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What should conditions on an alien planet be like, if the aliens of said planet move 3 or 4 times slower on their home planet than they could if they were to come to Earth? (Gravity, atmosphere, etc.) For example, on their home planet, they can run 5m/s, but when on Earth, they can reach 15m/s or 20m/s.

And since I don't want them to be shorter than average humans, I plan to design the aliens to be about 3 heads taller than average humans, while still maintaining their humanoid look, with no extra limbs to accommodate the speed. Will this conflict with the desired speed?

I am working on a story in which I try to give the aliens an advantage over the humans when they arrive to Earth (through portals), since their tech is at least 300 years behind modern humans.

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  • $\begingroup$ I think you have to sharpen your question a bit. Defining how will you evaluate the answers, for example. $\endgroup$ – L.Dutch Sep 30 '18 at 6:11
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    $\begingroup$ Is this move 3 or 4 times faster than humans or move 3 or four times faster than they could on their home planet? $\endgroup$ – Ummdustry Sep 30 '18 at 6:31
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    $\begingroup$ @Ummdustry Hey there, what I meant was "3 or 4 times faster than they could" when they arrive on Earth, sorry for the misunderstanding, my English isn't that great. I will edit the question now $\endgroup$ – Simon Sep 30 '18 at 6:40
  • $\begingroup$ @Jasper 10m/s. Dang my question is hard to understand I suppose. Sorry for the confusion, English is not something I'm decent at. I will edit the question now. $\endgroup$ – Simon Sep 30 '18 at 8:03
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You're looking at a higher gravity (and thus higher air pressure) on the aleins home-planet. you're aliens likely still look humanoid (since that's what you want them to look like and it's reasonable to assume that the humanoid COULD evolve under these conditions.) Specifically a surface gravity and an atmospheric density about three times larger than earths, the sort of conditions you would expect on a super-earth (https://en.wikipedia.org/wiki/Super-Earth) if you want to through in some sciency jargon.

How fast a person can go is basically determined by two things: that persons power-output and the restive forces working against them, it's actually a lot more complicated than that since mechanical advantage comes into it a lot however this isn't an unreasonable assumption to make from an engineering point of view, and your aleins could always wear high-tech walking aids (stilts) if the mechanical advantage problems are really that difficult. When making theese assumptions we find that dialing down the resistive forces by a factor of three or four would triple or quadrople velocity (though they would still accelerate and decelerate at more or less the same rate)

You're aliens likely look a bit like the "space dwarf" archetype. Short and stocky, though life has a LOT of variation and this isn't necessarily a given. Plus their high-tech stilts could always bring them up to human-eye level if that's important too you

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    $\begingroup$ Higher gravity and higher air pressure are independant, Venus has a lower gravity than Earth but air pressure is 90 times stronger. $\endgroup$ – Jean-Abdel Sep 30 '18 at 8:50
  • $\begingroup$ firstly i said air density, and while it's true that they aren't perfectly connected there is definitely a correlation, namely that larger gravitational fields normally attract more gas for an atmosphere and retain those gases longer, hence why not a single asteroid has any appreciable atmosphere and the largest objects are almost entirely atmosphere (stars, gas and ice gaints... etc...) $\endgroup$ – Ummdustry Sep 30 '18 at 16:53
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Just make the oxygen level on their planet much lower so they can't run to fast without getting out of breath, while on our they could easily run much faster due to the incresed oxygen level. Thus you could have a Earth-like gravity on your planet .

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    $\begingroup$ This could lead also to oxygen poisoning, if their respiratory system is too efficient when absorbing oxygen from the atmosphere. $\endgroup$ – SJuan76 Sep 30 '18 at 9:38
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    $\begingroup$ yes but you can easily handwave that by saying they don't have any issue with oxygen poisoning (at least for our 0.2 atm oxygen partial pressure) which is likely if their oxygen partial pressure is about 0.05 atm $\endgroup$ – Jean-Abdel Sep 30 '18 at 19:54
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Contrary to common belief, moving to lower gravity will make moving fast a challenge since larger forces exerted by our legs to counter gravity than to propel ourselves forward.
Think astronauts on the moon. Now imagine Usain Bolt on the moon. The start kick could launch him into orbit.

The aliens home world is of a lower gravity, so aliens resort to living underwater underfluid that's able to hold enough oxygen to breathe with lungs.

The lighter and oxygenated atmosphere of earth and the higher gravity leads to lower atmospheric drag and better foothold for the aliens to move with their large muscles from all the fly/swimming on their homeworld.

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    $\begingroup$ "Now imagine Usain Bolt on the moon. The start kick could launch him into orbit." The Moon's escape velocity is two orders of magnitude greater than what Bolt would be able to achieve there, therefore it's unlikely he would have the power to go into orbit. Even if it weren't so, nothing and no one can enter orbit of any body just by jumping. $\endgroup$ – Renan Sep 30 '18 at 18:18
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Earth's atmosphere is denser, has more oxygen, and has a lot less pollution than the aliens' native atmosphere. The extra density and oxygen make it much easier to breathe on earth, so they can go faster.

In Niven and Pournelle's The Mote In God's Eye, the moties have (over many millenia) polluted their atmosphere so badly that the entire planet reeks of ketones and other chemicals. The aliens' native planet might be polluted like the moties' planet.

These chemicals interfere with brain and lung function. Clean air would allow the aliens to be faster, and have better reaction times, than they are used to.

Also, the aliens might be from a part of their planet where it is hard to walk. For example, the terrain could be mostly swamp, or uncompacted sand, or dense vegetation, or craggy rocks. To deal with this, the aliens have built trails on their home planet, along which it is possible to move quickly. Most of Earth's landscape seems like such trails.

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