Critical mass near lightspeed

[Skyclad1uhm1]

When an object approaches the speed of light its mass increases. When you have nuclear material it has a critical mass at which it can sustain a nuclear reaction at a constant level.

But if you have a sub-critical amount of nuclear material approaching the speed of light, does the material approach critical mass? Or will it stay in the same state as the actual amount of matter stays constant?

 

[Jacky60]

Critical mass is about the fission reaction becoming self sustaining i.e. It is dependent on local density not absolute mass. Approaching the speed of light should not alter the local density of any given mass, it should all increase directly and spatially in proportion but will alter the absolute mass. Therefore no nuke as you sail superluminally (despite having a mass greater than that of the Universe) past the speed of light.

 

[Tuna-Fish]

Critical mass has, despite the name, nothing whatsoever to do with mass. Critical mass is a situation where less neutrons manage to escape the device than are created, leading to a rapid increase in neutron flux (remember, a neutron caught makes more neurons...) that keeps going up until the device disintegrates. As such, what matters is the density of neutron generation and the amount of stuff preventing their escape.

 

[Skyclad1uhm1]

Thanks, that's what I thought but wasn't sure about

And now that I think of it even if it would be at critical mass it wouldn't do anything at light speed as the movement within the atoms would stop too I guess (since otherwise the particles moving with the motion of the matter would have to move faster than the speed of light).

 

[Ferzerp]

Think of it more like "critical number of atoms of certain isotopes present in a given area"

That's a more accurate, but less attractive way of saying it.

 

[Paul98]

When an object approaches the speed of light its mass increases. When you have nuclear material it has a critical mass at which it can sustain a nuclear reaction at a constant level.

But if you have a sub-critical amount of nuclear material approaching the speed of light, does the material approach critical mass? Or will it stay in the same state as the actual amount of matter stays constant?

You are talking about relative mass( the mass that increases with velocity ), which is a term which physicists try to avoid, as we see the confusion here. The mass that we normally talk about is the rest mass of an object, that has to do with gravity, the m in the e=mc^2 equation,...

 

[Wizlem]

Thanks, that's what I thought but wasn't sure about

And now that I think of it even if it would be at critical mass it wouldn't do anything at light speed as the movement within the atoms would stop too I guess (since otherwise the particles moving with the motion of the matter would have to move faster than the speed of light).

In the materials frame of reference nothing will have changed, the mass of an object is the same with respect to itself no matter what. Unless of course its traveling at some speed with respect to itself, which is impossible. Another thing is, if an object is moving at some fraction of the speed of light (like 0.99c) with respect to you and another object is moving at the same speed with respect to the first, the second object is still moving less than the speed of light.

 

[Skyclad1uhm1]

In the materials frame of reference nothing will have changed, the mass of an object is the same with respect to itself no matter what. Unless of course its traveling at some speed with respect to itself, which is impossible. Another thing is, if an object is moving at some fraction of the speed of light (like 0.99c) with respect to you and another object is moving at the same speed with respect to the first, the second object is still moving less than the speed of light.

Hmm, that's something I have to think more about. It does make somewhat sense, seeing that we measure our own speed compared to the earth rather than looking at the expansion speed of this part of the universe, the speed at which the earth rotates and moves around the sun.

When you fire a laser at the surface of the moon and from there one is fired back you have 2 objects moving at a different speed, but the speed of the light beams are constant, based on the object it originated from right?

 

[Wizlem]

Light travels at the same exact speed(the speed of light) no matter who measures it or where it came from. This is the basis of all of special relativity.

 

[Tuna-Fish]

When you fire a laser at the surface of the moon and from there one is fired back you have 2 objects moving at a different speed, but the speed of the light beams are constant, based on the object it originated from right?

Wherever you put an observer, moving at whatever speed, he would always observe the light (in vacuum) moving at exactly light speed, regardless of where it originated. You can think of relativistic distortion as the universe twisting the time and space of the observer to make sure this stays true.