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Musings on Time Travel


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44 minutes ago, Kev M said:

So if m=0 then E=0

Therefore photons have no energy.....that cant be right.... or can it ?

Actual formula goes like this:

image.png.d8a4fc39ab577e28a58ba3a5feb84bce.png

Where m0 is rest mass

For photon with no rest mass above turns into

E = pc

Or other known form of that equation:

image.png.863ea52d6c94ef2add9642c1b3a8b445.png

(energy of photon depends on its frequency / wavelength).

In case particle is at rest, so momentum is 0, above equation simplifies to well known E=mc2

https://en.wikipedia.org/wiki/Energy–momentum_relation

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1 hour ago, Kev M said:

So if m=0 then E=0

Therefore photons have no energy.....that cant be right.... or can it ?

As you  say, that's not a correct interpretation as it only relates the Rest Mass and the energy equivalent 

The full relation has to include the contribution from elements with Momentum within the system, which includes the photons of light.

As outlined here, the full equation should be

image.png.593efce388c21cc35c9542b6eb5715df.png

meaning that for photons, the appropriate relation is E= 𝑝c, where 𝑝= the momentum of the photon = ℎ 𝜆 

Edited by Gfamily
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In my earlier comment I was careful to say that fundamental particles sometimes act as if they are point particles (e.g as in the photoelectric effect) and sometimes as waves (e.g. as in diffraction). We don't really understand what a particle is. They must be some sort of stable state of space-time ultimately.

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1 hour ago, vlaiv said:

Actual formula goes like this:

image.png.d8a4fc39ab577e28a58ba3a5feb84bce.png

Where m0 is rest mass

For photon with no rest mass above turns into

E = pc

Good explanation, that shows me where I misunderstood a bit....

 

However still have an issue with Photons, if they have energy and energy is a function of time.

The they must travel in time.....surely ?

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28 minutes ago, Kev M said:

Good explanation, that shows me where I misunderstood a bit....

 

However still have an issue with Photons, if they have energy and energy is a function of time.

The they must travel in time.....surely ?

Not sure energy is a function of time.  But photons certainly do travel in time as said earlier it is a matter of relative frames of reference. Photons will take approximately 11 minutes in our frame of reference to travel from the surface of the sun to Earth.  I'm not sure why you think they do not travel with a passage of time. 

Jim 

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21 minutes ago, Kev M said:

energy is a function of time.

Can you expand a bit on that please?

In most of the physics - energy is conserved so it can't be function of time.

Energy is tied to the time in following ways:

- conservation laws stem from different symmetries, and energy conservation stems from time symmetry (which just means that you can't start experiment at any given t0 - you can assign any number as start time and that won't change result of experiment).

- energy-time is uncertainty pair. You can't short lived system with well defined energy

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The more certain relationship is mass and time. The greater the mass, the slower time flows. The time we perceive is different to what you might experience in other parts of the universe.

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Just now, saac said:

I'm not sure why you think they do not travel with a passage of time. 

Well - there is this thought experiment - let's call it "If one could ride on a photon"  :D and it goes something like this: photon is traveling at the speed of light - so the rest of universe is traveling at the speed of light with respect to that photon in opposite direction.

Given that we have dilation of time for systems traveling very fast and when system is traveling at the speed of light - time slows down to a stop. Photon "sees" the rest of universe being effectively frozen in time. Whatever time "passes" in reference frame of that photon - it is irrelevant as nothing else could be used to time it - as everything else is at stand still - that leads to conclusion that no "effective" time flows in reference frame of a photon.

Extrapolation of this reasoning is that photon is just "energy transfer cord" between two space time coordinates - when particles exchange a photon they effectively "touch" across space and time (does this count as time travel? :D ).

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Just now, Mr Spock said:

The more certain relationship is mass and time. The greater the mass, the slower time flows. The time we perceive is different to what you might experience in other parts of the universe.

Only if you are away from that mass and looking at the clock next to that mass.

If you were standing on surface of neutron star - your clock would tick rather normally to you - it would be the rest of universe that is "sped up".

By the way - there is very interesting thing that this leads to - and that is:

falling into black hole will make all universe until its end, "play out" in front of your eyes. Similarly - for observer looking at you falling into the black hole - you'll sort of stay frozen close to event horizon slowly fading out of view (red shift).

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6 minutes ago, vlaiv said:

Well - there is this thought experiment - let's call it "If one could ride on a photon"  :D and it goes something like this: photon is traveling at the speed of light - so the rest of universe is traveling at the speed of light with respect to that photon in opposite direction.

Given that we have dilation of time for systems traveling very fast and when system is traveling at the speed of light - time slows down to a stop. Photon "sees" the rest of universe being effectively frozen in time. Whatever time "passes" in reference frame of that photon - it is irrelevant as nothing else could be used to time it - as everything else is at stand still - that leads to conclusion that no "effective" time flows in reference frame of a photon.

Extrapolation of this reasoning is that photon is just "energy transfer cord" between two space time coordinates - when particles exchange a photon they effectively "touch" across space and time (does this count as time travel? :D ).

Agreed not in the photon's frame of refence, but there are many others. If the photon does have an opinion to experience anything, which it does not, then we certainly do  and time it does experience :) 

Jim 

Edited by saac
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All this talk of of the photon galivanting around the universe isolated from time, while playful and poetic, is rather mute. Wherever we plant our feet to measure the photon's speed we will never do so in it's frame of reference.  We will experience and measure a finite time in its passage.  Physics is replete with these annoying hooks which catch popular imagination and quite often lead to wrong conclusions. 

Jim 

Edited by saac
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Nice easy read article on the hypothetical nature of time as a quantised property. Highly theoretical but I like the idea of discrete packets of time - argh there goes all that poetic interjection again :) 

https://www.scientificamerican.com/article/is-time-quantized-in-othe/#:~:text=1) There is no conclusive,and perhaps time as well.

Jim 

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8 hours ago, vlaiv said:

Can you expand a bit on that please?

In most of the physics - energy is conserved so it can't be function of time.

 

 

image.png.d8a4fc39ab577e28a58ba3a5feb84bce.png

 

c is a function of time , therefore E must be ?

 

 

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8 hours ago, vlaiv said:

Well - there is this thought experiment - let's call it "If one could ride on a photon"  :D and it goes something like this: photon is traveling at the speed of light - so the rest of universe is traveling at the speed of light with respect to that photon in opposite direction.

Given that we have dilation of time for systems traveling very fast and when system is traveling at the speed of light - time slows down to a stop. Photon "sees" the rest of universe being effectively frozen in time. Whatever time "passes" in reference frame of that photon - it is irrelevant as nothing else could be used to time it - as everything else is at stand still - that leads to conclusion that no "effective" time flows in reference frame of a photon.

Extrapolation of this reasoning is that photon is just "energy transfer cord" between two space time coordinates - when particles exchange a photon they effectively "touch" across space and time (does this count as time travel? :D ).

That sounds like interesting reasoning, no "effective time" as no means of measuring , I am happier with that explanation than no time at all.

"touch"....👍

 

 

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14 hours ago, Kev M said:

Indeed if the universe was infinite, then not only is everything possible but everything actually happens....an infinite amount of time each.

Obviously the universe cannot be infinite.

I think infinity is being imbued with some excessive powers in this thread.

Firstly, we don't know whether or not the universe is infinite in extent or not, and we may never know.

Secondly, the meaning of those words can be fluid to the extent that the statement becomes meaningless. Historically, "Universe" has meant different things, and even now can be interpreted in radically different ways (check out Max Tegmark). Some of this is speculative physics, some is metaphysics.

Thirdly, it's clearly the case that, even in an infinite universe, it's not true that everything must happen somewhere. Specifically, events that contradict the laws of physics cannot occur. Yes, you can posit alternate realms in which the laws of physics are different, but I would say there are fundamentals of logic/maths/internal consistency that can't be broken, whatever the physical environment.

Fourthly, even where an event or configuration is not expressly forbidden by natural laws, I would argue that it is still not the case that it must obtain somewhere in an infinite universe. I think I differ from Tegmark in this, but to me it seems obvious from considering maths, which has many examples. Does the decimal expansion of pi contain a sequence of a million consecutive "1"s? You could argue that it must do so somewhere, because the expansion is infinite, non-repeating and, so far as we know, "random" (a whole other can of worms there, but I mean that there are no statistical influences that force it into any kind of pattern). But it's perfectly possible that the expansion may contain no such sequence. The best that we may be able to state is that the probability of such a sequence occurring is vanishingly small. There's a whole realist/intuitionist argument hiding behind this; the latter group would argue that the question is literally meaningless because "the infinite decimal expansion of pi" does not exist as a completed entity and so cannot be the subject of that kind of discussion. The same kind of (positivist) attitude can be deployed against questions about the physical universe: must it be the case that, somewhere in an infinite universe, there exists a planet where, say, tribal life forms have, by chance, evolved to reproduce the Earth's capitalist/communist struggle, complete with their own word-for-word copy of the U.S. Constitution (Star Trek: "The Omega Glory")? The positivist might say that it's not even a valid question.

 

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I had read somewhere that one way of describing things is that everything travels through space-time at the same overall speed and so the more of that total speed that is used to travel through space the less is left over to travel through time and vice versa. So on earth where everything moves so slowly through space we therefore travel at high speed through time.

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13 hours ago, saac said:

All this talk of of the photon galivanting around the universe isolated from time, while playful and poetic, is rather mute. Wherever we plant our feet to measure the photon's speed we will never do so in it's frame of reference.  We will experience and measure a finite time in it's passage.  Physics is replete with these annoying hooks which catch popular imagination and quite often lead to wrong conclusions. 

Jim 

The whole point of relativity is that the speed of light is constant, irrespective of the frame of reference - the frame of reference quite simply doesn't matter. You can argue how accurately we can actually measure the speed of light but the simple concept that it is constant, and independent of the frame of reference, is all you need to derive special relativity. 

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1 hour ago, Paz said:

I had read somewhere that one way of describing things is that everything travels through space-time at the same overall speed and so the more of that total speed that is used to travel through space the less is left over to travel through time and vice versa. So on earth where everything moves so slowly through space we therefore travel at high speed through time.

While technically correct - it is not complete picture and can lead to misconceptions.

There is indeed 4-vector and it "rotates" (not quite the same as 4d euclidean that we could use sphere and unitary vector pointing in some direction) to align with certain directions / axis, but this line of thinking sort of implies that space-time of the background is "fixed" - much like fixed coordinate system - but it is not - every reference frame has it's own "interpretation" of things.

If I'm moving with respect to you at significant speed compared to speed of light - you will say that my vector is more aligned with "space" and that my time goes more slowly - but in my frame of reference - my direction will be completely in direction of time as I'll be stationary with respect to it - nothing will slow down for me - time will flow normally.

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6 hours ago, Paz said:

I had read somewhere that one way of describing things is that everything travels through space-time at the same overall speed and so the more of that total speed that is used to travel through space the less is left over to travel through time and vice versa. So on earth where everything moves so slowly through space we therefore travel at high speed through time.

Yes - when "at rest" (whatever that means), you are travelling through time at the speed of light.  And a photon moving through space at c is not moving through time at all.

Mass bends or dilates time; bent time causes gravity.

There's a lot on Youtube about this.  Difficult, counterintuitive, fascinating.  Just wish I understood it!  🤔

Doug.

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7 hours ago, iantaylor2uk said:

The whole point of relativity is that the speed of light is constant, irrespective of the frame of reference - the frame of reference quite simply doesn't matter. You can argue how accurately we can actually measure the speed of light but the simple concept that it is constant, and independent of the frame of reference, is all you need to derive special relativity. 

Well not quite Ian, the fixed speed of light is only half of the story.  Special relativity, more specifically time dilation is a consequence in part of the speed of light being fixed.  The relative nature of time is dependant not just on a fixed speed for light but also on the invariance of the laws of motion (Galilean Invariance) . Einstein referred to these two assumptions as his postulates upon which special relativity was derived. In short, once you accept that the speed of light is fixed and that Galilean Invariance holds true for observers in all  non accelerating (inertial) frames of reference, then it leads  to the inexorable conclusion (I love that word) that time is not absolute. Surprisingly the maths being no more complicated than elementary algebra.  So the frame of reference, one where the event takes place (our eponymous photon) and the observer (generally us) is everything and certainly do matter. Hence my previous point about the gallivanting photon and the misleading  and oft quoted statement that "it experiences no time".  In matters of the feasibility of time travel, accepting that means us time travelling, then what the photon experience is entirely relevant.  As far as we are concerned the photon certainly does takes it's time to traverse the galaxy.  Now of course all of that disappears if we are to imagine some way for us to take a ride on a photon as per Einstein's thought experiment then we would be isolated from time. That of course is impossible. 

Jim

Edited by saac
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Ok back to "musings" - braking laws of physics and symmetry not only possible but compulsory :)  For the moment I'm going to accept  the premise that time is somehow  linked to the state of all other particles, energy and fields in the universe. I don't think it is the whole story but for the moment it will suffice as a musing as per the OP.  So as discussed a few posts back, measuring, recording and reordering the state of the entire Universe is not something that we are going to do anytime soon.  So what if it would be good enough to make the task smaller , isolate a region of space from the rest of the universe - could we simplify this seemingly impossible task with a view of effecting time travel.  Here comes the plot of my first science fiction novel then :)   First we commission a space ship, the SS Physicus Erroneous, crewed by the inquisitive,  confused, and clouded over for another night members of SGL. Pointing our spaceship in the direction of Sagittarius A* we punch it  as per the Starship Enterprise.  Arriving at the event horizon of the super massive black hole we take one step further . Turning to face the view screen we see the whole of the future time line of the universe play out; , bummer  what do you know we had a clear sky two nights in a row over the entire UK next week!  We now arrive at the singularity, a place isolated from the rest of the universe (no naked singularity theory here).  This then is the place where we construct our time chamber, isolated from the rest of the universe. In our chamber we are able to measure and reorder the state of the few hundred particles which remain (virtual particles will look after themselves) . So here in this chamber, on the edge of a singularity, isolated from the rest of the universe we at last can make time dance to our will.  Just before we are ripped apart from existence we marvel in smug satisfaction that we reversed time for 20 minutes, the length of time the chamber has been in existence and the limit of our reordering of the particles therein :)   Well, 20 minutes of time travelling is better than nothing at all.  Ok so I suppose my novel owes more to Red Dwarf than Space Odyssey but how do we go about crowd funding the SS Physicus Erroneous ;) 

Jim

Edited by saac
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We are all travelling through time, most of us at the same speed. ISS crew travel a little faster through time than the rest of us. So time travel (in a forward direction) is an everyday occurrence, if you count a few nanoseconds of forward travel by astronauts 😀

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