Past Reflection

[petrushkagoogol]

Consider a person looking at his own reflection in a plane mirror at a distance of 1.5 meters from

himself. Light has to travel a total of 3 meters with a speed of 3x10⁸ m/s which means it takes 

100 microseconds to do so.

So even looking in the mirror means you are looking at yourself as you existed 0.01 microseconds ago. (who says the past and present cannot co-exist ?)

Do you agree ? :xmas_tree:

Edited December 26, 2016 by petrushkagoogol

[exchemist]

 

Consider a person looking at his own reflection in a plane mirror at a distance of 1.5 meters from

himself. Light has to travel a total of 3 meters with a speed of 3x10⁸ m/s which means it takes 

100 microseconds to do so.

So even looking in the mirror means you are looking at yourself as you existed 0.01 microseconds ago. (who says the past and present cannot co-exist ?)

Do you agree ? :xmas_tree:

 

Furthermore the electrochemistry of the synapses in your optic nerve and brain take a finite time to react to the impulse of photons striking the retina, before the person is able to form the image of himself in said mirror. So, if you want to be pedantic, nobody ever perceives "the present". 

 

Personally I see no point in such pedantry, however. It seems to yield no insights of value.

Edited December 26, 2016 by exchemist

[petrushkagoogol]

Furthermore the electrochemistry of the synapses in your optic nerve and brain take a finite time to react to the impulse of photons striking the retina, before the person is able to form the image of himself in said mirror. So, if you want to be pedantic, nobody ever perceives "the present". 

 

Personally I see no point in such pedantry, however. It seems to yield no insights of value.

 

Consider an extension to this thought experiment. If there is a photo-detector that beeps after it receives reflected light from the mirror at the same distance from the mirror as the man is, then consider the scenario that the man dies after 0.025 microseconds. If an external observer is watching the experiment, who can only hear audible signals and not see inside the test environment, he will hear the beep after 0.01 microseconds, in-spite of the fact that the person is actually dead. So it is not so bizarre as you might be inclined to believe.

[exchemist]

Consider an extension to this thought experiment. If there is a photo-detector that beeps after it receives reflected light from the mirror at the same distance from the mirror as the man is, then consider the scenario that the man dies after 0.025 microseconds. If an external observer is watching the experiment, who can only hear audible signals and not see inside the test environment, he will hear the beep after 0.01 microseconds, in-spite of the fact that the person is actually dead. So it is not so bizarre as you might be inclined to believe.

This seems to make no sense.

 

The sound waves travel (quite slowly) to the ear of this second, external, observer and then his ear, auditory nerve and brain have to process the signal before he is aware of it. So he will not hear it after 0.01 microsec.

 

And what has the death of the first man got to do with anything?  

[Maine farmer]

Furthermore the electrochemistry of the synapses in your optic nerve and brain take a finite time to react to the impulse of photons striking the retina, before the person is able to form the image of himself in said mirror. So, if you want to be pedantic, nobody ever perceives "the present". 

 

Personally I see no point in such pedantry, however. It seems to yield no insights of value.

The insight that we can only perceive the past is of value if you are trying to hit a moving target, in which case you have to aim for where the target will be and not where it was.

[xyz]

 

Consider a person looking at his own reflection in a plane mirror at a distance of 1.5 meters from

himself. Light has to travel a total of 3 meters with a speed of 3x10⁸ m/s which means it takes 

100 microseconds to do so.

So even looking in the mirror means you are looking at yourself as you existed 0.01 microseconds ago. (who says the past and present cannot co-exist ?)

Do you agree ? :xmas_tree:

 

I disagree for several reasons. 

 

 

1) There is a ''clear'' line of sight

 

2)time is adjoined from one increment to the next

 

3) darkness can be measured in its exact geometrical position relative to us. 

 

4) I see objects in no time at all because there is a clear line of sight, I can clearly see the start and end of journey positions simultaneously. 

 

 

5)Photons travel through time.

 

6) emr is adjoined from source point to our brains, emr is adjoined travelling through space, there is no ''gaps''.

Edited December 27, 2016 by xyz

[exchemist]

I disagree for several reasons. 

 

 

1) There is a ''clear'' line of sight

 

2)time is adjoined from one increment to the next

 

3) darkness can be measured in its exact geometrical position relative to us. 

 

4) I see objects in no time at all because there is a clear line of sight, I can clearly see the start and end of journey positions simultaneously. 

 

 

5)Photons travel through time.

 

6) emr is adjoined from source point to our brains, emr is adjoined travelling through space, there is no ''gaps''.

Good luck with this, Petrushka!  :)

Edited December 27, 2016 by exchemist

[fahrquad]

 

Consider a person looking at his own reflection in a plane mirror at a distance of 1.5 meters from

himself. Light has to travel a total of 3 meters with a speed of 3x10⁸ m/s which means it takes 

100 microseconds to do so.

So even looking in the mirror means you are looking at yourself as you existed 0.01 microseconds ago. (who says the past and present cannot co-exist ?)

Do you agree ? :xmas_tree:

 

The time it takes for light to travel is far too small to perceive, much less measure.  Why do you post such silly questions???  Look over your internet history.  Perhaps you need a chat forum instead of a science forum.

[fahrquad]

 

Consider a person looking at his own reflection in a plane mirror at a distance of 1.5 meters from

himself. Light has to travel a total of 3 meters with a speed of 3x10⁸ m/s which means it takes 

100 microseconds to do so.

So even looking in the mirror means you are looking at yourself as you existed 0.01 microseconds ago. (who says the past and present cannot co-exist ?)

Do you agree ? :xmas_tree:

 

 

The time it takes for light to travel is far too small to perceive, much less measure.  Why do you post such silly questions???  Look over your internet history.  Perhaps you need a chat forum instead of a science forum.

I should add that the measured speed light in a vacuum is 186,282 miles per second.  It travels a little slower at sea level due to the density of air.

Edited December 27, 2016 by fahrquad

[fahrquad]

Consider an extension to this thought experiment. If there is a photo-detector that beeps after it receives reflected light from the mirror at the same distance from the mirror as the man is, then consider the scenario that the man dies after 0.025 microseconds. If an external observer is watching the experiment, who can only hear audible signals and not see inside the test environment, he will hear the beep after 0.01 microseconds, in-spite of the fact that the person is actually dead. So it is not so bizarre as you might be inclined to believe.

When I died I was still conscious for about 5 minutes before I blacked out from oxygen deprivation.  I woke up in the emergency room shortly thereafter, at least I think it was shortly thereafter.  I could not raise my arm to look at my watch, but my eyelids were open in the direction of the wall clock in the emergency room.  Good times, good times.  <_<   No bright lights, no angels, no devils, and no deceased friends and relatives.  Just cold and black.

Edited December 27, 2016 by fahrquad

[petrushkagoogol]

This seems to make no sense.

 

The sound waves travel (quite slowly) to the ear of this second, external, observer and then his ear, auditory nerve and brain have to process the signal before he is aware of it. So he will not hear it after 0.01 microsec.

 

And what has the death of the first man got to do with anything?  

 

The light signal from the dead man is later by 0.025 microsec. When the light signal from the man in the alive state reaches the detector, the man is already dead. But the detector indicates otherwise. This is the paradox.

[CraigD]

The light signal from the dead man is later by 0.025 microsec. When the light signal from the man in the alive state reaches the detector, the man is already dead. But the detector indicates otherwise. This is the paradox.

Using the linked to definition “A paradox is a statement that, despite apparently sound reasoning from true premises, leads to a self-contradictory or a logically unacceptable conclusion,” I see no paradox, because no sound reasoning about this scenario leads to any contradictory conclusion.

 

It’s a cornerstone of modern physics that information takes time to get between places. The scenarios describe in this thread are no more paradoxical than one where you read a letter saying “I, your friend Alan, am alive” when Alan died shortly after sending the mail. Such things happen all the time.

[petrushkagoogol]

Using the linked to definition “A paradox is a statement that, despite apparently sound reasoning from true premises, leads to a self-contradictory or a logically unacceptable conclusion,” I see no paradox, because no sound reasoning about this scenario leads to any contradictory conclusion.

 

It’s a cornerstone of modern physics that information takes time to get between places. The scenarios describe in this thread are no more paradoxical than one where you read a letter saying “I, your friend Alan, am alive” when Alan died shortly after sending the mail. Such things happen all the time.

 

Perhaps instead of paradox I should rephrase the thought experiment as an assertion "Photons cannot provide up to the point information". 

So there is a limit that detectors based on incident photons can do.

So invariably this induces limits on human measurement based on photons .

[CraigD]

Perhaps instead of paradox I should rephrase the thought experiment as an assertion "Photons cannot provide up to the point information".

So there is a limit that detectors based on incident photons can do.

A better, more general statement would be “nothing can provide instantaneous information,” or a more exact “any information about a point distance D away must be at least D/c old.” (were c is the speed of light). This is the limit of detectors bases on anything, not just photons.

 

So invariably this induces limits on human measurement based on photons .

You say this like it’s a bad thing!

 

Were the speed of light much larger, or the universe much smaller or younger, we wouldn’t know as much about stars, galaxies, and the universe as a whole. Because the more distant a body, the older (so younger) its image, we’ve been able to observe each of the types of stars and galaxies at nearly every stage of their evolution, allowing us to refine our theoretical models of them to a high level of maturity.

[petrushkagoogol]

A better, more general statement would be “nothing can provide instantaneous information,” or a more exact “any information about a point distance D away must be at least D/c old.” (were c is the speed of light). This is the limit of detectors bases on anything, not just photons.

 

You say this like it’s a bad thing!

 

Were the speed of light much larger, or the universe much smaller or younger, we wouldn’t know as much about stars, galaxies, and the universe as a whole. Because the more distant a body, the older (so younger) its image, we’ve been able to observe each of the types of stars and galaxies at nearly every stage of their evolution, allowing us to refine our theoretical models of them to a high level of maturity.

 

Yes,of course,I agree.

 

However there is also a counter-argument.

 

Firstly, our visual cortex can only resolve those frequencies in the electromagnetic spectrum which deal with visible light.

We cannot, for instance, perceive the CMBR since we do not have receptors for microwave frequencies. So receptors for the

electromagnetic spectrum exist primarily in the visible range (though of course we can for instance perceive infra-red

frequencies through their heating effect).

 

Secondly, if we consider the subject of the experiment to be a heart patient, with the experimental set-up connected to a

computer, that relays a SMS to a physician, we should sample the output a few times (say 3), before we arrive at the patient

status. As you rightly pointed out, we cannot rely on instantaneous information. (application - telemedicine) 

[CraigD]

Firstly, our visual cortex can only resolve those frequencies in the electromagnetic spectrum which deal with visible light.

Its cells in the retina, in the eye, not the visual cortex, in the brain, that respond to EM radiation, but you’re right, the human eye can only sense EM radiation in the visual range, frequencies of about 4 x 10¹⁴ to 8 x 10¹⁴ hz (cycles/second). This is a surprisingly narrow range, less than an octave (1 doubling of frequency). Our ears can sense about 10 octaves, about 20 through 20000 hz, though, or course, sound is very different than light.

 

We cannot, for instance, perceive the CMBR since we do not have receptors for microwave frequencies. So receptors for the electromagnetic spectrum exist primarily in the visible range (though of course we can for instance perceive infra-red frequencies through their heating effect).

Though means other than our eyes, we can perceive EM radiation of many ranges, provided its intense enough. Stick you hand in a running microwave oven, for instance, and within a second or so, you’ll feel its on the order of 10⁹ hz EM radiation, as a very unpleasant burning sensation. The US military flirted with using about 10¹¹ hz beams to chase off unruly folks, hopefully without killing any, using a vehicle-mounted system called the ADS, but so far have used it only on volunteer test subjects.

 

On the other side of the visible spectrum, it’s possible that people have perceived very high-frequency (10¹⁷+ hz) EM radiation as flashes, due to ionizing collisions with retinae, nerves, or the brain. The phenomena is know as cosmic ray visual phenomena, and most scientists believe it’s caused not by photons, but by more massive neutrons or protons, but experiments with artificially generated x-rays (10¹⁶ to 10¹⁹ hz) suggest that weird sensing of photons might, rarely, occur.

 

Secondly, if we consider the subject of the experiment to be a heart patient, with the experimental set-up connected to acomputer, that relays a SMS to a physician, we should sample the output a few times (say 3), before we arrive at the patient status. As you rightly pointed out, we cannot rely on instantaneous information. (application - telemedicine)

As Exchemist notes many posts ago, our senses are so slow compared to the travel time of light involved in most everyday situations that such travel times can be considered practically zero. For a doctor using a telemedicine device, he seems as close to the patient when the patient is continents away as only a few meters. Connected with a lightspeed signal, the one-way signal time between any points on earth is never more than 0.14 sec.

 

Lightspeed travel time doesn’t become noticeable until the distances between sender and receiver is astronomical distances. For example, we can’t remote control robots the way we can on Earth, because the round-trip times for a light signal are about 182 seconds at the closest to over 45.6 minutes at the most distant. That’s why Mars robots must be controlled carefully, planning each move before executing it and seeing the results minutes later, with lots of automatic/autonomous systems set up to intervene if something unexpected happens. Telemedicine surgery between Earth and Mars seems an impossibility – it would have to involve a pretty much autonomous robot, with the remote operator/surgeon essentially a spectator.

 

No paradoxes or unexpected outcomes here.

 

I think the finite speed of light has a deep implication for physics, because were the speed of light infinite, as some early natural philosophers assumed, it would in principle be possible for a small volume of space to receive a near infinite amount of information in a small amount of time. I have a suspicion that, whatever space-time is ultimately made of, it’s something that can only handle finite amounts of information in finite volumes. Far from being a bad thing and a troubling limitation, the finite speed of light may be necessary to avoid “breaking” space-time.

[petrushkagoogol]

Its cells in the retina, in the eye, not the visual cortex, in the brain, that respond to EM radiation, but you’re right, the human eye can only sense EM radiation in the visual range, frequencies of about 4 x 10¹⁴ to 8 x 10¹⁴ hz (cycles/second). This is a surprisingly narrow range, less than an octave (1 doubling of frequency). Our ears can sense about 10 octaves, about 20 through 20000 hz, though, or course, sound is very different than light.

 

Though means other than our eyes, we can perceive EM radiation of many ranges, provided its intense enough. Stick you hand in a running microwave oven, for instance, and within a second or so, you’ll feel its on the order of 10⁹ hz EM radiation, as a very unpleasant burning sensation. The US military flirted with using about 10¹¹ hz beams to chase off unruly folks, hopefully without killing any, using a vehicle-mounted system called the ADS, but so far have used it only on volunteer test subjects.

 

On the other side of the visible spectrum, it’s possible that people have perceived very high-frequency (10¹⁷+ hz) EM radiation as flashes, due to ionizing collisions with retinae, nerves, or the brain. The phenomena is know as cosmic ray visual phenomena, and most scientists believe it’s caused not by photons, but by more massive neutrons or protons, but experiments with artificially generated x-rays (10¹⁶ to 10¹⁹ hz) suggest that weird sensing of photons might, rarely, occur.

 

As Exchemist notes many posts ago, our senses are so slow compared to the travel time of light involved in most everyday situations that such travel times can be considered practically zero. For a doctor using a telemedicine device, he seems as close to the patient when the patient is continents away as only a few meters. Connected with a lightspeed signal, the one-way signal time between any points on earth is never more than 0.14 sec.

 

Lightspeed travel time doesn’t become noticeable until the distances between sender and receiver is astronomical distances. For example, we can’t remote control robots the way we can on Earth, because the round-trip times for a light signal are about 182 seconds at the closest to over 45.6 minutes at the most distant. That’s why Mars robots must be controlled carefully, planning each move before executing it and seeing the results minutes later, with lots of automatic/autonomous systems set up to intervene if something unexpected happens. Telemedicine surgery between Earth and Mars seems an impossibility – it would have to involve a pretty much autonomous robot, with the remote operator/surgeon essentially a spectator.

 

No paradoxes or unexpected outcomes here.

 

I think the finite speed of light has a deep implication for physics, because were the speed of light infinite, as some early natural philosophers assumed, it would in principle be possible for a small volume of space to receive a near infinite amount of information in a small amount of time. I have a suspicion that, whatever space-time is ultimately made of, it’s something that can only handle finite amounts of information in finite volumes. Far from being a bad thing and a troubling limitation, the finite speed of light may be necessary to avoid “breaking” space-time.

 

This leads to an interesting conclusion.

 

1. Photons have time signatures.
2. Photons cannot be decoupled from time.
3. In view of this "Photons ARE time".

 

This leads to some interesting conclusions :

1. Black holes and other singularities, from which light cannot escape, can be abstracted from our notion of space-time.
2. We have a limit of the observable Universe, and, what is beyond that we really cannot tell.