# Astronomy deep underground

1. May 23, 2006

### Nereid

Staff Emeritus
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).

2. May 23, 2006

### Phobos

Staff Emeritus
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!

3. May 23, 2006

### 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?

4. May 24, 2006

### Chronos

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

5. May 24, 2006

### Nereid

Staff Emeritus
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)!

6. May 24, 2006

### Michael Mozina

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. :)

Last edited: May 24, 2006
7. May 24, 2006

### ratfink

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

8. May 25, 2006

### SpaceTiger

Staff Emeritus
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.

9. May 25, 2006

### Nereid

Staff Emeritus
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)?

10. May 25, 2006

### franznietzsche

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?

11. May 25, 2006

### SpaceTiger

Staff Emeritus
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.

Last edited: May 25, 2006
12. May 25, 2006

### 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. :)

13. May 25, 2006

### 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!

14. May 25, 2006

### 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. :)

15. May 25, 2006

### ratfink

Sorry, that should be contracting to a big crunch!

16. May 25, 2006

### ratfink

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

17. May 25, 2006

### ratfink

Sensble queston 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?

18. May 25, 2006

### franznietzsche

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).

19. May 25, 2006

### SpaceTiger

Staff Emeritus
Ok, I found it:

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

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.

Last edited by a moderator: May 2, 2017
20. May 25, 2006

### franznietzsche

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.

Last edited by a moderator: May 2, 2017