Speed Of Light Slowed Down.

Exactly what the title says. I didn't know this was possible.
I am not that clued up on the science but here is the article

www.bbc.co.uk...

I'm guessing they had this idea shortly after finding that those 'SLOW: CHILDREN PLAYING' signs had no effect.

Interesting read... I just don't understand how exactly they're observing photons... Or why they're trying to slow them down.

a reply to: alldaylong

S&F for the find! This is fascinating.

Though I cannot help but notice an uncertainty in the experiment. The pairs of photon (the two "racers") got one photon that passes in a mask. Perhaps that the delay is not caused by the photon travelling slower than c in vacuo, but actually because it interacts with the mask before continuing back on its normal speed, causing it to arrive at the "finish line" a tad after the unaffected photon?

a reply to: alldaylong

Absolutely fascinating!

I love this part:

Complicated? Oh yes. Which is why the researchers say it might help to think of a bicycle

Haha

a reply to: alldaylong

but but but...all the scientific guru guys are telling me the light speed is a constant...all our formulas depend on it.

originally posted by: MarioOnTheFlybut but but...all the scientific guru guys are telling me the light speed is a constant...all our formulas depend on it.

In a vacuum.

a reply to: swanne

actually because it interacts with the mask before continuing back on its normal speed, causing it to arrive at the "finish line" a tad after the unaffected photon?

What I was thinking. How do they tell? Also curious is how they determined an electron "changed shape"?

originally posted by: MarioOnTheFly
a reply to: alldaylong

but but but...all the scientific guru guys are telling me the light speed is a constant...all our formulas depend on it.

Obviously it's not a constant in everyday life.

You can see the speed of light get slowed down in everyday life. Just put a spoon in a glass of water, and notice how the spoon looks "bent" where it meets the water. This is due to refraction, and that refraction is due to the fact that the speed of light through water is slower than through air -- or more precisely, the photons moving through water interact with the molecules of water, causing them to take longer to get through the water.

Granted -- in the everyday life example I gave, the light speeds up again once it exists the medium that was making it slow down (the water), but in the case of the experiment in the article, it allegedly remained at the slower speed after exiting the medium.

Science news in mainstream media sigh...

www.sciencemag.org...

Our work highlights that, even in free space, the invariance of the speed of light only applies to plane waves.

Some have already speculated that the speed of light isn't a constant, but this experiment seems to prove it. Some of the questions I've seen:

- The mask used to shape the photon doesn't slow it. The experiment is easily replicated and testable anywhere. The shaping process has already been shown to not impact the photon.
- Formulas will all work just fine. In fact scientists will be able to improve predictions in cases involving short distance photon travel.
- Electrons are not shaped, light doesn't consist of electrons. The wave of a photon of light is being shaped. This technology has been around a long time now.

It seems the speed of light is simply another speed given current situations. What causes the speed to be as we see it is the question. Obviously the wave shape very slightly affecting it is a clue to a deeper understanding.

www.businessinsider.com...

Doing a quick search on google there are plenty of instances of scientists slowing the speed of light, so this is nothing new.



By use of a Bose--Einstein condensate, Danish physicist Lene Vestergaard Hau (Harvard University) succeeded in slowing a beam of light to about 17 metres per second, and, in 2001, was able to momentarily stop a beam.

Physicists Slow Speed of Light
news.harvard.edu...

Scientists Slow Down Speed of Light
abcnews.go.com...

a reply to: EnigmaAgent

This actually is something new. They aren't using a medium to slow light down. Both photons are travelling in a vacuum. What they are doing is shaping the wave of the photon, which is for some reason slowing it down even in a vacuum.

originally posted by: alldaylong
Exactly what the title says. I didn't know this was possible.
I am not that clued up on the science but here is the article

www.bbc.co.uk...

The article like many articles about science doesn't do a very good job of explaining what is going on.

The researchers looked at something called "Group Velocity" which is a characteristic that has already been observed going faster than light so it's no surprise to me it can also be observed going slower than light.

The paper gives a much better description of the research, even has a one-sentence summary:

Photons that travel in free space slower than the speed of light

One sentence summary: The group velocity of light in free space is reduced by controlling the transverse spatial structure of the light beam.

Their paper on page 7 implies the effect matches their theoretical predictions, so I don't expect much re-writing of textbooks:

the delays we measure can only result from the transverse structure of the beam and indeed are consistent with our theoretical predictions.

The Wikipedia article on FTL explains how group velocity can also be shown to be faster than light:
Group velocities above c

The group velocity of a wave (e.g., a light beam) may also exceed c in some circumstances. In such cases, which typically at the same time involve rapid attenuation of the intensity, the maximum of the envelope of a pulse may travel with a velocity above c. However, even this situation does not imply the propagation of signals with a velocity above c,[14] even though one may be tempted to associate pulse maxima with signals. The latter association has been shown to be misleading, basically because the information on the arrival of a pulse can be obtained before the pulse maximum arrives. For example, if some mechanism allows the full transmission of the leading part of a pulse while strongly attenuating the pulse maximum and everything behind (distortion), the pulse maximum is effectively shifted forward in time, while the information on the pulse does not come faster than c without this effect.

So we can make the group velocity of light travel faster or slower than the speed of light.

a reply to: Arbitrageur

Any ideas what is slowing down the photons? Given the experiment is in a vacuum there isn't supposed to be anything to slow it down. Is it because there can be no perfect vacuum so there will always be a little resistance to the wave? I dunno

The paper, thanks for the link, seems to say they predicted the slow down. It was due to the angle of the wave front. Getting too far for me to understand why it's a wee bit slower.

a reply to: noeltrotsky
They mention that the product of the phase velocity and the group velocity within a hollow waveguide is the speed of light squared. So if you speed one up the other must slow down for that equality to hold.

It's a geometric effect which means it might also have applications to other types of waves,as explained at the end of the paper:

Another citation from the paper noted in my previous post:

The effect can be derived from a simple geometric argument, which is also supported by a rigorous calculation of the harmonic average of the group velocity. Beyond light, the effect observed will have applications to any wave theory, including sound waves and, potentially, gravitational waves.

a reply to: Arbitrageur

I originally went for the group velocity explanation too, but the article specifically states this:

"So we measure the speed of a single photon as it propagates.

"And we find it's actually being slowed below the speed of light."

They claim that the two photons were not emitted in groups.

Draw a cube with x, y . z axis then on each xy, zx, yz plane draw a 45 deg triangle on each plane. Then connect the end point of the each planes diagonal to a common point in space that joins each planes diagonal component at equi distance in space. This arrangement draws a cube which is a geometric structure. If the speed of light is represented by the diagonal component that goes from the origin to the the common point then the transverse components are the diagonal lengths on each the xy, zx, yz plane. Reduce the transverse yz component or zx or xy component and the geometric effect the speed of light diagonal component reduces.

a reply to: AthlonSavage

The diagonal line isn't supposed to reduce (in vacuum) in the first place. That's why the speed of light is said to be constant no matter the frame of reference's shape. Its velocity is the same no matter the shape of all four dimensions.

It's light, not a rubber band.

Now you're going to ask, "then how come these fine men in the article succeeded in slowing Light down? " to which I will answer, "as I have already pointed out, the article is rather full of uncertainty - it could be that the second photon reached second place in the race not because it went slower in vacuum, but actually because it had to make a pit stop (the interaction with the mask). Or, as Arb has suggested, perhaps the experimenters measured the group velocity, which is not the same as the actual photon's velocity and can often go below the speed of light. "

a reply to: AthlonSavage

I thought that's what the researchers were predicting, a slow down due to the slight increase in distance due to this 'geometric' effect of changing the wave shape. So the researchers have proven that the photon travels 'along' the wave, and if you increase the length of the wave front you 'slow down' light???

a reply to: Arbitrageur

I noted that passage with interest when I saw 'gravitational waves'.