Question related to speed of light. (1 Viewer)

Rabbie said:

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

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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 said:

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.

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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.

However it does take time to get from one place to another. For example, it would take light 3 seconds to go around the earth.

Now, imagine the train, the Flying Scotsman, famous for traveling at 100 miles an hour (mph). This means in one hour it would be 100 miles away.

Now light travels at 670,616,629 miles per hour (mph) so in one hour it would be 670,616,629 miles away.

Now imagine writing this number down for how far light travel in a day, a week, a month, a year the numbers would be large, not a practical way of referring to lightspeed.

This impracticality of the large figures has led to the convention of referring to lightspeed as how far it travels in one year in miles this would be:- 5,878,499,817 and Convention dictates you refer to this as one light year.

As you can see it would take you 1 year to travel this distance and if the distance was 100 light years then it would take you 100 years to travel that distance.

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Uncle Gizmo said:

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.

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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.

Mike375 said:

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.

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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

Mike375 said:

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.

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This is how I understand it.

Rabbie said:

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

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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

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 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.

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Gravitational waves may reveal stringy Universe
http://arstechnica.com/science/2016/05/shining-gravitational-waves-on-strings/

So we now have a completely independent way to confirm that the universe is expanding.


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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...

The_Doc_Man said:

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...

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No I think you made a good point, it is clear to me that there is much we don't know and understand. We think we understand, but I'm pretty sure there's a lot more to learn. What we take for granted, what we take as gospel these days, people will look back and be amused at our naivety. Exactly the same as when we look back at Great Men like Isaac Newton and Copernicus. I don't detract from these great men, their minds I imagine were as good as the best Minds we have today. Our "apple falling"; our "where the Earth should be in relation to everything else" is... What is the universe? Where does it end? Did it begin? Will it collapse and reform? Is there anything really there. or is it just a hologram? So to play with ideas to propose things which are counterintuitive, against the grain, that skill is very important. You only have to look at the way Einstein formulated his ideas.. He played games in his head is imagination led him to his discoveries and even then one of his discoveries, I think he called it "spooky interaction at a distance". He just didn't believe it! He changed some of his equations to get rid of it. There's a lot of that in the science at the moment, there is a lot of stuff which you are unable to grasp. We need another Einstein, we need some to look at things differently, a new young mind.

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.

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!).

kevlray said:

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.

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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. Different frequencies of light take a slightly different route through this structure so there is a slight difference in the speed they travel through empty space. Now whether this means that the actual speed of light is fixed and is then affected by empty space or not I couldn't even begin to guess!

Uncle Gizmo said:

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.

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

Galaxiom said:

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

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No, it's a relatively New Idea probably why you haven't heard of it. I wasn't sure myself actually!

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

From which I have taken this extract:-
Previously, two different research groups had come up with the idea the speed of light might change, primarily because what we think of as a vacuum isn’t really empty. 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. 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.

Uncle Gizmo said:

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.

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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.

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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.

Uncle Gizmo said:

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.

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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.

Mike375 said:

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.

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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.

Frothingslosh said:

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

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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.

Galaxiom said:

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.

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That is what I was trying to illustrate.