## Can someone help me explain a speed of light question to a f

Discussions on classical and modern physics, quantum mechanics, particle physics, thermodynamics, general and special relativity, etc.

### Can someone help me explain a speed of light question to a f

I had a debate with a friend of mine regarding the speed of light, my opinion was that in the real world it would be impossible or at least extrememly unlikely for us (being people existing of mass) to move faster than the speed of light, no matter what technologic advances may be made in the future, this went further to me saying that lets imagine a world in which I could travel faster than the speed of light it would be possible for me to move from point (a) to point (b) and be seen in 2 positions at once by an observer my friend disagreed with this, obviously this is dependent on distances speed and time so for example if I moved from being 10 light years away to a distance of 2 light years away and that journey took me 8 light years then 10 years after I set off the observer would be able to see me at both my starting point and end point, simple maths and physics right! however now knowing he's lost, his argument is that because it can't be done then I can't prove this to be right could someone please help me find a way to explain that you can still use our laws of physics and maths to still prove what would happen in a theoretical world, obviously there is a huge part of science which is considered fact just because we KNOW what would happen if certain factors are met and that is what I need explaining to my friend.
jamessmckie
Forum Neophyte

Posts: 5
Joined: 20 Jan 2019

### Re: Tripping the Light Fantastic

Welcome JS,

Based on Relativity, you are correct that no object with rest mass can achieve universal speed limit c. To do so would imply the object had gained infinite energy (also seen as relativistic mass). Nevertheless, very low mass objects can be accelerated quite close to c. For example, the LHC in CERN, accelerates protons to "0.999999990 c".

You're wrestling with time dilation and length contraction as so many do, partly because opposite words are used to describe the same mathematical relation. There's a perspective switch built in there, but I won't belabor that.

Bottom line: At higher speeds the path length and trip duration (say from earth to another star system) can be made arbitrarily short - from the perspective of the traveler. However observers (say, on earth or at the destination) don't see any change in your path length and they count the trip duration according to your velocity: Δt = distance/v.

So what gives? Even at speed c, the observers would not see you in two locations at the same time. Even light quanta always travel to the future. They are seen to cross space and time in the same way: Δt = distance/c. However, the traveler (you or photon) ages less during the trip. Though a relative effect while at constant speed, it is locked in when the traveler stops in the rest frame of the observer (a deceleration due to applied force). The traveler is found to have aged less while experiencing a shorter path. Light doesn't age (because it experiences zero path).

It's important to distinguish between "the future" and "aging". The future implies the future (or age) of the cosmos, which is assessed by its size, background temp. or entropy. Aging for anything else, is time experienced on the way to that future. Thus, I'd have preferred if the Relativity literature referred to "length contraction" and "aging contraction" for consistency.

Active Member

Posts: 1969
Joined: 10 Oct 2012
Location: Times Square (T2)

### Re: Can someone help me explain a speed of light question to

Thanks for your reply, some of what you said went over my head I’m afraid and whilst you’ve probably explained why I wouldn’t be seen twice I’m not quite seeing how to expand on the example I gave, so let’s say I was on a planet 10 light years away from earth, if my friend on earth looked at that planet assuming he had rather good eyesight he would see me 10 years later, surely this is right as it would take 10 years for my image to reach him? Similarly if I was on a planet 2 light years away it would take 2 years before my friend on earth could see me? Now been as I can travel faster than the speed of light I can get to planet 2 in 8 years or less depending on how much faster than the speed of light I’m traveling at maybe I go much faster than the speed of light and get there in a year but wait about so that in any case I’m on planet 2 8 years after I set off, I believe that my friend would see me at both locations at once then see me set off from planet 1, I really don’t see how this isn’t correct and have seen other examples of this written by people with more scientific knowledge than me. Tbh I didn’t think I even needed this to be clarified but if I’m wrong I would like to understand why but just bare in mind probably need it explaining in a way someone with only a basic scientific knowledge can understand.
jamessmckie
Forum Neophyte

Posts: 5
Joined: 20 Jan 2019

### Re: Can someone help me explain a speed of light question to

There is a problem here with proper time. Proper time is your observation of time versus your friend’s observation of time. If each of you have a watch, “watch time” is your “proper time” and the two watches would not run in synchrony. There is no such thing as a universal rate of time.

The faster you travel, the slower your watch runs and the shorter your perception of distance becomes in the forward direction. If you could travel at the speed of light, your watch would come to a complete stop and your perception of distance would shrink to zero so you would leave one planet and instantly appear on Earth by your time but your friend would say the trip took ten years by his watch so he would not see you in two places at once.

If you actually could travel faster than light, you would travel back in time. As the old poem says,

"There was a young lady called Bright
Who could travel much faster than light
she departed one day
in a relative way
And arrived on the previous night"
bangstrom
Member

Posts: 793
Joined: 18 Sep 2014

### Re: Can someone help me explain a speed of light question to

Thanks again for your reply, I understand that the perception of time would differ for me (traveling faster than the speed of light) and the stationary observer on earth but I STILL don't see how the observer wuldn't see me twice, after looking into this over the last few days I've read other examples that suggest that it would be possible for the observer to see two images (or potentially more depending on the trip that was taken) at the same time (for him/her) though in different real world time with respect to me (the faster than light traveller) just as when you look at a star in the night sky you're not looking at it in the real world time but rather how it was at an age ago dependent on the distance it is away, I'm going to add a link to an article I read that seems to agree with my thoughts.

https://www.pbs.org/wgbh/nova/article/can-you-really-go-back-in-time-by-breaking-the-speed-of-light/
jamessmckie
Forum Neophyte

Posts: 5
Joined: 20 Jan 2019

### Re: "Being" Persistent

Since you're violating limit c anyway, let's go to infinite speed: travel taking no time at all, even from a stationary observer's perspective. It doesn't have to be far, let's say 2 feet. You move from where you're standing to a spot two feet to the right, instantly.

To your observant friend, light would stop coming from your first location, then instantly come from the new location. Chemically, an image on the retina persists about 1/16 of a second, so your friend might see two of you for about that long, minus the time for the new image to form.

Since you can travel instantly, you might as well keep it up, oscillating between the two locations. In that case you would be in a "superposition state" existing at both locations at the same time. Your friend would see two of you but at only half brightness, a bit ghostly. Similarly, standing on a scale at either location, you would only weigh half as much. The effect lasts as long as your oscillation persists.

That's the simple version. However, it ignores all the locations along your short path. It is arguably more likely that you would be smeared over that path (unless you managed to find and use a wormhole between the two locations). Being smeared, you'd be even dimmer and harder to recognize. If the scale is one foot wide and you are distributed over a two foot path, then at most it could measure half your weight.

A couple of points are worth noting.
1. Even at infinite speed, we never got you back in time before you started. In fact you had to continuously repeat your trip (persist in oscillating) to be noticeable. Otherwise your spread out existence would be lost in the past as your observers all move on to the future.

2. Strange as it seems, electrons exist in superposition, spread out over regions called "orbitals" around the nuclei of atoms. Their charge, mass and other properties are distributed probabilistically over the entire orbital.

3. For something to "exist" it must persist (over some observable period of time). Physics allows for "virtual particles" so named because they are purported to occur for too short a time to ever be observed directly. It is debatable if they are real or if some other explanation will supplant them.

Active Member

Posts: 1969
Joined: 10 Oct 2012
Location: Times Square (T2)

### Re: Can someone help me explain a speed of light question to

That makes sense and I can understand that being what would happen if you moved at infinite speed as you described however that’s not what I’m saying too clarify things further and give a somewhat modified version of my example, so this is from the perspective of the viewer on earth the year is 0 and the time is 0h0m0s I am on a planet 10 light years away and I am going to do a dance for 1 minute so in my opinion at Year 10 at 0h0m0s the observer will see me do my dance for 1 minute. Now after my 1 minute dance I am going to travel faster than the speed of light doesn’t matter how much faster but sufficiently so that I arrive on planet 2 which is 2 light years away before Year 8 0h0m0s now at this time I am going to do a dance for a minute, now back to Year 10 0h0m0s the observer on earth is about to watch me do my dance for a minute on planet 1 but assuming planet 2 is also in the observers field of view and obviously this also assumes the observers eyesight is sufficient to see that far as well as witnessing the 1m dance I did on planet 1 will also be able to see the 1m dance I did on planet 2 both at the same time from the observers perspective though clearly both dances were actually done years in the past. I am absolutely convinced this is what would happen and can only assume my first example didn’t explain things well enough hence this more detailed description, I remember seeing/reading a number of other examples of this over the years and that link I sent being just one that in my opinion demonstrates the same phenomenon as what I’m trying to show.
jamessmckie
Forum Neophyte

Posts: 5
Joined: 20 Jan 2019

I suppose the answer must then be yes.

If you can get ahead (and aside) of your own signal coming from a distant location, you should be able to accomplish a double signal.

A bullet on its way toward me from a distant location could graze a tuning fork yet still proceed faster than the speed of sound and strike another tuning fork closer to me. Set up just right, I could receive the sound from both at the same time.

Multiple simultaneous signals should, in principle, be possible anytime the signal source can propagate faster than the signal it sends.

Active Member

Posts: 1969
Joined: 10 Oct 2012
Location: Times Square (T2)

### Re: Can someone help me explain a speed of light question to

jamessmckie » January 23rd, 2019, 9:43 pm wrote: Now after my 1 minute dance I am going to travel faster than the speed of light doesn’t matter how much faster but sufficiently so that I arrive on planet 2 which is 2 light years away before Year 8 0h0m0s now at this time I am going to do a dance for a minute,

Your scenario is correct if could travel at the speed of light but the timing changes if you can travel faster. At faster speeds, you could arrive at planet 2 one minute or more before the light from planet 1 so your observer on Earth would see your one minute dance on planet 2 start before your dance on planet 1.
bangstrom
Member

Posts: 793
Joined: 18 Sep 2014

### Re: Can someone help me explain a speed of light question to

That's great, thanks to you both for your replies could just do with help now with the main reason of the original post in that I need to make my friend realise that just because it can't be done the logic and reasoning remains and is sufficient to say what would happen, obviously we both knew it was a theoretical world when we debated this so would you agree for him to simply say that 'it can't be done so you can't prove it' as a lame way of trying to save face and not admit he's wrong?
jamessmckie
Forum Neophyte

Posts: 5
Joined: 20 Jan 2019

### Re: The Hole Truth

I think your friend should join SPCF as well.

In the mean time, there's a class of faster-than-light phenomena which don't break any physical laws. Among them are projections, such as a laser, projecting a beam of light. The progress of a beam of light can easily be swept across the surface of the moon faster than speed c.

It's similar to shooting an automatic weapon at a barn. If you have very steady aim, all the bullets will hit the same spot. If you sweep its aim across the barn, the bullets will strike along a line. The faster you sweep the aim, the further apart the bullet holes will be. But that does nothing to change the speed of any individual bullet leaving the weapon.

So, a spot projected on the moon can proceed at any rate. The faster the sweep, the dimmer the beam will be as the photon "bullets" spread out. But that doesn't change the speed c of any individual photon.

If you oscillate the beam back and forth across the moon, you could create a visible line (like a laser level). That is in essence multiple images of the same projected spot being seen because it moves so fast.

Active Member

Posts: 1969
Joined: 10 Oct 2012
Location: Times Square (T2)

### Re: Can someone help me explain a speed of light question to

jamessmckie » Mon Jan 21, 2019 7:35 am wrote:for example if I moved from being 10 light years away to a distance of 2 light years away and that journey took me 8 light years then 10 years after I set off the observer would be able to see me at both my starting point and end point, simple maths and physics right!

As you move from x=10 to x=2, at c, earth friend observes you continuously from Et=10 to Et=18.
He continues observing you from Et=18 to Et=20.
He sees you at x=2 and x=10, but not simultaneously.
Even if you moved faster than c.
Perception follows emission.
phyti
Member

Posts: 103
Joined: 04 Jul 2006

### Re: Can someone help me explain a speed of light question to

phyti » January 31st, 2019, 1:17 pm wrote:
As you move from x=10 to x=2, at c, earth friend observes you continuously from Et=10 to Et=18.
He continues observing you from Et=18 to Et=20.
He sees you at x=2 and x=10, but not simultaneously.
Even if you moved faster than c.
Perception follows emission.

jamessmckie » January 23rd, 2019, 9:43 pm wrote: Now after my 1 minute dance I am going to travel faster than the speed of light doesn’t matter how much faster but sufficiently so that I arrive on planet 2 which is 2 light years away before Year 8 0h0m0s now at this time I am going to do a dance for a minute, now back to Year 10 0h0m0s the observer on earth is about to watch me do my dance for a minute on planet 1 but assuming planet 2 is also in the observers field of view and obviously this also assumes the observers eyesight is sufficient to see that far as well as witnessing the 1m dance I did on planet 1 will also be able to see the 1m dance I did on planet 2 both at the same time from the observers perspective though clearly both dances were actually done years in the past..

If he moves from 10 ly away to 2 ly away sufficiently faster than c, the observer on Earth could see could see him on both planets at the same time because emission from 2 ly from Earth would take place before the light from 10 ly away reached the same point. He is moving closer to the Earth bound observer at >c - not farther away.
bangstrom
Member

Posts: 793
Joined: 18 Sep 2014

### Re: Can someone help me explain a speed of light question to

bangstrom;

I stopped visiting here when Ralfis left with his personal theory, and forgot my reply to this topic. On review, and a spacetime graphic which is more convincing to me, if the anaut could attain constant light speed, he and his image would arrive simultaneously.
Within an SR environment, energy cannot be transmitted faster than c, which limits object speed. I avoid the fantasy scenarios, since SR is difficult in the real world.
phyti
Member

Posts: 103
Joined: 04 Jul 2006

### Re: Can someone help me explain a speed of light question to

Yes, if the anaut could attain constant light speed, he and his image would arrive simultaneously. The purpose of the question was to illustrate a rhetorical point rather than speculate about SR or anything physically valid so it was understandably a fantasy scenario.

My personal opinion is that SR is mathematically correct and gives valid results but the explanations describing what happens between a signal and receiver are themselves a fantasy scenario so fantasy is unavoidable in any discussion involving light speed and photons.
bangstrom
Member

Posts: 793
Joined: 18 Sep 2014