What Determines the Constant Speed of Light?

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Discussion Overview

The discussion centers around the reasons behind the constant speed of light, exploring its implications in physics, the definitions of measurement units, and the philosophical questions surrounding its value. Participants examine various theories, assumptions, and the relationship between light's speed and the properties of the universe.

Discussion Character

  • Exploratory
  • Debate/contested
  • Conceptual clarification
  • Mathematical reasoning

Main Points Raised

  • Some participants suggest that the lack of mass of photons contributes to the constant speed of light, questioning how speed can be added to massless particles.
  • Others argue that the speed of light is a constant due to its definition in terms of measurement units, specifically that the meter is defined as the distance light travels in a specific time interval.
  • One participant notes that the speed of light was recognized as a constant by Maxwell through the wave equation of electromagnetism, but questions why this value exists.
  • There are discussions about the implications of light traveling at infinite speed and how that would affect causality and the nature of the universe.
  • Some participants raise philosophical questions about why the speed of light has its particular value and whether this can ever be fully understood in physics.
  • One participant challenges the idea that the constancy of light's speed is merely an artifact of human measurement, arguing it is a fundamental property of the universe.
  • Another participant questions the constancy of light's speed when using non-standard units of measurement, such as cubits, highlighting the importance of defined units in the discussion.
  • There is mention of the anthropic principle as a potential explanation for the values of fundamental constants, though some find this unsatisfactory.

Areas of Agreement / Disagreement

Participants express multiple competing views regarding the reasons for the constant speed of light, with no consensus reached on the underlying explanations or implications. The discussion remains unresolved with various hypotheses and philosophical considerations presented.

Contextual Notes

Limitations include the dependence on definitions of measurement units, unresolved philosophical questions about the nature of constants, and the complexity of relating speed to reference frames in physics.

  • #91
Integral said: "We live deep in the suns "gravity well". this means that, according to GR, Space time is not flat in our vicinity."

At the Earth's surface, the gravitational force of the sun is nearly negligible to that of the earth. In fact it's even small compared to that of the moon, which is why tides are more dependent on the moon than the sun. So wouldn't any curvature of space time near the Earth be caused almost totally by the earth?
 
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  • #92
Clearly the sun has some significant effects. After all we orbit it, this is the gravitational well I am speaking of. As you mention the Earth has significant and noticeable gravitational effects, we walk the surface.

We still measure the speed of light as a constant.

Jdavel,
Are your comments addressing this issue or just to disagree with something I have posted?
 
  • #93
Integral,

No, I wasn't just trying to be contrary. Somebody else had suggested a thought experiment involving an accelerating rocket. I thought you had said that space time where we live is curved a lot more than it would be in a rocket because we live "deep in the sun's gravity well". But since the gravitational field at the Earth's surface is almost totally caused by the Earth itself, and since it only causes one g of acceleration, any effects predicted by GR would be greater in a rocket accelerating at more than one g, which is not unreasonable for a rocket. That's all I meant.
 
  • #94
Ok,
I think even Earth's gravitational well should be sufficient for the argument. After all any acceleration of more then 1 g for any significant amount of time is not a reasonable expectation of a human. (please do not quibble with the phrase "signifiant amount of time". significant >= 1 year. or what ever.

I am becoming more and more convinced that the SPEED of light will be a constant even if the velocity is not.
 
  • #95
The speed of light is, according to current theory, always constant. The speed of light, as measured by experiment, is always constant. The speed is 1 light-second per second. Feel free to convert to your favorite units ad infininitum.

If you'd like to advance a new theory as a replacement of existing mainstream theory, you are welcome to do so in the Theory Development subforum.

If you'd like to argue that existing experiments don't reflect reality, I suggest that you begin looking for a new universe in which the experimental results better suit you.

Case closed.

- Warren
 
  • #96
The illustration I referred to in my previous post from The Universe in a Nutshell is a spacetime diagram where the path of light rays' slopes vary from almost vertical near the surface of a star to 45 degrees further away. I was under the impression that the inverse of the (absolute value of the) slope in a spacetime diagram is proportional to speed. Yet this does not indicate a change in speed. I don't understand how that can be.
chroot said:
Case closed.
Fine.
 
  • #97
DrMatrix said:
I was under the impression that the inverse of the (absolute value of the) slope in a spacetime diagram is proportional to speed. Yet this does not indicate a change in speed. I don't understand how that can be. Fine.
You're forgetting time dilation.

- Warren
 
  • #98
What time dialation? There is only one reference frame and only one time axis. In order to say there is time dialation, don't you need to say time dilation with respect to something?
 
  • #99
Well

John Baez seems to be all over the map on this issue. I will quote a chunk from this page of his:

http://math.ucr.edu/home/baez/physics/Relativity/SpeedOfLight/speed_of_light.html


Since Einstein talks of velocity (a vector quantity) rather than speed it is not clear that he meant the speed will change but the reference to special relativity suggests he did[/color] mean so. This interpretation is perfectly valid but a more modern interpretation is that the speed of light is constant[/color] in general relativity.
The problem here comes from the fact that speed is a coordinate-dependent quantity, and is therefore somewhat ambiguous. To determine speed (distance/time) you must first choose some standards of distance and time, and different choices can give different answers. This is already true in special relativity: if you measure the speed of light in an accelerating reference frame, the answer will, in general, differ from c[/color].
In special relativity, the speed of light is constant when measured in any inertial frame. In general relativity, the appropriate generalization is that the speed of light is constant in any freely falling reference frame (in a region small enough that tidal effects can be neglected). In this passage, Einstein is not talking about a freely falling frame, but rather about a frame at rest relative to a source of gravity. In such a frame, the speed of light can differ from c[/color], basically because of the effect of gravity (spacetime curvature) on clocks and rulers.
If general relativity is correct then the constancy of the speed of light, in inertial frames is a tautology from the geometry of space-time. The causal structure of the universe is determined by the geometry of null vectors. Travelling at the speed c means following world-lines tangent to these null vectors. The use of c as a conversion between units of metres and seconds, as in the SI definition of the metre, is fully justified on theoretical grounds as well as practical terms because c is not merely the speed of light, it is a fundamental feature of the geometry of space-time.
Like special relativity, the predictions of general relativity have been confirmed in many different observations. The book by Clifford Will is an excellent reference for further details.
Finally we come to the conclusion that the speed of light is not only observed to be constant[/color]; in the light of well tested theories of physics, it does not even make any sense to say that it varies.

How is that for waffling?
 
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  • #100
My guess, is that Einstein was referring to the fact that light in SR is not only assumed to be constant speed but also constant velocity. In SR light always moves along a single coordinate axis, the only direction change occurs at the surface of a mirror. This pure rectilinear motion of a beam of light is strictly constant velocity. While it is not said specifically it is certainly built into his derivations. A nit picker could claim that the curvilinear motion of light in GR is a contradiction of SR. Because SR is developed for constant velocity light only.
 
  • #101
picass said:
Has anyone an idea why speed of light is constant ? Has this something to do with lack of mass of a photon because I cannnot imagine it to be possible to add speed to something that has no mass. As light emerges from energy fall of an electron, does speed of light have anything to do with rotation speed of an electron ?

The speed of light is constant because, unlike ordinary waves, light propagates without a carrier medium and you don't therefore have an absolute reference frame you could refer the speed to (at least not in a vacuum). The trick of nature is here that a light wave carries itself, or to be more precise, the electric wave carries the magnetic wave and vice versa (as given by Maxwell's equations). The irony is that even Maxwell apparently did not realize this and believed in the Ether Theory and a positive outcome of the Michelson-Morley experiment.
For related aspects see my webpage http://www.physicsmyths.org.uk/lightspeed.htm .
 
  • #103
Frankly I see no sign of waffling in his conclusive statement.
Finally we come to the conclusion that the speed of light is not only observed to be constant; in the light of well tested theories of physics, it does not even make any sense to say that it varies.
 
  • #104
I see no waffling in this statement:
This is already true in special relativity: if you measure the speed of light in an accelerating reference frame, the answer will, in general, differ from c.
But when you look at that quote next to Integral's quote, the man sounds like he's running for office.
 
  • #105
Integral said: "My guess, is that Einstein was referring to the fact that light in SR is not only assumed to be constant speed but also constant velocity."

Another possibility: Either Einstein or the German to English translator was a little sloppy with the German words for velocity and speed.

Anyway it's clear from the English translation of Einstein's own words, that what he was talking about is what we call "speed". From the 1905 paper: '...light is always propagated through empty space with a definite velocity c..." And from chapter 7, paragraph 1 of his book "Relativity The Special and General Theory": 'The assumption that this velocity of propagation is dependent on the "direction in space" is in itself improbable.' Neither of these is consistent with the term "velocity" being a vector.

By the way none of this has anything to do with whether, according to GR, the speed of light in a gravitational field is a constant. Surely the math of GR eliminates any ambiguity, just as the math of SR does.

But back on the topic to which this thread has drifted. It seems to me what you're saying implies that, according to GR, a photon's momentum can be altered by a G field perpendicular to its path but not by one parallel to it. That seems very strange to me, but maybe you're right, or maybe that's not what you're saying.
 
  • #106
jdavel said:
It seems ... that, according to GR, a photon's momentum can be altered by a G field perpendicular to its path but not by one parallel to it.
The proper momentum of a photon does not change in GR. One can infer, however, that the coordinate momentum of a photon changes by deflection in the first case and by red/blue shift in the latter case. Neither one of these indicates a change in velocity, since velocity and momentum aren't so intimitely related as they are in Newtonian mechanics.

I'm leaning towards the speed of the light attaining a value of c at the point of measurement regardless of the frame, but I haven't had time to get to the thought experiment proposed by Janitor.
 
  • #107
DrMatrix said:
I see no waffling in this statement:
Neither do I, its quite explicit: it says SR doesn't apply in non-inertial frames of reference. Its a re-statement of the domain of SR. That's not a surprise, not a shortcoming of SR, and not a statement against the constancy of the speed of light.

Integral's quote is equally clear. I don't see a problem here.
 
  • #108
jdavel said:
But back on the topic to which this thread has drifted. It seems to me what you're saying implies that, according to GR, a photon's momentum can be altered by a G field perpendicular to its path but not by one parallel to it. That seems very strange to me, but maybe you're right, or maybe that's not what you're saying.
A photon's momentum is altered by gravity in either direction. That does not, however, imply a change in speed because it is not a classical phenomenon.
 
  • #109
OK. Please reconcile the following with Integral's assertion that: "Consider this. In reality the Earth is a constantly accelerating reference frame, we measure the speed of light to be constant."
In special relativity, the speed of light is constant when measured in any inertial frame. In general relativity, the appropriate generalization is that the speed of light is constant in any freely falling reference frame (in a region small enough that tidal effects can be neglected). In this passage, Einstein is not talking about a freely falling frame, but rather about a frame at rest relative to a source of gravity. In such a frame, the speed of light can differ from c, basically because of the effect of gravity (spacetime curvature) on clocks and rulers.
I know how amused everyone was when I said it, so for your amusement I quote:
At the 1983 Conference Generale des Poids et Mesures the following SI (Systeme International) definition of the metre was adopted:
The metre is the length of the path traveled by light in vacuum during a time interval of 1/299 792 458 of a second.​
This defines the speed of light in vacuum to be exactly 299,792,458 m/s. This provides a very short answer to the question "Is c constant": Yes, c is constant by definition!

In post #97, chroot said that I forgot about time dialation. I am waiting for an answer to how can I account for time dilation when I have only one reference frame.
 
  • #110
You don't have just one. You have many, at different depths in the gravity well. Ever heard of gravitational time dilation?

- Warren
 
  • #111
chroot said:
You don't have just one. You have many, at different depths in the gravity well. Ever heard of gravitational time dilation?
Sure, I've heard of gravitational time dialation. Clocks deeper in a gravity well tick more slowly than a similar clock not so deep. Each clock has it's own reference frame with its own time axis. These we can compare. In the illustration, there is only one time axis. We have the path of a photon in a single reference frame. The slope of the photon changes. If the slope of the path in a space time diagram is not proportional to the speed (as I thought), what does the slope tell us?
 
  • #112
Janitor said:
A rocket is floating out in interstellar space. A laser is floating next to the rocket. On the outside of the rocket there is some device that measures the speed of light.
...
The astronaut hits the button to fire the engine, and away he goes.
...
The laser continues to shine at the rocket. The astronaut takes readings of the speed of the laser light as calculated by the apparatus on the side of his rocket.
I have examined this thought experiment in 1+1 D Kruskal space. For those of you who don't know, this space maps the rocket frame to a 90o wedge during the acceleration phase. The wedge border is a light cone that essentially designates an event horizon. Any light signal on the other side of the cone is causally disconnected from the 90o wedge accelerated frame. There is a subtle (and probably unexpected to most people) consequence of this space: acceleration is a function of proper distance from the event horizon. The acceleration, a, of the rocket corresponds to a particular proper distance, ds, from the horizon as: a = c2 / (-ds). For instance, assuming 10 g acceleration would put the rocket at ~0.1 ly from the horizon. I will assume a measurement device as I have described:


I will assume for my investigation that it has two points of detection (or possible four in the shape of a pyramid) separated by a known distance, and that it compares the detection times on clocks at each detection point by translating the clocks under negligible acceleration to a midpoint, and then adjusting for any time difference that appear among the clocks.
Assuming that the characteristic size of the measuring device is << the proper distance from the horizon, the measurement will be ~c, but just slightly less. This anomoly is, however, an artifact of the measurement process that I examined (specifically, the finite size of the detector), as the region surrounding any given point (event) in the accelerated frame can be transformed to an inertial region. I used a greatly exagerated detector size even for an extremely liberal acceleration (in other words, my detector size was 50% of the proper distance to the horizon), and I got c' ~ 0.8 c. Using reasonable numbers should result in c' = 0.999... c. Again, I want to emphasise that the difference in c' and c is strictly an artifact of the measurement process.
 
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  • #113
from The Universe in a Nutshell by Stephen Hawking, p. 115
When the star reaches a certain critical radius, the path will be vertical in the diagram, which means that the light will hover at a constant distance from the center of the star, never getting away.
Now, I know Integral has said that resorting to a black hole means that the point is lost. But Black Holes are a well established consequence of the General Theory of Relativity -- We're not talking fringe science. And it's not like I'm asking anyone to take my word here.

I'll also note that the light at the event horizon is not orbiting. Light orbits further out -- If I'm not mistaken, at 1.5 times the critical radius.
 
  • #114
Yes, light orbits a black hole at

r = \frac{3 G M}{c^2}

What's your point?

- Warren
 
  • #115
The point is that since light at the event horizon remains a constant distance from the star, it therefore has zero velocity, zero speed. Since zero does not equal c, the speed of light there is not c. Sorry if I wasn't clear. Does it make sense now?
 
  • #116
The light is still moving at c -- just around in a circle. Do horses going around a racetrack also have zero velocity and zero speed?

- Warren
 
  • #117
What has this got to do with the reasons behind a constant c?
 
  • #118
chroot,

Please re-read my post. I'm talking about light at the event horizon -- not light orbiting. The only reason I mentioned the radius where light orbited, was to avoid someone suggesting that the light at the horizon was going around in a circle. The light at the event horizon is not orbiting. It is not going around in a circle. You yourself pointed out that the radius where light orbits is further out.

The light is at a constant distance from the center and it is not going around in a circle. Zero velocity, zero speed.

To answer your question. No horses don't have zero velocity or zero speed, going around a race track.

Integral,

c is constant. The speed of light in GR is not necessarily c. You have insisted that the speed of light is constant in GR. It is not.
 
  • #119
Right, to an outside observer, it takes anything an infinite time to cross the event horizon. Again, what's your point?

You're apparently worrying about what people see from a distance. The speed of light is c only locally.

- Warren
 
  • #120
If by locally, you mean in an inertial reference frame where one can apply SR, then you are correct. I'm sure we're all in agreement that the speed (and the velocity) of light is constant in SR and the speed equals c.

Gravity and acceleration can affect the speed and the velocity of light. Light in a non-inertial reference frame does not, in general, travel at c. For example, at an event horizon the speed is zero.

My point is that the speed of light is not necessarily c in general relativity.
 

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