引力使星系聚集，但引力却比电磁相互作用力弱1040倍，亦因为引力微弱，在实验室难以研究。
Gravity keeps galaxies together , and it is about 1040 times weaker than the electromagnetic force . The fact that it 's so weak makes it all the more difficult to study it in the lab.
电磁相互作用力