Radiation pressure and this "Mass" thing

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skolnick1
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Radiation pressure and this "Mass" thing

Postby skolnick1 » Sat Sep 29, 2012 4:13 pm UTC

Back with another loco-bullshit alternative explanation for gravity and inertia.

So we have this cosmic background radiation, which we know is almost uniformly distributed and extremely diffuse, such that everything in the universe is constantly bombarded from all sides by low-energy photons coming "From infinity". Now recall the "Solar Pinwheel" demonstration (can't remember its name if it has one), or perhaps Randall's most recent "What-If", where we knock the moon out of orbit with super-lasers. Point is, light of any variety has momentum, and that momentum, when it's transferred, exerts a kind of "radiation pressure" on whatever it's hitting. So if we've got this background blackbody radiation hitting us from all sides and imparting roughly equal momentum, it makes sense that we wouldn't see any net effect from it as long as we're at rest. So consider two bodies near one another experiencing equal background-pressure from all sides. Each of them is going to absorb a minute amount of the energy from the photons passing through it, and so is going to change the mean amount of energy those photons have. Now each of these bodies sees marginally less radiation pressure from the side where the other one is located, and as such is "pushed" toward the other mass rather than "pulled" by it, and without any kind of Higgs mechanism we'd expect to see an apparently-attractive 1/r^2 force emerge (the portion of your visual field blocked by a spherical object is proportional to 1/r^2, and if the light is coming "From infinity", it seems safe to assume that the portion of light blocked might be as well). Can anyone tell me if this logic follows? And if not, why not?

Making a somewhat-sketchy assumption (I'll get to it in a moment), it seems to me that this radiation pressure would also explain inertial mass and the equivalence principle. If we're moving in any one direction, we should see the light ahead of us blueshifted and the light behind us redshifted. Since we'd see higher energy ahead and lower energy behind, that seems to imply that we'd feel a sort of radiation pressure "drag force" that would resist not just acceleration, but constant motion as well. This is the leap: if you imagine the atoms in a massive body wobbling about with a speed proportional to their temperature, then accelerating this body would heat it up some minor amount, and the faster-moving particles would then be less likely to interact with these background photons. (The same amount of matter, now hotter, is "less dense"--not a perfect analogy, but I can't think of a better way to illustrate it without diagrams.) This assumption would account for Newton's first law, as well as the eventual relativistic corrections for mass-energy that came around so many years later. If this isn't the mechanism, I'm curious as to why we don't see a cosmic background drag in constant motion.

So now we have a background field (which we can actually see) that would require applied force for matter to accelerate and would cause matter-dense objects to attract one another. If we can represent subatomic particles' probability of interacting with this field as proportional to their cross-sectional area, we may be able to do it all without ever involving a "mass" term, and may be able to explain the relative similarity of proton and neutron inertial masses, as well as their significant difference from that of the free electron.

Tear it apart, please! I need criticism to refine ideas and chop off the bits that don't work, otherwise I'll have to take this to one of my professors and get called a fool by someone whose opinion may actually influence my life and grades.
Thanks!

starslayer
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Re: Radiation pressure and this "Mass" thing

Postby starslayer » Sat Sep 29, 2012 5:34 pm UTC

Your logic starts off sound, and then starts to veer off in the second paragraph. However, instead of explaining why for now, I'll give you a way to test whether your idea is plausible:

Calculate the radiation pressure force of the CMB (I'll help you so you don't need to look this up: energy density of the CMB is u=4.2E-13 ergs/cm^3, and for an isotroptic radiation field, u=3P) on one side of a one meter square steel plate, say, and then compare it to the gravitational attraction it would experience on the Earth's surface or in orbit around the Sun or something (give the plate a mass of, say, 10 kg), and report back what you find. Then say whether you think this idea of yours is still a plausible explanation for gravity and mass.

Whenever you have a new physical hypothesis/idea/theory, ALWAYS put numbers to it.

skolnick1
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Re: Radiation pressure and this "Mass" thing

Postby skolnick1 » Sat Sep 29, 2012 6:21 pm UTC

when you say u = 3P, is that P referring to pressure?

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Re: Radiation pressure and this "Mass" thing

Postby SlyReaper » Sat Sep 29, 2012 6:25 pm UTC

One of the obvious objections is why bright things like stars still attract objects instead of repelling them, since they are significantly brighter than the CMB. Yes, they do exert some radiation pressure, but for all but the most theoretical circumstances (like, a 1 atom thick sheet of 100% reflective material), gravity overpowers the radiation pressure by many orders of magnitude.

The idea becomes slightly less silly if you posit some other universal pressure that isn't photons from the CMB, but something completely exotic that behaves the way the OP describes. Still kinda silly though, because it requires the addition of an unknown force or particle.
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starslayer
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Re: Radiation pressure and this "Mass" thing

Postby starslayer » Sat Sep 29, 2012 6:28 pm UTC

skolnick1 wrote:when you say u = 3P, is that P referring to pressure?
Yes.

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Re: Radiation pressure and this "Mass" thing

Postby skolnick1 » Sat Sep 29, 2012 6:40 pm UTC

SlyReaper wrote: for all but the most theoretical circumstances (like, a 1 atom thick sheet of 100% reflective material), gravity overpowers the radiation pressure by many orders of magnitude.



Wait, it changes with scale? How does that work? If you consider an individual atom, does the Gravity>>Radiation pressure relationship still hold?

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Re: Radiation pressure and this "Mass" thing

Postby skolnick1 » Sat Sep 29, 2012 6:48 pm UTC

SlyReaper wrote:The idea becomes slightly less silly if you posit some other universal pressure that isn't photons from the CMB, but something completely exotic that behaves the way the OP describes.

Oh hey how about Neutrinos? Don't we have a CNB as well? And "Neutrino Pressure" sounds nice and spacey.

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Re: Radiation pressure and this "Mass" thing

Postby starslayer » Sat Sep 29, 2012 7:10 pm UTC

skolnick1 wrote:Wait, it changes with scale? How does that work? If you consider an individual atom, does the Gravity>>Radiation pressure relationship still hold?
That depends heavily on the energy of the radiation field. But you should be able to figure this out for several different situations, too, using the same techniques you used for the steel plate, except now the cross section is on the order of a barn (the unit, not the building), and the mass is on the order of 1-100 protons.

Oh hey how about Neutrinos? Don't we have a CNB as well? And "Neutrino Pressure" sounds nice and spacey.
Neutrinos can pass through a light year of lead and not hit anything. This should tell you all you need to know about this.

Just as another random question, how much physics have you taken?

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Re: Radiation pressure and this "Mass" thing

Postby Charlie! » Sat Sep 29, 2012 7:12 pm UTC

Congrats! You've re-invented Le Sage's theory of gravitation! It's cool! And wrong! Because we don't measure the drag force it predicts, and relativity works really well! Exclamation point!
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Re: Radiation pressure and this "Mass" thing

Postby SlyReaper » Sat Sep 29, 2012 7:16 pm UTC

Charlie! wrote:Congrats! You've re-invented Le Sage's theory of gravitation! It's cool! And wrong! Because we don't measure the drag force it predicts, and relativity works really well! Exclamation point!

I knew I'd seen this theory somewhere before. It's good to know the names of things.
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Re: Radiation pressure and this "Mass" thing

Postby eSOANEM » Sat Sep 29, 2012 8:21 pm UTC

The biggest issue I have with this is that radiation pressure behaves nothing like mass.

It doesn't resist acceleration, only velocity. If this was truly how mass worked, instead of seeing most things moving at roughly constant velocities, we'd see most things moving in the CMB rest frame. We simply don't see this.
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skolnick1
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Re: Radiation pressure and this "Mass" thing

Postby skolnick1 » Sat Sep 29, 2012 9:20 pm UTC

starslayer wrote: Neutrinos can pass through a light year of lead and not hit anything. This should tell you all you need to know about this.


I dunno. Yeah, a light year is a big number, but it's only what, a billion or so times the radius of the earth? What do we know about the density of these background neutrinos? I've heard billions and trillions per second, but I'll admit that my source on that number is Douglas Adams. But if they're abundant but don't get easily stopped, wouldn't they affect something like the earth roughly equally all the way through?
As a side note, I've had up through 300-level modern physics, currently in electrodynamics and classical mechanics.

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Re: Radiation pressure and this "Mass" thing

Postby starslayer » Sat Sep 29, 2012 10:00 pm UTC

300-level means nothing to me. Have you at least had a year of freshman physics (I'm assuming you're in college/university)? If you have, you have all the tools you need to test this hypothesis. Even if you haven't finished freshman physics, by this point in the semester/quarter you've probably gone over enough in your classes to be able to test it anyway.

Since you now have a number for the mean free path of an average neutrino in lead (one light year), you should be able to calculate the cross section for interaction of a neutrino with an atom (assume that any atom has identical interaction probability to a lead atom). If you don't know the mean free path, or the probable distance a particle travels between collisions in a medium, it is λ = 1/(nσ), where n is the number density of particles and σ is the interaction cross section. n you can figure out from the average density of lead and the mass of a lead atom.

The temperature of the cosmic neutrino background is theorized to be 1.95 K; we can't actually measure it because there's no way we can detect such low energy neutrinos. If you assume that such neutrinos have identical interaction characteristics as what you just calculated above, and you know something about blackbody distributions*, you can calculate how many neutrinos there are, and then compare this to how many there would need to be in the CNB to have a comparable influence to "conventional" gravity on, say, the steel plate we talked about earlier. Call it 1 cm thick.

I think it is important for you to develop your skills in testing your own ideas and applying physics to questions outside class, presuming you're a physics/science/engineering major; that's why I'm not giving you a straight answer. It is equally important for you to be able to put numbers to your physical ideas. However, I am happy to put all my reasoning/calculations up if you get stuck, want to see them, or just don't have the time/inclination to do everything that random guy on the internet is telling you to do.

*If you don't, the energy density of an isotropic blackbody radiation field of temperature T is u=4σT4/c, where σ is the Stefan-Boltzmann constant (5.67E-5 ergs cm-2 s-1 K-4), and c is the speed of light (29979245800 cm s-1). You can approximate neutrinos as photons in many cases because their masses are tiny, no more than a few eV/c2, so they mostly travel at very close to c.

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Re: Radiation pressure and this "Mass" thing

Postby SlyReaper » Sat Sep 29, 2012 11:14 pm UTC

The googles tell me 300-level physics is 3rd year at a New Zealand university.

I'm certain there aren't any known background particle fluxes powerful enough to cause gravity via La Sage's mechanism. The reason I'm certain is because someone would have made the connection to gravity by now if there was. So even if you twerk the numbers and the concept to make it match observed data, it will require the addition of new particles. We already have a conceptually elegant model of gravity based on the curvature of spacetime (the old bowling ball trampoline thing). Unless you can prove the existence of this new background thing, or at least come up with some testable prediction that current theories don't have, then it's a non-starter.
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skolnick1
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Re: Radiation pressure and this "Mass" thing

Postby skolnick1 » Sun Sep 30, 2012 3:42 am UTC

thanks for the criticisms, guys. Though I think if it IS a "curvature", it's a little less "Balls on a trampoline" and more "eddies on a pond". Would it make sense to think of charge as analogous to whirlpools, the way light is a bit like waves?

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Re: Radiation pressure and this "Mass" thing

Postby Copper Bezel » Sun Sep 30, 2012 8:43 am UTC

Yes, but only because gyres explain everything.
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