Particle Physics (21 of 41) What is a Photon? 5. Photon Mass (or No Mass)

In this video I will explain how a photon with no mass can act like it has mass.

Next video in the Particle Physics series can be seen at:

[prabodhsrivastava]

U told. M=hf/c2 and if mass is zero then one thing in 'h' and 'f' must be zero. But 'h' is planck s constant can not be 0 then frequeny is 0 which is not possible

[niladri]

If rest mass of photon is 0 then from relativistic mass equation
M=m°/[(1-(v/c)^2)^1/2]
M=0/0
Which is in indeterminant form..
So, what is to be taken??
This or M=hf/c^2
???

[thrdel]

A photon doesn't " ...act as if it had mass...".
Energy is simply unlimited potential or infinite potential if you want. The moment any limits or boundaries are set, the unlimited becomes limited. A photon is just energy that has quantity and direction as limits (and some more limits). What else is speed or velocity other than limits set to the amount of time a certain amount of energy (a photon in this case) can be in one place in space and time ?
The moment any extra limits are place on the photon, such as proximity to other chunks of energy (gravity, change of the medium, etc.) there will be an interaction between them, in other words the limits become to change, more limits are added and so on.
Mass is like an extra dimension. It is zero when energy interacts with nothing but has a maximum value when that chunk of energy strongly interacts with more energy. And the photon becomes a particle. The limits have changed . Some dimensions decreased or became zero (speed), other dimensions went up from zero (like mass).
In a way, it's like the projection of a two dimensional shape (take a circle for example), when the width limit is reduced to zero, it becomes a line. The width has to have a max value for the dimension we call diameter to appear. In a similar way I suppose a photon is truly at the speed of light and has no mass when it doesn't interact with any other forms of energy.

[gibsonlespaul]

Did you conduct any experiments for the eclipse on August 21st?

I'm an undergraduate physics student taking my first Modern Physics course. I think it is crazy but awesome that we know so little about something we know so much about already. I saw in one of your comments that Einstein proved that light is affected by gravity in 1919, but do we know why it is affected by gravity if it has no mass?

Does light has mass 0 WRT the photon itself, and mass hf/c^2 to an observer, or does the light have mass 0 and hf/c^2 in the same reference frame? This really changes my understanding of light, and its something not normally talked about in lecture because we don't have time to cover all of the cool stuff.

I came into college really aspiring to go into Medical Physics, but the theoretical side seems endless.

[gibsonlespaul]

Did you conduct any experiments for the eclipse on August 21st?

I'm an undergraduate physics student taking my first Modern Physics course. I think it is crazy but awesome that we know so little about something we know so much about already. I saw in one of your comments that Einstein proved that light is affected by gravity in 1919, but do we know why it is affected by gravity if it has no mass?

Does light has mass 0 WRT the photon itself, and mass hf/c^2 to an observer, or does the light have mass 0 and hf/c^2 in the same reference frame? This really changes my understanding of light, and its something not normally talked about in lecture because we don't have time to cover all of the cool stuff.

I saw somewhere that you are in engineering with Raytheon. I'm aspiring for a career as a medical physicist or an engineer in some field, specifically aerospace. What is a good way to find research projects as an undergrad that hasn't taken hire level course like Analytical Mechanics, Theories of E&M, Optics, or Quantum Mechanics?

[cyclotron]

change in kinetic energy will be zero . no change .

[ncdarkness]

If you are familiar with newton cartan theory, when an object is pulled down by gravity (curvature of spacetime and Geodesic equation, instead of force field and newton's 2nd law. Or the "tidal force"), there will be a change in separation between geodesic curves wrt time. Example is when an object is free fallen along the gravitational field, the separation (or the dimension of the falling object along radial direction) wrt time stretches or increases. While the separation (or the dimension of falling object orthogonal to the radial direction) wrt time contracts or decreases. Then "total separation" or total change in volume of the object is zero.

In the case of light or photon, it seems "contract or decreases its length" as it pulled down by gravity because it increases its frequency hence decreases its wavelength and also total length of photon.

[ncdarkness]

If you are familiar with newton cartan theory, when an object is pulled down by gravity (curvature of spacetime and Geodesic equation, instead of force field and newton's 2nd law. Or the "tidal force"), there will be a change in separation between geodesic curves wrt time. Example is when an object is free fallen along the gravitational field, the separation (or the dimension of the falling object along radial direction) wrt time stretches or increases. While the separation (or the dimension of falling object orthogonal to the radial direction) wrt time contracts or decreases. Then "total separation" or total change in volume of the object is zero.

In the case of light or photon, it seems "contract or decreases its length" as it pulled down by gravity because it increases its frequency hence decreases its wavelength and also total length of photon.

[mehrannoorzaei]

Thanks à LOT Mr van biezen sir! For the dedication and amount of effort put into these videos!

[unknotmiguel]

.hmm.. i thought, that if photons have no mass, then Gravity should have no affect on the photon path, or speed. then why would light not be able to escape blackhole huge gravity fields...? if gravity requires mass... then gravity should not apply to photons, if they are truly mass-less..

[erichchristian]

Veritasium brought me here. I was challenging conventions and thinking light realistically and likely has a mass greater than 0 but is so minuscule it is negligible for what has been modern science. Theoretically, something that has no mass, does not exist. Yet, light is and exists, or if you want to be more technical I suppose the photon ray or 3d object and the photon space both are, with the ray being different and the space at the same time is yet perceivable as not.

[jovanovicoliver]

4:54 Yes, you can.
Pound–Rebka experiment

[theoreticalphysicsnickharv]

You say that: "a photon will actually gain kinetic energy and therefore the frequency would go up and that the wavelength would go down so actually it will have a slightly shorter wavelength". Why does the change from potential energy to kinetic energy form an increase in frequency?
Your videos are very good indeed!

[sakshamsachdev]

but the impact of gravity should be so less because the time for which photon remains close to earth is very less

[CatsBirds]

This lecture alone answered my so many questions regarding blackholes, relativity etc. A big thanks.

[daltonstanley]

Do you think the reason photons are affected by gravity might be because of the supposed gravitron you talked about earlier? That would make sense that a transfer particle is bouncing back in between the photon and large mass

[peterhall]

Hi Michel At 4:40 Can we measure that ? Actualy they already did . With the Pound–Rebka experiment in 1960 Using a tower at Harvard

[ptyptypty]

Michel.... this is an excellent Presentation!!.. Thank you!!.. I can almost imagine the Wavelength being longer with the Photon higher up.. and then seeing the wavelength contract more and more as the photon approaches the Earth... (or a much more massive body)....

[Lymphaofallcolours]

Thank you for your quick answer. The next video in the series clarified the thing completely.

[Lymphaofallcolours]

But, speed of photons is always constant, and so would be their mass, so their kinetic energy should be the same unless their wavelength would be increased. Am I wrong?