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## Main Question or Discussion Point

Just was wondering if there was something that pushed photons or if it just a property of light. Also what happens to photons from other stars when they reach our sun?

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Just was wondering if there was something that pushed photons or if it just a property of light. Also what happens to photons from other stars when they reach our sun?

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mathman

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jcsd

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selfAdjoint

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Now suppose p = 0; then the particle has no momentum => you are looking at it in its rest frame and the equation reduces to [tex]e = mc^2[/tex] which you might have seen before.

On the other hand suppose m=0; in this case the particle is massless and the equation reduces to [tex]e = pv[/tex], the energy is the size of the three-momentum times the speed of light. Plug this into the Lorentz transformation for energy and you get the velocity of the particle is c.

So these two predictions come from the relativistic energy equation, the energy of a particle at rest is [tex]mc^2[/tex] and the speed of a massless particle is c.

Of course in more general states you don't have either of these conditions, but the energy equation is still true.

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we know that nothing can travel faster than the speed of light.....but what is it about light that gives it this special status?? i mean it could have been sound...could have been some other thing......n if we if we think logically then anything travelling opposite to the direction of light at velocity "v" will see light going away from it at an speed " c+v"......i know this point has been time n again been said to be false when light comes into picture......but some how 1+1 will always be 2!!

regards, niranjan

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Nothing needs to be pushed for it to move. The speed of light can be calculated from Maxwell's equations. Seejackpot337 said:Just was wondering if there was something that pushed photons or if it just a property of light. Also what happens to photons from other stars when they reach our sun?

http://www.geocities.com/physics_world/em/mawell_eq.htm

It starts with the permittivity and permiability of free space. They appear in Maxwell's equations as the reciprocal of the the product of the two. That is written as the square of a quantity which is labeled "c". The equations for the E and B fields are then shown to be the solution of a wave equation and c is the speed of the wave.

Pete

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Not true. Photons have a mass, just theoretically no rest mass. So, particles with mass can travel at c as long as they have no rest mass.jcsd said:

niranjan, I like the way you think. Light is relative to the object, but physicists always find some mathematical way to prove it is constant. Simple logic says that if you are moving towards a light source the velocity of light would be dependent on that source.

[tex]\vec{v}_{light}=c+\vec{v}_{observer}[/tex]

Or if the object is moving away from the light source

[tex]\vec{v}_{light}=c-\vec{v}_{observer}[/itex]

Maybe they are the other way around, but logic says light speed should be dependent on velocity of the source of the observer or light source.

So, neutrinos are said to not travel at the speed of light? So, this means they have a rest mass, right?

Sorry, mentors, if my expression of opinion shouldn't go here but rather in a thread in Theory Development.

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marcus

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so at some level there is this basic assumption or observation about the universe:selfAdjoint said:You can derive the relativistic equation for energy [tex]e^2 = p^2c^2 + m^2c^4[/tex]. Here p is the magnitude of the three dimensional momentum, m is the invariant mass of the particle, and c, of course is the speed of light. I repeat that this equation follows from the postulates, the basic definition of relativity.

...

On the other hand suppose m=0; in this case the particle is massless and the equation reduces to [tex]\inline e = pc[/tex], the energy is the size of the three-momentum times the speed of light...

"there is a certain speed that is the same to all observers"

nothing said about light or massive or massless or any kind of boson, that would be too clunky----just that the universe has a preferred scale of speed that reads the same in all frames

later, after some algebra and thinking about particulars, we learn that massless things actually GO this speed, and that, as it so happens, light goes this speed

Correct me if I am wrong, but it seems as if the basic postulate does not even assume that anything travels at this speed, not to mention saying anything about light. I just says that in the universe there IS this speed which looks the same regardless of your perspective.

and the equations you mention derive from that, by some algebra

I'm sure this is oversimplifying, but it is a striking idea and immediately prompts one to ask "what if there were two invariant scales instead of just one?" Are there maybe several quantities in the universe that look the same to all observers---a speed and at least one other type of physical quantity.

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wolram

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I'm sure this is oversimplifying, but it is a striking idea and immediately prompts one to ask "what if there were two invariant scales instead of just one?" Are there maybe several quantities in the universe that look the same to all observers---a speed and at least one other type of physical quantity.

----------------------------------------------------------------------------------------------------

So what could this other type of physical quality be, the best i could

think of is a" physical meter", yard stick, universal mile ,what ever,

im sure time is irrelevant to physical quality, so what is missing?

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selfAdjoint

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hemmul

great!marcus said:"there is a certain speed that is the same to all observers"

good question. actually it brought me to a slightly crazy idea right away: from this algebra, it can also be seen, that it is impossible tomarcus said:I'm sure this is oversimplifying, but it is a striking idea and immediately prompts one to ask "what if there were two invariant scales instead of just one?" Are there maybe several quantities in the universe that look the same to all observers---a speed and at least one other type of physical quantity.

Does it make any sense?

One of the "difficult" questions about that is "why do we

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chroot

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chroot

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No,urtalkinstupid said:Simple logic says that if you are moving towards a light source the velocity of light would be dependent on that source.

[tex]\vec{v}_{light}=c+\vec{v}_{observer}[/tex]

Or if the object is moving away from the light source

[tex]\vec{v}_{light}=c-\vec{v}_{observer}[/itex]

Maybe they are the other way around, but logic says light speed should be dependent on velocity of the source of the observer or light source.

The bottom line is simply that your common sense is based upon observations of things around you (chairs, desks, people and so on) that are not moving very fast at all. You have no common sense about things moving near the speed of light. You cannot therefore rely on common sense beyond its limitations. Experiments show us how the universe works, not common sense.

And, no, your post does not belong here at all. Please don't make a habit of posting non-mainstream theories in the wrong forums here.

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This is considered to be always equal to 3 x 10^8 m/s in a vacuum for all wavelengths of light.

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chroot

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[tex]c = \frac{1}{\sqrt{\epsilon_0 \mu_0}}[/tex]

http://www.google.com/search?num=30&hl=en&lr=&ie=UTF-8&safe=off&q=1+/+sqrt(the+electric+constant+*+the+magnetic+constant)+=&btnG=Search

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marcus

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A physicist once explained in my hearing thatchroot said:

[tex]c = \frac{1}{\sqrt{\epsilon_0 \mu_0}}[/tex]

http://www.google.com/search?num=30&hl=en&lr=&ie=UTF-8&safe=off&q=1+/+sqrt(the+electric+constant+*+the+magnetic+constant)+=&btnG=Search

- Warren

[tex]\mu_0[/tex]

and

[tex]\epsilon_0[/tex]

are not real physical quantities

they are not actual measurable properties of empty space

but are fictions, their formal existence established by convention within the SI system

therefore according to him it does not explain why the speed of light is c

if one merely demonstrates that putting [tex]\mu_0[/tex]

and

[tex]\epsilon_0[/tex]

into Maxwell equations causes one to get out a speed which is c

So I have come to regard this equation:

[tex]c = \frac{1}{\sqrt{\epsilon_0 \mu_0}}[/tex]

as a kind of tautology or a stacked deck

and then the meter got redefined as the distance light in vacuo travels in

exactly 1/299792458 of a Cesium133 atomic clock second which means if you use metric units the speed of light cannot logically be other than

299792458 of those meters per second. It is therefore fruitless to ask why it is what it is. It is the standard speed. (for the metric system and probably for mother universe as well)

I guess maybe the thing to ask is not why is it what it is (why is it 299792458 instead of 299792459?)

but to ask why is it constant? why is it AFAWK the same everywhere in universe? why is it the same for all observers even if the buggers are moving vis-a-vis us? why is there a constant speed standard in the U?

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What other properties can spacetime have? What you're suggesting is that anything other than dimensions and how they curve is an artificial device? Can this be right? Can you define fields (scalar or vector) on spacetime without it being a property of the background without being arbitrary? In other words, where could such fields come from if they are not derived from spacetime itself? And if they can not be derived from the spacetime background, can they be real?

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chroot, take a look at this. I'm sure you won't think much of it. It makes sense to me. After reading Einstein's work and reading this. I've found the link I'm fixing to provide you to make more sense. That's just me though. :rofl: Here it is

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Nereid

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Sorry, mentors, if my expression of opinion shouldn't go here but rather in a thread in Theory Development.

chroot said:And, no, your post does not belong here at all. Please don't make a habit of posting non-mainstream theories in the wrong forums here.

I think chroot includes material such as that in 'aliceinphysics' as non-mainstream theories. If you would like to discuss the misunderstanding and misrepresentation of relativity that are evident in 'aliceinphysics', please start a thread in Theory Development.urtalkinstupid said:

chroot, take a look at this. I'm sure you won't think much of it. It makes sense to me. After reading Einstein's work and reading this. I've found the link I'm fixing to provide you to make more sense. That's just me though. :rofl: Here it is

You might also look through previous posts in PF; IIRC 'aliceinphysics' has been clearly shown to be inconsistent with observational and experimental results.

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chroot

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You absolutely must stop posting personal theories and crackpot websites to the general physics forums. Consider this your final warning.

If you'd like to discuss your train "paradox," please provide a little more detail -- about where the light is coming from, etc. -- and I will be happy to show you how there is no paradox at all.

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chroot

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The simple fact is that tens of thousands of people in the scientific community have thoroughly examined the theory of relativity for over a century, and there are no paradoxes in it. Any paradoxes you claim to find are only evidence of your own misunderstandings.

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urtalkinstupid, I dont understand why u are trying to say that light speed isnt constant. It has been proven experimentally sereral times. For example, light coming from stars has been measured from different parts of the world, and every time the speed of light ended up being the same exact number. You might think it might be simply because the earth's orbiting speed is so little compared to the speed of light...well it doesnt matter because chroot pointed out that light has been measured from pions moving at nearly the speed of light, the speed of light measured was once again the same. If light speed wasnt constant, then the time dialation equation predicted by relitivity would be wrong. But the fact is that the model for time dialation is very right and has been proven several times with precise atomic clock. So please, just dont argue about something that has been proven true in so many ways and so many times.

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chroot

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He's arguing against constant c not because he isn't aware of the experimental evidence; he's arguing only because to him it doesn't make "sense." As he's said, he is not concerned with reality.

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