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Old Mar 25th 2013, 09:05 AM   #1
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A thought experiment with speed of light

I have thought about a problem for a long time but can't get a valid answer.
Let us take a hypothetical particle which can move at the speed of light and we assume the speed of light to be 5 meter/sec for convenience. Now say the particle is 5 meter away from a detector and it is moving towards it at the speed of light (5 m/s in this case). At every moment the particle is emitting light. When the particle nearly hits the detector it will see the particle(because of the light emitted by it as the distance at that time is too small) and at the same time the light which was emitted before 5 sec also reach the detector. The question is what will the detector see, the particle hitting it or the particle that was 5 m away or a trail (because light is emitted at any moment throughout the journey)?
I myself created this situation and I get confused every time I think about it. Is there anything wrong in the problem? Or can we conclude from here that nothing except photon can travel at the speed of light?
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Old Mar 26th 2013, 08:11 AM   #2
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Originally Posted by tonzent View Post
I have thought about a problem for a long time but can't get a valid answer.
Let us take a hypothetical particle which can move at the speed of light and we assume the speed of light to be 5 meter/sec for convenience. Now say the particle is 5 meter away from a detector and it is moving towards it at the speed of light (5 m/s in this case). At every moment the particle is emitting light. When the particle nearly hits the detector it will see the particle(because of the light emitted by it as the distance at that time is too small) and at the same time the light which was emitted before 5 sec also reach the detector. The question is what will the detector see, the particle hitting it or the particle that was 5 m away or a trail (because light is emitted at any moment throughout the journey)?
I myself created this situation and I get confused every time I think about it. Is there anything wrong in the problem? Or can we conclude from here that nothing except photon can travel at the speed of light?
I don't see a paradox here .. if something is approaching at the speed of light, you can't see it till it hits you. [Rather like an airplane traveling faster than the speed of sound - you can't hear it till after it has passed.] Have you read up on the Doppler effect? That is related to a body in motion emitting photons.
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Old Mar 26th 2013, 08:32 AM   #3
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Thank you for the reply. I haven't read the doppler effect of light. I'll surely look it up and think about the problem.
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Old Aug 17th 2013, 07:24 PM   #4
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Wink Time Dilation Length Compression

In the General Theory of Relativity your theoretical object would need to obey the speed limit of light no matter what the situation. In order to accomplish this to properties of the object have to be modified which are its time and its length in the direction it is traveling. Your theoretical object is composed of atoms. The atoms that compose the theoretical object have electrons which move around them. The atoms also as a whole vibrate because of temperature. Some of the electrons in the atoms might be going in the direction of travel. Also sometimes the atoms will vibrate towards the direction of travel. Also the object is emitting light in the direction of travel. In order to keep the photons which are omitted towards the direction of travel from breaking the speed of light time dilation occurs to lengthen the time it takes for your object to move. If your object is moving at exactly the speed of light time dilation would be infinite which would reduce the speed of your object to zero and then the speed of light of photons emitted from your object would still be unbroken. Also length compression occurs in order to keep the parts of the object which are vibrating or rotating in the direction of travel so that the distance that they travel would reduce their speed in that direction. Your object would be therefore infinitesimally thin to reduce the speed of any vibration or rotation in the direction of travel to zero. Therefore your object would have an infinite density and any object with an infinite density is considered a black hole and no light can escape a black hole so the detector would not see anything.


On second thought, if your object was rotating and the axis of rotation was along the direction of travel and the object (a black hole) was able to absorb vast quantities of matter during its travel to the detector there might be a chance that it could create a gamma ray emission out its axis and if the detector could see gamma rays then it would see a gamma ray burst at the very moment it passes the event horizon. If the object were a small black hole spaghettification would occur and the detector may not have time to process the information it received, however if the object were a supermassive black hole spaghettification would not take place and it might be possible that the detector could process its information in time to see the gamma ray burst.

Last edited by arievlex; Aug 17th 2013 at 07:36 PM. Reason: Gamma ray emissions
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Old Apr 2nd 2014, 09:16 PM   #5
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@tonzent, thank you for contributing your thought provoking question.

I will try to answer your question simply and others more educated than me maybe able to correct my inaccuracies, yet I will try.

My understanding of your question is that something is emitting particles that are traveling towards you at the speed of light. I assume these particles are evenly spaced. I also assume you are holding a detector to detect the arrival of particles. Each particle traveling towards you also emits one or more photons, at equal times, and these also travel towards your detector.

Here are some things to ponder:

1. Particles traveling at the speed of light are frozen in time from the point of view of a bystander.

2. From (1) above this means that light speed particles have "no moving parts". If we think about this how can something thats frozen in time have moving parts? How can it do something that will emit a photon as it would take time to do this.

3. The best we can do is say its nearly the speed of light if we wish it to have any time to emit another particle towards the detector.

4. The newly emitted particle won't have time to get much of a headstart because its "handicapped" because of a very sluggish clock. Hence your detector would detect two (or more) particles in quick succession followed by the original particle that emitted them. Then it would wait awhile for the next stream of photons, in quick succession, followed quickly by the particle that emitted them.

This is my clumsy "low brow" explanation. Hopefully it makes some sense.

Originally Posted by tonzent View Post
I have thought about a problem for a long time but can't get a valid answer.
Let us take a hypothetical particle which can move at the speed of light and we assume the speed of light to be 5 meter/sec for convenience. Now say the particle is 5 meter away from a detector and it is moving towards it at the speed of light (5 m/s in this case). At every moment the particle is emitting light. When the particle nearly hits the detector it will see the particle(because of the light emitted by it as the distance at that time is too small) and at the same time the light which was emitted before 5 sec also reach the detector. The question is what will the detector see, the particle hitting it or the particle that was 5 m away or a trail (because light is emitted at any moment throughout the journey)?
I myself created this situation and I get confused every time I think about it. Is there anything wrong in the problem? Or can we conclude from here that nothing except photon can travel at the speed of light?
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Old Sep 30th 2014, 12:01 PM   #6
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The detector will see the light emitted first, because C is the speed of light FROM THE SOURCE, hence, if the particle is travelling at C... the light from the particle will be at C-square velocity. I KNOW this goes against accepted theory, but "accepted theory" is based on the FLAWED Michelson-Morley Experiment and latter experiments which only mirrored the flaw. C is the "source velocity" of light (think muzzle velocity of a bullet) or "Base Minimum Velocity" (Bmv); it is NOT the "Maximum Attainable Velocity" (Mav) of light (or anything else).

Another thing to consider: Special Relativity is the Physics of ILLUSION; that is, Einstein's theory describes things as the Observer SEES them, whether that which the Observer sees is a TRUE reflection of REALITY or not. In the olden, pre-Relativity, days ... the goal of "Hard Science" was to describe the Universe as it exists in Reality WHETHER AN OBSERVER IS PRESENT OR NOT TO SEE IT. Relativity places the Observer square in the middle of the Experiment then describes how the Observer views what occurs, and calls THAT reality! NOT necessarily so.
As someone who is studying Physics with a focus on Temporal Physics, I feel confident in stating TIME per se, NEVER stands still. Clocks may stop or slow down, but the movements and mechanical workings of clocks/particle oscillations do NOT alter the flow of TIME per se. You would do well to remember this ... sooner or later the Physics/Scientific Community-at-large is going to accept it as a given.

Last edited by logandiez; Sep 30th 2014 at 12:13 PM.
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Old Sep 30th 2014, 12:52 PM   #7
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Originally Posted by logandiez View Post
The detector will see the light emitted first, because C is the speed of light FROM THE SOURCE, hence, if the particle is travelling at C... the light from the particle will be at C-square velocity. I KNOW this goes against accepted theory, but "accepted theory" is based on the FLAWED Michelson-Morley Experiment and latter experiments which only mirrored the flaw. C is the "source velocity" of light (think muzzle velocity of a bullet) or "Base Minimum Velocity" (Bmv); it is NOT the "Maximum Attainable Velocity" (Mav) of light (or anything else).
Are you suggesting the current physics practice of using the Lorentz Transformation (which takes into account time dilation) is wrong and we should really be using the Galilean Transformation (which makes no such assumption)? In your view why was the Michelson-Morley experiment flawed and what should they have done differently in determining the value for C (speed of light)?

Also why would Muzzle velocity be less than the MAV velocity? In other words how can a bullet speed up when it has left the gun?

Last edited by kiwiheretic; Sep 30th 2014 at 12:55 PM. Reason: Address Muzzle velocity
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Old Oct 7th 2014, 11:54 AM   #8
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"Muzzle Velocity," for those who know firearms, it the velocity of the projectile as it leaves the muzzle of the firearm. As air resistance causes friction, muzzle velocity decreases; and, when you fire a projectile with an "accellerator" its velocity continues to increase after leaving the barrel until the accellerant is exhausted or until it impacts the target. Also, if you mount a firearm with X mv on a moving platform with Y velocity, then the projectile's mv=X, but the projectile's velocity through space is X+Y.
With light, the source is synonymous with the firearm, and the photons/lightwave synonymous with the projectile. Put your source on a moving platform that you yourself are moving on and you will only measure the velocity from the source; remove yourself from the platform and measure the light's velocity from a stationary position and its velocity will be the source velocity PLUS the platform's velocity. We have yet to conduct an experiment in which the detectors/researchers were not moving along at the same velocity as the source of the light being measured [at least not as far as i have been able to find in months of searching on the internet].
TIME, per se, DOES NOT DIALATE. Einstein, et. al., were and are flat incorrect on this point. CLOCKS can be slowed or sped up, but NOT Time itself. This incorrect conclusion come from when Researchers place themselves INSIDE their own experiments. When i was in school/college, the goal of scientific research was to describe the reality of the Cosmos as it exists WHETHER AN OBSERVER IS PRESENT OR NOT. That way you avoid the problem of optical and temporal ILLUSION that multiple, or even one, observer can experience. Einsteins Special and General Theory of Relativity places observers square inside the experiments then describes the illusional as reality (i HAVE read his published Theories in the original translations). We have/NEED to get away from that and get back to describing things AS THEY ARE rather than AS WE SEE THEM. Like that warning sign on some mirrors says: "objects may appear closer than they are."
The flaw in the M-M Experiment: (a) they used a single light source for two subordinate pulses; (b) the source and experiment were both at Earth velocity; (c) the subordinate pulse that was supposed to cross the Earth's solar orbital trajectory at a 90deg angle was actually only at an extremely shallow angular tangent; (d) they expected the tangential pulse to arrive later than the direct pulse, because they didn't expect Earth's velocity to be added to the former but did expect it to be added to the latter; and, (e) the Michelson and Morley were themselves within the framework of the experiment travelling at Earth velocity as were their detectors. Hence, the experiment was NOT a valid experiment to rely on as far as lightspeed being or not being a Constant OTHER THAN as relates to its SOURCE VELOCITY.

Last edited by logandiez; Oct 7th 2014 at 12:02 PM. Reason: to add a comment
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Old Oct 7th 2014, 02:34 PM   #9
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Originally Posted by logandiez View Post
"Muzzle Velocity," for those who know firearms, it the velocity of the projectile as it leaves the muzzle of the firearm. As air resistance causes friction, muzzle velocity decreases; and, when you fire a projectile with an "accellerator" its velocity continues to increase after leaving the barrel until the accellerant is exhausted or until it impacts the target. Also, if you mount a firearm with X mv on a moving platform with Y velocity, then the projectile's mv=X, but the projectile's velocity through space is X+Y.
You are mixing your terms and I think it's confusing you. Say we have a platform (frame X) moving at speed V in another frame Y. The bullet leaves the firearm at speed Vx from frame X and we measure it to have speed Vy = V + Vx in frame Y (in terms of Galilean relativity.) Likewise the momentum of the bullet measured in frame X will be mVx and measured in frame Y it will be mVy = m(V + Vx).

Originally Posted by logandiez View Post
With light, the source is synonymous with the firearm, and the photons/lightwave synonymous with the projectile. Put your source on a moving platform that you yourself are moving on and you will only measure the velocity from the source; remove yourself from the platform and measure the light's velocity from a stationary position and its velocity will be the source velocity PLUS the platform's velocity. We have yet to conduct an experiment in which the detectors/researchers were not moving along at the same velocity as the source of the light being measured [at least not as far as i have been able to find in months of searching on the internet].
TIME, per se, DOES NOT DIALATE. Einstein, et. al., were and are flat incorrect on this point. CLOCKS can be slowed or sped up, but NOT Time itself. This incorrect conclusion come from when Researchers place themselves INSIDE their own experiments. When i was in school/college, the goal of scientific research was to describe the reality of the Cosmos as it exists WHETHER AN OBSERVER IS PRESENT OR NOT. That way you avoid the problem of optical and temporal ILLUSION that multiple, or even one, observer can experience. Einsteins Special and General Theory of Relativity places observers square inside the experiments then describes the illusional as reality (i HAVE read his published Theories in the original translations). We have/NEED to get away from that and get back to describing things AS THEY ARE rather than AS WE SEE THEM. Like that warning sign on some mirrors says: "objects may appear closer than they are."
The flaw in the M-M Experiment: (a) they used a single light source for two subordinate pulses; (b) the source and experiment were both at Earth velocity; (c) the subordinate pulse that was supposed to cross the Earth's solar orbital trajectory at a 90deg angle was actually only at an extremely shallow angular tangent; (d) they expected the tangential pulse to arrive later than the direct pulse, because they didn't expect Earth's velocity to be added to the former but did expect it to be added to the latter; and, (e) the Michelson and Morley were themselves within the framework of the experiment travelling at Earth velocity as were their detectors. Hence, the experiment was NOT a valid experiment to rely on as far as lightspeed being or not being a Constant OTHER THAN as relates to its SOURCE VELOCITY.
This has all been said in another thread. I will simply say that, so far as I know the experiments have been done correctly, whether you like it or not the Lorentz transformations have been absolutely proven in Electrodynamics, so light will still obey it even if you still insist on it not being true outside of Electrodynamics. Until you do your experiment and essentially prove that the Maxwell equations are incorrect (very unlikely) then I am going to continue to say that you don't have a leg to stand on. And please read the MM experiment again and the comments we have made about it. You are still misunderstanding the entire point of it.

As far as SR and GR (and QM to boot) are concerned we are dealing with fields that exist across the entire Universe. Just how are we supposed to sit outside the problem and take measurements?

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Old Nov 16th 2014, 05:42 PM   #10
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Originally Posted by topsquark View Post
As far as SR and GR (and QM to boot) are concerned we are dealing with fields that exist across the entire Universe. Just how are we supposed to sit outside the problem and take measurements?
-Dan
Yes, if only it were that easy. It makes me curious that supposedly there is no way to judge motion through space because c is constant for all observers, but if one were to travel as fast as technically possible in a specific direction, and then travel as fast as possible in the opposite direction, wouldn't there be some way to determine what the center of velocity, and therefore some sense of a true "rest" frame? Perhaps out in intergalactic space someday!
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