# Light speed is not constant or accurate

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1. Mar 11, 2015

### Eric Ward

Hi all, this my first forum ever. I'm a self taught physicist. So pardon my ignorance. I pose this question. If speed :s = distance :d ÷ by time :t Then how can we possibly measure our cosmos accurately with a measure of speed aka "Light Speed". The problem I see with this is TIME is a variable is the speed equation that changes throughout the cosmos/space-time. For example. Time Dilation. Since we know that gravity effects light. As it pulls light into a black hole. If light were to move past a large object, let's say a star. Then gravitational time dilation would speed light up as it left the stars gravity as observed from the star. "Relative velocity time dilation" would also slows down light as it speeds past an observer. Now multiply that by the light traveling across billions of objects like stars, planets, black holes even by galaxies over millions of years. How accurate can our "light speed measuring stick" possibly be at that point. This also raises another question. If we want to go faster than light we must first understand what the actual speed is in a control with all gravity taken out of that equation. It also may be lights Achilles heel. A way for us to beat it. I obviously understand that we are susceptible to time dilation too so the whole faster than light debate will be for another day. Today I pose this question and a statement, if the fastest thing in the universe is not as fast as we think it is or even faster and we use it to measure the cosmos, our entire understanding of the size our universe will grow or shrink with it.

2. Mar 11, 2015

### nikkkom

As a start, have you read some introductory explanations such as

http://en.wikipedia.org/wiki/Speed_of_light ?

If you read it, please specify which specific part you dont understand or disagree with?

3. Mar 11, 2015

### Eric Ward

Thank you for your reply Nikkkom, I have read the wiki "speed of light" link. I'm disputing the "time" part of the equation. Light speed was calculated with time. Since time is malleable. It isn't constant. It speeds up and slows down per Time Dilation since it is a particle and a wave. So the distance it covers in a said time will be different if it gets close to a mass it should slow down as it moves past. Then as it moves from the center of gravity of the mass It will speed up relative to the observer on the mass. If light is susceptible to "time dilation". It's speed is not constant to an observer. Below is the wiki link to "time dilation" a proven theory nasa deals with every day.
http://en.m.wikipedia.org/wiki/Time_dilation

4. Mar 11, 2015

### Ophiolite

Eric, the central point is that the speed of light is constant. This has been very thoroughly validated. (Obviously we are talking about its speed in a vacuum.) Time dilation is something that effects the observer, not the light. Time dilation occurs because the speed of light is constant.

There are others here who can likely explain this much clearer than I can, should your doubts remain.

5. Mar 11, 2015

### ZapperZ

Staff Emeritus
This is awfully silly.

The phenomenon of "time dilation" is a DIRECT CONSEQUENCE of the postulates of Special Relativity, which explicitly includes Lorentz invariance, and the constancy of the speed of light. Somehow, you then turn around and use that consequence to show that speed of light is not a constant since time is "malleable"? You used something, which actually depended on c being a constant, to show that it isn't a constant? If c isn't a constant, you cannot use "time dilation", because it won't be correct either!

If c isn't a constant in all reference frame, your GPS will be horribly inaccurate!

There is a serious misunderstanding of SR here, and a very large hole in logic.

Zz.

6. Mar 11, 2015

### Eric Ward

Consider me enlightened! Thank you for your replies. Thank you for putting up with my ignorance. So your saying light is constant because the principals of time dilation are founded on light is a constant. Would that mean that both ideas are not accurate if they consider light as a constant. Keep in mind when we first measured light it was done on earth in earth's gravity. This should cause the speed to appear to be slower than it actually is to us the observers on earth. Or are you suggesting that a light is unaffected by gravity or time dilation? That's a real question. I really don't know. Thank you again. E

7. Mar 11, 2015

### jbriggs444

To be fair to OP, if it were true that the conclusion that "time is malleable" followed from the premise that the speed of light is constant and if it were also true that the speed of light is inconstant followed from the premise that "time is malleable" then one would have a proof by contradiction showing that the speed of light is inconstant.

In fact, however, the argument fails because "time is malleable" is a hand wave rather than something that SR actually says.

8. Mar 11, 2015

### jbriggs444

The speed of light measured locally is always c. Gravity neither speeds it up nor slows it down. [Nor even changes its direction. It is space-time itself that is curved. Light goes "straight" within curved space-time].

Time dilation, length contraction and the relativity of simultaneity conspire so that the speed of light is always measured to be the same regardless of your choice of inertial reference frame.

9. Mar 11, 2015

### Mentz114

If you measure the velocity of something in your lab - then that is the speed it is travelling. What does 'actually is' mean in the bolded part ?

10. Mar 11, 2015

### Staff: Mentor

I think you are looking for Shapiro delay: the time light needs to reach us from a planet behind the sun is a bit longer than it would be without the sun in between, for example. This effect is well-known, but it is negligible if we look at random astronomical objects - massive objects like our sun are incredibly rare, and we cannot measure distances of billions of light years better than a few percent anyway so a few years do not make a difference.

Another interesting case arises from different distances if light is curved due to nearby masses (gravitational lensing). This can lead to multiple images of the same object, including a time delay between those images. 14 months for the Twin Quasar, for example: whatever we see in the first image will be visible in the second image 14 months later.

The title was not a good choice.

11. Mar 11, 2015

### Eric Ward

"Actually is" means the speed light is traveling if it was being observed with no gravitational time dilation affecting it's observed speed. In a lab on earth or even in lower orbit it would be subject to gravitational time dilation. It would have to be observed away from all gravity to be as accurate. Which would be impossible because there will always be some type of micro gravity in space from something. Of course this is. This is all assuming the light is affected by gravity.

12. Mar 11, 2015

### Eric Ward

The title was on purpose. I wanted to get a larger group of people to view it. And get what you finally gave me. I'm familiar with lensing but I'm not with Shapiro Delay. If off to study that till the wee hours of the morning. Thank you! E

13. Mar 11, 2015

### Eric Ward

Copied from wiki on "Shapiro Delay". Thank you MFB. Wish I would have thought of it first.

Because, according to the general theory, the speed of a light wave depends on the strength of the gravitational potential along its path, these time delays should thereby be increased by almost 2x10−4 sec when the radar pulses pass near the sun. Such a change, equivalent to 60 km in distance, could now be measured over the required path length to within about 5 to 10% with presently obtainable equipment.

Calculating time delay..

In a near-static gravitational field of moderate strength (say, of stars and planets, but not one of a black hole or close binary system of neutron stars) the effect may be considered as a special case of gravitational time dilation.The measured elapsed time of a light signal in a gravitational field is longer than it would be without the field, and for moderate strength near-static fields the difference is directly proportional to the classical gravitational potential, precisely as given by standard gravitational time dilation formulas.

14. Mar 11, 2015

### ZapperZ

Staff Emeritus
I wish mfb didn't introduce this, because I can easily see how someone like you can completely misinterpret and misread such a thing as evidence that light changes its speed in such cases. This is where a little knowledge can lead to a wildly wrong idea!

The problem here is that you are trying to run when you can't even crawl. GR is extremely difficult to understand and the mathematics is daunting. SR is, actually, a much simpler idea with surprisingly simple mathematics! That is what you need to learn and understand FIRST! Diving into GR like this is like teaching a toddler how to climb a tree when he hasn't even learned how to stand up straight!

And no, as was mentioned, this was NOT a very good topic, regardless of your intention. There are better ways to get attention than throwing out a standard crackpottish title like this. Warren Siegel even included an entry in the same vein with regards to quacks. But besides that, there is a certain level of incredible disrespect here. One would think that with a lot of very intelligent physicists around, something as obvious as what you stated in the first post would have been caught already, if it were true! Seriously, how dumb do you think physicists are to not be able to see something that is right in their faces? But instead of taking a diplomatic approach and to first figure out if you had actually understood what you think you understood, you jumped right with such a proclamation that light speed is not accurate or constant!

There are tactful way to learn, if that is the intention. You don't insult the people that you wish to learn from.

Zz.

Last edited: Mar 11, 2015
15. Mar 11, 2015

### Eric Ward

My apologies Zapper Z. You teach me more than you know. I appreciate your disrespect. It is well deserved and does not offend me. I understand I am nothing, I can only hope one day to have the understanding that you've been afforded. I'm just a dumb human being on this planet curious about the world around me and hungry for answers. This was an attempt to understand if my idea had teeth and was at all relevant. I appreciate your reply and respect you immensely. Respectfully Eric

16. Mar 11, 2015

### Mentz114

When we say 'the speed of light is a constant' it means that anyone measuring this speed gets the same answer. So if the observer on earth did it, and another observer far away from matter did it - they both get 400.3462x1017 furlongs/fortnight, when they harmonise their units.

17. Mar 11, 2015

### Staff: Mentor

There is no way to measure this. If your lab on Earth is in a gravitational field, then you can measure the speed light is traveling in your lab, in the gravitational field, but there is no way to measure what the speed in your lab would have been if the field were not there. Even if you put your lab in a rocket and travel out into deep space, far away from all gravitating bodies, and measure the speed of light in your lab there, that still doesn't tell you what the speed would have been in the lab back on Earth if the Earth's field were not there.

As Mentz114 said, "the speed of light is constant" means everyone that measures it gets the same answer, no matter where they are. Strictly speaking, we should say that everybody in a local inertial frame gets the same answer--i.e., strictly speaking, your lab should be in free fall--for example, do the measurement in orbit, say on the International Space Station.

18. Mar 11, 2015

### Eric Ward

Oh and I don't think that light slows down i
Thank you, Mentz114 and Peter Donis. Great answers. So I'm getting the answer that Light does not slow down or speed up when it encounters gravity. Per Time dilation. As observed by an observer not traveling with the light. Then why does time dilation not affect light? If light is both a particle and a wave? I understand how it's not affected as a wave but I don't understand why it does not affect a light partical. Especially since light is supposed to be trapped beyond he event horizon (or the new apparent horizon) in a black hole.

19. Mar 11, 2015

### Eric Ward

I'm speaking of course in the eyes of the observer.

20. Mar 11, 2015

### Staff: Mentor

It shows properties of both, but it is neither.
Asking for time dilation does not make sense for objects at the speed of light.

21. Mar 11, 2015

### Eric Ward

Thank you mfb.

22. Mar 11, 2015

### phinds

Time dilation doesn't affect anything. Time dilation is something observed by a remote observer. You, for example, right now as you are reading this are MASSIVELY time dilated according to an accelerated particle at CERN. Do you feel any effect?

23. Mar 11, 2015

### Zentrails

Great pun relativistically speaking, but just to make sure the OP doesn't get confused I think you meant a particle traveling close to the speed of light rather than accelerated.

24. Mar 11, 2015

### Eric Ward

No, but if the particle at CERN was timing how long it took me to blink an eye and used that to measure its universe it would experience a slowed down timing of my blink due to its high speed. .... I understand light speed is constant. I just want to know if we are observing the correct light speed on earth. Great analogy btw!

25. Mar 11, 2015

### phinds

Traveling close to the speed of light relative to what? I meant accelerated relative to the Earth, which seems fairly clear since I specified CERN, and yes, that is "traveling close to the speed of light" but specifically relative to the Earth.

EDIT: Also, how do you get "pun" out of this? I must be missing something, since I see nothing at all about it that seems like a pun.