Really confused - basic special relativity

[Tomer]

Hey all,

this may sound dumb, but this question has been bothering me for a while now.
So we know that moving objects (or people!) measure time moving slowly compared to objects in rest (to them). We also know that distances appear to shrink.

Now, first this question: if we send a person to the nearest star, say, 5LY away, and he'll travel very rapidly (0.999c), which correlates to gamma ~22, he'd experience (theoretically) both time dilation and space contraction, right? So for us on Earth it would take t = (5/0.999) Y years to get there, right?
I'm having trouble calculating how long it would take him, in his system, because of these two effects. I couldn't understand if they cancel out, or add up, or are equivalent (that is, if he'll get there, according to his clock, after the same amount of time, t, or after t/22, or after t/22^2). I know well the lorentz transformations but I can't really define the event here. Is it "the person arrived the star"? If it is, when I'm trying to calculate it like I understand it, I get this strange result:
Earth system: x = 5LY, v =0.999c => t = 5/0.999 Y
But then:
x' = gamma * (x-vt) = ~22 * (5 -0.999 * (5/0.999) ) = 0 LY!

I don't get it at all :-)

My second question is: How does it feel like to be a photon? If space shrinks asymptotically to zero, does it "feel" like it's everywhere all the time? Another extraordinarily posed question, I know :-\

I hope I made some sense - I've had a little relativity when studying mechanics a few years back, I believe I could at least answer the first question alone back then (although probably just by using formulas like a robot), but I'm apparently very rusty :-)

Thanks a lot!
Tomer.

 

[Doc Al]

Hey all,

this may sound dumb, but this question has been bothering me for a while now.
So we know that moving objects (or people!) measure time moving slowly compared to objects in rest (to them). We also know that distances appear to shrink.

OK.

Now, first this question: if we send a person to the nearest star, say, 5LY away, and he'll travel very rapidly (0.999c), which correlates to gamma ~22, he'd experience (theoretically) both time dilation and space contraction, right?

Sure.

So for us on Earth it would take t = (5/0.999) Y years to get there, right?

Right.

I'm having trouble calculating how long it would take him, in his system, because of these two effects. I couldn't understand if they cancel out, or add up, or are equivalent (that is, if he'll get there, according to his clock, after the same amount of time, t, or after t/22, or after t/22^2). I know well the lorentz transformations but I can't really define the event here. Is it "the person arrived the star"? If it is, when I'm trying to calculate it like I understand it, I get this strange result:
Earth system: x = 5LY, v =0.999c => t = 5/0.999 Y
But then:
x' = gamma * (x-vt) = ~22 * (5 -0.999 * (5/0.999) ) = 0 LY!

The two events are: (1) spaceship leaves Earth & (2) spaceship arrives at star.

In the Earth frame, the distance traveled is 5 LY. But in the frame of the spaceship, the spaceship doesn't move at all. So Δx' = 0 makes perfects sense! The two events take place at the same location according to the spaceship.

You want to solve for Δt', the time between the two events according to the spaceship frame. So use a different Lorentz transformation.

You can also use the 'time dilation' and 'length contraction' short cuts, if you like. From the ship's viewpoint, the star is only 5/22 LY away and is traveling towards him at 0.999 c. Use that to figure out the time.

Alternatively, you realize that from the Earth's view the spaceship's clock is observed to run slow. Thus Δt = gamma*Δt'. The time measured on the spaceship clock is the time interval between those same two events.

You'll get the same answer any way you slice it.

My second question is: How does it feel like to be a photon? If space shrinks asymptotically to zero, does it "feel" like it's everywhere all the time? Another extraordinarily posed question, I know :-\

You might want to read our FAQ: https://www.physicsforums.com/showthread.php?t=511170"

 

[Tomer]

Thanks a lot, it's much clearer now. And this "funny" result I got was clear to Galileo 700 years ago and has nothing to do with relativity. *sigh*... :-)

I'll read the link - thanks again!

Edit: I just read the not very lengthy link. So, nothing makes sense. But the photon is still there! So (forgive me for the new-age) if the photon had emotions he would still experience something, even though our transformations crash, wouldn't it?! Or do we conclude that emotions cannot move in the speed of light?? :p
This is hardly satisfactory :-(

 

[Naty1]

From the link:

Time and length cease to have meaning in the limit v→c. In that limit, all time and length intervals shrink to zero. In the rest frame of a photon, the coordinates of any point in the universe at any time in the past, any time in the future is identically zero. That just doesn't make a bit of sense.

That's what the mathematics of science tells us.

Einstein did not develop mathematics, nor has anyone else, to deal with hypothetical emotions of inanimate objects. So far we have no experimental evidence that energy has emotions.

You might as well ask "What does the number 24 feel?"

 

[Tomer]

From the link:
That's what the mathematics of science tells us.

Einstein did not develop mathematics, nor has anyone else, to deal with hypothetical emotions of inanimate objects. So far we have no experimental evidence that energy has emotions.

You might as well ask "What does the number 24 feel?"

I'm of course aware of that, and that's why the word feel was surrounded with quotation marks. However, a number is a purely hypothetical concept whereas emotions, whatever they may be, are there.
By asking how does a "photon" feel I meant to ask how would a person feel going at the speed of light. This is simply not possible because a person has a mass, you'd say. But then I could talk about a "person made of photons", a weight-less person, an anorectic model maybe :-). If photons have energy without having any mass, why shouldn't "thoughts" or "emotions", as a possible form of energy, be able to arrive to the speed of light?

I realize this is mambo-jumbo, but I just wanted to give some meaning to the question. There's a non-hypothetical particle moving at the speed of light as far as we know it. It is therefore very hard not to ask, "how does the particle see the world", Unlike asking "how does it feel to be 24".

 

[ghwellsjr]

Why don't you look at https://www.physicsforums.com/showthread.php?t=524913" rather than make all of us go through the same thing yet another time?

 

[Tomer]

Why don't you look at https://www.physicsforums.com/showthread.php?t=524913" rather than make all of us go through the same thing yet another time?

I will, wasn't aware of this post.

 

[ghwellsjr]

You can find lots more by searching for "meaningless" on this forum.

 

[Doc Al]

You can find lots more by searching for "meaningless" on this forum.

:rofl:

 

[Tomer]

I have read it. I wouldn't want to start a new discussion if everyone here is so upset about it. It seems rather obvious that the matter is settled. I agree that with the current model it makes no sense to talk about these questions, just like it makes no sense to ask mathematically how much 1/0 is (being restricted to the classical real line).
However extensions to the real line have been made, and one can talk about the set which includes infinity. There 1/0 is defined. So instead of resolving it by saying "1/0" makes no sense, they altered the model.

Therefore, I don't think the question itself should be banned or laughed about. The question raises the possibility that the model might not be perfect, that there is something missing, that it doesn't cover everything. It could sharpen out things.
I think many of the posts there were a little hard. I count myself to be a follower of pure logic and wouldn't want to mix philosophy and physics. This is an obvious philosophical question. And still - the two feed one another. Isn't it legitimate to test physical theories with abstract notions, like suggested by others? They might not have the power to contradict or verify them physically, but they could raise new ideas regarding possible extensions of the theory or experiments. Philosophical ideas were and are the core of physical theories. Thought experiences were always an important tool in Physics.

I understand why it upsets some physicists to see a mixture of the two - and indeed, when one goes to speak about the "feelings" of a photon one should perhaps mention that he's diverging from classical physical models. But the question itself remains meaningful in our world, even if certain transformations of a certain model aren't currently defined under the restrictions of the question.

 

[WannabeNewton]

Well if you are going to use semi - riemannian manifolds to model space - time then you cannot avoid the issue. Not to mention , regardless of null geodesics having zero distance intervals on semi - riemannian manifolds, you still cannot lorentz boost to the frame of a photon to know what it "feels".

 

[ghwellsjr]

Tomer, this forum is devoted to helping people understand relativity. People who understand relativity understand why the question of what a photon experiences is meaningless. Are you interested in learning relativity?

 

[John15]

I have seen the problem with time and length contractions may times including with myself. Despite what the experts tell us the numbers don't make sense. To maintain the speed of light constant relatively then common sense tells me that if you are moving at 0.5 c then relative to an observer at rest, time and distance should both reduce by 50% but lorenz contractions as far as I can make out says that this does not happen till you reach near 90% c.
Also if time and distance both = 0 at c this means that distance(miles)= 0, time(per hour) =0 then at c speed(miles per hour) must = 0 not 186000 miles per second, which also makes no sense, at least with current interpretations.

 

[ghwellsjr]

I have seen the problem with time and length contractions may times including with myself. Despite what the experts tell us the numbers don't make sense. To maintain the speed of light constant relatively then common sense tells me that if you are moving at 0.5 c then relative to an observer at rest, time and distance should both reduce by 50% but lorenz contractions as far as I can make out says that this does not happen till you reach near 90% c.
Also if time and distance both = 0 at c this means that distance(miles)= 0, time(per hour) =0 then at c speed(miles per hour) must = 0 not 186000 miles per second, which also makes no sense, at least with current interpretations.

There's no "problem with time and length contractions" but there is a problem with your common sense. This forum does not tolerate relativity bashers. Wouldn't you rather learn relativity so that it will make sense to you?

 

[utesfan100]

I have read it. I wouldn't want to start a new discussion if everyone here is so upset about it. It seems rather obvious that the matter is settled. I agree that with the current model it makes no sense to talk about these questions, just like it makes no sense to ask mathematically how much 1/0 is (being restricted to the classical real line).
However extensions to the real line have been made, and one can talk about the set which includes infinity. There 1/0 is defined. So instead of resolving it by saying "1/0" makes no sense, they altered the model.

I have seen where the real numbers have been extended to include a maximal and minimal element, but I don't know which of these should be 1/0. Is this value positive?

If we have a field (the normal properties of addition and multiplication, without bothering to tie these concepts to a specific set of numbers) then defining a value to inf=1/0 can be shown to lead to contradictions.

0*2=0
inf*0*2=inf*0
1*2=1
2=1

1/0 is a mathematical contradiction, unlike the concept of a supremum you likely intended to represent by this. The ordinal w is a much better representation of the concept of this concept.

One can even take such an element and extend the algebra by the field properties and create the hyper real numbers.

 

[Dale]

Therefore, I don't think the question itself should be banned or laughed about.

The question isn't banned or laughed about, it is given the respect and attention that it deserves: a FAQ entry. The problem is that everyone who asks this frequently asked question ignores the answer and then tries to repeat the exact same discussion that occurs twice a week. There has been plenty of discussion on the topic: read it.

 

[Dale]

You can find lots more by searching for "meaningless" on this forum.

We should put that advice into the FAQ.

 

[bobc2]

...So we know that moving objects (or people!) measure time moving slowly compared to objects in rest (to them)...

Tomer.

This is about the third time I've noticed a comment like this. The first time, WannabeNewton noticed the confusion and clarified it.

Again: Moving objects or people do not measure time moving slowly. So, I can't tell whether you are just being a little careless with the way you state this or whether you don't have a fundamental understanding of time dilation in special relativity.

You might have said something to the effect that an observer at rest in his own reference frame observes that a clock moving relative to his rest frame appears to him (the rest frame observer) to be running slower than the clock at rest in his frame.

More generally, given obervers A and B in motion with respect to each other, each carrying a clock: A observes B's clock to be running slower than his (A's) own, and B observes A's clock to be running slower than his (B's) own.

Note that B does not observe that his own clock is running slow just because he is in motion relative to A (I thought that was what your original wording was implying).

 

[Dale]

Despite what the experts tell us the numbers don't make sense.

What is important is not what experts say nor what makes sense to you, but what experiments prove. Both you and the experts have to change your mind if experiments contradict you.

 

[ghwellsjr]

I have seen where the real numbers have been extended to include a maximal and minimal element, but I don't know which of these should be 1/0. Is this value positive?

If we have a field (the normal properties of addition and multiplication, without bothering to tie these concepts to a specific set of numbers) then defining a value to inf=1/0 can be shown to lead to contradictions.

0*2=0
inf*0*2=inf*0
1*2=1
2=1

1/0 is a mathematical contradiction, unlike the concept of a supremum you likely intended to represent by this. The ordinal w is a much better representation of the concept of this concept.

One can even take such an element and extend the algebra by the field properties and create the hyper real numbers.

I don't understand why you posted this. Are you trying to point out a flaw in Tomer's thinking or a flaw in the FAQ?

 

[Tomer]

Phew, who should I quote :-)
Ok...

1. I realize some of you are tired of these discussion popping every week. But you ought to realize that some of us are new to the forums and are not experiencing these discussions every couple of days. Saying "look for these discussions" is a bit unfair, because there's a huge amount of threads with interesting and relevant titles. I didn't expect a thread called "time traveling light" would contain contents similar to my current interest. Furthermore - it is also against the forum's rules to "hijack" threads and revive them as I understand, so am I not entitled to throw my dime even? Lastly - if it's so upsetting to some, curious people asking questions, also if they're not articulated with the symbols {$\exists\forall\wedge\neg$}, I don't see why they should comment. This is an open forum and nobody is forced into a discussion he doesn't want to take part in.

2. I'm not "bashing" relativity god forbid - even if I had other "ideas" I wouldn't, in my current position, actually try to contradict it. I was just curious about certain aspects of a certain particle and wasn't quite satisfied with the answers I've read in other posts - meaning I wanted to understand deeper what "meaningless" means. I'm not trying to contradict anyone, I tried to explain why I find the question meaningful, even if the transformations don't agree with it - and I might have been wrong. But instead of showing me where I'm wrong I just get cynical comments telling me to look for meaningless on the forums, and I don't quite understand why. First thing I'd encourage a physicist to do is to ask questions.

3. Utesfan100 - http://en.wikipedia.org/wiki/Real_projective_line

4. Bobc2 - that's what I meant, but thanks for the sharpening

 

[ghwellsjr]

Tomer, please respond to my question:

Tomer, this forum is devoted to helping people understand relativity. People who understand relativity understand why the question of what a photon experiences is meaningless. Are you interested in learning relativity?

 

[Tomer]

Tomer, this forum is devoted to helping people understand relativity. People who understand relativity understand why the question of what a photon experiences is meaningless. Are you interested in learning relativity?

Yes, I do, but in my own pace and way. I've asked this question here and not in "philosophy" forum because I wanted a physical interpretation. I tried to give physical meaning to the question - but your answers sound like you're defending some church.
I would be happy to get an answer on this one question: If a certain model isn't applicable to a certain domain or a certain problem, which obviously has a relation to the model, are questions about this certain problem, in light of the model, meaningless?
I came to this forum wanting to learn, not wanting to show you guys the light after doing a bachelor in physics and maths. But I think nativity and doubt are two important things to have, especially when we're just talking, and not building spacecraft s.

 

[Dale]

If a certain model isn't applicable to a certain domain or a certain problem, which obviously has a relation to the model

This sounds like a self-contradiction. How can a model both not be applicable to the domain and yet be obviously related to the problem?

 

[Dale]

1. I realize some of you are tired of these discussion popping every week. But you ought to realize that some of us are new to the forums and are not experiencing these discussions every couple of days.

We realize that, which is why we made the FAQ entry.

am I not entitled to throw my dime even?

The problem is that your dime is the exact same as every other dime that has been thrown in. You are not asking anything different nor presenting anything new other than what was addressed in the FAQ. For some reason, people who ask this particular question seem to think that they are saying something different from what is addressed by the FAQ. I guess the FAQ needs some work.

 

[rede96]

You can find lots more by searching for "meaningless" on this forum.

I thought I would take ghwellsjr’s advice and look up ‘meaningless’ specifically in the S/GR forum.

There was only one thread with the word ‘meaningless’ in the topic, which just so happened to related to the view point of a photon.

https://www.physicsforums.com/showthread.php?t=391425&highlight=meaningless"

Worth a quick read IMHO.

There were a total of 320 threads that used the word ‘meaningless’ over an 11.5 year period (First one from the search was Feb 2004.) I did a quick sample of 30 random threads to look up the use of the word ‘meaningless’ in the context of the thread.

Only 20% of threads used the word ‘meaningless’ in the context of describing the view point of a photon. The others were on a range of things from back holes to the twin paradox.
On average that is one thread every 138.35 days. Or about one post every 20 weeks.

However I did notice that the frequency of the view point of a photon type question increased to greater than 50% over the last 6 months or so. Even more so over the last couple of months.

I also had a good read through a lot of the threads and found them to very useful. In fact it is ironic that a search using the term ‘meaningless’ is probably one the most informative searches I’ve done!

We realize that, which is why we made the FAQ entry.

The problem is that your dime is the exact same as every other dime that has been thrown in. You are not asking anything different nor presenting anything new other than what was addressed in the FAQ. For some reason, people who ask this particular question seem to think that they are saying something different from what is addressed by the FAQ. I guess the FAQ needs some work.

I would agree with that, especially after reading through some of the search results. It seems that over time the responses to this type of question have become shorter and less detailed, (and less patient! ) leading to the standard ‘read the FAQ’ type answer.

We have a saying at work, if you always do what you always done, then you always get what you’ve always got.

I don’t know if this helps at all, but I thought I would put a few snippets below for anyone that is interested.



https://www.physicsforums.com/showpost.php?p=1684890&postcount=20"

Going the speed of light in Einstein Relativity is effectively the same as going at an infinite speed in Galilean Relativity. You can
imagine relative speeds going faster and faster and faster, but you can't really imagine going at an infinite relative speed--except that it takes zero time
to go from one place to another.

Effectively, from the photon's point of view (if it could be said to have a point-of-view, which really, it can't), it is going at that "infinite" speed. It
is going an infinite distance in zero time. From any other point of view, it is gong a finite distance in a finite time, at the speed of light.

I'm not sure what you mean by "each photon of light." Are you referring to a certain light in particular

Well, there is a big difference between looking in front of you and looking behind you. If you look behind you while traveling "at"
the speed of light, then it would appear that the Earth was staying with you. The light from behind you is just keeping up.

If you look at the variety of good relativity books out there, you'll see a place where this type of discussion ought to be, but is notably lacking. It's
the part of the twin paradox where they show the situation from the perspective of the stay-at-home twin, but they do not show the situation from the
perspective of the traveling twin.

You can see it has been tried in Wikipedia here http://en.wikipedia.org/wiki/Talk:Twin_paradox/Archive_13#Specific_example
but, even though it's a simple calculation, because it is considered "original research" Wikipedia can't publish it.

I've read one version of the twin paradox even, where the traveling twin is put into a locked cell with no windows, to assure that you don't question what
the traveling twin sees, because the traveling twin sees nothing at all!

When the traveling twin turns around, they will see the image of home shoot suddenly away during the acceleration phase, before rushing back superluminally
during the return.

If you imagine zooming across a room, near the speed of light, the far wall will appear to be much further away, and the space across the room in front of
you is likewise, far distant, but approaching superfast. The wall behind you would hardly appear to be receding at all.

If you want to measure the speed of light without regard to the Theory of Special Relativity, you don't worry about a frame of
reference. You just set an experiment which measures the round trip speed of light. This takes at least two photons. One going from you to a reflector and
one coming back from the reflector to you. But since you will have no way of knowing when the first photon started out, or how much delay there was for the
first photon to trigger the second photon, or how to know that the photon you sent out is the one that hit the reflector, it is not possible to measure even
the round trip speed of a pair of photons. Nobody ever did that and nobody would ever try to do that. Instead, we use bright flashes of light which are easy
to detect and we use long distances so that we can increase our accuracy.

The Theory of Special Relativity is where a frame of reference is defined and where the one-way speed of light (which includes photons) is defined to be c in
all directions within anyone frame of reference you arbitrarily choose. Everything is in that one frame of reference, no matter where it is or how fast it
is moving or accelerating. All light, including photons travel at c, by definition in that one frame of reference. If you want to consider a second
frame of reference, you need to use the Lorentz Transform to convert all your time, location coordinates from your first FoR to your second FoR. That's what
SR is all about.

We don't measure the speed of photons. If you tried to measure the speed of a photon, it would cease to exist. It doesn't speed away
to any distance at all. You cannot measure the speed of a photon. It's speed is defined to be c.

Yes, it's the relative motion that matters, as already pointed out. However, here's the thing ...

Relative motion only has meaning from the POV of material entity. The ability to perceive the passage of time has much to do with the ability to be held at
a state of rest, which of course requires rest mass (not effective mass). We assign the origin of a coordinate system as the reference for measurements,
including for motion. Coordinate system origins exist at a specific point in space at a specific time, because it's the nature of rest mass to exist as such
... and it matters not if the body is imagined or real, because imagined bodies assume the rest mass exists. So while it requires only relative motion
(under the context of invariant light speed) to produce relativistic effects, relative motion only has meaning (in the first place) per the POV of rest mass.
They are mutually linked concepts.

GrayGhost

Ok here's the deal.

You CAN assume the rest frame of a photon exists. However, IF you do, everything ELSE stops making sense. We KNOW Maxwell's Equations work in all frames of
reference. They've never been shown to fail, ever. IF you were to create this frame where a photon is at rest, in other words an inertial rest frame, you
would be looking at a static electromagnetic field. Such a field is in contradiction to Maxwell's Equations, such a field is NOT a solution to the
equations. That means Maxwell's Equations are wrong or the photon must not have a inertial rest frame one can observe from. What's more likely? The photon
has no inertial rest frame, consistent with our experimental observations, or that maxwell's equations are wrong , completely contrary to over a century of
millions if not billions of experiments showing it to be true?

To put it crudely, to make measurements we need a coordinate system (frame of reference ). It is usually simpler to have a
coordinate system in which we are at rest and for a timelike vector representing our motion we can always tranform to a system in which we are at rest. The
photon does not have a frame in which it is at rest or one which can be transformed into such a frame. So we cannot lay down a coordinate system from which a
photon can make measurements.

Matheinste.

'age' is sort of a meaningless concept for a photon. If you mean does time pass for a photon, the answer is that photons travel along paths of zero proper time in spacetime, so the answer is no. On the other hand, if you mean if we could somehow watch a photon traveling (from a frame that *wasn't* traveling at the speed of light) would time pass for *us* whilst we watch the photon, the answer is obviously yes. So we could arbitrarily assign an 'age' to the photon from our frame, say, it's time begins when it is emitted from the atom, and ends when it is absorbed by one. The problems are firstly that the age would be frame dependent (this is the whole idea behind proper time in the first place) and secondly we can't actually 'see' or observe a photon until it's been absorbed - it's path before measurement has no real meaning as far as we can make predictions about it. Therefore there is no process by which a photon could even be *given* an age, as far as I am aware. Meaningless concept.

Cheerio,

Kane O'Donnell

First, you are absolutely right that proper time along the photon trajectory is zero. Yet,
proper time is not the only meaningful definition of "time". Therefore, we don't know whether "time" experienced by the photon is equal to the proper time.

Second, when one says that photon "cannot be held in a state of rest", one actually means that there are no Lorentz coordinates with respect to which the photon is at rest. Yet, Lorentz coordinates are not the only meaningful type of coordinates. In particular, there exist light-cone coordinates, with respect to which photon is at rest.

If you never heard about light-cone coordinates, see
http://en.wikipedia.org/wiki/Light_cone_coordinates

Yes, you could say that.

But time stopping for the light doesn't mean an infinite speed from an external observer's point of view, it means infinite from an internal observer's point of view.

Light (photons) do not experience time at all. They "see" the entire universe in one instant - as if they really are traveling at infinite speed - in their own frame of reference, that is.


(Caveat: as many posts are likely about to tell you, it is meaningless to speak about photons "seeing" anything, or even a frame of reference moving at c. This is true, but as a fellow layperson, I understand how it can help to loosen the rules to make a leap of understanding. Keep in mind though, that faulty analogies (such as mine, and the clock Bernhard mentions) will eventually steer you awry.)

 

[Tomer]

This sounds like a self-contradiction. How can a model both not be applicable to the domain and yet be obviously related to the problem?

It could, perhaps, in such a way that a simple/complicated extension of the model would make it applicable to the problem. Or even without it - I find it strange that a certain model could not shed any light or raise any questions (maybe even philosophical ones) about that "unknown" domain of his - especially if we're talking about a single velocity - c - which is excluded from it (by that I mean an inertial rest frame moving with the speed of light).
An example: if we know that space "shrinks" as we move quicker, asymptotically to zero - it could make sense to define/assume/decide that (perhaps!) a photon is, in his system, everywhere, all the time, or something of the sort. This could have some consequences that - who knows - might be even measurable somehow.
This is just a dumb example, other examples might be better - but isn't that how models are being extended, especially in physics?
And Again - I'm not "arguing", I just wanted to hear some opinions of experienced physicists.

If one decides one day to explore the qualities of this domain - where would one start if not at special relativity?

The problem is that your dime is the exact same as every other dime that has been thrown in. You are not asking anything different nor presenting anything new other than what was addressed in the FAQ.

Well - that I didn't and cannot know.
With the FAQ - I admit I missed that. After being referred there I still found myself being bothered, which brought me to the question I posed in my last post, regarding the meaningfulness of predicting results not in the domain of a model using this model - whether it happens often, whether it's considered helpful, whether it has some logical sense behind it.
But what I absolutely cannot know is that every couple of days the same question is being raised. In which case I'd have to agree - maybe the FAQ needs some work! ;-)
If people are tired of my questions and find them completely irrational/already answered - then they're entitled to ignore me (which would be disappointing, but I wouldn't want to lose the legitimacy of posing them).

Rede96 - thanks. I'll read the links later, when I'll have more time.

 

[BruceW]

My second question is: How does it feel like to be a photon? If space shrinks asymptotically to zero, does it "feel" like it's everywhere all the time? Another extraordinarily posed question, I know :-\

The photon always travels along a null curve through spacetime. If we conveniently define the axes of spacetime, then we have c²dt² = dx². And so from a frame of reference where the photon is stationary, dt=0. This means that for a photon, it is absorbed at the same instant it is emitted (from its frame of reference). So the photons that reached our eyes from distant galaxies have left at the same time they reached us (from their frame of reference).

Of course, it doesn't matter how much time it takes from the photon's frame of reference, since a photon doesn't have a half-life, so it doesn't affect decay rate or anything like that.

 

[ghwellsjr]

Tomer, this forum is devoted to helping people understand relativity. People who understand relativity understand why the question of what a photon experiences is meaningless. Are you interested in learning relativity?

Yes, I do, but in my own pace and way.

OK. You asked:

...I could talk about a "person made of photons", a weight-less person, an anorectic model maybe :-). If photons have energy without having any mass, why shouldn't "thoughts" or "emotions", as a possible form of energy, be able to arrive to the speed of light?
...

You know, I'm sure, that every real person, no matter how fast they have been accelerated away from the earth, will still be overtaken by photons fired in their direction, correct? These photons will pass them at c, correct? But will any of these photons pass each other? I hope you will recognize that photons will not pass each other but will remain in the same order in which they were fired from the earth.

Now give a good long thought to your question about a "person made of photons". Are you thinking that these photons can interact with each other? Please explain in detail what your thoughts are concerning this "person made of photons".

 

[Tomer]

Now give a good long thought to your question about a "person made of photons". Are you thinking that these photons can interact with each other? Please explain in detail what your thoughts are concerning this "person made of photons".

Out of all the things I've said, this one was just a dumb metaphor. Of course I don't need an actual person made a photons to give meaning to "how does the photon experience time?". Particles experience time, for example, in that they decay. And clocks tick without having "emotions". It just seemed easier at the time of writing to give the photon human senses. I obviously wasn't aware I'm speaking with the most correct physicists out there :-) And I'm not dissing - it is clear to me one needs to be correct about it when actually wanting to understand, but I sort of assumed that what I meant is clear.

Anyway, the quotes themselves seem to contradict one another - and each of them is very interesting. (Thank you, Rede!). I can't say I'd be able to answer to one who asks me the same question a good answer. Maybe "it's meaningless" would be the easiest, but it wouldn't be the one I'd choose - not right now, at least. I'm overwhelmed with data (light-cone coordinates?!) and I guess it'll take time to digest it.

Thanks for the replies, and to all you upset professors - it wasn't my intention, at all. :-)

 

[WannabeNewton]

The photon always travels along a null curve through spacetime. If we conveniently define the axes of spacetime, then we have c²dt² = dx². And so from a frame of reference where the photon is stationary, dt=0. This means that for a photon, it is absorbed at the same instant it is emitted (from its frame of reference). So the photons that reached our eyes from distant galaxies have left at the same time they reached us (from their frame of reference).

Of course, it doesn't matter how much time it takes from the photon's frame of reference, since a photon doesn't have a half-life, so it doesn't affect decay rate or anything like that.

A photon is never stationary. In order for there to be a frame in which the photon is stationary, your reference frame would have to be at rest with respect to the photon which is impossible. Again, you cannot lorentz boost to the frame of a photon so saying how time is in "its frame" is completely meaningless. Proper time intervals being zero on a null geodesic doesn't translate literally to "time does not pass for a photon".

 

[ghwellsjr]

Out of all the things I've said, this one was just a dumb metaphor. Of course I don't need an actual person made a photons to give meaning to "how does the photon experience time?". Particles experience time, for example, in that they decay. And clocks tick without having "emotions". It just seemed easier at the time of writing to give the photon human senses. I obviously wasn't aware I'm speaking with the most correct physicists out there :-) And I'm not dissing - it is clear to me one needs to be correct about it when actually wanting to understand, but I sort of assumed that what I meant is clear.

Some massive particles like neutrons and muons decay but photons don't, so how exactly do you think photons experience time?

 

[Dale]

I mean an inertial rest frame moving with the speed of light

Here is the self-contradiction in a nutshell. Part of the definition of an inertial frame is that light moves at c. So indeed, that speed is singled out in the definition of inertial frames. And that is precisely why you cannot have an inertial frame moving at c, because then it wouldn't be an inertial frame.

 

[BruceW]

Proper time intervals being zero on a null geodesic doesn't translate literally to "time does not pass for a photon".

Why not? Between two events on a null geodesic, the space separation equals the time separation (with c=1).

 

[Tomer]

Some massive particles like neutrons and muons decay but photons don't, so how exactly do you think photons experience time?

No idea. If I knew I wouldn't have asked.
I wonder though, what the difference between "not experiencing time" and "experiencing time not passing" is. I know, philosophy. But it physicist are allowed to, and should, be philosophers from time to time.