what is the density of a neutron particle traveling a the speed of light?

Answer 1

Hi cosmocurious

The answers given above are only half right. It is correct that a neutron can't be accelerated to light speed because it has mass - to do so would require infinite energy.

However a particle travelling at c would *not* have "infinite mass". This misunderstanding arises from a lack of care with reference frames. The mass increase talked about is an increase in relative mass, or the mass measured by others observing the neutron, not the mass measured in the neutron's own rest frame (rest mass is in fact invariant - ie it does not change). This "mass increases as speed increases" is a common misunderstanding of relativity.

One simple way to demonstrate that only particles with zero rest mass can travel at c is to consider the energy-momentum 4-vector of such a particle. The length of the energy momentum 4-vector is identified with rest mass, and for particles travelling at c this length is always equal to zero (this is due to the structure of Lorentz space, or if you prefer it is a consequence of the speed of light being a speed limit).

The result of all this is that the rest mass of a neutron remains the same regardless of its relative velocity, hence its density remains the same as measured in its own rest frame.


Hope this helps!
The Chicken

Answer 2

Infinite. You cannot accelerate a particle with mass to the speed of light, the relativity equations don't allow it. You get infinite mass, which is another way of saying your math is all screwed up. You can get close though -- you know, .99999 percent the speed of light. But the thing is, you can add "ninths" behind that decimal point forever. The mass just increases, without limit. When it gets to light speed, you get infinity. I think "density" is not the right choice of words here. A neutron particle's "density" is that of an atomic nucleus. It doesn't matter if it's at rest or zipping along in a linear accelerator, it's density would be the same. However, it's mass, or "weight" if you like, would increase with velocity.

Answer 3

If the neutron WAS traveling at the speed of light it wouldn't have mass, and thus density because its now energy not matter.

Answer 4

1. A neutron has mass so it can't travel the speed of light.
2. Any object that has mass that somehow manages to travel the speed of light has infinite mass and therefore infinite density. This is WHY it can't travel the speed of light.

Answer 5

Water_Skipper is correct. And currently plans are being made to accelerate heavy nuclei like gold to a large fraction of the speed of light and slam them in head-on collisions. Their relativistic shortening and mass increase would give them such a high density that possibly they'd form tiny black holes on impact.

Answer 6

You might refine that question to traveling at 99.9 percent the speed of light.