Speed of light and dark matter

[ravikannaujiya]

what you say

This is not true. You can have any two of tachyons, relativity, and causality. So tachyons do not by themselves contradict relativity.

you can read below the animation of right side of this page on wikiapedia
http://en.wikipedia.org/wiki/Tachyon
now I would not say anything.

 

[Dale]

that's what wikipedia says:
...which (according to special relativity) would lead to violations of causality.[3] Potentially consistent theories that allow faster-than-light particles include those that break Lorentz invariance, the symmetry underlying special relativity, so that the speed of light is not a barrier.

Which agrees exactly with what I said in post 30 and does not support in any way what you said in post 3. The first bold says that if you have relativity and tachyons then you violate causality. The second bold says that if you have tachyons and causality then you violate relativity. As I said, you can have any two of tachyons, relativity, and causality.

You still have been completely unable to produce any reference even remotely supporting your comments of post 3. Do you now rescind those comments?

 

[ravikannaujiya]

I am still on the third post. But when you are trying to misinterpret the written word, then what can I say.
if you are saying that tachyon and relativity can exist together but then causality will be voileted.
tachyon and causaltiy can exist together but the relativity will be voiletd. Isn't it silly. when relativity always lead to causality.Outcome of relativity is cuasality. If you had any sense you can think that relativity and causality belong to one set of theory. One leads to another and have to satisfy each other . and you say tachyon may be with one of them and not with the other at the same time.
I gave you reference. The books I told you is a authentic science book, a book based on qualitative physics and you can not discard it by just saying "not a valid science reference". The book talks abou underlying philosophy behind the natural sciences. And if you disregard that, it does not mean that the book has no value.
and if you are right why we do not measure the speed of light (V+c) moving against the direction of light, instead we are just able to measure c.

 

[Parlyne]

I am still on the third post. But when you are trying to misinterpret the written word, then what can I say.
if you are saying that tachyon and relativity can exist together but then causality will be voileted.
tachyon and causaltiy can exist together but the relativity will be voiletd. Isn't it silly. when relativity always lead to causality.Outcome of relativity is cuasality. If you had any sense you can think that relativity and causality belong to one set of theory. One leads to another and have to satisfy each other . and you say tachyon may be with one of them and not with the other at the same time.

This is simply not correct. At its most basic, all that special relativity states is that the spacetime interval between any two events is the same in all inertial frames of reference, where the spacetime interval is defined to be
$$s = (c\Delta t)^2 - |\Delta \vec{x}|^2.$$
That's it. This only implies causality if you add the presumption that nothing moves faster than c. It does, however, tell you that nothing can move slower than c in one frame and faster than it in another; but, that does not preclude objects which move faster than c in all frames. And, in fact, the math allows for that possibility.

I will also note, in reference to several previous posts, that one does not, in fact, need to presume the invariance of the speed of light. In the formulation above, c is just a conversion factor between units of time and those of space. That it is the speed at which light moves, in this way of looking at things, can be derived from the masslessness of light.

 

[ravikannaujiya]

This is simply not correct. At its most basic, all that special relativity states is that the spacetime interval between any two events is the same in all inertial frames of reference, where the spacetime interval is defined to be
$$s = (c\Delta t)^2 - |\Delta \vec{x}|^2.$$
That's it. This only implies causality if you add the presumption that nothing moves faster than c.

Yes, it implies not only causality but this is also a Lorentz invariance which is the symmetry underlying special relativity. If Lorentz invariance breaks down, I don't think SR would work. The simple reason is that the symmetry is broken.

It does, however, tell you that nothing can move slower than c in one frame and faster than it in another; but, that does not preclude objects which move faster than c in all frames. And, in fact, the math allows for that possibility.

If we are talking about only math then fine. But physics would not allow to do it, if laws of physics are same everywhere.

I will also note, in reference to several previous posts, that one does not, in fact, need to presume the invariance of the speed of light. In the formulation above, c is just a conversion factor between units of time and those of space. That it is the speed at which light moves, in this way of looking at things, can be derived from the masslessness of light.

Pls just think of consequences in nature and laws of physics if speed of light need not to be invariant. Masslessness of the light resides in the nature of light itself, as it has zero rest mass but has momentum given by hλ or h*neu/c.
if you use relativistic mass formula, then
realtivistic mass m_v= rest mass/(√1-(v/c)²)
Now you can find what kind of mass you get if you put v=c or v > c.
for any patricle if you put v=c, you get that is not defined in mathematics.
If you choose V > c, you get that is not permisssible for mass in real world of physics.

 

[Dale]

I am still on the third post.

OK, please return and post when you believe you have some support for your incorrect claims of post 3.

But when you are trying to misinterpret the written word, then what can I say.
if you are saying that tachyon and relativity can exist together but then causality will be voileted.
tachyon and causaltiy can exist together but the relativity will be voiletd. Isn't it silly. when relativity always lead to causality.Outcome of relativity is cuasality. If you had any sense you can think that relativity and causality belong to one set of theory. One leads to another and have to satisfy each other . and you say tachyon may be with one of them and not with the other at the same time.

I disagree, but it is irrelevant.

The point isn't whether or not tachyons can exist, the point is that our measuring devices could, in principle, measure their speed if they did. I.e. our measuring devices are not inherently limited to measuring v<c, as you incorrectly claimed in post 3. We can, in fact, construct devices such that speeds > c can be accurately measured, regardless of whether or not anything actually exists which can travel that fast.

if you are right why we do not measure the speed of light (V+c) moving against the direction of light, instead we are just able to measure c.

Because in the reference frame of the measuring device the speed of light "against the direction of light" is c, not V+c.

Notice how your logic fails if you go "with the direction of light". In that case Newtonian physics predicts the speed of light to be c-V, and our measuring devices can clearly measure c-V. So, if it were merely a limitation of the device that is causing the non-Newtonian measurement then measurements not subject to the limitation should be Newtonian. But they are not, because in the reference frame of the measuring device the speed of light "with the direction of light" is c, not c-V.

 

[ravikannaujiya]

Yes our measuring device cannot give different value for speed of light irrespective of wherever we move.
But anything that moves faster than light could not be measured until and unless light from that object reaches to the measuring device as it would be simply unobservable object. That is why chereknov effect comes. When any particle moves faster than light, it produces a light waves (Chereknov radiation) that come before the object to measuring device. If light waves reach before the object how one can conclude that object moved faster than light.

 

[Dale]

But anything that moves faster than light could not be measured until and unless light from that object reaches to the measuring device as it would be simply unobservable object. ... If light waves reach before the object how one can conclude that object moved faster than light.

See my post 28 for a detailed example.

 

[ravikannaujiya]

the example of Tachyon that you took do not consider the Chereknove effect produced by it. Wikipedia also says that, "Because a tachyon would always move faster than light, we would not be able to see it approaching. After a tachyon has passed nearby, we would be able to see two images of it, appearing and departing in opposite directions. The black line is the shock wave of Cherenkov radiation, shown only in one moment of time. This double image effect is most prominent for an observer located directly in the path of a superluminal object (in this example a sphere, shown in grey). The right hand bluish shape is the image formed by the blue-doppler shifted light arriving at the observer—who is located at the apex of the black Cherenkov lines—from the sphere as it approaches. The left-hand reddish image is formed from red-shifted light that leaves the sphere after it passes the observer. Because the object arrives before the light, the observer sees nothing until the sphere starts to pass the observer, after which the image-as-seen-by-the-observer splits into two—one of the arriving sphere (to the right) and one of the departing sphere (to the left).

 

[Nugatory]

the example of Tachyon that you took do not consider the Chereknove effect produced by it. Wikipedia also says that, "Because a tachyon would always move faster than light, we would not be able to see it approaching. After a tachyon has passed nearby, we would be able to see two images of it, appearing and departing in opposite directions. The black line is the shock wave of Cherenkov radiation, shown only in one moment of time. This double image effect is most prominent for an observer located directly in the path of a superluminal object (in this example a sphere, shown in grey). The right hand bluish shape is the image formed by the blue-doppler shifted light arriving at the observer—who is located at the apex of the black Cherenkov lines—from the sphere as it approaches. The left-hand reddish image is formed from red-shifted light that leaves the sphere after it passes the observer. Because the object arrives before the light, the observer sees nothing until the sphere starts to pass the observer, after which the image-as-seen-by-the-observer splits into two—one of the arriving sphere (to the right) and one of the departing sphere (to the left).

Whether true or not, none of the above is in any way relevant to measuring the speed of an object, whether superluminal or not. I don't need to see the object coming, I don't need to see the object leaving, I don't need to be present at the arrival and departure events. All I need is an arrival time, a departure time, and a distance, and I can calculate the speed after the fact and at my leisure.

The procedure that I described in #8 and that DaleSpam has described above works this way.

 

[Dale]

the example of Tachyon that you took do not consider the Chereknove effect produced by it.

Irrelevant, as Nugatory pointed out.

I still await a reference supporting your post 3, or your admitting that post 3 was wrong.

 

[1977ub]

After a tachyon has passed nearby, we would be able to see two images of it, appearing and departing in opposite directions.

If such a thing were to happen, make a note of the time. When it happens to a confederate, he will do so as well. You'll have two sets of location & time coordinates. Voilà - velocity

 

[Parlyne]

Yes, it implies not only causality but this is also a Lorentz invariance which is the symmetry underlying special relativity. If Lorentz invariance breaks down, I don't think SR would work. The simple reason is that the symmetry is broken.

Who said anything about violating Lorentz invariance? The simple statement I gave not only entails Lorentz invariance, it also requires translation and time translation invariance, giving the actual symmetry group of SR - Poincare invariance. Tachyons do not violate this. They only violate causality, which is not inherent in either this formulation of SR or that in terms of Einsteins original postulates.

If we are talking about only math then fine. But physics would not allow to do it, if laws of physics are same everywhere.

Again, tachyons do not violate Poincare invariance; so, you are simply wrong here.

Pls just think of consequences in nature and laws of physics if speed of light need not to be invariant. Masslessness of the light resides in the nature of light itself, as it has zero rest mass but has momentum given by hλ or h*neu/c.
if you use relativistic mass formula, then
realtivistic mass m_v= rest mass/(√1-(v/c)²)
Now you can find what kind of mass you get if you put v=c or v > c.
for any patricle if you put v=c, you get that is not defined in mathematics.
If you choose V > c, you get that is not permisssible for mass in real world of physics.

I think you misunderstood my statement. The parameter c is a frame-invariant constant no matter how you formulate SR. My statement was simply that it only corresponds to the speed at which light actually travels because photons are massless. Were they not massless (and, strictly, we don't actually know that they are exactly massless - only that their mass is smaller than 10^-18 eV) their speed would vary with energy; but, in no frame would it be equal to c.

As for relativistic mass, it is a deprecated concept in the practice of physics, as it adds nothing of any use, but tends to lead to a great deal of confusion. The problem is that relativistic mass is really nothing more than energy divided by c². But, if we think about it that way, we can see that there is a perfectly consistent way to look at the kinematics of tachyonic particles if we just allow their masses to be imaginary valued. Then, since the Lorentz factor will also be imaginary valued, energy and momentum will be real valued and quite sensible.

 

[ravikannaujiya]

If such a thing were to happen, make a note of the time. When it happens to a confederate, he will do so as well. You'll have two sets of location & time coordinates. Voilà - velocity

You also can refer to wikipedia on that or any so-called reliable science reference.

Irrelevant, as Nugatory pointed out.

I still await a reference supporting your post 3, or your admitting that post 3 was wrong.

I know I am right whether you accept it or not. I gave you the book and if you do not rely on it, I have shown you some lines on wikipedea and you find it so called Nugatory, leave the matter apart. You can measure speed of anything more than that of light theoreticly or practically. But I don't see it anyway simply when nothing can move faster than light then how can I measure it.

Who said anything about violating Lorentz invariance? The simple statement I gave not only entails Lorentz invariance, it also requires translation and time translation invariance, giving the actual symmetry group of SR - Poincare invariance. Tachyons do not violate this. They only violate causality, which is not inherent in either this formulation of SR or that in terms of Einsteins original postulates.



Again, tachyons do not violate Poincare invariance; so, you are simply wrong here.



I think you misunderstood my statement. The parameter c is a frame-invariant constant no matter how you formulate SR. My statement was simply that it only corresponds to the speed at which light actually travels because photons are massless. Were they not massless (and, strictly, we don't actually know that they are exactly massless - only that their mass is smaller than 10^-18 eV) their speed would vary with energy; but, in no frame would it be equal to c.

As for relativistic mass, it is a deprecated concept in the practice of physics, as it adds nothing of any use, but tends to lead to a great deal of confusion. The problem is that relativistic mass is really nothing more than energy divided by c². But, if we think about it that way, we can see that there is a perfectly consistent way to look at the kinematics of tachyonic particles if we just allow their masses to be imaginary valued. Then, since the Lorentz factor will also be imaginary valued, energy and momentum will be real valued and quite sensible.

When you say Causality is voileted but not the SR, speed would vary with energy, allow masses to be imaginary then you can imagine anything you want. But I do not agree with you just because I don't find nature voilating some fundamental principle of causality but not the others. And you also need not to be agree with me... :)

 

[Parlyne]

When you say Causality is voileted but not the SR, speed would vary with energy, allow masses to be imaginary then you can imagine anything you want. But I do not agree with you just because I don't find nature voilating some fundamental principle of causality but not the others. And you also need not to be agree with me... :)

What makes you think that causality is fundamental in the first place?

More to the point, though, all I've been trying to point out throughout this discussion is that causality is a separate issue from the validity of SR, as SR is simply about Poincare invariance. I personally suspect that both are true (well, in terms of relativity, what really should be true is local, rather than global, Poincare invariance, as shows up in GR); but, that doesn't change that either could be violated independently of the other.

All that said, it's certainly not true that "you can imagine anything you want." The kinematics of tachyons are just as constrained as those of ordinary objects (or, bradyons, as the terminology would have it).

 

[Dale]

You can measure speed of anything more than that of light theoreticly or practically.

So do I correctly understand that you rescind your comment of post 3? If not, then you still need to post a valid reference supporting it. Neither the textbook nor the Wikipedia article made any comment supporting your post 3.

But I don't see it anyway simply when nothing can move faster than light then how can I measure it.

Agreed, but the point is that it is a limit on the physics, not a limit on the measuring devices. I.e. we don't measure anything going faster than c because we haven't found anything that goes faster than c, not because our devices are incapable of measuring speeds faster than c.

 

[Aadrish]

i have a question
why light waves are transverse waves?
as transverse waves are mechanical waves so these require a medium to propagate. while light don't require any material

 

[Dale]

i have a question
why light waves are transverse waves?
as transverse waves are mechanical waves so these require a medium to propagate. while light don't require any material

The defining feature of transverse waves is that they can be polarized, whereas longitudinal waves cannot.

 

[krash661]

there was an experiment from the ISS in the last couple of weeks that may help with this,
ALPHA MAGNETIC SPECTROMETER RESULTS
nasa.gov

Recent results from the AMS experiment
cern webcast

Potential Dark Matter Discovery a Win for Space Station Science
space.com

there are also these articles,

Homing in on Dark Matter
Three potential detections from deep underground could be from dark matter particles.
sky and telescope magazine

Researchers see potential hints of dark matter; gamma-ray flare up is cosmic coincidence
IOP-institute of physics

 

[Parlyne]

i have a question
why light waves are transverse waves?
as transverse waves are mechanical waves so these require a medium to propagate. while light don't require any material

Your mistake seems to be in the presumption that all transverse waves must be mechanical. But, there's no reason for such a presumption. As DaleSpam points out, transverseness implies polarizability; and, I'll just add that the dynamics of the electromagnetic field require that, in the absence of sources (that is charges or currents), the electric and magnetic fields must be mutually perpendicular and also perpendicular to the direction of energy transport (as determined from the extremely well-named Poynting vector).

 

[Parlyne]

there was an experiment from the ISS in the last couple of weeks that may help with this,
ALPHA MAGNETIC SPECTROMETER RESULTS
nasa.gov

Recent results from the AMS experiment
cern webcast

Potential Dark Matter Discovery a Win for Space Station Science
space.com

there are also these articles,

Homing in on Dark Matter
Three potential detections from deep underground could be from dark matter particles.
sky and telescope magazine

Researchers see potential hints of dark matter; gamma-ray flare up is cosmic coincidence
IOP-institute of physics

Despite the significant hype, the AMS results in no way point to dark matter any more than previous data about the same phenomenon had. The positron excess under discussion was actually first noted about 5 years ago by the PAMELA experiment; and, there was a flurry of papers at the time discussing the possible natures of dark matter models that could explain such a result. However, as was known then, the excess may well be the result of totally unrelated astrophysical processes, such as pulsars. And, because of the presence of galactic magnetic fields, the positrons can't be particularly reliably traced back to their source (or sources), meaning that we can't directly test whether they originate at the center of the galaxy (as a dark matter signal must) or from other more local sources. AMS has put out far more detailed and more precise data. However, at least as yet, the data shows no features that we're already present in the PAMELA data. Therefore, any attribution to dark matter is, at best, premature.

 

[krash661]

the best info is that nasa.gov and cern.

those were the actual releases.
i'm trying to find a written version of the actual release, but it's not as easy as i thought.

in that release there were questions and comments/answers that pertain to the tittle of this topic.

I'm trying to find it now.

 

[Bill_K]

The AMS paper is here.

 

[krash661]

Despite the significant hype, the AMS results in no way point to dark matter any more than previous data about the same phenomenon had. The positron excess under discussion was actually first noted about 5 years ago by the PAMELA experiment; and, there was a flurry of papers at the time discussing the possible natures of dark matter models that could explain such a result. However, as was known then, the excess may well be the result of totally unrelated astrophysical processes, such as pulsars. And, because of the presence of galactic magnetic fields, the positrons can't be particularly reliably traced back to their source (or sources), meaning that we can't directly test whether they originate at the center of the galaxy (as a dark matter signal must) or from other more local sources. AMS has put out far more detailed and more precise data. However, at least as yet, the data shows no features that we're already present in the PAMELA data. Therefore, any attribution to dark matter is, at best, premature.

just so i understand,

are you saying it was a waste of time and resources and useless ?

edit-

so the problem is finding the source of dark matter ?

edit 2 -

" We already were confident that there is an unknown source of positrons above 10 GeV. ",
is this what you are referring to ?
it was followed with this comment,

" What we wanted to know from AMS was whether the effect continues at even higher energy, well above 100-200 GeV, and whether their more detailed observations would give us insight into whether this increase is due to a new astronomical effect or a new particle physics phenomenon. "

 

[ZapperZ]

the best info is that nasa.gov and cern.

those were the actual releases.
i'm trying to find a written version of the actual release, but it's not as easy as i thought.

in that release there were questions and comments/answers that pertain to the tittle of this topic.

I'm trying to find it now.

Please don't make this type of "citation". You are not providing other members the proper information for them to look it up if they don't know what is going on.

Either you point to the EXACT source, or use the proper format that is used to cite a publication.

In case you missed an earlier post, there is a source for you to not only read a report on this result, but to actually download the exact PRL paper for free from the PRL website.

http://physics.aps.org/articles/v6/40

So one gets to actually hear this from the horse's mouth itself, rather than through some 2nd or 3rd hand news.

Zz.

 

[Parlyne]

just so i understand,

are you saying it was a waste of time and resources and useless ?

Not at all. I'm just saying that the reporting of this result as evidence for dark matter is not a responsible representation of the data. I happen to think the AMS is an awesome experiment; and, I was actually quite excited when the shuttle mission was added in order to deliver it. Furthermore, I take no issues with the actual data or the work being done with it. And, as more data is collected, it may actually be able to help answer the question of the source of the positron excess - both by extending our knowledge about it to higher energy and by looking for directional dependence.

so the problem is finding the source of dark matter ?

No, it's finding the source of the excess positrons. It certainly could be that they come from dark matter annihilation; but, they could also come from astrophysical processes that nothing to do with dark matter.

" We already were confident that there is an unknown source of positrons above 10 GeV. ",
is this what you are referring to ?
it was followed with this comment,

" What we wanted to know from AMS was whether the effect continues at even higher energy, well above 100-200 GeV, and whether their more detailed observations would give us insight into whether this increase is due to a new astronomical effect or a new particle physics phenomenon. "

I'm not sure exactly what you're quoting; but, it seems to get at the basics of the issue.

 

[krash661]

Please don't make this type of "citation". You are not providing other members the proper information for them to look it up if they don't know what is going on.

Either you point to the EXACT source, or use the proper format that is used to cite a publication.

In case you missed an earlier post, there is a source for you to not only read a report on this result, but to actually download the exact PRL paper for free from the PRL website.

http://physics.aps.org/articles/v6/40

So one gets to actually hear this from the horse's mouth itself, rather than through some 2nd or 3rd hand news.

Zz.

I can not provide sources.
and also the comment you quoted was me doing just that.
i believe it says,

" those were the actual releases.
i'm trying to find a written version of the actual release, but it's not as easy as i thought. ",

and then the actual paper was provided after that comment of mine..

pay attention here.

edit-
and also what you posted is not the actual release.

 

[krash661]

Not at all. I'm just saying that the reporting of this result as evidence for dark matter is not a responsible representation of the data. I happen to think the AMS is an awesome experiment; and, I was actually quite excited when the shuttle mission was added in order to deliver it. Furthermore, I take no issues with the actual data or the work being done with it. And, as more data is collected, it may actually be able to help answer the question of the source of the positron excess - both by extending our knowledge about it to higher energy and by looking for directional dependence.



No, it's finding the source of the excess positrons. It certainly could be that they come from dark matter annihilation; but, they could also come from astrophysical processes that nothing to do with dark matter.



I'm not sure exactly what you're quoting; but, it seems to get at the basics of the issue.

Interesting,
ok, i understand.

 

[krash661]

interesting,

I'm on the cornell university site,
on it there is a very short paper, actual there are a couple of them,
it's titled,

cosmology with mirror dark matter
From: Paolo Ciarcelluti

there is also

Does mirror matter exist?
From: Robert Foot
which dates back to 2002

does anyone know/understand about this and have thought's on it.
this is the first i ever heard about this.

i do not know how to give you a resource without posting links.

 

[Bill_K]

We just had a recent thread on mirror matter.

 

[krash661]

We just had a recent thread on mirror matter.

thanks.

 

[ZapperZ]

I can not provide sources.
and also the comment you quoted was me doing just that.
i believe it says,

" those were the actual releases.
i'm trying to find a written version of the actual release, but it's not as easy as i thought. ",

and then the actual paper was provided after that comment of mine..

pay attention here.

edit-
and also what you posted is not the actual release.

I have no idea what you just said here.

Link that Bill_K gave is exactly the link that is provided in the article that I pointed to. So how is this not "the actual release"? It is the SOURCE! The article I cited contains the link to that very same paper! I thought providing a link to the APS Physics article might be easier for someone without an extensive background in this field to understand what that paper is all about.

Please note that when you cite arXiv articles (assuming that this is what you mean when you said that you are on the "Cornell university site"), you should provide the article number!

Zz.

 

[San K]

the speed of light is a universal speed limit because no one could measure the speed of anything more than that of light, not even of the light.

we can measure speed faster than the speed of light. for example, as other posts have said above, by having two clocks some distance apart.

as another example:

quantum entanglement has been measured to be at (the) least 10,000 times faster than the speed of light.

 

[ash64449]

yes.we can measure if particles go faster than speed of light.particles whould create cherenkov radiation if it moved faster than speed of light.

 

[ghwellsjr]

yes.we can measure if particles go faster than speed of light.particles whould create cherenkov radiation if it moved faster than speed of light.

How does that help measure the speed of a particle going faster than light?