DCP: Objectivity is a myth because of quantum spookiness

[Physics Guy]

At present, the speed of photon travel in a vacuum is the fastest known, but that really does not mean anything necessarily. The "sacred" claim really should be nothing more than a flourish. They haven't been proven to exist, but if they do, gravitons could, according to some theories, travel faster than light.

No, it's not just that we haven't found anything else that goes faster than light in vacuum. We have a clear and simple theory, supported by an enormous amount of strong evidence, to say that the light-speed speed limit is not actually about light at all. Light can hit the limit, and so can gravitational waves, but the limit is not defined by light.

According to Einsteinian relativity, the speed of light is that speed because that speed is part of the relationship between space and time. It's what makes them the same, in many ways, yet also different in other ways.

The best way I know to explain how the speed of light makes space and time the same is to point out that the speed of light doesn't really have any particular value that could have been something else. It's really just a human convention to use different units for things that we perceive as being qualitatively different, the way we measure water in cubic feet while buying beer in pints. The number of pints in a cubic foot may sometimes be an important number, but it's not a constant of Nature. As far as physics is concerned, volume is volume, regardless of which liquid is occupying it, and so it's silly to use different volume units for two kinds of liquid. It's just a human convention, and if we were more logical creatures we would just drink beer in cubic feet, making the conversion factor between beer and water units be 1. I suppose if we drank beer in cubic feet we might not stay rational for long, so perhaps it's all for the best.

The speed of light (in vacuum) is the conversion factor between our preferred units for distance and our preferred units for time, like the number of pints in a cubic foot. We could measure all lengths in seconds, and then the speed of light would be one. The trouble is that a second of length would be a light second, nearly two hundred thousand miles, which is an awkwardly long unit for most human purposes. Or we could measure time in feet. The trouble then is that a foot of time is about a nanosecond, awkwardly short. So we use miles and seconds, and think that space and time are completely different kinds of things, when they're not.

On the other hand space and time are not exactly the same kind of thing, either—and just as their sameness was about the speed of light, the difference between them is about the speed of light, too. Space and time could both be measured in miles, but space miles and time miles still aren't as totally equivalent to each other as left-right miles and forward-back miles are equivalent. You can turn any mile in the left-right direction into a mile in the forward-back direction, or vice versa, just by turning your head 90 degrees. You can mix a space mile and a time mile into each other a bit, by speeding up or slowing down, but there is nothing you can do to turn a pure duration, with no spatial length, into a pure spatial distance, with no duration. There's a difference that can't go away.

If you hold up two fingers a foot apart, that's a foot of distance between them; if you close your eyes and wait for a second before opening them again, the interval between closing and opening is a second of time. There is a qualitative difference between those two in-between intervals. We could consistently measure them in the same units if we wanted, but there is nothing that we can do which would be like turning our heads by 90 degrees, to turn the time interval into the space interval, or vice versa.

The reason we can't do that is that the speed of light cannot be exceeded: it is this speed limit which defines the distinction between space and time. Or to put it the other way around, the reason we cannot travel faster than light is that traveling faster than light isn't traveling. It's already being there.

Having said all that, it may in principle be possible to travel faster than light by warping space and time themselves. The idea would be not to move at all, in fact, but rather to sit still in a patch of space that was itself moving through the rest of the universe. Surprisingly, this is not inconsistent with Einstein's general theory of relativity (the Alcubierre warp drive is a solution to Einstein's field equations). To be consistent with general relativity, however, this warp drive would require some bizarre form of matter, which probably cannot exist, to form the moving space bubble. And even though the space bubble moving faster than light is a solution to the equations, that solution doesn't say how such a bubble could be started in the first place, or stopped so that passengers could leave. Those parts of the operation may actually be impossible, as far as I know. Or maybe a finite-run Alcubierre bubble would actually just amount to a kind of wormhole, which is another hypothetical way to reach distant places quickly, by finding a short cut through which they are actually close, like having the Andromeda galaxy inside your wardrobe instead of Narnia.

[Doctor Steuss]

No, it's not just that we haven't found anything else that goes faster than light in vacuum. We have a clear and simple theory, supported by an enormous amount of strong evidence, to say that the light-speed speed limit is not actually about light at all. Light can hit the limit, and so can gravitational waves, but the limit is not defined by light.

Wasn't there an experiment a few decades ago with a laser pulse through cesium gas that had one of the wavelengths exceed the speed of light? Or was it found to be flawed... or am I (very likely) misremembering it?

[Physics Guy]

Yes and no. There are some technical shenanigans to be played with the definition of "speed". For instance there are two kinds of velocity that can be defined for waves of any kind: phase velocity and group velocity. Phase velocity is the speed at which crest and trough pursue crest and trough within a long train of waves.

Group velocity is the speed at which a clump of waves moves as a whole. You get a clump of waves like that, for instance, if a power boat speeds past you on a lake. Each spreading line of its wake is usually a string of half a dozen crests and troughs, that move together; the leading crest and trough are pretty shallow, and so are the trailing ones, while the ones in the middle are bigger.

For sound waves and light waves there is usually hardly any difference between the two kinds of velocity—for light in vacuum there is exactly no difference—so the distinction doesn't matter, but for surface water waves the difference can be quite dramatic. The phase velocity can be quite a bit higher than the group velocity. So you're sitting in a canoe, a powerboat zooms past you some distance away, and you watch the waves of its wake coming toward you. They're racing toward you quite fast, crest after crest; but somehow they're taking a long time to actually reach you. As they get closer you can see that those fast-moving crests are just kind of emerging out of the water as shallow ripples, back at the trailing edge of the wake, growing bigger as they move toward you, but then shrinking again and dying out, at the front edge of the wake. The strip of wake is moving towards you at a comparative crawl, while the wave crests keep running through it faster. This happened to me once. It was weird.

Anyway, it can also happen for some other more exotic kinds of waves, like light waves in weird optical media, that phase velocity and group velocity are substantially different. Only the group velocity is limited by the speed of light in vacuum. Phase velocities higher than "c" are allowed, because they don't represent the speed at which any detectible change is being transmitted—they're an ongoing pattern that is already established, everywhere, so nothing new is happening anywhere. A group of waves, with a moving front edge, could be a signal; that signal cannot go faster than c.

In a certain sense even the group velocity can sometimes be higher than c, if a pulse of waves moves through a nonlinearly absorptive medium. Then the back edge of the pulse gets attenuated more than the front edge, shifting the peak of the pulse toward the front. With this little added boost, the peak of the pulse can be made to "travel" faster than the speed of light in vacuum.

It's really a crock, of course. It's like chopping off the end of a carrot and saying that the center of the carrot has just jumped an inch instantaneously, defying Einstein, because once you cut off the back end of the carrot, only the remaining part still counts as "the carrot", and with the back end gone, its center is further forward than the center of the original whole carrot. But if you took the right textbook definition of "group velocity", you could indeed say, by that definition, that you had sent a light pulse with group velocity faster than the speed of light in vacuum.

I think that's probably the experiment you are remembering. I used to know some of the people who did it. They encoded some music in their light pulses, so they could talk of transmitting "superluminal Mozart" or something.

There are some things that really can go faster than light—for certain values of "things". Shadows, for instance; or the point of first contact between the two blades in a pair of scissors as it closes. You can follow the motion of the edge of a shadow or of that scissors contact point, so your mind does kind of register them as moving things, but they are not things in a physical sense. The crucial point is that their motion cannot transmit information between the points between which they move.

You can swing a laser pointer, for instance, and make the red dot zip across the wall faster than light. Or at least, relativity wouldn't prevent you from doing that—your wrist just wouldn't be up to it. If you bolted a laser pointer to a car tire spinning at 6000 rpm, and it was a good enough pointer to make a visible dot on a satellite 1000 km overhead, that dot would zip across the satellite's hull at about 2c, a hundred times every second.

That dot wouldn't be carrying any information from one edge of the satellite to the other, because it isn't being generated in any controllable way at the edge of the satellite. It's being generated by you back down on Earth, with the laser pointer, and it's only reaching the satellite at the speed of light. The situation of the two edges of the satellite, with the red dot going between them faster than light, islike that of two people a hundred miles apart from each other, listening to the same radio station as it broadcasts from a transmitter that is located somewhere between them. If their distances to the transmitter happen to differ by only a foot, the slightly more distant listener will hear each note of music only a nanosecond after the nearer one hears the same note, but this does not allow communication between the two of them across a distance of a hundred miles in a nanosecond, which would be superluminal, because neither of them can affect what the other one is hearing. The red dot sweeping across the satellite is like that, as far as communication from one side of the satellite to the other is concerned, so it can "move" faster than light.

[Alphus and Omegus]

At present, the speed of photon travel in a vacuum is the fastest known, but that really does not mean anything necessarily. The "sacred" claim really should be nothing more than a flourish. They haven't been proven to exist, but if they do, gravitons could, according to some theories, travel faster than light.

No, it's not just that we haven't found anything else that goes faster than light in vacuum. We have a clear and simple theory, supported by an enormous amount of strong evidence, to say that the light-speed speed limit is not actually about light at all. Light can hit the limit, and so can gravitational waves, but the limit is not defined by light.

According to Einsteinian relativity, the speed of light is that speed because that speed is part of the relationship between space and time. It's what makes them the same, in many ways, yet also different in other ways.

The best way I know to explain how the speed of light makes space and time the same is to point out that the speed of light doesn't really have any particular value that could have been something else. It's really just a human convention to use different units for things that we perceive as being qualitatively different, the way we measure water in cubic feet while buying beer in pints. The number of pints in a cubic foot may sometimes be an important number, but it's not a constant of Nature. As far as physics is concerned, volume is volume, regardless of which liquid is occupying it, and so it's silly to use different volume units for two kinds of liquid. It's just a human convention, and if we were more logical creatures we would just drink beer in cubic feet, making the conversion factor between beer and water units be 1. I suppose if we drank beer in cubic feet we might not stay rational for long, so perhaps it's all for the best.

The speed of light (in vacuum) is the conversion factor between our preferred units for distance and our preferred units for time, like the number of pints in a cubic foot. We could measure all lengths in seconds, and then the speed of light would be one. The trouble is that a second of length would be a light second, nearly two hundred thousand miles, which is an awkwardly long unit for most human purposes. Or we could measure time in feet. The trouble then is that a foot of time is about a nanosecond, awkwardly short. So we use miles and seconds, and think that space and time are completely different kinds of things, when they're not.

Thanks for the clarification. My point in discussing the hypothetical speed of gravity in this context is that "light speed" is nothing special. There is obviously nothing sacred about it, especially since gravitational waves do travel at the same rate, as do other forms of electromagnetic radiation, such as radio waves.

Peterson and his muse seem not to understand the significance of this. As far as we currently know, these massless particles can reach this speed because they have no mass. Trying to draw theological conclusions from this fact makes no more sense than using scattered goat entrails for discerning the will of Zeus.

There is no scientific knowledge to be gained from using definitional fallacies to claim that biblical sayings about "light" and "God" are anything other than ancient creation hypotheses. They hold no more explanatory value than the quaint theories of Thales, who hypothesized that everything was about water.

The cosmology of Christianity was very obviously derived from Neo-Platonic monism as well, so it really is rather embarrassing for present day Christians to try to go back to that tradition, considering its obviously nonscientific ideas.