Arrow of time and travel at the speed of light

[BernieM]

If a guy gets on a rocket that travels at the speed of light for a finite period of time (in relation to other observers not moving at C) the guy in the rocket experiences no passage of time. In this scenario his leaving and arrival of the rocket happens at the same time in his frame, correct?

Yet an observer watching this happen, sees the guy get on the rocket at point A, some time passes, and he gets off at point B? His progress has a past and this establishes an arrow of time for him from the observer's viewpoint. Is that correct?

And if this is so, which way does the arrow of time point for the guy in the rocket? Aren't both the point of origin and destination equally plausible as either? That point A where he departed and point B where he ended his journey are one and the same and can be swapped around?

In a possible real version of this problem, if a photon is exchanged between two particles (electromagnetic or strong force carrier photon for example), are two photons exchanged, one from particle A to particle B and one from particle B to particle A?
or, is one photon exchanged that satisfies both particle A and B that they have both sent and received a separate force carrier photon (that is one photon between them, that appears to be both arriving and departing both particles simultaneously)?
Or is it simply one photon moving from one particle to the other with no reciprocal exchange?
Or is it the only other option I can think of, that one photon is sent out from one of the particles, it is received by the other particle and then it sends out a photon in exchange after it receives one.

 

[Orodruin]

If a guy gets on a rocket that travels at the speed of light for a finite period of time (in relation to other observers not moving at C) the guy in the rocket experiences no passage of time. In this scenario his leaving and arrival of the rocket happens at the same time in his frame, correct?

Incorrect, there is no frame that travels at the speed of light relative to the other frames. Light does not have a rest frame.

Yet an observer watching this happen, sees the guy get on the rocket at point A, some time passes, and he gets off at point B? His progress has a past and this establishes an arrow of time for him from the observer's viewpoint. Is that correct?

The world line of a massless particle cannot be parametrised by proper time. However, it is perfectly possible to find an affine parametrisation that goes from earlier to later times in all frames, giving a proper time ordering to all events along that world line.

In a possible real version of this problem, if a photon is exchanged between two particles (electromagnetic or strong force carrier photon for example), are two photons exchanged, one from particle A to particle B and one from particle B to particle A?
or, is one photon exchanged that satisfies both particle A and B that they have both sent and received a separate force carrier photon (that is one photon between them, that appears to be both arriving and departing both particles simultaneously)?
Or is it simply one photon moving from one particle to the other with no reciprocal exchange?

First of all, you would do well to forget about exchange of photons as being throwing little balls between two charges. It is at best a popular scientific trick and at worst misleading. This can be described in a proper manner only with quantum field theory and it is not helpful to think about it in the way you have presented.

Second, as I stated above, a time ordering can be imposed on s light like world line even if the proper time along it is zero.

 

[Dale]

If a guy gets on a rocket that travels at the speed of light for a finite period of time (in relation to other observers not moving at C) the guy in the rocket experiences no passage of time. In this scenario his leaving and arrival of the rocket happens at the same time in his frame, correct?

Nothing with mass can travel at c, so the best you can do is "close to c" and "experiences very little passage of time"

And if this is so, which way does the arrow of time point for the guy in the rocket? Aren't both the point of origin and destination equally plausible as either? That point A where he departed and point B where he ended his journey are one and the same and can be swapped around?

All frames agree on the direction of the arrow of time. In all frames the origin comes before the destination.

In a possible real version of this problem, if a photon is exchanged between two particles (electromagnetic or strong force carrier photon for example), are two photons exchanged, one from particle A to particle B and one from particle B to particle A?

Different events on a light pulse still have a frame invariant order. The math is more tricky, involving affine parameters instead of proper time, but there is no ambiguity about which event was emission and which was reception for a light pulse.

 

[Simon Bridge]

[edit: oh wow - everyone jumped on this ... probably because you appear to have been thinking about it at length and otherwise quite well. Perhaps we need a sticky or insights article or something?]

If a guy gets on a rocket that travels at the speed of light for a finite period of time (in relation to other observers not moving at C) the guy in the rocket experiences no passage of time. In this scenario his leaving and arrival of the rocket happens at the same time in his frame, correct?

No - the statement "a guy travels at the speed of light" makes no sense in relativity.
Time has no meaning and the reference frame of a photon also has no meaning.

You can make this sensible by saying that the "guy" travels very close to the speed of light ... so that his travel time is too small for him to measure on his clock.

Yet an observer watching this happen, sees the guy get on the rocket at point A, some time passes, and he gets off at point B? His progress has a past and this establishes an arrow of time for him from the observer's viewpoint. Is that correct?

No - the observer's clock establishes the arrow of time for that observer before the journey even started. This is how that observer can tell which event came first.

And if this is so, which way does the arrow of time point for the guy in the rocket?

In the same direction as his clock ticks.

Aren't both the point of origin and destination equally plausible as either? That point A where he departed and point B where he ended his journey are one and the same and can be swapped around?

In the event of instant teleportation in a way that violates the postulates of special relativity - yes.
What you are noticing here is what happens when you postulate something that makes no sense in physics even though it makes a valid English language sentence.

In a possible real version of this problem, if a photon is exchanged between two particles (electromagnetic or strong force carrier photon for example), are two photons exchanged, one from particle A to particle B and one from particle B to particle A?

The reference frame of the interaction is the particle's frame ... so the events are distinct, and occur in a specific order. The particles are not traveling at the speed of light, so the observation above does not apply.

Or is it the only other option I can think of, that one photon is sent out from one of the particles, it is received by the other particle and then it sends out a photon in exchange after it receives one.

... there is another option: your model is too simple.
Charges do not "send out" photons ... instead there is a probability that there is a photon present to be interacted with.
These photons are virtual - they are not "really" there in the sense that your chair is really there.
They are probably best thought of as artifacts of a step in a calculation - like the intermediate steps when you do long division: you can use those steps to work out the final answer but none of those bits need ever exist as a stage in physically dividing an actual object.

But there are experiments when you actually fire a photon at a target and the target responds - in that case, for the reasons you have noticed, it makes no sense to look at things from the reference frame of the photon.

 

[BernieM]

The rocket I was referring to would have been a massless rocket. Thought experiment one.

So in regard to the clocks of the observer and the traveller traveling at C, who sets 'Now', the current moment? I think each sets it's own now, correct? As there is no universal clock keeping track of time?

But I see an answer I was looking for that makes sense of a few things for me. That was the purpose, to gain understanding. Earlier I posted a longer model of the problem and got it zapped because it presented a non-allowed theory (actually my view of how I saw things with my current understanding and wanting people to show me where it was in error.)

 

[Ibix]

What are you going to build a massless rocket and massless clock out of? You can't do it, I'm afraid.

Trying to think about the perspective of something traveling at the speed of light will just give you a headache. There is no way to describe it in relativity. Light would have to be both stationary and moving at c for such an observer, which is obviously self-contradictory.

 

[BernieM]

What are you going to build a massless rocket and massless clock out of? You can't do it, I'm afraid.

It wasn't a problem for Einstein in his thought experiments.

 

[Ibix]

I take it you are referring to the "riding a motorcycle next to a pulse of light". That's not a thought experiment so much as a fantasy. As Einstein later realized, it's impossible to describe doing such a thing.

None of his actual thought experiments involve anything traveling at the speed of light except light itself. And he never describes anything from the perspective of something traveling at the speed of light because it's self-contradictory to do so, as I noted above.

 

[BernieM]

I take it you are referring to the "riding a motorcycle next to a pulse of light". That's not a thought experiment so much as a fantasy. As Einstein later realized, it's impossible to describe doing such a thing.

None of his actual thought experiments involve anything traveling at the speed of light except light itself. And he never describes anything from the perspective of something traveling at the speed of light because it's self-contradictory to do so, as I noted above.

Here is his first: Source is http://www.pitt.edu/~jdnorton/Goodies/Chasing_the_light/
It says:
How could we be anything but charmed by the delightful story Einstein tells in his Autobiographical Notes of a striking thought he had at the age of 16? While recounting the efforts that led to the special theory of relativity, he recalled

"...a paradox upon which I had already hit at the age of sixteen: If I pursue a beam of light with the velocity c (velocity of light in a vacuum), I should observe such a beam of light as an electromagnetic field at rest though spatially oscillating. There seems to be no such thing, however, neither on the basis of experience nor according to Maxwell's equations. From the very beginning it appeared to me intuitively clear that, judged from the standpoint of such an observer, everything would have to happen according to the same laws as for an observer who, relative to the earth, was at rest. For how should the first observer know or be able to determine, that he is in a state of fast uniform motion? One sees in this paradox the germ of the special relativity theory is already contained."

So far as I know Einstein isn't massless and he speaks of pursuing a beam of light with the velocity of C. However that can be read that the light is traveling at C, though that would be redundant. So I read it as he is pursuing the light by moving the speed of light himself (how else would he get in phase with it).

 

[Simon Bridge]

The rocket I was referring to would have been a massless rocket. Thought experiment one.

Doesn't matter ... it contains an observer able to tell the time. Therefore there is a clock at rest in the reference frame of the massless rocket. This is what does not make sense. The argument is not that it is impractical or physically unrealizable but that it is a logical contradiction with the postulates of special relativity.
You cannot have something both traveling at some speed and also being stationary to the same observer.

It wasn't a problem for Einstein in his thought experiments.

Appeal to authority: OK Einstein would be a valid authority on his own theory...please cite an example where Einstein postulated a massless clock in a thought experiment that accepted the postulates of special relativity?

The "riding a motorcycle next to a pulse of light" thing? The story, you cited, did not involve a clock, pre-dated special relativity, and concluded that the situation contained contradictions so another theory may be needed. You will notice that, in the thought experiment, he posited that the light was stationary ... this is in violation of the postulate that the speed of light is the same to all observers so what is your point? It is possible to imagine things that are nonsense? I agree.

Maybe we are not being clear - I admit it is tricky to wrap the mind around...
Nobody is saying the sentence does not make sense as an idea in every possible framework, or that it does not make sense in terms of galilean relativity ... we are saying it is nonsense in terms of special relativity. The statement, after accepting the Einstein postulates, is a contradiction in terms. Just like the thought experiment you cited as an example.
It you start from a nonsense assumption/assertion, you will get nonsense conclusions. Which is what happened.

Have a look at the original paper:
https://www.fourmilab.ch/etexts/einstein/specrel/www/
... it is amazingly accessible. There is a popular treatment that should address some of the things you are interested in here:
http://www.physicsguy.com/ftl/html/FTL_intro.html

But you also asked about the relationship between time and clocks.
In a nutshell: time is what is made by clocks. If it makes time it is a clock: that is what a clock is.
For experiments we like clocks that also mark intervals of time that every inertial observer agrees are always the same as each other.
The question of "now" is handled under 'simultaniety" and is a tricky subject that quickly turns into philsophy. You will find the concepts like "now" or "the present" do not get used all that much in relativity.
The "arrow of time" stuff is usually related to how clocks make time - what is it about physical clocks that can be said to "make time" like I put it so glibly above and how come it is always one-way? That sort of stuff. This is not especially tied up with special relativity, despite the loose talk about FTL=time-travel, and gets into discussions about entropy, evolution of the mind, and how memories get laid down. All fun stuff, and all too big for this thread.

But there is no Universal clock keeping a Universal proper time that we know of... we agree there.

But I see an answer I was looking for that makes sense of a few things for me.

Which was? <all ears>

That was the purpose, to gain understanding. Earlier I posted a longer model of the problem and got it zapped because it presented a non-allowed theory (actually my view of how I saw things with my current understanding and wanting people to show me where it was in error.)

That is fine - though with established theories you are better looking for ways you do not understand what the theory says and ask about that rather than try to postulate you own theory in opposition and ask people to shoot it down.

 

[Ibix]

"...a paradox upon which [Einstein] had already hit..."

Did you read what you quoted? The man himself says that the idea is paradoxical. The fully developed version of this is that you can't travel at c nor describe the perspective of something traveling at that speed.

Maybe I overstate my case to call the above a fantasy. At most, though, it is a proof by contradiction that it is impossible to travel at the speed of light.

 

[Nugatory]

So far as I know Einstein isn't massless and he speaks of pursuing a beam of light with the velocity of C. However that can be read that the light is traveling at C, though that would be redundant. So I read it as he is pursuing the light by moving the speed of light himself (how else would he get in phase with it).

Yes, and he also says in the same text that the result is a physical impossibility: "an electromagnetic field at rest though spatially oscillating", something that is forbidden by Maxwell's laws of electromagnetism. The point of the story that the assumption that you can travel at the speed of light is internally inconsistent; it leads to the logical contradictions that @Ibix points out in #6 of this thread.

 

[Dale]

So in regard to the clocks of the observer and the traveller traveling at C, who sets 'Now', the current moment? I think each sets it's own now, correct? As there is no universal clock keeping track of time?

As others have mentioned, you cannot have a clock traveling at c.

However, even for a pulse of light, where there is no proper time between the emission and the absorption, the emission and the absorption events are still separate events with the emission occurring before the absorption in all frames. Because there is no proper time along the path, you cannot use proper time as the affine parameter, and instead you use some other affine parameter.

 

[BernieM]

snip - (this was mosly superfluous stuff that gets me off track)
But there is no Universal clock keeping a Universal proper time that we know of... we agree there.
OP Quote -> "But I see an answer I was looking for that makes sense of a few things for me. <-
Which was? <all ears>

For me learning requires that I can take what I have learned and extend the implications of it into an area that is unknown, and arrive at the proper conclusions. If I can't do that, then I haven't learned anything, I am just a parrot citing information in meaningless snippets of parroted script. If I do have an understanding that is correct, then I should be able to properly predict or explain other things that I currently don't understand.

The answer you gave me was that I had missed an optional scenario, which was the apparently the correct one; that these photons are all over the place and are bumped into rather than being sent out like messenger photons. That in the process of bumping into one of these photons, the information between the particles is exchanged.

Did I get that right? Or am I still missing an important point there?

Perhaps then you can help me with this:

When short lived particles sprayed down on Earth from the upper atmosphere from gamma ray collisions in the upper atmosphere, reach the earth, due to the fact that they see the distance as shorter than we see it, the particles nonetheless get here. Our view seems to say they shouldn't get here. We reconcile it with time dilation in our frame. But the actual thing that got the particle from up there to down here was the length contraction, not the time dilation of time we observed. Time dilation that we observed didn't participate with the particle. Or did it?

It's odd that of space and time, it is time that gets tossed around the most as possibly not being a real property, but possibly one made up by man. I don't see that with space. In the event mentioned above, the particle deals with space and gets to the Earth without consideration of time. It is us as the observer to the event that has to play with clocks, and we call it time dilation so we can rationalize it, put it in harmony with our view or frame of reference.

Is that right? But the particle didn't play with a clock, it spanned space. It apparently had less space to span. And our tinkering with time dilation didn't help it bridge that distance. So isn't the particles frame or viewpoint the only true valid frame for an event? UNLESS the observer is actually a participant in the event?

The photon, a true point particle, sees no distance between the beginning and the end of it's journey. As with the subatomic particles reaching the earth, I would think the photon doesn't need to tinker with a clock anyhow to get from point A to point B and that clock tinkering is for us so we can rationalize it in our frame. So a photon is supposedly at the beginning and end of it's journey due to distance contraction? There was no distance to cover, so it's there at both ends simultaneously? But there's that word in the sentence 'simultaneously' which deals with time right? And our view of the event?

How do I get it out of there? I thought like the subatomic particle, it experiences the ultimate in length contraction. That doesn't need time does it? So you see there where I have some huge misunderstanding and could certainly use being set right here. To get the proper view of these things.

As one poster said:

snip - Trying to think about the perspective of something traveling at the speed of light will just give you a headache. There is no way to describe it in relativity. Light would have to be both stationary and moving at c for such an observer, which is obviously self-contradictory.

That doesn't work for me. Headache or not I feel I have a need to understand it on a conceptual level.

Ultimately, all of this I feel, is required for me to understand the original problem I began with many many years ago, which is:

Is potential an actual quality in this universe such as space and time? (Not potential energy or potential space or any other thing, but pure potential viewed as a substance or inherent property in the universe) Does it exist and is it required for other substances to exist (like energy)?

It has been observed long before math came about. Potential is not the outcome of mathematics and mathematical theory, I think. If not, is it a substance? A thing? A property? A quality? Or is it possibly a construct of the mind, like time has been suggested to be?

Is it useless even to try to answer the question? Perhaps it is impossible. But isn't it the journey that life is all about? Aren't we supposed to attempt to understand the incomprehensible?

Potential is usually defined as that which may exist yet is not. I think maybe it is more likely, that which WILL exist yet is not.

Is it required that a 'force' of potential precedes the existence of anything/everything?

Or is this just another one of those things (as was said earlier) that makes sense in English, but makes no real sense in physics or the universe?

Now don't think that this is a theory of mine. If it was, I would tell you how it worked and how it explains everything. It's not. It's a question, pure and simple. But I think it is inherently very hard to answer (at least it has been for me.) It goes along the lines of a discussion I had with a theoretical physicist at the University of Idaho, Moscow back in the late 90's, in which we discussed tachyons. I proposed to him that since tachyons apparently move in reverse through time, that one might put a detector at a distance away from the experiment that might generate them, and start it detecting before the experiment is turned on. Pretty simple right? haha. Well, his argument about detecting them was that until he knew their properties, he didn't know how to make a detector to detect them.

And so in a way this problem of potential seems to fall into that same sort of trap. By nature it seems not to have any identifiable property. But does time or space?

In recent history I have had some time to devote to try and answer this question for myself, and in the process have run across other dilemmas that relate to the answer I am looking for, like the photon dilemma I mention above.

 

[Dale]

But the actual thing that got the particle from up there to down here was the length contraction, not the time dilation of time we observed. Time dilation that we observed didn't participate with the particle. Or did it?

No. Time dilation did get the particle "from up there to down here" in our frame. In the muon's frame it was length contraction that got the particle "from up there to down here". Neither view has any theoretical justification for being considered the "actual" reason. Both frames and their associated explanations are equally valid.

 

[BernieM]

Does the muon getting from point A to the ground (point B) require an observer, or the rest frame?
If so what contribution does the rest frame make for the travel of the muon?

 

[Nugatory]

Does the muon getting from point A to the ground (point B) require an observer, or the rest frame?
If so what contribution does the rest frame make for the travel of the muon?

"No" for the first question, and "none" for the second.

The physical facts are:
1) A muon was created at a particular point in spacetime (which happens to be in the Earth's upper atmosphere).
2) The muon's path intersected the surface of the Earth at a particular point in spacetime.
3) The proper time along the muon's path between those two points in spacetime has some particular value which is less than the muon lifetime.
These facts have nothing to do with observers or reference frames; they'd be facts even if there were no observers around.

A reference frame is just a rule that we use to to assign times and positions to these events. For a very prosaic example... If I said that a firecracker exploded ten meters my left and you said that it exploded five meters to your right, we wouldn't be confused, we'd just conclude that you were standing 15 meters to my left, we're both right, there's nothing wrong with either of our perceptions of reality, we're just measuring distances relative to different starting points That's different reference frames at work.

Relativity is less intuitive because it attaches different times as well as different positions to events, but the principle is the same - both pictures are just as good, and there is no physical content in the difference between them.

 

[BernieM]

"
- snip -
2) The muon's path intersected the surface of the Earth at a particular point in spacetime.
3) The proper time along the muon's path between those two points in spacetime has some particular value which is less than the muon lifetime.

2) A shared point that coincides with both the muon's length contracted spacetime and Earth's non-length contracted spacetime?
3) Proper time? Universal clock? Proper to who or what? Is this the observer frame? Local universe frame? I am not quite following.

 

[Ibix]

Proper in the Latin sense of "one's own" (propio if you speak Spanish), like property. The muon's own time - or time measured by a clock moving with the muon if you want to be precise.

 

[PeterDonis]

A shared point that coincides with both the muon's length contracted spacetime and Earth's non-length contracted spacetime?

There aren't two spacetimes. There is only one. The Earth frame and the muon frame are different coordinate charts on the same spacetime, just as Cartesian and polar coordinates are different coordinate charts on the same Euclidean plane. Points in spacetime are independent of which coordinate chart you use, just as points in the plane are independent of which coordinate chart you use.

Proper time?

Proper time is the arc length along a particular curve in spacetime, in this case the muon's worldline (the curve in spacetime that describes its history). Arc length along a curve is an invariant, independent of the coordinates you choose; this is just as true in spacetime geometry as it is in ordinary Euclidean geometry.

 

[Nugatory]

2) A shared point that coincides with both the muon's length contracted spacetime and Earth's non-length contracted spacetime?

Yes. The easiest way to see this is with a spacetime diagram - these are invaluable for visualizing and understanding this stuff. I'll post one up tonight if someone else doesn't beat me to it

There will be four interesting points in the diagram: Muon is created; muon reaches the muon detector at the surface of the earth; the location of the muon detector at the same time that the muon is created using a frame n which the muon detector and the surafce of the Earth are at rest; and the location of the muon detector at the same time as the muon is created using a frame in which it is the muon that is at rest and the Earth that is moving..

The last two are different points in spacetime because of the relativity of simultaneity: two different events that happen simultaneously ("at the same time") using one frame will not, in general, be simultaneous in other frames. If this is new to you, take a moment to google for "Einstein train simultaneity", as the relativity of simultaneity is essential to understanding all relativity thought experiments and paradoxes.

(And now would be a good time to mention that points in spacetime are generally referred to as "events", to avoid confusion with points in space)

3) Proper time? Universal clock? Proper to who or what? Is this the observer frame? Local universe frame? I am not quite following.

Proper time along a path through spacetime is defined as the amount time that a clock moving along that path would record. If you use different frames you'll find that the two events at the endpoints have different space and time coordinates, but the proper time between them always comes out the same. In this case, we're talking about the path between the "muon created" event and the "muon detected", so the proper time along that path is the amount of time that the muon experiences; it's less than the muon lifetime so the nuon makes it to the surface of the Earth in all frames.

 

[BernieM]

Well that helps clear up some of the muon questions.
A photon moving at C is the most extreme possible case of what happens to the muon then? With the exception of the mass and charge limitations of the muon I mean. So with a photon there will be maximum (infinite?) length contraction?

 

[Dale]

Does the muon getting from point A to the ground (point B) require an observer, or the rest frame?

An "observer" is a commonly understood (in relativity) shorthand for "reference frame" usually specifically a reference frame where a designated object (the observer) is at rest. The muon itself can be used as the designated object, or the earth, but designating an object as "the observer" and using its reference frame is purely a matter of convenience with no physical consequences.

If so what contribution does the rest frame make for the travel of the muon?

It changes the description, much like the choice of using magnetic north vs celestial north might change the description of a hiking path.

 

[PeterDonis]

So with a photon there will be maximum (infinite?) length contraction?

No. There is no inertial frame in which a photon is at rest, so the concept of "length contraction" cannot be applied.

 

[BernieM]

No. There is no inertial frame in which a photon is at rest, so the concept of "length contraction" cannot be applied.

If I am understanding you properly then I would assume that a neutrino (or maybe an electron) which has mass, would experience the most length contraction (similar to a muon in that scenario,) of all the particles that exist? So explain the tie to the inertial frame for the photon to have length contaction. Why does it need a rest frame to have the length contraction if other frames are not required or relevant to it?

And if it can't have length contraction, why is it always stated that in the view of a photon, there is no distance between the end and beginning of it's trip, regardless of the distance it has travelled; that the end point, origin, and all points between are the same point to a photon. Isn't that length contraction? Or is that some other concept? Or are these analogies of how a photon perceives the universe incorrect in some fundamental way?

 

[PeterDonis]

If I am understanding you properly then I would assume that a neutrino (or maybe an electron) which has mass, would experience the most length contraction (similar to a muon in that scenario,) of all the particles that exist?

The amount of length contraction is frame-dependent; it doesn't depend on the type of particle, it depends on the particle's velocity relative to the observer.

So explain the tie to the inertial frame for the photon to see length contaction.

We have a FAQ on this:

https://www.physicsforums.com/threads/rest-frame-of-a-photon.511170/

why is it always stated...

Where is it "always stated"? What valid source (textbook or peer-reviewed paper) says this? Yes, pop science treatments might use this misleading language, but that just illustrates why you don't want to learn actual science from pop science treatments.

Or are these analogies of how a photon perceives the universe incorrect in some fundamental way?

Yes. I strongly advise you to take some time to work through an actual textbook on SR. Taylor & Wheeler's Spacetime Physics is one such.

 

[Dale]

And if it can't have length contraction, why is it always stated that in the view of a photon, there is no distance between the end and beginning of it's trip, regardless of the distance we perceive it to have travelled

It isn't always stated. Pop-sci authors may often say it, but it is not stated in professional scientific sources. When you see it, that is an indication that the author is more interested in conveying excitement than in conveying science. (this is also why pop-sci sources are not acceptable here)

 

[Mister T]

For me learning requires that I can take what I have learned and extend the implications of it into an area that is unknown, and arrive at the proper conclusions. If I can't do that, then I haven't learned anything, I am just a parrot citing information in meaningless snippets of parroted script. If I do have an understanding that is correct, then I should be able to properly predict or explain other things that I currently don't understand.

If the speed of a light beam is independent of the speed of its source, then it follows that it's also independent of the speed of an observer. Thus, if you chase after a light beam, no matter how fast you travel in your attempt to catch it, it will forever recede from you at speed c. If you really do want to learn what Einstein was getting at when he spoke of that childhood thought experiment, understanding the above is a good start. Once you do, and it's by no means an easy thing to do, then you will understand that it's not ever possible to move alongside the light beam so that it's at rest relative to you.

What follows from that understanding is that the speed c is the fastest speed possible, and that as an observer you can come as close as you want to it, but you can never achieve it. It therefore makes no sense to speak of what things would happen to someone or something moving at speed c. This is what Einstein learned from his thought experiment.

It's odd that of space and time, it is time that gets tossed around the most as possibly not being a real property, but possibly one made up by man.

Lots of people don't like it when I tell them this, but both space and time are human inventions. In that sense they are real. And of course they are physical properties in the sense that we humans have to define how we are to go about measuring them. Otherwise they make no physical sense.

Is potential an actual quality in this universe such as space and time? (Not potential energy or potential space or any other thing, but pure potential viewed as a substance or inherent property in the universe) Does it exist and is it required for other substances to exist (like energy)?

Energy is another example of a property invented by humans, as witnessed by the fact that it is up to us to provide the definition for how we are to measure it.

If you can do the same with this "potential" then it would be on the same footing. But it isn't recognized as a physical property in the same sense as space, time, or energy.

 

[BernieM]

-snip-

Lots of people don't like it when I tell them this, but both space and time are human inventions. In that sense they are real. And of course they are physical properties in the sense that we humans have to define how we are to go about measuring them. Otherwise they make no physical sense.

Energy is another example of a property invented by humans, as witnessed by the fact that it is up to us to provide the definition for how we are to measure it.

If you can do the same with this "potential" then it would be on the same footing. But it isn't recognized as a physical property in the same sense as space, time, or energy.

My leaning on the potential aspect was that it is an absolute reference (without location or participation in space or time in any respect) with an absolute value of nothing, from which other things in the universe gain their value or state. Like the 0 on a number line makes the number line workable. How do you define the zero on a number line without at least one other number? Does it have meaning without other numbers? Big philosophical headache.

But as I said, I don't even know really how to define potential without a reference, like 'potential energy' or 'electrical potential', etc.

 

[Mister T]

How do you define the zero on a number line without at least one other number?

I pick a point and label it zero.

Does it have meaning without other numbers?

Yes. You need other numbers only if you wish to establish a scale.

But as I said, I don't even know really how to define potential without a reference, like 'potential energy' or 'electrical potential', etc.

That's why it's not physics.

 

[BernieM]

How do you define time or space without reference to something else?

 

[Dale]

Lots of people don't like it when I tell them this, but both space and time are human inventions.

I don't particularly like this either. The words "space" and "time" are human inventions, as are the mathematical and other symbols used in relativity. But the things that those symbols and words represent are not human inventions.

 

[BernieM]

Like space and time, I think potential is a pure property of value as well, but as I said, I haven't found a way to express it in a scientific way yet (maybe never will?.) In a way dark energy and dark matter are similar. They are postulated to exist by effects we observe in our universe, yet by nature, since they are 'dark' and haven't yet been observed, it's kind of hard to say what they are or aren't in reality. Perhaps potential is just dark energy or something, or a manifestation of it. Potential seems to be an objective form of probability/possibility, on which quantum mechanics is based. So I would think that QM would have to deal with 'from where does probability in the universe arise' on a theoretical level.

But I won't belabor potential so I can keep out of trouble here on the forums.

In regard to the photon scenario, I have gained some insight that has value to me such as the rest frame contradiction. As well as the other option in my original post that set me straight there. In general the answers have been helpful and this thread has been productive for me. Although the scenarios people present of 'how a photon sees the universe' have been marginalized, I have yet to actually find a 'proper' model presented in a relatively simple to understand way. I would find that very helpful. That if the currently offered scenarios are incorrect in some way, it would be valuable to have a correct representation made.

One final question comes to mind:
If every particle in the universe were to be considered an observer, wouldn't every observer perceive a different universe than every other observer?

 

[Mister T]

How do you define time or space without reference to something else?

What you define is the operation used to measure them.

 

[Drakkith]

If every particle in the universe were to be considered an observer, wouldn't every observer perceive a different universe than every other observer?

I'd say that each observer perceives the universe slightly differently since all other objects are positioned differently and may have different properties (like relative velocity) with respect to the observer. In other words, electron A sees electron B on its right side and nothing on its left, while B sees A on its left and nothing on its right, leading to unique perspectives.