
Help me out here: why is the EM drive so surprising? - CarolineW
We&#x27;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, &quot;thrust&quot; is observed in the opposite direction.<p>What am I missing?
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gus_massa
If 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.msg13...](http://forum.nasaspaceflight.com/index.php?topic=36313.msg1302455#msg1302455)
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.

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Tomte
It'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.

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CarolineW
I 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
See 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.

