Why would the mass of a particle drop to zero if all other mass in the universe disappeared?

Just out of pure logic, I assume what he is getting at is that mass is essentially a measure of how much energy it takes to change an objects velocity. If you take everything out of the universe, the object cannot move because there is nothing for it to move relative to, therefore it's mass is a moot.

Hmmm this is a fun one. I'm not sure if it makes sense. So long as an object exists it has mass, regardless of other objects around it. The measure of it is against a gravitational field. I suppose without other gravitational fields it would lack mass, but it would still have substance, so it would in theory still have mass, just not measurable against other forces. I doubt it'd drop to zero, because to be zero it would have not not exist. It just becomes something you can't measure for lack of gravity. However it would have it's own gravity(say the bowling ball example). so it would have mass, albeit small.

So I guess because you can't quantify it if there's no gravity. However since it would produce gravity it would have mass.

I suppose I see it like the earth, it has mass, but you can't weigh it, so you could say it's weightless. But against another gravity field it would be measurable I think by the amount of force it would take to stop it from descending further(I think that's how weight works).

So if it was alone in the universe, it would still have mass, you just couldn't weigh it.

The same goes for any minute particle left, it would still have mass but you cannot weigh it. If anything else came into existence it would draw it to itself.

To have no mass you have to lack existence I think.

Well if u see i don't think there will be a change in the mass.

Speaking in terms of relativity, if all mass of universe disappeared then, relative to the massless universe it has some mass. I believe u are considering that the mass of the individual quasers and mesors of the system in consideration is not lost. Moreover it depends probably all on the system's velocity and relative position.

Conservation of power. Like a superb style of issues in technological know-how, we don't comprehend why photons behave the way they do, we basically comprehend how they behave. And because it happens, they have a tendency to stay in this universe.

May be the velocity of the moving particle have equaled the velocity of light