# Mass of a photon

Dale
Mentor
I think (may be wrongly) that math comes later than concepts. I am after the concept here....
I think you are wrong on this. The math IS the concept, in the best language that we have developed for understanding and communicating such concepts. Any natural language description is merely a rough translation.

I think I surely would have to apply force just as now I apply force to stop a ball.

1. Isn't this mass?
A force is a change in momentum. What does the equation tell you about momentum and mass? Are they the same thing?

2. When I stop a ball it doesn't stop existing ( i.e. lose its mass). Why would a photon become massless if I bring it to rest?
It can't be brought to rest, as has been explained multiple times already.

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Dale
Mentor
From the replies it seems that photons are already moving at the speed of light when they are produced...

Why? What is this going on... a magic show!
Yes photons are already moving at c when they are produced. This is not magic, it is conservation of momentum and energy. The same considerations determine the initial speed of any newly created particle.

ZapperZ
Staff Emeritus
From the replies it seems that photons are already moving at the speed of light when they are produced...

Why? What is this going on... a magic show!
Drop a stone into a pool of water and look at the ripples produced. Do you see it "accelerating" from zero velocity, or does the wave already move at a certain speed when it is produced?

This "magic" is already all around you, even in such ordinary observation as water waves. It is so familiar even if you are not aware of it.

Your question on how something can exert a force, or needs to have a force exerted on them, and yet not have mass, is why I often tell people that you can't learn physics in bits and pieces. You see, way back in the 1800's, before the concept of photons even came into existence, light was known to be a classical wave, described via Maxwell Equations. There was no concept of particles here.

Yet, even back then, they already have a knowledge of "radiation pressure", meaning they already have a description of light pushing against something, i.e. imparting a force. Yet, nowhere in the formulation of light at that time was there ANY involvement of "mass" for this entity.

Texts on classical E&M still have this concept. So already, the insistence that having a force must equate to having a mass is faulty.

Zz.

nasu and Dale
Slightly OT but for the newbs, one poster mentioned ways to produce photons;

There is a type of candy you can eat in a dark room with yr mouth open and produce light, hopefully someone will post a link.

Nugatory
Mentor
From the replies it seems that photons are already moving at the speed of light when they are produced...
Why? What is this going on... a magic show!
Not a magic show, because it's real.... And that makes it incomparably more exciting and interesting than any sleight-of-hand magic show.

Yes, photons are always moving at the speed of light - they don't start at zero and accelerate to ##c##.

While we're on this subject... If I am in a spaceship flying past you at half the speed of light (##.5c##) and I emit a flash of light in the direction of travel, the light will be moving at ##c## relative to both of us. This effect was first observed in 1850 and is one of the most solidly confirmed experimental results in all of science (although of course the experimenters didn't use spaceships). It is explained by the same math that leads to the energy/momentum/mass formula that we've been posting. However, until you've gotten to where you can do the math for yourself, you're pretty much stuck with trusting what you hear from the people who have learned the math.

I think (maybe wrongly) that math comes later than concepts. I am after the concept here.
The concepts come from the math and not the other way around. Math is the language of physics, so trying to understand the physics before you do the math is like trying to understand a book written in a language you don't know by looking at the pictures.

Dale
Thanks everyone for their intelligent answers.

I hope nobody gets irritated by my questions. I will continue to post in this thread under this assumption only....

Right now I am in 'thinking mode'.

Will take some time to post...

Drakkith
Staff Emeritus
I will continue to post in this thread under this assumption only....
What assumption?

phinds
Gold Member
2019 Award
Thanks everyone for their intelligent answers.

I hope nobody gets irritated by my questions
. I will continue to post in this thread under this assumption only....
.
To expand slightly on Drakkith's question:

Perhaps it is just a language difference but please be clear: There ARE no "assumptions" in the answers you have been given. You have been presented with the current knowledge of physics, none of which are taken as "assumptions".

What assumption?
Assumption is that 'nobody gets irritated by my questions'.

Drop a stone into a pool of water and look at the ripples produced. Do you see it "accelerating" from zero velocity, or does the wave already move at a certain speed when it is produced?

This "magic" is already all around you, even in such ordinary observation as water waves. It is so familiar even if you are not aware of it.

Your question on how something can exert a force, or needs to have a force exerted on them, and yet not have mass, is why I often tell people that you can't learn physics in bits and pieces. You see, way back in the 1800's, before the concept of photons even came into existence, light was known to be a classical wave, described via Maxwell Equations. There was no concept of particles here.

Yet, even back then, they already have a knowledge of "radiation pressure", meaning they already have a description of light pushing against something, i.e. imparting a force. Yet, nowhere in the formulation of light at that time was there ANY involvement of "mass" for this entity.

Texts on classical E&M still have this concept. So already, the insistence that having a force must equate to having a mass is faulty.

Zz.
Well, my thinking mode is over (as if I am a scientist...lol)

I actually dropped a stone and the ripples were instantaneous (if it's the right word).

These ripples must have to do something with the nature of medium (water).

Similarly, I reason that the instantaneous speed of photons has something to do with the medium (space).

Rest is 'blind faith' in what you say.

Thanks again.

The concepts come from the math and not the other way around. Math is the language of physics, so trying to understand the physics before you do the math is like trying to understand a book written in a language you don't know by looking at the pictures.
I was listening to Lawrence Krauss....

He said.... I paraphrase...

...no matter how elegant the math..no matter how elegant and symmetrical the theory............if it does not match the results of experiments, it is debunked..

So, what you say is partially correct, IMHO.

ZapperZ
Staff Emeritus
I was listening to Lawrence Krauss....

He said.... I paraphrase...

...no matter how elegant the math..no matter how elegant and symmetrical the theory............if it does not match the results of experiments, it is debunked..

So, what you say is partially correct, IMHO.
No, it is 100% correct, because you misunderstood what Krauss said.

How do you think experiments verify theory? Experiments needs the MATH from the theory to be able to verify it. The math in the theory gives precise VALUES that experiments can check! Handwaving description are seldom tested in experiments, because they are flimsy and gives no QUANTITATIVE prediction!

I'm an experimentalist. I depend on the math that theory provides to not only verify experiments, but also makes sense of what I measure!

The more you try to strengthen your case, the deeper the hole you're digging yourself into. I suggest you stop before you bury yourself completely. And you are approaching the "irritating" level.

Zz.

Nugatory
phinds
Gold Member
2019 Award
Assumption is that 'nobody gets irritated by my questions'.
A very good assumption. We don't get irritated by questions, just by people who refuse to listen to what we say and you are not doing that at all.

phinds
Gold Member
2019 Award
I was listening to Lawrence Krauss....

He said.... I paraphrase...

...no matter how elegant the math..no matter how elegant and symmetrical the theory............if it does not match the results of experiments, it is debunked..

So, what you say is partially correct, IMHO.
Krauss is paraphrasing Feynman, who said it about 50 years ago (or since you are paraphrasing Krauss, he might have been quoting Feynman directly). If you haven't watched any of Feynman's lectures online, I recommend them.

No, it is 100% correct, because you misunderstood what Krauss said.

How do you think experiments verify theory? Experiments needs the MATH from the theory to be able to verify it. The math in the theory gives precise VALUES that experiments can check! Handwaving description are seldom tested in experiments, because they are flimsy and gives no QUANTITATIVE prediction!

I'm an experimentalist. I depend on the math that theory provides to not only verify experiments, but also makes sense of what I measure!

The more you try to strengthen your case, the deeper the hole you're digging yourself into. I suggest you stop before you bury yourself completely. And you are approaching the "irritating" level.

Zz.
I think I will be buried now...

The following is what 'Russ Waters' said in my thread 'A question about units'

"No, this isn't about how the universe works, it is just how math works. There is no implication that any particular equation reflects how the universe works and the structure of math doesn't necessarily say anything at all about the universe."

ZapperZ
Staff Emeritus
I think I will be buried now...

The following is what 'Russ Waters' said in my thread 'A question about units'

"No, this isn't about how the universe works, it is just how math works. There is no implication that any particular equation reflects how the universe works and the structure of math doesn't necessarily say anything at all about the universe."
You seem to be learning about physics via a series of quotes, as if they are words from god. This appears to be something you really like. Well then, I'll give you another quote for you to live by:

Warren Siegel said:
Science is not just knowing "what goes up must come down", but when and where it comes down.
Do you think, without understanding the mathematics, you can predict "....when and where it comes down...."?

Where would you like to be buried?

Zz.

' I actually dropped a stone and the ripples were instantaneous (if it's the right word)."

In fact they were not, you can measure the speed easily;

Speed = frequency X wavelength.

Besides, quoting out of context is not a good idea anyway.
In that thread the context was about the internal structure and "rules" of math, which have indeed a good amount of conventional.
You can make up perfect mathematical "objects" which do not necessarily correspond to objects in reality.
As a natural language was also mentioned in that post, you can think that you can use English to describe things from reality as well as describing fictional things, impossible things or even to make meaningless sentences. Even whole paragraphs.
But when is comes to the internal rules of the language, like why do we put an "s" for nouns in plural form, there is no point to ask if this "objectively" right or wrong.
It may be change, by consensus, without any relevance to the "reality".

russ_watters
Nugatory
Mentor
"No, this isn't about how the universe works, it is just how math works. There is no implication that any particular equation reflects how the universe works and the structure of math doesn't necessarily say anything at all about the universe."
That is also true, and in no way conflicts with what we've been trying to tell you. The universe is described by math, but not all math describes the universe.

russ_watters
Where would you like to be buried?

Zz.
In your backyard....so that I can haunt at will....

Dale
Mentor