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# Re: [Phys-l] Absolute four-momentum of massless particles

Fine.
In this case, out of the two possibilities to interpret your result for a
photon:
1) P = m U , vector equation; your conclusion: |P|= m |U| = 0 (Scalar equation,
True)
2) P = m U , vector equation, your conclusion: P = m U = 0, (vector equation,
Wrong)
only the wrong one, 2), remains in your calculations.

Moses Fayngold,
NJIT

________________________________
From: John Denker <jsd@av8n.com>
To: Forum for Physics Educators <phys-l@carnot.physics.buffalo.edu>
Sent: Fri, October 1, 2010 10:13:45 PM
Subject: Re: [Phys-l] Absolute four-momentum of massless particles

In the context of
https://carnot.physics.buffalo.edu/archives/2010/10_2010/msg00011.html

On 10/01/2010 09:28 PM, Moses Fayngold grasped at the following straw:

1) The symbol U is used as the norm |U| of 4-velocity (and the same for P).

Absolutely untrue.
No such thing occurred in the calculation I did.
I have reviewed the message, and cannot imagine how
anyone could misinterpret in this way.
The equation P = m U is a vector equation.
When I write P = 0 it means every component of P is zero.
If I had meant |P| I would have written |P|.

this is one of the most elusive
and thereby most dangerous errors,

It is particularly elusive in cases where it did not occur and could
not possibly have occurred.

Bottom line:

Understanding arithmetic is a prerequisite for understanding relativity.
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