Some Thought Experiments (simple ones)

[Deepak Kapur]

1. A clock is placed on a light beam.

Will the clock stop?

If yes, why doesn't the beam stop altogether?


2. A ruler is placed on a light beam.

Will it's length shrink to zero? Why/Why not?


3. A massless particle is placed on the beam.

What would happen?


4. An electron is placed on the beam.

What would happen?

The things to be placed on the beam can be even more varied.

 

[HallsofIvy]

I am afraid you will have to be more explicit. HOW do you "place a clock on a light beam"? Wouldn't it just fall right through?

If you mean "move a clock at the speed of light", that's impossible. Nothing with mass can move at the speed of light. And asking "what would happen if they could" is like asking "Suppose relativity were wrong. What would relativity say about this situation?"

As far as (3) is concerned, where you say "A massless particle is placed on the beam", you are essentially putting a photon into a light beam! Nothing new would happen.

 

[Deepak Kapur]

I am afraid you will have to be more explicit. HOW do you "place a clock on a light beam"? Wouldn't it just fall right through?

If you mean "move a clock at the speed of light", that's impossible. Nothing with mass can move at the speed of light. And asking "what would happen if they could" is like asking "Suppose relativity were wrong. What would relativity say about this situation?"

As far as (3) is concerned, where you say "A massless particle is placed on the beam", you are essentially putting a photon into a light beam! Nothing new would happen.

I think your contention is wrong because thought experiments are always highly hypothetical and don't tend to be contained in restrictions.

That's why they are called thought experiments after all!

 

[dx]

I think your contention is wrong because thought experiments are always highly hypothetical and don't tend to be contained in restrictions

It's ok for thought experiments to be practically difficult, but they cannot be impossible in principle. To say anything about a thought experiment, you have to be in a conceptual framework in which they are in principle possible.

 

[JulianM]

Deepak,

I am also a confused learner, and I did like your questions, because I have the same ones.

I am not sure I have any answers, so this reply is more of trying to understand the questions.

First, when you say put a clock on the light beam - should I assume you mean the clock is traveling at the speed of light ?

If that is your question it seems to me to be very valid. Certainly the wave frequency of light and various particles have been used to measure time, so in a way we can consider a beam of light to be a clock itself.
So I think your question, and therefore all the following questions, are very worthy of discussion.

Is what you are asking - what happens to various items when they travel at the speed of light ?

If so then I'd be pleased to explore the issues through discussion.

 

[Deepak Kapur]

Deepak,

I am also a confused learner, and I did like your questions, because I have the same ones.

I am not sure I have any answers, so this reply is more of trying to understand the questions.

First, when you say put a clock on the light beam - should I assume you mean the clock is traveling at the speed of light ?

If that is your question it seems to me to be very valid. Certainly the wave frequency of light and various particles have been used to measure time, so in a way we can consider a beam of light to be a clock itself.
So I think your question, and therefore all the following questions, are very worthy of discussion.

Is what you are asking - what happens to various items when they travel at the speed of light ?

If so then I'd be pleased to explore the issues through discussion.

You got it right my dear!

 

[Deepak Kapur]

It's ok for thought experiments to be practically difficult, but they cannot be impossible in principle. To say anything about a thought experiment, you have to be in a conceptual framework in which they are in principle possible.

I am nobody to talk of Einstein, but when he did his thought experiments, they were indeed impossible in principle. It's only afterwards that a conceptual framework in which they became possible was devised.

1. Light is at least 'something' if not matter. How could anything move with the speed of light?

2. Photons of light have been proved to possesses particle nature, How can these 'particles' travel with the speed of light.

3. The photons of light must be involed in some vibration, oscillation, etc. When time stops at the speed of light, how are such processes possible in the case of photons.

I have now asked direct questions rather than giving them the shape of 'thought experiments'.

 

[starthaus]

2. Photons of light have been proved to possesses particle nature, How can these 'particles' travel with the speed of light.

Because their rest mass is zero.

 

[Frame Dragger]

What you have is an active imagination, not a thought experiment. There is a difference, and when you open with "on a light beam" you're going to get a reaction.

That said, you have now asked questions, but given #1, it's clear you are leaping to conclusion with 2 & 3.

As for what light IS, it's just part of the spectrum of Electromagnetism. You haven't asked questions, you've made assumptions, and want things to fit them. You need to learn many MANY more of the basics (such as how to read the diagram that would answer your question) before you form assumptions about what MUST be.

 

[dx]

I am nobody to talk of Einstein, but when he did his thought experiments, they were indeed impossible in principle. It's only afterwards that a conceptual framework in which they became possible was devised.

Nope. For example, Einstein's thought experiment where he considered what he would observe if he traveled with a light beam was performed in the framework of Galilean/Newtonian mechanics. In that framework, it is possible to move at any speed. By doing this thought experiment, he discovered that the galilean transformation formulas did not allow Maxwell's electrodynamics to obey the principle of relativity of uniform motions.

1. Light is at least 'something' if not matter. How could anything move with the speed of light?

2. Photons of light have been proved to possesses particle nature, How can these 'particles' travel with the speed of light.

Light particles have no mass. This is why they can move at the speed of light.

3. The photons of light must be involed in some vibration, oscillation, etc. When time stops at the speed of light, how are such processes possible in the case of photons.

You are imagining photons to be like ordinary objects like balls etc. They are not. They don't vibrate or oscillate. Secondly, a photon is a quantum mechanical idea, and you can't even really think of it as haveing a definite speed or trajectory, so it's better if you stick to classical electrodyanmics when you consider such questions.

 

[Frame Dragger]

Nope. For example, Einstein's thought experiment where he considered what he would observe if he traveled with a light beam was performed in the framework of Galilean/Newtonian mechanics. In that framework, it is possible to move at any speed. By doing this thought experiment, he discovered that the galilean transformation formulas did not allow Maxwell's electrodynamics to obey the principle of relativity of uniform motions.



Light particles have no mass. This is why they can move at the speed of light.



You are imagining photons to be like ordinary objects like balls etc. They are not. They don't vibrate or oscillate. Secondly, a photon is a quantum mechanical idea, and you can't even really think of it as haveing a definite speed or trajectory, so it's better if you stick to classical electrodyanmics when you consider such questions.

Damn it, that's practically like sacrificing a goat to summon the Bohmians! :rofl: (no offense Demyst and Zenith!)

I think maybe, geometry would be a good place to start too; geodesics do make things far more clear.

 

[Deepak Kapur]

Because their rest mass is zero.

So, at the speed of light why is their mass of the value that it actually is? Why not more or less? Why not infinite?

 

[starthaus]

So, at the speed of light why is their mass of the value that it actually is? Why not more or less? Why not infinite?

What do you mean? The photon rest mass is zero, period.

 

[DaveC426913]

So, at the speed of light why is their mass of the value that it actually is? Why not more or less? Why not infinite?

Not sure what you're asking here. Are you asking why is their mass what it is? Or why is the speed of light what it is?


In the latter case, that is the 64 thousand dollar question.

 

[Deepak Kapur]

What you have is an active imagination, not a thought experiment. There is a difference, and when you open with "on a light beam" you're going to get a reaction.

That said, you have now asked questions, but given #1, it's clear you are leaping to conclusion with 2 & 3.

As for what light IS, it's just part of the spectrum of Electromagnetism. You haven't asked questions, you've made assumptions, and want things to fit them. You need to learn many MANY more of the basics (such as how to read the diagram that would answer your question) before you form assumptions about what MUST be.

Ok. I ask again.

You mean to say that light is energy. Does it mean that energy (being at least something) is able to move with the speed of light.

So, what about mass energy equivalence. Mass can be taken to be as a concentrated (unqualified term, I suppose) form of enegy.

 

[DaveC426913]

Ok. I ask again.

You mean to say that light is energy. Does it mean that energy (being at least something) is able to move with the speed of light.

So, what about mass energy equivalence. Mass can be taken to be as a concentrated (unqualified term, I suppose) form of enegy.

You have not asked a question.

 

[Deepak Kapur]

Not sure what you're asking here. Are you asking why is their mass what it is? Or why is the speed of light what it is?


In the latter case, that is the 64 thousand dollar question.

I am asking both the questions.

 

[DaveC426913]

I am asking both the questions.

In both cases, they are the 64 thousand dollar question. We don't know.

 

[Frame Dragger]

You have not asked a question.

Indeed... I'm wondering if this isn't the crank's version of the Socratic Method. :grumpy: If not, this is still going nowhere with someone who has no interest in anything but confirmation of their bias.

 

[DaveC426913]

Indeed... I'm wondering if this isn't the crank's version of the Socratic Method. :grumpy: If not, this is still going nowhere with someone who has no interest in anything but confirmation of their bias.

Possibly, but I can give him/her the benefit of the doubt. I think it's more a matter of him/her seeing apparent discrepancies and wanting to explore them but not having enough knowledge to formulate well-defined questions. His/her questions place the burden on us, the answerers, to supply the definitions and terminology.

 

[Deepak Kapur]

You have not asked a question.

I think you are not inclined to give an answer, or you recognise questions only with the help of the 'interrogation sign'. Otherwise lack of questionmarks doesn't mean that there is no question.

 

[yuiop]

1. Light is at least 'something' if not matter. How could anything move with the speed of light?

Here is an example of "something" that can move at the speed of light or even exceed the speed of light:

Consider a long straight shoreline and a wave coming into the shore that is not exactly parallel to the shore. If you call the intersection of wave with the shoreline as it hits the shore a thing, then that thing can move along the shoreline at greater than the speed of light. The inportant thing to note here is that thing (the intersection) is not a physical object made of mass and nor can the intersection be used to transmit useful information along the shoreline at superluminal velocities.

Now consider another analogy. Imagine you have a large calm pond. You throw a pebble into the middle of it and the ripples spread out from where it lands and eventually arrive at the shore. Now none of the molecules of water that were near where the pebble landed are in the ripple that arrives at the shore. What arrives at the shore is energy and information. The individual molecules in the water just bob up and down, but do not travel horizontally. You can think of light as a disturbance in electromagnetic fields that propogates at c and transmits energy and information, but you can not think of the distubance as a particle with rest mass moving at c.

 

[DaveC426913]

I think you are not inclined to give an answer, or you recognise questions only with the help of the 'interrogation sign'. Otherwise lack of questionmarks doesn't mean that there is no question.

[sarcasm]Yes, it's my fault you're not getting answers.[/sarcasm]

I cannot psychically guess what parts of your statement you don't understand and want clarification on. The interrogation sign is the indicator of that.

You spend time responding and criticizing instead of spending that time formulating your questions.

 

[Deepak Kapur]

Nope. For example, Einstein's thought experiment where he considered what he would observe if he traveled with a light beam was performed in the framework of Galilean/Newtonian mechanics. In that framework, it is possible to move at any speed. By doing this thought experiment, he discovered that the galilean transformation formulas did not allow Maxwell's electrodynamics to obey the principle of relativity of uniform motions.



Light particles have no mass. This is why they can move at the speed of light.

You are imagining photons to be like ordinary objects like balls etc. They are not. They don't vibrate or oscillate. Secondly, a photon is a quantum mechanical idea, and you can't even really think of it as haveing a definite speed or trajectory, so it's better if you stick to classical electrodyanmics when you consider such questions.

I am not doing so. I think according to QM, waves are associated with photons. So, these waves must be involving some activity, some process (even if there is a standing wave). How is this possible when time itself has stopped?

Plz don't feel pestered.

 

[Frame Dragger]

I think you are not inclined to give an answer, or you recognise questions only with the help of the 'interrogation sign'. Otherwise lack of questionmarks doesn't mean that there is no question.

@DaveC: I'd agree, but see above. The combativeness is unusual in someone who is merely curious, but uninformed such as me. I know at least, when to ask and listen, not play word games with the people who are trying to help me.

That said, I suppose I look for cues in the writing style and approach, and I'm still getting a feeling. The thread title, the reference to "Who am I to talk about Einstein" followed by talk of Einstein.

I see smoke, but no fire yet. That said, wise man evacuate building filled with smoke, not wait for flames to lick backside! Fortune cookies... the silliest wafers on earth.

There should be a mechanism in this forum that doesn't ban or kick, but kind of puts some people in a, "how to learn" boot camp. Then again, I do tend to get a bit militant in these situations.

@Deepak Kapur: He gave you an asnwer that NOBODY enjoys saying, namely: "WHO KNOWS?" Would you prefer that kev or Dave LIE to you? You're asking "Why" instead of "How", in a very complex, contentsted, and incomplete region of physics. Why not take a moment to consider that to ask the right questions is ALWAYS step 1. You need to learn basics so that you have a foundation upon which you can build, and the best way to judge that, is by what questions you ask, and how you respond.

I'm not criticizing your intellect, just your approach. Is that so terrible?

EDIT: There is something called Wave-Particle Duality, which is in an of itself a major branch of research, and an outstanding quandry for physics! I know for a fact that there are dozens here who would walk you through the inclined plane, through GR and QM... you're eschewing that for reasons I can only guess at.

 

[Frame Dragger]

I think you are not inclined to give an answer, or you recognise questions only with the help of the 'interrogation sign'. Otherwise lack of questionmarks doesn't mean that there is no question.

@Deepak: You do realize that everyone in this thread is TALKING to you, and not REPORTING you?! Shouldn't that tell you that maybe you shouldn't just spit in DaveC's eye?!

 

[DaveC426913]

Plz don't feel pestered.

Then don't put the blame on us if you're not getting the answers you expect.

I think according to QM, waves are associated with photons. So, these waves must be involving some activity, some process (even if there is a standing wave). How is this possible when time itself has stopped?

You can think of a photon as a connection between two points in spacetime. Since, in this 4-dimensional matrix, time is simply another dimension, and not something that ticks by, effectively everything is static.

A photon is akin to a piece of string attached at two points. This piece of string has a helical shape whose crest-to-crest distance can be measured.

Note that the piece of string and its undulation is completely static; it involves no movement, it involves no passage of time.

 

[Deepak Kapur]

Here is an example of "something" that can move at the speed of light or even exceed the speed of light:

Consider a long straight shoreline and a wave coming into the shore that is not exactly parallel to the shore. If you call the intersection of wave with the shoreline as it hits the shore a thing, then that thing can move along the shoreline at greater than the speed of light. The inportant thing to note here is that thing (the intersection) is not a physical object made of mass and nor can the intersection be used to transmit useful information along the shoreline at superluminal velocities.

Now consider another analogy. Imagine you have a large calm pond. You throw a pebble into the middle of it and the ripples spread out from where it lands and eventually arrive at the shore. Now none of the molecules of water that were near where the pebble landed are in the ripple that arrives at the shore. What arrives at the shore is energy and information. The individual molecules in the water just bob up and down, but do not travel horizontally. You can think of light as a disturbance in electromagnetic fields that propogates at c and transmits energy and information, but you can not think of the distubance as a particle with rest mass moving at c.

I know that even shadows can move at greater speed than the speed of light. But it's not a question of TFL. I am seeing 'light' as 'something' that is capable of accelerating space objects, of causing burns on skin when pased through a magnifying glass etc.

I leave that for a moment.

Disturbance is also a form of enegry. So ou mean energy does not come in the the purview of things just as is the case with shadows.

But I have a feeling that even shadow will be considerd a 'thing', say 500 years from now.

You are free to squirm at this!

 

[Frame Dragger]

I know that even shadows can move at greater speed than the speed of light. But it's not a question of TFL. I am seeing 'light' as 'something' that is capable of accelerating space objects, of causing burns on skin when pased through a magnifying glass etc.

I leave that for a moment.

Disturbance is also a form of enegry. So ou mean energy does not come in the the purview of things just as is the case with shadows.

But I have a feeling that even shadow will be considerd a 'thing', say 500 years from now.

You are free to squirm at this!

I'll just laugh, thanks. By the way, how does your "TFL" shadow operate? I'm truly fascinted.

 

[DaveC426913]

But I have a feeling that even shadow will be considerd a 'thing', say 500 years from now.

You are free to squirm at this!

OK, we've got your number now. Only took 30 posts for you to tip your hand. (Well done FD, you called it in post 19.)

 

[Frame Dragger]

OK, we've got your number now.

Yah, and I take back the "no one has reported [him]" thing. Sometimes I hate being right about people from first impressions... it makes me more likely to be harsh in an innapropriate situation. The old, "Remember the hits, forget the misses, fallacy.

@Deepak: Thanks for lowering the tone *insert deeply insulting curse*.


Oh look, he ran away. Oooh, I can TASTE the line through his name :grumpy:

 

[DaveC426913]

EDIT: Hey now Dave... I thought he was out of his gourd waaay before #30. :rofl:

I amended my post.

 

[Frame Dragger]

Ah, my bad... I get... heated in these situations. I don't like seeing intellectual capital (you and kev) wasted this way. This is the kind of thing that turns helpful and outgoing people like you two, into angry pricks like me.

 

[yuiop]

But I have a feeling that even shadow will be considerd a 'thing', say 500 years from now.

You are free to squirm at this!

Certainly a shadow is thing, but a difficult thing to catagorise. It depends on you definition of "thing". If we define "thing" as something with rest mass then a shadow is not a thing and by that definition a photon is not thing either, but normally the word "thing" is not so narrowly defined.

So what is shadow? Is it a thing with zero rest mass and zero energy? Hmmmm... not sure (See later definition by Dave). It can be used to transmit information, but it can not be used to transmit useful information at greater than c.

You seem to be under then impression that nothing with energy can travel at the speed of light. This is not correct. In fact light is a form of energy and can only travel at c in a vacuum. The restriction for relative velocities is that nothing with mass can travel at c. Photons do not have mass. Here is a simple demonstration:
$$E = \frac{mc^2}{\sqrt{1-v^2/c^2}}$$

$$m = \frac{E\sqrt{1-v^2/c^2}}{c^2}$$

where $m$ is mass. For a photon, v=c and the mass must be zero.

<Edited to remove the qualification "rest" applied to mass, as that implies there are other kinds of mass.>

 

[DaveC426913]

So what is shadow? It is a thing with zero rest mass and zero energy.

This definition may get you into trouble in unforseen ways.

It is probably better to remain cognizant of what a shadow is: an area where photons are blocked. Those photons still obey the laws of physics.