Where does a photon get it's infinite energy from?


by emirhasa
Tags: energy, infinite, photon
DaveC426913
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Feb3-12, 07:07 PM
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Quote Quote by bahamagreen View Post
Dave:"As the car accelerates in space, it cannot move faster than 60mph..."

So, you assume the conclusion? How does this explain?

[edit - just saw emirhasa's last note after writing this... Dave, I'm not meaning to pile on, leaving my questions as written :)]
OK, I am guilty of making the wrong assumptions about who I'm addressing.

It is very common on PF to get members from all walks of life asking questions, and every question requires some assumptions about the level of knowledge of the asker (this has to be done, there's no universal way to address everyone).


Based on the ideas put forth in the opening post, I made an educated guess that emirhasa was fairly unknowledgeable about photon propagation. (Sorry emirhasa, but your first post was pretty off). I chose to answer the question in a way that a layperson would understand. We do this all the time on PF.

The car analogy is a poor analogy but it is often enough to make it clear to laypeople why c is not so much an exceedible 'speed limit' as an ultimate 'end of the line'.

Apologies to both of you.
DaleSpam
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Feb3-12, 08:22 PM
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Quote Quote by emirhasa View Post
I know that given the state that it has no rest mass, and a speed, it will go with the speed of c. What I ask is, when given no rest mass, and if it has to move a speed ,why wouldnt it be like 399,987,362 m/s.
I suspect that this will not satisfy you, but the answer for this question is simply that we have defined meters and seconds such that the speed of light is 299 792 458 m/s.

http://www.bipm.org/en/si/si_brochur...2-1/metre.html

The value of any dimensionful universal constant is merely an artifact of our units. We can make the speed of light have any numerical value, such as 399,987,362 simply by choosing our units such that it is true. There is nothing more to the value of a dimensionful constant than that. For convenience, we usually use units where c=1.

I suspect what you are more interested in is why the fine structure constant has the value it does. That is unknown at this time:

http://math.ucr.edu/home/baez/constants.html
PeterDonis
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Feb3-12, 08:26 PM
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Quote Quote by DaveC426913 View Post
Here is a way to think of c as a limit.
I hate to throw a spoke into the wheels, but I have to object to this way of thinking of it. The reason I object is this: moving at c is *not* a limiting case of moving at a speed closer and closer to c, while still less than c.

Suppose I'm at rest, and I fire a photon in the x direction, and at the same instant I fire a bullet in the x direction at .9999999999999999c. To me, the photon is only barely outpacing the bullet; so it would seem that if the bullet just moved "a bit faster", it would be keeping up with the photon.

But to the bullet, the photon is moving in the x direction at c, and I am moving in the minus x direction at .9999999999999999c. The bullet is just as much "at rest", and just as far "away from c", as I am, and it can't move at c by just moving a bit faster; it would have to move "c faster" (if that makes sense), just as I would.

The way I would answer emirhasa's question about "why c is c" is this: don't think of c as a speed. Think of it as a conversion factor between "distance" units and "time" units. In "reality", time and distance are in the same units, but because of the way our particular cognitive systems are constructed, we perceive them as having different units, distance in "meters" and time in "seconds", so we have to have a conversion factor between the two. The particular number that the conversion factor turns out to be is a contingent result of how our conventional time and distance units got determined in the first place; it's not really a matter of physics, but of history. [Edit: I see DaleSpam says pretty much the same thing.]
emirhasa
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Feb4-12, 08:27 AM
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Quote Quote by DaleSpam View Post
I suspect that this will not satisfy you, but the answer for this question is simply that we have defined meters and seconds such that the speed of light is 299 792 458 m/s.
Not the answer I searched for I am completely aware of that alone.
Quote Quote by DaleSpam View Post
I suspect what you are more interested in is why the fine structure constant has the value it does. That is unknown at this time:
That is the actual question, that is the answer I was looking for. I thought that the people know the reason behind why it has the value it does because it's what I implied to a 100 times.

And as a reply to the last post, sorry but that's not the answer I was looking for for the 4th time in this thread, you haven't read the previous ones?

Doesn't matter anyway, I guess if it is unknown nowadays I can accept it as such.
DaleSpam
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Feb4-12, 11:40 AM
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Quote Quote by emirhasa View Post
Not the answer I searched for ... that is the answer I was looking for. ... that's not the answer I was looking for for the 4th time in this thread,
Given that you are not, are, and are not getting the answer you are looking for despite asking four times, you may wish to consider that your question is unclear. You may also wish to consider that the answer you are looking for is incorrect.

Based on your reply, I have no idea which, nor if my previous response helped at all.
juanrga
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Feb4-12, 12:16 PM
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Quote Quote by emirhasa View Post
On top of that, I have one more question...
Why doesn't a photon move faster than the speed of light? Why can't it actually move faster? I know the explanation regarding for e.g. some object humanly made or any other why it can't move faster. But why is the photon's limit that particular speed? If it has no mass, it will not gain mass with accelerating. That is, what defines the actual speed of light?
The velocity of an object is given by first Hamilton equation of mechanics v=∂H/∂p

For a photon H=|p|c. Therefore solving the equation of motion for a photon gives |v|=c
nitsuj
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Feb9-12, 11:58 AM
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Quote Quote by DaveC426913 View Post
Likewise, in an unrelated analogy, trying to go faster than c is like gonig to the North pole then trying to go more North. You're not "stopped" from going, there just isn't a "more North" than the North pole.
I have had a simular disagreement with Dalespam, northpole is a location, north is a direction. the direction isn't eliminated because you've reached a location.

Of course I understand what the annalogy is referring too.

It's the line "there isn't anymore north then the north pole" that doesn't sit well with me.

What does the north pole have to do with a north direction? Is it that the direction is relative to Earth, in that all latitude lines point to the north/south, and a "north direction" is a relative term?

Not trying to be argumentative or whatever, just I think you'll be able to better see my perspective and be able to point out what I am getting hung up on with such a simple concept.
DaveC426913
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Feb9-12, 12:35 PM
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Quote Quote by nitsuj View Post
I have had a simular disagreement with Dalespam, northpole is a location, north is a direction. the direction isn't eliminated because you've reached a location.

Of course I understand what the annalogy is referring too.

It's the line "there isn't anymore north then the north pole" that doesn't sit well with me.

What does the north pole have to do with a north direction? Is it that the direction is relative to Earth, in that all latitude lines point to the north/south, and a "north direction" is a relative term?

Not trying to be argumentative or whatever, just I think you'll be able to better see my perspective and be able to point out what I am getting hung up on with such a simple concept.
It is a very loose analogy. It is only meant to point out the very rudimentary concept that it is possible to not be able to get to a certain place, despite the fact that nothing is preventing you from getting there. Most people wonder why you can't "just fly 1mph faster".

I think you get this. The analogy may be beneath you (you are more sophisticated than the audience the analogy is aimed at). But I'll spell it out.


Consider driving in a car. You head "North" and someone tells you "you will eventually reach a point where you will not get any farther North, no matter how much you drive".

You respond with "I don't see what's stopping my car. It's got a huge engine and can drive over anything. As long as the engine is running and the wheels are spinning, I can continue to drive. So what would stop me from driving North as long as I want?"

"Well, nothing's going to get in your way, it's just that there is no more North than the North Pole."

"Well I could always get there and then just drive one more mile, right? No, I can't, can I?"


Compare with:

Consider flying in a spaceship. You accelerate continuously and someone tells you "you will eventually reach a point where you cannot get any faster than c no matter how much you accelerate".

You respond with "I don't see what's stopping my spaceship. It's got a huge engine and can accelerate forever. As long as the engine is running and the rocket is exhausting, I can continue to accelerate. So what would stop me from accelerating as long as I want?"

"Well, nothing's going to get in your way, it's just that there is no faster speed than c."

"Well I could always get there and then accelerate one more mph, right? No, I can't. Ahhhh!"
nitsuj
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Feb9-12, 12:57 PM
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Thanks for the reply DaveC426913,

I understand the analogy, I think it is quite literally the same as a simple spacetime diagram. In the analogy the directions north and west respresting dimensions; one time and one spatial.

It is a good analogy, it represents the space/time relationship well, and you presented it well.

My question is specifically about why it's said "when at the north pole, you cannot go in a north direction.", not at all about relativity, probably more to do with semantics I supose.

And in that case probably not worth much more discussion, just a point of contention I guess.
DaleSpam
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Feb9-12, 03:55 PM
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Quote Quote by nitsuj View Post
What does the north pole have to do with a north direction?
It is the point at which there is no direction which is north. Suppose you are standing at the north pole, how should you stand so that you are facing north?

Quote Quote by nitsuj View Post
Is it that the direction is relative to Earth, in that all latitude lines point to the north/south, and a "north direction" is a relative term?
Yes. "North" is a vector in the tangent space at each point in the manifold, specifically it is the vector corresponding to a positive dθ. Each point in the manifold has a different tangent space so each point in the manfold has a different north vector. Because of the coordinate singularity at the north pole there is no such vector in the tangent space at the north pole.
nitsuj
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Feb9-12, 05:11 PM
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Quote Quote by DaleSpam View Post

Because of the coordinate singularity at the north pole there is no such vector in the tangent space at the north pole.
Ah I see, simple enough.

Thanks for clarifying DaleSpam!


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