Speed of Light

[Eagle]

I have a couple of questions regarding this. I remember reading that Einstein as a boy asked himself the question that if a boy was holding a mirror in front of his face while running at the speed of light would he see a reflection? You see if he is travelling at the speed of light then the light rays cannot reflect off the mirror back into your eyes as the light rays cannot hit the mirror in the first place.

 

Firstly, is there a flaw in this reasoning?

 

And secondly, If the boy is travelling at the speed of light what will he see? Will he see nothing but a blank mirror? Will he see nothing at all? Will he see the last image he saw on the mirror before he cracked the speed of light?

 

*Note* These are theoretical questions and do not imply that I believe a boy can run at the speed of light. Lol.

 

Thanks, Eagle

[Klaynos]

I have a couple of questions regarding this. I remember reading that Einstein as a boy asked himself the question that if a boy was holding a mirror in front of his face while running at the speed of light would he see a reflection? You see if he is travelling at the speed of light then the light rays cannot reflect off the mirror back into your eyes as the light rays cannot hit the mirror in the first place.

 

Firstly, is there a flaw in this reasoning?

 

Yes, boys have mass and no massive object can move at the speed of light. It would take an infinite amount of energy to achieve it.

 

And secondly, If the boy is travelling at the speed of light what will he see? Will he see nothing but a blank mirror? Will he see nothing at all? Will he see the last image he saw on the mirror before he cracked the speed of light?

 

*Note* These are theoretical questions and do not imply that I believe a boy can run at the speed of light. Lol.

 

Thanks, Eagle

 

As he can't move at the speed of light you can't really as the question.

 

If he is travelling at 0.999999999999999999999999999999*speed of light relative to you then in his reference frame he is still not moving because it's his reference frame. He would always measure c to be the same, as long as he is moving at a constant velocity.

[swansont]

Or, to put it another way, once you posit something that is physically impossible, you can probably justify any answer you want.

[theCPE]

As he pointed out he knows that it is physically impossible for the boy to travel with such speed, thus he probably was wanting the theoritical answer.

 

 

The thought experiment is what led Einstein to discover/determine that the speed of light is constant for all observers. Meaning if the boy or any object were traveling the speed of light with a mirror in front of them, they would still see their reflection as if they were standing still.

 

Light still has the same speed of 3x10^8 m/s whether you are at rest or moving.

[losfomot]

Since the boy is HOLDING the mirror there is no relative velocity. The boy and the mirror are stationary relative to one another, no matter how fast the boy might be capable of running.

[swansont]

As he pointed out he knows that it is physically impossible for the boy to travel with such speed, thus he probably was wanting the theoritical answer.

 

That's just it, though. There is no valid theoretical answer. Gamma diverges. What does infinite length contraction "look like?"

[theCPE]

That's just it, though. There is no valid theoretical answer. Gamma diverges. What does infinite length contraction "look like?"

 

Again...the point of the question is, is the speed of light relative to the observer. With the answer being no.

 

We all know many physical impracticalities and impossibilities of the thought experiment, the point of thought experiments isn't to provide precise definitive calculations....they are too help visualize or comprehend concepts.

[Klaynos]

Again...the point of the question is, is the speed of light relative to the observer. With the answer being no.

 

We all know many physical impracticalities and impossibilities of the thought experiment, the point of thought experiments isn't to provide precise definitive calculations....they are too help visualize or comprehend concepts.

 

But there cannot be an observer. There is nothing here to visualise because it cannot happen.

[timo]

..., the point of thought experiments isn't to provide precise definitive calculations...

Imho, calculations is what makes the difference between thought experiments and thoughts.

[theCPE]

I don't think there was a moment that the OP thought there could be an actual observer in said environment, excellent answer.

[Eagle]

As he pointed out he knows that it is physically impossible for the boy to travel with such speed, thus he probably was wanting the theoritical answer.

 

 

The thought experiment is what led Einstein to discover/determine that the speed of light is constant for all observers. Meaning if the boy or any object were traveling the speed of light with a mirror in front of them, they would still see their reflection as if they were standing still.

 

Light still has the same speed of 3x10^8 m/s whether you are at rest or moving.

 

Thanks for that theCPE. I'm sorry i was not on when the reply's came in but theCPE hit my question dead on. All i was looking for was a answer that could explain how the light could travel to give him a reflection. I specifically put in that i know that a boy cannot travel at the speed of light so any who said something irrelevant should read post's more thoroughly and try to be helpful instead of critisizing. By the way, if my questions were so stupid then maybe you'd like something better to think about?

 

Explain to me the frame of reference for light. Does light need a frame of reference? thanks

[Spyman]

Nobody is critisizing or calling your questions stupid...

 

They are only trying to explain that if the rules for a theory is broken, then the theory can't predict any results for the event causing the break down.

 

According to relativity all sorts of weird things could happen, but since the predictions of the theory are wrong, the theory no longer apply. Anyone can tell you what they guess would happen, but you can't demand a theory to supply an answer for a situation the theory itself claims to be impossible.

[someguy]

I like that question and its a good one to ask, questions like this are how you will achieve a better understanding of relativity. We could also say what if a boy was traveling incredibly close to the speed of light.. would there be a delay in the face he sees? that would still work with relativity.

 

I think if he were moving at the speed of light i would have to initially say that the mirror would be black since no light from behind you could make it to the mirror even if you emitted light. if you were very near the speed of light... then i would think there would be a delay and you would be essentially looking into the past of your face. the farther away you hold the mirror the farther in the past you are looking at.

 

but come to think of it. light is kind of a strange thing. If someone is shining a light at you and you're traveling near the speed of light, light doesn't come at you any faster. I think this has to do with the warping of your time frame. so it may be possible actually that the boy's time is warped moving at that speed that it would compensate for the delay and no delay would be apparent. when you think about it.. for all we know we could be moving just on the cusp of light speed. it depends what you compare yourself to to determine your speed. just comparing yourself to light is not enough you need two anchor points.

 

Time slows down for you as you move faster and the limit is the speed of light so if you were moving at the speed of light time will have warped so much for you that moving would be instantaneous so really you wouldn't be anywhere.. or rather you'd kind of be everywhere until you slow again. Light doesn't really age it kind of is everywhere.. from its perspective. we can "see" it moving. that's how i understand it to be at any rate. And I'm pretty sure that's why it sometimes must be looked at as a wave and sometimes a particle.

[phyti]

I have a couple of questions regarding this. I remember reading that Einstein as a boy asked himself the question that if a boy was holding a mirror in front of his face while running at the speed of light would he see a reflection? You see if he is travelling at the speed of light then the light rays cannot reflect off the mirror back into your eyes as the light rays cannot hit the mirror in the first place.

 

Firstly, is there a flaw in this reasoning?

 

And secondly, If the boy is travelling at the speed of light what will he see? Will he see nothing but a blank mirror? Will he see nothing at all? Will he see the last image he saw on the mirror before he cracked the speed of light?

 

*Note* These are theoretical questions and do not imply that I believe a boy can run at the speed of light. Lol.

 

Thanks, Eagle

 

Any motion alters the process of light serving as a messenger.

As he moves faster, the light takes longer to reach the mirror.

Remember light speed is independent of the source, so light is chasing a moving target. However your reference frame, which includes you and the mirror, are effected by time dilation. At the particle level the light is also chasing moving targets, thus the biological functions, neurons, etc. are effected. Time does not slow down but the activity does and you are not aware of this. Only observers not in your ref. frame notice this.

Think of it like length contraction. If your spacehip contracted to 1/2 length.

everything in it would also contract to 1/2 length including your ruler.

When you measured the length of the ship you would not detect a change from its design length.

When you time a light signal to the mirror and back, you use a shorter time

(unknowingly) to calculate the distance. Now the distance is also shorter by the same factor. When you divide the short distance by the short time, you get c, the speed of light. From your perspective everthing seems normal.

Ultimately whatever clock sytem you use, at light speed a clock cycle cannot be completed, therefore it is indeterminate.

[swansont]

Any motion alters the process of light serving as a messenger.

As he moves faster, the light takes longer to reach the mirror.

Remember light speed is independent of the source, so light is chasing a moving target. However your reference frame, which includes you and the mirror, are effected by time dilation. At the particle level the light is also chasing moving targets, thus the biological functions, neurons, etc. are effected. Time does not slow down but the activity does and you are not aware of this.

 

The boy is holding the mirror, so they are not moving with respect to each other, so there is no "moving target" in his frame. If the boy is in any valid reference frame, he will see it take the same time for the light to reach the mirror.

 

There are no "moving targets" in the biological system, either. The whole implication of time dilation is that it is time that slows down, as observed by a stationary observer.

[phyti]

The boy is holding the mirror, so they are not moving with respect to each other, so there is no "moving target" in his frame. If the boy is in any valid reference frame, he will see it take the same time for the light to reach the mirror.

 

There are no "moving targets" in the biological system, either. The whole implication of time dilation is that it is time that slows down, as observed by a stationary observer.

 

My quote:

"However your reference frame, which includes you and the mirror,"

does not imply they are moving with respect to each other.

 

"Remember light speed is independent of the source"

This is the principle of constant light speed (in vacuum) from SR.

 

The light is moving and therefore not part of the reference frame of the boy.

The constant speed for any observer is the result of the constant speed and time dilation. This can be demonstrated in a simple light clock.

No one gets ahead or behind anyone else relative to time.

Time does not accelerate or decelerate, moving clocks just parse it into different length intervals.

[swansont]

My quote:

"However your reference frame, which includes you and the mirror,"

does not imply they are moving with respect to each other.

 

I said they were not moving with respect to each other. Which has to be true if they are in the same frame of reference.

 

"Remember light speed is independent of the source"

This is the principle of constant light speed (in vacuum) from SR.

 

The light is moving and therefore not part of the reference frame of the boy.

 

Light is not part of any frame, but any inertial observer will observe its speed to be the same, relative to them.

 

 

The constant speed for any observer is the result of the constant speed and time dilation. This can be demonstrated in a simple light clock.

No one gets ahead or behind anyone else relative to time.

Time does not accelerate or decelerate, moving clocks just parse it into different length intervals.

 

Well, no, not really. There is no universal time.

[lakmilis]

Errors by Phyti:

 

Any motion alters the process of light serving as a messenger.

As he moves faster, the light takes longer to reach the mirror.

 

No, as swansont points out, since the mirror and lad are both at rest w.r.t. each other, light does *not* take a longer time to reach the mirror (the relative distance would be the same within the frame, and the light travelling at c. Note that the obvious constraint is that this reference frame has always v < c no mathher how minute the difference).

 

Remember light speed is independent of the source, so light is chasing a moving target. [/Quote]

 

 

It is important to remember that when thinking of such as the particular thought experiment provided, that one keeps in mind, one is *not* observing the boy as he observes light (hence, speaking from a different frame of reference), but keeping in mind, one is meant to *be* the boy in this question (mirror/boy same reference frame).

 

lak

 

Again, No; the light is only 'chasing' a moving target if you are referring to a frame outside the boy/mirror!

However your reference frame, which includes you and the mirror, are effected by time dilation. At the particle level the light is also chasing moving targets, thus the biological functions, neurons, etc. are effected. Time does not slow down but the activity does and you are not aware of this. Only observers not in your ref. frame notice this.[/Quote] Exactly. So it is kind of a raa, your above quoted comment

 

Think of it like length contraction. If your spacehip contracted to 1/2 length.

everything in it would also contract to 1/2 length including your ruler.

When you measured the length of the ship you would not detect a change from its design length.

When you time a light signal to the mirror and back, you use a shorter time

(unknowingly) to calculate the distance.

 

Now the distance is also shorter by the same factor. When you divide the short distance by the short time, you get c, the speed of light. From your perspective everthing seems normal.

Ultimately whatever clock sytem you use, at light speed a clock cycle cannot be completed, therefore it is indeterminate.

Again you are arguing with the wrong frame in mind (an additional external one.)

[lakmilis]

Well, no, not really. There is no universal time.

 

Swantsont, please keep in mind that Relativity does *not* conclude that there is no absolute/universal time. It suggests that all [inertial] reference frames are equivalent. However, this is a question about metaphysics. We sometimes in science as in anything else, extend derivations to larger systems.

 

If one provided the logical hypothesis that All subsets of a set X are equivalent, and only X is not a subset of a set, not itself, then relativity would hold for all the given subsets bar X. The question then is, if X exists, relativity holds for all but X which would be 'absolute'. Relativity says X is a subset of itself and therefore a subset (ergo equivalent), but does not prove this conclusively as such. Of course, one has a good case for stating the opposite of my argument as stated just above, as all these subsets are typically belonging to the dimensional magnitude of Relativity. but if X perhaps has a higher order of dimensions, then X can exist and relativity holds to the subsets (Minkowski space).

 

We sometimes forget that these things belong to philosophy, which science is a subset of. With this though, I do not mean to say, we are always left to merely believe or not, if X exists. I agree derivations of that assumption must yield some pattern we can predict if X exists.

 

X does exist, and in fact is abundantly present in mathematics, but we are at the start of the 21st century. Similar paradigms of early 20th century will also come around as time goes by.

 

lak