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August 16, 2007

Faster Than the Speed of Light

Sounds very odd indeed to me:

A pair of German physicists claim to have broken the speed of light - an achievement that would undermine our entire understanding of space and time.

According to Einstein's special theory of relativity, it would require an infinite amount of energy to propel an object at more than 186,000 miles per second.

However, Dr Gunter Nimtz and Dr Alfons Stahlhofen, of the University of Koblenz, say they may have breached a key tenet of that theory.

The pair say they have conducted an experiment in which microwave photons - energetic packets of light - travelled "instantaneously" between a pair of prisms that had been moved up to 3ft apart.

I'll admit that my physics isn't up to date (not that it ever was, O Level only) but isn't the point of Einstein's equations that you cannot accelerate a particle, a particle with a rest mass of greater than zero, to the speed of light, for that would require infinite energy? As photons have no rest mass (??) this doesn't in fact apply to them? And that the equations certainly do allow faster than light travel...it's the acceleration to it that is not possible?

Also, (from reading those Asimov et al books of science essays) isn't there some well known example of radio waves moving faster than the speed of light? Through a board (or matrix?) of some kind? The only problem being that information still moves through it slower than that limiting speed?

Ah well, no doubt someone who actually knows about these things will be along soon enough to explain it all to us.

August 16, 2007 in Science | Permalink

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Comments

"As photons have no rest mass (??) this doesn't in fact apply to them?"

Well, my physics may be mildly better, but is extremely rusty. With that caveat, I am tempted to say "no".

The famous e=mc^2 equation explains that energy is related to matter and matter is related to energy - there is an equivalence between them. A photon that has energy can, for the purposes of relativity be said to have a - very very very - small amount of mass.

This is I think the point of the wave/particle duality. The photon in question is only referred to as "a" photon, because it is considered - in this particular limited set of circumstances - to be behaving more as a particle than a wave. If it were a wave, you would be talking about "a" photon, because it would be a continuous stream of energy.

Or not, as I'm sure many of your erudite readers will no doubt enlighten (ho ho) us.

Posted by: Cleanthes | Aug 16, 2007 9:41:16 AM

As photons have no rest mess it implies that they have to travel AT the speed of light. After all, that makes sense as they ARE light.

(Anything with no rest mass would have to travel at c )

There's something called Quantum mechanics, which has at its core something called the Heisenberg Uncertainty Principle. This in turn implies something Einstein called 'Spooky action at a distance', which implies information exchange at faster than c (not a particle as we'd understand it travelling faster than c).

Relativity and Quantum Mechanics both have their weirdness - as they operate in the realms of the fast and the small - realms which our brains, evolved to deal with the scale of the African Savanah, and the speeds of predator and prey there aren't equipped to deal with. On those scales, both relativity and QM approximate to classical physics (which is good, as if they didn't we'd be having problems) - and where they differ, all experiments done back them up.

As some of the predictions are weird, they've been tested really well, to accuracies of 1 part in 10^10 in some cases, that's like predicting the width of the earth and being right to within the thickness of a human hair (or thereabouts).

The big trouble is that we know that one or both is not the end of the story - as there is an area of overlap where they contradict, e.g. this 'spooky action' of Einstein, or alternatively in the fast/high gravity but small scale realm (e.g. small black holes).

Hence, Physicists are looking for a grander theory which approximates to both relativity and QM, just as relativity and QM approximate to classical - or a Grand Unified Theory.

For more on QM, I can recommend: 'In Search of Schrodinger's Cat' by John Gribben.

Or to look at it another way. Occasionally, Science throws up experiments which contradict our understanding. This is interesting, and not something to be ignored (for example, the ultra-violet catastrophe in black body radiation). 'Spooky action' doesn't allow information able to travel faster than light - people have thought that they got around this in the past, but AFAIK it has been shown to be an error in the experiment.... however, that doesn't mean it will always be so.

Hope this helps.

Posted by: Murk | Aug 16, 2007 9:55:00 AM

"For more on QM, I can recommend: 'In Search of Schrodinger's Cat' by John Gribben."

I second that.

Posted by: Kay Tie | Aug 16, 2007 10:01:09 AM

Of course, this particular example is Quantum Tunnelling, not Spooky Action, but still.... I think the above still applies (I'd written the above as a stream of consciousness).

Apologies.

Posted by: Murk | Aug 16, 2007 10:03:21 AM

Also as far as the information thing, I thought that had already been disproved with the quantum entangled pair issue. Basically you can get two photons (I think it is photons) to effectively "link" themselves together, then separate them. However when you read the state of one photon the other photon immediately assumes the same state regardless of distance, hence information travels instantly

Posted by: Mark | Aug 16, 2007 10:45:00 AM

In this case what I thin is happening is that you have two entangled photons (or bits of photons). One goes a normal path from sender to receiver. The other takes a path through two prisms and demonstrates the same interference effect with the first one even though the path it has to have travelled in the same time is 3 foot longer.

So the photon has travelled faster than the speed of light but the information encoded has not.

(My explanation derived from http://www.cnet.com.au/broadband/cable/0,239035953,339281235,00.htm which seems to be about as good as it gets unless you shell out for the New Scientist article itself)

I have to say the desc

Posted by: Francis | Aug 16, 2007 12:00:40 PM

Chaio, Kwait and Steinberg did a similar experiment, over a much shorter distance, at Berkeley in 1993.

The basic problem of trying to explain anything to do with quantum physics is that it's all contrary to "common sense", which is why the common factor amongst all quantum physicists since about 1930 is a puzzled expression! :-)

For a good, if deliciously quirky, description of what Chaio et al were up to, have a look at http://www.dhushara.com/book/quantcos/qnonloc/qnonloc.htm

Posted by: pogo | Aug 16, 2007 1:48:04 PM

The important consideration being that information cannot travel faster that the speed of light in vacuum. It can, technically, travel faster than the speed of light in the medium you are observing (electrons in super-conductors?).

The quantum entanglement thing doesn't actually transmit information - the two entangled particles are already in the appropriate states - observing one merely causes the other to behave as if it had also been observed.

Quantum tunnelling is something completely different - it is effect that the blurred quantum wave function of a particle can have that means that its position, if observed / resolved, has a finite chance of being the other side of a close but, according to non-quantum physics, insurmountable energy barrier. This can be used to lead to useful effects and devices (resonant tunnelling diodes and scanning, tunnelling electron microscopes.)

(Flame-proof boxers on in case any actual physicists are reading this.)

Posted by: Surreptitious Evil | Aug 16, 2007 1:54:17 PM

TW

Reverting to a later post of yours concerning the data behind the science of climate change ("The Guardian on Steve McIntyre"), at least the guys at the University of Koblenz will have to provide sufficient information to make their experiment replicable and thus falsifiable before they achieve any scientific traction. Alternatively they can always get some failed overweight politician to make a film "proving" their contentions.

Posted by: Umbongo | Aug 16, 2007 2:13:21 PM

Have I ever told you that we've got the recipe for Frau Schrodinger's chocolate cake? It's pretty dire - no wonder he chased other women.

Posted by: dearieme | Aug 16, 2007 3:23:00 PM

Mark:
'However when you read the state of one photon the other photon immediately assumes the same state regardless of distance, hence information travels instantly'

Not quite. The state of the two photons is entangled, in that if you make a measurement of one, it affects the measurement of the other.

However, this is in such a way that it doesn't allow useful information transfer, this is because each measurement that's made affects the thing that's been measured.

Here's an analogous example. If we have two coins, one heads and the other tails. If I take one and look at it to find it is heads, I know that you have tails. Also, you know that I had heads. HOWEVER, this doesn't allow me to send you information as I cannot affect who had which coin.

It's a bit more complex than that, as the measurement affects the thing, but that'll do for now.

(Comments that coins can be flipped over are missing the point here, if you prefer, think of a red tiddlywink and a green tiddlywink)

Posted by: Murk | Aug 16, 2007 11:20:54 PM

RF waves travel faster than the speed of light in waveguides. I remeber the physics being explained when I did my training many years ago, but don't remember any of it now.

Tim adds: That's the bunny I was thinking of.

Posted by: Neil | Aug 17, 2007 12:06:01 AM

When RF waves travel faster than light, it is the _phase_ velocity (Vp). So Vp > c (this is the speed of the wave crests) but the _group_ velocity (the speed of the wave packet) carries the information and energy and for wave-guides Vp*Vg = c*c. This is what people mean when they say that it is "information" which is limited by 'c'.

Actually Vp measures the up-down motion of the packet -- a completely analagous situation is when you point sweep a laser across the face of the moon at a fast enough angular speed the dot on the moon's surface is travelling faster than light but the photons which create the dot travel at the speed of light from your laser to the moon.

Get it ?

Posted by: gh | Aug 17, 2007 3:47:23 AM

"which is why the common factor amongst all quantum physicists since about 1930 is a puzzled expression!"

I will use this in a book someday, Pogo. If you want credit, give particulars.

Tom

Posted by: Tom Kratman | Aug 17, 2007 3:52:15 AM

Tom... Sorry, I don't think that it's my original expression, it's at best a paraphrase of a comment I'm sure I read somewhere years ago. It just seemed so apposite that it stuck in my memory.

But Boy! is it true! :-)

Posted by: pogo | Aug 17, 2007 1:28:21 PM

Doc Taylor might know.

Posted by: Tom Kratman | Aug 17, 2007 11:16:45 PM

Near Light Speed Travel possible.


This is very close to the formula that we want. We want to know the value of τ when the traveler has made it halfway to the destination, because then the deceleration starts. If the total distance is X, then the total travel time T is given by

(8) X / 2 = (c2 / g) [cosh (0.5 g T / c) – 1]

T = (2 c / g) cosh–1 (1 + 0.5 g X / c2)

If X = 4.3 light-years, then T = 3.6 years. Dozens of stars could be reached in five to six years. In fact, a traveler could even go the Andromeda galaxy in under 29 years if a constant acceleration could be maintained

Posted by: mthomas | Aug 19, 2007 12:42:47 AM

one of Einstein's postulates of relativity is that no matter what reference frame you measure it in light can never be faster than c.

You could be in a spaceship traveling at 1/2*c then shine a light in front of you and someone that you fly past would measure that light to be going no faster than c.

Posted by: jean | Aug 21, 2007 4:04:28 AM

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