Particle Physics (26 of 41) What is a Photon? 10. Momentum and Velocity
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- čas přidán 5. 09. 2024
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In this video I will calculate the momentum and velocity of a mass-less photon.
Next video in the Particle Physics series can be seen at:
• Particle Physics (27 o...
Your explanations are very simple and elegant. Shows a mastery of the subject
Thank you. We are glad you found our videos! 🙂
We are really thankful for such a imaginative and crystal clear explanation with numericals... :)
shouldn't we subtract final momentum from initial momentum ? and get a negative momentum
It depends on what you are trying to calculate. Here we just wanted to know the magnitude of the change, which is then imparted onto the electron.
You are generally very good but how could you say at the very outset that a photon has no mass without immediately qualifying it by saying that a photon has no REST mass. Every travelling photon at speed C must have mass because E=mc^2 where E = moving photon whose mass = E/C^2 = hf/C^2. Thus photonic mass is directly proportional to frequency and shows BOTH gravity and inertia.
Your previous lecture on Compton was very good.
Photon have NO mass. (They do have momentum).
Thank you very much for responding.
What you say is often stated as such. But....
If Photon has Energy then Einstein said (All) Energy = MC^2. Which means Energy MUST have BOTH mass AND speed (the mass is called effective or travelling mass) UNLESS you have an equation where E is NOT = MC^2 or you deny light has Energy.
And photon's momentum is MC.
People get confused as to how can ANYTHING with mass be travelling at speed C? Good question.
Yes if an objects REST mass is NON zero, say an electron, then Relativity says it can never achieve speed C as the Relativity divisor becomes zero with infinity as answer.
But photon's REST mass is indeed ZERO. So at speed C its mass is NOT infinite but 0/0. So what is 0/0? Good question. Well it is NOT infinite nor zero.
0/0 actually means "no single right answer but MULTIPLE right answers". (some call it indefinite because there are multiple answers) That is true for photons. Photons have NO SINGLE effective mass. It's effective or travelling mass depends on its frequency i.e it's mass is directly proportional to it's frequency = hf = E. So there are multiple right answers regarding the effective mass of a travelling photon.
To say photon has no mass means you are EITHER DENYING that photon has Energy or you are denying that E = MC^2.
It was photon's (light's) gravitational mass which was used to show that the sun bent light in 1919 which made Einstein famous worldwide. Photon's effective mass has both inertia and gravitation.
Finally consider: When an electron and positron annihilate to form gamma ray photons where does their mass go since mass is a conserved quantity. It goes in the masses of the gamma ray photons. Mass cannot disappear.
The sun too is constantly losing 5.5 million tonnes of mass/sec, mainly (tho not exclusively), because it is emitting light.
i wasted my life learning and worshiping biology. This should be my thing. SAD!
Biology is a great field as well!
Nice bow tie :)