Starts With A Bang! » relativity 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 Can you slow time down? http://startswithabang.com/?p=1514 http://startswithabang.com/?p=1514#comments Thu, 19 Feb 2009 02:04:33 +0000 ethan http://startswithabang.com/?p=1514 We’ve talked before about how you can make time around you speed up. Go into a spaceship, travel close to the speed of light, and come back to where you started. When you land your spaceship, you will have aged “normally” for you, but everyone who remained on Earth (even your identical twin) will be much older.

This is because the speed of light is the same everywhere and at all times in the Universe. If you travel fast, and by fast I mean close to the speed of light, time for you has to slow down relative to time for people who are at rest. So when you move close to the speed of light, travel back to Earth and come to rest here, you’ll be younger than people who’ve been on Earth the whole time. You are effectively speeding the time of everyone around you up.

Well, reader Tony writes in and asks the following related question:

If I was in a rocket and set a course and speed that countered all motion, including Earth, Solar, Galactic, and Universe, would time then speed up?

Well, the Earth moves around the Sun, the Sun moves around the Galaxy, and the Galaxy hurtles through space. So could you, if you found out how quickly everything was moving and counteracted it, could you effectively slow the time of everyone around you down?

The answer appears to be yes, but not by nearly as much as Neo here can. The problem is, for any measurement of time to make sense, people need to be in the same place at two different points in time. So whatever you do, you need to come back to the place you started at. If you want to do this for the planet Earth, it’s only the Earth’s movement that you can counter.

The Earth moves around the Sun at roughly 30 kilometers/second (67,000 miles per hour), which is pretty fast, but only about 0.01% of the speed of light. So if you sat in Earth’s orbit and turned on your rocket thrusters so that you stayed in exactly the same position while the Earth continued to go around the Sun, time would continue to pass slightly quicker for you than for people still on Earth! They’re moving at 30 km/s, while you’re there, stationary, at 0 km/s. When the Earth finally catches you, though, one year later, what does your clock say?

Your clock is off. By 0.16 seconds. Yes, that’s right, you spent an entire year and you found that your clock was off by 0.16 seconds thanks to all of your efforts. Want to know what’s even worse? Time didn’t slow down on Earth relative to you. After all that, you’re still 0.16 seconds younger than Earth. Turns out that because you left Earth and returned to Earth, the only velocity that matters is your velocity relative to Earth.

So I’m sorry, Tony, because your question is a good one and made a lot of sense. But it turns out the Universe doesn’t work that way, and that if you leave Earth and return to Earth, you can’t make time pass faster for you than it does on Earth. Pretty sad, but hopefully now you know not only what the answer is, but a little bit about why.

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Is the Speed of Light truly constant? http://startswithabang.com/?p=1419 http://startswithabang.com/?p=1419#comments Fri, 23 Jan 2009 18:53:11 +0000 ethan http://startswithabang.com/?p=1419 This is an interesting question, for a number of reasons. First off, many of the things we think of as fundamental constants may not be constant. First off, most macroscopic physical things change over time. Second off, the conditions in the Universe are not the same today as they were in the past; the Universe was hotter, denser, and more energetic in the past. And third, we already know that some physical constants, such as the fine structure constant, do change at extremely high energies. This is where the idea of the unification of forces comes from:

Well, the fine structure constant is made up of a combination of three other fundamental constants: the electron charge, Planck’s constant, and the speed of light. One of these must change, at least a little bit, as a function of either energy or time. Well, yesterday, a scientist named Lorenzo Iorio published a paper on the limits of changes in the speed of light. This is really interesting for a number of reasons. Ready?

If the speed of light was faster in the past, the Universe is much younger than we think it is now. It also means that it’s possible that there was no inflation. Since our understanding of the Universe hinges on the constancy of the speed of light, this is an important thing to measure.

It may be possible for other constants to change over time, too. I know of one person who things that Planck’s constant changes over time as well. Her theory is that the speed of light and Planck’s constant both change proportional to (the age of the Universe)^(1/3). Although I disagree with her conclusions, it is certainly a possibility.

Or, that is to say, it was until this recent paper came out. If you watch the planets move in orbits around the Sun, the point of closest approach, or their perihelia, changes over time in a very predictable way.

This change, observable in Mercury, Venus, Earth, and Mars over multiple centuries, is very sensitive to the speed of light! So you predict the motion of the planets based on the laws we know, and then you look for anomalies, which will give you evidence that the speed of light is changing.

These measurements are incredibly precise over the last century or so, and the conclusion is reached that over that time, with more than a 99.7% certainty, the speed of light has not changed at all.

Phew! So it’s still possible that the speed of light changed in the past at some point, but this piece of research indicates that, as best as we can tell, it isn’t changing now. So you can keep looking for evidence of changes, past and present, if you like, but I’m convinced that it’s 299,792,458 m/s now, that it always was in the past, and that it always will be in the future.

That’s why we do the experiments, though, because you never know what new evidence is going to come up! And speaking of what comes up, the new Carnival of Space is up, and the Martian Chronicles is hosting. How appropriate that I just wrote about terraforming Mars! Have a great weekend, folks!

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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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Warp Drive: Is it Really Possible? http://startswithabang.com/?p=1255 http://startswithabang.com/?p=1255#comments Mon, 15 Dec 2008 18:28:02 +0000 ethan http://startswithabang.com/?p=1255 Ever since we realized that our Universe extended far beyond the extent of Planet Earth, mankind has longed to travel to distant planets, stars, or even other galaxies, perhaps going as far as the edge of the known Universe.

We’ve designed spacecrafts that, in theory, can travel these great distances. The problem with all of these designs is time dilation, or the fact that while you go off on your space journey, time passes much faster for everyone who stays back on Earth, so that if you wanted to go off and return, people on Earth would age tens, thousands, or even millions of years while you were gone on your journey.

Well, reader Howard Mauch writes in and wants to know whether warp drive is really possible. Warp drive is the idea that we can bend space so significantly in front of our spacecraft that we can travel forward great distances in short amounts of time, without suffering the effects of time dilation:

This was, of course, made famous by Star Trek. But is this at all physically feasible? The quick answer is no. Why not? Because although space can be curved, and we can theoretically connect two distant points to travel instantaneously between them, nothing can safely travel through them. Take a look at this picture, where it looks like you can safely travel from one side to another through this short-cut:

The big problem is that the curvature you see here represents a gravitational field. The more curved a piece of this diagram is, the stronger the gravitational field is. And the stronger the field is, the greater the forces are on you. In the diagram above, the forces are so strong they will not only crush you, they will tear individual atoms apart. The only solution? We need some way to create stable, flat spacetime inside of this curved area. Is this possible?

Well, it’s possible in electricity; you put an electrical conductor around your ship and you block all electric fields inside the conductor. Want to do the same thing for gravity? You’d need to put a gravitational conductor around your ship. No big deal, right? Except that there’s no such thing as a gravitational conductor, because there’s only one type of mass (positive), where there are two types of electric charge (positive and negative). So either invent something with a negative mass (which doesn’t exist) and build a gravitational conductor, or everything, even in theory, will be destroyed by the gravitational forces that would allow you to travel via warp drive.

NASA is more optimistic about this. I believe they are mistaken in their optimism, but who knows what new physics will be discovered, and what new possibilities will arise from them?

And if you need a bigger fix for your space reading needs, check out the latest Carnival of Space, where many interesting space delicacies await you!

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Q & A: The Speed of Light http://startswithabang.com/?p=1176 http://startswithabang.com/?p=1176#comments Mon, 24 Nov 2008 22:25:48 +0000 ethan http://startswithabang.com/?p=1176 It’s 103 years after Einstein first formulated his Theory of Special Relativity, which explains what happens to objects near the speed of light. But SWAB reader Jacinth wants to do one better, and asks:

What will happen if we can actually travel at the speed of light?

It’s a great question, and provides a lot of learning opportunities. First off, let’s take a look at what happens to regular matter when we bring it close to the speed of light. There are three major things:

1. Lengths contract. This works for everyone. If I move close to the speed of light, then anyone who sees me sees that my length is smaller. But from my point of view, everything that I see is moving towards my rear close to the speed of light, and also looks like it has a smaller length.

2. Time slows down. We call this time dilation, and again, it works for everyone. It means that if I’m moving close to the speed of light, everyone who sees me sees that time is traveling more slowly for me: my clocks run slower, I age slower, my heart beats slower, etc. But I see the same thing: everyone else looks like their clocks are running slower, they’re aging slower, etc. But if I go away close to the speed of light and then come back to Earth at Earth’s speed, we find out that on my journey, although I’ve aged normally, much more time has passed on Earth. (Incidentally, this is what Paris Hilton was worried about.)

3. It takes more energy to accelerate your speed. Some of you who know a little physics know that the rest energy of a particle is E=mc2. Some of you also know that Kinetic Energy = 1/2 mv2. But when you get close to the speed of light, it takes more and more energy to move quickly. In the graph above, the purple line is the old formula for kinetic energy, but the red line is the real (relativistic) energy. Notice that you never quite get up to the speed of light, but that the energy it takes approaches infinity.

So that’s what happens when something made of normal matter approaches the speed of light: it sees lengths contract, times slow down, and it requires more energy to change its speed. Alternatively, things that have no mass (like photons, or perhaps gravitons), have to move at the speed of light.

But let’s say you had a spaceship, and decided to actually go at the speed of light, somehow. What would happen?

Well, if you used all the energy in the Universe for your spaceship, you could probably get up to speeds incredibly close to the speed of light. How close? The speed of light is exactly 299,792,458 meters/second. And you could get to within about 1 x 10-30 meters/second of that value — pretty good. If you got that fast, though, what would happen? First, the entire Universe would contract to appear to only be a few billion kilometers across — less than one light year! Second, time would slow down so much, that as you would only age a few seconds, the Universe would age literally trillions of years. Galaxies would merge, stars would be born and explode in the blink of an eye. And finally, you may get to see the fate of the Universe firsthand; if the Universe has an end, you would slow down time for yourself so much that you might not only see it, you might do it in just a few seconds.

So, in conclusion, not only can’t you move at the speed of light, there’s good reason not to!

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Special Relativity and Paris Hilton http://startswithabang.com/?p=1100 http://startswithabang.com/?p=1100#comments Fri, 07 Nov 2008 21:50:10 +0000 ethan http://startswithabang.com/?p=1100 Are you kidding me? Who ever thought that Paris Hilton and Albert Einstein would be in the same sentence, much less have things to say on the same topic?

And yet, here we are in 2008, and theweek.com reports the following:

Paris Hilton is worried about Einstein’s relativity theory. The hotel heiress is one of 157 people to have already paid a $200,000 deposit for a seat on Richard Branson’s Virgin Galactic commercial space-flight service, but this week confessed to being “very scared” that if the spaceship flies too fast, its passengers will experience relativity’s predicted distortion in the passage of time. “With the whole ‘light-years’ thing, what if I come back 10,000 years later and everyone I know is dead?” said Hilton. “I’ll be like, ‘Great. Now I have to start all over.’”

Want to know the amazing thing? Paris Hilton has nailed one of the most important concepts in all of physics! She’s right, when you move close to the speed of light, time slows down for you. If you go on a spaceship, travel close to the speed of light, and come back to Earth, you’ll only age a little bit, but people on Earth will age much more.

Let’s take a look at just how this works. The main idea is that light always moves at a constant speed. If I send you a light beam, it gets to you in a certain amount of time. But if we’re moving very quickly, that light beam is limited by the speed of light. We’ll see the same amount of time pass, but really, because everything is moving so quickly, time passes at a slower rate. Take a look at the illustration below, where the green side (on the left) is someone at rest, while the red side (on the right) is moving close to the speed of light.

See how it takes light more time to travel on the right? That’s what we mean when we say that it appears that time slows down when you move close to the speed of light. But there’s a caveat that Paris missed:

You have to move close to the speed of light!

And this is the problem. While you can check out an online calculator about time dilation, the fact is that the puny spacecrafts that we build don’t move anywhere close to the speed of light. The best tests we’ve ever done show that time slows down, but for any Earth vehicles, the effects are, at most, a few seconds. So don’t sweat it, Paris, we’ll still know you when you come back.

But, if you want a little bit more in the way of details, check out this video…

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Not So Fast to the Next Star… http://startswithabang.com/?p=694 http://startswithabang.com/?p=694#comments Wed, 16 Jul 2008 19:48:25 +0000 ethan http://startswithabang.com/?p=694 Why not? After all, I wrote on Monday that if we can keep accelerating at 9.8 meters/second2 (the acceleration on Earth) for the whole journey (accelerating for the first half and decelerating for the second), we can get to a star 20.5 light years away in under 7 years. People have even drawn up designs for what a nuclear-fusion powered spacecraft would look like:

Well, that part is still true! But it isn’t feasible, not even with nuclear fusion technology. The big problem is everyone’s favorite law: The Conservation of Energy. This is going to take one equation to understand, the equation for Kinetic Energy:

KE = 1/2 * mass * velocity2.

The big problem is that if I want to get my spacecraft up to a certain speed, it takes a certain amount of energy. But if I want 10 times that speed, it takes 100 times that energy. An engine has a finite rate-of-energy output, meaning a finite power output. Let’s draw you up a little chart to help you understand, for a spacecraft of 100,000,000 kg (50,000 tonnes) in mass:

Table for Energy Required to Change Between Speeds


Starting SpeedEnding SpeedEnergy NeededPower Needed
0 km/s100 km/s5 x 1017 Joules49,000 GigaWatts
100 km/s200 km/s1.5 x 1018 J147,000 GW
200 km/s500 km/s1.05 x 1019 J343,000 GW
500 km/s1,000 km/s3.75 x 1019 J1,030,000 GW
1,000 km/s10,000 km/s4.95 x 1021 J5,400,000 GW

So the big limitation is that the power you need to keep your acceleration constant goes up and up and up as you want to go faster and faster. To get all the way up to 10% of the speed of light requires about 4.5 x 1022 Joules of total energy, or, for a hypothetical 10 GW engine running on fusion power, a little over 140,000 years. It would also take about 500 million tonnes of hydrogen to power it.

So, I’m sorry for the early optimism; this is still a big difficulty! We’d need a much bigger, more powerful engine, and capable of cannibalizing a much larger amount of hydrogen that I had realized! Maybe Ian O’Neill and Star Trek had the right idea after all?

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Why doesn’t Light Age? http://startswithabang.com/?p=1781 http://startswithabang.com/?p=1781#comments Wed, 02 Apr 2008 09:05:12 +0000 ethan http://startswithabang.com/?p=1781 There’s a graduate student that I’m sort-of mentoring/working with at Arizona, named Xiaoying Xu (hi Xiao!). She’s bright and curious, and she asks some very good questions. She asked me one yesterday that’s pretty tough to wrap your head around:

How do I explain to someone why light doesn’t age?

Well, here on Earth, time progresses at a certain speed. That is, if I measure how many seconds tick by as the Earth revolves once around the Sun, I’ll get 31,556,926 seconds. (31,558,150 if I’m measuring a sidereal year.)

But let’s say in the course of that year, I put you on a rocket ship, and you come back exactly one Earth revolution later. I send you off at 12:00 AM on New Year’s day. Well, if you’re moving at typical rocket ship or satellite speeds (a few kilometers/second), your clock will be about one-hundredth of a second faster after a year due to the time dilation effect of special relativity, hardly noticeable.

Big deal. But what if you start moving fast? If you move at 10% the speed of light (30,000 km/s), your clock will say that it’s about 44 hours earlier than mine. While I ring in the New Year, you think it’s 4 AM on December 30th. If you get up to 90% the speed of light, I get a kiss and champagne while you think it’s early morning on June 9th. At 99.99% the speed of light, only 5 days will have passed for you while I’ve lived a whole year. And at 99.9999999% the speed of light, an entire year passes for me in just 23.5 minutes for you.

So as things move faster, time passes slower for them. Now, being made of matter (and having mass), we can never move at the speed of light. But things that do travel at the speed of light never have any time pass for them. Now as if that wasn’t neat enough, most things that we know of will decay eventually. Things that we like, such as neutrons. But if they move faster, they live longer. That’s how cosmic rays known as muons can actually reach the surface of the Earth, because they move at 99.999% or more the speed of light! But what about light? Well, that’s the one thing in the Universe that we know will never decay. Protons might decay (we know that if they do, their half life is over 1035 years), electrons might decay, but particles of light can’t. Because time doesn’t pass for them!

And that is why light doesn’t age.

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