Understanding Gravity
william chan
Posts: 1,326
Here are some questions about gravity which i can't answer.
1. Does electrons attract each other gravitationally?
2. Does anti-matter repel each other gravitationally?
3. What about quarks and neutrinos? Do they attract each other gravitationally?
4. Do photons attract each other gravitationally?
5. If a photon moves away from a body of mass at the speed of light, how does it exchange gravitons with that body? If gravitons are not exchanged, how does that body of mass attract the photon?
1. Does electrons attract each other gravitationally?
2. Does anti-matter repel each other gravitationally?
3. What about quarks and neutrinos? Do they attract each other gravitationally?
4. Do photons attract each other gravitationally?
5. If a photon moves away from a body of mass at the speed of light, how does it exchange gravitons with that body? If gravitons are not exchanged, how does that body of mass attract the photon?
Comments
-Phil
Quoting wikipedia:
"For example, a massive particle can decay into photons which individually have no mass, but which (as a system) preserve the invariant mass of the particle which produced them. Also a box of moving non-interacting particles (e.g., photons, or an ideal gas) will have a larger invariant mass than the sum of the rest masses of the particles which compose it."
http://en.wikipedia.org/wiki/Mass_in_special_relativity
Let us know when you find the answers.
http://forums.parallax.com/showthread.php?t=124732
1. Does electrons attract each other gravitationally?
Yes. This is especially noticeable at astronomical distances where the other forces become irrelevant. Of course, protons and neutrons are a lot heavier so they contribute a lot more, but the electrons are pulling a little too when galaxies collide.
2. Does anti-matter repel each other gravitationally?
No. The anti in antimatter refers to electrical charge; gravitationally it's the same as regular matter.
3. What about quarks and neutrinos? Do they attract each other gravitationally?
It's not possible for quarks to exist in isolation; they always occur in combinations that form more familiar particles. When those particles have mass, they interact gravitationally; when they don't, they don't.
It's been an object of controversy for some time whether neutrinos have mass, but the scale seems to have settled on the side of "they do." This means they interact gravitationally. But they're very small and move very fast so it would be really hard to verify this experimentally.
4. Do photons attract each other gravitationally?
No. Photons do not have mass, and therefore do not generate a gravitational field. However, they do have momentum (it's a long story) which is why gravitational fields affect them.
5. If a photon moves away from a body of mass at the speed of light, how does it exchange gravitons with that body? If gravitons are not exchanged, how does that body of mass attract the photon?
It's programmed that way into the Big Computer.
No, seriously, in quantum physics that's where you end up; the universe works in crazy strange ways and at the end of the day lately physicists shrug and say "well that's how it seems to work, we can't explain why it works that way but when we use the equations they predict what the experiments produce."
Oh, IANAPhysicist but My Dad Was, and I keep up on things.
5 is a really good question. Relativity says that photons travel in straight lines in curved space-time, so no exchange of gravitons is required in that theory. Quantum mechanics says that all forces are mediated by the exchange of virtual particles. So these two theories seem incompatible and a Nobel prize awaits the person who unravels this knot.
One thing we do know is that gravity bends space-time and those deformations travel in waves at the speed of light. If gravitons or gravity waves traveled faster than the speed of light, it would allow for FTL communication by moving large masses around. This would violate causality and be a bad thing for historians.
There are a lot of things in QM that simply don't make sense in a billiards parlor particle exchange way of looking at things (two slits one photon *cough*). Somewhere we have a fundamentally wrong idea about how the universe works, and we're throwing ideas about it that are as relevant as vacuum tube parameters are to P8X32A design. There's a Nobel Prize waiting for the guy who figues out how the Young double slit experiment works with single photons, much less the gravity on a fleeing photon thing.
In any event I probably should have said that we know gravity can't travel faster than the speed of light due to the causality problems and it is predicted to travel at the speed of light.
Is there any link to any site or experiment that confirms that electrons really do attract each other gravitationally?
Somebody just mentioned that a regular particle like a neutron can decay into photons.
To conserve the invariant mass and to conserve gravity, shouldn't photons be attracting each other and other surrounding masses gravitationally?
At the sub-atomic level, the debate between the 'strong' electrical charge forces and the 'weak' force of mutual attraction of mass (another way of saying gravity) remain in hot debate.
I have much more difficulty with the 'strong' forces. What really is an electrical charge? Why are photons and electrons so similar, yet not the same thing from two points of view. It does seem that the particle decay offers a clue to what are really the fundamental sub-atomic particles and that photons are very troublesome to theorist.
And what about magnetism? It seems to come with the alignment and flow electrical charge, but have a lot in common with gravity as it pulls objects to it. (But of course gravity doesn't repel objects.)
wonder, wonder, wander.....
As for decay via photon emission, that gets into some of the great subtleties of relativity. Energy and mass are equivalent and one can be converted to another, but energy doesn't exert a gravitational attraction. Momentum and mass are not the same in relativistic (not even quantum) physics, and so ideas like "conservation of mass" that make sense in macroscopic terms do not apply when you are talking about these kind of particle interactions. It's possible for a particle to lose some mass by emitting a photon and the photon doesn't have mass. There's no way to explain this in English except to say "the math permits it." The math would have to, since we figured out the math by observing what particles do.
Loopy, the "strong" forces as you call them are pretty well understood, but again it's all about the math. None of it really makes any sense at all in terms of everyday experience, but at least with the electromagnetic/weak nuclear and strong nuclear forces there is a lot of consistency which dovetails with other at first seemingly unrelated observations of particle behavior. A lot of it has to do with quarks which can't actually exist on their own, only in certain combinations that form observable particles but they can rearrange themselves in sufficient proximity make different observable particles. The forces are how particles interact; asking how two electrons interact to repel one another as if they're two styrofoam balls that must be puffinga air or something to do it is really missing the point. They just do. That's the way they are written into the Big Computer.
It would be interesting to see a compendium of "we're pretty sure it's true but never actually observed it" just to see if all the "negative spaces" in our many non-observations end up forming a pattern of some sort, a picture maybe of something quite unexpected.
The universe demonstrates extreme consistency. For example, we are very sure that the physical constants such as h and mass of the electron and so on are the same billions of light-years away and back in time as they are here. Do you know how we know that? The atomic spectra are very sensitively dependent on those constants. We can read those spectra with great precision and the lines of absorption and emission from quasars and distant galaxies are exactly the same as the ones we see in labs here on Earth. We know very clearly how those spectra are formed so it means that in those distant galaxies all the physical properties we take for granted here are the same. That means a lot of other properties we can't directly observe will also be the same, because those are derived from the same source constants in ways that are very fully understood. That in turn has broad implications for things like the life cycles of stars which depend on atomic properties.
So yes, even though we've never directly measured the gravitational attraction of an electron, it would actually be quite silly to propose that they don't contribute. One would then have to ask why protons and electrons in magnetic fields act exactly as if they possess the mass fractions we suppose they do when they are assembled together and very obviously generating a measurable gravitational field as an atom.
There are things we don't know about the low level organization of the universe, and some of those things point eerily to an arrangement of information that is connected in ways that don't seem sensible to our macroscopic expectations -- but that's because we didn't form our expectations at quantum scale. One thing we can say is that whatever remains to be discovered has to explain why what we know so far is so consistent. I'm fond of mysteries and edge cases myself, but people tend to forget that anything physics is missing has to fit in an extremely well-developed framework (or explain, as relativity did with Newtonian motion, why the old framework is a special case).
Roger,
I really enjoy your comments on this physics stuff. It's fun even though I have no idea what I'm talking about.
So I sometimes wonder if mathematics is the only language that can adequately describe the structure of the universe. There might be other languages, model-making human brain functions that we have yet to evolve, mushy-middling synapse activity that can truly grok all that double-slit stuff. It's somewhat mind-boggling that we humans can understand anything about the universe beyond throwing spears at fleeing ungulates, and it's an age-old argument going back to at least Plato that debates the very nature of our understanding of things. So, yes, we have to work with what we've got, but I also know there's plenty of wiggle room for the mushy stuff to ooze through.
I don't know if this is a good example, but it seems to me that for a long time CP symmetry was something everyone "just knew" was fundamental.... until people started actually looking at the data.
From wikipedia:
"Until 1956, parity conservation was believed to be one of the fundamental geometric conservation laws (along with conservation of energy and conservation of momentum). However, in 1956 a careful critical review of the existing experimental data by theoretical physicists Tsung-Dao Lee and Chen Ning Yang revealed that while parity conservation had been verified in decays by the strong or electromagnetic interactions, it was untested in the weak interaction. They proposed several possible direct experimental tests. The first test based on beta decay of Cobalt-60 nuclei was carried out in 1956 by a group led by Chien-Shiung Wu, and demonstrated conclusively that weak interactions violate the P symmetry or, as the analogy goes, some reactions did not occur as often as their mirror image."
http://en.wikipedia.org/wiki/CP-violation
From what I remember, this CP thing was just a simple matter of people actually taking a look at something everyone just assumed was the way they assumed it was.
Long story short, I like to keep an open mind about how our minds might work.
I guess you could say "Gravity Sucks!"
Bill
Bill: You are technically and philosophically correct!
As ever, I am awed and humbled by the collective practical and theoretical brainpower of Forum members, as typified in this thread. You guys are friggin' geniuses.
Scientists miss low-hanging fruit that is right in front of their faces for decades. Continental drift, K-T impactor anybody? CP symmetry is a relatively minor thing by comparison but it's the kind of thing that would shock you to the core if you have a certain kind of faith in a certain kind of mathematical expression of how the Universe works. The thing is we know there has to be an asymmetry somewhere because the Universe seems to be almost completely made up of matter with hardly any antimatter, and you'd thing that would be a Big Fat Clue. But it's possible to get so hung up on the elegance of a model that its failure to align with reality calls reality into question instead of the model. (Guilty Party #1: Albert "God does not play dice with the universe" Einstein.)
(A friend of mine recently said that Albert was only half-wrong, God does play dice but the dice are loaded.)
I regard it as possible, though not exactly likely, that the Universe is a truly colossal huge lie, that it is like a video game engine trying to pretend to be much more than it is and silly stuff like the single-photon double-slit results are what happens when it gets caught in its lie. In such a case almost anything could be possible; the Universe could seem to be extremely consistent because that's the lie it is fervently trying to tell us, but at the same time extreme violations of the rules could be possible so long as they do not collapse the entire simulation. In that case all of the crazy nonscientific things people believe about ghosts and angels and ESP and so on could actually be true, and science would be powerless to investigate because as a tool, science does not work unless the Universe is truly consistent.
And while I don't assign a very high probability of that latter possibility being the truth, I don't assign it zero probability either -- particularly when events seem to suggest the existence of weird probability vortices. And for the most part this unscientific lying universe seems to be the consensus opinion of the majority of my fellow humans, no matter how unlikely I personally think it is ... even if they haven't thought out all of the unpleasant ramifications of such a world.
This is not gravity. The gravitational force is something like 40 orders of magnitude weaker than the electrical force. Electrical forces cancel out when there is more and more matter, but gravity just keeps building up & up. Feynman was looking for a quantum theory of gravity, but as of 1985 the theories were in trouble, and were untestable anyway. I don't know if things have changed much since then.