Re: why is neutral dark matter invisible?


kajlina wrote:
I am reading Michio Kaku's "Parallel Worlds". In the book, he says that
"If dark matter coinsists of subatomic particles, they must be stable
and neutral in charge(otherwise they would be visible), and they must
interact gravitaionally."
Why neutral in charge means invisible? Does this apply to other
particles? Neutrons are also neutral in charge.

Hi - - They must be neutral, ie: have NO electrical charge, because if
they had any charge at all, they'd respond to the electromagnetic
interaction, which would then guarantee that they'd be inherently
<visible>... capable of absorbing and emitting photons of EM. Neutrons
and neutrinos are electrically neutral and therefore cannot interact
via
the EM force (via photons of any kind).

The regular, normal particles we are familiar with cannot fill the bill
as
'dark matter' candidates for various reasons: Neutrinos are neutral but
are so low in mass they can't serve as the dark matter gravitational
potential source (even though they exist in huge numbers). The
neutrons
you mention are neutral, but they beta decay on their own on average
in ~ 1,000 seconds, producing particles which DO interact via the EM
force (protons and electrons).

The present thinking is that whatever dark matter particles are, they
must only interact via the grav'l force (some sort of WIMPs).
I personally do not believe that WIMPs are the answer however. There
is a possibility that the dark matter potential emanates from baby
universes linked to the black holes in the galaxies of our universe.

Cheers.

.

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