Arrow of time and travel at the speed of light

  • #26
PeterDonis
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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.
 
  • #27
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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)
 
  • #28
Mister T
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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.
 
  • #29
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-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.
 
  • #30
Mister T
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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.
 
  • #31
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How do you define time or space without reference to something else?
 
  • #32
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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.
 
  • #33
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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?
 
  • #34
Mister T
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How do you define time or space without reference to something else?
What you define is the operation used to measure them.
 
  • #35
Drakkith
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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.
 
  • #36
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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.
A proper model of what? Of how a photon sees the universe?

There isn't one. It is improper and self contradictory at its core.
 
  • #37
PeterDonis
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This seems like a good point at which to close the thread.
 

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