Question

-Why can't we be certain that the Andromeda Galaxy exists today?

-How do sound and light waves differ?

-What is the frequency of a radio wave that has a wavelength of λ = 100 m?

-Which of these is the same for all forms of electromagnetic (EM) radiation in a vacuum (i.e., empty space)?

wavelength, frequency, velocity, photon energy

-Which of these is emitted when an electron falls from a higher to lower orbital?

another electron, a positron, a neutrino, a photon

Answer 1

-Why can't we be certain that the Andromeda Galaxy exists today?

Ans : As we know Information moves at the speed of light in this universe. No getting around that. Therefore, there is no common "now" with us and the Andromeda galaxy, the "now" is separated by two million years of ability of information about one to get to the other. What we can say is that two million years ago the Andromeda galaxy existed, and ours exists now, so it is not an illogical leap to assume Andromeda is moseying along just fine like we are. But we cannot know for sure, so our "now" is Andromeda's two million years ago, and vice versa. Andromeda has no idea if the Milky Way is still here or not, just that it is seeing the light of the Milky Way from 2 million years ago, which means the Milky Way existed THEN. That is as much as can ever be known due to the cosmic speed limit.

-How do sound and light waves differ?

Ans: sound waves are mechanical waves which requires a medium to travel. But light waves are electromagnetic waves which do not require to travel.

-What is the frequency of a radio wave that has a wavelength of λ = 100 m?
Ans : use, c = lamda*f

==> f = c/lamda

= 3*10^8/100

= 3*10^6 m

-Which of these is the same for all forms of electromagnetic (EM) radiation in a vacuum (i.e., empty space)?

Ans: velocity

-Which of these is emitted when an electron falls from a higher to lower orbital?

Ans: a photon