I was recently watching an episode of The Universe, the show from the History Channel, which detailed ten different ways to destroy the Earth (Season 4 Episode 6). One of the proposed destruction techniques was to instantly stop the rotation of the Earth. This would cause everything on the surface of the Earth to be violently thrown as the ground beneath it instantly stands still. It would be quite a violent stop. For some perspective, in New England, where I am located, you would be thrown at a velocity of about 800 miles an hour to the East. It is very unlikely that anybody could survive an impact with anything at that speed. That is not to mention that every building would be thrown at that velocity as well. However, the contributors to the show, professional astrophysicists, claimed that if you could survive the initial stop it is still unlikely that you could survive the aftermath. Specifically, the atmosphere would not be stopped, so there would be windspeeds of up to 1000 miles per hour as it continued rotating at its initial velocity. The show claims that this wind would create so much energy that it would heat up the atmosphere enough to melt rock. This claim seemed a bit too dramatic to be true, so I decided to do some calculations to see how credible it is. Brace yourselves, quite a bit of math is ahead as we fact-check…The Universe! Continue Reading
For those of you outside the physics community something big is happening next week that you may not know about. It is the annual March Meeting of the American Physical Society (APS). It is the single largest physics conference in existence and it just so happens to be taking place in Boston this year. To give you an idea how big, last year, there were over 7700 submitted abstracts meaning that there were that many people with posters or giving a talk about their research. That does not include the thousands more physicists that go to keep up with current research and foster collaborations with other groups. Needless to say, I will be there almost all of next week soaking it all in as it will be the first conference I have ever been to. I will try to post updates about the meeting throughout the week so stay tuned!
There has been quite a bit of press about construction of a space elevator by Obayashi Corp, a Japanese construction company, lately. As exciting as an elevator to space would be, let’s just look at some of the hurdles that such a project would have to overcome from a scientific point of view.
What may not be obvious to non-physicists is how you would keep an elevator to space from falling back down to Earth. It would, in some sense, be the tallest thing mankind would have ever constructed, but you can’t just build a really tall building into space. Instead, you would have to assemble a giant platform in geosynchronous orbit (altitude 36,000km) and then lower the elevator cable from space. This would allow you to build the elevator from the top down. However, the plans proposed by Obayashi propose extending the elevator to 96,000km. This means that if the top of the elevator were not connected to Earth in any way, it would orbit slower than the Earth rotates. When you connect the two, the Earth would actively pull on the top platform, speeding it up to match the rotational speed of the Earth. This makes sense from a construction point of view; having the platform want to fly away from the Earth means that you can pull on the cable in order to climb it without pulling the elevator down on top of you.
Even though the top of the elevator has to be pulling on the Earth to keep tension in the cable, it is a productive exercise to figure out just how much tension would be produced from something at that altitude. First, we need to make a few assumptions. Let’s assume the counterweight at the top of the elevator has about the same mass as the International Space Station (450,000kg) and for now let’s ignore the mass of the cable connecting the counterweight and the Earth.
It is easy enough to write down the force on the counterweight due to the gravity of the Earth. Newton figured it out centuries ago:
Fearth = -G M m/r2
Meanwhile, the force from the Earth accelerating the counterweight to match the rotation speed of the Earth is given by:
Frotation = m ω2 r
where ω is the rotation velocity of the Earth.
The sum of these two forces is what acts on the counterweight:
Ftotal = -G M m/r2 + m ω2 r
At an elevation of r=102,400km (the altitude plus the radius of the Earth), Ftotal = 227,799 Newtons. This is equivalent to the weight at Earths surface of a 25 ton object. In construction terms however, that is not too bad; many things weigh much more than that are suspended by cables. However, the cables used to make such an elevator would have to stretch almost 100,000 km; about 100 times longer than any current cable in existence! Not only that, but when you factor in the weight of the cable itself, it is not feasible unless the material it is made of has a much higher tensile strength/mass ratio than steel. Currently the only material that has such a high ratio is carbon nanotubes, but the longest continuous piece of carbon nanotubes was 2cm long made at MIT, not even close to useful lengths.
So, due to the limited manufacturing capability of carbon nanotubes, the space elevator is likely to remain a lofty dream for the foreseeable future. This doesn’t mean we can’t imagine what it would be like to visit the top of the space elevator. For instance, if you were way up at the counterweight, you would not be weightless like the astronauts in the ISS. Instead, you would actually feel a weak force pushing you away from the Earth, about 1/20th that of gravity on the surface of the Earth. This means that in order to not fly away and off into space, you would either have to stand with the top of your head facing the Earth or somehow tether yourself to the counterweight. This would have to be considered when building the elevator car. When you start your journey on the Earth, you would experience normal gravity. However, that gravity would slowly decrease until you reached and elevation of 36,000km where you would be weightless. Then, as you climbed more, you would experience a force pushing you into the ceiling. That means what started as the ceiling became the floor and when you travel back down to the planet, you would have to switch one more time. So, controls, storage, facilities, etc. in the elevator car would need to be able to operate regardless of the apparent direction of the force of gravity.
Which mode of transporation is more awesome: The Knight Bus or The Cat Bus?
For those of you unfamiliar with these two “vehicles,” the Knight bus is from the world of Harry Potter. It picks up wayward wizards on the British Isles and takes them wherever they want to go in magical style. The Cat Bus is from the world of Hayao Miyazaki’s My Neighbor Totoro. It picks up little girls that get lost and takes them home. To see the two in action, check out the videos:
I think this argument is pretty cut and dry. The Knight Bus is a superpowered triple-decker bus that can take you anywhere you want to go on the British Isles. It is the pinnacle of wizardly ground transportation. The Cat Bus on the other hand is a freak of nature. Just take a second and try to imagine the anatomy going on inside the cute and fuzzy exterior. What does the Cat Bus’s skeleton look like, especially considering the fact that it can just open up a door in its side at will. That brings me to another point, the Cat Bus is alive. It would need to eat and expel waste. If you think car exhaust smells bad, then just imagine a Cat Bus garage. My final point is that we can easily tell the Cat Bus is some accident spawned from a radioactive wasteland just by the fact that its eyes are its headlights! That’s right, glowing eyes. So, in conclusion, the Knight Bus represents a pinnacle of man’s achievement while the Cat Bus represents the worst side of nature’s folly and therefore the Knight Bus is more awesome than the Cat Bus.
To check out the counterpoint, go here.
The next neighbor on the block we are setting out to meet can be, without a doubt, considered mayor of the celestial neighborhood: The Sun.
Just the Facts:
The Sun is by far the biggest thing in our solar system. It contains 99.8% of all of the mass in the solar system. The Sun is about 93 million miles away from the Earth, just the right distance away so that we are not too hot or too cold. However, if the sun were to suddenly stop shining, we would not know about it on Earth for about 8.3 minutes because that is how long it takes light to get here from the Sun. There are two things that are vital to keep in mind when discussing the Sun; it is MASSIVE and HOT. The radius of the sun is a bit over 430,000 miles. That is almost twice the distance between the Earth and the moon. Meanwhile, the temperature at the surface is a sweltering 10,000 degrees Fahrenheit (5800K) while at the core it is an unconceivable 28 million degrees Fahrenheit (15.6 million Kelvin).
Where did it come from?:
To understand where the Sun comes from, we need to know what is inside of it. The Sun contains about 73% Hydrogen and 25% Helium. However, there is still 2% of the Sun that is made up of heavier elements like iron, lithium, carbon, and basically any other element you can think of. The Sun creates it’s energy by taking Hydrogen atoms and fusing them into Helium atoms thus releasing energy and light. In fact, the famous equation E=mc2 is what governs energy creation in the Sun. Every second in the Sun, 700 million tons of Hydrogen atoms are converted to 695 million tons of Helium atoms. The missing mass is converted into energy through the proportionality constant the speed of light squared, c2. If the sun were big enough, it could also fuse Helium atoms together to make Lithium and so on down the periodic table, creating heavier and heavier elements. However, due to the structure of heavier atoms, Iron is the most massive element that can be produced in the center of a star. However, we know that heavier elements than Iron can be found in our solar system, Uranium for example. There is only one way that elements heavier than Iron can be created naturally; in a supernova. That’s right, our solar system was created from the remains of a supernova.
Our Sun and the rest of our solar system was created from the stuff left over from a dead star that exploded as it died. The gas and dust that was left over from the supernova slowly gathered together through gravity. As most of the material gathered in one central point, friction heated the material up. Once it got hot enough, Hydrogen was fused into Helium and our Sun was born. If this is still confusing, I will defer to Stephen Hawking and Benedict Cumberbatch to explain the origins of our solar system:
Why should I care about it?:
Well, I think we all know that we owe the existence of life to the Sun, but here are some other interesting things about the Sun that impact our lives.
Reason 1: The Sun is Dynamic (and dangerous)
The Sun looks pretty boring from the surface of the Earth; it is round and bright. However, it is so much more than a glowing ball. The surface is churning with tremendous energy, equivalent to many many nuclear warheads and all of this energy manifests itself in several ways.
On Earth, we have earthquakes, where two tectonic plates slide against one another; but on the Sun there are sunquakes, caused by the massive turbulence of superheated gas churning within the sun. The rapid movement of gas causes shockwaves that propagate through the Sun. They are so powerful that we can actually measure them using satellites because the surface of the sun is constantly shaking and moving due to them. In fact, this is exactly what NASA’s SDO project does.
In addition to sunquakes, there are events called flares, prominences, and coronal mass ejections. These three events are related to one another and are all caused by fluctuations in energy and the Sun’s magnetic field. Basically these events shoot plasma out of the sun. If the material loops back and returns to the Sun, it is called a prominence. If it extends straight out of the sun, it is called a flare. Finally, the material that is flung out of the sun through a flare is called a coronal mass ejection (CME). These CMS’s can be especially important for Earthling’s because if they are powerful enough, they could knock out any satellites in its way. For this reason, there are many satellites orbiting the sun giving a space weather forecast to try to avoid such complications.
Reason 2: The Sun is our Protector
Remember how those CME’s are caused by the Sun’s magnetic field? Well it turns out that we rely upon the magnetic field of the Sun to protect us from the harsh radiation of space. The magnetic field of the Sun is incredibly powerful and incredibly complicated. It is so powerful in fact that its influence can be felt far outside of Pluto. It creates a shell around our solar system called the heliosphere. If we were to travel outside of the heliosphere, we would be bombarded by radiation from deep space. These particles don’t normally reach Earth because they are deflected away by the Sun’s large magnetic field. On a side note, Voyager I is close to becoming the first man-made object to ever travel outside of the heliosphere.
Summing it all up:
The Sun can giveth and the Sun can taketh. On one hand, the Sun is necessary for life on Earth though the light and energy heating our planet and making photosynthesis possible. On the other hand, CME’s pose a risk to space travel and satellites. Also, in about 5 billion years or so, the Sun will begin to run low on Hydrogen fuel. When that happens, it will expand in size, engulfing and incinerating our planet. So, next time you wake up to a bright sunny day, remember that there is a lot more than meets the eye when it comes to the Sun.