**Ask a monkey a physics question thread**

[MadMojoMonkey]

I have questions about my Einstein book. Some of them aren't related to quantum physics, should I start a new thread?

I think this thread is fine, but if you want a more specific conversation on a topic, then a new thread is fine, too.

[Eric]

I think this thread is fine

Ok, cool. Like I said, Einstein by Walter Isaacson has a lot to think about.


We shine a light into space. I get on a spaceship going in the same direction at 10k miles per second.


The light is moving away from both me and the earth at 186k miles per second, right?
We're not used to thinking this way, we're used to thinking of something moving away from me at just 176k (186 - 10).
The key is time, right? My relative time is different than the earth's relative time.


Page 119 talks about sound being constant at 770 miles per hour. How is sound the same as light and how is it different?

[MadMojoMonkey]

We shine a light into space. I get on a spaceship going in the same direction at 10k miles per second.

(A)The light is moving away from both me and the earth at 186k miles per second, right?
We're not used to thinking this way, we're used to thinking of something moving away from me at just 176k (186 - 10).
(B)The key is time, right? My relative time is different than the earth's relative time.

A) Yes, what you describe with (186 - 10) is called Galilean Relativity.
I'll use 186k mps as the approximation for our discussion.

B) The key is both time and space. The distances (outside your ship) in your direction of travel are shorter.
It is the combined effects of both time dilation and space contraction that contribute to the constant speed of light.

(Nicely put, OngBonga.)

Page 119 talks about sound being constant at 770 miles per hour. How is sound the same as light and how is it different?

Actually, this is a difficult post because we're touching on a lot of concepts that a freshman course in physics would spend weeks on. Please bear with me if I'm long winded and feel free to follow up on parts where I'm terse.

***
Part 1: What do we mean by constant?

The speed of sound is not constant. It can be treated as constant at 770 mph for most airplanes, though. This a close approximation for the speed of sound at STP (Standard Temperature and Pressure). Variations in the chemical composition (primarily humidity, on Earth) of the air or the ambient air pressure or any average flow (wind) will affect the speed at which sound waves travel.

Both sound and light move at different speeds in different mediums.
What's that I said? Light moves at different speeds?
Yes.

The "constant" speed of light is the maximum speed in vacuum. Light travels more slowly when it passes through a medium. E.g. As light passes through a pane of glass with a refractive index of 1.5, the light travels at a speed of c/1.5 = (2/3)*c ~= 124k miles per second. As light passes through air at STP, which has a refractive index of 1.000293, the light move more slowly, at a rate of c/1.000293. This is ~55 mps slower than its speed in vacuum.

The lowest possible refractive index is 1. There is no material through which light moves faster than c.

When we talk about the "constant" speed of light, we're talking the speed in vacuum and about what you described in terms of relative motions of observers.

The important point here is that the speed of sound is relative to particles in a region interacting with each other. Any variations which increases or decreases the frequency or violence of particle collisions is going to alter the speed of sound.
The speed of light is a property of the electromagnetic fields. The EM fields exist everywhere in the universe (so far that we've checked) and are not merely restricted to regions where there is "stuff."

***
Part 2: Similarities between light and sound

Sound is like light in that both propagate as waves. This means they follow the same basic mathematics as far as being solutions to the wave equation.

The wave equation explicitly states the relationship between the derivatives of a function w.r.t. space and time. It predicts how any function will evolve through space over time, given the properties of the restorative forces and dissipating forces.

***
Part 3: Differences between light and sound

The most striking difference is what is waving.

A special note is that when we talk about the constant speed of light in vacuum, we are talking about waves propagating through a region with no dissipating forces. This gives the waves a theoretically infinite range.

There is no analogue for sound waves. Sound waves propagate through a medium of particles which dissipate the wave's energy by creating heat, inelastic deformations, etc.

Sound is an alternating region of pressure in a medium. In air, it is the statistical summary of all the molecular interactions which result in particles exchanging energy. Sound is a longitudinal wave, meaning that it's action is along it's direction of travel.

Light is an electromagnetic wave. What is waving are the electromagnetic fields. These fields are intangible in the sense that they are not made of atoms. EM waves are transverse waves, meaning that the action is perpendicular to the direction of travel.