A Time Travel Dialogue 1) How do I become a normalist? 2) If time travel was possible how come we don't see time travelers from the future? The objection to time travel you raised is a strong empirical argument that I used to use, but is outweighed by stronger logical arguments based on the possibility of altering the past (an implication of normalism). Assuming the physics is correct, there are many reasons why we have not encountered time travelers. First, it is much easier to send small particles back in time than objects as massive as ourselves. Secondly, the needed acceleration might have to be provided by a strong gravitational field such as a black hole. If Tb is not reached, the time travelers will never escape the hole into past time. Thirdly, the physics does not predict the possibility of travel back to the future, so one could be trapped in the past. Fourthly, it is easier to go to the recent past than the remote past, so that we may be a difficult destination when the technical problems are overcome. These, and other technological factors, may discourage people from ever making the attempt. . How to become a normalist? Study the website, flip over all decisions (no deliberating), assume nothing as to the future, or the present, or the past, and act accordingly. The result is called your nature. I am a firm believer that just about anything is possible with a particle. When talking about time travel most people assume it involves sending humans through time on earth and then back to the future. Sending an atom back 10ms in a black hole is not very glamorous. Human beings could go back. But the risk is commensurate with the reward, so people may not want to try. But . . . the real reason that there are no time travelers among us is that the future does not exist, in any sense, at any time. The paradox is strong, and satisfying. This is a brilliant argument. I can't believe I didn't think of it. I should be studying more normalism. I would think that if people can't return to the future they would want to go back only a short distance to prevent an accident or make money on the stock market. $100,000 is Microsoft options in 1983 would be worth $150 billion today. To say the future does not exist is to say you can only go back in time (a position that Hawking has apparently adopted, for different reasons) , which, again, may be costly (because of induced g-fields as Tb is approached). And as to normalism and the market--it obviates the need. Also, without actually doing the calculations, the process of accelerating large masses (e.g., Americans) back into time could consume the present universe, which is a (logically) nice concept. It actually might be happening all the time, as objects accelerate into black holes, and we don't notice it because of the scale on which it is occuring, so that the universe can be described as a density of spacetime strings vibrating with only the present as a fixed or nodal point. Each string would extend back to the beginning of time (big bang or whatever), with harmonics concentrated in the recent past. (This could/should turn out to be relatable to conventional string theory--but is not the same). The real point is that, like Planck's black body analysis, we may be able to say that time is quantized backwards, and that is why the universe hasn't collapsed.