# Exploring the Physics of Light: Questions from a 14-Year-Old

• Cbray
In summary, photons have mass, but not rest mass, and if you reach the speed of light, it would require division by zero and be undefined.
Cbray
Few questions:
When approaching c , why does time slow down? When traveling at c and time stops, how does light have time to reach a certain destination?

What does a photon look like? Why doesn't it have mass, what are they made up of? Is there an equation for a photon? Does light bend because gravity is like a curve inwards on space 'fabric'?

In the sun, what removes the electrons from the Hydrogen atoms for them to become Hydrogen Nuclei?

Sorry for so many questions, I'm interested in physics (Sorry if they are basic questions, I'm only 14).

Cbray said:
Few questions:
When approaching c , why does time slow down? When traveling at c and time stops, how does light have time to reach a certain destination?

This is a very common misconception, and it needs to be straighten out.

Let's say I'm traveling at some velocity v, with respect to you. I see my clock being no different than usual. However, You look at MY clock and see it being slower than yours. At the same time, I look at your clock, and see it being slower than mine! After all, according to me, YOU are the one who is traveling at the velocity v (although in the opposite direction).

Both you and I see our own clocks as not being affected, i.e. no different than usual. So you see, someone who is traveling close to the speed of light see NO SLOWDOWN IN HIS/HER OWN TIME!

Why this happens? It is because of the way we define time (and space) as stated in Special Relativity.

Zz.

ZapperZ said:
This is a very common misconception, and it needs to be straighten out.

Let's say I'm traveling at some velocity v, with respect to you. I see my clock being no different than usual. However, You look at MY clock and see it being slower than yours. At the same time, I look at your clock, and see it being slower than mine! After all, according to me, YOU are the one who is traveling at the velocity v (although in the opposite direction).

Both you and I see our own clocks as not being affected, i.e. no different than usual. So you see, someone who is traveling close to the speed of light see NO SLOWDOWN IN HIS/HER OWN TIME!

Why this happens? It is because of the way we define time (and space) as stated in Special Relativity.

Zz.

Okay, so technically, traveling at the speed of light, our time is so slow its actually still, but for people like my parents, their time is much faster? Is that right?

I was just made clear on the fact that photons do have mass, but not rest mass. Also from my understanding based on the time dilation equation if you were to reach c then it would require division by zero and is undefined. When you look at the graph of the time dilation equation as you approuch c on the graph time gets infinitly slow. I believe c is the limit as it requires an infinite amount of energy to accelerate something to it, someone correct me if I am wrong.

Cbray said:
Okay, so technically, traveling at the speed of light, our time is so slow its actually still, but for people like my parents, their time is much faster? Is that right?
No, that's not right. And don't say "traveling at the speed of light", you can't do that.

But your parents could say you are traveling near the speed of light with respect to them and they would observe your time as being slowed way down while theirs is normal. But at the same time you could say that they are the ones that are traveling near the speed of light, in the opposite direction with respect to you, and you would observe their time as being slowed way down while yours is normal.

Nobody's time is faster in Special Relativity as long as the observer is not accelerating. Everybody's time is normal, as long as they are not accelerating, but they observe everyone else who is traveling with respect to them as having their time slowed down.

So the speed of light appears normal to everyone (as long as they are not accelerating) no matter how fast they appear to be going relative to someone else. When you are traveling at any speed, it seems to you like you are not moving at all and Special Relativity says that you can assume that you are not moving at all, everyone else is. Of course, they can say the same thing about you.

Trevormbarker said:
I was just made clear on the fact that photons do have mass, but not rest mass. Also from my understanding based on the time dilation equation if you were to reach c then it would require division by zero and is undefined. When you look at the graph of the time dilation equation as you approuch c on the graph time gets infinitly slow. I believe c is the limit as it requires an infinite amount of energy to accelerate something to it, someone correct me if I am wrong.
You are wrong about photons having mass. They also do not have a rest mass because they are never at rest. They always travel at exactly c, the speed of light, by definition.

ghwellsjr said:
You are wrong about photons having mass. They also do not have a rest mass because they are never at rest. They always travel at exactly c, the speed of light, by definition.
They have momentum and therefore mass do they not?

Trevormbarker said:
They have momentum and therefore mass do they not?

Oh lord..

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Cbray said:
Oh lord..

I read that and I understand light has no rest mass because it is never at rest, but it has energy and therefore mass? I am sorry if I am totally confused here but does it not have energy which can be shown by the equation e=pc. My understanding is that energy goes hand in hand with mass.

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In a sense, you could say that the mass of an atom decreases by the amount of energy contained in a photon when the atom emits a photon, and that energy could be converted back to a mass when the photon finally strikes another atom, but while in transit, it is pure energy with no mass.

Trevormbarker said:
I read that and I understand light has no rest mass because it is never at rest, but it has energy and therefore mass? I am sorry if I am totally confused here but does it not have energy which can be shown by the equation e=pc. My understanding is that energy goes hand in hand with mass.

Zz.

Cbray said:
Okay, so technically, traveling at the speed of light, our time is so slow its actually still, but for people like my parents, their time is much faster? Is that right?

I don't think you're getting this. The clock that is in the same reference frame as yours doesn't slow down. Your time doesn't slow down. Your parents' time, if they're in the same reference frame as yours, doesn't slow down!

You are traveling very fast right now, when compared to a creature in a very far galaxy somewhere. Do you see your clock slowing down?

May I suggest that you learn a bit more relativity (it is difficult to teach it here since it requires diagrams, lots of diagrams)? Try this link:

http://www.oberlin.edu/physics/dstyer/Einstein/SRBook.pdf

Zz.

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Cbray said:
Oh lord..

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ZealScience said:

Unless you're willing to define what you mean by "mass", that isn't correct either.

What we normally accept as "mass" is "invariant mass". This is what defines the mass of elementary particles, etc. This is zero for photons.

If you insist that there is a mass because it has "momentum" or "inertia", or worse still, it has a "relativistic mass", then you need to search this forum to see why those are faulty arguments to claim that photons have "mass".

Zz.

I have read some posts and many posts say that if something has energy it must have relativistic mass" as you cannot have mass without energy and vice versa. I am new to the forum so if I am overlooking some major concept I am not aware of it! however I will read the FAQ and see if I am horribly mistaken claiming photons have non-inertial mass
EDIT: Ok! I just read the FAQ and it makes sense but could you possibly explain that if the energy of the photon can by described by E=pc , p being momentum does it not need to have mass to have momentum?

Trevormbarker said:
They have momentum and therefore mass do they not?

Trevormbarker said:
but could you possibly explain that if the energy of the photon can by described by E=pc , p being momentum does it not need to have mass to have momentum?

Trevormbarker said:
I have read some posts and many posts say that if something has energy it must have relativistic mass" as you cannot have mass without energy and vice versa. I am new to the forum so if I am overlooking some major concept I am not aware of it! however I will read the FAQ and see if I am horribly mistaken claiming photons have non-inertial mass
EDIT: Ok! I just read the FAQ and it makes sense but could you possibly explain that if the energy of the photon can by described by E=pc , p being momentum does it not need to have mass to have momentum?

As with any aspect of physics, we know more now than we did before. While we used to require mass for a quality called "momentum", we now know that there's a more general definition of momentum that requires no mass. Another example is in solid state physics called "crystal momentum". No mass there either!

Zz.

ZapperZ said:
Unless you're willing to define what you mean by "mass", that isn't correct either.

What we normally accept as "mass" is "invariant mass". This is what defines the mass of elementary particles, etc. This is zero for photons.

If you insist that there is a mass because it has "momentum" or "inertia", or worse still, it has a "relativistic mass", then you need to search this forum to see why those are faulty arguments to claim that photons have "mass".

Zz.

But OP is talking about the reason why photons has momentum. If you avoid relativistic mass, you are talking irrelevant topic.

In addition, there is only inertial mass which is invariant. Like quarks would have much more mass in a baryon or meson than free ones.

ZealScience said:
But OP is talking about the reason why photons has momentum. If you avoid relativistic mass, you are talking irrelevant topic.

In addition, there is only inertial mass which is invariant. Like quarks would have much more mass in a baryon or meson than free ones.

Huh?

Would you like me to show you that even Einstein, after his GR paper, stopped using the term "relativistic mass", because he considered it to be a very inaccurate description? And would you also like to see more papers that criticized the use of that term? If you do, do a search on here!

I had already explained why "momentum", in the generalized sense, need not have mass. What else do you want?

Zz.

ZapperZ said:
Huh?

Would you like me to show you that even Einstein, after his GR paper, stopped using the term "relativistic mass", because he considered it to be a very inaccurate description? And would you also like to see more papers that criticized the use of that term? If you do, do a search on here!

I had already explained why "momentum", in the generalized sense, need not have mass. What else do you want?

Zz.

But in modern physics, I think they apply the equation p=E/c, so that de Broglie's equation can hold, but E is related to mass, as well as gravitational effect of photon. You can say momentum can generate gravitational field, but I think it is still mostly generated by energy.

And what about the momentum of fermions like you say? I heard that modern experiments measure the mass of electrons by their energy. So this mass is relativistic, because it is different from the inertial mass and varies from place to place due to difference in energy. I don't think modern physicists would like to use "inaccurate" results.

## 1. What is light and how does it travel?

Light is a form of electromagnetic radiation that is visible to the human eye. It is made up of tiny particles called photons, which travel in waves. These waves move at a constant speed of about 299,792,458 meters per second, also known as the speed of light.

## 2. How does light interact with objects?

Light can interact with objects in three main ways: absorption, transmission, and reflection. When light is absorbed by an object, it is converted into heat energy. When light is transmitted through an object, it passes through without being absorbed or reflected. When light is reflected, it bounces off the surface of an object, allowing us to see it.

## 3. What is the difference between transparent, translucent, and opaque objects?

Transparent objects allow light to pass through them without being scattered, so we can see clearly through them. Translucent objects allow some light to pass through, but it is scattered in different directions, making the object appear blurry or hazy. Opaque objects do not allow any light to pass through, so we cannot see through them at all.

## 4. How does light affect color?

Light is made up of a spectrum of colors, including red, orange, yellow, green, blue, indigo, and violet. When light hits an object, the object absorbs some of these colors and reflects others. The colors that are reflected determine the color of the object that we see. For example, a red apple appears red because it reflects red light and absorbs all other colors.

## 5. What is the role of light in our daily lives?

Light plays a crucial role in our daily lives. It allows us to see the world around us, helps plants grow through photosynthesis, and provides us with energy through solar power. Light also allows us to communicate through technologies such as fiber optics, and it is used in various medical procedures. Overall, light is essential for our survival and for many aspects of our modern society.

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