Unknown to many, phenomena can travel faster than light

[Ummon]

http://www.nytimes.com/2007/06/20/op...erland&emc=rss

ON Thursday, on the summer solstice, the Sun will celebrate the year’s lazy months by resting on the horizon. The word solstice derives from the Latin “sol” (sun) and “sistere” (to stand still). The day marks the sun’s highest point in the sky, the moment when our shadows shrink to their shortest length of the year. How strange to think that these mundane friends, our ever-present familiars, can actually go faster than the sun’s rays.

I remarked on this recently to my husband as we sat on the porch with our shadows pooling by our chairs. Nothing can go faster than light, he insisted, expressing what is surely the most widely known law of physics, ingrained into us by a thousand “Nova” programs.

That is the point, I explained: Nothing can go faster than light. A shadow isn’t a thing. It’s a non-thing. It’s the absence of light.

Special relativity dictates that we cannot move anything more quickly than the particles of light known as photons, but no law says you can’t do nothing faster than light. Physicists have known this for a long time, even if they generally do not mention it on PBS documentaries.

My husband looked troubled, as did my sister and some friends I regaled with the story that evening. Like the warp drive on “Star Trek,” faster-than-light travel is supposed to be a science-fiction fantasy. Isn’t it?

They are right about the travel: According to relativity, no physical substance can exceed the speed of light because it would take infinite energy to accelerate anything to such a velocity.

Yet the laws of physics pertain only to that which is. That which isn’t is not bound by relativity’s restraint. From the point of view of relativity, a shadow (having no mass) is a non-thing, an existential void.

It’s quite easy to conjure up a faster-than-light shadow, at least in theory. Build a great klieg light, a superstrong version of the ones set up at the Academy Awards. Now paste a piece of black paper onto the klieg’s glass so there is a shadow in the middle of the beam, like the signal used to summon Batman. And we are going to mount our light in space and broadcast the Bat-call to the cosmos.

The key to our trick is to rotate the klieg. As the light turns, the bat shadow sweeps across the sky. Round and round it goes, projecting into the void. Just as the rim of a bicycle wheel moves faster than its hub, so too, away from the source our bat shadow will fly faster and faster, a consequence of the geometry that guarantees the rim of a really big wheel moves faster than a co-rotating small wheel.

At a great enough distance from the source, our shadow bat will go so fast it will exceed the speed of light. This does not violate relativity because a shadow carries no energy. Literally nothing is transferred. Our shadow bat can go 10 times the speed of light or 100 times faster without breaking any of physics’ sacred rules.

My sister leapt to the heart of this apparent paradox: Why isn’t the light itself traveling faster than the speed of light? Isn’t it also rotating in space? Actually, no. The bulbs that produce the light are spinning, but the light particles leave the source at 186,000 miles a second, the vaunted “speed of light.” Once emitted, the photons continue to travel at this speed directly away from the source. Only the shadow revolves around the great circle. The critical point is that no object, no substance, defies light.

My husband was right to object that you’d need one spectacular klieg to produce a detectable shadow thousands of miles out in space. Still, the theory is sound.

The anthropologist Mary Douglas noted that all systems of categorizing break down somewhere, unable to incorporate certain forms. By standing beyond relativity’s injunction, shadows suggest the limits of all classification schemes, a tension that even modern science cannot completely resolve.

In the terms recognized by relativity, shadows are non-things. Yet before the invention of clocks, shadows were the most important means for telling time. Weightless and without energy, shadows can nonetheless convey information — though they cannot, despite our giant klieg, be used for faster-than-light communication. That’s because the shadow’s location cannot be detected until the light, moving at its ponderous relativistic pace, arrives.

“Here there be monsters,” said the medieval maps, signaling the limits of reason’s reach. As a map of being, physics is flanked by the monsters of non-being whose outlines we glimpse in the paradoxes of quantum mechanics and in the zooming arc of a shadow bat going faster than light.

More properly, anything which doesn't convey matter-energy or information, can travel faster than light.

[David Deas]

Oh. Yeah. You know. Check it out. I just spun around in a circle *really* fast like an figure skater. Relative to the Moon my angular velocity was faster than the speed of light!

[Ummon]

All movement is relative, like some guy called Einstein said. :wink:

By the way: inappropriate example, seeing you moving conveys information.

[Juvenal]

This is tosh...

Suppose I put my (fire-proof) bat shaped cut-out on the surface of the Sun, it will be 8.5 minutes before the shadow shape will be visible on Earth.

If I move the bat, it will still be 8.5 minutes before you see the movement on Earth.

If I replace the Sun with a lighthouse, then I could have it rotate such that the light beam should be sweeping around Earth's orbit faster than light. In actual fact the light beam bends like a catherine wheel. Any shadow in the beam would follow the beam.

From Earth, my lighthouse beam might appear to be travelling faster than light, but this effect is like a wave in water - the actual light beam is moving outwards (at the speed of light), the "thing" moving "faster than light" around the orbit is just a succession of photons emitted at different times.

Obviously the beam (or shadow) cannot carry any information from where I first see it on Earth to any other part of the orbit, because the information is actually travelling outward from the Sun - not around the path swept by the beam.

[Ummon]

It is the relative movement of the bat (increased by angular movements of earth and sun) on earth's surface and not the movement of the shadow towards earth, which is faster than light.

The article is not very well written, but it is the first I could find on the subject. A subject which, by the way, has been mathematically demonstrated. If need arises, I will look for a formal proof in the next days.

http://www.spacebanter.com/archive/i...p/t-67359.html

Nothing is wrong with your logic- a shadow can certainly move faster
than the speed of light. There are all sorts of phase effects that also
move faster than light- consider something like a giant pair of
scissors; the cutting point can move much faster than light as the
scissors close (even an ordinary pair of scissors will allow you to move
the cutting point at faster than the speed of sound- but you don't hear
a sonic boom, do you?) In these cases, no physical object is moving
faster than c.

A good test for whether something is violating the c limit is whether
any _information_ is being conveyed faster than light. In the case of
your shadow, it pretty clearly is not. That is, there is no way that
someone at the Earth could in any way use the moving shadow to send
information to Pluto.

A shadow isn't really anything in a physical sense, so it isn't
surprising there are multiple ways of interpreting this. I was
considering the edge of a shadowed region (which is pretty much what the
OP was describing). So let's say I'm standing by an omnidirectional
light source. I give it lots of time to send light outward- so that
observers in two different solar systems several light years apart, but
the same distance from me, see it. Now I block my source from one side,
taking a few seconds to do so. That will produce a moving shadow, and
one of the observers will enter the shadow a few seconds before the
other. You could say that the shadow "moved" from one observer to the
other very quickly- much faster than c. Of course, no information is
being carried between the two observers, and no individual photons are
moving faster than c.

[Juvenal]

It is the relative movement of the bat (increased by angular movements of earth and sun) on earth's surface and not the movement of the shadow towards earth, which is faster than light.

I presume you mean the movement of the shadow of the bat.

What I was trying to point out is that nothing is actually moving anywhere at greater than light speed.

When you see a mexican wave in a football stadium, nobody actually has to leave their seat - the wave is your interpretation of the relative movement of multiple objects.

A shadow or light beam has the same property. If I shine a light beam toward Mars, then redirect it toward the Moon - nothing has actually moved from Mars to the Moon, so it is meaningless to talk about the lateral speed of the light beam (or any shadow in it).

[Medicus]

It's actually possible to travel faster than light.

Mind you, your ship would have to be the size of an atom to do it...

[Thanatos]

It's actually possible to travel faster than light.

Mind you, your ship would have to be the size of an atom to do it...

No, David Tennant, it's not possible. Your phone box ain't going nowhere.

[Medicus]

No, David Tennant, it's not possible. Your phone box ain't going nowhere.

David Tennant? Who is this man you speak of? My name is the Doctor. Just the Doctor.

Back on-topic, the Wiki has something on it here.

[Ummon]

Phenomenon: it means "something which appears". While the absence of something is not something, on the other hand you see, the threshold between light and the lack thereof is a clearly distinguishable phenomenon.

[Juvenal]

Phenomenon: it means "something which appears"...

I was illustrating that my attention might qualify as a phenomenon. The location of my attention is wherever it appears to me that it is - a phenomenon does need an objective location because we have already agreed that it doesn't have to be a "thing" which obeys the laws of physics.

I don't understand how you can locate my attention in my brain - my brain looks nothing like the Andromeda galaxy. I think you are confusing my attention with the brain activity of which it is an emergent property.

My second illustration has my lighthouse beam move from Mars to the Moon faster than the speed of light. The phenomenon here is the appearence of the beam sweeping around. The physical nature of the beam is actually a stream of photons moving away from the source, so nothing actually moves from Mars to the Moon, except the phenomenon.

I think that the fundamental cause of the debate in this thread is that we have never agreed on whether we are using the physical or the philosophical meaning of Phenomenon. Ummon seems to lean toward the philosophical meaning (see quote above), whereas most of the rest of us are thinking of physical phenomena (i.e. observable events).

If we can agree that we are discussing physical phenomena, then maybe we can decide if the movement of a shadow qualifies as one. Personally I don't think it does... but maybe I'm wrong.

[Spurius]

I think I remember something about the group velocity of light being higher in a plasma than in a vacuum. No information transfer either, but that's the most common type of thing, that the speed of light is surpassed, but only as a 'group velocity'.

http://en.wikipedia.org/wiki/Group_velocity

Yup, I remembered correctly.

[TheKwas]

At a great enough distance from the source, our shadow bat will go so fast it will exceed the speed of light. This does not violate relativity because a shadow carries no energy. Literally nothing is transferred. Our shadow bat can go 10 times the speed of light or 100 times faster without breaking any of physics’ sacred rules.

My sister leapt to the heart of this apparent paradox: Why isn’t the light itself traveling faster than the speed of light? Isn’t it also rotating in space? Actually, no. The bulbs that produce the light are spinning, but the light particles leave the source at 186,000 miles a second, the vaunted “speed of light.” Once emitted, the photons continue to travel at this speed directly away from the source. Only the shadow revolves around the great circle. The critical point is that no object, no substance, defies light.

Anyone else notice the different criteria being used here? The light is being judged based on the speed of a photon, while the shadow is being judged on the speed of the 'phenomenon' (as Ummon just explained it). Of course, if I judge the speed of the light based on the phenomenon of the light,rather than the photons, they would both be going the same speed, no?

Seems like just an odd game of semantics.

[Ummon]

Actually, by necessity, presence there is limited by the speed of light, and absence, not so. This means, that of course, light can be traced back to photons which travel in space, shadow on the other hand exists only as absence of light.

But semantic games aside, phases can travel this way, and they are implicit to light waves. We should not mistake examples with cathegories. As I said: everything which doesn't convey matter or information. A border cannot exist without something at its limit. That something travels at the speed of light.

[Juvenal]

Actually, by necessity, presence there is limited by the speed of light, and absence, not so...

...Not so, my lighthouse beam can move from illuminating Pluto to illuminating Mercury within a few seconds.

Also, I can look at the Andromeda galaxy, then move my attention to the cup of tea at my elbow almost within the blink of an eye.

Once you start talking in terms of "phenomena" rather than actual things, then the speed of light, or any other law that affects "things" need no longer be a factor.

[Ummon]

The light you see is x seconds old. The shadow you see has no time.

[chris_uk_83]

The light you see is x seconds old. The shadow you see has no time.

Of course it does, since the shadow requires photons of visible light to strike the surrounding area, thus making it visible by contrast. You can't have a shadow with no light source, and light takes time to travel.

I agree though that it doesn't really make much sense to talk about these 'phenomena' moving faster than the speed of light; it's pretty obvious that you can do it, as Juvenal points out so eloquently. It does not however, mean that you can transmit information from one place to another faster than the speed of light.

You can shine a light on the moon, then move it to a star 5 light years away in a second, but no information has travelled from the moon to the star, and the star hasn't actually received any light at all; it won't either, for 5 years.

So yes, it's easy to show; but it's meaningless to debate.

[Ummon]

Of course it does, since the shadow requires photons of visible light to strike the surrounding area, thus making it visible by contrast. You can't have a shadow with no light source, and light takes time to travel.

Actually, you will see a shadow without any light in the environement, because your neurons willl sporadically fire (that is anyway a subjective illusion).

The volume of the shadow can be said to extend in four dimensions, and light moves and forms the border, if one really wants to be poetic, too.

I agree though that it doesn't really make much sense to talk about these 'phenomena' moving faster than the speed of light; it's pretty obvious that you can do it, as Juvenal points out so eloquently. It does not however, mean that you can transmit information from one place to another faster than the speed of light.

You can shine a light on the moon, then move it to a star 5 light years away in a second, but no information has travelled from the moon to the star, and the star hasn't actually received any light at all; it won't either, for 5 years.

So yes, it's easy to show; but it's meaningless to debate.

Well, not everything which is meaningless today will always be so, and it was a curiosity to report, I guess.

[Spurius]

Pondering group velocities made me think on how a school of fish reacts faster than any individual fish's reaction time can explain.

There it DOES convey information, somehow. The sum is faster in reaction to stimuli than the parts.

[chris_uk_83]

Indeed it can, but it still doesn't do it faster than the speed of light.