I have heard that if there will exist backwards time travel in some reasonable form in the future, presumably they would send the technology back so that we have it earlier. Since we don't have it, presumably we will never discover this technology. Is this true?
I think it is useful to stop thinking about time as a quantity. Time is simply cause-effect relationships. Yes, time slows down in a mathematical way as you approach speed of light, but this can be explained by a slowing of the physics giving rise to cause-effect relationships.
Try this out: time
We can be sure we don't have a full picture of how time works. In relativity, time is a dimension, and space-time behaves in a way that the dimensions interpenetrate and transform one to the other. In quantum mechanics time is treated as universal and absolute. Thermodynamics gives rise to the arrow of time, and time irreversability - both relativity and quantum mechanics are time-symmetric and events are reversible. Various attempts are being made to approach this problem directly like the purification principle, or through quantum gravity approaches like spin networks in loop quantum gravity.
Our understanding of time travel is founded in general relativity, and we just don't know how gravity affects quantum system because it is such a weak force on the quantum scale. Relativity seems to show gigantic energies are involved, like the Tipler cylinder, or highly entangled blackholes. Or exotic physics like negative energy, which may be limited to the quantum scale. At the energy involved, our descendents concerns would likely be completely unintelligible to us. It is also conjectured the travel couldn't go to a point before the closed time-like curve was made.
So far nothing explicitly forbids time travel. We can't form any strong conclusions either for or against, because we know our physics isn't complete, and it may just involve giant energies, or be limited to quantum scales.
From string theory, it might be possible to travel into parallel dimensions with different conditions. I don't know of any investigations into this.