# Reaching C

1. Jul 7, 2011

### Cbray

Few questions:
When approaching c , why does time slow down? When travelling 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).

2. Jul 7, 2011

### ZapperZ

Staff Emeritus
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.

3. Jul 7, 2011

### Cbray

Okay, so technically, travelling 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?

4. Jul 7, 2011

### Trevormbarker

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 im wrong.

5. Jul 7, 2011

### ghwellsjr

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.

6. Jul 7, 2011

### ghwellsjr

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.

7. Jul 7, 2011

### Trevormbarker

They have momentum and therefore mass do they not?

8. Jul 7, 2011

### Cbray

Oh lord..

Last edited by a moderator: May 5, 2017
9. Jul 7, 2011

### Trevormbarker

I read that and I understand light has no rest mass because it is never at rest, but it has energy and therefore mass? Im 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.

Last edited by a moderator: May 5, 2017
10. Jul 7, 2011

### ghwellsjr

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.

11. Jul 7, 2011

### ZapperZ

Staff Emeritus

Zz.

12. Jul 7, 2011

### ZapperZ

Staff Emeritus
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 [Broken]

Zz.

Last edited by a moderator: May 5, 2017
13. Jul 7, 2011

### ZealScience

Last edited by a moderator: May 5, 2017
14. Jul 7, 2011

### ZapperZ

Staff Emeritus
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.

15. Jul 7, 2011

### Trevormbarker

I have read some posts and many posts say that if something has energy it must have relativistic mass" as you cannot have mass with out energy and vice versa. Im new to the forum so if im overlooking some major concept im 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?

16. Jul 7, 2011

### bcrowell

Staff Emeritus

17. Jul 7, 2011

### ZapperZ

Staff Emeritus
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.

18. Jul 7, 2011

### Trevormbarker

19. Jul 8, 2011

### ZealScience

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.

20. Jul 8, 2011

### ZapperZ

Staff Emeritus
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.

21. Jul 8, 2011

### ZealScience

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.