# The idea behind a reverse shock

1. May 25, 2013

### Kidphysics

So in a supernova explosion for example (5th slide) http://www.astro.princeton.edu/~burrows/classes/541/blastwavesChisari.pdf

Ambient medium is accelerated, compressed and heated. It pushes back into the
ejecta, creating a reverse shock.

Why does it do this and not just emit a drag force? Why exactly does it push back? Are there conservation of momentum reasons also?

2. May 25, 2013

### Andrew Mason

This reverse shock is required by Newton's Third Law. The matter that provides the force driving the outward acceleration of matter experiences an equal but opposite (inward) force from the matter being accelerated outward.

AM

3. May 25, 2013

### Kidphysics

Yes but it seems this would just be modeled like a drag force.. you push a pillow across a bed and its restivene, but you don't get a reverse shock from it, everything in the system is translated forward.

4. May 25, 2013

### Andrew Mason

I don't understand the analogy. What is "restivene" (typo?)? What does a pillow being pushed across a bed have to do with matter exploding through a gas cloud?

The physics of a supernova is not simple. These are complex phenomena and I don't fully understand it. But from the article you have cited, it appears that the matter being ejected in the supernova explosion is initially inside a dust or gas cloud. The exploding matter compresses the gas cloud and the gas/dust heats up and the pressure in the cloud increases. The pressure increases to a point that it exceeds the forward pressure of the ejecta so there is a wave of pressure in the reverse direction.

AM

5. May 26, 2013

### Kidphysics

Cool thank you for the response (resistive was the word) what I am confused with is that even if there is pressure on the outside cloud why would the shock be driven inword?? For example, if a bomb goes off doesnt EVERYTHING fly away? Unless the bomb goes off the shockwave compresses the air, PASSES the compressed air and then the air expands I have no idea why anything would be traveling radially inword from a radially outword blast. Isnt it a tad counterintuitive?

6. May 26, 2013

### Andrew Mason

The paper does not say that the ejecta from the super nova reverses direction. It just slows down a bit and heats up due to the reverse pressure in the ambient dust/gas medium created by the forward shock wave.

The paper you cited is just a power-point presentation so it doesn't provide much detail. There are more complete explanations available such as this one. Astrophysics requires expertise in many areas of physics. Good luck in trying to understand it all!

AM