In the news

My cod General Relativity meets your cod Quantum Mechanics here on the internet.

I suspect we will have a Grand Unified Theory of fish and chips by supper time.

I went to the first post in this thread and it was literally like travelling back in time.

In which the cod is both dead and alive

I was watching Stranger Things last night and it's 1983 in the Upside Down but 1986 in 'present day' Hawkins (although it's actually 2022) so point proved I think.

No spoilers please, I’m standing further away from the telly than you so that episode hasn’t reached my eyes yet.

You're correct about saying you see everything in the past - it's just a matter of relative distance. You don't want to over complicate it and think about quantum mechanics or general relativity, you can get a good approximation just using Newtonian mechanics. The equation you want is speed = distance / time. We know that light travels at 3*10^8 metres per second, and we know that the distance between you and your phone is about a metre from your eyes. As a result we can calculate that the time elapsed between the light being emitted by your phone and it reaching your eyes is 1/3*10^8 seconds, or 0.00000000336 seconds. This is near instantaneous so you don't notice it.

A scenario where you do notice it is if you're watching a someone hit something from a noticeable distance away (i.e. a cricket game) - where you see the ball get hit and then momentarily later hear the sound of it. It's because sound travels significantly slower than light and so you experience the moment of the ball being hit twice, because it's being reported to you via two different ways (sound and light). Each time, however, you were experiencing the instant at which the ball was hit at that moment in time.

Going back to the example of the phone - the light we see from the sun was emitted around about 8 minutes ago (due to the distance between you and the sun) so if you were to stand the same distance from your phone to the sun and get someone to change what was displayed on the screen - you observing the phone from that distance would not see the phone screen change until 8 minutes later. Therefore, you are not seeing the phone in its current state, but you are always seeing the phone reported to you as it was 8 minutes ago.

When you apply the principle to the galaxies, they are so mind-bogglingly far away that the light hitting your eyes is as it was emitted billions of years ago. So yes, you are looking back in time - in the same way that you are always looking back in time, because your eyes are simply receptors for light which has been emitted by a physical object at a point in time.

You are correct that the concept of a universal "now" is somewhat flawed - but only when you think about time as something constrained by human reporting such as GMT or UTC. Particles experience less time the closer to the speed of light they are travelling, and nothing can travel faster than the speed of light, but in the context of the calculation of speed = distance / time, we know that the speed of light is constant, and as we are measuring the snapshot of when the light was emitted from the object we are observing there is a finite distance between the emitter and the observer - therefore the time being calculated is from the frame of reference of that light which has travelled that distance. That light has been travelling for x number of years therefore it is x number of years old and therefore we are looking x number of years back in time.

I am a physicist, for what it's worth.

I for one are glad you're a physicist, because if you could explain it that well and WEREN'T a physicist I'd begin to feel mighty inadequate...

You are correct that the concept of a universal "now" is somewhat flawed - but only when you think about time as something constrained by human reporting such as GMT or UTC. Particles experience less time the closer to the speed of light they are travelling, and nothing can travel faster than the speed of light

Thanks for the long reply, I appreciate it.

I was aware that Newtonian physics is what is being used to calculate the 13billion year "age" of the light.

But it's the bit I have quoted where, for me, the complicating nuance comes in. For those photons no time has passed since they travel at c, with the associated Lorentz trabnsformation. But if a putative rocket flew from those galaxies, leaving at the same time as the photon, travelling at c minus some infinitesimal delta v, the occupants would experience some time lapse before they arrived here, but it wouldn't be 13 billion years.

All of that aside. I suppose the thing that really gets me is how we could define the moment that is "now" on those distant galaxies, with reference to "now" here.

What would it mean to say that we are seeing them in the present, when to do so would necessitate violating causality due to the speed of light and special relativity?

I just don't really believe in a universal concept of "now".

I'm very happy to be shown how I am flawed in my thinking...

I'm very happy to be shown how I am flawed in my thinking...

I point you to any of your bike builds to demonstrate your flawed thinking, your sense aesthetics is terrible. 😀

I just don't really believe in a universal concept of "now".

At moment(s) of observation? Just enjoy the pretty pictures, they went to a lot of effort to fake them, any fool knows space ain't real.

Everybody knows you can't see space so how do they take pictures of it? Riddle me that!

It's difficult to explain using words, that's why physics is handled mostly in mathematical notation - but I'll try.

You're using the reference frame of the photons as your measure for time which is the issue - ignore the photons and the fact that they don't experience time. You don't care about that, because you care about the reference frame of the observer. The observer is not travelling at the speed of light, and so it does experience time.

Think of it as a two people in a field at night and it's completely dark. One of the people in the field is on a horse, the other is on foot. The person on foot is trying to catch the person on the horse, but the horse keeps moving. The person on the horse shouts whilst the horse is moving, at that point in time at which the person on the horse shouts, the horse does a poo (bear with me). The person on foot walks, with a trundle wheel and a stopwatch, towards where he heard the shout and then steps in the poo. He knows the horse was there at the point in time when the person shouted, because it was a loud shout and it caused the horse to do a poo. The person on foot now has three very important bits of information. The time elapsed from the observers frame of reference since the shout occurred (person on foot is the observer), the distance to the physical location at which the shout occurred and that the horse existed at the point in time when the sound was emitted (see poo). Why is the poo important? Because of the fact that it proves that the horse existed at the point at which the sound was emitted - from the perspective of the observer, he now has the knowledge that it took ten seconds for the sound to reach him from the horse, so therefore the minimum amount of time (from the observer perspective) that the horse has existed is ten seconds.

The important thing here, is that it doesn't matter how fast the sound travelled, all that matters is that a period of time has elapsed for the observer.

When we apply this to the galaxy thing, it kind of does its own poo. We know that the universe is expanding at a certain rate, and it's very fast. Fast enough to apply the doppler effect to light and change the frequency at which it is observed. We look at the light emitted by the galaxy and we say, oh - cool, it's not the colour we were expecting it to be, and so it must have been red-shifted. That means that it was travelling at a certain speed at the point of emission. This is the poo. We are the observers - we've seen the poo, and we know how far away the galaxy is, therefore we can conclude that although this light itself has experienced no time, both the emitter and the observer have experienced time because these were not travelling near the speed of light. Therefore we can reasonably conclude that the emitter existed at the point in time it emitted and from the reference perspective of the observer that was X billions years ago - therefore from the perspective of the observer that object is X billion years old.

The light is the messenger - we don't care if it experiences time, but the fact that it does not experience time does not change the fact that the thing that does emit it does experience time. It may not be the same amount of time being experienced by the observer, but everything is measured from the observer frame of reference - so for us, looking at it, that galaxy has been in the state that it's in for x billion of years.

Your thinking isn't flawed - there is no universal concept of now, time bends and changes around things with very large mass and things that travel very fast - but it's just sort of the wrong question to ask. Because now is now, everywhere, it's just it might be a different now.

It's always about poo with you physicists isn't it?

Talk shit get big banged

Oops, sorry! It's not a big one - if I told you why it's 1983 that would be a real spoiler.

Great few episodes btw - one silver lining of still being whacked out after having Covid two weeks ago.

So all that 'living in the moment' stuff is bollocks then?

I suspected as much.

nothing can travel faster than the speed of light

The Millennium Falcon tho?

So rude

Top chat this morning (or whenever it is. Or isn't).

Nadhim Zahawi using NZ4PM as his social media campaign hashtag

Click for a surprise http://www.nz4pm.com

Let’s get physical, (meta)physical!

Have we mentioned the speed of nerve transmission, or brain waves? Iirc our nerves convey electric impulses from our body to our central nervous system at ≈ 80 m/s. Even with things happening truly ‘now’, we don’t perceive them until fractions of a second later, because of that lag, and our brains process that moment as ‘now’.

Click for a surprise http://www.nz4pm.com

Not sure why I clicked but this was blocked by my work spam filter. I am now on some sort of list somewhere?

Nothing NSFW if that's your fear?

1 Attachment

Penny Mordaunt's campaign bought that domain and others before he did so it redirects to her campaign.

Ha! That's pretty good