Naked Science Forum
Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: ARVIND on 13/01/2013 09:38:54
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I am not concerned whether light has mass or not. But, physicists say that anything that travels at the speed of light, its time slows down. This is known as time dilation. But, ligt itself travels at the speed of light. So, does this mean that ligt also dilates time.
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But, physicists say that anything that travels at the speed of light, its time slows down.
Not at all, maybe popular books, not physicists. Infact it's false.
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... physicists say that anything that travels at the speed of light, its time slows down.
That's not what physicists say. What they say is that time as measured in frame of reference moving at speed v < c runs slower than time as measured in a frame of rest. It can also be shown that nothing can be at rest in a frame which is moving at the speed of light. Or simply put; moving clocks run slow when compared to clocks at rest.
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If I wanted a debate, I had a lot to wrote about the way you explained it, Pete, but I'm sure you already know it can be put in a better form, so I prefer not to be finicking this time [:)]
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If I wanted a debate, I had a lot to wrote about the way you explained it, Pete,…
You didn’t like the way I explained it? If so you should have posted your own explanation in hopes of making the answer more clear.
I’m not to happy with my explanation either so here goes again
With regards to “physicists say that anything that travels at the speed of light, its time slows down.”
This is wrong. What physicists really do say is that that nothing can travel at the speed of light. What he must have meant to say (or what he really did hear) was that “physicists say that when an object is moving its time slows down.” Which is correct, but only when it is understood that it only has meaning when its referring to what his time is being compared to and its time is being compared to that of observers who are at rest in the frame in which the other observers are moving.
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What he must have meant to say (or what he really did hear) was that “physicists say that when an object is moving its time slows down.” Which is correct, but only when it is understood that it only has meaning when its referring to what his time is being compared to and its time is being compared to that of observers who are at rest in the frame in which the other observers are moving.
But saying "when an object is moving its time slows down" is however incorrect, for two reasons:
1. It suggests to people that the fact he/she is moving, slows down its clock. False, because if he/she is moving with respect to X, then X is moving with respect to he/she. So, which clock is "slowing down"? And this generates tons of paradox or people attacking the SR claiming it is false....
2. The act of moving doesn't slow down time, it's only that when the twin A and the twin B met again (after B has made a trip with his starship and back), their clocks signs different times. But this only because they have made different path in the spacetime, the twin A made a "longer" path and so his clock obviously signs a longer time interval.
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But saying "when an object is moving its time slows down" is however incorrect, ...
What you did was take that comment out of context. You ignored the part that followed which said but only when it is understood that ... and what followed that. You also made a mistake here in assuming that time dilation (slowing down of clocks) only has meaning when its referring to two twins. I.e. you're confusing the Twin Paradox with time dilation and slowing down of clocks.
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But saying "when an object is moving its time slows down" is however incorrect, ...
What you did was take that comment out of context.
I know, I've read your post carefully. But, you know, having experience with forum discussions of SR, I can state it's better not to suggest to laymen that "time slows down if you are moving", even if you explain what it means.
As well as the OP, *a lot* of people believes that "the act of moving slows down your clock". You also made a mistake here in assuming that time dilation (slowing down of clocks) only has meaning when its referring to two twins. I.e. you're confusing the Twin Paradox with time dilation and slowing down of clocks.
I'm not confusing it, I use Lorentz equations too. What I mean is that *physically* you cannot say which clock is actually signing more time, until you compare them to the same two events in spacetime. The "twin paradox" is just the simpler example of what happens.
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]I know, I've read your post carefully.
Then please don’t do it again, okay my friend? Taking a comment out of context means to not understand it since a large portion is missing. The correct statement is
“Consider two observers, one at rest in the inertial system S’ and one at rest in the inertial system S. Let S be moving with speed v relative to S’. Then time has slowed down in system S’ when compared to system S. This means that all clocks in S are running slower that clocks at rest in system S as observed by observers at rest in system S.”
That is what my statement means when its not taken out of context.
Lightarrow – Sometimes I get the feeling that you’re trying to correct me when you already know that what I posted was not incorrect. I hope you’re not doing that on purpose of if so as a joke.
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The only thing about light that would dilate time is its momentum. Just as light is influenced by gravity, light influences gravity. If you put a gram of matter inside of an otherwise evacuated Schrödinger catbox (impervious to anything, including information) it would have the same mass (and gravitational influence) as if the entire gram were converted into light, bouncing around inside the box.
Light, itself, does not experience time.
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The only thing about light that would dilate time is its momentum. Just as light is influenced by gravity, light influences gravity. If you put a gram of matter inside of an otherwise evacuated Schrödinger catbox (impervious to anything, including information) it would have the same mass (and gravitational influence) as if the entire gram were converted into light, bouncing around inside the box.
Light, itself, does not experience time.
I assume that you’re referring to light generating a gravitational field and that field generating time dilation. If the source is radiation then it can be momentum but it need not be. Its perfectly satisfactory to generate a gravitational field with a photon gas which has zero momentum density. All such a gas has to generate a gravitational field is energy and pressure. Those are the only components of the stress-energy-momentum tensor which don’t vanish for such a distribution of radiation..
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]I know, I've read your post carefully.
Then please don’t do it again, okay my friend?
Ok, didn't intend to annoy you.Lightarrow – Sometimes I get the feeling that you’re trying to correct me when you already know that what I posted was not incorrect. I hope you’re not doing that on purpose of if so as a joke.
No, the reason is that these concepts are always misinterpreted by laymen, so I believe it's important to clarify them, before legions of people grows believing relativity must be false.
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Light, itself, does not experience time.
That statement doesn't mean anything.
1. It's impossible to create a frame of reference comoving with light.
2. If you travel at speeds very near to c, you will still experience time, no matter how close to c is your speed.
And this could be one of the many examples of people that misunderstand relativity [:)]
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]I know, I've read your post carefully.
Then please don’t do it again, okay my friend?
Ok, didn't intend to annoy you.Lightarrow – Sometimes I get the feeling that you’re trying to correct me when you already know that what I posted was not incorrect. I hope you’re not doing that on purpose of if so as a joke.
No, the reason is that these concepts are always misinterpreted by laymen, so I believe it's important to clarify them, before legions of people grows believing relativity must be false.
Oh! Okay then. Good enough!
Please don't mind me. I get annoyed easily latel. It's this damn flu.
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2. If you travel at speeds very near to c, you will still experience time, no matter how close to c is your speed.
Your speed never can get 'close to c', because you will always see the photon moving away from you at exactly 'c' no matter what the speed of other non-zero rest mass objects relative to you is.
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Your speed never can get 'close to c', …
When light arrow said “If you travel at speeds very near to c,” he was referring to the speed as measured by an observer who wasn’t co-moving with the object whose speed we are speaking of. He’s speaking about the speed as measured by an observer who is at rest in the frame in which the moving body has speed v. As measured by such an observer any object can travel arbitrarily close to c without violating any law of physics.
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Light, itself, does not experience time.
That statement doesn't mean anything.
1. It's impossible to create a frame of reference comoving with light.
2. If you travel at speeds very near to c, you will still experience time, no matter how close to c is your speed.
And this could be one of the many examples of people that misunderstand relativity [:)]
Einstein had no problem using the perspective of light in his thought experiments. The statement is correct. Nothing that travels through space at c experiences time.
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Einstein had no problem using the perspective of light in his thought experiments.
I'd be very interested to see an example where he did that. Can you provide one?
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Light, itself, does not experience time.
That statement doesn't mean anything.
1. It's impossible to create a frame of reference comoving with light.
2. If you travel at speeds very near to c, you will still experience time, no matter how close to c is your speed.
And this could be one of the many examples of people that misunderstand relativity [:)]
Einstein had no problem using the perspective of light in his thought experiments. The statement is correct. Nothing that travels through space at c experiences time.
Maybe you have read some popular book or seen a movie on TV, but I'm talking of physics, instead.
Study physics, not popular books, then we can go on with the discussion...
Regards.
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lightarrow
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Light, itself, does not experience time.
That statement doesn't mean anything.
O course it does. That's a very common expression in special rleativity.
E.g. - http://www.askamathematician.com/2011/07/q-does-light-experience-time/
It's not as though light thinks but then again electrons don't think either and wonder "Gee. Time sure seems to be slowly down now that I' all set for my vacation this comming up friday." lol!
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Interesting reading, however this seems to go into a circular type of argument.
Let's say light does not experience time. Then one cannot define a clock from the photon point of view, and therefore neither a ruler. With no operational time and ruler, how can we define frame of reference for a photon in the first place? If we cannot define a frame of reference for a photon, then how can we actually tell anything about its proper length and time?
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[quote If we cannot define a frame of reference for a photon, then how can we actually tell anything about its proper length and time?[/quote]
Could one not equally well argue that if light cannot have a defined F of R, neither time nor space has any relevance for light?
If that were the case, light would exist in infinity!
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Light, itself, does not experience time.
That statement doesn't mean anything.
O course it does. That's a very common expression in special rleativity.
E.g. - http://www.askamathematician.com/2011/07/q-does-light-experience-time/
It's not as though light thinks but then again electrons
You can create a frame of reference comoving with an electron, you CAN'T do it with a photon. Re-read you books of physics.
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So to a photon created say at the big bang, or even 400,000 years later, no time has passed in its frame of reference, yet on earth, 13.7 billion years have passed?
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Lightarrow ... When people say "that question does not make sense", I sometimes feel sympathetic, but as quantum theory does not make sense anyway, that's hardly a reasoned reply.
Travelling close to the speed of light does slow "relative time" if I can call it that, so what's wrong with a thought experiment about travelling on a photon? Einstein did it.
By the way, thanks to you for all the patient replies to what will seem as senseless questions.
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You can create a frame of reference comoving with an electron, you CAN'T do it with a photon. Re-read you books of physics.
You shouldn't be condescending when you're as wrong as often as you are. Plus being condescending is rude and that violates forum rules.
In this case the comment that light does not experience time means precisely this, no more and no less: the proper time interval between two events on a null geodesic (i.e. the worldline that light travels on) is zero.
When something has a precise meaning it is said to be meaningful, by definition. Just because you don't understand things like this it doesn't mean that it’s wrong.
Let me give you an analogy from something you hold dear to you heart. The zero proper mass of a photon. The proper mass of a particle is that mass as measured in its rest frame or low velocity frame. The mathematical relationship between the particle’s inertial energy E and momentum p is (assuming c = 1) E2 - p2 = m2. Therefore when E and p are measured one obtains the proper mass of the particle. That’s why it’s often referred to as its rest mass. Since a photon can never be at rest or move at low speed its mass can never be measured. It’s only meaning is mathematical. Yet we define its mass according to the m in E2 - p2 = m2 being m = 0. But we don’t go around saying that it makes no sense to give zero mass to a photon. Or if we did you’d be a hypocrite. But you seem to want it both ways, don’t you?
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Maybe you could look at in form of change. A photon doesn't change as far as I know, not intrinsically at least. You have the expansion red shifting it, assuming that exist, but the redshift is a wave related phenomena, although it must concern a individual light quanta too. Photons are defined as existing at a 'source and a sink', but not in between. Why this is, is because there is no known way to measure a photon without destroying it. Everything you ever will see are carried by photons, they are the information carriers, all physical reactions are also carried by those photons, including all senses we have. And we know they have a source, at least indirectly, by the recoil matter shows when emitting a photon. We are also able to measure their impact 'hitting', and by that we find that photons 'travel'. The rest are experiments, theory and mathematics, defining how they travel, what properties we find them to have, etc. But if we go by experiments then a photon have a source and a sink, and they don't change in any way measurable between those two, excluding the definition of a expansion red shifting them.
So, change?
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From that notion you may be able to build a relationship between what change, with the help of those unchangeable photons, as matter. But it doesn't answer what the arrow is, because as soon as you have a change you must have a arrow allowing for it to change. Although it might be seen as a definition of spectrum of 'time', from something that seems timeless intrinsically (photons), to all other processes that indeed have a arrow to them. So maybe timelessness exist in our daily life, giving cause to change and motion? Weird :)
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Light, itself, does not experience time.
That statement doesn't mean anything.
1. It's impossible to create a frame of reference comoving with light.
2. If you travel at speeds very near to c, you will still experience time, no matter how close to c is your speed.
And this could be one of the many examples of people that misunderstand relativity [:)]
Einstein had no problem using the perspective of light in his thought experiments. The statement is correct. Nothing that travels through space at c experiences time.
Maybe you have read some popular book or seen a movie on TV, but I'm talking of physics, instead.
Study physics, not popular books, then we can go on with the discussion...
Regards.
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lightarrow
The book I'd recommend you check is "Relativity" by Albert Einstein. In it he describes riding a tram away from a clock tower. He imagined the tram going faster and faster. He realized that, from his perspective, the clock would seem to slow down. And, at the speed of light, the clock would seem to have stopped.
Are physics students not taught how to formulate thought experiments? It seems to be a lost art.
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Maybe you could look at in form of change. A photon doesn't change as far as I know, not intrinsically at least.
The mentalpicture that I have is that the photon is oscillating as it moves through space. The polarization of light oscilates as it moves through spae. For a good description of QED using this notion see QED by Feynman.
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So to a photon created say at the big bang, or even 400,000 years later, no time has passed in its frame of reference, yet on earth, 13.7 billion years have passed?
When you say "in its frame of reference", are you referring to the photon? If the answer is yes, there isn't ("a photon's frame of reference").
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Lightarrow ... When people say "that question does not make sense", I sometimes feel sympathetic, but as quantum theory does not make sense anyway, that's hardly a reasoned reply.
Travelling close to the speed of light does slow "relative time" if I can call it that, so what's wrong with a thought experiment about travelling on a photon? Einstein did it.
This is what is written in popular books; yes, Einstein "tried", maybe, to imagine what would happen to "ride a light beam", but what he actually found is what would happen travelling in a spaceship at a very high speed. In case you are interested *in this*, the answer is that you *would* experience time as normally as you are experiencing it now. The fact that when you will come back to Earth you find all the people there with more age than you (and their clocks too, of course) is because they have made *a longer* trip through spacetime (not very simple to explain here what it means). By the way, you can read "Spacetime Physics" - Taylor - Wheeler; it's a Very simple book but, at the same time, describes the physics correctly.
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The book I'd recommend you check is "Relativity" by Albert Einstein. In it he describes riding a tram away from a clock tower. He imagined the tram going faster and faster. He realized that, from his perspective, the clock would seem to slow down. And, at the speed of light, the clock would seem to have stopped.
Are physics students not taught how to formulate thought experiments? It seems to be a lost art.
Probably you haven't understood, so I'll be more explicit. When I read "Nothing that travels through space at c experiences time" and I reply that it's meaningless because it's not possible to create a frame of reference which travels at c, I refer to travelling *exactly* at c. Is it more clear now?
Furthermore, even if you intend "travelling at a speed very near to c" (and in this case you *can* find a frame of reference co-moving with you) saying that "you don't experience time" is incorrect and, at least, misleading (see previous posts of mine).
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Can something oscillate without being a wave Pete, or at least have wave properties?
Maybe it can?
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That one is tricky, considering the duality.
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Not being a wave I mean, although presenting us with a duality. Thinking of it as a excitation in a field now, and the field I imagine as a set of points creating a space, in 3D for us, going on and off (4D as the arrow is there), slightly whacky but still :) It fits my thinking.
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Can something oscillate without being a wave Pete, or at least have wave properties?
Maybe it can?
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That one is tricky, considering the duality.
I've never heard of it. But that doesn't mean that it can't happen. In the case of photons its the direction of the photon's polarization that is oscilating.
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Can something oscillate without being a wave Pete, or at least have wave properties?
Maybe it can?
Sure, a pendulum oscillates and is clearly not a wave. (A plot of it's position vs. time does look like a wave, however.)
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Can something oscillate without being a wave Pete, or at least have wave properties?
Maybe it can?
Sure, a pendulum oscillates and is clearly not a wave. (A plot of it's position vs. time does look like a wave, however.)
Oops. Of course. There are countless examples of osciallators such as crystals, clocks, etc.