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Moonshane

Book - why does E=mc2 and a question about light speed

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I am currently reading the Cox/Forshaw book of the above title (well the first bit anyway) and am currently working through time, space and light speed. I understand the maths of how the following statement works but cannot understand it in a practical sense.

The book states that if we could travel a a fraction under the SOL (something like 99.9999%) then we could travel to the Andromeda galaxy (2.5m light years away) in about 50 years due to the effect of light speed travel effectively 'reducing the distance traveled' as time slows down for the person traveling at light speed.

On the basis of the content of the book, this would be the same for a particle, e.g. a photon.

Therefore at the speed of light (which a photon will clearly travel at in space) it will take 50 years for that photon to reach our eye and that we will see the galaxy as it was 2.5m light years ago. BUT what I don't 'get' is that assuming the galaxy was formed 50 years ago, and emitted photons then surely they would arrived in our eye today if it only takes......

actually, writing this down, I have just realised what I think the answer is. the 50 year journey is obviously 50 years as far as the photon is 'concerned' but it's still 2.5m years to us as we are effectively stationary as regards the photon?? have I understood this correctly?

It's funny how writing something down sometimes helps to understand something - assuming I have!

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Well not quite. From the photons point of view it takes it no time to get to us from the galaxy as the proper distance from the photons perspective is zero. For a traveler at near the speed of light the proper distance is finite but much smaller than the 2.5m lyr as measured by a stationary observer. From the travelers perspective it would take the 50 years due to both time dilation and length contraction.

Not sure this will help but hope it does. Andrew

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Thanks Andrew it really does. So if traveling at light speed time effectively stands still? That had not quite clicked with me. What an incredible concept this is and all on the back of a proverbial fag packet.

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Subjective time, the time you feel, does not change one bit, of course (as long as you are not accelerating!). It's only when you compare clocks with people with different motion histories that the relativistic effects show up. But yes, if you want to live longer than others, start running rings around them.

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A good example of how this is used now is the detection of Muons. If we could see them they would disappear shortly after coming through our atmosphere. But they travel at extremely quick speeds so they do actually make it to the earths surface and we can detect them.

Sion

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Travel as close as you like to the speed of light and Special Relativity will predict your temporal and spatial perceptions. Travel at the speed of light and SR is no good to you. A broader theory is required. A theory which doesn't spit back a division by zero error in yer face.

There is also an inherent danger in trying to pair-wise compare brethren of the quantum scale with non-quantum scale objects.

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This is where I think the limits of mathematical modelling can shape our beliefs about things. We have this nice mathematical model of SR which only really works as close as you like to the speed of light but then throws a division by zero error at the speed of light as the previous poster said.

Although science works by making mathematical models and testing them with experiments and gaining conidence with the models such that you can use them to predict things, I think on a cosmic scale we don't really know what happens at the boundary cases except for what the models can predict. So I tend to take these predictions with a pinch of salt because in all probability, if we could measure the boundary cases we'd maybe find discrepanies with the models.

So results like, it taking a photon zero time to reach us (from the photon's reference frame) dont make a lot of sense to me and I wonder if these come from boundary cases and are just predictions that might one day turn out to be incorrect. Or maybe I should go read the book the OP refers to...sounds interesting.

Savid

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For an example of time distortion we have a modern practical example in the ubiquitous Sat Nav system we now use. The satellite clocks are all in synch but offset from ground time to allow for time slowing due to their orbital velocity and speeding up tp compensate for the reduction in gravity due to their orbit.

Incidently I didn't find Cox/Forshaws clock analogy on the question of electron/photon positions that illuminating. How did others find it?

Edited by astrofox

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I thought it was o.k, not the best i hve come across (think Hawkins has explained it more elegantly). Annoying thing when i was reading that book was that i got 3/4 through it then heard the discovery of neutrinos travelling faster than light and thought no point readin rest of that then. I havent got round to picking it up again yet.

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I really enjoyed this book, made several concepts a lot clearer. Also the follow up book.

I recently went to a lecture by Jeff, and after some high expectations was rather disappointed. OK - it wasn't designed for the public, but it didn't have the clarity of his books. He's clearly intelligent but perhaps not the communicator that Brian is.

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I thought it was o.k, not the best i hve come across (think Hawkins has explained it more elegantly). Annoying thing when i was reading that book was that i got 3/4 through it then heard the discovery of neutrinos travelling faster than light and thought no point readin rest of that then. I havent got round to picking it up again yet.

The faster than light neutrinos were subsequently found to be caused by mistakes in the equipment used. Once corrected, the speed of neutrinos was found to be consistent with the speed of light.

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The faster than light neutrinos were subsequently found to be caused by mistakes in the equipment used. Once corrected, the speed of neutrinos was found to be consistent with the speed of light.

Yes, shame that.

I was hoping that that result would be reproduced, allowing the SoL limit to be removed and a host of new theories to move us beyond that and into the star trek 'verse....

Alas it wasn't to be, not yet anyway...

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Yes, shame that.

I was hoping that that result would be reproduced, allowing the SoL limit to be removed and a host of new theories to move us beyond that and into the star trek 'verse....

Alas it wasn't to be, not yet anyway...

Ah, well I just finished reading a book called Faster Than The Speed of Light, which is written by a cosmologist proposing that the speed of light is in fact not constant, or at least was a lot faster in the very early universe. This paradigm is called the Varying Speed of Light (VSL). He explains the ideas and where they lead to in terms of current theories. It's a pretty good read and not particularly technical if you're up for reading some different viewpoints on things. I downloaded it from the Apple online book store.

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