# Im not too clever, but faster than light?

Hurkyl
Staff Emeritus
Gold Member
Consider the following setup:

You have a circular room, 4 meters in radius.
A pillar stands in the center of the room.
You have a light source moving at one-half light speed in a circular path around the pillar with a radius of one meter.

How fast does the pillar's shadow travel along the wall of the room?

baywax
Gold Member
Consider the following setup:

You have a circular room, 4 meters in radius.
A pillar stands in the center of the room.
You have a light source moving at one-half light speed in a circular path around the pillar with a radius of one meter.

How fast does the pillar's shadow travel along the wall of the room?
Don't tell me the shadow is travelling at 2 x c.:surprised

russ_watters
Mentor
Why not...?

baywax
Gold Member
Why not...?
Its just an expression.

Let me guess, because there is no mass to a shadow it is possible for it to travel faster than c without discombobulating (another expression).

baywax
Gold Member
How about a finger turning a light switch to the on position? Does it travel at the speed of light?

The theory does not contradict itself - the explanations for laypeople often do though. Don't confuse them.

P.S. Having said all that, I'd doublecheck your textbooks.
- Are you sure they were proposing a rigid rod? Or are you mashing things in your memory?
My University textbooks from my physics degree course say it. [Not books for the layman]. And I distinctly remember thinking about this as soon as I read it. ie that relativity is flawed because it's a very circular argument, and is full of contradictions.

I think Einstein used the same explanation in his small booklets on relativity, not sure if he did so in the original 1905 paper though. Anyway I still don't believe in SR. Though the math obviously fits the data.
ie, I'm not convinced SR has a good logical explanation. Just an idea that works (though defies logical explanation IMHO), and it's cute because it all follows from the simple concept of the constancy of c, that's all.

- They might have been proposing it as a sort of 'reducto ad absurdum' - i.e. showing how it is impossible.
.nope. absolutely not.

How fast does the pillar's shadow travel along the wall of the room?
It travels at the same speed as the light coming from the light source, regardless of how quickly that light source travels. I must be missing something big time because to insinuate that a shadow can travel faster than the light producing that shadow seems, to me at least, surreal.

baywax
Gold Member
Why not...?
Are there any examples of a light source travelling at .5 c around a pillar in a rotunda? Because if there are no examples then there is only the idle and rather nerdy speculation that a shadow will travel faster than the speed of light

A shaddow CAN move faster than c,
because it carries no energy or information (it cannot exert a force etc)
so all is well.

However, if this is a fact, then it suggests a shaddow could be made to travel at infinite speed, and could be used to precisely synchronise all clocks thoughout the universe. The clocks could not otherwise be synchronised by the usual method of a pulse of light carrying energy.

Which in turn IS a contradiction of SR as far as I know.

SF
A shadow won't travel at infinite speed, that's just an illusion.
A shadow is defined by absence of light and that's limited by the speed of light.

If the sun were to dissapear, we'd only know that after ~8 minutes. Same about the shadow cone.

As you stand on the ground and rotate a laser beam (just like the shadow cone) parallel to the ground, the path of the photons will define a spiral, expanding outward from you with the speed of light.

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Hurkyl
Staff Emeritus
Gold Member
It travels at the same speed as the light coming from the light source, regardless of how quickly that light source travels. I must be missing something big time because to insinuate that a shadow can travel faster than the light producing that shadow seems, to me at least, surreal.
If the shadow in my example travelled at the speed of light, then it would go once around the room every 84 nanoseconds, while the lightsource goes once around the room every 48 nanoseconds. Do you really believe that?

Or what if I made the lightsource move very slowly... say, 1 meter per second. Do you still think the shadow should be travelling at the speed of light?

Basically, why the heck should the speed of light have anything at all to do with the speed that a shadow moves?

Hurkyl
Staff Emeritus
Gold Member
A shaddow CAN move faster than c,
because it carries no energy or information (it cannot exert a force etc)
so all is well.

However, if this is a fact, then it suggests a shaddow could be made to travel at infinite speed, and could be used to precisely synchronise all clocks thoughout the universe.
Why do you think that?

The local conditions that cause the shadow are limited by the speed of light. If you want to "send" a shadow to Proxima Centauri that travels at twice the speed of light, you will have to spend over two years sending preparatory information before you can "send" the shadow.

A shadow won't travel at infinite speed, that's just an illusion.
? A shadow is only the unlit region. I think the boundary of the shaddow can travel faster than light even though the photons inside the lit region don't.

A shadow is defined by absence of light and that's limited by the speed of light.
I don't think that's true. See above. There's no reason why the boundary cannot exceed c. The water trail of a hose-pipe played across some concrete can travel a good deal faster than the water droplets that make up the stream of water.
If the sun were to dissapear, we'd only know that after ~8 minutes.
I agree that's probably true. But it's not quite the same thing as the speed of a boundary between regions of the lit & the unlit (a shaddow) which is caused say by an object moving at speed much less than c past an adjacent light source is it. The further you are from the source the faster the shaddow will travel. I don't think it has anything to do with c.
The light is not required to move faster than c.
As you stand on the ground and rotate a laser beam (just like the shadow cone) parallel to the ground, the path of the photons will define a spiral, expanding outward from you with the speed of light.
so ? how does that show that the boundary between the lit and unlit cannot move across a surface (or any place) at a speed > c ? I don't think it does. Maybe I missed something there. Any-one shed some light on this ?

Why do you think that?

The local conditions that cause the shadow are limited by the speed of light. If you want to "send" a shadow to 'someplace', you will have to spend over two years sending preparatory information before you can "send" the shadow.
It's irrelevant that the shaddow has to be prepared. And I don't think you follow the argument. There is nothing in relativity that says a shaddow can't pass over an object at > c. (as far as I am aware)
It would however be difficult (impossible) to switch it off instantaneously, in exactly the same way that water keeps coming out of a hose-pipe after you turn the supply of photons off. ;-)

If the shadow in my example travelled at the speed of light, then it would go once around the room every 84 nanoseconds, while the lightsource goes once around the room every 48 nanoseconds. Do you really believe that?
You seem to have misread my post. I am not interested in the speed of the light source moving around the room. I am interested in the speed of the light coming from the light source. If you look more closely at my post you will find I view this example as bizarre, to quote "I must be missing something big time because to insinuate that a shadow can travel faster than the light producing that shadow seems, to me at least, surreal."
After viewing the last couple of posts I will be bailing out until sanity has been restored

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SF
See the attached image.

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You seem to have misread my post. I am not interested in the speed of the light source moving around the room. I am interested in the speed of the light coming from the light source. If you look more closely at my post you will find I view this example as bizarre, to quote "I must be missing something big time because to insinuate that a shadow can travel faster than the light producing that shadow seems, to me at least, surreal."
After viewing the last couple of posts I will be bailing out until sanity has been restored
Mosassam, the shadow on the wall is just a pattern. It's just like if you were pointing a flashlight or a laser to a wall and in semantical sense "moving the spot of light around" (And erroneously assuming "identity" to the spot of light or a shadow btw... "Fallacy of identity" :)

Anyway, by rotating the flashlight, you can obviously make the spot of the light that the flashlight produces to move at any given speed at all (if you care to measure such a "speed" at all), without the "photons" moving faster than C. (Or trivially call something a shadow and say it is moving at any arbitrary speed)

That is to say, when you move the spot of light from one position to another (at arbitrary speed), the spot of light does not carry any information. If there is a fly at location A, who wants to inform a fly in location B about something, this cannot happen by you moving the spot of light from A to B (the bug would have to inform you first, and then that could happen at speed C only... you can figure it out)

Sanity restored? :)

ZapperZ
Staff Emeritus
This is what will happen when actual physics question is asked in the Philosophy forum. If someone would have asked this in the Physics forums, we could have given a clear answer already, which had been available online years and years ago, since this is a very frequent question.

This issue has already been tackled (and put to bed) in this FAQ:

http://math.ucr.edu/home/baez/physics/Relativity/SpeedOfLight/FTL.html#3

As a general reminder, our guidelines regarding over-speculative posts and crackpottery applies throughout PF, including the Philosophy forums. While we may have given a bit of a freedom in discussing various philosophical aspects of the sciences, misinformation and outright fabrication of scientific facts and principles are not and will not be tolerated. So if you attempt to answer, for example, a specific physics question even in here, and you do not have a clear understanding of the physics involved, please defer to someone else who is more knowledgeable in that particular area. If not, you will get this same layer upon layer of confusion, unless creating such utter confusion is your intention in the first place.

Zz.

DaveC426913
Gold Member
Simpler analogy to the superluminal shadow:

Point a laser pen at the left edge of the Moon, and then instantly flick it to the right edge. The spot will have moved faster than c.

It does not violate relativity because no one part of it has moved faster than c. Also, you can't transmit information at >c this way.

exactly my point dave,
no energy moves > c just the edge of the shadow. hence you could use the edge of the shadow that's cast from the 'lighthouse' (or laser beam) to synchronise all events by this >c shadow which could in theory be made to travel as fast as you wish (>>>>c)

exactly my point dave,
no energy moves > c just the edge of the shadow. hence you could use the edge of the shadow that's cast from the 'lighthouse' (or laser beam) to synchronise all events by this >c shadow which could in theory be made to travel as fast as you wish (>>>>c)
Exactly so. This is not that different from synchronizing events by sending two photons to opposite directions.

Perhaps a practical example is helpful.

Say, you synchronize two light sensors - attached to opposite ends of a wall - by flicking a laser pen across them. For simplicity, let's say we are standing in the exact middle between the light sensors.

When the light hits the first light sensor (A), call that event A. And when the second sensor (B) is hit, call that event B.

Provided you flicked fast enough, now we have two events that are separated by a space-like distance (and in lab-frame A happens before B).

Yes, in terms of relativity this would also mean there are inertial frames in which event B occurs before A. (Like if judged from the frame of an object that is moving from sensor A towards sensor B. Call it "2nd frame")

But even in this "2nd frame" the "A-photons" were "sent off" from the laser pen before the "B-photons". Just that A-photons fly much longer distance (since in this frame sensor A is moving away; escaping the photons all their journey, while sensor B is moving towards the photons, shrinking the distance they need to move)

And yes, in this "2nd frame" the spot of light would be found to start from sensor B and move towards sensor A; opposite to how it moves in lab-frame.

If this still seems odd, remember the spot of light does not have an identity to itself. I.e. you can call it a "thing" only in semantical sense, but physically it is more proper to view it as a stable pattern that we just happen to call "spot of light". (When a building casts a shadow, is it the "same shadow" from one day to another, or from one moment to another? Same thing with this spot of light)

This is what will happen when actual physics question is asked in the Philosophy forum. If someone would have asked this in the Physics forums, we could have given a clear answer already, which had been available online years and years ago, since this is a very frequent question.
Philosophical issues concerning light and shadow have been opened up here, if confusion exists (which it does) it remains philosophical.

While we may have given a bit of a freedom in discussing various philosophical aspects of the sciences
Would this be the Royal "WE" or the fascist "WE"?
misinformation and outright fabrication of scientific facts and principles are not and will not be tolerated.
Please point out any "misinformation and outright fabrication of scientific facts" that has occurred on this post. You may find that "facts" has a loose interpretation on the Philosophy forum.

ZapperZ
Staff Emeritus
Philosophical issues concerning light and shadow have been opened up here, if confusion exists (which it does) it remains philosophical.
No, it wasn't. What happens with a "shadow" of light, etc. is NOT subject to "philosphical" interpretation. It is solved using the special relativity AND algebra. The confusion exists because people didn't understand either.

Would this be the Royal "WE" or the fascist "WE"?
Now don't start. You know fully well what you got into when you signed up on PF. I pointed out exactly which part of the PF Guidelines that one needs to pay attention to, and the fact that you are here explicitly implies that you agreed to those rules, or are telling me your word means nothing here?

Please point out any "misinformation and outright fabrication of scientific facts" that has occurred on this post. You may find that "facts" has a loose interpretation on the Philosophy forum.
Sure. You made one yourself when you wrote

The information involved in the non-local effects proposed by Bell's Theorem (and as experimentally demonstrated) travels faster than light. It appears to be instantaneous over large distances.
Spend any considerable amount of time in the QM forum on Bell-type experiments and you'll see the message repeated several times that NOTHING travelled faster than light in those experiments. Anton Zeilinger, in all his publications on Bell-type experiments, never once suggested such a thing.

The answer to these questions are not obtained via philosophical discussions. They are obtained via invoking physics principles and experimental observations. This is different than trying to figure out that they mean beyond what physics has given, which would fit in this forum and which I don't care to touch. However, some people have confused between the two.

Please read the issue surrounding the current crackdown in the Philosophy forums in the Feedback section.

Zz.

Spend any considerable amount of time in the QM forum on Bell-type experiments and you'll see the message repeated several times that NOTHING travelled faster than light in those experiments. Anton Zeilinger, in all his publications on Bell-type experiments, never once suggested such a thing.
As a layman/newbie philosopher I do not understand this point:
Does the measurement of the properties of one particle in a Bell corellation instantaneously affect the properties of the second particle, or not?
I ask this because, as I understand it, it does (obviously my understanding can be considered minimal) and this opens a very large can of philosophical worms (for me at least).

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ZapperZ
Staff Emeritus
As a layman/newbie philosopher I do not understand this point:
Does the measurement of the properties of one particle in a Bell corellation instantaneously affect the properties of the second particle, or not?
And this is why you shouldn't be using it without understand what it is you are using. This seems to be very prevalent on here, and resulted in the usage of physics ideas in rather strange and, at best, misleading way.

Note that the Bell experiments are trying to illustrate the entanglement properties of QM. Within that formulation, there is nothing that connects the entangled properties from one to the other. So how could something travelled when nothing is?

Also note that you can easily do this with classical properties. Take a particle, split it into two spontaneously, and measure the momentum of one. You immediately and instantaneously know the momentum of the other, no matter how far away it is. There's nothing strange here and no one argues about FTL transfer of anything. It is all conservation laws. The same applies to Bell-type experiments. What makes it different and "strange" is the superposition principle that is inherent in the system. Unlike a classical system in which the properties are actually well-defined even before a measurement, a quantum system does not have such property. It is this superposition of all possible outcomes that makes these two situations different. That is a very crucial ingredients that most people missed when they don't look at the details.

Zz.

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