How do photons lose their energy to the universe as they cross it?
Do photons shed their energy by emitting it as extremely
weak photons or interact with nearby particles or each other so weakly
it only appears continuous? The amount of energy lost by photons
the big bang must be enormous. Similarily with a gravitational field, are
photons able to lose energy by exchanging gravitons in undetectable
until they "evaporate" when their wavelength becomes nearly infinite?
The Universe is expanding in time, as Edwin Hubble
discovered in 1929. As the photons travel across the Universe, the
space that they travel through expands, thereby stretching the
wavelength of the photon. Longer wavelengths mean less energy. This
is called the cosmological redshift.
Photons can also change energy by traversing a gravitational potential
well, called the gravitational redshift. As far as gravitons and
photons are concerned, it is important to realize that they are both
force mediators - meaning the photon is a quanta which "transmits" the
electromagnetic force. So, when 2 positively charged particles to
repel each other, they are exchanging photons. Similarly, when 2
masses attract one another, they exchange gravitons. To our knowledge,
gravitons and photon do not interact with each other. If you have read
somewhere that the gravitational field of an object, say a black hole,
has an effect on the magnetic field around it, this is true but it
arises from geometrical effects (the black hole warps space around it
and electromagnetic fields can be compressed or diluted depending on
their location) alone.
There is a useful website which may help answer your
The author has separated his Q&A into 3 parts - beginning, intermediate
and advanced questions.