4 ms·
Help me out here: why is the EM drive so surprising?
We've known for a long time that photons have momentum. Is it not the case that the EM drive is simply chucking out photons in one direction, momentum is conserved, "thrust" is observed in the opposite direction.
What am I missing?
- Tomte 10y agoIt's not losing photons. Inaccurately speaking: Those photons are "chucked" in one direction resulting in thrust. And they get back to their origins without resulting in any thrust in the opposite direction.
- CarolineW 10y agoI can see in the description that the photons are being bounced around, but it's consuming power, and no description I've read convinces me that the photons are genuinely completely contained. Again, all the descriptions are incomplete and ultimately unhelpful, but I was wondering if anyone had done the sums about energy consumption versus thrust obtained by emitting photons, even if said photons had first been bounced around in a complicated manner, and then escaped. Are they sure it's not losing (or even loosing) photons?
- T-A 10y agoSee last column in table at http://emdrive.wiki/Experimental_Results http://emdrive.wiki/Experimental_Results ("Force/Power Multiple of Photon Rocket"). The measured thrust (when one is claimed) is much larger than what an ideal photon rocket would produce.
- gus_massa 10y agoIf you use a photon thruster (aka a laser or a flashlight in the right direction), then the maximal thrust is at most 1/c = 3.33 nano-Newtons per Watt = 0.0033 uN/W = 0.0033 mN/KW. Someone compiled here a list of experiments: http://forum.nasaspaceflight.com/index.php?topic=36313.msg1302455#msg1302455 http://forum.nasaspaceflight.com/index.php?topic=36313.msg13... The list is a few years old, but you can compare the current result you read with 0.0033 mN/KW. The results in the table vary from 3x to 200000x the theoretical maximum of a photon thruster. I think that the typical current value is x4000. It's a theoretical upper bound that is very easy to calculate using special relativity, and it depends on very fundamental laws, not tricky details of the universe. The theoretical bound is so fundamental, that it's even true if you use a graviton thruster instead of a photon thruster. This example is not totally made up, someone have an alternative impossible massless drive that "push against the far away stars". They claimed also a thrust/energy ratio > 0.0033 mN/KW, so it was also impossible from a theoretical point of view. Also, for the EmDrive, someone proposed that it emits twin photons that somehow cancel each other, but the pair still carry momentum. That explanation extremely wrong.