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Light and time- what i think about it

  1. Sep 2, 2006 #1
    i think this was said by Lurch, a pf member.
    well, this is what i got after i looked up 'time' in wikipedia:In physics, the Planck time (tP), is the unit of time in the system of natural units known as Planck units. It is the time it would take a photon travelling at the speed of light to cross a distance equal to the Planck length.[1] The unit is named for Max Planck.
    so, according to this, the photon does take time to travel.
    time is everything, isn't it? we have used units for our convinient because time is the indefinite continued progress of existence and events in the past, present, and future, regarded as a whole.. u say that photon takes no time to reach it's destination. but everyrthing is happening in time, don't u think? it's just that it travels so fast that it does not reach out calculations. our units. if a photon has to travel from a star to Earth, then it should travel in time. because time is always happening no matter how fast anything travels in it.
    this is what i think. please tell me what u think, so that i can correct myself if i'm wrong.:smile:
  2. jcsd
  3. Sep 3, 2006 #2


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    Hi, Varsha;
    Once again... objective and subjective time are not the same thing. From the photon's perspective, no time passes. From external frames of reference, it does.
  4. Sep 3, 2006 #3
    ok...this is confusing...fact is fact isn't it? so, how is there, "photon's perspective" and "external frames"? photon is no living thing so it has no views...so how do u say that? in "inertia" i understand, that it depends on the persons views. but this...? and we can't experience what a photon does (again, i can't say that a photon can experience anything) because we can never travel that fast...so shouldn't there be just one thing happening? either the photon takes time or it doesn't? please don't mind if i'm irritating u by asking the same things again, but i'm really desperate to get this cleared...
  5. Sep 3, 2006 #4


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    Geez, man... don't ever worry about irritating me with questions. People who refuse to acknowledge the answers are irritating. (And I don't mean my answers, because my answers are not always correct. When, on the other hand, half a dozen professionals make the same point and it's ignored... :grumpy: )
    All that I can assume here is that you haven't a grounding in Relativity. Every single particle in the universe has its own reference frame, because it's in motion relative to the others. This does not in any way suggest that it's aware of that. It's just that no matter what reference frame you choose to base yourself from, your measurements of the others will not coincide with those of the others regarding each other or you.
    Try to find a few good books about Einstein's work, preferably those written by his self.
  6. Sep 3, 2006 #5
    thanks for the recommendation.:smile: i will surely look for it.
    i think i'm getting it...though i won't completely understand it unless i read some books related to it. but i'm sure i'll come up with more doubts...:biggrin:
  7. Sep 3, 2006 #6
    I read one years ago that was a translation of some lectures the great man gave at Berlin in the 1930's, (just before he left for the USA) it was the best explanation I have ever seen but I have not been able to find a copy since - probably because I can't remember who the translator was. Dammit!
  8. Sep 3, 2006 #7


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    This was Einstein's insight that changed the world forever.

    Events are not universal. Neither is time. They are dependent on the frame of reference.
  9. Sep 3, 2006 #8


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    Wow! If you ever find it again, let me know. I'd love to read that.
  10. Sep 3, 2006 #9

    Andrew Mason

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    It is still in print:
    "[URL [Broken]
    The Principle of Relativity, Dover[/URL]

    Last edited by a moderator: May 2, 2017
  11. Sep 3, 2006 #10
    Ya, same here.
  12. Sep 3, 2006 #11


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    Thanks, Andrew. I'll try to find a copy. Locally, I mean. I don't generally buy over the net. Might have to in this case, but there's a good chance of my neighbourhood store being able to order it.
  13. Sep 3, 2006 #12

    I am no physicist, and my graps of relativity is shaky at best. I'll probably get slammed by my betters for inaccuracies, but perhaps, in a sense, this makes me the best person to answer the initial part of your questions. I say initial because grasping the full meaning of relativity is a process, IMO, not an all-or-nothing issue.

    First, the thing about time is that it is a very subjective measure. Common expressions like "Time flys when you're having fun" is a profound thing when you realise that an idividuals perception of time can be very different from someone else's.

    Of course, you might argue that the actual passage of time was the same for everyone, so perception was illusory. What then is the point? The point is that it actually works the other way around. A variation in actual time can be perceived by you as "normal" time by your watch. You assume the flow of time is constant, so your perceptions are based on that.

    I've read that if we watched a body fall into a Black Hole, we would never see it actually reach its destination, because of time dilation. The flow of time for that body is different to ours. Apparently, proximity a gravity source slows the passage of time. So if you are watching this event from a "safe distance", it would seem to you that the falling object slows down. To you, the object would eventually appear to be just hanging there (if you could still see it).

    For the object, time is moving along "as usual". If it was observing you, it would seem that you were speeding up, the whole universe would be "running" faster and faster.

    I have not the least notion of why time behaves like this, but this illustrates that time does flow differently depending on your point of view. "Normal time" is that of the observer. "Observer" is a very common word you will encounter in matters of relativity; as I hope I've explained, the point from which an event is observed makes all the difference.

    So, to come back to your first quote, I do not agree. To us (the observer), the photon would appear to travel faster than expected because we are in a gravity well (the Earth's), while the photon is relativly free of of gravity. BUT WAIT! Isn't the speed of light a constant? Ah, so what happens is the spectrum of light we see which is the photon is shifted, but we perceive the photon as moving at the "right" speed - c.

    For the photon, well now, what happens there? Again, something travelling at c ALWAYS travels at c. Is it not said that anthing travelling at c has infinite mass? IF that is so, then from the photon's perspective it isn't travelling at all, but exists everywhere at once. There isn't any "trip" from the photon's perspective, so discussing how long it takes is a meaningless question - it hasn't gone anywhere since it is everywhere.

    For an object moving at less than c, time is an interesting subject to discuss; the object has some velocity, some specific position in the universe, and some time it will take to get where it's going. Our perception, from a relatively stationary platform, deep in a gravity well, will be very different to the perception of the object making the trip, but such a comparison has some meaning.

    For the Photon, however, I suspect time is irrelevant. From its point of view, it is everyWHERE, and I am thinking it is also everyWHEN.

    Hence, no matter what you do along the lines of chasing after the photon or running away from it, you'd always measure its speed as c, it will just be red- or blue-shifted in your perception of it.

    To the Photon, you don't exist by any meaning of the word you or I can understand!
  14. Sep 3, 2006 #13


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    I remember seeing a quote from Einstein himself back in the 70's (when I saw it, not when he said it). I have to paraphrase a bit because I can't be sure that I'm remembering the exact words. Essentially, he said "Ten minutes sitting on a pretty girl's lap is not the same as ten minutes sitting on a hot stove." :biggrin:
  15. Sep 4, 2006 #14
    this brings me to another question (no big surprise)
    how does gravity affect time? i mean, gravity is a force (if u go according to Newton) or it's a curvature of space (Einstien). but time? it's not a force or...how exactly do u relate time to gravity?
  16. Sep 4, 2006 #15


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    Clocks slow down with gravity.
    You could read about blackholes, the GPS system or there was an experiment with atomic clocks at the top and bottom of a tower showing this.
  17. Sep 4, 2006 #16


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    As you've said, you know that gravity bends space. Well one thing that Einstein said was that space and time are intertwined, forming what we call
    (very unoriginnally) "space-time." So when space is bent by gravity, causing orbits etc., time is also bent, changing the rate at which is goes by for a person in the gravity well. Keep in mind the person in the well can't tell that time is moving slower. He still thinks hes moving through time at a normal rate and everyone else has sped up. You can't feel time move slower because when time moves slower, you brain processes information slower.

    Also, we can't say that the person in the gravity well is experiencing "abnormal time" and the people outside are experiencing "normal time" Both reference frames are correct. When the person inside says that the outside is moving faster, hes correct. When the people outside say that the inside man is slowing down, they are also correct
    Last edited: Sep 4, 2006
  18. Sep 4, 2006 #17
    could u explain this? how could he say that they are interwined?
    and if we consider Newton's law of gravitation, can we say that gravity affects time?
  19. Sep 4, 2006 #18

    Andrew Mason

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    To understand conceptually why gravity affects time, you have to understand is the Principle of Equivalence. Einstein postulated that there was no difference to the laws of physics in an inertial frame and in gravitational free-fall.

    Imagine a long pointed tube of length L. A light signal is sent from the tail to the nose. What speed does the observer in the nose measure the light to travel?

    If it is not accelerating (inertial frame), the speed of the light is c so the observer in the nose measures the time of travel to be L/c.

    If it is accelerating due to gravity, falling nose first in a gravitational field with acceleration g, the light will take a little longer to reach the nose because when the light travels the distance L in time L/c, the nose will have moved a distance [itex]s = \frac{1}{2}gt^2[/itex] further away.

    But the Principle of equivalence says that the two frames are equivalent, so the observers in the gravitational and inertial fields both measure the laws of physics, including the speed of light, to be the same.

    If that is the case, then the observer in the gravitational field must measure time differently (more slowly).

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