I recently asked a question in Space Travel SE where we were exploring ideas on how to get a space craft that weighs 1 kg to travel at 0.1c (one tenth the speed of light). At this rate, it would be possible to reach Proxima Centauri, the nearest star after our sun, in 43 years. We concluded that repeated gravity assists with the solar system would become impossible once the spacecraft significantly exceeds solar system escape velocity, which is much much less than 0.1c.

While composing the question and much reflection afterwards, I couldn't help asking myself if there are objects in our Milky Way galaxy that are 1 kg or greater in mass that are traveling at speeds of 0.1c or greater. Without taking relativistic effects into consideration since they are still insignificant at 0.1c, the kinetic energy using E=0.5mv2 that one kilogram of mass traveling at 0.1c would be 449.4 trillion joules. This is the amount of energy equivalent to an earthquake of 7 on the Richter scale or a large hydrogen bomb.

Although such an object would not strictly be categorized as a relativistic bomb, it would still cause a very significant explosive release of energy upon collision with a massive object. The only events that come to mind that would be capable of accelerating a 1 kg or greater mass object to 0.1c or greater speed would be supernova. There may be others like gamma ray bursts, quasars, etc.

I am having trouble understanding if generation of such relativistic bombs can be generated by supernova explosions. What physical laws would prevent this or make it unlikely? If it is indeed possible, why don't we observe them? How can we observe them if possible?

How large would a mass have to be, traveling at 0.1c, that if it struck the earth, would cleave the earth in 2?

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    $\begingroup$ Re, "...cleave the earth in 2" The Earth is not a solid object. It's soft and squishy with a thin, solid crust. If the Earth were squarely struck by an object with more KE than the Earth's gravitational binding energy, it would not "cleave in 2", it would splash into many fragments. $\endgroup$ – Solomon Slow Nov 17 '17 at 17:42
  • $\begingroup$ Supernovas release on the order of ~$10^{53}$ ergs (or ~$10^{46}$ J), or something along those lines. Roughly 1% of that energy goes into the shock wave, which sounds small but ~$10^{51}$ ergs is not small! Most of the energy goes to neutrinos but that is beside the point. If you assume that ~1 solar mass is ejected as the blast wave material, then you can determine the kinetic energy (and speed) of that material assuming its total kinetic energy is ~$10^{51}$ ergs. $\endgroup$ – honeste_vivere Nov 22 '17 at 18:31
  • $\begingroup$ @jameslarge somehow your comment reminded me of this old cartoon pbs.twimg.com/media/COYoNDZUwAA1Ke9.jpg $\endgroup$ – WetSavannaAnimal Nov 22 '17 at 22:59

After reflecting on the question, 2 reasons came to mind as to why relativistic bombs are unlikely in our galaxy.

  1. While a supernova explosion would indeed have the energy needed to accelerate 1kg or greater solid masses to relativistic speeds or at least to 0.1c, the dying star pre-supernova and post-supernova is composed of plasma, where it would not be possible for anything to be in a solid state. Any cooled off plasma would still be thousands of degrees too hot to be anything other than gaseous plasma.

The plasma nuclei and alpha particles are indeed sent out with lots of energy at near the speed of light and we witness these as cosmic rays. It is simply not possible for stellar plasma to accrete together to form any kind of solid mass.

Ok, what about the small rock fragments and mini asteroids that were orbiting close to the dying star pre-supernova? While the supernova explosion would likely blow many apart and vaporize them, it is not impossible that some of these stay in tact and accelerated to 0.1c or greater from the explosion.

If such rocks were indeed accelerated to 0.1c or greater, would they survive the journey through many years of travel through space, which brings up the next reason.

  1. Space is not a complete vacuum.

The density of atoms per volume in interstellar space averages about 1 hydrogen atom per cc (cubic centimeter). This is a weighted average ranging from 1 atom per 10cc on the low side to 1000 atoms/cc in the galactic core region. 1 atom/cc is very very low and any friction encountered by spacecraft at usual speeds are nano-scale. This may not be the case at 0.1c or greater. Indeed at a certain relativistic speed or greater which can be calculated, any such rock would 'burn up' just like solid mass entering our atmosphere from space.

While 0.1c would not instantly vaporize a space rock, it may gradually get eroded away on its long journey. The interstellar gas friction does not increase linearly with speed of travel, but rather as the square of the speed since the kinetic energy is directly proportional to the square of the speed. The momentum of each atom strike becomes significant at such high speeds.


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