The speed of light is not the fastest.

[ajtribick]

I think this explanation is basically correct, although there are certainly far more knowledgeable people here than me.

While photons don't have mass, they do have an energy associated with them, E=hf, where E is energy, f is frequency and h is Planck's constant.

This energy is equivalent to an amount of mass given by E=mc??, where E is energy (again), m is mass and c is the speed of light.

Equating energy gives: hf=mc??

Now momentum is mass times velocity, for a photon this velocity is c, so we can rearrange this equation by dividing both sides by c to give the momentum of a photon:

p=hf/c

Where p is the momentum. Thus when an object emits or absorbs a photon it accelerates so that momentum is conserved.

Remember that the photon is in fact massless, the mass used in the middle of all this working out is a convenient way to visualise the energy to do momentum calculations, it is not "real" mass, otherwise the photon wouldn't be going at the speed of light and we'd all be in trouble.

[Matt McIrvin]

There are two different definitions of mass that are sometimes used in the context of relativity. One of them is the so-called "relativistic mass" which is just the total energy of an object divided by c^2. This increases with velocity for ordinary objects (though the increase is tiny for speeds that are small compared to light), and it is nonzero for photons.

However, while this "relativistic mass" definition continues to appear frequently in introductory and popular books, it is rarely used by modern physicists, who usually prefer to use the word "mass" to refer to the "intrinsic mass" or "rest mass", which is the quantity

sqrt(E^2/c^4 - p^2/c^2)

where p is the momentum. You can think of it as not including the kinetic energy.

This mass is a constant independent of velocity. For ordinary objects, it is the same as the relativistic mass when the object is at rest. For things that move at the speed of light in vacuum (such as photons), it is zero.

[rthorvald]

YOu still havent explained the fact that mother's break the laws of physics by being at the exact place at the exact time that a child does something wrong

Well, according to Douglas Adams, the only thing that travels faster than light is bad news. But nobody uses it to travel between the stars in fear of getting a bad reception, wherever they arrive...

-rthorvald

[Rassilon]

Well in my own humble opinion I believe it is possible to travel faster than light just as its possible to travel faster than sound....Before supersonic travel no one knew just HOW it could be done...so it was labeled impossible...

In General Relativity, Einstein proposed light speed (usually 186,000 mps) as the fastest speed that matter could move. In Special Relativity, he specified that matter could move faster, but does not in our observable universe.

imho...true light speed would be to the observer simply the velocity of light where as the traveller would experience the time distortion effect...He would effectively transverse great distances without feeling the effects of time whereas all his friends and relitives would age and eventually die in a matter of moments relative to the travellers concept of time at the moment of exact light speed...Now if one would travel faster than this I suppose he would grow younger the faster he travelled until he eventually winked out of existance....

As for the way around this limit other forms of travel would have to be introduced other than brute force (point A to point B) such as hyperspace or even the tesseract or folding space and time...instead of moving to point B you fold space and time and bring point B to point A...This would require an alternate dimention and a science beyond our reach in this day and age...We have yet to even use nuclear engines on spacecraft let alone light drive engines...so looking at the timescale in my opinion this is what it would look like...

mid-21st century - nuclear engines / solar sail
possible late 21st century - fusion engines / ion drive may be possible
mid 22st century - ion drive / sublight travel
22nd - 23rd century - possible intrduction to hyperspace (if it exists)
25- 30th century would produce a possibility of the tesseract

This of course is if we survive as a species that long...

[Size_Mick]

I remember seeing a documentary show where they split a beam of some sort (actually carrying music) with an apparatus which made one part of the beam go through a vacuum, and the other part through a solid bar of some sort of metal. The beam that traveled through the metal bar got to its destination first, faster than the speed of light, and the music, though degraded, was still audible. This show apparently challenges the statements made by some of you. So, what gives? Sorry that I can't remember the name of the show but I think it was a PBS show within the last 5 years.

P.S. How can light have no mass but still be affected by gravity??

[Drake]

I

P.S. How can light have no mass but still be affected by gravity??

Einstein's thought experiment is the most elegant way I know to explain this. I'll do my best...

Imagine you are in a box with no windows. Now you want to figure out what is going on. You notice a flashlight shooting a beam out of the wall and v-e-r-y carefully measure how high it is from the floor, and do the same to where the beam hits the other wall. Now, there is a measureable time difference between when the beam leaves the flashlight and when it hits the other wall. If you are in freefall, everything is moving at the same rate, and the beam hits the opposite wall at the same height as it leaves the flashlight. (OK, we are making some assumptions that the beam doesn't spread, that it is aligned perfectly and all that, but ignore that.) But if we are in a spaceship that is accelerating, during the time the beam takes to travel from one wall to the other the spaceship goes a little faster, leaving the beam a little farther behind, and so the beam hits the other wall a little bit lower, which we can measure. Now Einstein's brilliant leap here is that a windowless spaceship accelerating at 1g can in no way be distinguished from a windowless box sitting on Earth in a vacuum chamber (there is some reasoning as to why that must be, but skip that for now). So, light is bent by Earth's gravity just as if we were inside of a spaceship accelerating at 1g.

So, light bending due to gravity has nothing to do with mass, but it is due to the speed of light not being infinite.

[Evil Dr Ganymede]

Thus when an object emits or absorbs a photon it accelerates so that momentum is conserved.

Remember that the photon is in fact massless, the mass used in the middle of all this working out is a convenient way to visualise the energy to do momentum calculations, it is not "real" mass, otherwise the photon wouldn't be going at the speed of light and we'd all be in trouble.

I thought most of the "push" on solar sails came from the other particles in the solar wind that do have mass? How would the momentum imparted by charged and neutral particles onto a sail compare to simple 'light pressure'?

[t00fri]

In Einstein's /general/ relativity, the gravitational force affects the movement of light, but a photon mass is not needed for this effect:

remember:

General Relativity says:
-----------------------
-- Mass = source of the gravitational force

++++++++++++++++++++++++++++++++++++++++
-- mass tells space-time how to curve
-- the curvature of space-time tells particles how to move
++++++++++++++++++++++++++++++++++++++++

In other words: Some heavy masses in the universe lead to a (local) deformation of the geometry of space and time. Photons and other massive particles move on geodesics of that deformed space-time (like rivers flowing in mountaneous countryside) and --because of this-- implicitly feel the presence of the gravitational force.

Think of gravitational lensing, for example. Here the "straight" light path is deformed by massive foreground galaxies, analogously to what happens when light passes through normal glass lenses. The deformation of space is effected by the heavy mass of the foreground galaxies and leads to the familiar image distortions of light sources positioned behind the foreground galaxies.

Bye Fridger

[Drake]

I thought most of the "push" on solar sails came from the other particles in the solar wind that do have mass? How would the momentum imparted by charged and neutral particles onto a sail compare to simple 'light pressure'?

Evil Dr. G...

From http://www.planetary.org/solarsail/faqs.html

"The solar wind is made up of ionized particles ejected by the Sun. These particles move much slower than light. A solar sail does not stop or reflect them, although they also may impart some of their momentum to the solar sail. However, the force from the solar wind is less than one percent of that from light pressure."

[leonardo]

With respect, this question is being answered on the assumption that there is a problem,, there isn't. Einstein started with two axioms:

There is no such thing as an absolute reference platform,

No information can travel faster than the speed of light


As soon as you start questioning an axiom, it can no-longer be an axiom.

[Bob Hegwood]

Okay, I just hafta ask this now...

If we managed someday to get an astronaut travelling at the speed of light,
then what would happen if he turned on a flashlight then?

Are you guys saying that its beam would not even be seen?

Sorry, but I really am just curious about this interesting discussion.

Thanks, Bob

[Cham]

Bob, this is a question Einstein asked himself, when he was a teenager !

According to special relativity theory, whatever the speed at which you're moving, the light emitted by your flashlight will go... at the speed of light ! Light is moving at c for every observer.

If you move, say, at 99.9999999999999999% light speed relative to Earth, your flashlight will emit light which moves at 100% c relative to you. What is freaking is that the same light will move at 100% c relative to people on Earth !! Weird, isn't ? That's relativity theory, man !

But it's a goddam good theory. It's verified each day, in all big physics laboratories in the world since 1905 (or so).

This means that time will not flow the same for you as for the people on Earth. You wont notice a difference until you stop and return to Earth. Earth will then be the planet of the Apes !

[t00fri]

Bob,

please don't blush

But once for all I want to tell you that you keep asking pretty damn good (physics) questions...

Bye Fridger

[Bob Hegwood]

If you move, say, at 99.9999999999999999% light speed relative to Earth, your flashlight will emit light which moves at 100% c relative to you.

So, are you saying that it is impossible to obtain 100% of the speed
of light with an object containing some mass? Again, I don't know beans
about relativity (or physics in general) but this is a fascinating
discussion.

This means that time will not flow the same for you as for the people on Earth. You wont notice a difference until you stop and return to Earth. Earth will then be the planet of the Apes !

So, since time has slowed down for me - as the astronaut - I would
see the light beam as normal?

Thanks, Bob

[Bob Hegwood]

please don't blush

Sorry Fridger, but

Believe it or not, I really do enjoy these discussions. I may not
understand about 75% of what I read, but the 25% I do understand
is simply fascinating.

Thanks Doctor Schrempp, for encouraging us Brain-Dead types.

Take care, Bob

[Cham]

Bob,

time is slowing down for the astronaut, but only relative to the Earth observer. The astronaut himself wont notice ANY difference at all in his own laboratory. For him, light will behave as normal, as always it does. There's simply no way the astronaut can learn he's moving at that 99.9999999% c speed, without watching outside.

And yes, it means that any kind of matter cannot reach the 100% c limit. It's just impossible without destroying the matter with its information content.

[t00fri]

Bob,
...
For him, light will behave as normal, ...

Aha! Observers are male . What else...

Bye Fridger

[Bob Hegwood]

For him, light will behave as normal, as always it does.

But if that's the case, then the light from his (or her) flashlight will
be travelling faster than the speed of light, will it not? Or, if I'm misunderstanding
again, you're saying that since his/her time has slowed down, then the light
from the flashlight is really not travelling faster than the speed of light because
time is passing in a slower fashion... Is this correct?

Seems to me that an outside observer would view these events as
transpiring at a velocity that is faster than light. So I guess it
depends on your point of view too. Isn't there something in physics which
states that the observer of any phenomena has an effect on the
observed?

And yes, it means that any kind of matter cannot reach the 100% c limit. It's just impossible without destroying the matter with its information content.

Now this I can believe. It's just that I can't believe that something can't travel faster than the speed of light.

At any rate, you've made my head hurt now, so I think I'll go try to
amuse myself with a brewsky for a while.

Thanks very much for the education though. I still think this is simply
fascinating stuff.

Take care, Bob

[maxim]

The speed of light is one of the constraints of the existence of our universe, or more special of the timespace continuum we are living in. So nothing in here can travel faster than speed of light.

There are of course theories about things travelling faster than speed of light (tachyons?) but these wouldn't be part of the timespace continuum as they can't exist here. These theories are BTW currently the only hope of people who'd like to see mankind spread to the stars. These theories had been the reason why a 'hyperspace' was proposed where such things could exist. But this is still more of metaphysics than real physic - and sience as far away from any proofs about it. So you can easily imagine something that that IS faster than light - but it would leave our universe immediately.

Concerning the astronaut:
It's often forgotten to mention that not only the experience of time changes for a moving observer, but also the experience of space. These two are interdepended. So a 99.x percent traveller would not only act slower (seen from outside) but also his space will deform in a way that the light he emitts will travel the same distance at a time (on his scale) as for any other observer in the universe. As seen from outside the light will /crawl/ away from him, but his local scale became so short, that it travelled the same way (no not /way/ but measurement units) on the local scale as on the scale of the outside observer.

The astronaut himself would experience himself standing still and the outside observer moving at 99.x percent of light.


maxim

[Spaceman Spiff]

Er... people, people,

you won't shed any light on this situation if you don't start with an understanding of the Michelson-Morley experiment (1887) (http://scienceworld.wolfram.com/physics/Michelson-MorleyExperiment.html), which explains why Einstein developed his Theory of Special Relativity (1905).

This theory was developed from just one assumption: no matter how fast you travel and where you are going, you will always get the same measurement for the speed of light.

This was what was observed in the Michelson-Morley experiment, and was not expected at the time, as no one thought material objects couldn't travel faster than light.

As Cham rightly says of Bob Hedgewood, asking what you would see travelling at the speed of light is what Einstein used to explain the Michelson-Morley experiment, and is a great question to ask.

The finding that no material object could accelerate to beyond the speed of light is an outcome of the theory, not an assumption of it.

And finally, back to Kikinho's original post...

I've heard that a man made an experiment with microwave light and the speed of light started to increase a bit.

... because there's no supplied reference to this experiment, I can't check up on this, but I know that microwave and radio antennas do this: emit 'evanescent modes' in their 'near fields'. These are technical terms for which one would have to learn electromagnetic theory and antenna design. The point about evanescent modes is that they have a group velocity slightly faster than c, but their phase velocity is not. This is because the wavefront is travelling obliquely to their direction from the antenna's phase centre. Also, evanscent modes cannot be used to carry information faster than light. All this can be calculated using Maxwell's equations for Electromagnetism.

This is not accepted as an observation falsifying Einstein's Special Theory of Relativity, because the 'speed of light limit' refers to phase velocity, not group velocity.

The chap who did this experiment was probably mixing evanscent and real modes of microwaves.

Spiff.