When many people first learn that the speed of light is constant — that it’s the same everywhere in the universe, and is measured the same by all observers — it all seems kind of arbitrary. It’s strange enough that the speed of light is a “cosmic speed limit,” beyond which no object can go. But, how can the speed of anything possibly be a constant value? When I’m in an airplane and I walk forward up the aisle, I am going faster than the plane itself, something that could be verified by an observer on the ground. My total speed, relative to the ground, will be the speed of the plane plus my walking speed. (Subtract the speed if I’m walking in the other direction, toward the tail.) Yet, if I measure the speed of a light beam coming from a laser aboard a plane or a spaceship, I’ll always get the same result of 186,000 miles per second, whether the beam is facing forward, backward, or sideways, or even if it’s beamed from the spaceship in any direction and measured on the ground. Why?
It actually has to be this way. A world in which the speed of light varied might be impossible logically (see below), or at least could be so messed up and incoherent that complex structures such as galaxies and intelligent life might not be possible.
Consider a thought experiment: You set up a rapidly rotating beacon in deep space, with a radio antenna on it. (Radio waves are an invisible form of light.) The antenna is transmitting a radio signal consisting of a sequence of natural numbers: 1, 2, 3, and so on. Far away from the transmitter, you tune in your receiver and wait for the beacon to start spinning and transmitting the sequence. What kind of signal will you receive?
That depends on whether the speed of light is constant or not. Let’s imagine that it isn’t. In that case, the speed of the radio signal in your direction would change according to the velocity of the transmitting antenna, as with the example of walking up the aisle of an airplane: As the beacon spins, sometimes the antenna would be coming toward you — which would “throw” the signal faster in your direction — and sometimes it would be moving away, which would subtract some miles per hour from the signal’s speed. So, parts of the signal would be traveling toward you faster than other parts. Naturally, if the speed of different portions of the signal is different, the faster portions will reach you sooner than the slower portions. This effect would worsen the farther away you are from the beacon, and the faster the beacon is spinning. You might end up receiving a steady sequence of numbers like this:
1 - 2 - 3 - 7 - 8 - 9 - 4 - 5 - 6 - 10 - 11 - 12 - 16 - 17 - 18 - 13 - 14 - 15 ...
Here, the 7 - 8 - 9 segment is arriving sooner than the 4 - 5 - 6 segment, even though it was actually transmitted later. If this were a television signal rather than a sequence of numbers, with the antenna on a large rotating disc, the program would be really messed up. A political candidate might be seen giving a concession speech, followed by a speech where he seems confident he’ll win. I’m not sure that causal impossibilities would result from incoherent information flying around the universe,* but surely the formation of large structures such as galaxies, responding gravitationally to far-away moving bodies, would be affected (if gravity would even exist in such a universe).
Now let’s consider what happens in a world where the speed of light is constant. In this case, all portions of the signal are transmitted at the same speed relative to you, even as the beacon rotates, so no portion reaches you faster than any other. The numbers arrive in the correct sequence, just as they were transmitted. However, the only way this can possibly work (as Einstein showed) is if measurements of time and distance change for various observers. For a transmitting antenna on a rotating beacon, this produces a relativistic Doppler effect — a slowing down and speeding up of the signal, almost like a vinyl record being played back off-center, something like this:
1 ---- 2 --- 3 -- 4 - 5 - 6 -- 7 ---- 8 --- 9 -- 10 - 11 - 12 -- 13 ---- 14 --- 15 ...
If the TV antenna on the disc were transmitting American Idol, you’d see the show from beginning to end without interruption — no unexpected spoiler — but the slowing down and speeding up might make it sound as if the singer and the band can’t stay on key to save their lives (which in reality happens sometimes).
As it turned out, the constant speed of light was confirmed by the Dutch astronomer Willem de Sitter in 1913, through observations of double star systems, where two stars are rotating around each other closely. He realized that if the speed of light varied as the stars advanced toward us or receded away, the orbits would appear erratic. The system might appear blurry or scrambled and incoherent, and from great distances the laws of motion would appear not to work at all. However, such was not the case in any system that de Sitter observed, and this was used as evidence to support Einstein’s special theory of relativity and the constancy of the speed of light. All observations made to date support the same conclusions.
Personally, I think that the constant speed of light is a hint that the universe is fundamentally informational (the “it from bit” hypothesis proposed by the great physicist John Wheeler). This idea says that matter/energy and spacetime emerge from a deeper layer of existence that’s based purely on information. In our universe, information in the form of light always passes from point A to point B in a coherent, sequential fashion, for every observer — it’s space and time (and even mass) that all change accordingly to suit it. There might be a lesson there.
* I haven’t been able to come up with a paradox of cause and effect, resulting just from some information traveling faster than other information. In the days when some news came by rail and some by telegraph, you might have heard that Lincoln’s assassin had been caught before you heard that Abe had been shot — but nobody went back in time to kill John Wilkes Booth. Still, I suspect there is a thought experiment that would show it couldn’t work. If you have any ideas, please comment.
Showing posts with label John Wheeler. Show all posts
Showing posts with label John Wheeler. Show all posts
Monday, January 14, 2013
Monday, February 6, 2012
The Zero Universe
The world is like a great theater where we watch history unfold. This colossal story features a cast of billions, who not only witness the arc of its epic plot, but also actively take part in its creation. Look around you; it’s raucous and noisy inside this theater we call “the world.” But, if it were possible for us to step outside the theater and take a look, there would be nothing.
Inside, the theater is a mind-boggling swirl of information, actions, and reactions; from the outside, as seen by a cosmic Google Earth beyond space and time, it’s completely empty. I am not just making a cute metaphor here — this is a real feature of our universe, with profound implications.
It seems that in the final analysis, when all things are considered, the universe adds up to exactly zero. For one example (there are others at the bottom), consider the relationship between space and time. Bear with me as I review an idea from high-school geometry. If we have a triangle that includes a 90° angle, we can use the Pythagorean theorem to determine the length of the diagonal from the length of the right-angle sides:
a2 + b2 = c2
where a and b are the lengths of the right-angle sides, and c is the length of the diagonal. A nifty geometrical diagram proves why this is true. If you make a square box along each of the three sides, then each box has an area of the length of that side, squared. The area of the smaller squares adds up to the area of the largest square: If a = 3 and b = 4, then c = 5.
The Pythagorean theorem works in three dimensions, too. If you see a blimp in the sky, you can calculate the exact straight-line distance to the blimp by knowing its altitude above the ground (the value z), as well as how far east-west (x) and north-south (y) you'd have to go to get right under the blimp:
x2 + y2 + z2 = d2
where d is the distance to the blimp. I don't have a 3D diagram, but you can prove it for yourself with a little effort.
Now it gets interesting. This trick extends to four dimensions. Time is typically cited as the fourth dimension. Does the decidedly non-geometric idea of time work into the Pythagorean theorem? Incredibly, it does — but first, you have to convert the time measurement into a distance-like measurement. Then, the total distance you’re calculating is the spacetime distance in the bizarre four-dimensional world where east-west, north-south, up-down, and earlier-later mean the same thing, only in eight different directions. Represented by the letter s, spacetime distance (also known as a Minkowski interval) is determined by an amazing formula. Let’s break it down:
x2 + y2 + z2 – (ct)2 = s2
As before, x is the distance (for example) to the east, y is north, and z is up, but we’ve added a fourth term for time (t), which gets multiplied by a constant, c. Notice the minus sign before the term for time. When it comes to distance through spacetime, elapsed time counteracts spatial distance, and vice versa: If we travel a distance through space, and do it in a very short interval of time,* the distance traversed is effectively reduced. This is why a space traveler could reach stars across the galaxy within their lifetime if they got close enough to the speed of light. Time goes in the opposite “direction” of space!
That constant, represented by c? It’s the same c that represents the speed of light in equations such as E = mc2. What better number to convert units of time (seconds) into a distance-like measurement — after all, we know that for light, there are 186,000 miles per second. See what Einstein did there? The speed of light is more than just a speed; it’s a universal conversion factor that turns time into a distance-like measurement. By treating time as a negative and multiplying it by c, we can exchange time and space in our formulas as readily as nature exchanges them. That’s what special relativity is all about.
This extra meaning of the speed of light has interesting consequences. If you could look out the window of a rocket going at the speed of light, you wouldn’t “c” a thing — the entire universe would vanish to a point through Lorentz contraction. In order for space and time to enter into what we call “reality,” they must be measured by an observer, something not traveling at the speed c. In the real world, that’s anything with mass. For any observer with mass (make your own couch-potato joke), zero spacetime distance separates into the components familiar to us: a measurable amount of space and a measurable amount of time.
It’s as if the presence of mass causes “zero” to pull apart into the familiar ideas of spatial distance and temporal duration, like taffy. But since the universe is by definition everything there is, you have to be inside the universe to witness this incredible stretching apart of zero, to experience space and time as different things. If you were taking in the all-seeing “God’s-eye view” from a timeless, spaceless, massless perspective outside, you would see the same thing the speed-of-light traveler sees — nothing. To witness the action, you have to be inside the theater, in your seat.
Space and time cancel out to exactly zero for the universe as a whole. But that’s just one example of the zero-sum nature of the physical world. A few others:
• The mass–energy of everything in the universe is exactly balanced by the universe’s gravitational energy. The latter is expressed as a negative number, just as time is in the spacetime formula. A while back Alex Filippenko, who’s a familiar smiling face to science-TV geeks, co-wrote an essay about how this means the universe may have come from “nothing at all.” Like the pulling apart of space and time, mass–energy and gravitational energy were also pulled apart in the Big Bang.
• For similar reasons, the net charge of the universe is generally believed to be zero, with the number of positively charged particles equaling the number of negative. (This is unproven.)
• Certain pairs of phenomena, like electricity and magnetism or mass and the curvature of space, are linked such that they seem to keep each other in check. The great physicist John Wheeler was fascinated by these “automatic” connections, pointing out how they are constrained together by zero sums, the way the ends of a see-saw are always the same total distance from horizontal. “That this principle should pervade physics, as it does,” he asked in 1986, “is that the only way that nature has to signal to us a construction without a plan, a blueprint for physics that is the very epitome of austerity?”
On the one hand, it’s surprising that quantities totaling zero show up again and again in nature. But on the other it makes sense, if the universe is a closed system incorporating everything there is. As a teen I remember being into the Taoist idea of Yin and Yang — I thought that in the final analysis, the universe as a whole couldn’t be anything but perfectly balanced. On a level deeper than I imagined, I may have been right.
* Slow speeds (which mean long elapsed times) cause the time part of the formula to overwhelm the space part, resulting in large spacetime distances. Spacetime distances only get small when you approach the speed of light, for example, covering 186,000 miles in 1.1 seconds — then the (negative) time part almost cancels the space part.
Inside, the theater is a mind-boggling swirl of information, actions, and reactions; from the outside, as seen by a cosmic Google Earth beyond space and time, it’s completely empty. I am not just making a cute metaphor here — this is a real feature of our universe, with profound implications.
It seems that in the final analysis, when all things are considered, the universe adds up to exactly zero. For one example (there are others at the bottom), consider the relationship between space and time. Bear with me as I review an idea from high-school geometry. If we have a triangle that includes a 90° angle, we can use the Pythagorean theorem to determine the length of the diagonal from the length of the right-angle sides:
a2 + b2 = c2
where a and b are the lengths of the right-angle sides, and c is the length of the diagonal. A nifty geometrical diagram proves why this is true. If you make a square box along each of the three sides, then each box has an area of the length of that side, squared. The area of the smaller squares adds up to the area of the largest square: If a = 3 and b = 4, then c = 5.
The Pythagorean theorem works in three dimensions, too. If you see a blimp in the sky, you can calculate the exact straight-line distance to the blimp by knowing its altitude above the ground (the value z), as well as how far east-west (x) and north-south (y) you'd have to go to get right under the blimp:
x2 + y2 + z2 = d2
where d is the distance to the blimp. I don't have a 3D diagram, but you can prove it for yourself with a little effort.
Now it gets interesting. This trick extends to four dimensions. Time is typically cited as the fourth dimension. Does the decidedly non-geometric idea of time work into the Pythagorean theorem? Incredibly, it does — but first, you have to convert the time measurement into a distance-like measurement. Then, the total distance you’re calculating is the spacetime distance in the bizarre four-dimensional world where east-west, north-south, up-down, and earlier-later mean the same thing, only in eight different directions. Represented by the letter s, spacetime distance (also known as a Minkowski interval) is determined by an amazing formula. Let’s break it down:
x2 + y2 + z2 – (ct)2 = s2
As before, x is the distance (for example) to the east, y is north, and z is up, but we’ve added a fourth term for time (t), which gets multiplied by a constant, c. Notice the minus sign before the term for time. When it comes to distance through spacetime, elapsed time counteracts spatial distance, and vice versa: If we travel a distance through space, and do it in a very short interval of time,* the distance traversed is effectively reduced. This is why a space traveler could reach stars across the galaxy within their lifetime if they got close enough to the speed of light. Time goes in the opposite “direction” of space!
That constant, represented by c? It’s the same c that represents the speed of light in equations such as E = mc2. What better number to convert units of time (seconds) into a distance-like measurement — after all, we know that for light, there are 186,000 miles per second. See what Einstein did there? The speed of light is more than just a speed; it’s a universal conversion factor that turns time into a distance-like measurement. By treating time as a negative and multiplying it by c, we can exchange time and space in our formulas as readily as nature exchanges them. That’s what special relativity is all about.
This extra meaning of the speed of light has interesting consequences. If you could look out the window of a rocket going at the speed of light, you wouldn’t “c” a thing — the entire universe would vanish to a point through Lorentz contraction. In order for space and time to enter into what we call “reality,” they must be measured by an observer, something not traveling at the speed c. In the real world, that’s anything with mass. For any observer with mass (make your own couch-potato joke), zero spacetime distance separates into the components familiar to us: a measurable amount of space and a measurable amount of time.
It’s as if the presence of mass causes “zero” to pull apart into the familiar ideas of spatial distance and temporal duration, like taffy. But since the universe is by definition everything there is, you have to be inside the universe to witness this incredible stretching apart of zero, to experience space and time as different things. If you were taking in the all-seeing “God’s-eye view” from a timeless, spaceless, massless perspective outside, you would see the same thing the speed-of-light traveler sees — nothing. To witness the action, you have to be inside the theater, in your seat.
Space and time cancel out to exactly zero for the universe as a whole. But that’s just one example of the zero-sum nature of the physical world. A few others:
• The mass–energy of everything in the universe is exactly balanced by the universe’s gravitational energy. The latter is expressed as a negative number, just as time is in the spacetime formula. A while back Alex Filippenko, who’s a familiar smiling face to science-TV geeks, co-wrote an essay about how this means the universe may have come from “nothing at all.” Like the pulling apart of space and time, mass–energy and gravitational energy were also pulled apart in the Big Bang.
• For similar reasons, the net charge of the universe is generally believed to be zero, with the number of positively charged particles equaling the number of negative. (This is unproven.)
• Certain pairs of phenomena, like electricity and magnetism or mass and the curvature of space, are linked such that they seem to keep each other in check. The great physicist John Wheeler was fascinated by these “automatic” connections, pointing out how they are constrained together by zero sums, the way the ends of a see-saw are always the same total distance from horizontal. “That this principle should pervade physics, as it does,” he asked in 1986, “is that the only way that nature has to signal to us a construction without a plan, a blueprint for physics that is the very epitome of austerity?”
On the one hand, it’s surprising that quantities totaling zero show up again and again in nature. But on the other it makes sense, if the universe is a closed system incorporating everything there is. As a teen I remember being into the Taoist idea of Yin and Yang — I thought that in the final analysis, the universe as a whole couldn’t be anything but perfectly balanced. On a level deeper than I imagined, I may have been right.
* Slow speeds (which mean long elapsed times) cause the time part of the formula to overwhelm the space part, resulting in large spacetime distances. Spacetime distances only get small when you approach the speed of light, for example, covering 186,000 miles in 1.1 seconds — then the (negative) time part almost cancels the space part.
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