# i have a question...

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i have a question... maybe im just crazy, and i have no basis for this quesiton. i have no idea. 8 different physics teachers i have emailed. and nobody believes that i can be right, or they have never answered back:

In science, a vacuum is hated by nature. A true vacuum is perfect nothing. Why we're able to talk about true vacuums if they aren't actually anything is a good philosophy question to bring up next time you're at a party. Scientists have decided that it is impossible to create a true vacuum. First, we can't seem to get rid of every single atom from a jar no matter how hard we try. The fact that energy and mass are equivalent doesn't help. Energy can still get into the best container, so even a jar that has no atoms or subatomic particles still isn't a vacuum because of all the energy flying through it. Quantum mechanics also won't allow a perfect vacuum, because the walls of a vacuum chamber emit blackbody radiation (energy) if they aren't at absolute zero and the uncertainty principle prevents any space from being without any sort of matter. Partial vacuums, which have some matter and energy and do exist in the real world, are often called vacuums though. For example, there is an interstellar medium between stars, but we can functionally call it a vacuum.

It means we can define a quantity c, the speed of light, which is a fundamental constant of nature.

(from an einstein site: http://www2.slac.stanford.edu/vvc/theory/relativity.html)

now if i got my physics lessons right... i thought that:

1) everything had mass, even energy and particles, for anything with no mass is non existent,

2)nothing with mass can move the speed of light,

3) we measured the speed of light in a true vacuum,

and 4) we measure things that move faster than light thru an object (such as fake, manamde elements)....

now somebody correct me... if its true that everything that has mass cannot reach the speed of light, and EVERYTHING, INCLUDING PARTICLES (this would include LIGHT particles) has mass, how can light particles be going the speed of light if its physicaly impossible for anything with mass to do so?

also, if einstein said it can be measured in a true vacuum (again, correct me if im wrong) but a true vacuum is impossible.... how can we accuratley measure the speed of light if particles cant go the speed of light without being in a paradoxical situation ( a true vacuum that can never exist)?

finaly... since i got on the topic of false vacuums, it IS physicaly impossible to create one, (if you dont get it look at 2nd section of post)and since it is impossible i restate my question:

how can we create a true vacuum (impossible), with something that has mass(that cannot go the speed of light) and go the speed of light (thus a paradox) to accuratly measure the speed of light?

can someone help me with this conundrum? i have asked my teachers since freshmen high school, and i have gotten the same answer: einstien cant be wrong....

now i didnt get good grades in physics, but this seems to be a very valid and simple question.. when i brought it up with my fellow classmates, they said it seems like a very obvious flaw, if you seriously think about it.

so can i please PLEASE get an explanation (not claiming to be an expert, just confused)

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The flaw in your reasoning is quite simple: Photons are massless.

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Actually photons have mass. It's given by E=MC^2 and E=hf.

(Energy, mass, speed of light, Planck's constant and frequency)

They are atracted by gravitational fields (The effect is usually refered to as "gravitational lensing"). They also carry momentum and exert radiation pressure.

There is a cop out on the "nothing can travel at the speed of light" What is forbidden is accelerating anything with a rest mass (ie a mass, even when it's stationary) to the speed of light. Photos have zero rest mass (a slightly meaningless idea since a photon at rest doesn't exist).

The fact that we cannot produce a perfect vacuum does not stop us being able to establish the speed of light in a vacuum.

That sounds like a paradox, but it isn't.

We can measure the speed of light in air at one atmosphere pressure, we can do the same at half an atmosphere, a tenth, a thousandth and so on.

Then we can plot a graph of speed vs pressure and draw a straight line through the points. We can carry that line on until it hits the axis for "pressure = zero" and read the value of the speed in a vacuum from that.

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Actually photons have mass. It's given by E=MC^2 and E=hf.

That is not correct. The full form of the equation is

$E^2 = m^2c^4 + p^2c^2$

For a massless particle such as the photon, this reduces

$E = pc$

They are atracted by gravitational fields (The effect is usually refered to as "gravitational lensing").

Per general relativity, gravity curves spacetime. Light follows a geodesic in this curved space. In other words, a light beam curves because space itself is not "straight". That light has been observed to behave this way is taken as confirmation of relativity.

They also carry momentum and exert radiation pressure.

Just because photons have momentum does not mean they have mass. Photons have energy, given by $E = pc$ for a massless particle. Equating this result with the quantum mechanical result $E = hf$ yields the momentum of a massless photon, $p=hf/c$.

There is a cop out on the "nothing can travel at the speed of light"

No cop out. Photons (or any other massless particles) in vacuum travel at the speed of light and at no other speed.

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its hard to argue, photons have mass. but could it be a mass we have no means to understand?

why is it assumed all energy waves move at C. is there an assumed reason why one could not travel faster or slower?

if the means for acceleration is moving at C, what would keep it from performing with in its form?

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its hard to argue, photons have mass.

Photons are massless. But don't just take my word for it.

http://en.wikipedia.org/wiki/Photon

In modern physics, the photon is the elementary particle responsible for electromagnetic phenomena. It mediates electromagnetic interactions and makes up all forms of light. The photon has zero invariant mass and travels at the constant speed c, the speed of light in empty space. However, in the presence of matter, a photon can be slowed or even absorbed, transferring energy and momentum proportional to its frequency.

http://math.ucr.edu/home/baez/physics/Relativity/SR/light_mass.html

Does light have mass?

The short answer is "no", but it is a qualified "no" because there are odd ways of interpreting the question which could justify the answer "yes".

Light is composed of photons so we could ask if the photon has mass. The answer is then definitely "no": The photon is a massless particle. According to theory it has energy and momentum but no mass and this is confirmed by experiment to within strict limits. Even before it was known that light is composed of photons it was known that light carries momentum and will exert a pressure on a surface. This is not evidence that it has mass since momentum can exist without mass.

http://math.ucr.edu/home/baez/physics/ParticleAndNuclear/photon_mass.html

Is there any experimental evidence that the photon has zero rest mass?

If the rest mass of the photon was non-zero, the theory of quantum electrodynamics would be "in trouble" primarily through loss of gauge invariance, which would make it non-renormalizable; also, charge-conservation would no longer be absolutely guaranteed, as it is if photons have vanishing rest-mass. However, whatever theory says, it is still necessary to check theory against experiment.

It is almost certainly impossible to do any experiment which would establish that the photon rest mass is exactly zero. The best we can hope to do is place limits on it. A non-zero rest mass would lead to a change in the inverse square Coulomb law of electrostatic forces. There would be a small damping factor making it weaker over very large distances.

The behavior of static magnetic fields is likewise modified. A limit on the photon mass can be obtained through satellite measurements of planetary magnetic fields. The Charge Composition Explorer spacecraft was used to derive a limit of 6x10-16 eV with high certainty. This was slightly improved in 1998 by Roderic Lakes in a laborartory experiment which looked for anomalous forces on a Cavendish balance. The new limit is 7x10-17 eV.

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As D H has pointed out, two different definitions of mass are being used. One defined by $E = mc^2$ (relativistic mass) and one defined by $E^2 = m^2c^4 + p^2c^2$ (rest or invariant mass)

The problem arises when you try and use them interchangeably. The invariant mass term tends to be more useful, since the mass measurement does not depend on your frame of reference, and is usually what is meant by mass in a physics discussion.

Rigor should demand that you note if you are going to use the relativistic mass instead, but this important step is often omitted.

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why is it assumed all energy waves move at C. is there an assumed reason why one could not travel faster or slower?

Electrodynamics doesn't work, for one. Maxwell's equations require motion at c (as gven by the permeability and permittivity of free space), in all inertial frames, for EM waves.

edit: I see D H has posted a more complete response while I was composing these two answers.

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It looks to me like a matter of definition.

"The photon has zero invariant mass "

The need to qualify the variety of mass that a photon lacks as invariant implies that there is some other sort of mass (presumably variant) that a photon does have.

OK so a photon at rest has no mass. A photon "at rest" does not exist.

(Even allowing for "stationary" photons in Bose Einstein condnsates, if it's "at rest" from your point of view it's doing roughly 1000 MPH from mine because I'm on the other side of a spinning plannet.)

Can anyone explain why that pair of statements is inconsistent with the assertion that no photon which exists has zero mass?

Anyway, the mass or lack of mass of a photon (at rest or otherwise) doesn't matter in terms of answering the question

"how can we create a true vacuum (impossible), with something that has mass(that cannot go the speed of light) and go the speed of light (thus a paradox) to accurately measure the speed of light?"

We don't need a true vaccuum to measure c. (Though actually I think I may have made a mistake in my answer yesterday; I think you need to plot 1/c vs p and extrapolate to p=0 then calculate c)

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yes, i know its said photons have no mass. i don't disagree, but in the sense they can carry, why couldn't this be a form of matter we don't consider. its now said to photon can carry matter itself that we do understand.

yes, i understand that waves travel at C. why C or what keeps them from going C 2. there must be something that prevents anything from traveling faster. sound is limited by means of travel for example.

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Physicists have more-or-less abandoned the concept of relativistic mass. It is messy and unnecessary. Defining relativistic mass as "E=mc2" is superfluous. One might as well just use energy. The relativistic mass does not work with "F=ma". Physicists used to have concepts of longitudinal and transverse mass to enable the use of "F=ma".

Sound waves travel at the speed of sound because of the "stiffness" and density of the underlying medium: $v_{sound}=\sqrt{\frac C \rho}$. There is no aether that "carries" electromagnetic radation. Characteristics of space itself -- the permittivity and permeability of free space -- give rise to the speed of light: $v_{light} = \frac 1 {\sqrt{\varepsilon_0 \mu_0}}$.

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