Does photon have mass ?

  • Thread starter nvrmnd
  • Start date
  • #1
3
0
According to a book Sir Arthur Eddington wrote something like "photons have mass" ..Is that true? I know that photons don't have mass. Anyone read something about this?
 

Answers and Replies

  • #2
jtbell
Mentor
15,643
3,689
When did Eddington write that book? I bet it was back in the days before most physicists switched from thinking in terms of "relativistic mass" to thinking in terms of "invariant mass."
 
  • #3
3
0
That book is not Eddington's..It is about physics but it says that " after his observations in Africa, Eddington says light have mass ".
 
  • #4
679
2
That book is not Eddington's..It is about physics but it says that " after his observations in Africa, Eddington says light have mass ".
I suppose it just wanted to say "photon can be affected by gravitation".
If somebody tries to ask if photon has rest mass, the truth is we're not really sure, but we treat it as massless in almost all mainstream theories.
 
  • #5
ZapperZ
Staff Emeritus
Science Advisor
Education Advisor
Insights Author
35,847
4,664
According to a book Sir Arthur Eddington wrote something like "photons have mass" ..Is that true? I know that photons don't have mass. Anyone read something about this?
That book is not Eddington's..It is about physics but it says that " after his observations in Africa, Eddington says light have mass ".
Please start by reading the FAQ thread in the General Physics forum.

Zz.
 
  • #6
K^2
Science Advisor
2,469
29
Short version: Inertial mass, yes. Rest mass, no.
 
  • #7
201
44
Short version: Inertial mass, yes. Rest mass, no.
Since the OP's question is answered, let me use the thread for a related question.

Is there anything else in the universe - detected or predicted - that has only inertial mass, but no rest mass?
 
  • #8
3
0
Please start by reading the FAQ thread in the General Physics forum.

Zz.
Ty, ZapperZ
 
  • #9
K^2
Science Advisor
2,469
29
Is there anything else in the universe - detected or predicted - that has only inertial mass, but no rest mass?
Anything that travels at the speed of light. Off the top of my head, gravitons. Can't really think of anything else.
 
  • #10
Theoretical physics can be molded to say that photons don't have mass. Certainly photons are different from particles that travel at speeds usually far less than light. Those differences do matter. However, the problem with asking or answering the question about what has mass and what does not have mass is that no one knows what mass is. It was originally adopted as an indefinable property with its own unique indefinable units of measurement. The adoption process did not represent knowledge, it represented lack of knowledge.

In the equation f=ma there are three properties. Acceleration consists of measurements of distance and time. Its units are meters per second squared. No problem so far. Both time and distance are indefinable properties. They are the properties from which all physics knowledge results. In other words, it is changes in the patterns of changes of velocity that tell us what the universe is doing from a mechanical perspective. Mass represents that we learned from those patterns that objects resist acceleration to varying degrees. Force represents that we reasonably deduce that there must be a cause for acceleration.

The problem we contginue to face is that while we may feel comfortable with both distance and time, we cannot know what to make of force and resistance to force. They are both mysterious. In order to proceed forward with theory it was decided that one must be accepted as another fundamentally indefinable property. The choice was to make accept mass as that indefinable property. Afterwards, force could be defined in terms of mass and acceleration. However, the mystery remains after all this time. No one knows what mass is. No one knows that it received the proper theoretical treatment when f=ma was interpreted.

A choice was made that was not based upon empirical evidence. It used empirical evidence, but, the choice was made because it was not known what that empirical evidence was telling us. What do measurements of distance and time tell us about what mass is? Whatever that answer is, it represents a better path to follow than to just guess that mass is a fundamentally indefinable property. That is what I think.

James


James
 
  • #11
108
0
Theoretical physics can be molded to say that photons don't have mass. Certainly photons are different from particles that travel at speeds usually far less than light. Those differences do matter. However, the problem with asking or answering the question about what has mass and what does not have mass is that no one knows what mass is. It was originally adopted as an indefinable property with its own unique indefinable units of measurement. The adoption process did not represent knowledge, it represented lack of knowledge.

In the equation f=ma there are three properties. Acceleration consists of measurements of distance and time. Its units are meters per second squared. No problem so far. Both time and distance are indefinable properties. They are the properties from which all physics knowledge results. In other words, it is changes in the patterns of changes of velocity that tell us what the universe is doing from a mechanical perspective. Mass represents that we learned from those patterns that objects resist acceleration to varying degrees. Force represents that we reasonably deduce that there must be a cause for acceleration.

The problem we contginue to face is that while we may feel comfortable with both distance and time, we cannot know what to make of force and resistance to force. They are both mysterious. In order to proceed forward with theory it was decided that one must be accepted as another fundamentally indefinable property. The choice was to make accept mass as that indefinable property. Afterwards, force could be defined in terms of mass and acceleration. However, the mystery remains after all this time. No one knows what mass is. No one knows that it received the proper theoretical treatment when f=ma was interpreted.

A choice was made that was not based upon empirical evidence. It used empirical evidence, but, the choice was made because it was not known what that empirical evidence was telling us. What do measurements of distance and time tell us about what mass is? Whatever that answer is, it represents a better path to follow than to just guess that mass is a fundamentally indefinable property. That is what I think.

James


James
I think you are right: no one knows what mass is. But mass is a consequence of energy (mass is energy), so I would say that nobody knows what mass is because nobody knows what energy is. In Feynman Messenger Lectures (http://research.microsoft.com/apps/tools/tuva/index.html), he tells about energy. That energy is a quantity that is conserved, a quantity that nobody knows why does exist, and that is "hidden" in different forms (that's why we have different kind of energies).

I might think mass, as you say, as a indefinable property. Mass exists and we know this but we do not know why does it exists. Magnetism and electricity does exist, we know, but we do not know why does it exist (I mean, why it was "invented", why is it necessarily a part of our universe).

Rafael Andreatta
 
  • #12
Hi Taturana,

I think you are right: no one knows what mass is. But mass is a consequence of energy (mass is energy), so I would say that nobody knows what mass is because nobody knows what energy is. In Feynman Messenger Lectures (http://research.microsoft.com/apps/tools/tuva/index.html), he tells about energy. That energy is a quantity that is conserved, a quantity that nobody knows why does exist, and that is "hidden" in different forms (that's why we have different kind of energies).

I might think mass, as you say, as a indefinable property. Mass exists and we know this but we do not know why does it exists. Magnetism and electricity does exist, we know, but we do not know why does it exist (I mean, why it was "invented", why is it necessarily a part of our universe).
Thank you for your reply. I think that mass cannot be energy. In other words, resistance to force is not force. Certainly there are important relationships; however, it is the interpretations of the relationships that must be made correct first before abiding by their fruits as facts. I know that it is said that it is a form of energy. However, I would like to stick with f=ma for a while. It is the beginning of theory, and, it is essential that it be interpreted properly whatever the truth is. All other theory follows from it.

Theory is the practice of imagining causes. We do not know what cause is. So, theory enters as a substitute. Force is a cause. We do not know what force is. Energy is force times distance. Or, energy is equal to 'mad' from the equation. So, it does not appear that mass is a form of energy. Rather, it appears that mass times acceleration times distance is equal to the sum of force times distance that we call energy.

James
 
  • #13
601
0
However, I would like to stick with f=ma for a while. It is the beginning of theory, and, it is essential that it be interpreted properly whatever the truth is. All other theory follows from it.
f=ma is only correct in classical physics, as soon as you start talking about relativistic concepts such as if photons have mass you can't use it any longer.

In general you can say that mass is one component of the energy constituents in an object, the other is kinetic energy. The difference between kinetic energy and mass is that kinetic energy is also associated with momentum while mass is just energy. Things like inertia and gravitational charge is associated with the total energy content of the object rather than the total mass, the only reason we use total mass in classical is because the other energy content is usually so small that it is negligible compared to the mass.
 
  • #14
77
0
According to a book Sir Arthur Eddington wrote something like "photons have mass" ..Is that true? I know that photons don't have mass. Anyone read something about this?
Long before the web started an article appeared in Scientific American on a paper by an American student who had deduced (mathematically) that photons have a mass that (at that time) was about 2/3 of the lowest mass value measurable by experiment.

Unfortunately far to many papers get lost or ignored (take this week,s example of a paper on falling leaves, thought to be original; but the original paper was mentioned in a children's book by Enid Blyton in 1954; so far no one has found the original paper).

That said, the real problem arises from the failure to define 'mass' and how mass operates.
 
  • #15
K^2
Science Advisor
2,469
29
James, you are way off. While what you say applies to Newtonian physics, it is entirely outdated with respect to modern physics.

Inertial mass is the ratio of body's momentum to its velocity. Since momentum is a conserved quantity, it's convenient to define a quantity that is rate of change of momentum for a particular body, and we call it force. (In fact, this is how Newton originally defined it.)

F = dp/dt = d(mv)/dt = v dm/dt + m dv/dt = v dm/dt + ma.

Here, m is inertial mass, rather than rest mass. For v<<c, inertial mass is equal to rest mass, and is therefore constant. dm/dt = 0, and you have your F = ma. For v~c, this is no longer true.

dm/dt = m₀ dγ/dt = m₀va / (c²-v²)^(3/2) = mva / (c²-v²)

And so in general, 2nd law looks a little different.

F = ma (1 + v²/(v²+c²))

So there is absolutely no question of what mass is, where it comes from, and how it transforms. The only question is why there has to be a non-zero rest-mass, and that's a question for particle physics. As far as mechanics goes, it's easy enough to show that masses are additive, and from there, once you accept that field equations for elementary particles results in non-zero rest mass for many particles, everything in mechanics falls into its places.
 
  • #16
Hi Klockan3,

f=ma is only correct in classical physics, as soon as you start talking about relativistic concepts such as if photons have mass you can't use it any longer.
F=ma represents whatever roles force, mass, and acceleration play. It is not limited to unrelativistic theory.

In general you can say that mass is one component of the energy constituents in an object, the other is kinetic energy. The difference between kinetic energy and mass is that kinetic energy is also associated with momentum while mass is just energy. ...
I can't say that. Mass is not just energy. Mass is mass. It is not separable as energy rather than momentum. Energy is force times distance. Momentum is force times time. Both apply to the very same event. Whether the event is potential, i.e. will possibily happen in the future, or is occuring, or has occured does not separate energy from momentum. They are both sum totals applying to the same event. They do not have substance. The event involves force distance and time.

Things like inertia and gravitational charge is associated with the total energy content of the object rather than the total mass, the only reason we use total mass in classical is because the other energy content is usually so small that it is negligible compared to the mass.
Mass is resistance to force. It matters not what the context is. Explaining what mass is involves explaining why objects resist force? No one knows why objects resist force anymore than they know what force is.

James
 

Related Threads on Does photon have mass ?

  • Last Post
Replies
20
Views
6K
  • Last Post
Replies
1
Views
6K
  • Last Post
Replies
14
Views
4K
Top