On: Relativistic Spaceship

[jvanderbot]

That's silly. My flashlight shoots stuff at speed of light all the time. Mass matters.

Shooting just enough mass at very high velocity is not much different than shooting a lot more mass at lower velocity, in terms of force.

[LoganDark → jvanderbot]

> Mass matters.

Exactly. You try accelerating 200kg up to anywhere close to the speed of light (say 80%). That is a lot of force.

Technically even photons can exert force on objects, but they have such a small mass that it's a difficult effect to observe.

[jvanderbot → LoganDark]

This is getting overly pedantic.

Opening comment said that you'd need absurd amounts of mass to accelerate a person to near light speed.

I said that the velocity of ejecting that mass mattered. That if you could push out mass at speed of light, you'd need a lot less mass.

Not that you'd push out the same mass at speed of light. Or that you could arbitrarily push things out at speed of light. Sheesh.

[hotpotamus → LoganDark]

From memory, photons are massless, otherwise they could not move at light speed. They do have momentum though.

[eru → hotpotamus]

Photons have mass, but no rest mass. (Or something like that.)

[scubbo → eru]

Momentum but no mass, if I recall my physics correctly (low certainty).

Wait, hang on, we have access to an appreciable-chunk of the world's knowledge at our fingertips...

https://en.wikipedia.org/wiki/Photon - "Photons are massless[...]In empty space, the photon moves at c (the speed of light) and its energy and momentum are related by E = pc, where p is the magnitude of the momentum vector p[...]Current commonly accepted physical theories imply or assume the photon to be strictly massless"

[tsimionescu → eru]

Typically momentum is thought as mass times velocity, and since photons do have momentum, there was a desire to give them some kind of "relativistic mass".

In more recent times, it has been seen as easier to use just one concept of mass, and to redefine momentum entirely. So, photons have a mass of 0, and we don't need to specify "rest mass". But they do have momentum.

[LoganDark → hotpotamus]

Oh, thanks for the correction.