What is the true meaning of constant speed of light? Why is the Speed of Light Constant?

The speed of light is the foundation of many important theories, such as special relativity. At the first glance, scientists thought the speed of light might be measured to distinct values when viewed from different perspectives. However, the Michelson-Morley experiment told us the speed of light doesn't show any difference in any direction. One the other hand, Shapiro time delay confirmed massive objects can slow down light passing by. Is the speed of light constant? What is the true meaning of constant speed of light? All in 8 minutes.

Images in this video are designed by macrovector / Freepik.

[christopherkopperman]

The reason why the speed of light is constant is much simpler to understand. It isn't the speed of light at all but the speed of causality. Because we experience time and time is a calculation in speed there must be a minimum amount of time between when a thing is in one place and when it is in another. If it was instant then time and reality as we know it would collapse. But if there is a minimum time there must be a maximum speed based on that time. That is the speed of causality. Now, since light has no mass it can really only move at the maximum speed. Zero mass means that any amount of energy would accelerate it to max possible speed. This is true for all massless "particles"; photons, gluons, and if it turns out there is a gravity particle that one too. This is why you have causality issues, time travel, and all that other sci-fi stuff when you do calculations faster then light. Not because light is special but because the universe we perceive is moving forward through time.

[craigfordyce]

The constant speed of light is the greatest sacred cow in science.

[ChrisDIYerOklahoma]

I worked on radar for 21 years. We learned all about the Doppler Effect. We had radar coverage "issues" in the east when the sun rose and to the west when the sun was setting. It was only for about 20-30 mins or so. The angle of the sun to the radar propagation angle lined up at those times...causing loss in radar coverage. Not a big deal. But, if one wanted to evade being detected on radar...fly toward the radar with the sun to your back at those times. Thankfully, there are redundant systems that counteract those kinda threats!

[preparedsurvivalist]

When light curves around the sun it takes a longer linear trajectory than a straight line (called a geodesic) to get to its destination. Its a curvilinear manifestation of the bending of space time caused by large celestial bodies (i.e. general relativity). And whenever light seemingly slows down, this means time dilation is also in effect, as time and lightspeed maintain a constant equivocal correlation no matter the reference frame or circumstance.

[ramithewest]

What "constant in the local frame" means ? ... it should be in respect to a specific way of measurement ? .. what is that measurement method ? ... It's stunning how many and often scientists copy-cat that statement without due scrutiny ... Which is very ambiguous

[uskapyaar]

There's an error in video @ 2:30 lambda shown should be decreasing not increasing as the source is moving towards object and frequency is increasing hence lambda should decrease

[premendratiwari]

My brain has hanged. Need more time to understand.

[rogerscottcathey]

We cannot, as yet, exclude the possibility of a variable speed of light so longs we cannot reduce the magnitude of space or vacuum to test. It is unscientific to treat the concept of the constancy of the speed of light as a dogma or proven fact. It simply isn't proven

[romado]

Michelson-Morley's experiment was not null. The values that were measured were smaller than what was expected. They did have direction of about 17h sideral.

[TheTransporter]

It's not the speed of light, it's the speed of information and causality.

[vansf]

Examples of how the idea of relative speeds is an illusion, but not any expression of reality:
Person A is driving from North to South at the average speed Sp1= 50 km/h, while person B is driving in the opposite direction at the average speed Sp2 = 30km/h, and person C is driving in the same direction at the same speed Sp3 =50km/h, and person D standing by the road.
All the speeds Sp1, Sp2, Sp3 are actual and unchanged values from the observation of each insiders of each of such motored vehicles, and those actual speeds are the foundation to calculate how long it will take each of them to get to their different destinations, while to the observation of the person B, the person A is moving at the speed Sp1 = Sp1 +Sp2 = 50km/h + 30 km/h= 80km/h, and similarly from the observation of person A, person B seems to be moving at the Sp2 = Sp2 + Sp21= 30km/h +50km/h = 80 km/h, but the speed 80kn/h is not the actual speed of vehicle1, nor the speed of vehicle 2, but it is the combination of the speeds of the 2 vehicles moving in 2 opposite direction, and thus, the combined speed is not the foundation for neither of them to calculate how long it will each of them to get to their different destinations. So the combined speed is redundant or merely an illusion in this context. Likewise, person C is moving in the same direction as that of person A at the same speed Sp3 = 50km/h which is the actual speed, and the foundation for both of them to calculate how long it will take each of them to get their different destinations, but not that their speeds Sp1 and Sp3 are both zero km/h which is merely an illusion due to the fact that both of them are moving at the same speed in the same direction, which is if both of them were in the same car. So, the peed = 0km/h is merely an illusion and can never be any foundation for anyone of them to calculate the periods of time nor the distances they will travel. From the observation of person D standing by the road, all the 3 speeds Sp1, Sp2, Sp3 are the actual speed because the concept of speed is established on the displacements of moving objects or particles, but not on any speed of any moving thing. The actual speed of a moving object is dependent on the values of its displacement or the change of its location with respect to the change of time
V(t)=( D2 -D1)/ t2-t1.
There are some applications of the idea of combined speeds of objects moving in opposite directions, such as 2 missiles being launched from two different locations. One is an attacking missile, and the other is a defending or countering missile. You can apply the idea of combined speed to estimate when you should launch the defensive missile to destroy the attacking missile before it can reach your land. But again, all such combined speeds are not any actual speed to be based on for calculation of actual capabilities

[michaeljorgensen]

Reading through the comments it seems obvious that many people missed the part in the video where it says the speed of light is NOT UNCONDITIONALLY equal to c. In their own minds they leap to the wrong conclusion that the speed of light is variable. By CONVENTION we measure the speed of light locally as if there was an observer at each point along the path of a photon and likewise we do NOT measure the speed in a global reference frame. That is what people seem to be failing to grasp....even though this video explains it to them.

[frankyjayhay]

In plain English that I can understand pure empty space has a property (permittivity and permeability) that determines the speed of light so the speed of light is constant simply because empty space is constant, total emptiness must look the same for everyone however fast they're travelling.
The big mystery is how you can have a property without any substance. If space was any substance at all however tenuous then the speed of light would be relative to that and not be constant for everyone.
That's surely the starting point for relativity, I'd love to see a video about how electromagnetism relates to empty space with the great animations that we see here.

[curtaustin]

Regarding the analogy: Surface waves do not travel at a constant velocity - their speed increases with wavelength. The best explanations will be found in discussions of the "hull speed" of displacement boats.

[williamwalker]

According to Relativity observers on a moving train and a stationary train platform will disagree on the size of the Train and the passage of time on the Train. This is complete logical contradiction if the size and the passage of time of the train are real. If the size of the train is real, the Train can not be both contracted and not contracted. The same goes for the observed passage of time on the Train. If these effects are observed, then the only possible conclusion is that it is an optical illusion. Things that are real must appear to be same from all frames of reference, if not then it is an illusion.

[rbwinn]

Speed of light measured by scientific or atomic time will always have the same speed. But consider the Galilean transformation concept of time. It was based on the rotation of the earth, one second being 1/86400th of the time it takes for the earth to rotate. Now consider a clock in a flying airplane. Einstein says this about that clock. The time which elapses between two strokes of the clock is not one second, but 1/sqrt(1-v^2/c^2) seconds, a somewhat larger time. As a consequence of its motion, the clock goes more slowly than when at rest."
So if we have a GPS clock on the surface of the earth that agrees with the rotation of the earth, then the clock in the airplane does not. We then have a difference in the concept of time used by Galileo and Newton and the concept of time used by scientists today. If we take the Galilean transformation equations
x'=x-vt
y'=y
z'=z
t'=t
these equations say nothing about the time of the clock in the airplane. To show the time of that clock, we would need to use a different set of Galilean transformation equations with different variables for velocity and time. If we are using the time of the clock in the airplane to show relativity from the frame of reference of the airplane, the equations would be
x = x' - (-vt/n')n'
y = y'
z = z'
n = n'
n' is the time of the clock in the airplane, and (-vt/n') is the velocity of the ground relative to the airplane. If we are saying that both clocks show light to be traveling at c=186, 000 miles per second, according to the times of the clocks, then we would say x=ct and x'=cn' instead of x=ct and x'=ct' as Einstein and Lorentz did. Then we get
x'=x-vt
cn' = ct - vt
n' = t-vt/c
which is the same as the numerator for Lorentz's equation for t' if we substitute in x=ct.
t-vt/c = t-vct/c^2 = t-vx/c^2
This is the part of Lorentz's equation that results in a slower clock in Special Relativity. The length contraction is a result of the denominator of Lorentz's equation.
Since there is no length contraction in the Galilean transformation equations we just have n' = t(1-v/c).
If t is GPS time, then we can see how faster velocities relate to slower clocks. A clock on Mercury would be slower than a clock on earth because Mercury is traveling at 30 miles per second in its orbit, while the earth is traveling at 20 miles per second. Outer planets would have faster clocks than a GPS clock on earth because they would be traveling slower in their orbits. If we compare the result obtained by this use of the Galilean transformation equations for the time of a clock on Mercury to the time obtained for that clock using the Lorentz equations, the results will agree to several decimal places. Using the Galilean transformation equations with t', which is what Isaac Newton did with his absolute time, results in a greater disparity of results.
I have tried to communicate with scientists to get an opinion concerning this since I first thought of it in high school, but science of today is a religion, not a study of reality. If this mathematics works for the solar system, it should work for theoretical science in computing atoms, molecules, etc. Lorentz first derived his equations in calculating electromagnetic fields. Maxwell derived all of his equations using Galilean relativity. He died before the Michelson-Morley experiment took place. But scientists of today will not discuss the Galilean transformation equations at all. They are more interested in keeping the flow of money to themselves from governments than they are in mathematics and science. Do you think they are going to admit that a high school graduate discovered something they missed? I don't.

[dialectic-scietist]

06:00 and how the did the measurement? Is there any electromagnetic measurement way to measure the speed of light in one direction? No, so prove to me if the light in the one direction didn't travel 6 time the c and when it is returning the speed wasn't 1/6? That is the reason for the Einstein famous paper where he said that is it impossible to measure the speed of light in one direction, because to do such a measurement you need some synchronization signal of the measuring units to travel faster than light. For that reason we use the same light as a signal and after the time measurement we're dividing the time/2 to say this is the time that the light needs to go or to return the distance. This is also a dogma in our measurement because we do not know something moving faster than the c. About the bending of light around the Sun there isn't Ether, but there are different layers in the Sun atmosphere and the limit between them creates the phenomenon of Refraction of the light which can bent and change even its speed. In diamond, the speed of the light is the half of c.

[bernardbotteron]

Great video, but it does not answer the title question: « why is light speed constant ? ». This fact is measured in experiments and confirmed by the resulting theory(ies) of relativity producing correct predictions with that assumption, but the basic question: (why? ) remains unanswered

[iiii_xuti]

Why is the speed of light is constant even when the wavelength and frequency have different values?

[hillaryclinton]

The speed of light decreases over time.... a Nobel has been awarded for this little nugget. If space expands, the distance traveled is less and the same redshift occurs....