Super luminous entities

Mr-T

There is a relativistic insect crawling up a chain. There is a light shining on the insect at a 90 degree angle with the chain. There is a shadow being cast on a wall behind the insect. If the insect is traveling up the chain with a speed .9c then the speed the shadow moves is greater than c. How can it be that this is the case?

ghwellsjr

A shadow is not a thing. Why do you think this should be a problem?

Mr-T

The speed that light travels should be unchanging from any vantage point. I suppose if I am to understand this concept I must understand what a "thing" is and why a shadow is not a "thing". But to avoid a response that is vague and general I shall restate the question in a way that will highlight what is truly crucial in my understanding of this phenomenon.

What is a "thing" as it appears to us? Why does a shadow not fit the criteria of what makes something a thing?

PeterDonis

It is. But the speed of light is not the same as the "speed" of the shadow.

I'll answer that question with another question. The insect is made of atoms. The light that shines past the insect is made of photons. What is the shadow made of?

mfb

You can use a laser pointer, point it on the moon and tilt it quick enough to get a laser spot "moving" with more than the speed of light on its surface (you will not see the spot, however). You can do the same with a flashlight and some shadow-casting object, and get a moving shadow.
No object (!) moves with >c, it does not violate relativity.

Mr-T

Thank you for the replies,

To PeterDonis,

See the thing is, we are quantifying over the "non-thing", the shadow, because it has velocity, and then of course it has momentum. And it is certainly massless. The two things that relativity has to say about the speed of light is: (1) The speed of an object with mass travels at a speed less than an object without mass. (2) Massless objects travel at a constant speed c.

It is undesirable to quantify over the absence of something, namely photons in this case. But, it appears we have because we have given this absence a velocity. How can relativity theory deal with this?

I suppose I shall try to elaborate on the question pertaining to what the shadow is made of. The shadow is really just a region with comparably less photons than the surrounding regions, hence it appears "darker". Technically, this forces the shadow to be a "thing" for a shadow is still comprised of photons. I suppose there are two possible routes I can take to arrive at an answer but I shall only confidently talk about the first because of my lack of knowledge.

1: As the angle between the insect and the light increases, the length of the shadow increase with some proportion to the angle between the insect and the light. If we add the increasing growth of the shadow to the speed of the insect we will get a speed greater than c.

mfb

Object is the important point. The shadow is not an object - and a bright spot is not an object either, even if it is defined as the presence of something.

There is nothing to deal with. No particle and no information is transmitted with superluminal speed.

PeterDonis

If you define "velocity" appropriately for the shadow, yes; but this "velocity" is not subject to the limitation that it must be less than the speed of light. This is because this...

... is false. Or at least, if you're going to claim that the shadow has momentum, you're going to have to justify it in detail; it's certainly not obvious (so your "of course" is a huge misstatement). The light that shines past the insect has momentum, but that light is not the same as the shadow.

Again, the light that shines past the insect is massless, but that's not the same as the *shadow* being massless. If the shadow is massless, what is it, exactly, that is massless? Photons? The light is made of photons, but the shadow isn't. See further comments below.

*You* have tried to give "the absence of photons" a velocity; but that's your problem, not relativity's. Relativity theory deals with this by saying that the shadow is not a "thing", so any "velocity" you define for it does not have to be less than the speed of light.

Ok, so what is a "region" made of?

No, it isn't. Or at any rate, you haven't shown that it is. All you've shown is that we can pick out a "region" that has fewer photons than surrounding regions. That's not the same as saying that a region is made of photons.

Yes, we do. So what? Unless you can show that this is equivalent to saying that "something" moves faster than light, you haven't shown any contradiction with relativity. All you are showing is that the boundary of some "region" you've picked out "moves" faster than light. Relativity theory says the region, or its boundary, isn't a "thing" because it isn't made of anything.

nitsuj

There it is, a shadow is a region, so is space, so is length.

Reminds me of Phinds comment regarding it being impossible to measure the length of purple, in an earlier thread.

The region is being "defined" FTL, not traveling a velocity. With your reasoning the same could be said for your body, that your height is a velocity FTL be cause the top and bottom are simultaneous, sounds like nonsense right?

Mr-T

Peter your points have been understood,

I suppose our thought process was flawed at the start by assuming the shadow has a velocity.

If we are to assume the shadow is not a "thing" and therefore does not have an upper speed limit. Also, relativity does not account (for lack of a better way to state it) for "non-things".

By EPRs standard of what an element of reality is: "An element of reality is a phenomenon if without in any way disturbing a system, we can predict with certainty (i.e., with probability equal to unity) the value of a physical, then there exists an element of physical reality corresponding to this physical quantity." (Einstein has accepted this condition)

So, what makes the shadow an element of reality is the fact that we can predict the rate at which the shadow expands with probability equal to unity.

If we propose that a shadow is not a "thing" then we are saying that Einstein has a problem with relativity for because the shadow is an element of reality but relativity does not justifiably explain this observed phenomenon.

To progress the learning, what is the reason why we are not accepting the shadow as a thing?

DaleSpam

This is a little incorrect. Consider the example of an eclipse. There is a 3D volume between the moon and the earth with considerably less photons than the surrounding volume, but this volume is NOT a shadow. The shadow is the intersection of this volume with some appropriate surface, like the surface of the earth.

A shadow has neither energy nor momentum, so its speed is not limited by relativity in any way. Things with energy and momentum come in three "flavors", things with more energy than momentum (massive objects, v<c), equal energy and momentum (massless objects, v=c), and less energy than momentum (don't exist, v>c). The categorization of an object with momentum and energy as one of those three categories is both frame invariant and constant over time.

ghwellsjr

Let's consider doing the same thing in outer space with a machine gun firing bullets at a distant target and making holes in the target such that the progress of the holes is faster than the speed of light. Is a hole a thing?

Mentz114

Even if a hole is a thing - each hole is a different thing, and the progression of holes is not the track of one thing travelling at some definable speed.

Mr-T

The criteria of what it is to be thing-like is ad hoc. All of the responses answer why the shadow is not a thing only because we "know" that things cannot be traveling faster than c.

This is a problem. The premises of these explanations do not "need" to be true in order for the conclusion to be true.

From basic logic: (false statement) implies (true statement)
This is always true, no matter the initial conditions.



In other words,
Everyone seems to be under the same impression that an intrinsic characteristic of a "thing" is that it is not able to travel greater than c. So, no superluminal "thing" can ever be found.

So the problem is avoided, of finding a superluminal thing, by saying that any "x" that is superluminal is not a thing. This amounts to saying, "It isn't a thing because I say so."

I remain unconvinced by this argument. Therefore, any theory accepting c as the upper speed limit cannot handle discovering a superluminal thing. Thus, any theory that cannot handle a superluminal thing cannot be a complete theory (quantum mechanics falls here as well).

DaleSpam

Of course it is ad hoc. "Thing" is not a well defined scientific term so any use will naturally be ad hoc.

There are two options:

1) Do not provide a rigorous scientific definition of "thing". Then SR does not make any well defined predictions of the behaviors of "things" and any discussion will be ad hoc. This is not a limitation of the theory, but a limitation of the definition of "thing".

2) Provide a rigorous scientific definition of "thing". Then SR can make well defined predictions, and depending on your definition that prediction could include a limit on the speed of "things".

Right now, we are in situation 1. That is why I put my previous response in terms of well defined terms like energy and momentum and not "thing"-ness.

Mr-T

To DaleSpam,

I completely agree with your first point but I am curious, is there any basis for undermining the definition of thing as opposed to undermining relativity?

It appears to me that if one could not make an appropriate definition for something as simple as "thing" then that same person could not create an appropriate physical schema which is a guideline for how our universe acts. Unless of course, the theory is flawed.

The theory would be flawed in the sense that it cannot account for a natural occurrence found in nature.

PeterDonis

It's a pity Einstein isn't still around so we can ask him whether he would accept the additional premise that no "element of reality" can travel faster than light. His answer would have been interesting. But since he isn't still around, we'll have to do the best we can without him; see below.

Now you are assuming that "element of reality" is equivalent to "thing" in the sense you're using that term (i.e., a "thing" can't move faster than light). It isn't. By Einstein's criterion, the charge on the electron is an "element of reality". How fast does it move?

See above for why "element of reality" is not the same as "thing". But even if we leave that out, where have we shown that relativity can't explain the shadow's behavior? It explains it just fine; you gave the explanation yourself. Relativity's explanation simply doesn't say that the shadow can't "move" faster than light. What's the problem?

Because it's not made of anything. As far as relativity is concerned, that's really the only answer that can be given. Relativity is not a theory of all the different types of "things" that can exist. If you really want to know why a shadow isn't a "thing" while an ant and a photon are, you need to look at the standard model of particle physics, which catalogues all the fundamental "things" we know of that all other "things" are made of. A shadow isn't made of any of them, so it's not a thing.