B How Does Einstein's Theory Affect the Age of Light?

[Francis Ward]

If, as Einstein tells us, time dilates as we travel, and that dilation increases massively at speeds approaching the speed of light, then light, which travels at c would, to the static observer, never age. To us, light would take infinitely long to reach us.

 

[mfb]

There is no reference frame where light is at rest. "Time that passes for light" is not a meaningful concept.

To us, light would take infinitely long to reach us.

No. The time dilation you discussed is for the traveling object, not for us. Send a spacecraft at 99.99% the speed of light to Alpha Centauri (4 light years away), and it will be there in about 4 years, while the clocks on board show a much smaller time difference.

 

[Simon Bridge]

Welcome to PF,
You are mixing up reference frames. Try again, but this time be more careful.
Note: light cannot be an observer: it is what we use to observe things. See mfb: it is not meaningful to talk about time that passes for light.

 

[Francis Ward]

Thanks for the replies folks. As you see, I am new to this relativity thing!
According to the 'Thought experiment' which Einstein used to demonstrate the dilation of time, the time dilation is relative to the static observer. The 'person' traveling on the train ages slower when compared to the static observer. However to the person on the train, the 'light clock' ticks at exactly the same rate. Note also the muon decay experiment which 'proves' the dilation of time. As far as the muon is concerned, it lived it's prescribed length of time. As far as we, the static observer is concerned it lived 29 times longer because it was traveling at close to the speed of light.
I am curious about the 'it is not meaningful to talk about time that passes for light' - why not? We talk about light years, and that it takes 4 years for light to travel from here to Alpha Centuri. If light is not subject to time, then it is meaningless to talk about how fast it can travel, or how long it takes for it to get somewhere.
As a final (for now) and related question, in the Thought Experiment, where on the mirror on the opposite side of the traveling light clock would we expect the light to impinge? Does it impinge at the point directly opposite the point it left the other mirror (in other words travel both across the train and with the train) or does it impinge marginally back from that point (ie not affected at all by the movement of the train)? This is relative to the person on the train.

 

[Simon Bridge]

The 4 some years for a light signal to get from here to alpha centauri is time experienced by us, not the light.
The time dilation effect is a comparison between two clocks. The muon experiment is a case in point, the experimenters are comparing the muon clock (its decay time) with a clock on the ground. What is the clock for light?

Also note: all observers are stationary in their own reference frames. All observers measure the same speed for light, but not for muons.
There is a precise language that goes with relativity that is designed to avoid the kind of confusions you have.

 

[Francis Ward]

Again, thank you.
Thoughts on the train question?

 

[PeroK]

Again, thank you.
Thoughts on the train question?

In the train's reference frame, it's not moving. The light is bouncing back and forwards in a straight line from the middle of one mirror to the middle of the other.

On your other point. Relativty explicitly precludes light (EM radiation) having a reference frame and having measurements of space and time. And, if anything else could travel at the speed of light, it would also have no concept of space and time. Within relativity, the equations that govern an observer's measurements of space and time are mathematically undefined for an observer moving at the speed of light, $c$, with respect to another observer.

The key thing is the "gamma factor": $\gamma = \frac{1}{\sqrt{1- v^2/c^2}}$.

If you try to put $v=c$ in that equation, corresponding to the reference frame of something traveling at $c$, you get $\gamma = \frac{1}{0}$, which is mathematically undefined.

 

[Francis Ward]

The clock for light? Maybe it is just the same as the clock for any stable particle. To my mind muon is not a clock for muon, it is a measure of the passing of time under a fixed set of conditions based on the life of muon particles. Change those conditions and you change the physical way the muon particles react. As such it would be considered an unreliable clock.

 

[Francis Ward]

In the train's reference frame, it's not moving. The light is bouncing back and forwards in a straight line from the middle of one mirror to the middle of the other.

In which case, the light is moving both forwards and sideways? Because the opposite mirror has moved v*t from the moment the light left the first mirror.

 

[PeroK]

In which case, the light is moving both forwards and sideways? Because the opposite mirror has moved v*t from the moment the light left the first mirror.

That's from your perpective outside the train. From the perspective of those on the train, it's you that is moving sideways.

This is a critical issue regarding the notion of relative vs absolute motion. Many people think this relates to Einstein's relativity, but in fact it was Galileo who first realized all motion is relative.

Let's suppose that the light on the "moving" train doesn't behave in the same way as the light on a "stationary" train. Well, the "stationary" train is orbiting with the Earth. So, we ought to see all sorts of strange effects here on Earth, with light and other objects trying to move "properly" relative to a stationary observer at rest with respect to the sun, say. You might point a torch in one direction, but the light would go off in another because you are moving with the Earth.

But, the sun itself is moving round the galaxy and the galaxy is moving towards Andromeda.

So, who's actually at rest here and who is actually moving with what absolute speed in which absolute direction?

 

[phinds]

The clock for light? Maybe it is just the same as the clock for any stable particle.

You misunderstand what a clock is. This is a VERY common misconception. A clock, first and foremost, is something made of matter. Matter cannot travel at c, so there is no such thing as a clock traveling with light, thus light has no clock.

 

[jbriggs444]

In which case, the light is moving both forwards and sideways? Because the opposite mirror has moved v*t from the moment the light left the first mirror.

Suppose that you are in a jet airplane flying west from New York to Los Angeles at noon.

You are flying first class and pick an olive from your complimentary beverage, tilt your head back and toss it in the air, catching it in your mouth. Do you expect to be able to do so?

Is the olive traveling vertically upward from your hand and vertically downward to your mouth?
Or is it traveling on a diagonal path at ~400 mph westward relative to the rotating earth?
Or is it traveling on a diagonal path at ~300 mph eastward relative to the Earth's center?
Or is it traveling on a diagonal path at some thousands of miles per hour westward relative to the sun?

Yes, it is doing all of these.

 

[PeroK]

Suppose that you are in a jet airplane flying west from New York to Los Angeles at noon.

You are flying first class and pick an olive from your complimentary beverage, tilt your head back and toss it in the air, catching it in your mouth. Do you expect to be able to do so?

Is the olive traveling vertically upward from your hand and vertically downward to your mouth?
Or is it traveling on a diagonal path at ~400 mph westward relative to the rotating earth?
Or is it traveling on a diagonal path at ~300 mph eastward relative to the Earth's center?
Or is it traveling on a diagonal path at some thousands of miles per hour westward relative to the sun?

Yes, it is doing all of these.

I'll try that next time I'm flying first class!

 

[Francis Ward]

I do that every time I fly first class.
But we cannot attribute the actions of a material item, such as an olive, to those of light. Light, we are told, cannot be influenced by the speed of it's source (we cannot add to the speed of light by putting the torch on the front of the train). Hence it should not be influenced either by the sideways motion of the train. As a person sitting on a train I would expect, within my frame of reference, for a ball I throw against the opposite wall to hit where I aim and bounce back to me. That is because it has applied to it the two motion components of the train and my arm. Light should not, we are told, behave like that. So why does it (according to the thought experiment) do so? Could it simply be that in fact it does not, and that in fact we simply do not have the apparatus to be able to detect what light is in reality doing? Could it be that the theoretic universal speed limit, c, is only there because all of our observational capabilities are constrained by light? Who really knows what the movement of the earth, galaxy etc is doing to the light that is all around us? It is all a matter of perception, and probably very little to do with reality.

 

[Drakkith]

Could it simply be that in fact it does not, and that in fact we simply do not have the apparatus to be able to detect what light is in reality doing? Could it be that the theoretic universal speed limit, c, is only there because all of our observational capabilities are constrained by light?

No. It doesn't matter how you actually accomplish the measurement of the time passed for an object or the measurement of its velocity. You will still have time dilation and there will still be a speed limit of c.

Who really knows what the movement of the earth, galaxy etc is doing to the light that is all around us? It is all a matter of perception, and probably very little to do with reality.

If what we regularly observe doesn't coincide very closely with reality, then reality is very strange indeed. That would mean that the laws of physics that we have now and that we've tested to a very high precision are grossly incorrect despite working extremely well for us here on Earth. Our observations of distant places in the universe would somehow make sense according to these incorrect laws, even though they would be very wrong. That's a serious contradiction.

 

[Francis Ward]

No. It doesn't matter how you actually accomplish the measurement of the time passed for an object or the measurement of its velocity. You will still have time dilation and there will still be a speed limit of c.
If what we regularly observe doesn't coincide very closely with reality, then reality is very strange indeed. That would mean that the laws of physics that we have now and that we've tested to a very high precision are grossly incorrect despite working extremely well for us here on Earth. Our observations of distant places in the universe would somehow make sense according to these incorrect laws, even though they would be very wrong. That's a serious contradiction.

Why then does it seem that the thought experiment contradicts what we are taught about the movement of light? It seems that the light IS being influenced by the velocity of the train, because otherwise it would not, to the observer on the train, impinge at the centre of the opposite mirror. It would impinge a distance (albeit very small) behind the centre of the mirror.

 

[jbriggs444]

I do that every time I fly first class.
But we cannot attribute the actions of a material item, such as an olive, to those of light. Light, we are told, cannot be influenced by the speed of it's source

True enough, its speed is not influenced by the speed of its source. But its direction can be influenced by its source. If you have ever used a flashlight and shone its beam first left then right, you may be aware of this.

It may be worth paying close attention to the design of the aiming device that you use to emit the pulse of light on the train. How exactly is your hypothetical emitter designed? [I would suggest a flashlight, a fast shutter and a blackened tube through which the pulse travels -- that should be simple to analyze].

 

[Francis Ward]

True enough, its speed is not influenced by the speed of its source. But its direction can be influenced by its source. If you have ever used a flashlight and shone its beam first left then right, you may be aware of this.

It may be worth paying close attention to the design of the aiming device that you use to emit the beam of light on the train. How exactly is your hypothetical emitter designed? [I would suggest a flashlight and a blackened tube through which the light shines -- that should be simple to analyze].

Yes, the light on the wall will move as the flashlight is moved. But the light that is emitted at the instant the flashlight is at 90 degrees to the wall, should not deviate from that path, regardless of how fast I move the flashlight. Einsteins thought experiment implies (to my feeble mind) that it does.

If the train is going 300 km/h and the distance across to the opposite mirror is 10m (very big train) then the light will take 33.356 nano seconds to travel across. In that time the train will have progressed a massive 2.78 micro-meters relative to the starting position of the light. So, to the person on the train, should the light not impinge 2.78 um behind the position it would if the train was stationary?

 

[jbriggs444]

Yes, the light on the wall will move as the flashlight is moved. But the light that is emitted at the instant the flashlight is at 90 degrees to the wall, should not deviate from that path, regardless of how fast I move the flashlight. Einsteins thought experiment implies (to my feeble mind) that it does.

You must analyze the scenario carefully. The above is not careful. How does the aiming on your hypothetical flashlight work, exactly?

 

[PeroK]

Why then does it seem that the thought experiment contradicts what we are taught about the movement of light? It seems that the light IS being influenced by the velocity of the train, because otherwise it would not, to the observer on the train, impinge at the centre of the opposite mirror. It would impinge a distance (albeit very small) behind the centre of the mirror.

You're simply misinterpreting the statement 1) "the velocity of light does not depend on the velocity of its source". It's interesting, because a while ago someone else had exactly the same thought as you.

With you're interpretation, I guess, all light beams must move in a single direction. Let's assume you set up a target and shine a light beam towards it. That establishes the velocity (speed and direction of light). Then, all other beams of light must go in that direction. Even if you turn round and shine your torch in the opposite direction, the light ought to come out of the back of your torch and towards your target.

Or, if a friend stood 1m to your left and shone their torch at the target, you would have violated the above statement, as you now, patently, have two different velocities for light. The same speed perhaps, but definitely differenet directions, hence different velocities. So, light does not always have the same velocity, only the same speed.

Clearly, however, that is not what is meant by statement 1). A beam of light from a torch can be sent out in any direction you like. Now, imagine the beam is shone along inside a tube (let's say, by an observer standing next to the tube). He will see the light move along inside the tube without touching the sides. But, so will an observer moving with respect to the tube and the light source.

Now, you could argue that the direction is "the same" in each case (it's physically in the direction of the tube). But, clearly, light cannot be moving "straight in front of" both observers at the same time. That is, of course, impossible.

 

[Francis Ward]

You must analyze the scenario carefully. The above is not careful. How does the aiming on your hypothetical flashlight work, exactly?

Not sure quite what you mean by careful analysis :)

You're simply misinterpreting the statement 1) "the velocity of light does not depend on the velocity of its source". It's interesting, because a while ago someone else had exactly the same thought as you.

With you're interpretation, I guess, all light beams must move in a single direction. Let's assume you set up a target and shine a light beam towards it. That establishes the velocity (speed and direction of light). Then, all other beams of light must go in that direction. Even if you turn round and shine your torch in the opposite direction, the light ought to come out of the back of your torch and towards your target.

Or, if a friend stood 1m to your left and shone their torch at the target, you would have violated the above statement, as you now, patently, have two different velocities for light. The same speed perhaps, but definitely differenet directions, hence different velocities. So, light does not always have the same velocity, only the same speed.

Clearly, however, that is not what is meant by statement 1). A beam of light from a torch can be sent out in any direction you like. Now, imagine the beam is shone along inside a tube (let's say, by an observer standing next to the tube). He will see the light move along inside the tube without touching the sides. But, so will an observer moving with respect to the tube and the light source.

Now, you could argue that the direction is "the same" in each case (it's physically in the direction of the tube). But, clearly, light cannot be moving "straight in front of" both observers at the same time. That is, of course, impossible.

You are correct, velocity is vector, speed scalar. However, that aside, I am not talking about flashlights, which emit a continuous beam of light, I talking about a small pulse of light emitted from a source across a 10 m train, which is traveling at 300 km/h. If that light hits the opposite wall precisley opposite the source, then it has traveled not only 10m across the train but 2.98um from the point at which it left the source. That, as I understand it, is the premise of the thought experiment. Is that correct?

 

[jbriggs444]

Not sure quite what you mean by careful analysis :)

A starting point would be to answer the question I've asked three times now: What is the design of your aiming device?

Edit: That's a dead serious question. I am prepared to discuss at least three possible designs: A blackened tube, a parabolic reflector or the flat end of a ruby laser. Without having accepted the theory of special relativity to start with, the blackened tube model is the best choice since its operation is simple and uncontroversial.

 

[PeroK]

Not sure quite what you mean by careful analysis :) You are correct, velocity is vector, speed scalar. However, that aside, I am not talking about flashlights, which emit a continuous beam of light, I talking about a small pulse of light emitted from a source across a 10 m train, which is traveling at 300 km/h. If that light hits the opposite wall precisley opposite the source, then it has traveled not only 10m across the train but 2.98um from the point at which it left the source. That, as I understand it, is the premise of the thought experiment. Is that correct?

It makes no difference whether it's a single pulse or a continuous beam. No, it hasn't moved $2.98 \mu m$ in the reference frame of the train, it has moved precisely 10m. In the reference frame in which the train is moving, it has moved slightly further. So, if the speed is the same in both frames, then less time must have elapsed in the train's frame when measured from the platform frame. And there you have time dilation.

 

[Francis Ward]

It makes no difference whether it's a single pulse or a continuous beam. No, it hasn't moved $2.98 \mu m$ in the reference frame of the train, it has moved precisely 10m. In the reference frame in which the train is moving, it has moved slightly further. So, if the speed is the same in both frames, then less time must have elapsed in the train's frame when measured from the platform frame. And there you have time dilation.

But I didn't say it had moved 2.98 um in the reference frame of the train, I said from the point at which it left the source. I know that this is in fact the stand-point of the observer outside of the train, and so to them it is perceived to have traveled further. But I am trying to understand the real situation, not the one of perception. Perception is simple to grasp but has little bearing on reality.

 

[PeroK]

But I didn't say it had moved 2.98 um in the reference frame of the train, I said from the point at which it left the source. I know that this is in fact the stand-point of the observer outside of the train, and so to them it is perceived to have traveled further. But I am trying to understand the real situation, not the one of perception. Perception is simple to grasp but has little bearing on reality.

The reality is both. Distance traveled is relative to reference frame. Nothing (whether it's light, cars, penguins or whatever) can move the same distance in all reference frames. That would be impossible. Just imagine yourself sitting still. You have not moved at all, as far as you're concerned. But, to someone on a train you have move at 200km/h perhaps. And, to take the example from earlier posts, you are spinning through space.

The reality is that distance traveled is relative to reference frame and that has been known since Galileo.

 

[Francis Ward]

The reality is both. Distance traveled is relative to reference frame. Nothing (whether it's light, cars, penguins or whatever) can move the same distance in all reference frames. That would be impossible. Just imagine yourself sitting still. You have not moved at all, as far as you're concerned. But, to someone on a train you have move at 200km/h perhaps. And, to take the example from earlier posts, you are spinning through space.

The reality is that distance traveled is relative to reference frame and that has been known since Galileo.

Thanks again PeroK. I do understand that. Speeds and distances are relative to the frame of reference. And, according to Galileo, there is no fixed point in the universe. But, would that not debunk any big bang theory? For a singularity to form the origin of our universe, that has to be confined to a point in space. A point from which all time and distance originates? That being the case, then yes, there is an absolute reference point from which to measure all time and speed and distance.

 

[jbriggs444]

Thanks again PeroK. I do understand that. Speeds and distances are relative to the frame of reference. And, according to Galileo, there is no fixed point in the universe. But, would that not debunk any big bang theory? For a singularity to form the origin of our universe, that has to be confined to a point in space. A point from which all time and distance originates? That being the case, then yes, there is an absolute reference point from which to measure all time and speed and distance.

No, that is a common but incorrect understanding of the Big Bang. The initial singularity would not be a point even if it were part of the space-time described by the theory. See many articles in the cosmology section.

 

[PeroK]

Thanks again PeroK. I do understand that. Speeds and distances are relative to the frame of reference. And, according to Galileo, there is no fixed point in the universe. But, would that not debunk any big bang theory? For a singularity to form the origin of our universe, that has to be confined to a point in space. A point from which all time and distance originates? That being the case, then yes, there is an absolute reference point from which to measure all time and speed and distance.

I'm glad to see you've given up trying to debunk relativity, but I suspect you'll have no more luck debunking the big bang theory!

Try this for starters:

https://www.physicsforums.com/insights/big-bang-happen/

 

[Herman Trivilino]

As a person sitting on a train I would expect, within my frame of reference, for a ball I throw against the opposite wall to hit where I aim and bounce back to me.

And if it happens in your frame of reference, it happens in all the others. There is no frame of reference in which the ball does not return to you, if it indeed does return to you in your frame of reference.

The same is true of a beam of light.

That is because it has applied to it the two motion components of the train and my arm.

The same is true of a light beam.

Light should not, we are told, behave like that.

Do you have a reference for that claim? It's a demonstrably false assertion.

So why does it (according to the thought experiment) do so?

For the same reason the ball does.

Could it simply be that in fact it does not, and that in fact we simply do not have the apparatus to be able to detect what light is in reality doing?

No. And no.

Could it be that the theoretic universal speed limit, c, is only there because all of our observational capabilities are constrained by light?

No. Many of our observations are not constrained by light in any way.

Who really knows what the movement of the earth, galaxy etc is doing to the light that is all around us?

The Principle of Relativity asserts that there is no effect caused by uniform motion. In other words, that anyone inertial reference frame is as good as any other. It could be that this assertion is wrong, but there is no experimental evidence to support that stance.

It is all a matter of perception, and probably very little to do with reality.

It first and foremost has everything to do with reality, regardless of anyone's perception.

 

[Simon Bridge]

When attempting a critique it is important to understand what is actually being said, otherwise we end up inadvertently attacking a straw man.

The important rule is that the speed of light is the same for all observers - not that the velocity is the same.
Remember the speed is the magnitude of the velocity vector.
There is no rule to say that relative motion cannot change anything about light. For instance you can change the kinetic energy and momentum carried by the light by changing the motion of the source, and you can also change the direction that the light travels in... put more precisely: different observers may determine different energy, momentum, and direction, for light; even though they always measure the same speed.

This thing about the speed has fun consequences when you compare observations made in different reference frames.

For instance - a directed pulse of light is fired to a detector in the ceiling of a rail car ... that is moving along tracks.
At the instant the pulse is fired, a flash bulb goes off so everyone can see when that happened. Everyone will agree call that this happens at t=0.
When the pulse hits the detector, another flash bulb goes off. Everyone agrees to take note of when and where the flash goes off, in terms of their personal coordinates and clocks. For the sake of simplicity, everyone agrees to account for the speed of light travel times when working out the times that things happen.
Bear this in mind when considering the following discussion. I am going to attempt to illustrate the stuff about what is apparent vs what happens in reality... I'll start by talking about what is apparent to one observer without making claims about what is real or "actually" happening.

According to Alice, standing on the tracks, the pulse was emitted at the floor of the car when it was right next to her, but was detected at the ceiling of the car some distance down the tracks. Therefore the trajectory of the pulse looks to have been at a diagonal to the tracks. But was that actually the case, or is this just what it looks like and the reality is different?

She may reason that the pulse of light should have just shot straight upwards from her position ... after all, if she held the emitter and fired a pulse, that is what would happen. Maybe, she speculates, there is a spherical pulse of light and the part the detector intercepted is not the part that kept going upwards; maybe it only appears that the the pulse was diagonal?

She can prove it by having a second detector directly above the first at t=0, but which does not move with the car. This detector does not go off, indicating that no part of the pulse lagged behind the train. It is also possible to repeat this experiment at speeds arbitrarily close to the speed of light, or using rail-cars that are arbitrarily tall ... this makes the observed (by Alice) horizontal displacement quite large so what is "actually going on" is going to become more and more apparent. Lastly, the experiment was rigged with a highly directional pulse, the physics of which Alice can examine and see that it behaves as reported.

She is forced to conclude that the pulse of light traveled diagonally with no spreading, no lagging, nothing.

How can this happen? Easy - the emitter was cleverly devised so that the pulse was actually aimed ahead of the detector-position. This is, after all, how a marksman hits a moving target.

After the experiment is over she talks to her good friend Bob who rode along on the train. They compare notes over coffee.
Bob is like, "No, I aimed the pulse straight up like you said to."
Moreover, the time between emitting and detecting the pulse was less by Bob's clock.

Which report is the reality and which the seeming?
It it possible to devise an experiment to demonstrate which is the real reality?

Notes:
1. it may be that there is something else in mind, in which case please state it clearly - there are lots of fun situations that can be, and have been, devised;
2. the above example is not as careful as it should be - this is why the standard thought experiment involves a return trip. The whole thing can be reformulated with the emitter also being a detector and the ceiling detector replaced by a mirror for anyone who wants to be more rigorous.

 

[Francis Ward]

I'm glad to see you've given up trying to debunk relativity, but I suspect you'll have no more luck debunking the big bang theory!

Try this for starters:

https://www.physicsforums.com/insights/big-bang-happen/

Actually I am not trying to debunk anything. Just asking questions and trying to grasp concepts which at first sight are riddled with inconsistencies.

 

[Francis Ward]

When attempting a critique it is important to understand what is actually being said, otherwise we end up inadvertently attacking a straw man.

The important rule is that the speed of light is the same for all observers - not that the velocity is the same.
Remember the speed is the magnitude of the velocity vector.
There is no rule to say that relative motion cannot change anything about light. For instance you can change the kinetic energy and momentum carried by the light by changing the motion of the source, and you can also change the direction that the light travels in... put more precisely: different observers may determine different energy, momentum, and direction, for light; even though they always measure the same speed.

This thing about the speed has fun consequences when you compare observations made in different reference frames.

For instance - a directed pulse of light is fired to a detector in the ceiling of a rail car ... that is moving along tracks.
At the instant the pulse is fired, a flash bulb goes off so everyone can see when that happened. Everyone will agree call that this happens at t=0.
When the pulse hits the detector, another flash bulb goes off. Everyone agrees to take note of when and where the flash goes off, in terms of their personal coordinates and clocks. For the sake of simplicity, everyone agrees to account for the speed of light travel times when working out the times that things happen.
Bear this in mind when considering the following discussion. I am going to attempt to illustrate the stuff about what is apparent vs what happens in reality... I'll start by talking about what is apparent to one observer without making claims about what is real or "actually" happening.

According to Alice, standing on the tracks, the pulse was emitted at the floor of the car when it was right next to her, but was detected at the ceiling of the car some distance down the tracks. Therefore the trajectory of the pulse looks to have been at a diagonal to the tracks. But was that actually the case, or is this just what it looks like and the reality is different?

She may reason that the pulse of light should have just shot straight upwards from her position ... after all, if she held the emitter and fired a pulse, that is what would happen. Maybe, she speculates, there is a spherical pulse of light and the part the detector intercepted is not the part that kept going upwards; maybe it only appears that the the pulse was diagonal?

She can prove it by having a second detector directly above the first at t=0, but which does not move with the car. This detector does not go off, indicating that no part of the pulse lagged behind the train. It is also possible to repeat this experiment at speeds arbitrarily close to the speed of light, or using rail-cars that are arbitrarily tall ... this makes the observed (by Alice) horizontal displacement quite large so what is "actually going on" is going to become more and more apparent. Lastly, the experiment was rigged with a highly directional pulse, the physics of which Alice can examine and see that it behaves as reported.

She is forced to conclude that the pulse of light traveled diagonally with no spreading, no lagging, nothing.

How can this happen? Easy - the emitter was cleverly devised so that the pulse was actually aimed ahead of the detector-position. This is, after all, how a marksman hits a moving target.

After the experiment is over she talks to her good friend Bob who rode along on the train. They compare notes over coffee.
Bob is like, "No, I aimed the pulse straight up like you said to."
Moreover, the time between emitting and detecting the pulse was less by Bob's clock.

Which report is the reality and which the seeming?
It it possible to devise an experiment to demonstrate which is the real reality?

Notes:
1. it may be that there is something else in mind, in which case please state it clearly - there are lots of fun situations that can be, and have been, devised;
2. the above example is not as careful as it should be - this is why the standard thought experiment involves a return trip. The whole thing can be reformulated with the emitter also being a detector and the ceiling detector replaced by a mirror for anyone who wants to be more rigorous.

Hi Simon,
That is precisely what I am trying to understand, and thank you for taking the time to depict it so well. I come from an engineering background and as I said I am new to pondering the whole relativity stuff. So my views are initially constrained by a very logical approach eg if you add speeds in a single direction there should be no limit how fast you can go.

So, pondering the many comments my initial post had generated, leaves many questions. For instance, that raised by the question of relative velocities, such as, to a person on the ground, observing an aircraft passing over head, they see it moving at, say, 600 km/h. However to the person on the aircraft, observing the person on the ground they see precisely the same of the person, that he is moving at 600 km/h. Simple and obvious stuff

Now, if we replace the aircraft with a rotating podium on which an observer, A, stands, and that podium is rotating at 6000 rpm. Then place another observer, B, a distance of 478 km from the podium. If my maths is right to A they will simply see B rotating at 100 hz, but to B they will see A flash past them at marginally higher than c, 100 times per second.

Now replace B with a light source emitting a straight source of light, which does not diverge. Build a circular wall at 478 km radius from B, on which the spot of light can project. To all observers in the frame of reference of the wall, that spot of light would travel at marginally higher than c (again assuming my maths is correct).

Clearly, in both cases, as c cannot be exceeded, what is seen is not what is really happening.

 

[Drakkith]

Now, if we replace the aircraft with a rotating podium on which an observer, A, stands, and that podium is rotating at 6000 rpm. Then place another observer, B, a distance of 478 km from the podium. If my maths is right to A they will simply see B rotating at 100 hz, but to B they will see A flash past them at marginally higher than c, 100 times per second.

Rotating reference frames have to be handled very carefully. You can construct physical laws in which the universe is moving around you, but physics will be MUCH more complicated. In this case, the speed of light will have to be modified by some factor which increases with distance. In any case, it will still be a fact that a beam of light can never be beaten in a race by anything, no matter how you construct your thought experiment.

Now replace B with a light source emitting a straight source of light, which does not diverge. Build a circular wall at 478 km radius from B, on which the spot of light can project. To all observers in the frame of reference of the wall, that spot of light would travel at marginally higher than c (again assuming my maths is correct).

I'm having trouble following your setup, but it sounds like the classic example of shining a laser beam across the surface of the Moon from Earth. You can move the laser such that the spot sweeps across the surface at any arbitrary velocity. But so what? The light isn't traveling in that direction, it is traveling from the emitter to the surface, not across it, so c is never violated.

 

[Francis Ward]

I'm glad to see you've given up trying to debunk relativity, but I suspect you'll have no more luck debunking the big bang theory!

Try this for starters:

https://www.physicsforums.com/insights/big-bang-happen/

But even if time existed before the Big Bang, there is still another reason not to imagine the Big Bang as happening at one point in a preexisting empty space. Observations of the universe show a nearly complete lack of structure on very large scales, and the cosmic microwave background is also extremely uniform (with fractional temperature differences on the order of 10-5). For this reason, realistic cosmological models must be almost exactly homogeneous, meaning that no point in space has properties that differ very much from those of any other point. Therefore the best evidence is that the Big Bang happened uniformly, everywhere at once.

Reference https://www.physicsforums.com/insights/big-bang-happen/

An omnipresent big bang ...

 

[weirdoguy]

Is that a problem for you?

 

[Francis Ward]

Is that a problem for you?

No. On the contrary, it is probably the most magnificent concept I have heard. The imagery is awesome. Do you have thoughts on how such an event would be initiated?

 

[Drakkith]

No. On the contrary, it is probably the most magnificent concept I have heard. The imagery is awesome. Do you have thoughts on how such an event would be initiated?

Please stick to the main topic. If you have questions or comments regarding the BBT, then please make a new thread in the cosmology section.

 

[GeorgeDishman]

Well one could say an answer to the original question is that the oldest 'light' is the cosmic microwave background ;-)

 

[Simon Bridge]

Hi Simon,
That is precisely what I am trying to understand, and thank you for taking the time to depict it so well. I come from an engineering background and as I said I am new to pondering the whole relativity stuff. So my views are initially constrained by a very logical approach eg if you add speeds in a single direction there should be no limit how fast you can go.

... that would be common sense logic rather than logic based on very careful observation.
Common sense is what tells you the Earth is flat ... though, to be fair, common sense is pretty good for day-to-day experience.

So, pondering the many comments my initial post had generated, leaves many questions. For instance, that raised by the question of relative velocities, such as, to a person on the ground, observing an aircraft passing over head, they see it moving at, say, 600 km/h. However to the person on the aircraft, observing the person on the ground they see precisely the same of the person, that he is moving at 600 km/h. Simple and obvious stuff

... as an engineer you would know that those speeds are estimates. They should be quoted with their uncertainty.
So ... off those figures, the speed would be $600\pm0.5$kmph. This is important to notice since, for such slow speeds, the difference between common-sense and relativity is usually much smaller than the uncertainty.
The next thing I want you to notice is that you have described two inertial reference frames (apart from gravity being in both of them) ... so they are equivalent from the point of view of the physics you can do in them. The only way to distinguish them is to notice that the flying aircraft is making a lot of noise and is using up fuel ... but I'm sure you can think of a way to get rid of such clues.

Now, if we replace the aircraft with a rotating podium on which an observer, A, stands, and that podium is rotating at 6000 rpm. Then place another observer, B, a distance of 478 km from the podium. If my maths is right to A they will simply see B rotating at 100 hz, but to B they will see A flash past them at marginally higher than c, 100 times per second.

You no longer have the kind of equivalence you had with the aircraft example since A is in a non-inertial frame.
Rotating frames are accelerating.

The rotating observer thing is handled in general relativity.
http://abacus.bates.edu/~msemon/SemonMalinWortel.pdf
(Slideshow discussion... http://luth.obspm.fr/~luthier/gourgoulhon/fr/present_rec/imcce_syrte10.pdf )
... however, it is probably best to get used to special relativity first, otherwise you are trying to make links to things that are harder to understand from a fuzzy understanding of something else. The tldr answer is that the common-sense, euclidean, geometry you are used to becomes non-euclidean for rotating observers.

Now replace B with a light source emitting a straight source of light, which does not diverge. Build a circular wall at 478 km radius from B, on which the spot of light can project. To all observers in the frame of reference of the wall, that spot of light would travel at marginally higher than c (again assuming my maths is correct).

... there is nothing in relativity to contradict this, and the effect has been observed in Nature.
ie. http://www.mtu.edu/news/stories/201...may-help-illuminate-astronomical-secrets.html

Another example would be two spacecraft flying in opposite directions away from a space station, each at 0.6c wrt the station. Clearly an observer in the station will see the distance between the spacecraft getting bigger at the rate 1.2c.
Distant galaxies can also exceed c - due to cosmological expansion.

All observers measure the same speed for light in a vacuum ... this does not mean that nothing, no effect of any kind at all, can travel FTL, just that no message can be sent FTL.

 

[GeorgeDishman]

Now replace B with a light source emitting a straight source of light, which does not diverge. Build a circular wall at 478 km radius from B, on which the spot of light can project. To all observers in the frame of reference of the wall, that spot of light would travel at marginally higher than c (again assuming my maths is correct).

Replace B with a machine gun. Does any individual bullet move along the wall at greater than c? Or does each bullet move from the gun to the wall at the usual muzzle velocity, well below the speed of light?

You say you come from an engineering background, I'm in that discipline too, so make your light source an LED connected to a pulse generator. What happens to each flash of light emitted?

The situation you are describing is a variant of a paradox called the "Superluminal Scissors".

 

[Francis Ward]

Replace B with a machine gun. Does any individual bullet move along the wall at greater than c? Or does each bullet move from the gun to the wall at the usual muzzle velocity, well below the speed of light?

You say you're an engineer, me too, so make your light source an LED connected to a pulse generator. What happens to each flash of light emitted?

The situation you are describing is a variant of a paradox called the "Superluminal Scissors".

HI George,
Yes, that is exactly how I had been viewing it. I shall look up the Superluminal Scissors - thank you
One question that have not been able to clarify though is the impact of the sideways motion in that pulse of light. In this case, as Simon rightly put it, better to stay with the simpler non-rotational motion. So, going back to the light emitter traveling at 90 degrees to the direction of the light. With the 'physical' things, such as the bullets or the ball being fired/thrown, I can see clearly that the velocity of the train 'adds' to the velocity of the ball/bullet and gives a resultant. Can we apply the same principle to the pulse of light? As Brian Cox put in his book, does the light get a 'helping hand' from the motion of the train? My understanding, based on the fact that if you switch the direction of the light emitter to one that is parallel to the direction of the light, you cannot 'add' to the speed of the light, is that is should not. This was refuted by an earlier responder, saying that is was 'a demonstrably false assertion', and Einsteins light clock thought experiment seems to support this, inasmuch as the light travels from mirror centre to mirror centre, exactly as it would if the train were stationary. (Which of course, to the observer on the train, it might well be)

 

[Francis Ward]

... that would be common sense logic rather than logic based on very careful observation.
Common sense is what tells you the Earth is flat ... though, to be fair, common sense is pretty good for day-to-day experience.

... as an engineer you would know that those speeds are estimates. They should be quoted with their uncertainty.
So ... off those figures, the speed would be $600\pm0.5$kmph. This is important to notice since, for such slow speeds, the difference between common-sense and relativity is usually much smaller than the uncertainty.
The next thing I want you to notice is that you have described two inertial reference frames (apart from gravity being in both of them) ... so they are equivalent from the point of view of the physics you can do in them. The only way to distinguish them is to notice that the flying aircraft is making a lot of noise and is using up fuel ... but I'm sure you can think of a way to get rid of such clues.You no longer have the kind of equivalence you had with the aircraft example since A is in a non-inertial frame.
Rotating frames are accelerating.

The rotating observer thing is handled in general relativity.
http://abacus.bates.edu/~msemon/SemonMalinWortel.pdf
(Slideshow discussion... http://luth.obspm.fr/~luthier/gourgoulhon/fr/present_rec/imcce_syrte10.pdf )
... however, it is probably best to get used to special relativity first, otherwise you are trying to make links to things that are harder to understand from a fuzzy understanding of something else. The tldr answer is that the common-sense, euclidean, geometry you are used to becomes non-euclidean for rotating observers.

... there is nothing in relativity to contradict this, and the effect has been observed in Nature.
ie. http://www.mtu.edu/news/stories/201...may-help-illuminate-astronomical-secrets.html

Another example would be two spacecraft flying in opposite directions away from a space station, each at 0.6c wrt the station. Clearly an observer in the station will see the distance between the spacecraft getting bigger at the rate 1.2c.
Distant galaxies can also exceed c - due to cosmological expansion.

All observers measure the same speed for light in a vacuum ... this does not mean that nothing, no effect of any kind at all, can travel FTL, just that no message can be sent FTL.

Hi Simon,
Once again thank you for the time and effort you put into your responses. It is highly appreciated.
Can you explain the inertial/non-inertial frame? Do you refer to moving items with mass, and therefore inertia vs those without mass and therefore zero inertia?
Regards
Francis

 

[Simon Bridge]

Hi Simon,
Once again thank you for the time and effort you put into your responses. It is highly appreciated.
Can you explain the inertial/non-inertial frame?

Discussion: https://www.physicsforums.com/threads/what-is-an-inertial-frame-of-reference.183267/
Wikipedia actually takes some care: https://en.wikipedia.org/wiki/Inertial_frame_of_reference
tldr... it's like this: you are in a well equipped lab in a closed box and you are tasked to conduct an experiment to discover the state of motion of the box.
If there is no experiment that will do this, the box is in an inertial frame.

For instance, you can tell if the box is accelerating by observing the slope in the level of water in a container in different parts of the box... thus, an accelerating box is not in an inertial frame. Uniform motion, however, cannot be detected - thus: an inertial frame. For rotating observer: imagine the box is around the observer, rotating with her. Is there any experiment done inside the box that will determine that the box is rotating?

This is the idea in a nutshell.

 

[Drakkith]

So, going back to the light emitter traveling at 90 degrees to the direction of the light. With the 'physical' things, such as the bullets or the ball being fired/thrown, I can see clearly that the velocity of the train 'adds' to the velocity of the ball/bullet and gives a resultant. Can we apply the same principle to the pulse of light? As Brian Cox put in his book, does the light get a 'helping hand' from the motion of the train? My understanding, based on the fact that if you switch the direction of the light emitter to one that is parallel to the direction of the light, you cannot 'add' to the speed of the light, is that is should not. This was refuted by an earlier responder, saying that is was 'a demonstrably false assertion', and Einsteins light clock thought experiment seems to support this, inasmuch as the light travels from mirror centre to mirror centre, exactly as it would if the train were stationary. (Which of course, to the observer on the train, it might well be)

That's right. From the emitter's frame, the light pulse moves directly out from it, with no motion to either side or forward or backwards. From the frame of an observer in which the emitter is moving, the light pulse will move diagonally in such a way that the sideways component of its velocity is the same as the emitter's velocity. One thing to note: the emitter cannot travel at 90 degrees to the direction of the light. It either has no motion (emitter's frame) or its motion matches the sideways motion of the light, in which the angle between the two is less than 90 degrees.

Honestly, if you're set on learning special relativity, you should grab one of the many books dedicated to teaching it. Sitting down and working through a well-written book that contains diagrams, equations, thought experiments, and real-world experiments will beat trying to make your way through dozens of online references and forum posts/threads hands down. Not only that, but once you learn a little bit of the fundamentals, you can very easily take what you've learned and ask more specific questions here on the forums.

I'm sure someone here can recommend a good book on SR if one hasn't already been recommended in the thread.

 

[GeorgeDishman]

Can we apply the same principle to the pulse of light? As Brian Cox put in his book, does the light get a 'helping hand' from the motion of the train? My understanding, based on the fact that if you switch the direction of the light emitter to one that is parallel to the direction of the light, you cannot 'add' to the speed of the light, is that is should not.

Yes, you can apply the same principle but you have to use the relativistic maths, not conventional version that simply adds the velocities which only works for low speeds. For light parallel to the direction of motion, the combination of c and v gives c and still in the same direction. For light at any other angle, the angle changes but the speed remains c. That change of angle essentially throws the light from a moving source forward, it is sometimes called the "relativistic headlight effect" or "relativistic beaming" or aberration. A real world example occurs on the jets from some super-massive black holes, see the comparison of M87 and 3C31 in the following article. The jet moving away from us is so dim we cannot see it for this reason.

https://en.wikipedia.org/wiki/Relativistic_beaming

 

[Herman Trivilino]

Hi Simon,
So my views are initially constrained by a very logical approach eg if you add speeds in a single direction there should be no limit how fast you can go.

You can add speeds in a single direction, and there is no limit on the sum that you get when you do that.

But there are other ways besides addition that one can combine numbers. The logical question to ask is which is the right way to combine them when trying to figure out how fast something is moving. If B moves with respect to A at speed $v_{AB}$ and C moves with respect to B with $v_{BC}$, then what is the speed $v_{AC}$ that C moves with respect to A?

One possible way to figure that out is by adding $v_{AB}$ to $v_{BC}$. When you do that you of course get a speed, but it's not equal to $v_{AC}$.

Note that common sense is just something that appears to be true because it has been demonstrated to be true, and therefore appears to be obvious.

But in fact this way of combining speeds to get $v_{AC}$ works no better than it does when combining slopes. If you're on a roof that has a slope of 5/12, meaning it rises 5 inches for every 12 inches of run, and you place a rod on that roof so that it is tilted at a slope of 5/12 relative to the roof surface, then the rod will not have a slope of 10/12 relative to the horizontal. Instead, you would add the angles, which are the inverse tangents of the slopes. (Note though, that all you ever deal with is very small slopes, you find that adding the slopes works just as well as adding the angles. You don't notice that one way works better than the other until you have some experience with larger slopes. The same is true of slow speeds!)

Likewise, you would add the inverse hyperbolic tangents of the speeds. That's the right way to combine them.

 

[Herman Trivilino]

HI George,
My understanding, based on the fact that if you switch the direction of the light emitter to one that is parallel to the direction of the light, you cannot 'add' to the speed of the light, is that is should not. This was refuted by an earlier responder, saying that is was 'a demonstrably false assertion', and Einsteins light clock thought experiment seems to support this, inasmuch as the light travels from mirror centre to mirror centre, exactly as it would if the train were stationary. (Which of course, to the observer on the train, it might well be)

You do not add the speeds, you combine them in the way I described in my previous post. Likewise, when the direction of object's motion (light beam or ball) is tilted relative to the horizontal direction of the train's motion, you combine the horizontal components of the velocities in the same way.

 

[Battlemage!]

You can add speeds in a single direction, and there is no limit on the sum that you get when you do that.

But there are other ways besides addition that one can combine numbers. The logical question to ask is which is the right way to combine them when trying to figure out how fast something is moving. If B moves with respect to A at speed $v_{AB}$ and C moves with respect to B with $v_{BC}$, then what is the speed $v_{AC}$ that C moves with respect to A?

One possible way to figure that out is by adding $v_{AB}$ to $v_{BC}$. When you do that you of course get a speed, but it's not equal to $v_{AC}$.

Note that common sense is just something that appears to be true because it has been demonstrated to be true, and therefore appears to be obvious.

But in fact this way of combining speeds to get $v_{AC}$ works no better than it does when combining slopes. If you're on a roof that has a slope of 5/12, meaning it rises 5 inches for every 12 inches of run, and you place a rod on that roof so that it is tilted at a slope of 5/12 relative to the roof surface, then the rod will not have a slope of 10/12 relative to the horizontal. Instead, you would add the angles, which are the inverse tangents of the slopes. (Note though, that all you ever deal with is very small slopes, you find that adding the slopes works just as well as adding the angles. You don't notice that one way works better than the other until you have some experience with larger slopes. The same is true of slow speeds!)

Likewise, you would add the inverse hyperbolic tangents of the speeds. That's the right way to combine them.

Thanks for that. This is now my favorite analogy.

 

[GeorgeDishman]

The "stacked doorstop" analogy :-)

 

[Herman Trivilino]

This is now my favorite analogy.

Thanks, but I can't take all the credit. Most of it is adapted from Taylor and Wheeler. I threw in the part about the roof.