The speed of light?

[Martin]

A good tutorial article available online

 

"Expanding Confusion: common misconceptions of cosmological horizons and the superluminal expansion of the Universe"

 

http://arxiv.org/astro-ph/0310808

 

It is by Lineweaver and Davis.

[JaKiri]

Well, you showed me.

[Martin]

JaKiri thanks for responding!

 

']Um' date=' [i']what[/i].

 

I call bullshit on this one, this goes against everything I know on relativity. If you can prove me wrong, by all means do so.

 

Tycho, there are two theories of Relativity

the 1905 "special" theory does not allow FTL speeds

einstein's later theory, the 1915 "general" theory of rel

allows FTL recession speeds

 

the later theory also allows a preferred frame of reference

and a universal idea of simultaneity, and as developed in the

Friedmann equations model actually predicts this

(the standard cosmology metric has a preferred frame which is the

CMB frame, stationary w/rt "hubble flow" or w/rt the expansion of the U, which is essentially same as being stationary with respect to microwave background)

 

and cosmologists constantly make use of the CMB frame and

the notion of a universal absolute time (simultaneity and time are not

"relative" in the sense of the 1905 theory)

 

when people talk about "Relativity" it is important for them to

make it clearly explicit what theory they mean because the two theories

say different things (although because one is local and the other global they do not actually contradict each other)

 

if people just say "Relativity" then confusion can arise

 

recession speed is different from local speeds of encounter

the point is that over vast distances there can be a significant amount of curvature between you and it

and special rel does not apply to curved frameworks, it only applies to local flat frameworks

[JaKiri]

Don't forget 'relativity' can also refer to Newtonian concepts, so it's a bad word to use.

[YT2095]

just as a side topic offshoot for a sec, Question for Jakari: Gamma particles are said to have no mass, do they also travel at lightspeed?

[Dave]

Gamma radiation is just part of the EM spectrum, so a gamma "particle" would in effect be a photon.

 

Unless I'm being silly.

[JaKiri]

Gamma radiation is just part of the EM spectrum' date=' so a gamma "particle" would in effect be a photon.

 

Unless I'm being silly.[/quote']

 

You're right.

 

All exchange particles travel at light speed. Conjecturally at least.

[Dave]

A-level physics to the rescue again

[YT2095]

Ok, thnx for that guys, it was rellated but not really thread topic worthy.

Back to where we left off....

 

Don't forget 'relativity' can also refer to Newtonian concepts, so it's a bad word to use.

[[Tycho?]]

Well Martin, it seems I owe you an apology. I can't say I really trust those java calculators when I use numbers that really mean nothing to me, but I'll take your word for it while I look it up more.

[[Tycho?]]

Haha, I do a google search on the subject, and the first link I get is one to this very thread...

 

 

...and it does indeed seem that you are right there Martin. Neat.

[Dave]

I never knew GR could account for ftl recession. Learn something new every day

[Martin]

']Haha' date=' I do a google search on the subject, and the first link I get is one to this very thread...

[/quote']

 

glad to hear it, suggests SFN has acquired visibility on google

some of the "guests" must be google bots doing their job

I have a feelin SFN is set to grow---has a nice open not-too-serious

atmosphere

[TheMadHatter0]

Could someone tell me-

1) What is the speed of light measured by?

2) What possible factors did play an affect on how the light was measure?

3) Is there proof that nothing can travel faster than the speed of light?

 

I think that Martin pretty much explained how this works.

 

1) The metric system was defined as being in reference to the speed of light. One meter was the distance that light traveled in a certain fraction of a second (what I want to know is why they didn't just make it a nice' date=' round 3 x 10^8...).

I think that #2 has pretty much been explained as well.

 

Here's some info on #3:

Going back on my HS physics class (and please correct me if I slip anything up.. it's been a little while), a guy named Lorentz came did some research based on Einstein's theories of relativity and came up with some equations that are now called the "Lorentz Transform."

And, basically, if you use these formulas, any object having [i']rest[/i] mass (meaning mass when the object is not moving... here's where you get to mess with a ton of wonderful "plane of reference" issues) increases in mass the closer it gets to the speed of light.

When the object reaches the speed of light, a proper calculation returns the object's new mass is infinity.

The Lorentz transform also deals with object length as well. As it has been proven, an object does get smaller the faster it goes. In this case, though, a proper calculation will return a length of zero. Put the speed at faster than than light, and you get a negative length.

 

This has led some scientists to believe that when an object does reach the speed of light, it turns into pure energy (as by Einstein's E = mc^2 equation).

 

 

But, if you take an object without rest mass (i.e. a photon), you can theoretically make the object travel faster than light.

This has already been accomplished... I believe an experiment was performed a few years back where a photon was accelerated to 300x the speed of light.

And, yes, it did get to the end of the tunnel thingie before it left.

While this may not seem possible with simple Newtonian mechanics, there's some equations (that I have forgotten) that do demonstrate this result.

 

Taking all this into effect, some scientists have pretty much given up on trying to achieve faster than light speeds and have just started fiddling around with quantum teleportation.

Fun stuff.

 

-Ian

[Martin]

Ive done enough talking on this thread for a while and

there are probably half a dozen people around who can explain the SR speed limit---or know another SFN thread where it is derived

 

if something has positive rest mass it cant go c

it has to go some speed less than c

 

I dont know how anything could be turned into pure energy

by being accelerated----I suspect it couldnt but dont know

 

if something has zero mass, thinking of a photon, I dont see how

it can go any speed but c

although in various media light waves can have different speeds, and phase velocity and group velocity can be different

I still cant picture anything which people would call a photon travelling faster than c.

 

so I am going to pass, on what MadHatter is talking about here

and hope someone else responds

 

the Special Rel speed limit is totally real for me and I believe in it with absolute conviction----it just does not apply over vast distances of curved space, to the case of recession speeds (the rate those distances increase)---so it confuses me to be told about something going 300c in an earthbased experiment.

Somethings wrong, or something i didnt get.

 

...

...But' date=' if you take an object [i']without[/i] rest mass (i.e. a photon), you can theoretically make the object travel faster than light.

This has already been accomplished... I believe an experiment was performed a few years back where a photon was accelerated to 300x the speed of light.

...

...

[TheMadHatter0]

if something has zero mass' date=' thinking of a photon, I dont see how

it can go any speed but c

although in various media light waves can have different speeds, and phase velocity and group velocity can be different

I still cant picture anything which people would call a photon travelling faster than c.

[/quote']

 

Okay, I may have been wrong here.

I did some quick research, and I found two different articles that contradict each other concerning this phenomenon.

Here's what looks to be info on the actual experiment itself:

http://www.dhushara.com/book/quantcos/qnonloc/qnonloc.htm

 

And here's an article saying it can't happen:

http://www.cabot-biz.com/photonics/ftl.html

 

And here's an article about Tachyons, or Tachyon-like particles (theoretical particles that go faster than light):

http://www.sciam.com/askexpert_question.cfm?articleID=000657D8-67D9-1C71-9EB7809EC588F2D7

 

With that, I think I'm ready for bed.

-Ian

[Martin]

I'm drifting that way too

it gets confusing with all these different sources

(better than having none at all tho)

 

tonight I ran into a wonderful quote from robert frost

It may have been in an article you gave a link to

 

"We dance round in a ring, and suppose,

but the Secret sits in the middle and knows."

[JaKiri]

Tachyons don't necessarily have 0 mass, though.

[Sayonara]

glad to hear it' date=' suggests SFN has acquired visibility on google

some of the "guests" must be google bots doing their job

I have a feelin SFN is set to grow---has a nice open not-too-serious

atmosphere[/quote']

SFN has been visible on google and growing steadily for the better part of two years.

 

I really don't get where your obsession with the Who's Online list comes from

[swansont]

1) The metric system was defined[/i'] as being in reference to the speed of light. One meter was the distance that light traveled in a certain fraction of a second (what I want to know is why they didn't just make it a nice, round 3 x 10^8...)

 

The meter was originally defined in terms of the distance from the equator to the north pole, being 10,000km. From that they defined the standard meter in terms of an iridium-platinum bar. The number was a little off because they had not accounted for the oblateness of the earth, so the actual measured distance is not quite 10,000 km.

 

I've never read where c enters into this, or if without this discrepancy c would be a round 3e8.

 

Today, of course, c is defined and the meter is a certain number of wavelengths of light from a specified atomic transition. But that's not where it originated, AFAIK.

 

 

But' date=' if you take an object [i']without rest mass (i.e. a photon), you can theoretically make the object travel faster than light.

This has already been accomplished... I believe an experiment was performed a few years back where a photon was accelerated to 300x the speed of light.

 

No, objects with zero rest mass travel at c. Travelling at other speeds means there is a mass term - if v>c then this term is imaginary.

 

The superluminal experiment to which I think you are referring was a case of anomalous dispersion and did not in any way constitute a case of a photon being accelerated to move faster than c. It was basically reshaping a pulse of light (many photons) so that the peak was further ahead afterwards than in the original pulse.

[5614]

i found this site via google.. if u search for any subject in google which has a similar thread in this site, then this site comes up near the top of the google list...

 

back to the topic:

 

i cant remember any proof, but im 99.9recurring% sure that the only things which can travel at the speed of light it something without mass, and that nothing can travel faster....

is there anything which proves any of that wrong.... [ignore the speed of an object in relative to you] just is the light / [any object] moving faster than light

[Martin]

...

 

i cant remember any proof' date=' but im 99.9recurring% sure that the only things which can travel at the speed of light it something without mass, and that nothing can travel faster....

[/quote']

 

I think that's right. although some people have conjectured that a type of object they call a "tachyon" might exist (same root as tachometer----tachy greek for speedy) which HAS to always go faster than light. From our point of view it can never slow down

 

so the upper speed limit for us, namely c, serves as a lower speed limit for them

 

but right now the existence of tachyons is, at best, speculative

[PerpetualYnquisitive]

The speed of light, one of the most sacrosanct of the universal physical constants, may have been lower as recently as two billion years ago - and not in some far corner of the universe, but right here on Earth.

 

The controversial finding is turning up the heat on an already simmering debate, especially since it is based on re-analysis of old data that has long been used to argue for exactly the opposite: the constancy of the speed of light and other constants.

 

A varying speed of light contradicts Einstein's theory of relativity, and would undermine much of traditional physics. But some physicists believe it would elegantly explain puzzling cosmological phenomena such as the nearly uniform temperature of the universe. It might also support string theories that predict extra spatial dimensions.

 

Full article: http://www.newscientist.com/news/news.jsp?id=ns99996092

[Martin]

...

 

Full article: http://www.newscientist.com/news/news.jsp?id=ns99996092

 

see also another recent newscientist article with more or less the opposite conclusion

 

http://www.newscientist.com/news/news.jsp?id=ns99996057

 

BTW these articles report studies of possible change in alpha

 

which does not directly imply a change in the speed of light

 

when journalists get hold of reports about change in alpha they tend to bend them into something about change in c

because readers know what the speed of light is

and dont know what alpha is

 

since c is one of the constants entering into a formula for alpha

you can always find someone interested in speculating that IF

a change in alpha can be proven then it MIGHT have resulted from a change in c

 

but it could just as well have happened that alpha changed without any change in the speed of light at all-----alpha is a number approximately equal to 1/137 which indicates the strength of electromagnetic interactions---it is like Coulomb's constant expressed in planck units

 

the strength of electric attraction/repulsion could conceivably change slightly over time (compared, say, with gravitational interaction) without the speed of light changing

 

Anyway, the issue of alpha changing is pretty open, you just have to read both sides and see who you think is doing the solidest research. new scientist is presenting one side and then the other just a few weeks apart---they really shouldnt drag the speed of light into it,

but I guess they do that in order to tap a wider audience so its understandable

[TheProphet]

First: I just stumbeld apone this forum, and im swedish so don't, so aparden my english and the newbie stuff =)

 

I'd like to give u some sort of proofe for this. But all i remember is that Allen Greene wrote in his book (the elegant universe). That Light most surely can and will deviate light speed. He dosen't state why but that on the large scale these, as also interperated to back and forth in time, deviations cancel out! The neat thing altough with this moving back and forth most surely has to do with the "purpouse" of light itself. According to me light is the "ringbearer", or the perfect energytransporting entity! Why, well first of: since it never travels in "time". With that said i belive i must emphasize it further!

Due to Einsteins SP relativity lightspeed is constant, why he didn't emphasize for light but for all non lightspead particles (us). Light on the other hand(to explaine why it's so perfect) always travel at same speed and thus never travel in "time" since time in it's "absolute" frame och reference is ZERO. A praticle that never feel nor know what time is never loses energy under it's travel and so forth... therefore it's perfect =)

To stretch this "fact" further, photons can really do some amazing thinghies! Like being at 2 places (temporarily back and forth in time!) att the same instance or amazingly seeme to know whats coming(at time Zero, would u se somethings coming huh? it's already there then i guess ). This would explain experiments at berkley, where photons seems to be seeing forward into time. It might also explain Feynmans electron - positron - electron simultanity dilemmas, altough just might.

 

That's my first post.. now i'll sour this forum thorugh!