Hey, science smarties!

[Vanamonde]

This site seems to attract a lot of knowledgeable people and science enthusiasts, so maybe someone can answer some long-lingering questions for a semi-self-educated layman like me.

1) If space is filled with an infinite number of virtual particles at all times, how come every point in space doesn\'t have infinite mass?

2) Why can\'t the dark matter that alters galaxy rotation consist of scads of rogue planets? I mean, we\'re pretty sure there are lots of rogue planets out there now, right? Why invent this weird stuff and assert that 90% of the universe can\'t be seen? To me, dark matter is too similar to the old idea of ether: something completely indetectible except in that it conveniently accounts for this one phenomenon. It\'s like physics invisible friend; 'Oh, it\'s there, it just doesn\'t want you to see it right now.'

3) Cosmic background radiation. It\'s got to be coming toward us, or our instruments couldn\'t detect it. But if it\'s said to be energy thrown out by the big bang, why isn\'t it heading away from us? What exactly is it our instruments are receiving?

4) Gyroscopes. I mean, WTF? Doesn\'t Newton\'s second law say that the reaction should be opposite to the action? Not 90 degrees in another direction. So are gyroscopes black magic or what?

[CaptainArbitrary]

1) If space is filled with an infinite number of virtual particles at all times

It isn\'t. At the risk of sounding tautological, 'virtual particles' aren\'t real. What I mean is, they are a mathematical model used in quantum field theory to describe interactions between fields in terms of bosons. This works extremely well —in terms of actually sitting down and doing math and working problems —but if taken too far, it can lead to wrong conclusions. Like for instance, there\'s an effect whereby two plates placed very close together in a perfect vacuum will experience a surprising tendency to move toward each other and close the gap between them. This has been described —badly —as being an effect of 'virtual particles' popping into existence in the vacuum and bouncing off the plates, pushing them around. To put it bluntly, that\'s a complete crock. It\'s just that the real story involves the ground state of vacuum fields and is thus a lot harder to explain to people who aren\'t quantum field theorists.

So the safest thing to do is remember that 'virtual particles' are a mathematical tool, not anything that actually exists.

2) Why can\'t the dark matter that alters galaxy rotation consist of scads of rogue planets?

Two reasons. First, there\'s no good evidence to suggest that 'rogue planets' should even be a thing, outside science fiction stories. And second, because dark matter doesn\'t behave the way 'rogue planets' would behave if they existed.

To me, dark matter is too similar to the old idea of ether: something completely indetectible except in that it conveniently accounts for this one phenomenon.

That\'s completely wrong. Dark matter is absolutely detectable. We\'ve observed it directly, through gravitational lensing of the Bullet Cluster. It\'s just that it doesn\'t interact electromagnetically, making it difficult to detect. Neutrinos also don\'t interact electromagnetically, making them difficult to detect. Is it sensible to cast aspersions on neutrinos and suggest that they\'re fantastical? Of course not. Don\'t be silly.

3) Cosmic background radiation.

Isn\'t anything like what you think it is. It isn\'t light that was 'thrown out' by the Big Bang, because there was no 'out' for the Big Bang. The Big Bang wasn\'t an event that happened at a place. It occurred everywhere in the universe simultaneously. During that time, the scale factor of the metric was much smaller than it is now —at least a thousand times smaller, and before recombination much smaller than that indeed. Because of this, the universe was much denser than it is today. It was still infinite, just like it is now, but everything in it was much closer together. Because of this, near the time of recombination, all of infinite space was filled with a dense, hot hydrogen plasma.

What else is filled with a dense, hot hydrogen plasma? The sun is. In fact, at the end of the Big Bang, about 370,000 years after the moment of creation, the entire universe was pretty much the temperature and density of the outermost layer of the sun. And just like the sun, it glowed. All those electrons and protons bouncing around interacted like crazy and shed off their momentum as electromagnetic radiation — at the time, visible light.

But then came a moment — pretty much all at once, everywhere —when the scale factor reached a point such that the density of the universe was no longer quite great enough to sustain this hot hydrogen plasma. Suddenly electrons and protons could get together and form atoms that would last for longer than the nanosecond or two it took for some wandering particle to smash into them and knock them apart as had happened constantly before. When this happened, the mean free path of a photon shot up dramatically, and the universe became, for the first time in history, transparent to light.

The cosmic microwave background is that light. It\'s the light that was being emitted by the entire universe at the time of recombination, at the end of the Big Bang. At that time, nearly fourteen billion years ago, it was visible light, blackbody radiation of about 3000 °K, not at all different from sunlight. But over the history of the universe, the scale factor has continued to grow, meaning that light has gotten 'stretched out' (not really, but sort of) to the point where it\'s now a thousand times less energetic than it was then. That\'s why we can only see it with sensitive instruments, when in fact it was once exactly as bright as you imagine it would be if you imagine a sun that\'s infinitely large.

4) Gyroscopes. I mean, WTF? Doesn\'t Newton\'s second law say that the reaction should be opposite to the action?

It does not, no. That\'s Newton\'s third law, and in fact that\'s not really a useful statement at all. It\'s kind of true, sometimes —that the vector sum of all forces within a closed system is zero — but it\'s false just enough to make it pretty much useless except as a historical artifact. What\'s true is Noether\'s theorem, which says that for every continuous symmetry in nature there is a corresponding conserved charge. Newton\'s third is a terrible special-case example of that —continuous symmetry of translation giving rise to conservation of momentum —but as you rightly point out, it fails to predict basically anything involving the conservation of angular momentum, which is what you get whenever there\'s a continuous symmetry of rotation. Once you stop thinking about simplistic Newtonian mechanics and accept that angular momentum is a conserved quantity, the gyroscopic effect falls right out onto the floor. It turns out to be really very simple and obvious.

[Vanamonde]

Ugh! I hate it when something I thought I understood turns out to be somebody\'s bad analogy. I know it\'s hard to explain some of this stuff in plain terms, but it\'s just so annoying to find out I\'ve been told something that isn\'t true. But how does one get it right? By asking dumb questions. To wit:

1. If virtual particles are just a useful fiction, how do they get anything done? Don\'t they at least have to have the *properties* of actual particles, however ephemeral their lifespans? And since one of the properties of normal particles is mass, shouldn\'t virtual particles have mass as well?

2. Rogue planets are not wholly a fictitious idea: http://www.time.com/time/health/article/0,8599,2072290,00.html I don\'t actually believe that this is what dark matter consists of, but I am curious about how the possibility can be ruled out.

Dark matter is absolutely detectable.

But as I said, 'conveniently accounts for this one phenomenon' but has no other apparent properties. Just the gravity needed to make galaxies rotate as observed. Isn\'t that somewhat like ether, which they thought was necessary to explain light but was otherwise indetectable?

Don\'t be silly.

Of course I\'m being silly. That\'s why I was posting this on a website devoted to crashing little spaceships full of green people, instead of, say, Scientific American. I don\'t actually believe that I am right and all of the scientific establishment is wrong. I want to improve my understanding by finding out why I\'m wrong.

Is it sensible to cast aspersions on neutrinos

I do regret and apologize if I have in any way besmirched the good name of dark matter or neutrinos. They both pass through me constantly, and have never done any harm in the process. My aspersions upon these harmless and inoffensive particles were indeed uncalled for. I am ashamed.

3. So the background radiation was released from everywhere in space at all directions and just the fraction of it that was randomly heading our way reaches our detectors? Okay, I think I get that.

4. I checked several sources before posting, and I suppose the numbering is arbitrary, but action/reaction is traditionally ennumerated as Newton\'s 3rd. I\'m afraid I still don\'t get how gyroscopes fling the reaction 90 degrees from the action, though. It has occurred to me that it might involve the same principle that requires me to make Kerbal orbital-plane-change burns a quarter of an orbit before the effect becomes fully evident, though I struggle with visualizing how the forces play out.

[semininja]

As far as I, as a relative layman, am concerned, the only understandable explanation of gyroscopes is 'because physics.' Anything more than that, I think, would require at least some progress toward a degree in physics or engineering...

[CaptainArbitrary]

Ugh! I hate it when something I thought I understood turns out to be somebody\'s bad analogy.

Yeah, man, I know. It stinks, but we live in an age where the difference between reality as it really is and the things we experience in everyday life becomes unavoidably apparent. We still use analogies and metaphors and models to understand things, but like they say, the map isn\'t the territory.

1. If virtual particles are just a useful fiction, how do they get anything done?

They don\'t. I\'m saying they literally do not exist. They are nothing more than a mathematical technique for solving problems in quantum electrodynamics. If you pretend a boson propagates between two fermions, you get results that match experimental observations. But there is no actual boson there. If you draw the Feynman diagram, it sure looks like that wavy line represents a boson — and mathematically, it represents a boson term in your path integral. But that doesn\'t mean there\'s an actual real boson there. That\'s why they\'re called 'virtual particles.' Because it\'s the opposite of actual particles. They\'re purely a mathematical technique, not an actual physical phenomenon.

But as I said, 'conveniently accounts for this one phenomenon' but has no other apparent properties.

Dark matter has exactly the properties it\'s predicted to have by theory. Neutralinos are superpositions of bosonic superpartners, and they do in fact — on paper, according to the math — behave precisely like dark matter does. It\'s just the fact that we haven\'t actually detected them (again, because they\'re weakly interacting) that even keeps this topic alive.

Isn\'t that somewhat like ether, which they thought was necessary to explain light but was otherwise indetectable?

Well no, that\'s completely unlike the ether, because that\'s not at all what the ether was like. But there\'s no point getting sidetracked into a discussion of ether theories of light; they\'re old news. The point is it\'s become fashionable for people who know a little about the history of science to go 'Dark matter? Why, that\'s just like the ether!' when in fact that\'s completely wrong.

3. So the background radiation was released from everywhere in space at all directions and just the fraction of it that was randomly heading our way reaches our detectors? Okay, I think I get that.

Eh, kind of. I mean, once you really get down into a deep understanding of the essential nature of light, you discover that it\'s kind of a philosophical question as to whether an unabsorbed photon can even be said to exist at all. Until it interacts with something, it can really only be described as the potential energy of the electric field at a given point in space, which as it turns out is not essentially different from the potential energy of the electric field at any other point in space. So asserting that there\'s light out there that we can\'t see \'cause it\'s not heading toward us … well, that gets pretty fuzzy pretty fast.

It\'s way better to say that we can point our detectors at the sky and observe a mostly isotropic intensity distribution, with a frequency distribution that\'s consistent to the limit of measurement with an ideal blackbody at about 3°K.

I\'m afraid I still don\'t get how gyroscopes fling the reaction 90 degrees from the action, though.

Conservation of angular momentum, like I said before. If you take something with angular momentum pointing in one direction, then apply a torque to it perpendicular to its angular momentum, the net force on the spinny thing has a component that\'s perpendicular to both the torque and the angular momentum. This is the origin of gyroscopic precession — the tendency of gyroscopes to turn when you try to twist them — and it\'s what makes them useful for things like inertial reckoning and attitude control. If you want, you can look up the math and see how it all works out for yourself, but the short answer is no, there is nothing mysterious going on there. It\'s really very straightforward, once you see how it works.

[Yourself]

Conservation of angular momentum, like I said before. If you take something with angular momentum pointing in one direction, then apply a torque to it perpendicular to its angular momentum, the net force on the spinny thing has a component that\'s perpendicular to both the torque and the angular momentum. This is the origin of gyroscopic precession — the tendency of gyroscopes to turn when you try to twist them — and it\'s what makes them useful for things like inertial reckoning and attitude control. If you want, you can look up the math and see how it all works out for yourself, but the short answer is no, there is nothing mysterious going on there. It\'s really very straightforward, once you see how it works.

Back as an undergrad we were actually show a complete derivation of angular motion starting only from Newton\'s laws (the strong form, assuming that forces between point masses are equal, opposite, and aligned with the vector connecting the points) and an arbitrary set of point masses (more specifically we were walked through an analysis of the system and then we defined terms in the resulting equations to be angular momentum, torques, and moments of inertia). I really wish I could remember this derivation, but I remember it taking quite a bit of time to complete. The concepts of angular momentum and inertia really fall right out of Newton\'s laws

[thorfinn]

They don\'t. I\'m saying they literally do not exist. They are nothing more than a mathematical technique for solving problems in quantum electrodynamics. If you pretend a boson propagates between two fermions, you get results that match experimental observations. But there is no actual boson there. If you draw the Feynman diagram, it sure looks like that wavy line represents a boson — and mathematically, it represents a boson term in your path integral. But that doesn\'t mean there\'s an actual real boson there. That\'s why they\'re called 'virtual particles.' Because it\'s the opposite of actual particles. They\'re purely a mathematical technique, not an actual physical phenomenon.

I wouldn\'t be so categorical. This is one of these points where the notion of 'reality' and 'existence' start to require lots of qualifiers; if you get a bunch of physicists in the same room and ask them OP\'s question about virtual particles, you\'ll get a bunch of different answers (and yes, I\'ve done that ) The problem of the energy density of the vacuum is still controversial, and most probably ties into the bigger problem of quantum gravity - regarding which, as you probably know, we\'ve been stuck shooting almost in the dark for the last few decades

[sneakeypete]

as an engineer i\'m quite a fan of the engineering approach; Close enough within tolerances.

[Phoxtane]

Gyroscopes are like magnets, IMO. Nobody knows how they effing work! ;P

[Endeavour]

How does a magnet work!? ???