OK, we all know that gravity exists even though we do not know how it works, but I have a couple of questions.
The moon and sun have a big pull on the earth and seas as we see from the changing tides. When the sun and the moon are both pulling together on the one side of the earth causing a high tide, how do you explain that you get a high tide on the other side of the earth where there is no gravitational pull?
Does the gravitational pull of the Sun and the Moon affect our weight making us lighter when we have a high spring tide and heavier when we have a low tide and would we be able to jump higher in the air when we have a high spring tide than on a low tide where there is less gravitaional pull?
Is gravitational pull instantaneous as it is not an electromagnetic force?
Is there any way that we can experiment to see if gravitational force is instantaneous or not?
The Sun and moon pull more on the near side of the earth than on the far side, because the gravitational force goes as the inverse square of the distance. Remember that the moon pulls on the earth, not just the oceans, so the oceans only bulge due to the differences in force between the near and far sides of the earth. The near side experiences more force relative to the earth, and the far side experiences less. So relative to the earth, the near side attracts toward the sun and moon, and the far side is less attracted than the earth itself, so it is less attracted than the earth is, and so is attracted away from the earth.
Yes. But you won’t notice it. The affect of the tide on your weight is only about + or - 0.000001%.
No forces are instantaneous. All forces move at the speed of light.
The only way that I know of are gravitational waves, which have not been detected because it is a very difficult thing to measure. But theoretically this is very well understood. No forces are instantaneous.
Consider three “objects”: the water on the Earth closest to the sun, the Earth itself, and the water on the Earth farthest from the Sun. The water “under” the Sun is closer to the Sun than the Earth is, so the sun pulls on it more and it lumps up there. But the water “opposite” the Sun is even farther from the Sun than the Earth is, and so the Earth gets pulled away from that water. Thus, relative to the Earth, the water makes two “humps.”
The Sun and the Moon do exert gravitational forces on you, yes; so you could probably jump highest at noon on the day of a new moon, since the Sun and the Moon are both trying to pull you away from the Earth. Similarly, at midnight on the day of a full moon, the Earth, Sun, and Moon are all lined up in trying to pull you toward the center of the Earth. These effects are miniscule, though, and have very little to do with the tides.
According to General Relativity, which is our best theory of gravity to date and works pretty damn well everywhere it’s been tested, the speed of gravity is the same as the speed of light. Experimentally, we have some indirect evidence that the speed of gravity is pretty close to the speed of light; the rate at which the Hulse-Taylor binary pulsar is losing energy depends on how fast gravity travels, and our measurements of it put the speed of gravity to be within 1% or so of the speed of light. It turns out to be hard to unambiguously measure the speed of gravity, though; there have been a few more contentious results over the past few years that claim to have measured the speed of gravity more precisely, but they’re not universally accepted.
Mike, I don’t go along with the sun pulling the earth as surely the earth is in a stable ortbit around the sun or does it get pulled out of orbit once every 28 days when bothe the sun and the moon pull on the earth.?.
I am also curious about two other things that are beyond chance and one is that the earth only sees one side of the moon, meaning that one rotation of the moon is exactly the same as its orbit around the earth, approximately every 28 days.
The second thing is that when we have an eclipse of the sun, the moon is an absolute perfect fit to totally obscure the sun, not too small and not larger.
Being “in a stable orbit around the Sun” means “falling into the Sun constantly but moving sideways fast enough to never hit it”. Strictly speaking, the Sun and the Earth are both in orbit around their common center of gravity. Because the Sun is many times more massive than Earth, that common center of gravity is inside the Sun. It’s not in the middle of the Sun: the center of gravity between the two bodies moves around, based on the location of Earth itself. The Sun bobs a little bit because that center of gravity moves around. There is a tidal effect on Earth because of the Sun, and there is a tiny tidal effect on the Sun because of Earth.
Similarly, the Earth and the Moon are in orbit around their common center of gravity. The Moon is much less massive than Earth, so again it appears mostly that the Moon is the one in orbit. And the Earth bobs a little bit because the common center of gravity moves around, and there is a tidal effect on Earth, and there is also one on the Moon (but it has no oceans to make it visible).
There’s an explanation here, and at thousands of other places on the Web.
Belief has little to do with it. I can assure you that you do go along, if you live on Earth.
Yes, it’s because of tidal locking. After a while, the mass lumps in the Moon aligned with the Earth-Moon center of gravity, it was less hassle that way. Earth is slowing down, too, but it’s still spinning because it’s the more massive partner.
Yes, it’s nice when that happens, but there are also annular eclipses.
That one is just a coincidence. The moon has been slowly pulling away from the earth ever since it was formed. Why? Tides again.
Yes, it’s fascinating that the two bodies’ apparent sizes are identical when homo sapiens happens to exist. But beyond chance? What’s your answer? Gods? Aliens? An intergalactic theme park?
No only gravity (very probably) and electricity and magnetism (parts of the same thing) move at the speed of light. The other two forces, the strong and weak nuclear force are carried by mesons that have mass and move at less than the speed of light, I’m pretty sure, though since they’re virtual particles I could be wrong.
The strong force is not carried by mesons (that’s a discarded theory). The strong force is carried by gluons, which are massless. The weak force operates over distances so short that for a long time it was modelled as a point interaction. The weak force has a range of less than 10^-16 meters, which is so short that it is sort of meaningless to ask whether its mediation is at, or just near, the speed of light. Yes, the mediators are highly virtual, which means that they are not propagating like a normal particle that must go slower than the speed of light. In fact, owing to their virtuality W/Z bosons (mediators of weak force) and photons can both go slower or faster than the speed of light (sending information faster than light is impossible, however). But on average both the weak force and the other forces all propagate at the speed of light.
As Heracles has noted, the fit is pretty good, but hardly perfect. On May 20, an eclipse occurs with the Moon’s apparent diameter only 94% of the Sun’s.
Not how I’d phrase either of those points. First, the strong force can still be modeled as being carried by mesons, and doing so gives you decent approximations for many purposes. We know now that it’s more complicated than that, and that it’s only an approximation, but then again, the more sophisticated techniques we use also break down in a lot of situations (annoyingly, including anything low-energy), so a simpler approximation isn’t all that bad, comparatively.
Second, it isn’t quite precisely the strong force that’s mediated by gluons; it’s the color force. The color force is much, much stronger than the strong force, but in any physically realizable situation, the color forces on various parts of a system will all almost cancel out, and it’s just the residual that doesn’t cancel that we call the strong force. It’s qualitatively similar to the van der Waals force, which is really just a manifestation of the electromagnetic force not quite canceling out, except that with electromagnetism, it’s possible to construct other situations where it doesn’t cancel.
But, like I said, it’s a discarded theory. For a while they thought the strong force was actually mediated by mesons. Now they know it isn’t, though it is still sometimes a useful approximation.
In my area at least (HEP), no distinction is ever made between “strong force” and “color force.” No distinction is made on wikipedia either, or in any of my books on QCD. Both are synonyms for the force mediated by gluons corresponding to SU(3) gauge symmetry. Historically there is a distinction in that “strong force” was initially only known to be what held nuclei together. Later, it was discovered that protons and neutrons are color neutral, and the force that holds together nuclei is a residual effect of what is now generally just referred to as the strong force. It is not wrong to say that the strong force holds both nuclei and hadrons together, it’s just that in the case of nuclei the force, like you say, is more like a van der waals effect.
As noted, the moon does not fit neatly over the sun at all times. When an eclipse happens when the moon is at or near apogee (the furthest point in its orbit from Earth) the moon covers too small a piece of sky to entirely cover the sun at any point in the eclipse, we call it an “annular” eclipse. “Annular” comes from the Latin “annul”, which means “ring”, because only a ring of the Sun appears in the sky-- the moon is blocking the rest. In fact, annular eclipses are slightly more common than total eclipses. Here’s some cool pictures of this.
That would be beyond chance except that it’s not chance at all. When the moon formed, it was so hot as to entirely molten. The heaviest elements sank to the bottom and the lightest elements floated to the top, precisely like the Earth, except that the moon has since cooled off and frozen solid, unlike Earth. While this happened, though, the heaviest elements shifted slightly toward the Earth, meaning that the densest and heaviest elements are not exactly in the center, and so the moon’s center of gravity is off-center, as it were. Because of this, the lightweight crust is actually thinner on the near side of the moon and thicker on the far side. The heavier side (the near side) of the moon is therefore drawn more strongly toward the Earth, and over billions of years, the extra energy required to maintain the moon’s original rate of rotation is used up, so the moon ends up with only one side facing the Earth at all times. It’s a phenomenon called Tidal Locking and it’s well understood. Many bodies in the solar system are tidally locked in this way.
If that’s not clear, imagine you’ve got a bicycle wheel, and you can hold on to the axle with one hand and hold the wheel flat above the ground, so that it can spin freely. If you then tie a weight onto the wheel, you can spin the wheel and it will spin just as long as it would have if you had not tied the weight to it and which direction the weight is facing when the wheel stops will be entirely random, because since no point on the wheel is closer to the ground than any other; the weight does not cause the wheel to tend to stop in any particular place. However, if you tilt the wheel so that one side of the wheel (let’s say the south side) is slightly lower, the wheel will stop with the weight facing south every single time. Now, since the moon is in orbit around the earth, imagine that while spinning the wheel, you walk around a tree (representing the earth) and you tilt the wheel not south, but directly toward the tree. The wheel will settle with the weight facing the tree every single time, and once the wheel has settled, the weight will remain facing the tree each time, and once it has settled, the wheel will make one rotation for each revolution you make around the tree. The wheel is the moon, the weight represents the moon’s frozen tidal bulge, the tree is the earth, and the tilt in the wheel represents the Earth’s gravitational pull.
Now, you might ask, why doesn’t the Earth have a frozen tidal bulge that locks its rotation into sync with the Sun? The answer, of course, is that it doesn’t have one yet, but it will! As the Earth cools over billions of years, the precise same thing will happen. (Of course, the Earth may be consumed by the expanding Sun as the sun becomes a red giant first. I’m not sure which will happen first.)
I would say “insufficiently detailed” rather than “discarded”. If you draw a Feynman diagram for the real QCD process of interaction between, say, a proton and a neutron, you could isolate a part of the diagram and identify it as a pion connection the two nucleon lines, with all the of the details of the gluons absorbed into a pion-nucleon vertex.
On the topic of tidal bulges: Most moons that we know of are tidally locked to their parent planets, and Pluto and Charon are both tidally locked to each other. It’s quite a common state of affairs. Mercury and Venus would both be tidally locked to the Sun, too, except in both cases they ended up with a different sort of pseudo-lock instead: Mercury is in a harmonic lock, such that every time it’s closest to the Sun it’s got its long axis lined up on the line to the Sun, but alternating which way from one pass to the next. And Venus got almost all the way to a lock (its rotation is almost the same period as its revolution), but it caught some interference from the Earth: Whenever Venus and Earth pass closest to each other, Venus shows the same face towards the Earth.
