confirmed until beyond our lifetimes. It's one of the best feelings a scientist can get, is to be on the cutting edge and make an educated guess, and turn out to be right!
So I'm curious now: of all the educated guesses scientists are making, what do you think are the ones likely to turn out to be right?
What's in a name? That which we call a rose
By any other name would smell as sweet. -W. Shakespeare
After writing about the 80th Birthday of Pluto becoming a planet, I was asked about Pluto's planetary status, and whether I thought it deserves to be a planet or not. Let me just recap for you, very briefly, what this argument is all about.
Pluto, when it was discovered back in 1930, was the only object in the Solar System found out beyond Neptune. Although we imaged it, observed it, and surveyed the whole sky for other objects, it remained the only Solar System object out beyond Neptune until the 1970s, when Pluto's giant moon, Charon, was discovered.
But this all changed severely in the 1990s. A whole zoo of "trans-Neptunian" objects were discovered, many of them of comparable mass and size to Pluto. In 2003, Eris was discovered, and this was big news to people who were worried about what counts as a planet and what doesn't. Why? Because Eris was larger than Pluto!
So, the argument went, if Pluto was a planet, then Eris needed to be one also. If Eris wasn't a planet, then Pluto couldn't be one either. So what do we do? For nearly three generations, we all learned that there were nine planets in the Solar System; was it necessary to change all of that?
Putting aside what the IAU decided -- which was to demote Pluto to "dwarf planet" and promote Eris and others to "dwarf planet" -- let's think about it. What's really going on in our Solar System?
Close to the Sun, we've got four rocky planets -- two with moons -- and nothing else. If there's anything interesting going on from the Sun out to about 300,000,000 km away, it's probably happening on Mercury, Venus, Earth, Earth's Moon, Mars, or (possibly) Mars' moons Phobos and Deimos. So it makes sense to have four inner planets. Beyond that?
Well, sure, you've got the four gas giants. Obviously, they dominate the outer Solar System, and they clearly deserve planetary status. Each one is many times the mass and size of Earth, has a system of rings around it, and has a plethora of moons (both large and small) orbiting it.
But the Solar System is much more than just these four inner planets and the four outer planets, isn't it?
In between Mars and Jupiter, we've got thousand upon thousands of asteroids, moving too quickly and of too low a mass to merge into a giant planet, much the same way Saturn's rings won't merge into a giant Moon. The largest of these asteroids is almost 1000 km across, and massive enough to have pulled itself into a sphere! Say hello to Ceres, as seen by Hubble.
Out beyond Neptune, there's Pluto, Eris, Makemake, Haumea, and many other large bodies in the Kuiper belt, in addition to a bunch of smaller frozen rocks, some of which will one day become moons of the gas giants or comets!
Does the fact that Pluto lives in the Kuiper belt make it less special than if it were the only one out there? Does the fact that Ceres isn't a standalone object, but has a whole host of neighbors make it less special than if it were on its own? Does Saturn's moon Titan -- with an atmosphere thicker than Earth's -- become any less special in your eyes because it wasn't able to eat up Saturn's rings with its own gravity?
The answer to all of these questions, of course, is no. These objects are special based on their own merits, not because of what's going on around them. And if you don't like the fact that the IAU called Pluto a "dwarf planet," just call it "the planet Pluto." The purpose of language is to communicate, and to be understood. I don't adhere to MLA writing rules when they don't suit me, and I think it's the height of foolishness to expect that people will stop calling Pluto a planet (or stop thinking of it as a planet) when they refer to it.
So a recap of my thoughts? Call it whatever you want. It's a fascinating member of our Solar System, the first trans-Neptunian object ever discovered, the pioneer in our understanding of the Kuiper Belt, and it remains an ignition switch to some of the best parts of our imaginations. If you want to say "dwarf planet Pluto," we can still have a good chat about it, and the same goes if you say "the ninth planet," "the former planet," or just "Pluto." It doesn't change what it is, or how much we all care about it, and that fact is more powerful than any IAU ruling will ever be. Pluto has better things to do than care about whether you call it a planet or not.
So call it whatever you want; if I were Pluto, I'd be too busy orbiting the Sun.
Yesterday, I wrote to you about part 5 of The Greatest Story Ever Told, about how the Universe came to have more matter than antimatter in it. And many of you correctly responded that I had given too much detail and not enough explanation.
So, I want to try again for all of you. Here's the explanation, starting at the beginning.
The Universe inflated first, stretching it flat and making it uniform, both everywhere in space and in all directions equally. Then inflation ended, and all the energy that was making it inflate got dumped into particles and radiation. This part, when inflation ended, could be called the "classical" big bang.
The radiation and these particles were incredibly energetic, even compared to anything we've ever created in a laboratory. It was so energetic that there were no atoms, nuclei, or even protons or neutrons. What you need to do, if you want to understand what the Universe looked like at this time, is imagine the tiniest point particles and all of their antiparticles, flying around in great abundance, as close to the speed of light as the conservation of energy allows them. This means electrons, muons, taus, all six quarks, neutrinos, and all of their antiparticles in equal abundance, in addition to whatever else may be out there.
They're also way more abundant at this time -- by about a factor of a billion -- than matter particles are today. But if this was all we had in the Universe -- matter and antimatter in equal amounts -- they would annihilate away (like matter and antimatter are wont to do), until there were so few particles left that they couldn't find each other in this huge and expanding Universe.
But this didn't happen; if it did, only one in every hundred billion particles that exist now would still be here, and half of them would be antiparticles. (This is down by a factor of about 1020 from the particles & antiparticles that existed at the end of inflation.) So something had to have happened that made the Universe choose matter over antimatter, and at the level of about one extra matter particle for every billion matter/antimatter particles out there, otherwise nearly everything would have annihilated away into photons (i.e., light) by now.
So what happened? Well, we know that there are a few constraints between matter and antimatter. First off, if you have an unstable matter particle, its corresponding antimatter particle is also unstable. Because of symmetries between matter and antimatter, they need to have the same total decay rate, which means they need the same lifetime and they need to be able to decay into the opposite, corresponding particles. And finally, whatever it is that they decay into, there is a conservation law telling us that the net number of baryons (protons & neutrons) must equal the net number of leptons (electrons and neutrinos) that you create.
This works out to be amazingly convenient, because we do have a Universe with equal numbers of protons and electrons! But, I wanted to tell you how this happened. There are many different ways, including (here come some scientific names) GUT baryogenesis, leptogenesis, electroweak baryogenesis, and the Affleck-Dine mechanism.
But let's make up a way to do this that's even simpler than any of these, just to show you how this is possible. Imagine that we have a new particle called Y, which is neutral and unstable, and its antiparticle, Y*. This is both necessary and reasonable; in fact, it seems unreasonable to imagine that we've discovered every single particle in the Universe, considering that there's such a significant energy range left to explore.
We need to produce a lepton for every baryon we produce, and an anti-lepton for each anti-baryon we produce. We also have to conserve charge, and we need for the Y and the Y* to allow the same types of decays. The one way they're allowed to be different is known as CP violation, which means that particles can decay through one decay route more frequently than their antiparticles do, and antiparticles can decay through an alternate route more frequently.
So let's say the Universe is full of -- among other things -- Y's and Y*'s. The Y's can decay into either a proton (one baryon) and an electron (one lepton), or an anti-neutron (one anti-baryon) and one anti-neutrino (one anti-lepton). In both cases charge is conserved and we meet our conservation law about baryon number equaling lepton number. What do the Y*'s do? Well, our conservation laws tell us they must decay into either an anti-proton (one anti-baryon) and a positron (an anti-lepton) or into a neutron (one baryon) and a neutrino (one lepton).
But remember what CP-violation lets us do: it allows the Y's to decay into protons and electrons more frequently than they decay into anti-neutrons and anti-neutrinos, and it allows the Y*'s to decay into neutrons and neutrinos more frequently than they decay into anti-protons and anti-electrons. It means that if we have a thousand Y's and a thousand Y*'s, 501 Y's can become protons and electrons while 499 become anti-neutrons and anti-neutrinos, while 501 Y*'s can become neutrons and neutrinos, while 499 become anti-protons and positrons. The protons and antiprotons, neutrons and anti-neutrons, and electrons and positrons will all find each other, annihilating away. But what will be left over? Two extra protons, two extra electrons, two extra neutrons, and a bunch of neutrinos and anti-neutrinos.
In other words, more matter than antimatter. I realize this is a difficult topic and I'm sorry I don't have a simpler way to explain it, but that may be because I don't have a simpler way that I understand it. So ask your questions here, and if there are enough good ones that are answerable in a reasonable length, I'll take them on at the end of the week. In the meantime, I hope this clarification helps!
For every one billion particles of antimatter there were one billion and one particles of matter. And when the mutual annihilation was complete, one billionth remained - and that's our present universe. -Albert Einstein
Welcome back to our series, The Greatest Story Ever Told, where we're recounting the physical history of the Universe, from beforethebigbang up through the present day. We're currently in a hot, dense, expanding Universe, filled with equal parts matter and antimatter, bathed in radiation, and it's been only a tiny fraction of a microsecond for all of this to happen.
But the Universe we live in today isn't equal parts matter and antimatter. In fact, every galaxy we observe in the Universe is made out of matter and not antimatter. The laws of nature that we've discovered are pretty symmetric between matter and antimatter, and we believe that the Universe started out with equal amounts of matter and antimatter. So how are we here? If there were equal amounts and the Universe was very dense, eventually nearly all of the matter and antimatter would find their antiparticles, and would annihilate, leaving a Universe that was practically empty except for radiation (photons).
Although there are many different ways to make slightly more matter than antimatter, they all have the following properties, known as Sakharov conditions:
You need to be able to create or destroy baryons (protons, neutrons, etc.),
You need particles and antiparticles to have slightly different properties from one another, and
You need to be out of thermal equilibrium.
This is not hard. First off, if you're in an expanding, cooling Universe, you're always going to go out of thermal equilibrium, so that one's a given. But what about the other two? How could this possibly happen, and still obey all the laws of physics we currently observe?
Let me lay out the simplest scenario for you of how to make more matter than antimatter, and if you want to know the word physicists use when we talk about this process, it's called baryogenesis.
And it doesn't take anything divine, either. I'm going to assume that we have electrons (charge -1), positrons (charge +1), and that protons and neutrons are made up of quarks.
A proton has two up quarks (charge +2/3 each) and one down quark (charge -1/3), while a neutron is made up of one up quark and two down quarks, while antiprotons and antineutrons are made up of two anti-up quarks (charge -2/3) and one anti-down (charge +1/3), and antineutrons are two anti-down and one anti-up. So if we want more matter than antimatter, we need to make more quarks than antiquarks, and more electrons than positrons.
How can we get this? Imagine a particle -- I'll call it X -- that has a charge of +4/3, and can decay either into two up quarks or one positron and one anti-down quark. It also has an anti-particle -- X* -- that has a charge of -4/3, and can decay into two anti-up quarks or an electron and a down quark.
So our possibilities are:
X --> up + up
X --> positron + anti-down
and
X* --> anti-up + anti-up
X* --> electron + down
The early Universe is full of all the particles that can exist, including X's (or things very much like them.) If the X goes into two ups 50% of the time and into a positron and anti-down 50% of the time, then we'll get the Universe we want if the X* goes into two anti-ups 49.99997% of the time and into an electron and a down quark 50.00003% of the time.
Is this possible? Yes; it's called CP-violation, and we've observed it in many different cases.
So even if everything starts out perfectly symmetric between matter and anti-matter, all you need is a slight difference between particles and anti-particles, consistent with what we observe, and you'll be guaranteed to have a Universe with either more matter than anti-matter or more anti-matter than matter!
(And if it were the other way around, you'd never know, except you'd be made of anti-matter, and you'd likely be calling it matter!)
Once you've got this problem solved -- making the matter in the Universe -- you can get on to turning it from a hot, dense, expanding soup into the Universe we see today.
Come back for part 6, where we'll take the next step on our journey!
Until one has loved an animal, a part of one's soul remains unawakened. -Anatole France
Those of you who own pets probably know that there's nothing to grab your animal's attention like rewarding them with food. A Japanese game show has taken this to a whole new level, and somehow, I can't look away.
I love the part where the first dog is just drooling and drooling with dozens of treats on its head. Enjoy your weekend!
In a nutshell, a nearby massive body (like the Moon or the Sun) pulls on the Earth's center due to its gravity. But the portion of the Earth that's closest to that massive body gets pulled with a slightly greater force, while the portion that's farthest gets pulled with a slightly smaller force. This differential force, known as a tidal force, causes objects to be stretched out, and causes our oceans to bulge at the points nearest and farthest from the Moon, where the tidal forces are greatest.
But what about more extreme cases? All you need for tides to be more extreme is to have three things in place: the object causing the tides needs to be massive, you need to be close to it, and you yourself need to be long. Let's start in our own Solar System, looking at our most massive planet, Jupiter. The nearest large object to Jupiter is its innermost moon, Io. Let's take a look at Io.
This was Io when New Horizons passed it just three years ago. Do you know what that bright spot on the unlit side is? That's a volcano! But Io is even smaller than Earth's Moon, which has a cooled core and is volcanically dead, and it's even farther from the Sun. Do you know where it gets the energy to have active volcanoes on it? Tidal heating from Jupiter.
But are these tidal forces ever strong enough to not only heat things up, but to completely tear objects apart? Let me introduce you to an old friend of mine from back when I was in high school.
Say hello to Comet Shoemaker-Levy 9, as imaged in 1993 (shortly after its discovery). Shoemaker-Levy 9 was extraordinarily interesting, because shortly after its discovery, they realized it was headed right towards Jupiter. And when I say "right towards", this is what I mean.
That's right, Shoemaker-Levy 9 became the first comet we ever observed directly to collide with another planet. But this image above is not a time-lapse photo! Instead, what we observed was that as the comet approached Jupiter, the tidal forces tore it apart, and stretched it out into a long, straight series of fragments. Take a look at what we saw when we pointed the Hubble Space Telescope at it back in 1994, shortly before the collision.
Amazing, that even in our Solar System, objects can be completely torn apart by tides!
But why stop there? We can go more massive, closer, and experience the greatest tidal forces in the Universe by approaching a black hole. After all, a black hole is a mass that's anywhere from a fraction of the mass of our Sun to over a billion times the mass of our Sun. What if you wanted to push the limits, and fly towards a black hole with the mass of our Sun?
Oh yeah, space would look funny for a while, due to the distortion. But at some point, you'd really start to physically feel it! Just like the Earth gets thinner and stretched more, so will you. And it will be more and more severe the closer you get!
So what happens to you as you get closer and closer to the center of this black hole? Well, in order (and the whole process -- from the first item on the list until the last -- takes under a minute):
your extremities (head, arms, legs) are torn from your torso,
the individual muscles, tendons, ligaments, etc., are ripped apart from your body,
your individual cells are torn apart from one another,
the organelles inside each cell are ripped apart, destroying the cells themselves,
the individual molecules from your body are ripped apart into atoms,
the tidal forces tear your atoms apart into nuclei and electrons, and finally
the individual nuclei are ripped apart into, eventually, quarks and gluons.
And if there's anything smaller than electrons, quarks, and gluons, the tides will eventually rip everything into their most fundamental constituents.
And a fun fact for you? At the event horizon of a black hole (as opposed to falling into the singularity, as above), the tidal forces are greatest for the smallest masses of black holes. In fact, if you went up to the center of our galaxy and went right up to the event horizon of our supermassive black hole, Sagittarius A*, weighing in at around 3 million Suns, the tidal forces on you would be less than the force of Earth's gravity is right now! This is because the event horizon for a more massive black hole is so much farther away than for a less massive one; it turns out that distance is more important than mass alone.
So if you want to destroy something with tides, just send it as close as possible to as massive an object as possible.
And keep this in mind if you've got dreams of traveling through a wormhole: you've got to survive the tides!
When you get into a tight place and everything goes against you, till it seems as though you could not hang on a minute longer, never give up then, for that is just the place and time that the tide will turn. -Harriet Beecher Stowe
Last week, our longtime reader Pamela asked if I could explain how the tides work. As you all know, when the tide comes in at the ocean, the water level appears to rise (and can do so significantly), while at low tide, the water level appears to drop.
This goes in a cycle twice per day, with the ocean level reaching its highest point twice daily (high tide), having the water recede over a period of six hours until it reaches its lowest level (low tide), and then having the water level rise again over a period of another six hours until it reaches the next high tide. Variations in the height of the water level are typically on the order of three meters (maybe ten feet) each day, depending on a couple of factors, which I'll go into below.
The reason we have any tides at all are twofold: the Earth is pretty big and gravity cares how far away you are. The farther away you are from something, the weaker gravity's pull is on you. If you were to take a look at our Solar System, and you were to move the Earth out to where Pluto is, you'd find that the force of gravity from the Sun on the Earth would be an astounding 1,600 times weaker than it is today, as Pluto is 40 times as far away as Earth is from the Sun!
If you were to look at everything in our Solar System and ask what affects the Earth the most, gravitationally, you'd think to look at two things: the Moon, because it's massive and it's very, very close to us, and the Sun, because it's extremely massive, even though it's quite far away. Let's start by considering the Moon.
The Earth is quite far from the Moon, at an average distance of 384,400 km. When we speak about this distance, however, we are talking about the distance from the center of the Earth to the center of the Moon. But one edge of the Earth will always be closer to the Moon by 6,370 km (the radius of the Earth), and the opposite edge will always be farther from the Moon by the same amount. This means -- after a little math -- that the force of gravity of the Moon on the far side of the Earth is about 3.2% weaker at the far edge of the Earth than it is at the center of the Earth, and about 3.4% stronger at the edge of Earth nearest the Moon than it is at the center. This difference in forces between the near edge, the center, and the far edge defines what we call tidal forces.
This means the effect of the Moon's gravity on Earth is to try to flatten it a little bit at the poles and wherever Moonset/Moonrise is occurring, and to stretch it at its nearest point (when the Moon is directly overhead) and its farthest point (exactly 12 hours from the Moon's apex). This force is weak enough that it wouldn't be a big deal at all if the Earth were simply a solid ball; the tidal forces from the Moon are unable to stretch rocks and dirt by more than a few millimeters. But the Earth is covered in water, which changes its shape extremely easily!
(Image Credit: Steve Gaunt.)
So while the solid ground of the Earth remains in its roughly spherical shape, the oceans bulge by just a few meters in two spots around the equator: at the point closest to the Moon and at the point farthest from the Moon. As the solid ground rotates, each point on the Earth passes through the side closest to the Moon and the side farthest from the Moon once per day: these are your two high tides.
The two times that correspond to Moonrise and Moonset are your two low tides per day. And the closer to the equator you are, the more severe your tides are, while the closer to the poles you are, the less drastic your tides are!
But the Moon isn't the only gravitational body in our Solar System affecting the tides on Earth. While none of the other planets, moons, asteroids or comets in the Solar System matter, the Sun does!
The tidal forces from the Sun are weaker than those from the Moon, but are still quite strong, causing tides that are about 30% as strong as the Moon's. When the Sun and the Moon are lined up, during a New Moon and during a Full Moon, you get the highest high tides and the lowest low tides, known as Spring Tides.
But when the Sun and Moon are at right angles to each other (during the Moon's first and last quarter, or when it appears half-full), you get the lowest high tides and the highest low tides, known as Neap Tides.
In fact, if you're meticulous, you can measure the water level over a long period of time, and can see not only the high tides and the low tides, but also where the Spring Tides and Neap Tides occur. Take a look at this data from Bridgeport, Connecticut.
And that's how tides work! I freely admit that there are small, subtle details that come into play if you want to predict the times and heights of the tides extremely accurately. But just by considering the gravity of the Sun, Earth and Moon, and by calculating the force on the oceans, you can do an incredible job of predicting all of the above about the tides. Thanks to Pamela for a riveting question, and I hope you all enjoyed the answer!
Free energy will promulgate a forward leap in human progress akin to the discovery of fire. It will bring the dawn of an entirely new civilization -- one based on freedom and abundance. -Sterling Allan
There is, of course, no such thing as truly free energy, or energy that we can take out of nothing and use for something, which is why perpetual motion machines not only don't work, but are physically impossible. (Although it is amusing to try to get as close as possible.)
But as many of you have noted about dark energy, there is a non-zero amount of energy that seems to be inherent to space itself. As the Universe expands, it appears to create more space, and hence, more energy.
Now, the energy density is tiny. So tiny that we didn't even discover the existence of dark energy until 1998, and if you were to compare it to the energy stored in, say, the mass of a human body, you would have to spread a human being out over the entire inner solar system (to fill a sphere the size of the orbit of Mars) just to get the same density as dark energy, which is about two protons per cubic meter.
Now, while energy is ill-defined in general relativity, we understand energy and momentum well enough (as well as more complicated properties of metric spaces, which I will not go into) to know that dark energy should have the following properties based on our current observations:
It should have a constant energy density everywhere in space.
It should be impossible to add to or take away from that energy density.
That energy density should also remain constant throughout time.
However, a recent paper has come out on the arXiv (and was discussed over on Cosmic Variance earlier today) that seeks to test that first assumption: is dark energy a constant everywhere in space?
Using a hypothetical improvement on a technique called atom interferometry (illustration above), they are proposing that changes in dark energy density could lead to changes in atomic motions, and could hypothetically exert a force on atoms.
Now, there are all sorts of reasons to believe that dark energy doesn't exert a force on atoms. Namely, the following big ones:
Dark energy, as far as we can tell, affects the expansion of space and nothing else, meaning it shouldn't exert a force on atoms.
You can only exert a force (whether you're dark energy or not) if your field changes from point-to-point. On the other hand, dark energy is observed to be a constant everywhere in space.
But, it isn't like we have a better proposal out there to try to perform some laboratory test on dark energy. Does it couple to matter? We don't think so, but we haven't tested it sufficiently to know for sure. Is it a constant everywhere in space? We think so, but we don't know if it clumps (even a little) around masses like the Earth or the Sun.
There is so much we don't understand about dark energy, and how the Universe's expansion on the largest scales relates back to what we can observe in a laboratory on Earth that this is possibly the most exciting prospect to come out concerning dark energy all year!
So do I think this is likely to produce anything new? No, probably not. There are many good reasons to believe that we know what we're talking about, but that doesn't really matter. What matters is that -- in order to know anything for sure -- we need to do the experiment. This idea deserves to get a little bit of a buzz, if for no other reason than we need to throw ideas around about dark energy, and be open to the notion that what we're seeing is so bizarre it could really turn our view of the Universe on its head.
But there still isn't any truly free energy out there, not even if dark energy does change from point-to-point. Which is too bad... because if there was some, I could then move to phase 3. I'll keep dreaming.
When you look at yourself from a universal standpoint, something inside always reminds or informs you that there are bigger and better things to worry about. -Albert Einstein
I woke up this morning with the Sun in my face, which marks the first time all year that that's happened.
(The irony, that I'm now listening to the Grateful Dead's "Looks Like Rain.")
And after a few seconds had passed (you know, it takes me a few seconds to realize that I'm not still in my dream, trying to navigate through some bizarre hotel corridor), I realized I couldn't read the numbers on my old LED alarm clock. After pulling the clock out of the sunshine, I could read it perfectly, and it occurred to me just how important perspective is to your sense of vision, and how easily your rods and cones can fool you.
In fact, this simple trick is how nearly all optical illusions work: confusing your eye with the images that surround the one you're focusing on. For example, take a look at the picture below, with a lit checkerboard and the shadow from an object falling on it.
Would you guess that the squares labeled "A" and "B" are actually the same color as one another? In fact, you probably don't even believe it.
In fact, you probably -- looking at the image below -- also wouldn't believe that the face of the white slab is actually the same exact shade as the face of the grey slab, would you?
But surely I'm full of it, and you aren't going to take my word for it. At any rate, perspective wouldn't be able to fool your eyes with color the same way I'm contending it can fool them with brightness, would it?
There's no way that the central brown section on the top face is the same color as the central yellow section on the leftmost, near face, is there?
And yet, it's all a question of perspective. If I remove and/or sufficiently dim the objects surrounding the regions in question, you'll see what I'm talking about. Let's utilize my massive paintbrush skills to black out part of the topmost image.
Amazed that the squares "A" and "B" are actually the same color and shade? Take a look at what happens to the image of the grey tile and the white tile when I remove everything except the central faces of those images.
Yes, they really are exactly the same shade. And that colored cube? What happens if I darken everything except that brown tile and the yellow tile?
Isn't that something else? So don't believe everything you see, at least not before looking with a little more scrutiny. Nah, that sentiment is good for rainy days. I'm feeling a little lighter because of the sunshine, so just get out there and enjoy how brilliantly your eye and your mind fills in the gaps, and creates colors and shades that help us better perceive our surroundings here on Earth!
