Starts With A Bang! » Gravity http://startswithabang.com Ethan Siegel's blog/video blog about Cosmology, the Universe, and everything else Sat, 04 Apr 2009 20:12:38 +0000 http://wordpress.org/?v=2.3.2 en How to build a time machine? http://startswithabang.com/?p=1378 http://startswithabang.com/?p=1378#comments Wed, 14 Jan 2009 19:16:41 +0000 ethan http://startswithabang.com/?p=1378 The science channel has, for lack of a tactful way to put it, some pretty bad science on it sometimes. However, they raise public awareness, do a lot of good things most of the time, and most recently, have put up a very interesting interactive web feature called Build Your Own Time Machine.

Some of it is really interesting, and some of it is really unrealistic. Let’s cut through the… uhh… bullbutter… and let’s see what physics says is practically possible, theoretically possible, and what’s theoretically impossible.

1. Traveling forward in time. Of course we travel forward in time, we’re doing it right now! But if you travel at high speeds, like in the rocket shown above, you can travel forward in time faster than everyone else. This happens to astronauts, in fact, but they only travel forward by hundredths of a second after months in space. In order to travel forward a significant amount of time, you need to move close to the speed of light. This is something we can do for subatomic particles right now, but for something the size of a human, it’s only theoretically possible, not practically possible, because it would take too much energy to do it.

Science Channel Score — Good Science: 1 Bad Science: 0

2. Traveling backwards in time. According to conventional physics, there’s no way to do this. You can travel forward in time at a different rate, but backwards? As far as we know, you can’t do that with this Universe. The only conceivable way? Bend spacetime so severely that you create a wormhole. The science channel got that right, too.

Science Channel Score — Good Science: 2 Bad Science: 0

Now, sci-fi writers love to invent scenarios where this just might work. Why? So you can become your own grandfather, of course, like this guy:

(Image Credit: the infosphere.) But given what we know about physics is this even theoretically possible? To be honest, the answer is probably no. The following things would all need to be true to make this possible:

  1. Wormholes would need to actually be able to exist in our Universe. But let’s assume quantum gravity let’s it all work out. What else?
  2. You’d need to find a way to pass through the wormhole without being crushed. The science channel recommends using negative energy to grow the wormhole so it won’t crush you.
  3. The wormhole would have to not only connect one part of the Universe to another, the connection would have to be between different times.

Now, why do I think this may not even be theoretically possible? First, we’ve never seen one nor any evidence for a wormhole ever (contrary to what the Science Channel says on this: strike one).

Science Channel Score — Good Science: 2 Bad Science: 1

But the big prohibitive theoretical thing about this? We’d need to understand quantum gravity to know whether it’s possible to make one. Wormholes happen at very small scales, very large energies, and involve gravity. We don’t have a physical theory that makes sense combining those three. Therefore, it’s a really big assumption to say that these are even possible.

Second, there’s no such thing as negative energy. The science channel really botched this one: they contend that the Casimir Effect, the force that repels two parallel plates placed close together, is based off of negative energy. This is wrong. A repulsive force does not mean negative energy: the energy is in fact positive.

Science Channel Score — Good Science: 2 Bad Science: 2

And finally, putting enough energy to make a hole in spacetime does not mean that you’re going to move anywhere through time. Work on doing it for a single particle first, which has been ongoing for decades, has been 100% unsuccessful.

Now, the facts I’ve given you here won’t sell any books, but they’re 100% right, which is 50% better than what you get from the science channel, and 100% better than most of the science on the history channel. And with more halloween costumes than any other astrophysicist!

So why isn’t there an Ethan Siegel channel yet? Any philanthropists reading? Hellloooo?

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Why doesn’t Earth lose its atmosphere? http://startswithabang.com/?p=1141 http://startswithabang.com/?p=1141#comments Mon, 17 Nov 2008 22:28:35 +0000 ethan http://startswithabang.com/?p=1141 So we’ve got this giant wet rock hurtling through space at incredibly high speeds. It spins, and it revolves around the Sun. Why, with all of this, do we still have something as tenuous as our atmosphere? Pereira2 writes us:

How it’s possible the atmosphere follow[s] Earth during
its travel through… space?

Let’s think about this. There are a lot of things going on here, so let’s make sure we deal with all of them, and deal with them properly.

1. The Earth is moving really fast. Just like Galileo said. How fast? Each year, the Earth travels 584 million miles to go around the Sun, for a mean speed of about 67,000 miles per hour (108,000 km/hour). But unlike objects on Earth that move that quickly, there’s no drag force in space. Since space is so empty, there’s nothing to burn the atmosphere off the way that meteors burn up when they get too close to the Earth. So instead the Earth, which moves at the same speed as its atmosphere, can keep its atmosphere intact simply through the force of its gravity.

2. The Earth is spinning quickly. So why doesn’t the atmosphere get flung off of the planet? Although the Earth rotates very fast, once a day, that only gives the surface of the Earth moving at a pathetic 1000 miles-per-hour (1700 km/hour). Sounds fast, but it isn’t a problem for two reasons. First off, just like a centrifuge, spinning things (like the Earth) tend to throw off things on their surface (like the atmosphere). But the Earth’s gravity is about 300 times stronger than this centrifugal force, so the atmosphere is fine. Second, even though the Earth is spinning, the atmosphere spins with the Earth, due to the law of conservation of angular momentum. So again, our atmosphere is fine.

3. But objects like Comets lose everything when they orbit the Sun! Why doesn’t the Earth lose its atmosphere the same way? Although the Sun’s light does hit the Earth’s atmosphere, giving individual atoms and molecules tremendous amounts of energy, the energy from the Sun just isn’t enough at this great distance to overcome Earth’s gravity. The atoms in the atmosphere still stay bound to the planet. For comets, they’re tiny, and their gravity is tiny, so they leave a wake of debris (comet tails) through space. But the Earth is big enough, it’s gravity is strong enough, and gosh-darn-it, people like it, and so its atmosphere stays right here with us on our journey forward in space and time.

And for all the space-related news that you can handle, check out this week’s Carnival of Space. We’re already up to Carnival #79!

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Q & A: Gravity and the Power of a Theory http://startswithabang.com/?p=1037 http://startswithabang.com/?p=1037#comments Mon, 20 Oct 2008 21:25:47 +0000 ethan http://startswithabang.com/?p=1037 How do you know what makes a scientific theory good and useful? We can put everything that a theory does into two categories. One one hand, theories make predictions, that is, they tell us what’s going to happen if your theory is correct. But on the other hand, theories also require assumptions. The most powerful theory imaginable would assume nothing and predict everything. Of course, that’s not feasible, so we do the best we can with what we have and what we know. Wikipedia actually has a pretty good article about the predictive power of a theory, and there’s recently a telegraph article about a combined science and art project that looks at this (see image below for an example).

But SWAB reader Rene writes in, and wants to know about gravity. Namely, he wants me to consider alternative theories of gravity. Let’s start with the simple stuff, and remember to put it in Richard Dawkins’ context of the predictive power of a theory. Let’s take a look:

Now, of course Dawkins is being tricky when he says that “genes exist” is all that Darwinian evolution requires as its assumptions, since the mechanism is clearly more complicated than that. But Newton’s gravity was really simple; its assumptions were as follows:

  1. Everything in the Universe that has mass emits a gravitational force.
  2. The force that every object exerts on every other object obeys Newton’s law of Universal Gravitation.

And with that, we were able to explain nearly all the gravitational phenomena on Earth and in space. We were even able to predict the existence of Neptune from it! The “power” of Newton’s theory was huge.

But eventually we made enough discoveries of things that Newton’s gravity couldn’t explain. Why was the perihelion of Mercury precessing? Why did clocks run differently in different gravitational fields and at high velocities? When Einstein came along, he came up with a new theory of gravity that not only explained these things, it also explained everything Newton’s theory of Gravity explained, plus it made new predictions that have since been verified, including the bending of light by matter (below), frame dragging, and the decay of gravitational orbits.

But now, nearly 100 years after General Relativity, we have new discoveries that both Newton and Einstein’s theories cannot explain. Why is the expansion of the Universe accelerating? Why is there more gravity than matter can account for? If we want to explain all of these new things, we need at least two more assumptions: dark matter and dark energy. Now, Dark Matter explains a lot of things, as I’ve been over, and so it seems that we need that. But what about dark energy? Really, I like Karl Gebhardt’s answer:

Dark energy is our ignorance of what’s going on in the universe right now. What I always like to say is that dark energy is only a phrase, and don’t get hung up on the words dark and energy. It may not be dark, and it may not be energy. All it is is our ignorance of how the universe may be expanding, and we don’t know what it is at this point.

So, Rene, I don’t have a better answer for you than that, but as far as gravity goes, Newton’s theory was incredibly powerful, but didn’t explain everything. Einstein’s general relativity was a little more complicated, but explained a whole host of new things. Adding dark matter means there’s one more particle (at least) that’s 5 times as abundant as protons, neutrons, and electrons, but it solves a pretty large number of problems, too, in a way that other alternatives don’t.

But dark energy? It’s one assumption to solve one problem. Not a powerful theory. And so, cosmologists have jobs for a while longer, as we all work to try to figure it out.

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Trying to Understand Gravity http://startswithabang.com/?p=943 http://startswithabang.com/?p=943#comments Mon, 29 Sep 2008 21:26:14 +0000 ethan http://startswithabang.com/?p=943 Sure, gravity sounds like a pretty simple idea, now that we’re used to it. But, how does it work?

Think about it for a minute. What is gravity? It’s the idea that anything at all, with any mass or energy at all, in the whole Universe, is attracted to everything else with mass or energy in the Universe. This is true for familiar things that are near, but not touching Earth,

and it’s also true for things that are on (and touching) the surface of the Earth,

and it’s even true for objects that have nothing to do with the Earth at all:

But how does this work? Or in other words:

How can two things that don’t come into contact with one another exert a force on one another?

Newton didn’t know the answer to this, and he made up the phrase “action-at-a-distance” to explain it.

Nice try, wig-boy. I know that giving something a fancy name doesn’t actually explain what it is! (As an aside, that’s exactly what we’ve done now, over 300 years later, with dark energy.) So we go from wig-boy to wall-socket licker:

And this time, we actually get a deeply profound answer: all objects with mass and energy are connected through spacetime. So the Earth bends space around it, and that’s why things that are closer to the Earth are more attracted to it.

This same principle works with everything, including the gravity from the Sun, and even light from distant stars! If you look up at the sky near the Sun during a solar eclipse, you will find that stars are out of position, because the Sun’s gravity even bends starlight!

And so now, 400 years after we tried to answer the question of how gravity works, we realize that we still don’t have an answer for what happens at very small distances. This is what people working on quantum gravity are trying to discover, and honestly, this is the big hope of people who work on string theory: that strings will solve this problem.

Will it? It hasn’t so far, although no other solution looks promising. Any ideas as to how gravity really works? Hopefully, we’ll find one theory that explains it successfully for both strong and weak fields, and for small and large distances.

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A Discovery of Gravitational Waves? http://startswithabang.com/?p=1783 http://startswithabang.com/?p=1783#comments Fri, 04 Apr 2008 09:05:43 +0000 ethan http://startswithabang.com/?p=1783 Two summers ago, I was in Les Houches, France, for a summer school that turned out to be one of the best experiences of my life. Seriously, we’d wake up every day and this was the view from the school:

Well, the University/Institution that ran the school sends periodic updates to me. And they linked me to this release. Here’s the interesting and (if it’s true) sensational claim that the release makes:

Recently, a team of theorists … performed a new analysis of all available CMB and LSS data including the WMAP and Sloan data and favor an inflation model where exist primordial gravitational waves: the amount of the ratio r between these waves and the density fluctuations is non zero in their model. … In the frame of their model, the team obtains the inflaton potential which best fits the data together with the most probable value for the primordial gravity wave ratio r ~ 0.055. This value is within the reach of forthcoming CMB observations.

So now in English: based on the most recent data from the microwave background and from galaxy surveys, we can figure out some of the parameters of the theory (inflation) that set up the big bang. There are fluctuations in the energy density (corresponding to places where galaxies will and won’t form), and we see those; those are the hot and cold spots in the microwave background. But what we haven’t yet seen are fluctuations that are characteristic of gravitational waves left over from inflation/the big bang. Inflation has a very different prediction for gravitational waves than the big bang without inflation, so this could be very strong evidence for inflation if they find it. They can’t measure the waves outright, but they can measure how strong the gravitational wave fluctuations are compared to the matter/energy fluctuations. They cleverly name this r, for ratio.

Now, in most simple models of inflation, r is teeny-tiny, and we’ll never see it. But what they claim is to have evidence for r being at least 0.016 (at the 95% confidence level). Here’s the graph of their results overlayed over the constraints from the microwave background:

Everything above the line that says “h=-0.99″ is the stuff that’s allowed at 95% confidence level; this means there’s only a 5% chance, all things being equal in their analysis, that r is below that line. (ns is measured, by the way, and is about 0.97 +/- 0.02.) The scientific paper is available here, however, they do make an assumption here, that if it’s false, invalidates their conclusions. Their assumption? That the potential that gives rise to inflation is a polynomial of the form:

V(x) = A + Bx + Cx2 + Dx3 + Ex4.

Is this a good assumption? No. It’s a shame that you have to be not just a scientist, but a scientist well-versed in inflation theory to realize that a big sweeping claim like this is probably wrongheaded. But now you know something that probably only a few thousand people in the world know! Happy Friday!

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Bring him to me! http://startswithabang.com/?p=1746 http://startswithabang.com/?p=1746#comments Fri, 29 Feb 2008 22:06:20 +0000 ethan http://startswithabang.com/?p=1746 The Milky Way galaxy is a relatively big spiral galaxy. So is Andromeda. There are about 20 dwarf galaxies that are gravitationally bound to us; combined with us, all of this makes up the local group. But Andromeda is moving towards us, and eventually, it’s going to merge with us. I’ll once again show you a video of what this merger might look like:

But what would we see, here in the Milky Way, as Andromeda got closer and closer to us? Right now, Andromeda looks like this:

But Andromeda is also very far away: about 2.3 million light years (770 kpc). The center of it is tiny on the sky, but the whole galaxy, as seen above, is actually 4 degrees across, or eight times larger than the diameter of the full moon! Its apparent magnitude is 4.4, which means it can barely be seen with the naked eye (anything less than 6 can be seen with your eye) if your vision is good and there’s no light pollution.

But the Universe will continue to age, and gravity will basically tell Andromeda “Get over here!” When this happens, Andromeda not only gets closer to us, but also starts to appear bigger and brighter in the sky. What does this mean? Let’s play Zeno’s Paradox, and see what happens when it gets halfway to us, and then halfway of that distance, etc.

  • About 1.9 billion years from now, Andromeda will be 385 kpc away from us. It now has an apparent magnitude of 2.9, which means it’s just barely visible from most urban neighborhoods, and appears slightly brighter than our own Milky Way does. It now takes up 8 degrees on the sky, making it 16 times as large as the Moon in diameter.
  • About 2.7 billion years from now, it will be within 190 kpc of us. That’s still well outside the Milky Way, which is less than 20 kpc in radius. It’s now quite bright, though, with an apparent magnitude of 1.4, making it as bright as one of the brightest stars in the sky, Regulus. It is now 16 degrees on the sky. If, at this point, it were oriented face-on to us, it would take up about 1% of the entire visible night sky.
  • About 3.2 billion years from now, it will be under 100 kpc from us. It now takes up 1/20 of the entire night sky, and only the Moon, the Planets, and three stars, Sirius, Canopus, and Alpha Centauri are brighter than Andromeda appears.
  • 3.4 billion years from now, Andromeda will be within 50 kpc of us, on the verge of beginning to merge with us. (Remember, it has a radius of about 20 kpc, too.) Its apparent magnitude is -1.5, meaning that it is now brighter than any star in the sky. It will now take up about one-fifth of the night sky, and will just begin to create new star-forming regions in the outskirts of the galaxies, where the gas begins to merge.

And then the merger happens. What will that do to us? Take a look:

Although we don’t know exactly what’s going to happen, it’s a good bet that we won’t want to be here for it. Time to find a new galaxy… or at least a temporary home outside of ours while that merger takes place!

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How old is the Sun in Galactic years? http://startswithabang.com/?p=1744 http://startswithabang.com/?p=1744#comments Wed, 27 Feb 2008 21:23:36 +0000 ethan http://startswithabang.com/?p=1744 The Moon goes around the Earth, the Earth goes around the Sun, and the Sun goes around the center of the Milky Way. We know the Moon takes about 4 weeks to make its trip around the Earth, and that causes the Moon phases:

We also know that the Earth takes one year to go around the Sun, and that causes the seasons:

We also know that the Earth has been around for about 4.5 billion years, which means it has gone around the Sun about 4.5 billion times. Well, now I ask the question(s):

How long does it take the Sun to go around the Milky Way? How many times has it done that so far, and how many times will this happen before the Sun finally dies?

Well, we know that we travel in (roughly) a circle around the center of the Milky Way, and that our radius from the center is about 8 kiloparsecs, or roughly 26,000 light years. That means our Solar System (including the Sun) needs to travel a distance of 1.55 x 1018 kilometers to go around the Milky Way once. If we know how fast the Sun is moving, we can figure out how long a Galactic Year is. Well, we can both measure and calculate its velocity to be 220 kilometers/second, and so we can just do the math, knowing that there are 31,556,952 seconds in a Gregorian Year, and we find that it takes about 223 million years to make one galactic year.

So, if the Sun is 4.5 billion years old, that makes it about 20 galactic years old. If the Sun has a total lifetime of around 10 billion years, then it has a total galactic age of around 42 galactic years.

What? Did I just say the answer is 42?! Well, this means that one possible question is “What is the Sun’s lifetime in Galactic Years?”

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Q & A: The Age and Size of the Universe? http://startswithabang.com/?p=1739 http://startswithabang.com/?p=1739#comments Fri, 22 Feb 2008 01:30:40 +0000 ethan http://startswithabang.com/?p=1739 Alright; this is a question I’ve been putting off for various poor reasons, but Starts With A Bang! reader Andy asks:

If Im looking at something, the light from which has taken 15 billion years to get to me, and there was only an opaque ball of radiation and stuff 15 billion years ago, why do I see formed galaxies? Shouldnt the age of the universe be: TIME LIGHT FROM OBJECT TAKES TO REACH ME + TIME TAKEN TO FORM OBJECT IM LOOKING AT?

In other words, how can I see things like galaxies that are 15 billion light years away, if the Universe isn’t even 15 billion years old?! This is a damned good question, and something that took me about two years in graduate school to figure out the answer to.

First off, how old is the Universe? Well, you can take a look at the oldest stars that we see, and you know the Universe has to be at least that old. So far, of all the stars we’ve been able to accurately date, the oldest is HE 1523-0901, coming in at 13.2 billion years old. (It’s identified in the image at right.)

Want to get more accurate than that? There are other methods, too, like looking at radioactive element abundances (at left). If we know how these elements were created, and we know their half-lives, we can figure out how old something is by measuring how much of that radioactive material is left. That’s how we know that the oldest rocks on Earth are 3.8 billion years old, for example. We can apply these methods to the Milky Way, and we find that it is between 12.3 and 17.3 billion years old. But can we be more certain than that?

Yes. Because we measure the temperature of the Cosmic Microwave Background (2.725 K), and we know what the Universe is made out of today: 73% dark energy, 27% dark matter, and maybe 0.01% radiation (photons and the like). Put those together, and you can calculate how old the Universe is today, as compared to an arbitrarily high temperature, and you find that it’s between 13.5 and 13.9 billion years old: pretty accurate for my tastes!

So, now we know how old the Universe is. Does that mean that it’s 13.7 billion light-years in size? Surprisingly, no. Take a look at the “model universe” below, which is a balloon with coins (that can represent galaxies, if you like) glued onto it:

Let’s pretend that we are the quarter at the center, and we’re looking at the dime on the left. When the Universe was younger, it was smaller, and the dime was closer to us (left panel). The dime emits light at us, and the light starts traveling towards us along the balloon. But as the Universe ages (middle panel) and ages even more (right panel), the balloon expands. This means two things for us:

  • the light emitted gets redshifted on its way towards us, and
  • the light has to travel a longer distance to reach us than it would have were the Universe not expanding.

So when we see the light from the dime today, and someone tells you how far away it is, it’s not always easy to tell whether they mean

  1. how far away was it from us when the light was emitted
  2. how far away is it now that we observe it, or
  3. how long has the light been traveling towards us, and what is that time multiplied by the speed of light?

When you read a press release, the “distance” they usually (but not always) give is the third option, which is always younger than the age of the Universe times the speed-of-light. But, if you want to know how far is that object from us today, that’s the second option, and that number can be much greater, up to 46 billion light years in any direction from us.

Now, you might ask, does this mean that space is expanding faster than the speed of light? The answer, my dear friend, is yes. Take that brain-buster to your physics teacher and watch him/her go into denial; it’s awesome! (It is, of course, because the Universe expands in a very bizarre, complicated, but moreover counterintuitive way; see the illustration at right.) Then send them to my webpage and to Ned Wright’s page for the more technical explanation.

And if your brain ain’t broke yet, check out the latest Carnival of Space, where they have my post on why we need dark matter!

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Why we need Dark Matter in the Universe. http://startswithabang.com/?p=1735 http://startswithabang.com/?p=1735#comments Tue, 19 Feb 2008 20:20:55 +0000 ethan http://startswithabang.com/?p=1735 Last week, Pamela Gay over at Star Stryder pointed me to a press release which claimed that, among other things, perhaps dark matter wasn’t necessary. So I wrote a guest post on her blog explaining why it was. Apparently, some people still aren’t convinced. So I will lay out for you all the reasons I can think of why we need it, and explain what happens if you try to do without it.

1. Cluster Velocity Dispersions. When we take a look at galaxies, we often find hundreds or even thousands of them clustered together, like in the Coma Cluster (at left). We can measure how quickly those galaxies are moving around their center, and we can determine, based on the laws of gravity, how much mass is in these clusters. When we do these calculations, we find that Ωm, or the amount of total matter in the Universe, is about 20 to 30% of its critical value. (Note: Ωb, or the amount of matter in protons, neutrons, and electrons, is known from Big Bang Nucleosynthesis to be between only 4 and 5%.)

2. The Power Spectrum of the Universe. We can measure how many galaxies there are at a bunch of different positions in the Universe. There are big surveys like the 2-degree Field Galaxy Redshift Survey and the Sloan Digital Sky Survey, which have allowed us to construct maps of where all the galaxies in a large volume of the Universe are (see the image at right, where each little dot represents a galaxy). We can then determine, based on the clustering of these galaxies, how much total matterm) and how much normal matterb) there is. We find that Ωm is about 23% and Ωb is about 3 to 4%.

3. The Cosmic Microwave Background. The leftover radiation from the big bang is about 2.725 degrees Kelvin, and it’s approximately that temperature everywhere. But some spots are just a tiny bit hotter or colder (on the order of 10 microKelvins). We can learn a lot about cosmology from these temperature anisotropies, and one of them is how much dark matter and how much normal matter we have in the Universe. What we find from this is that the amount of total matter is about 27% and the amount of normal matter is 4-5%.

4. Gravitational Lensing Data. Again, let’s come back to my favorite nail-in-the-coffin of “we can just change the laws of gravity and do away with dark matter” theories: colliding galaxy clusters. There’s the Bullet Cluster,

there’s Abell 520,

and the cluster CL0024+17,

and the important thing is that we find that we have matter exerting gravity where no normal matter exists. Even if you fine-tune a theory of gravity to work for one of these three geometries, it doesn’t work for the other two. I will sum this up: there is no theory of modified gravity that can explain these three observations without also including dark matter.

5. Rotation of spiral galaxies. When a spiral galaxy rotates, we would expect, if there were no dark matter, for the outer parts to revolve around the center with a much slower velocity than the inner parts, just as the outer planets of our solar system revolve around the Sun with slower velocities than the inner planets. But we don’t see that. In fact, as far as we can tell, the velocities of rotating galaxies remain constant no matter how far out you go (see the image at right). This means either the laws of gravity are wrong for galaxies or there is some extra mass, like dark matter. But if you think the laws of gravity are wrong, I urge you to reread point number 4.

I realize this is especially confusing given the number of crackpots out there, who ignore some of these points and then claim we don’t need dark matter. I realize that there are alternative theories that can explain *some* of these theories, and there may even be alternatives to General Relativity like Bekenstein’s Tensor-Vector-Scalar gravity that can possibly explain points 1, 2, 3, and 5 without dark matter. But point 4 is really the killer. You want there to be gravity where there’s no matter? They haven’t made a theory that can do that in a way that’s consistent with observations, and people have been trying.

There are other tests that provide evidence for dark matter as well, but I think the five above should convince any skeptic of the facts that

  • Dark Matter explains all of these observations, while
  • modifying gravity cannot explain all of these observations.

Are you convinced? Leave me a comment about it. Still not convinced? Leave me a comment and tell me why!

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What if I were made of Dark Matter? http://startswithabang.com/?p=1729 http://startswithabang.com/?p=1729#comments Thu, 14 Feb 2008 01:25:59 +0000 ethan http://startswithabang.com/?p=1729 I’ve been talking about dark matter a lot, and yet there’s still so much to explain about it. For example, dark matter and normal matter (protons, neutrons, and electrons) have a few things in common:

  • They both have mass.
  • They both feel the effects of gravity.
  • They both cause their own gravity.

But that’s where the similarities end. I can make a long list of the ways that dark matter and gravity are different from one another, but I prefer to give you an example. Imagine the following scenario: you stand up from your seat, walk towards the wall, and smack right into it. You might wind up looking like this.

On the other hand, what if you were made of dark matter? Well, you’d try to stand up, but that wouldn’t work. You see, protons are charged particles, electrons are charged particles, and even neutrons (which are neutral) are made up of charged particles. This means that when they get close to one another, the electromagnetic force keeps them from passing through each other. Dark matter doesn’t feel the electromagnetic force, and so as you pushed down on the floor, the floor wouldn’t push back, and you’d pass right through it. How much stronger is the electromagnetic force than gravity? 1038 times stronger. The odds of gravity acting as strong as the electromagnetic force, even for an instant, are the same as picking the winning numbers to the New York lotto jackpot six times in a row.

In fact, if you were made of dark matter the center of the Earth would pull you towards it, just as it would for regular matter. But without the electromagnetic force to stop you, you would simply fall through the Earth towards the center. In fact, without the electromagnetic force, there would be nothing to hold your body together, and if you managed to raise your hand, there would be nothing to keep your hand attached to your body; it would simply fly off into the air, bound to the Earth by nothing more than gravity. Here’s something weird: since there’s no electromagnetic interaction on dark matter, it wouldn’t get trapped at the center of the Earth; you would pass through the center, come out the other side (presumably in Indonesia or something for where I am), and then fall back through the center of the Earth, and wind up back where you started, and you would do this for eternity.

Maybe this gives you a little handle on what would happen to you if you were made of dark matter, and just how different this stuff is from everything we’re used to.

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