Question related to speed of light.

[Mike375]

If you travel at the speed of light it will take you 100 years to reach a planet 100 light years away. That's how long it takes a photon from that star to reach us

I am under the impression that from the perspective of earth and the distant planet it will take 100 years but for our intrepid traveller he will arrive at the distant planet instantly, assuming he has jumped on a light beam and there is 0 time getting to the speed of light.

 

[Mike375]

I think you are under the impression that light gets from one place to the other instantaneously.

That is how it appears to us here on earth, and as far as we are concerned for everyday thinking it is instantaneous.

Instantly from the perspective of someone riding the light beam.

If I could start off at the speed of light and start my clock then when I arrived at a star 100 light years away my clock would show zero time has passed. But from the perspective of earth and the planet 100 light years away then 100 years has passed.

Thus if I loop around that distant planet and back to earth my clock will still show zero but 200 years will have passed on earth.

 

[Rabbie]

I am under the impression that from the perspective of earth and the distant planet it will take 100 years but for our intrepid traveller he will arrive at the distant planet instantly, assuming he has jumped on a light beam and there is 0 time getting to the speed of light.

Not sure where you get this impression from. If you have a reference it could be helpful if you post it. It does not seem intuitively obvious to me that your supposition is correct so further elucidation would be helpful

 

[speakers_86]

Instantly from the perspective of someone riding the light beam.

If could start off at the speed of light and start my clock then when I arrived at a star 100 light years away my clock would show zero time has passed. But from the perspective of earth and the planet 100 light years away then 100 years has passed.

Thus if I loop around that distant planet and back to earth my clock will still show zero but 200 years will have passed on earth.

This is how I understand it.

 

[Mike375]

Not sure where you get this impression from. If you have a reference it could be helpful if you post it. It does not seem intuitively obvious to me that your supposition is correct so further elucidation would be helpful

Try this one.


https://www.physicsforums.com/threads/does-time-essentially-stop-if-you-could-travel-a-c.51790/

Or enter in Google....

does time stop if you travel at the speed of light

 

[The_Doc_Man]

As a thought experiment, maybe time stops for a photon... maybe not. (See later in this post for my take on the answer.)

In the real world, time stoppage can't happen for persons because persons have non-zero rest mass. Persons can NEVER travel at the speed of light in an unwarped space-time region. Only an object with zero rest mass (e.g. a photon) can do so. Even a thought experiment needs to include some aspect of real-world properties - otherwise it is merely science fiction.

In order for your thought experiment to have meaning, you must include some sort of warping of space-time to allow you to violate the implied infinity of energy required to bring a real object to the speed of light - because in your E=mc^2 equation, dilation ALSO affects mass. As speed increases, time-flow decreases but effective mass INCREASES. This also begs the question of whether the speed you actually reach would be the speed of light in that warped space-time volume of space-time.

Since we are dealing with non-zero rest mass for the object doing the traveling, our thought experiment must take into account that current theory says that object can't reach the speed of light. Therefore, time would not stop, it would just dilate a lot.

Now, here is the 64$ question - does time flow for a photon? My counter-question is, how would we tell?

Well, the real difficulty in answering this question is that time doesn't exist independently. It is a measure of change, a concept of mutability. Movement is a form of change and we measure time based on movement (sometimes, as e.g. a light-year's true meaning). Vibration is a form of change (repetitive though it might be) and we use atomic clocks to measure time based on vibration. Radioactive decay is a statistical process that can be used to express time in terms of half-life periods. Chemical processes reach natural equilibria based on time and reaction probability. I.e. a reaction with a lot of energy to "spur it on" occurs quickly. Low-energy reactions are slower. That ticking object on the mantle or desk measures time as a function of the conversion rate of potential energy (in a wound-up spring) to kinetic energy (in the mechanical clock's escapement gears).

In vacuo, a photon does not appear to change. That is, photons from close stars and photons from distant stars seem to have the same properties even though our understanding of light says that the light from the closer stars is younger (by a lot) than the light from the farther stars. If there were no apparent changes in the nature of the photons, we would have to say that we have no basis for any assumption of time flow in those photons.

You know of course that I can't leave it there. This line of thinking opens up a REAL can of worms. Since this is the Watercooler, let's have a typically wild digression. Suppose that light really WAS somehow changing by interacting with time... what would such change look like?

According to our theory of light interacting with something, it can only do so by losing or gaining energy because it has no rest mass to be changed by the interaction. But if a photon's energy changes, that light's corresponding frequency has to change as well. The laws of thermodynamics suggest that in vacuo, if light were to change somehow, it could only lose energy, never gain it, because you can't create energy from nothing. Note that adjacent pairwise particle creation is symmetric - one particle and one anti-particle, so there is balance. Photons ON THE AVERAGE would not gain energy in the presence of spontaneously generated particle pairs. More precisely, whatever one photon gained, another photon could lose.

So... here's the question to be thrown into the mix. Let us suppose for this argument that light can decay over time. It means that it would lose energy and would therefore undergo a shift towards redder colors (and towards infra-red as it gets further.) So... do you want to propose to an astronomer that Ed Hubble was wrong about the meaning of the red-shift experiment?

The stars aren't moving apart at all... it is that LIGHT is decaying and in so doing is red-shifting based solely on distance traveled.

 

[The_Doc_Man]

Aw, c'mon, Uncle! I guess it is hard to detect that my earlier response was typed at least in part with my tongue deeply embedded in my cheek, cheeky bugger that I am...

 

[kevlray]

It is interesting that there is an article on the internet questioning whether or not some of our constants (speed of light) are really constant.

 

[speakers_86]

That's what is great about theoretical physics. Nothing is really set in stone! There is a theory called the variable speed of light (VSL) that says the speed of light varies over great periods of time (too large to really measure directly). It doesn't have much support, but if correct, it would negate the need for inflation. That is what I like about the theory, but inflation seems to be settled (until it's not of course!).

 

[Galaxiom]

I believe the speed of light does vary through empty space. It's something to do with the route the light takes between the structure of space itself.

Sounds a bit too much like the defunct theory of Luminiferous Aether.

 

[Galaxiom]

They discussed the possibility that space is actually "a great big soup of virtual particles that wink in and out of existence in tiny fractions of a second," reports Livescience.

Nothing new there. That is from The Standard Model.

Those winking particles would impede light and cause fluctuations. But those two papers were theoretical. The new paper actually includes observations that show some photons were slowed.

Similarly, photons certainly would be slowed if they interacted with virtual particles. I guess over a very long distance a photon could hit enough to make a small difference. Clever that they could measure it. Will have a read tonight but it doesn't sound like anything fundamentally new.

Interacting with vps is quite different from negotiating a path through the structure of space.

 

[Mike375]

Those winking particles would impede light and cause fluctuations. But those two papers were theoretical. The new paper actually includes observations that show some photons were slowed.

Does that really change the speed of light?

For example let's say a car has a top speed of 100 mph and so on a straight road 100 miles long it will take one hour for the trip. However, if every few minutes the driver gives the brakes a quick jab then obviously the trip takes more than one hour.....but the top speed of the car is still 100 mph.

 

[Frothingslosh]

Does that really change the speed of light?

For example let's say a car has a top speed of 100 mph and so on a straight road 100 miles long it will take one hour for the trip. However, if every few minutes the driver gives the brakes a quick jab then obviously the trip takes more than one hour.....but the top speed of the car is still 100 mph.

Light has been empirically tested and proven to change speed depending on the density of the medium through which it moves. Hell, that's a large part of why a prism works the way it does.

So yes, light travelling through any sort of medium more dense than vacuum really will slow down.

 

[Galaxiom]

So yes, light travelling through any sort of medium more dense than vacuum really will slow down.

This does not change the fundamental value of the speed of light constant (c). The slowing is the delay caused by absorption and reemission of particles in the medium.

 

[Mike375]

This does not change the fundamental value of the speed of light constant (c). The slowing is the delay caused by absorption and reemission of particles in the medium.

That is what I was trying to illustrate.

 

[Galaxiom]

I eventually found this article:-
http://www.smithsonianmag.com/smart-news/speed-light-can-vary-180953949/

What a rubbish article.

 

[The_Doc_Man]

After a quick peek at the Smithsonian article, I have to agree with Galaxiom. While there might be something there, it has been so heavily glossed over that it has no apparent scientific value.

The critical experiment of sending two beams through two paths is where we run into the scientific weakness. If the fiber is a constant consistency throughout its length, then the index of refraction of the medium will have a highly predictable effect on the time it takes light to traverse the fiber. The other path mentioned therein has no firm description but says it does something to the light's structure and then undoes it. The question is, what it does and how it does it.

I have a further quibble with their use of "structure of light." A beam of light has a shape because it is the sum of many individual sub-beams (on down to the photon level OR to the energy-packet wave-front level, pick your poison). But when you break it down to the lowest-level interactions, OF COURSE they take time.

If you are going to change things in your light beam in some way, that change implies interactions, which in turn imply some sort of electron-cloud involvement. The interaction is usually that something gets absorbed and re-emitted. Without this kind of interaction, the light components wouldn't react at all. The trick is that there is a configuration change in the atoms that absorbed the light and a corresponding change when the light gets emitted again. This configuration change is not instantaneous. We have two words, fluorescence and phosphorescence, to describe what is essentially a continuum of speeds of re-emission. Phosphorescence is the "slow" interaction, fluorescence is faster. But NEITHER of them are actually instantaneous. So if you have an interaction to change the light, OF COURSE it takes longer to finish the path. But did that actually change the structure of the light, or of the light beam that is the aggregate of the photons / wave fronts?

For that reason, I consider the article a bit weak. Or maybe more than a bit.

 

[Frothingslosh]

This does not change the fundamental value of the speed of light constant (c). The slowing is the delay caused by absorption and reemission of particles in the medium.

I neither said nor implied that it did. I simply pointed out to Mike that light DOES slow down based on medium.

 

[Galaxiom]

I neither said nor implied that it did. I simply pointed out to Mike that light DOES slow down based on medium.

The discussion was about "The Speed of Light", meaning "c", the speed of light in a vacuum.

In fact it is the only speed that photons travel. Any other measured speed of light is corrupted by the time between absorption and reemission when the photon hits something.

To improve on Mike's car analogy, lower speeds are measured if the driver pulls over between stints of driving at 100 mph.

 

[Frothingslosh]

Discussions often involve tangents, and if you'll look at the post directly above mine, you'll note he asked if a denser medium actually changes the speed of light.

That is why c is specifically defined as 'speed of light in a vacuum', not 'speed of light'.

So again, nothing in my response either stated or implied that the value of c was changeable in the slightest; I simply confirmed that light does change speeds based on medium, and that if light really did encounter a region of space with appreciable density, that yes, the effective speed at which that light traveled really would decrease.