travelling at the spead of light

[Spaceman Spiff]

i didn't mean for this thread to start any arguments...

Ah, don't worry, it's just four cosmologists discussing the universe and coming up with five (hundred) theories. Bound to happen. Not your fault. Did you get an answer to your first question you understand?

The answer to your second is: depends on how precise your measuring kit is, but I recall in physics that relativistic effects weren't normally considered until 10% - 30% of the speed of light. However, we can tell that satellite clocks run faster than clocks on Earth, but only with super-accurate atomic clocks. It's important for the GPS system.

Sorry, there's some confusion here. Photons DO have kinetic energy : K = h f, where "f" is the frequency and "h" is Planck's constant. However, The relativistic Newton equation do not apply to them as photons do not have an intrinsic mass.

OK, I'm rusty on the ol' physics, but I think the energy of photons is not kinetic energy. Kinetic energy is related to transfering energy into something to make it move. As far as I can see (maybe I'm blind! ) the energy that a photon has is energy tied up in the electric and magnetic fields that are oscillating. That's more like potential energy. I think the only way you can call a photon's energy kinetic here is if you turn the idea of what kinetic energy is on its head.

The only error is the interpretation of m_0. It isn't "Mass initial of something". It's the "proper mass", "true mass", "intrinsic mass" of the particle.

'Rest mass' is a term I remember being used, but I read in another relativity debate that someone claimed the term has fallen from grace on the grounds it was confusing and that there should be only one type of mass, no matter how fast particles went. I'm not sure why. The way I see it is that the mass of a stationary electron is due to some energy bound up in this particle that is neither what we conventionally call kinetic or potential, or even electrical. I've seen this 'rest mass' called 'frozen energy' and maybe what's happened is that more familar types of energy are converted into something we just historically experience as 'mass'. Perhaps for the electron, this 'frozen' energy is related to the electron's spin? *

When you accelerate an electron, it gets heavier because the kinetic energy weighs something, and that appears to make the electron heavier.

Also, the explanation about photon interferences isn't valid, because electrons can interfere too (because of quantum mechanics), and yet they aren't "everywhere at the same time because of their absence of mass".

OK, I threw this in (ooh, ooh, another of those physics debates! ) because I got this 'explanation' from a physics teacher once. Thought I'd throw it to the cats to play with.

I'm still not sure what can make me decide whether that's 'right' or 'wrong.' It's one of the few combinations of relativity with quantum mechanics I've heard of, and since it can't be the case that both theories are true, it might be a clue about the nature of the universe beyond these theories.

At least what about this: cheekily, I point out that we can only observe the electron interference pattern via photons, and electrons can only interact with each other by exchanging photons, and they already know what them electrons done before they done it.

I think alot of the misconception about the status of photonic mass is due to the scientific reluctance to say that Einstein's Theory of Special Relativity is incorrect on the matter.

It's good to be sceptical of Einstein's theories of relativity, even to try disproving them. It's the thing he wanted most...

But photons do have mass, with its observable effects- ...

Photon don't have a mass. But it has energy and momentum ! We CAN have energy without mass !

What is mass, really ? m = E/c^2 ? ...

Hmmm, so one of you's wrong? After many years of reading the same ol' "E = mc?? means that a given mass m converts into energy E," I wonder if popular explanations have got it back to front. I think what this equation really means is that all energy weighs something: m = E/c??.

It's the energy of the photon that has mass.

In turn, this mass distorts spacetime.

Spiff.

* I'm really guessing here!

[BlindedByTheLight]

"Physical meaning" is a subtle notion. What I mean is this :

Equation (1) doesn't have any physical meaning because it's simply a definition. It is not a RELATION between TWO observable things ("m" isn't observable, or measurable if you prefer). Equation (2) IS a relation between two independant entities, which can BOTH be subject to experiments. Just replace (1) into (2) so "m" vanish entirely from the equation. What remains are "E" and "m_0", which are the true observable things here.

Just to make sure I have this right, you are saying that the m_0 is real but m_0 TIMES gamma (to create just m) is not an observable quantity. However, m_0 TIMES gamma DOES have an observable effect on E in E=mc2? Which, if I'm not mistaken, means that the E is relative-motion dependent?

Equation (2) is exact and comes from special relativity. The "m" which is entering this equation is exactly the same as in (1), WHEN APPLIED TO A MASSIVE PARTICLE such an electron. The formula still apply to a photon, but then, the "m" doesn't mean anything. It may be called "mass of the photon", but this is really arbitrary and doesn't have any experimental meaning without gravity.

I understand what you are saying here - that the "m" doesn't apply to a photon... but is there a fundamental reason why this is so? Or just an arbitrary omission to make sure the theory is consistent?

[Cham]

Just to make sure I have this right, you are saying that the m_0 is real but m_0 TIMES gamma (to create just m) is not an observable quantity. However, m_0 TIMES gamma DOES have an observable effect on E in E=mc2? Which, if I'm not mistaken, means that the E is relative-motion dependent?

Equation (2) is exact and comes from special relativity. The "m" which is entering this equation is exactly the same as in (1), WHEN APPLIED TO A MASSIVE PARTICLE such an electron. The formula still apply to a photon, but then, the "m" doesn't mean anything. It may be called "mass of the photon", but this is really arbitrary and doesn't have any experimental meaning without gravity.

I understand what you are saying here - that the "m" doesn't apply to a photon... but is there a fundamental reason why this is so? Or just an arbitrary omission to make sure the theory is consistent?

It is so because, for a photon, m_0 -> 0 while "gamma" -> infinity (v -> c). The product gamma times m_0 is then undefined.

You must understand clearly that the "m" in formula (1) is just an intermediate quantity. In itself, it doesn't have a meaning. Lets take another example. Suppose in classical mechanics you write the kinetic energy as

K = mv^2/2

Then, someone comes and write the exact same formula as

K = m c^2 Exp(J) /2

where he defined J = 2 Log(v/c) for some reason.

Then, will you say that the quantity "J" in this equation has a fundamental character ? It's just an intermediate quantity without any physical meaning. Okay, this is just an artificial example, but it's the same for "m" in equation (1). We can go without it, it's useless and it's confusing. A lot of people get wrong interpretations because of this "m". What we should write are these formulas :



See ? No "m" there.

Because of the appearence of "m_0", those formulas cannot be applied to photons. To them, we must apply the (5) and (6) formulas.

[BlindedByTheLight]

Thank you, Cham. I think I understand what you're saying (if this is correct):

The product of m_0 (light's mass or lack thereof) TIMES gamma is meaningless because, at the speed of light, gamma is an undefined infinity.

However, I'm still a bit fuzzy on what all this has to do with whether or not light has any m_0 to begin with.

Certainly, if you just say... "Well, it has been measured... and has no mass. End of story" I can accept that. But working from the theory I'm a bit confused why this should be so - and why the meaningless infinities couldn't just be a reflection of a limitation of the theory vis-a-vis light?

In other words, for the sake of argument, let's say light has a tinsy little m_0 mass... but when you try and plug that into Einstein's:

E = gamma TIMES m_0 TIMES c squared

... and the forumla fails... could it not simply be said that it's the formula's problem? And not ask light to adjust its fundamental nature so the formula works?

I say this because it seems to me that, if light has zero m_0 then that in and of itself would render Einstein's formula moot as far as photons go EVEN BEFORE one got to the meaningless infinities from a speed-of-light gamma... since clearly light DOES have energy and timesing by zero is zero.

P.S. Thank you for forumla 3b... I always wondered how relativistic changes fit into the momentum formula.

[Cham]

The product of m_0 (light's mass or lack thereof) TIMES gamma is meaningless because, at the speed of light, gamma is an undefined infinity.

Certainly, if you just say... "Well, it has been measured... and has no mass. End of story" I can accept that. But working from the theory I'm a bit confused why this should be so - and why the meaningless infinities couldn't just be a reflection of a limitation of the theory vis-a-vis light?

In other words, for the sake of argument, let's say light has a tinsy little m_0 mass... but when you try and plug that into Einstein's:

E = gamma TIMES m_0 TIMES c squared

... and the forumla fails... could it not simply be said that it's the formula's problem? And not ask light to adjust its fundamental nature so the formula works?

[Cham]

The product of m_0 (light's mass or lack thereof) TIMES gamma is meaningless because, at the speed of light, gamma is an undefined infinity.

Certainly, if you just say... "Well, it has been measured... and has no mass. End of story" I can accept that. But working from the theory I'm a bit confused why this should be so - and why the meaningless infinities couldn't just be a reflection of a limitation of the theory vis-a-vis light?

In other words, for the sake of argument, let's say light has a tinsy little m_0 mass... but when you try and plug that into Einstein's:

E = gamma TIMES m_0 TIMES c squared

... and the forumla fails... could it not simply be said that it's the formula's problem? And not ask light to adjust its fundamental nature so the formula works?

Well, of course, relativity theory is a scientific theory after all, and some experiments may find it is wrong. But I seriously doubt it, because a HUGE amount of data fits the theory perfectly now. If an experiment shows the contrary, we may suspect today that the experiment itself was badly done, or data has been wrongly interpreted, or some effect wasn't taken into account by the experiment authors, or that the error bars are wrongly defined, etc...

Remember, the photon (until now) has m_0 = 0 AND v = c, so the product gamma TIMES m_0 is ill defined.

If we trully find photons to have a tiny mass, guess what ? It wont even put the relativity theory into trouble, like the public believe. We will just have to adjust the basic evolution equations (Maxwell equations) by adding a parameter in them (the value of m_0). According to relativity theory, if the photon has a small mass, it could not travel at exactly the velocity v = c. it would have a velocity v < c and then gamma isn't infinite. The product gamma TIMES m_0 will be legitimate then.

But then, you'll ask what is the "c" parameter ? If light has a small mass and its velocity is v (smaller than c), what the hell is "c" ? That's the funniest thing about relativity. In this theory, "c" ISN'T THE SPEED OF LIGHT ! It's a fundamental universal constant inscribed into the fabric of vacuum itself ! It just happens that, if a particle has a rest mass m_0 = 0, then IT HAS to travel at the velocity v == c. It is not a coincidence that light is traveling at the velocity v = c, because its mass is so tiny, that's all !

[BlindedByTheLight]

Thank you, again, Cham. VERY interesting stuff - especially about c and its relationship to lightspeed. One final thing, though - since I think my initial question was not fully addressed:

Well, of course, relativity theory is a scientific theory after all, and some experiments may find it is wrong. But I seriously doubt it, because a HUGE amount of data fits the theory perfectly now. If an experiment shows the contrary, we may suspect today that the experiment itself was badly done, or data has been wrongly interpreted, or some effect wasn't taken into account by the experiment authors, or that the error bars are wrongly defined, etc...

Remember, the photon (until now) has m_0 = 0 AND v = c, so the product gamma TIMES m_0 is ill defined.

I wasn't suggesting that "what if" we find out the photons have a tiny mass. I was asking WHY we say photons do NOT have any m_0.

In other words, I see three options:

1) Einstein's theory itself suggests light has no mass
2) We have experimentally come to this conclusion
3) Both

I ask this because you seemed to be suggesting (in earlier posts) that Einstein's formula indicated photons could not have any m_0. Though now it seems you are saying the theory COULD accomodate an small m_0 without much of a problem.

So, then, why exactly do we say light has no mass? Experimentally-observed? Or predicted by theory? Or both? And, if predicted by theory... Einsteins? Maxwells?

Thanks

[d.m.falk]

In other words, for the sake of argument, let's say light has a tinsy little m_0 mass... but when you try and plug that into Einstein's:

E = gamma TIMES m_0 TIMES c squared

... and the forumla fails... could it not simply be said that it's the formula's problem? And not ask light to adjust its fundamental nature so the formula works?

I say this because it seems to me that, if light has zero m_0 then that in and of itself would render Einstein's formula moot as far as photons go EVEN BEFORE one got to the meaningless infinities from a speed-of-light gamma... since clearly light DOES have energy and timesing by zero is zero.

And thus you hit on what I was saying in the first place: The continuous attempts to fit the anomalous nature of photons within Einstein's Theory of Special Relativity, rather than trying to quantise the nature of photons themselves. A better way of putting it is that Einstein was on the right track, but came up with the wrong conclusion. Yes, photons have energy, and thus they have mass. This is observable, and has been measured. However, it conflicts with Einstein, and just as you observed, you can't really have something from nothing- The exchange between mass and energy exists on all levels. A photon's mass is just marginally above that of a quark's. Heck, I'd even say that it's best to state that mass and energy are just two states of matter, which would be accurate, giving the mutual exchange that usually results between both, and the energy for the exchange is subtracted from the total. (This is why, for what it's worth, we have atomic bombs.)

Like I said, we're still learning- We don't have all the answers yet, and we probably never will. Most likely, we've barely scratched at understanding our Universe!

d.m.f.
(Been arguing about photon mass since I was in 8th Grade, almost 30 years ago... That an observable mass has been measured in my lifetime satisfies me. That people still can't accept that empirical data conflicts Einstein on this matter bothers me. That we haven't accepted that c does not equal infinity, and thus at c, you're not in all places at once (or all places along the trajectory) or everythiung stops- Light takes a hellova long time to get from one place to another! Our owqn Sun is a mere 8 minutes away- And 93 million miles is NOT that far away!)

[BlindedByTheLight]

I don't mean to sidetrack you, Cham, with this post... I'm still very curious about my prior post but...

That people still can't accept that empirical data conflicts Einstein on this matter bothers me

d.m.falk... I believe that Cham's earlier point was that, even in the event empirical data shows that photons have mass (does it? anyone else care to chime in?) - Einstein's theories can still accomodate that.

Oh - Cham... just thought of something. Guess I'll throw it in here. As noted, I'm trying to see how/what a theory predicts (don't really care whether or not it's ultimately true...)

But, assuming, for the sake of argument, that photons do really travel at speed c... are you saying that Einstein's E=MC2 formula would predict they have no mass because a zero'd M would be required to cancel out the undefined infinity from a c-velocity gamma? Else, the result would be nonsensical?

I guess my math skills are rusty on this point. I guess the deeper mathematical question is, if there is a undefined number like the kind that can appear with gamma (at the high end of the velocity spectrum)... does that render the whole equation moot? Or can you deal with it mathematically by TIMESING it by a zero - rendering the undefinable part a moot point anyway? I guess if the answer is yes (is it?) that would mean that Einstein's formula (ultimately true or false) IMPLIES that photons have zero mass?

[Cham]

I wasn't suggesting that "what if" we find out the photons have a tiny mass. I was asking WHY we say photons do NOT have any m_0.

In other words, I see three options:

1) Einstein's theory itself suggests light has no mass
2) We have experimentally come to this conclusion
3) Both

I ask this because you seemed to be suggesting (in earlier posts) that Einstein's formula indicated photons could not have any m_0. Though now it seems you are saying the theory COULD accomodate an small m_0 without much of a problem.

So, then, why exactly do we say light has no mass? Experimentally-observed? Or predicted by theory? Or both? And, if predicted by theory... Einsteins? Maxwells?

Thanks

Well, the photon has a 0, or near zero, rest mass because of option 3). When Einstein made his special relativity theory, the photon wasn't yet known at that time. Light was only of a wave nature, until Einstein himself suggested its quantum nature. Whatever the nature of light, He accepted the empirical fact that light travel in vacuo at the velocity "c" and appeared to be the same whatever the source and the observer state of motion. Today, relativity theory strongly suggest (but doesn't prove) that photons should travel at the velocity "c". But the theory can easily accept the other way (v < c) if experiment gives a tiny mass to the photon. It just happened, historically, that Einstein discovered the "c" constant inscribed in vacuo using the properties of light, but this is just an happy coincidence. Today, many physicists believe we should change the name of that "c" constant. "c" is the "Einstein constant", not light speed.

To respond to your query : why is the photon has a m_0 = 0 proper mass ? We currently don't know ! Until now, it's just an empirical fact. Why is the electron having that particular mass m_0 != 0 is also a mystery. We know it's not just the electrical charge, it's not the spin, it's ... something else. It's not gravity, maybe the strings, we just don't know yet (don't talk to me about strings, I hate this theory and I just don't "believe" in it). Actually, the origin of inertia is the most profond mystery in all of physics and the source of all troubles. Inertia and vacuum (proper mass and origin of the fundamental constants, that is) is where the real physics is hidding.

EDIT : This brings me to my personal obsession. The ultimate mystery : Inertia, Vacuum and the physical Constants of nature. Here's a small addon I made last year to show the fundamental constants of nature on the faces of an empty cube (vacuum inside).

[BlindedByTheLight]

Thanks, Cham. I saw that add-on before. Very cool. As for inertia... well, that was my next question. You mean I'm going to have to wait for the answer! Yuck...

Anyhow, when you wrote:

Today, relativity theory strongly suggest (but doesn't prove) that photons should travel at the velocity "c".

Is that a fairly difficult to understand suggestion? Or is it simply the suggestion I alluded to before... that it appears that light travels at speed c and, therefore, the m_0 must be zero so E=MC2 doesn't become non-sensical?

[Cham]

Is that a fairly difficult to understand suggestion? Or is it simply the suggestion I alluded to before... that it appears that light travels at speed c and, therefore, the m_0 must be zero so E=MC2 doesn't become non-sensical?

Yes, you may look at it this way. But the suggestion comes more strongly from Maxwell theory (which is fully relativistic) and its conformal (scale) invariance. Conformal invariance is a very nice mathematical symetry, but it would be totally ruined if the photon had a mass.

This is related to inertia. Masses ruins the conformal symetry of equations. Physics would be more "beautifull" with conformal invariance as a general symetry of nature. Why is there any mass out there is part of the mystery. But it just happens that some equations (Maxwell) are scale invariant.

[GoHeelsWeeeehoooo]

does anyone here think we will ever send people to another star or will it prove to either be just oo long a trip or too difficule to do, etc

[d.m.falk]

I don't mean to sidetrack you, Cham, with this post... I'm still very curious about my prior post but...

That people still can't accept that empirical data conflicts Einstein on this matter bothers me

d.m.falk... I believe that Cham's earlier point was that, even in the event empirical data shows that photons have mass (does it? anyone else care to chime in?) - Einstein's theories can still accomodate that.

And I never said his theory was fundamentally wrong; only that the conclusion is incorrect.

Oh - Cham... just thought of something. Guess I'll throw it in here. As noted, I'm trying to see how/what a theory predicts (don't really care whether or not it's ultimately true...)

But, assuming, for the sake of argument, that photons do really travel at speed c... are you saying that Einstein's E=MC2 formula would predict they have no mass because a zero'd M would be required to cancel out the undefined infinity from a c-velocity gamma? Else, the result would be nonsensical?

I guess my math skills are rusty on this point. I guess the deeper mathematical question is, if there is a undefined number like the kind that can appear with gamma (at the high end of the velocity spectrum)... does that render the whole equation moot? Or can you deal with it mathematically by TIMESING it by a zero - rendering the undefinable part a moot point anyway? I guess if the answer is yes (is it?) that would mean that Einstein's formula (ultimately true or false) IMPLIES that photons have zero mass?

Even the most basic of mathematics will tell you that anything multiplied or divided by zero is zero.

d.m.f.

[Cham]

Even the most basic of mathematics will tell you that anything multiplied or divided by zero is zero.

d.m.f.

Ooops ! That one is very funny !

[BlindedByTheLight]

Even the most basic of mathematics will tell you that anything multiplied or divided by zero is zero.

Thanks d.m.f... I figured that anything that was multiplied by zero was zero... I just wasn't sure if the presence of a term that is nonsenical (ie. gamma at lightspeed) rendered the whole equation moot.

However, is something that is DIVIDED by zero, zero?


GoHeelsWeeeehoooo: will we reach other stars? are they too far? who's to say? even if we can never travel faster than the space shuttle, there is no fundamental reason we still cannot reach the stars - either through suspended animation, generational ships or just by building progressively farther space stations and "island hopping" to the next star. Easier said than done... but not impossible. Even if these journey's take hundreds, thousands, or millions of years... what else are we going to do? The time will pass anyway - it could just as easily pass on an inter-stellar voyage.

Whether there is political or social will for this to occur is another matter entirely.

Of course, that is all assuming there are no fundamental breakthroughs in propulsion systems. If we were to be able to radically increase our speeds, however... the journey can be trimmed down to years, not thousands of years.

Furthermore, there might (might!) be some as-yet-unknown understood mechanism to allow faster-than-light travel. Wormholes, warp speed-type stuff, things we haven't even considered. From a layman's POV, I do not believe there is much hope, however, given our current theoretical understanding of how the universe works. But any good scientist will probably tell you... you never can speak in absolutes on such things. I'm sure others will weigh in on this, matter, however.

To answer your question simply... will WE (the users of the this forum) go to other stars? I wouldn't bet on it.

Steven

[Cham]

To answer your question simply... will WE (the users of the this forum) go to other stars? I wouldn't bet on it.

Steven

I can fly directly to any star in the galaxy right away. My spaceship engine is Celestia driven.

[d.m.falk]

Well, the photon has a 0, or near zero, rest mass because of option 3).

"Near-zero" is not "zero"- It either has mass or it does not.

To respond to your query : why is the photon has a m_0 = 0 proper mass ? We currently don't know ! Until now, it's just an empirical fact. Why is the electron having that particular mass m_0 != 0 is also a mystery. We know it's not just the electrical charge, it's not the spin, it's ... something else. It's not gravity, maybe the strings, we just don't know yet (don't talk to me about strings, I hate this theory and I just don't "believe" in it). Actually, the origin of inertia is the most profond mystery in all of physics and the source of all troubles. Inertia and vacuum (proper mass and origin of the fundamental constants, that is) is where the real physics is hidding.

We know now that all particles have mass, and all forces do not. Even the electron's mass was known well before I was born (of which I'll be 40 in two months). Even quarks have mass. Gluons do not, however, because they're not particles, but rather forces.

(See, it doesn't mean Einstein's theory is wrong, but how it's interpreted that's gotten sticky.)

d.m.f.

[BlindedByTheLight]

We know now that all particles have mass, and all forces do not. Even the electron's mass was known well before I was born (of which I'll be 40 in two months). Even quarks have mass. Gluons do not, however, because they're not particles, but rather forces.

(See, it doesn't mean Einstein's theory is wrong, but how it's interpreted that's gotten sticky.)

d.m.f.

Is not the photon simply the means by which the electromagnetic force is expressed?

So would that mean, if the statements in the above quote are true, that a photon does NOT have mass?

[d.m.falk]

does anyone here think we will ever send people to another star or will it prove to either be just oo long a trip or too difficule to do, etc

Short answer: Yes.

Long answer: It'll take some time, but by the end of this century, we should be capable of getting interstellar probes out at a significant fraction of the speed of light- At least 10% c, and maybe as high as 50%, which should make manned flight possible... Lengthy, but possible.

Perhaps early in the 22nd Century, we may be able to feasably "warp" through space, which actually means that the direct speed might never come close to the speed of light, but that we can "shorten" the distance by simply warping space i9tself, giving the convenient effect of faster-than-light travel, without violating relativity as we know it. Only now, we can deal with the math, but we don't have the technology, and won't until after practical quantum computers are built in the next few decades.

d.m.f.