Unless they've found a way around the second law they would still have to reject waste heat. The second law is probably one of the very best scientific laws in

Unless they've found a way around the second law they would still have to reject waste heat. The second law is probably one of the very best scientific laws in terms of having been solidly and repeatedly proven. The most efficient generators known are certain kinds of batteries and fuel cells at over 90 percent conversion. It's conceivable that some kind of exotic betavoltaic nuclear process like (I think) boron fusion could beat this by basically making electrons directly. Still the second law says nothing can be 100%. Let's assume 95%, which would be amazing. If you had a propulsion system that required 10 megawatts of power, you'd be rejecting 500 kilowatts of heat at 95% efficiency. So that's a 500 kilowatt infrared light bulb! 10 megawatts is probably conservative. The only hypothetical candidates for exotic propulsion I know from physics have outrageous energy requirements. Even assuming many breakthroughs I would still assume them to be high. I can see a 25 watt light bulb at some distance at night. If (the positive hypothesis) what you saw really was artificial and really was maneuvering like that, it would have been very bright in the infrared. I do think we can assume that anything visiting us is inhabiting the same universe and is subject to its laws. It's the only sane assumption. The alternative is things like "we are living in a simulation" and then anything goes so why bother.