A question about gravity and photons

Originally posted by CLPrime
reply to post by Moduli

 

"Relativistic mass" is most certainly a term used in physics...where do you think I learned it?

It is not, it is a term used in pop-sci books, which is where I think you learned it. In my entire carrier in theoretical physics I have not seen one published paper use this term because it is not used by any competent physicist who doesn't live in the 1900s (and even then it was only used a few times because it was known to not be a good description).

reply to post by Moduli


Lord Moduli...I'm not doing this with you again. Feel free to answer any further questions that beezzer may have. And by all means, flex your superiority complex...that is what you came here to do, right?

Whoa guys!

Just wanted to expand a little bit!

This stemmed from a discussion at work, am learning alot here.

Originally posted by CLPrime
reply to post by Moduli

 

Lord Moduli...I'm not doing this with you again. Feel free to answer any further questions that beezzer may have. And by all means, flex your superiority complex...that is what you came here to do, right?

Irony! I am not the one getting offended by making mistakes. Don't get so offended, I don't even care if your explanation is wrong, I came here to explain things correctly, that's it.

reply to post by Moduli


You can correct every word I've ever said, I don't care. I've been wrong before, and I will be wrong again. That's not my problem with you.

Anyway...research tells me that "Relativistic mass" is most certainly used in physics, though some physicists disagree with that usage. Obviously, my education in physics has involved none of those critics.

Originally posted by CLPrime
Anyway...research tells me that "Relativistic mass" is most certainly used in physics

My career in physics tells me no one has used this terminology for a century outside of pop-sci books, and even when it was used it was understood to be an incorrect usage that was used only for brevity.

though some physicists disagree with that usage.

All physicists disagree with the usage because it is wrong. It is incorrect. It is not a matter of terminology or taste, it is incorrectly erasing half of an equation because you do not understand it, which is not the correct thing to do.

"Relativistic mass" is simply not well-defined and does not mean anything. It's not even well-defined for one single observer, because it's half of a transformation! It's like rotating something and only keeping the new x-coordinate and throwing away the y one. You don't get to do that, it's nonsense.

Originally posted by Moduli

... even when it was used it was understood to be an incorrect usage that was used only for brevity.

I see. So posting in a non-technical thread on a message board has no call for brevity?

Originally posted by CLPrime
I see. So posting in a non-technical thread on a message board has no call for brevity?

The kind of brevity that involves abbreviating highly technical terminology with poorly defined century out of date and incorrect jargon that one must be an expert to understand? No, it has no call for that.

If you think it's a good idea, you clearly need to have your choleric humors looked at.

Why do I suddenly feel like Howard asking Sheldon and Leonard a question!

(will just stare at Penny)

Originally posted by beezzer
Why do I suddenly feel like Howard asking Sheldon and Leonard a question!

(will just stare at Penny)

It's a fine question, just one with a very simple answer: a photon's mass is exactly zero. It does however still have non-zero energy and momentum.

Originally posted by Moduli

Originally posted by beezzer
Why do I suddenly feel like Howard asking Sheldon and Leonard a question!

(will just stare at Penny)

It's a fine question, just one with a very simple answer: a photon's mass is exactly zero. It does however still have non-zero energy and momentum.

edit on 31-10-2012 by Moduli because: (no reason given)

(I'm finaly waking up a little)

If a photon has no mass and falls into a black hole due to the curvature of space-time, then is there anything than can puncture space-time and simply travel in a straight line regardless of outside influence?

reply to post by Moduli


A follow-up question, so energy, say as a laser beam, has no mass? I'm talking about waveforms and such, not potentials of energy.

reply to post by Moduli


Which is what I said. If you'll re-read my initial post carefully, including the part that comes after what you quoted, you'll see that it's momentum and energy that I attributed to the photon. I even equated "relativistic mass" with energy. My implication was that "relativistic mass" is illusory. What I wasn't expecting was to have to defend my over-simplified answer to someone who only cares about showing off how smart they are.

And yes, now I feel like Leonard arguing with Sheldon. Just don't try to blow up my head.

Originally posted by beezzer
reply to post by CLPrime

 

If photons have momentum, does that mean they accelerate?

PS, thanks for the answer.

Photons do not accelerate. They have a set speed and they begin and start at that speed (only the medium they travel through alters it). Their behavior is counterintuitive in fact. Imagine for a minute you are on a train going 30mph and throw a baseball 10mph. The ball will be going 40mph correct? So what happens when you shine your flashlight? It should go the speed of light + 30mph like the ball right? Seems like we just found a way to break the speed of light! Only it doesn't. The speed of the train does not impact the speed the light goes. So if you had a train going 5 mph less than the speed of light and you turned the flashlight on, the light coming out would appear to only be going 5mph! Kind of mindblowing.

As others have pointed out, photons have no rest mass.

Originally posted by beezzer
If a photon has no mass and falls into a black hole due to the curvature of space-time, then is there anything than can puncture space-time and simply travel in a straight line regardless of outside influence?

This question doesn't really make any sense. Photons always travel in straight lines, that's what defines a straight line. An outside observer may see them apparently traveling in a curve, but if you were following along the light as it moved along you'd notice you had never actually changed directions.

Originally posted by beezzer
A follow-up question, so energy, say as a laser beam, has no mass?

That's correct, it has energy and momentum, but no mass.

Originally posted by OccamsRazor04

Originally posted by beezzer
reply to post by CLPrime

 

If photons have momentum, does that mean they accelerate?

PS, thanks for the answer.

Photons do not accelerate. They have a set speed and they begin and start at that speed (only the medium they travel through alters it). Their behavior is counterintuitive in fact. Imagine for a minute you are on a train going 30mph and throw a baseball 10mph. The ball will be going 40mph correct? So what happens when you shine your flashlight? It should go the speed of light + 30mph like the ball right? Seems like we just found a way to break the speed of light! Only it doesn't. The speed of the train does not impact the speed the light goes. So if you had a train going 5 mph less than the speed of light and you turned the flashlight on, the light coming out would appear to only be going 5mph! Kind of mindblowing.

As others have pointed out, photons have no rest mass.

edit on 31-10-2012 by OccamsRazor04 because: (no reason given)

Now THAT is a brilliant concept!

Originally posted by CLPrimeMy implication was that "relativistic mass" is illusory.

It's not "illusory" it's false. It's like trying to "simplify" an explanation of a disease by describing it as an imbalance of humors. It's indefensibly wrong.

Originally posted by Moduli

Originally posted by beezzer
If a photon has no mass and falls into a black hole due to the curvature of space-time, then is there anything than can puncture space-time and simply travel in a straight line regardless of outside influence?

This question doesn't really make any sense. Photons always travel in straight lines, that's what defines a straight line. An outside observer may see them apparently traveling in a curve, but if you were following along the light as it moved along you'd notice you had never actually changed directions.

Apologies. (I really am trying to keep up)

Originally posted by beezzer
A follow-up question, so energy, say as a laser beam, has no mass?

That's correct, it has energy and momentum, but no mass.

Is momentum a constant?

reply to post by Moduli


Are you really going to argue the difference between "illusory" and "false"? Should I go get my thesaurus?

Originally posted by CLPrime
reply to post by Moduli

 

Are you really going to argue the difference between "illusory" and "false"? Should I go get my thesaurus?

Hopefully you keep it next to your stock of formaldehyde, because you really need to re-balance your humors.