B Speed Of Light

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1. Nov 6, 2015

SrayD

I have seen this before, and just saw a thread about the sped of light. The problem is that they say, "light has no mass" then conclude incorrectly on a number of issues. What may be worse is that no replies seem to address the source of confusion, light does have mass, zero rest mass yes, but mass it does have. The last I saw, was it should go infinitely fast have no mass; again no reply addressed the rest mass issue. WHY??

2. Nov 6, 2015

ZapperZ

Staff Emeritus
When you say that you have no money, what does it mean? Is it equivalent to Money = \$0.00?

So what is the problem here?

This is semantics, not physics.

Secondly, it doesn't go infinitely fast due to the validity of Special Relativity. That is why it has trumped classical/Newtonian physics, which is where you "saw" this infinite velocity.

Has this "addressed the rest mass issue"?

Zz.

3. Nov 6, 2015

Staff: Mentor

"Mass" in modern relativity means "rest mass". For what used to be called "relativistic mass", in modern relativity, we just say "energy", which is the same thing but a lot less confusing. In SR, things with zero rest mass move at the speed of light (which is the same in all inertial frames); things with nonzero rest mass move slower than light (and their speed is frame-dependent). I'm not sure what the issue is; all of this is straightforward and consistent.

4. Nov 6, 2015

Mister T

There's rest mass and relativistic mass. When people use the word mass they mean what you are calling the rest mass. For a few decades when introductory physics textbooks used the word mass they were referring to the relativistic mass. About 20 years ago that began to stop. For reasons that included getting the physics right, they began using only one kind of mass and no longer use the separate concepts of rest mass and relativistic mass.

That's not based on a confusion over rest mass, it's based on a confusion over motion. The Aristotalian notion that lighter bodies naturally have faster speeds. Following that, it makes intuitive sense that a zero-mass body would have an infinite speed. Of course, that's gibberish.

Last edited: Nov 6, 2015
5. Nov 9, 2015

SrayD

Thanks for the replies.
I do not agree with ZapperZ use of money in place of light. Zero money then have none, but zero mass light, still have some light. Also, I posted after seeing someone reason that light should go infinitely fast, since it has zero mass. Trying to school me on relativity & how it "trump's" classical physics, was a bit insulting.
MisterT- The post I read showed that the initiator reasoned incorrectly about motion based on their stating a fact that light has no mass; I was getting to source of their error. I did not elaborate on that in my post, but should have.

I'm just saying that energy has mass, end of story. The threads that I saw did not seem to address that. The reasoning of the initial poster, to the thread I saw, thinks that light or energy has zero mass, in total, no relativistic semantics. They meant zero, none at all.

6. Nov 9, 2015

ZapperZ

Staff Emeritus
Why is it "insulting"? Telling you that there's something better out there is insulting?

Your understanding of the analogy is faulty. I can easily say that the entity called "amount of money" is the issue here. You may have zero money, or no money, but the entity "amount of money" still exists. Same thing with light. Having zero mass or no mass is the same.

We are not talking about set theory here such as an empty set { } versus a set of zero {0}. We are talking about "semantics". In terms of the physics, it makes no difference what you call it. Rest mass of light is ZERO. If you don't like to call his "no mass", then fine. But do not confuse the physics!

No, this is not the end of the story. That often-misunderstood equation means that energy can be transformed into mass, and mass can be transformed into energy, with that equation providing the exact quantity of conversion. It doesn't mean that all energy has mass, because the FULL relativistic equation contains another part that most people ignored, which is the momentum part (read the FAQ), i.e. energy of the motion!

But that's silly! E=mc^2 is already a "relativistic semantics"! You used it above! Yet, you don't wants us to use it to explain what's wrong with this!

This thread is much ado about nothing! Unless you think that there is a confusion on what to do with the physics when one says "no mass" versus "zero mass" (and I want actual evidence that this is an issue), then I do not see the point in this.

Zz.

7. Nov 9, 2015

Mister T

A single photon is an example of some thing that has energy but no mass.

8. Nov 9, 2015

Staff: Mentor

And we are saying that is not correct; "mass" means "rest mass", and not everything that has energy has nonzero rest mass.