# Something's wrong in < Intro to QM >, by Griffiths

1. Feb 5, 2012

### liumylife

Hi, I am Liu, a undergraduate in mainland China, I post out this idea and want to
see how you gentleman think, here it is:

Griffiths says, in his book < Introduction to Quantum Mechanics >, if an insect fly
in a path perpendicular to the direction in which lightbeam goes, the velocity of
its shadow is proportional to the distance between light source and shadow.

I think the problem is, the velocity of insect's shadow is same as insect's v.

From perspective of wave theory, you can draw a picture below:

－－Ｇｏａｌ　ｐｌａｎｅ－－－
｜｜｜＼　＼｜｜｜｜｜｜｜｜｜
｜｜｜｜＼　＼｜｜｜｜｜｜｜｜
｜｜｜｜｜＼　＼｜｜｜｜｜｜｜
｜｜２｜｜｜＼１＼｜｜｜２｜｜
｜｜｜｜｜｜｜＼　＼｜｜｜｜｜
｜｜｜｜｜｜｜｜＼　＼｜｜｜｜
｜｜｜｜｜｜｜｜｜｜－＞｜｜
Ｖ
Two parts of the space is divided, 1 is for in which light doesn't exist, opposite the 2,
and is the insect.
Now think of the move of "slash tunnel", it is like "be pushed by light", hence the end of
the tunnel, or the shadow, is moving at the insect's speed on the goal plane.
Am I right?

Sorry for grammar error, if any.

Last edited: Feb 5, 2012
2. Feb 5, 2012

### 256bits

It would be true for Griffith if he is talking about a central light source that radiates spherically - the rays are not parallel.

3. Feb 5, 2012

### liumylife

Thank you for replying, I get it.

So further question, once you have a shadow travelling with v larger than
speed of light, we now set a negative film far far away, rolling like a tape,
and shot bullets in front of the source in certain frequencies, meanwhile we
are very lucky that there's a supercomputer interpreting those shadow
frequencies into human language, didn't we just give information over the
speed of light ?

4. Feb 5, 2012

### James_Harford

What you think the answer is to this question may help: A radio station is transmitting information in all directions. If houses A, B, and C are all 50 km from the station and are the vertices of an equilateral triangle with the station at its center, can one say that information received at A was transmitted faster than the speed of light to B and C?

5. Feb 6, 2012

### liumylife

Right, that is much clearer. Thanks.

6. Feb 6, 2012

### sophiecentaur

The shadow will not be a straight like but, at great distances, a spiral shape, spreading outward at the speed of light. A distant receiver will see flashes (as from a lighthouse) and there timing will be delayed due to the transit time for the light.

7. Feb 6, 2012

### James_Harford

Clearer in that I've restated your problem, or in that you saw my point? :-)

8. Feb 6, 2012

### liumylife

I saw your point. Thank you, everyone.

Last edited: Feb 6, 2012