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How to build a time machine?

January 14, 2009 on 12:16 pm | In Gravity, relativity | 29 Comments

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?


Why doesn’t Earth lose its atmosphere?

November 17, 2008 on 3:28 pm | In Gravity, Q & A, Solar System | 46 Comments

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!


Q & A: Gravity and the Power of a Theory

October 20, 2008 on 2:25 pm | In Gravity, Q & A | 12 Comments

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.


Trying to Understand Gravity

September 29, 2008 on 2:26 pm | In Gravity, Q & A, String Theory | 38 Comments

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.


A Discovery of Gravitational Waves?

April 4, 2008 on 2:05 am | In Gravity, Scientific papers | 1 Comment

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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