# Mass= ?

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When I think the word mass I invision a blob that can be large or small, which is not very discriptive, and probably has little to do with mass.

Is the more specific meaning,   mass = (The resistance to change, in motion.) ?

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That is the most useful description.

As to what it actually is ?
It is a property of 'things'.
And, to the best of our understanding, certain 'things' obtain this property through an interaction with the Higgs field.
( look up the Higgs mechanism )

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Sure, remember the term mass in kinetic energy, 0.5mv^2, is actually the resistance of a "thing" to change its velocity, just like in electromagnetism 0.5(epsilon)E^2, epsilon is the resistance of a medium to change the electric field ('s magnitude). This kind of analogies work everytime you want to know what a physical quantity represent and you can get to a explicit equation for the energy (same with 0,5kx^2 in springs, 0.5CV^2 in capacitors, etc).

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On 8.12.2018 at 8:43 AM, jajrussel said:

When I think the word mass I invision a blob that can be large or small, which is not very discriptive, and probably has little to do with mass.

It's layman definition of volume..

Two different objects with the same shape and the same volume, can have significantly different mass. Example is water with 1 g/cm^3 density, and iron with 7.9 g/cm^3 density, gold with 19.3 g/cm^3. The same volume, but 19x bigger mass.

Edited by Sensei

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This is semantics of course, but bodies with mass are often defined as being matter.

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Mass can be considered to be the kind of energy.

A photon has basically only kinetic energy so it doesn't have rest mass.

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Isn’t saying something has zero mass exactly the same thing as saying it has no resistance to acceleration? Meaning that it shouldn’t take much if just a whisper so to speak to turn a photon? Meaning that it would have to accept the energy of some force and slow down? Since it can’t go faster.

wouldn't it have to be the same for any particle without mass?

I suppose it could just as easily be said that it would react to force by giving kinetic  energy thus accelerating by slowing down?

On 12/23/2018 at 4:20 PM, Itoero said:

Mass can be considered to be the kind of energy.

A photon has basically only kinetic energy so it doesn't have rest mass.

Which in order to maintain existence as a photon it would have to retain some reasonable measurable amount of kinetic energy?

or experience an equal exchange during the interaction? Which I’m guessing should display as a time? As in it should take some amount of time for the interaction that occurs?

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1 hour ago, jajrussel said:

Isn’t saying something has zero mass exactly the same thing as saying it has no resistance to acceleration? Meaning that it shouldn’t take much if just a whisper so to speak to turn a photon?

Well, not necessarily turn into a photon, but that is a good way of understanding why massless objects must move at the fastest speed possible.

1 hour ago, jajrussel said:

wouldn't it have to be the same for any particle without mass?

Yes, all massless particles travel at c.

1 hour ago, jajrussel said:

Which in order to maintain existence as a photon it would have to retain some reasonable measurable amount of kinetic energy?

It has energy as an intrinsic property, proportional to its frequency. I'm not sure that this can really be described as kinetic energy (mv2/2) because photons all travel at the same speed and they don't have mass.

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14 minutes ago, jajrussel said:

Isn’t saying something has zero mass exactly the same thing as saying it has no resistance to acceleration?

There is just no force strong enough to accelerate a photon.

59 minutes ago, jajrussel said:

Meaning that it would have to accept the energy of some force and slow down

Photons interact with particles and can loose or gain energy. This changes the speed of light but the photons always move at c. Compton scattering, discovered by Arthur Holly Compton, is the scattering of a photon by a charged particle, usually an electron. It results in a decrease in energy (increase in wavelength) of the photon (which may be an X-ray or gamma ray photon), called the Compton effect.

1 hour ago, jajrussel said:

Which in order to maintain existence as a photon it would have to retain some reasonable measurable amount of kinetic energy?

Correct, I think.  Certain interacting can completely absorb the kinetic energy of photons. Like in the Photoelectric effect.https://en.wikipedia.org/wiki/Photoelectric_effect

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6 minutes ago, Itoero said:

There is just no force strong enough to accelerate a photon.

Doesn't a mirror do exactly this?

Why do folks so often forget that acceleration is a vector and that change of direction is an acceleration?

Too much Christmas pud?

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15 minutes ago, studiot said:

Doesn't a mirror do exactly this?

Why do folks so often forget that acceleration is a vector and that change of direction is an acceleration?

Too much Christmas pud?

A mirror doesn't accelerate photons. Acceleration is the rate of change of velocity of an object with respect to time.

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Yeah, you're right of course, but its not like throwing a ball up in the air; it doesn't slow down, come to a stop, change direction and speed up again.
What's Christmas pud ?

There are two classes of things; those which can be in an inertial frame, and those that can't.
Things that can be 'framed', can accelerate and stop according to forces.
Things that can't, are constrained to move at c ( the speed of light ).

Since 'frameless' things can never be at rest, talking about their rest mass is non-sensical, so we call them massless things, and a photon is one such thing.
There are properties that all things have in varying amounts, mass and energy, which are like two sides of a coin, and are closely related ( by E=mc^2 ). All things ( because of quantum effects ) need to have a minimum amount of energy, but it doesn't seem to work that way with mass.
And of course, any moving energy is the same as moving mass; both have momentum.
This is why we can say that a photon, while having NO rest mass, has the equivalent of mass at its natural speed, c .

Edited by MigL

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3 hours ago, Strange said:

Well, not necessarily turn into a photon, but that is a good way of understanding why massless objects must move at the fastest speed possible.

Yes, all massless particles travel at c.

It has energy as an intrinsic property, proportional to its frequency. I'm not sure that this can really be described as kinetic energy (mv2/2) because photons all travel at the same speed and they don't have mass.

Note it was turn a photon not turn into a photon

2 hours ago, studiot said:

Doesn't a mirror do exactly this?

Why do folks so often forget that acceleration is a vector and that change of direction is an acceleration?

Too much Christmas pud?

In my opinion that is exactly what a mirror does.

I have noticed the same capacity to forget the definition of acceleration displayed on occasion.

Pertaining to the Christmas pudding, maybe  if you called it figgy pudding it might be easier to google... Note - I may be making an assumption based on recent media lectures on how to make it. My favorite being a daily soaking of Brandy for five consecutive days. If I remember correctly in my early twenties there was a Christmas spent in Germany where I might have passed as a living example of that  recipe except in my case it was  tequila and peppermint schnapps, with beer chasers. It’s kind of one of my favorite memories likely due to the fact that as a memory it was probably more imagination than memory. Actually, I’m sure it was...

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2 minutes ago, jajrussel said:

Note it was turn a photon not turn into a photon

Oops. I misread that. But it would take energy to turn a photon because they do have momentum.

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3 hours ago, Strange said:

Oops. I misread that. But it would take energy to turn a photon because they do have momentum.

Yes.

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15 hours ago, Strange said:

But it would take energy to turn a photon because they do have momentum.

Are you sure?

How does that work?

18 hours ago, Itoero said:

A mirror doesn't accelerate photons. Acceleration is the rate of change of velocity of an object with respect to time.

Exactly right, which is why a change of direction is an acceleration.

That's not an arguable thing it is a definition, first taught (or should be) in high school.

15 hours ago, jajrussel said:

Note it was turn a photon not turn into a photon

+1 for encouragement.

Well caught.

18 hours ago, MigL said:

What's Christmas pud ?

We had a lovely dancing cognac flame on ours.

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3 hours ago, studiot said:

That's not an arguable thing it is a definition

Can you show me this definition? Light can accelerate , photons can't, they always travel at c.

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18 minutes ago, Itoero said:

Can you show me this definition?

Quote

Acceleration is the name we give to any process where the velocity changes. Since velocity is a speed and a direction, there are only two ways for you to accelerate: change your speed or change your direction—or change both.

Quote

To accelerate an object is to change its velocity, which is accomplished by altering either its speed or direction (like in case of uniform circular motion) in relation to time.

Quote

Recall that velocity is a vector—it has both magnitude and direction. This means that a change in velocity can be a change in magnitude (or speed), but it can also be a change in direction. For example, if a car turns a corner at constant speed, it is accelerating because its direction is changing. The quicker you turn, the greater the acceleration. So there is an acceleration when velocity changes either in magnitude (an increase or decrease in speed) or in direction, or both.

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19 hours ago, jajrussel said:

In my opinion that is exactly what a mirror does.  I

I thonk you can argue this from a classical physics perspective, but photons are quantum, and you have a much harder time applying Newtonian physics.

You can’t really say it’s the same photon coming out as going in. Same thing for refraction.

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1 hour ago, Strange said:

Acceleration is the name we give to any process where the velocity changes. Since velocity is a speed and a direction, there are only two ways for you to accelerate: change your speed or change your direction—or change both

So then every time a photon interacts with a particle it accelerates? That's odd to say. The Wikipedia definition is then incomplete. Then light can accelerate because it changes direction or because it speeds up?

And what when change in direction (refracting) increases velocity?

In physics, acceleration is the rate of change of velocity of an object with respect to time.

This should be : In physics, acceleration is the rate of change of velocity (-vector) of an object with respect to time.

Edited by Itoero

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3 minutes ago, Itoero said:

The Wikipedia definition is then incomplete.

In physics, acceleration is the rate of change of velocity of an object with respect to time.

This should be : In physics, acceleration is the rate of change of velocity (-vector) of an object with respect to time.

That is pretty much what says.

(What amazed me when looking for those definitions is how bad most explanations of acceleration are; even the ones aimed at children tend to be incredibly confusing and bury a simple concept in all sorts of irrelevant detail.)

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On 26.12.2018 at 6:08 PM, Itoero said:

A mirror doesn't accelerate photons. Acceleration is the rate of change of velocity of an object with respect to time.

Absorbed photon is accelerating mirror. Accelerated mirror is changing its frame of reference. After remission, newly created photon (reversed direction), should have slightly lower energy than incoming photon.. if you will use special relativity calculations..

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1 hour ago, Itoero said:

In physics, acceleration is the rate of change of velocity of an object with respect to time.

Exactly so.

So the rate of rotation - which is a reflection or refraction - is the rate of change of velocity.

QED.

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2 hours ago, Strange said:

That is pretty much what says.

(What amazed me when looking for those definitions is how bad most explanations of acceleration are; even the ones aimed at children tend to be incredibly confusing and bury a simple concept in all sorts of irrelevant detail.)

I really liked the Khan Academy link you gave - thanks for that :-).

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2 hours ago, Sensei said:

After remission,

Fix: after reemission..

(missing 'e' character)