The Scientific Indian

A question about Matter

Consider a pebble in space. Assume it is very lucky in that it never gets hit by anything in space, neither does it fall into a star, planet or meet with any such physically violent end. How long will it last as a pebble (i.e. retain it’s shape)? In other words, how long would atoms stick together if untouched by external forces? Do feel free to rephrase the question if needed.


  1. #1 JStein
    April 14, 2008

    Damn. An impressive philosophical thought.

    The fact is, things only change their form when reacted upon by an outside force (this is a part of Newton’s law of inertia). So, by definition, it will retain it’s shape indefinitely, which is to say, in perpetuity. Fascinating response, though.

    I’m definitely going to post this on my site, very interesting.

  2. #2 Gerard Harbison
    April 14, 2008

    Where in space? (Intergalactic, interstellar, near a star?)

    It matters a great deal.

  3. #3 joolya
    April 14, 2008

    JStein: Will it though, really? Wouldn’t you expect that random quantum fluctuations or whatever on the outer surface of the pebble would cause atoms to pop off every so often? So that after a long enough time, the size of the pebble would actually decrease, and over the course of the age of the universe might even actually … dissolve?

  4. #4 qetzal
    April 14, 2008

    What’s it made of? If we completely ignore external forces, I can think of things like radioactive decay and some sort of sublimation-like processes. Those would be highly dependent on composition.

    I guess that individual atoms or molecules might gradually ‘evaporate’ from the pebble at some vanishingly small rate and be lost into space. Not doubt the rate would be almost infinitesimal.

    The only other thing I can think of: I recall that even protons and neutrons have finite lifetimes, right? But I guess that would be a much, much slower way for a pebble to ‘decay’ even compared to sublimation?

  5. #5 Dunc
    April 14, 2008

    Currently an open question, as far as I understand it. Various Grand Unified Theories involve the decay of baryons, but none of them are on particularly sound footings yet.

  6. #6 Ben M
    April 14, 2008

    It’s going to depend on sublimation. Whatever the object is made of, there’s some nonzero vapor pressure which will allow individual atoms to leap off the surface and fly away.

    However, just like all evaporation, this process results in cooling. Are you allowing your hypothetical pebble to be in thermal equilibrium with, e.g., the Cosmic Microwave Background? If so, the pebble will eventually evaporate entirely—the energy lost through sublimation will be replenished from the photon bath, and the temperature will always be finite and nonzero. If not, the object will cool down asymptotically towards absolute zero, at which the sublimation rate should be zero and the pebble will last forever.

    Shape changes aren’t terribly different, but the transformation of a pebble from an irregular shape into a perfect sphere doesn’t require energy—in principle it could occur at absolute zero, with atoms tunneling away from peaks and towards valleys (there’s usually a potential energy associated with surfaces, and a spherical shape is the ground state with respect to this energy) thereby smoothing out the pebble.

    That’s all for an object in a really idealized no-outside-influences vacuum. Real interstellar or intergalactic space is fairly busy—there are hard UV photons, x-rays, cosmic ray protons, etc., all of which can violently knock out an atom or two per collision. My guess is that, in practice, this is the main process which will transform a real pebble in space.

    I have no numbers on any of this, sorry. I recall reading an article (Physics Today, perhaps?) years ago on the survival of the Voyager golden record; IIRC there was a calculation showing when the inscriptions would erase themselves, but I can’t even remember which of the above mechanisms was dominant.

    There is no evidence (yet) for the decay of protons and stable nuclei; experiments show that the half-life of a proton cannot be shorter than 10^35 years. That’s a really, really, really long time.

  7. #7 selva
    April 14, 2008

    Gerard, anywhere really. Let’s stipulate no external forces (forces include ALL radiation in space).

    quetzal, I was imagining a garden variety pebble (is it mostly silicon?).

    Ben M, no thermal eq. with space. I am going with your ‘half-life of proton’ thought. I am not clear where it is leading though. This pesky – albeit completely theoretical – pebble could outlast the Universe?

  8. #8 Ambitwistor
    April 15, 2008

    This question was raised about 10 years ago on the sci.physics Usenet newsgroup, in this thread. Lots of discussion of vapor pressure, no consensus, although see the calculation of Geoffrey Landis here. (He got ~10^1357 years for a brick to fully dissipate.) The thread did consider the cosmic background radiation and the existence of intergalactic dust.

  9. #9 selva
    April 15, 2008

    Ambitwistor, many thanks. I was able to only partly grasp the discussion. But, am sure some of those reading this would find it fascinating.

  10. #10 JStein
    April 15, 2008

    One of the stipulations of your post was that it does not meet with a disasterous end, and basic Newtonian laws suggest that matter is conserved as long as things are not met with external forces.

    Assuming that it is not met with external forces, and that it exists in a vacuum, it should retain shape in perpetuity, purely on the basis that, for anything to happen to it, it would have to be acted on by an outside force.

    Metal does not oxidize if kept inside a vaccuum, and for the same reason I see no grounds for which the pebble should change its molecular structure.

    This is, though, really a matter of innate inertia. If the matter is not acted on by an outside force, then nothing will happen to it. Really, though, it’s much simpler:

    If nothing happens to the pebble, then nothing happens to the pebble.

    We’re speaking in hypotheticals here, so I’m taking the philosophical approaches, because I am treating quantum forces as forces in general, and assuming that they do not act on the pebble either.

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