Can Deep Underground Astronomers Determine the Mysteries of the Universe?

[Nereid]

If you were trapped deep under the surface of a geologically dead rocky (say, Mars, or Mercury) or icy (say, Callisto, or Charon) planet-sized body, say 10 to 1,000 km down, what could you determine about the rest of the universe (and the place of your home in it)?

Assume that you have available to you today's level of physics, and the technology to build instruments similar to those we have (but no, you are trapped, and cannot get within 100 km of the surface, say, and neither can any of your instruments).

 

[Phobos]

Yikes...extrapolating cosmology from neutrino flux and the periodic table (as determined from the rocks/ice around you and assuming there is some radioactivity to experiment with)? Gravity (and therefore relativity?) experiments by digging deeper? Gravity wave detectors? Interesting thought experiment!

 

[Rach3]

That's asking the wrong question. Absent from the surface, what would they know about their own planet? The interesting question would be, not what exists beyond the surface, but does the surface exist? Is it in fact a spherical shell? (This might eventually be inferred from experiments with gravity, which would be accessible to them). What is beyond the surface, if not more rock? (To them this would be as deep and unanswerable a question, as, what is dark matter made of?) Perhaps the surface is made of eleven-dimensional strings?

 

[Chronos]

The embedded physicists would be quick to realize they are not at the center of an infinite, uniformly rock-dense universe.

 

[Nereid]

Well the existence of a surface would we well within the capabilities ... just as we can determine the structure of the Earth, from analysis of sound waves, so our rock/ice dwellers could do the same (though their home not being geologically active means they'd have to make their own sound; no worries, they can set of H bombs).

Neutrino astronomy yes; particle physics and chemical elements, yes.

Suppose we add that there are several colonies of such folk, at widely separated locations (but all still deep within the rock/ice). Suppose we also add good record keeping, stretching back 10,000 years, and constant technological capability that whole time.

Would a LIGO work? Our LIGO hasn't detected anything (yet)!

 

[Michael Mozina]

If you were trapped deep under the surface of a geologically dead rocky (say, Mars, or Mercury) or icy (say, Callisto, or Charon) planet-sized body, say 10 to 1,000 km down, what could you determine about the rest of the universe (and the place of your home in it)?

I could determine that I'm not living in the most advantageous place to do astronomy. :)

A gravity meter would be pretty useless since a small movement of dust at the surface would make it seem like the whole universe was moving. :)

 

[ratfink]

If you hung a pendulum you should be able to tell that you where in orbit - ie it would appear to swing outwards,.

 

[SpaceTiger]

If you were trapped deep under the surface of a geologically dead rocky (say, Mars, or Mercury) or icy (say, Callisto, or Charon) planet-sized body, say 10 to 1,000 km down, what could you determine about the rest of the universe (and the place of your home in it)?

Cosmic ray fluxes should be negligible at those depths, so I think they're pretty much stuck with neutrinos. Although seismology would allow them to determine their place in their own planet/moon, it's hard to imagine them having a very deep understanding of gravity, so GR is probably out of the question. They wouldn't know to look for gravitational waves.

As for neutrinos, we can so far detect the sun and nearby supernovae, but the latter was only because we knew when to look. They'd need much better neutrino detectors than we currently have to say much of anything about astronomy.

 

[Nereid]

There are certainly some challenges - for example, deep underground, how do you create a (rock/ice free) cavity? A vacuum??

But we still have Foucault's pendulum, and the Coriolis force? With today's technology, but being trapped deep underground, would the acceleration due to rotation be detectable? Surely in Mars, very likely in Callisto and Charon, but maybe not in Mercury?

Never mind how, deep underground, you would ever come up with a theory like GR, suppose you did, you could always do a Pound-Rebka experiment to test it, right (assuming you could create a big enough cavity)?

 

[franznietzsche]

As for neutrinos, we can so far detect the sun and nearby supernovae, but the latter was only because we knew when to look. They'd need much better neutrino detectors than we currently have to say much of anything about astronomy.

Maybe I'm missing something, but wasn't supernova 1987A picked up by neutrino detectors first (one of which could detect the direction the neutrinos came from), and optically several hours later?

 

[SpaceTiger]

Maybe I'm missing something, but wasn't supernova 1987A picked up by neutrino detectors first (one of which could detect the direction the neutrinos came from), and optically several hours later?

I don't recall the order in which they were announced, but my point was that the experiment wasn't repeatable -- without the oft-seen optical counterpart to back it up, the event may have been dismissed as an anomaly. The three detectors together collected a total of 25 neutrinos and I'm not aware of any localization (do you have a reference for this?). A good underground scientist would be hesitant to associate this (by itself) with an astronomical event.

Of course, if you give them enough time, then a supernova will go off in the nearby Milky Way, at which point they might have a tough time explaining it as anything other than astronomical.

 

[Michael Mozina]

This topic reminds me of Plato's "The Allegory of the Cave".

It seems unlikely that folks from such a world would even consider the concept of "astronomy" to have any merit. They would probably never have seen any observational evidence to believe that anyting exists beyond the solid confines of their world. The entire world would probably seem very "self contained" and the neutrino detector would probably never get funded. :)

On the other hand, one might be able to figure out that there was a moon circling the world (assuming there is one orbiting this world) based on tidal changes observed in water. Of course this presumes a large body of water exist under the surface. :)

 

[ratfink]

As the planets slowly spriral into the Sun, the planets would warm up.
The speed of sound would increase.
It would take shorter and shorter times for the echoes of their voices to bounce off the walls.
They would say, "Eh up!" the cave is expanding!

 

[Michael Mozina]

Ok, I would like to apply for funding to build a sonar/radar transmitter/receiver to be located 101km from the known surface. The intent is to transmit waves through the surface at regular intervals in the hopes of receiving return reflections from nearby objects. This project has the advantage of being duplicated all along the surface. Big Government money in that one!

The second project would involve creating a neutrino "laser" of sorts. The intent is to create Bose-Einstein type condenstates of neutrinos that form quantum packets which can be aimed directionally. The hope is that we will see reflections of these neutrino packets from quantum scattering effects, both inside our planet and from outside the planet. I haven't a clue if that particular idea will even work (i.e. do neutrinos "scatter"?), but QM principles suggest it might work and there is a bonus as well. The neutrino transmitter program gives us an another good reason to fund a sensitive neutrino reciever program as well. It is sort of a two for one funding deal, and hey, you never know, it just might work. :)

 

[ratfink]

Sorry, that should be contracting to a big crunch!

 

[ratfink]

Now if the planet was to cool down with age then they would think that the cave was expanding!

 

[ratfink]

Sensble question here,
How good are rocks (silicates) at absorbing microwave radiation? If there were small amounts of the CMB arriving in your cave you could tell how you were moving relative to the CMB (direction and velocity). Blue shifted in front of you, redshifted behind you. Isn't the Earth moving n a direction towards the Virgo cluster?

 

[franznietzsche]

I don't recall the order in which they were announced, but my point was that the experiment wasn't repeatable -- without the oft-seen optical counterpart to back it up, the event may have been dismissed as an anomaly. The three detectors together collected a total of 25 neutrinos and I'm not aware of any localization (do you reference for this?). A good underground scientist would be hesitant to associate this (by itself) with an astronomical event.

Of course, if you give them enough time, then a supernova will go off in the nearby Milky Way, at which point they might have a tough time explaining it as anything other than astronomical.

Kamiokande was the one of the ones that picked it up and was capable of determining the direction the neutrinos came from based on which photomultiplier tubes picked up the event (this is from my modern physics notes, I'd have to look around for another source).

 

[SpaceTiger]

Kamiokande was the one of the ones that picked it up and was capable of determining the direction the neutrinos came from based on which photomultiplier tubes picked up the event .

Ok, I found it:

http://prola.aps.org/abstract/PRD/v38/i2/p448_1"

They had directional information for the charged particles created/scattered in the neutrino interactions, but most of the detections were antineutrinos:

$$\bar{\nu}_ep^+ \to e^+n$$

The COM energy in this interaction is much greater than the neutrino energy (order 10 MeV), so the outgoing positrons are roughly isotropic and the neutrino direction can't be determined. They claim a detection of one neutrino scattering:

$$\nu_ee^- \to \nu_ee^-$$

which is expected to scatter the electron in the direction of the incoming neutrino. This event is consistent with being in the direction of SN1987A, but only to within ~20 degrees. They don't list directionality as being evidence for associating the neutrino burst with the supernova, so I guess they didn't feel that the one neutrino scattering was very compelling.

 

[franznietzsche]

Ok, I found it:

http://prola.aps.org/abstract/PRD/v38/i2/p448_1"

They had directional information for the charged particles created/scattered in the neutrino interactions, but most of the detections were antineutrinos:

$$\bar{\nu}_ep^+ \to e^+n$$

The COM energy in this interaction is much greater than the neutrino energy (order 10 MeV), so the outgoing positrons are roughly isotropic and the neutrino direction can't be determined. They claim a detection of one neutrino scattering:

$$\nu_ee^- \to \nu_ee^-$$

which is expected to scatter the electron in the direction of the incoming neutrino. This event is consistent with being in the direction of SN1987A, but only to within ~20 degrees. They don't list directionality as being evidence for associating the neutrino burst with the supernova, so I guess they didn't feel that the one neutrino scattering was very compelling.

Well, I wouldn't necessarily consider one data point compelling etiher. But I didn't know that only one of the events provided any directional data.

 

[Rach3]

I suspect they wouldn't know about universal gravitation, without astromonical observations to suggest it. The prevalent theory would be that entire universe is made of rock; borings into their surroundings would support this (unless they reached the surface, which our scenario here excludes.) Objects at lower levels, closer to the COM, would weigh less (linearly in r); they would have a phenomenological explanation for this ("mass ~ distance from point X"). This would be a huge mystery; why is this point X they extrapolate, so special? Why is mass disappearing as you approach X? Then someone might notice, inertial mass is conserved! But weight is not! Their primitive concept of "gravity" would be this mysterious ratio of weight/intertial mass. Extrapolating out linearly, there would be places where (gravity) is arbitrarily strong - this would also be pathological. Their cosmology would be a severe mystery to them, just like our own accelerating universe mystifies us. The most obvious solution for them, would be to compactify their universe; there would be no reason for them to think of an infinite universe, with gravitational acceleration being linear in r out to infinity (!). Thus I believe their mainstream model of a universe, for most of their history, would be compact.

What would suggest an extraplanetary world to them? Tidal forces in gravity, would find an alternative explanation easily enough, in their compact-universe model. The problem with living deep underground, is that there's really no EM radiation reaching you at all! Particle physics would have to be the answer. They look at radioactive materials, and conclude something big happened billions of years ago. Harnessing nuclear fusion, someone might point out that, certain features of their terrestial distribution of elements are suprisingly consistent with a radical new idea - that at one point, everything came from a giant nuclear reactor operating at 3x10^7 Kelvin. Was this the early universe? Impossible; it couldn't have cooled down in a compact universe. Nucleosynthesis would be the key to their discovering the outside world, I predict.

Particle physics comes along by itself pretty well. They'd discover natural magnets, and static electricity, and a few years later they have cyclotrons and LINACs. Their first neutrino would be a single, unrepeatble event (like that monopole someone thought they saw...). Some brave soul, with too much funding, builds a super-huge target (like our Super-K) and confirms that neutrinos are real, though very rare! Over time, mysterious bursts of neutrino activity would find no convenient explanation. Another mystery, to them as profound as, say, dark matter.

Of course how could they possibly extrapolate the real nature of the universe, from these strange phenomena?

 

[Rach3]

It becomes perfectly obvious to them, when someone observes that nuclear reactions produce neutrinos. This might have been an accident; someone moved the nuclear reactor, for example, and the neutrino flux went up in the experiment across the hall. The simplest explanation, is that the cosmic neutrinos are produced by nuclear reactions somewhere. But where? Some grad student works out a consistent explanation; it involves a much larger universe, with very distant, hot plasma objects. These are distinct from there own rocky world; then suddenly they must figure out what are the boundaries? They re-examine their cosmology; linear-in-r gravity no longer works. Someone draws a connection with 1/r^2 forces from electrodynamics, the calculations work!

Someone else suggets, why not universal graviation? Torsion experiments are done, the theory is vindicated dramatically! Someong gets a Nobel. Then the concept of a gravitationaly-bound sphere of matter is proposed. It becomes trivial to calculate the size of their world-sphere - it's a few thousands of kilometers in radius. This is a reasonable, accessible scale. What is outside of it? How distant are the cosmic nuclear reactors? Since there's obviously an interplanetary vacuum, it is hypothesized that astronomical objects could actually be observed optically! Then their civilization tunnels to the surface (straight up, physicists advise), and this scenario ends. It was ridiculous, IMO, to really think they wouldn't eventually tunnel their way out.

/speculation

 

[Chronos]

Agreed, Rach3. The curious scientists would eventually tunnel to the surface of their planet attempting to explain gravitational anomalies - and say "holy sh*t" when they first glimpsed the sky.

 

[Nereid]

Thanks Rach3! You've also managed to bundle in a fair amount of the 'social' aspects of the development of science (theories which rest on, at best, marginal detections spaced at irregular intervals tend to be rather unsuccessful).

My preconditions included 'today's technology', and I see no strong reason to deny the development of all of today's physics, given 10,000 years of sufficiently intense research ... with one exception.

That GR is testable deep underground (Pound-Rebka can be done anywhere, for example) is insufficient, I feel, to ensure that it would 'get up'. Nor is the fact that SR would be a breeze any guarrantee that the extension to GR would happen.

But back to our rock-trapped astronomers (OK, maybe it's a stretch to call them such ...).

If there were an equivalent to the http://www.phys.canterbury.ac.nz/research/laser/ring_publications.shtml" , how much of the solar system could be detected? From Callisto, certainly Jupiter and the Sun (and very likely Ganymede and Saturn); from Charon, Pluto and the Sun; from Mars, the Sun (and Jupiter?); from Mercury, the Sun and Jupiter.

With neutrino astronomy no more sophisticated than today's Earthly neutrino detectors, 'day/night' variations would be a snatch, and for Callisto and (maybe) Charon, periodic occultation by Jupiter/Pluto would serve as independent confirmation of the astonomical theory.

Sound imaging of the surface would surely show the craters (just as we use seismology to determine the structure of the Earth), and over a sufficiently long time period, the link between these craters and impacts could be teased out.

So, the road to astronomy would likely be blazed by the counterpart of geology (and neutrino physics).

So, perhaps some fuzzy, vague, confusing things about our solar system could be (eventually) teased out, from deep underground.

Could the existence of rest of the universe be reliably worked out?

No, no EM penetrates ~100 km of rock; it may be that some EM penetrates ~100 km of ices. Nor do cosmic rays. However, perhaps there's a diurnal heat pulse signature in the deep rocks? Perhaps, unbeknownst to Earthly astronomers, there are DM particles which can be detected deep underground? (more later)