Changing the Half-Life of Radioactive Substances Is there a way to extend or reduce the half-life of a radioactive object? Perhaps by subjecting it to more radiation or some other method.
 A: Have a look at the paragraph "radioactive decay"  . 
The half life is characteristic of each radioactive nucleus and depends on the basic interactions holding the nucleus together.
It depends on the quantum mechanical probabilities of transition from one energy level to another, sometimes changing element in the periodic table. 
Thus, to affect the half life, one would have to affect the basic interactions of the decay mechanism. There have been speculations on what would happen if the QFT vacuum is different, as in the Casimir effect, (a simpler explanation here), but I have not been able to find an experiment.
The simple answer is, no, the half life cannot change.
A: Short answer: yes, the decay rates could be changed considerably by environment. However, generally the energies required for this are comparable to the energy output of the nuclear reaction in question and so are (usually) not achievable in laboratory, but the relevant processes are of great importance in astrophysics for stellar nucleosynthesis (particularly in supernovae).
Long answer: The main problem with such effects is the discrepancy between energy of typical nuclear reaction (from several keV's to dozens of MeV's) and energies per reactant particle  achievable in laboratory. Temperature 300K correspond only to energy 0.025 eV so temperature effects would be extremely small correction to nuclear reaction energy levels. Chemical bonds have energies around several eV which again is several orders of magnitude smaller than energies needed to affect typical nuclear reaction. 
Let us consider the beta decay (ordinary one, not electron capture). It results in emission of electron of a certain energy distribution (and neutrino which we will ignore). But electrons obey Fermi-Dirac statistics so if prior to the decay event there is already an electron with a certain momentum and spin quantum numbers then another electron with exact same quantum numbers cannot be produced by the decay process. So, if we have electron degenerate gas around the unstable nucleus then the distribution of electrons modified to exclude the already occupied electron energies and overall decay rate would be changed. 
In everyday environment the Fermi energy of electron gas is of the order of eV, so in phase space the prohibited zone for beta decay is only a small spot near the origin  inside the large ball of allowed electron states. However, if we compress the matter, then the density of electrons increases along with the Fermi energy. Ultimately, if the Fermi energy of degenerate electron gas is greater than total energy released by beta decay then this beta decay would not occur at all -- we thus have stabilized the nucleus.
Let us do some calculations. Take for instance beta decay of tritium. The electron spectrum has a maximum electron energy of 18.6 keV with an average energy of 5.7 keV. So in order to suppress the decay fully we need to have Fermi energy of electron gas equal to 18.6 keV. From wikipedia page on Fermi energy we  have
$$E_F = \frac{\hbar^2}{2m} \left( \frac{3 \pi^2 N}{V} \right)^{2/3}, $$
which gives us electron number density $N/V = 1.152\times 10^{28} \text{cm}^{-3}$, which in turn corresponds to mass density needed of about $58 \text{kg}/\text{cm}^3$ if we are assuming only tritium is present or ~19 kg/cm$^3$ if we have small percentage of tritium together with normal hydrogen. If we only try to measurably slower the decay rate by having Fermi energy of 6keV we will need density of about 3 kg/cm$^3$.
These are, of course, enormous densities, however, much greater densities of degenerate electrons do exist inside white dwarfs, and therefore beta decays (at least with relatively low energies) would be greatly suppressed there. Another environment where this suppression effect is of great importance is the core collapse supernovae where the r-process is responsible for production of considerable number of heavy nuclei that exist in the universe. One of the things that enable it is the suppression of beta decay.
As to having this effect observable locally around us, one possibility is ultradense deuterium and protium. This hypothesized and possibly observed (the number of peer reviewed papers is quite high) states of matter are supposed to have densities of around 100kg/cm$^3$, so potentially this could provide the electron densities to stabilize or slow the beta decay rate.
Another similar effect is bound state beta decay since bound electrons around nucleus could be interpreted as the degenerate electron gas and so ionization would increase the possible allowable states of electrons.
All above concerns the $\beta^-$ decay, but for gamma decay we have Induced gamma emission which could potentially be exploitable (as in hafnium bomb -- so far purely theoretical construct).
A: The simple answer is no, we can't change the half life. There's no technology available to us that can affect energy levels in the nucleus enough to make a change to the half life.
Having said that, I've always wondered if the Mossbauer effect could change the half life. Mossbauer spectroscopy measures tiny changes in the energy levels of nuclei due their chemical environment. If you can change the spacing of energy levels in a radioactive nucleus you could in principle change the probability of transition between them and therefore change the half life. However I've never heard of this effect being observed, and I suspect the shifts of energy levels would be too small to make any significant difference. You can only observe the shifts because Mossbauer spectroscopy is exquisitely sensitive.
A: Yes. It is true, as others have stated, that half-life, as such, is intrinsic and basically immutable. But, as you guessed, by bombarding it with further radiation, elements can be transmuted faster (and into different isotopes) than they would on their own. Also, elements that are not radioactive on their own can be transmuted this way.
A practical artifical version is neutron bombardment. This is proposed as a way of reducing nuclear waste, rather than waiting for centuries for it to decay on its own.
Neutrino bombardment also can do this. Even though there is no practical method to generate enough artifical neutrinos to cause transmutation, this does work as a practical method of detecting neutrinos from space. 
A: I think in some sense the half life can be increased. If you could increase the speed of the radioactive element close to the speed of light then the time in its reference frame would slow down and hence its half-life would increase. This technique is what used at the LHC for detecting atomic or sub atomic particles of very small half-life.
A: Yes there are independent variables for half life.
The electric charge of the radioisotope.
Also neutrino flux.
Edit:
Say in the example of a radiotope that decays by electron capture you could in theory completely ionize electrons off the specie thus reducing the decay rate.
Or say focus a beam of nutrions from a neutrino gun upon some other radioisotope to reduce the  half-life.
Perhaps moving the source out of our earths atmosphere and closer to the sun would also have this effect.
