… and the seasons move ahead. Strange year. All summer long, there were hardly any hummingbirds at the bird feeders. Instead, brilliantly-colored orioles dominated them. Now the orioles have gone south, as have most of the swallows that spent the summer raising the next generation. And out of nowhere, an infestation of hummingbirds has suddenly descended – furiously drinking pints a day of hummingbird nectar, like students at a kegger.
Those who attribute preternatural senses to birds argue this early migration of birds means we who cannot fly south are in for a hard winter. Time will tell.
The passage of the seasons reminds me of how much I miss the insights of John Walker. Back in Newton’s day, the assumption was that gravity acted instantaneously regardless of distance. In effect, the force of gravity travels at speeds vastly exceeding that of light. Then Einstein came along, and asserted that nothing can travel faster than light. Modern measurements of gravitational waves do indeed suggest they move at about the speed of light.
People occasionally point out this time delay means the Earth does not feel the pull of gravity from where the Sun is, but instead from where it was 8 minutes ago. By the time we get out to Neptune, whose orbit is very well described by Newton’s instantaneous gravitational attraction, the planet is feeling the pull of gravity from where the Sun was over 4 hours ago. And remember, the Sun is moving around the galaxy at a speed of around 435,000 miles per hour. Thus, Neptune feels the gravitational attraction not where the Sun is, but from about 1,700,000 miles behind its current position. Yet Newton’s equations work!
This seems to be one of those topics which is not much discussed in polite circles. That is unfortunate, since there probably is much that could be learned from it. John, I miss your erudition.
But your mention of our mutual friend and gravity, reminds me of his gravitational experiments in his basement of a plank with two masses on it supported by a step ladder, attracted to two other fixed masses on the floor.
Newton’s laws work because, in the appropriate limits, the field equations of general relativity reduce to Newton’s law of gravitation: the Newtonian approximation. Much like Wayne Gretzky and the puck, Neptune “knows” where the sun is going to be. However, if the sun jumped to a different spot sometime in the last four hours, then all bets are off.
This also happens in electromagnetism. Even though the electromagnetic field propagates at the speed of light, nevertheless, the field lines of a moving charge radiate out from the location of the charge is, not from where it was when the signal would have reached a distant observer. This is even true at relativistic velocities (speeds approaching light’s), though the field lines are somewhat bunched. This is seen in this figure from the Feynman Lectures, viz.,
Of course, if the charge accelerated some time before it got to that spot, the field at a distant location would still look as if it arrived from the original location. The disconnect between the two is the origin of electromagnetic radiation. In gravitation, it is likewise the origin of gravitational radiation. In both cases, the field jumps in a different direction and that is called radiation: a disturbance propagates at light speed, as illustrated for electromagnetism in this animation.
I remember having a hard time explaining this to a colleague, who thought that, intuitively, the field at a distant location would correspond to a radial vector from where the charge was when it “emitted” that field line. Well, that’s not the way Mother Nature worked things out, intuitive or not.
My circles may not be polite but such topics were discussed.
Thanks for the explanation, Dr L. It still leaves me in the “puzzled” camp. If we look at an observer in the “midnight” position on that animation, he would see the field radiating out from the original position of the emitter until – suddenly – the apparent position of the emitter changes when the new information moving at the speed of light reaches him.
Presumably if we played with the numbers, we could make the apparent jump in position of the emitter look faster than the speed of light – but that is not something we apparently ever observe (outside of UFOs, of course).
Maybe underlying my confusion is the requirement – in a world that has rejected the aether – to have immaterial fields (lots & lots of them!) pervading the universe which are still able to convey energy from place to place. The math works, but what is really going on?
Fields are nothing more than a way to visualize the mathematics. They are real or observable in the sense that the math says there is a field at a certain point, there will be certain effects there: you will know them by their fruits. This is the same kind of reality that electrons have. You can’t see an electron any more than you can see a gravitational or electromagnetic field. Is an electron more material than its electric field?
This raises more questions than it answers. How do you know your sense perceptions signify what you think they do? How do I even know that you, or anyone else, exists? This way lies madness.
As one of my favorite and, sadly, recently deceased bloggers wrote, “Idle hands do the Devil’s work and the best proof of that is philosophy.” I am a practical man. While I enjoy musing on the philosophical significance of non-real properties/nonlocality in quantum mechanics — much more counterintuitive than classical fields — at the end of the day, I have to use the equations to do calculations and understand the results of experiments. So far, it’s been working out okay. Things that we all use every day* are testament to this.
Agreed ! The math works. But then, the math of epicycles worked too for describing the movements of the planets, even though the concept was entirely unrealistic.
Today, we have no choice but to use the equations we have that “work”. However, there is a real physical world, and the better we understand what is actually going on, the more accurate the next round of mathematical approximations can be.
Not exactly, Epicycles were a decent approximation. There was nothing “unrealistic” about that. The problem is they were ad hoc, lacking predictive power outside their original domain. Contrast this with Newtonian gravitation or general relativity, which make predictions that can be checked and have been verified outside their original data set. These two things are not the same.
There is no “real physical world” besides the mental models we make of it. Every observation you make requires a model to interpret your sense perceptions. A trace on an oscilloscope or a track in a bubble chamber is meaningless without an underlying model. Even apparently “direct” observations require a model. If you see a rainbow in he sky, is it a supernatural miracle or the refraction of light by raindrops? What do your perception of colors represent?
Primary cultures had very different models for natural phenomena that worked for them. More to the point, they would find scientific explanations to be absurd. What is this gravity of which you speak? Clearly, the gods move the sun and moon around while Earth stands still. The idea that the Earth moves is preposterous: contrary to even casual observation.
The proof that the current approximation has something to do with “what is really going on” is that it works great, even in new domains. And when it doesn’t work so well in some new domain, the approximation has to be modified. That has been the process of getting more accurate rounds of approximations.
While I agree with much of what you wrote, I think that particular statement was a little exaggerated. Of course there is a real physical world – even if it turns out in the end to be a real simulated physical world in some alien teenager’s super-duper cell phone.
I think there is a real physical world but we can only apprehend it via our mental models. Thus, for each person, the physical world is the mental model.
I have noticed that other people sometimes have dramatically different models of the world — some wildly divergent from my own or greatly simplified. They make predictions and draw conclusions from these models that are often at odds with reality in the sense that these models have poor predictive value. Nevertheless, this is their reality. Though I might claim that my model is better in some case, they are unswayed or uninterested in my allegedly better model. Truth is not the supreme value I’d like to think it is. Or, as the poet A. E. Housman observed, “The faintest of all human passions is the love of truth.”
However, this discussion has strayed a bit. You seemed disturbed about the reality of “immaterial fields pervading the universe.” So let me ask you this: are electrons any more real than these fields? How about molecules? Viruses? Bacteria? Not one of these things can be perceived with the naked eye, which means some model of the universe is required to transform human sense perceptions into these objects.
It is said that Ludwig Boltzmann committed suicide because he couldn’t convince skeptics like Mach that atoms were real. At least, that’s the romantic version of the story my professors liked to tell. That was only about a century ago. Galileo said, “E pur si muove” and Boltzmann said S = k log W. Were they right? I’d like to think so.
You are right! This started with a sudden influx of hummingbirds, leading to ruminations about the attraction between Sun and Earth causing the seasons. I now have to report a dramatic reduction in the number of hummingbirds swarming around the feeders. No doubt about it – those babies are going South! We may be in for a long hard winter.
