B Did the Big Bang expand faster than light?

[brajesh]

I read that our telescopes pointing in any direction show light coming from the early days of the Big Bang (like 13.7 billion years ago).
So did the expansion of the Big Bang to "fill" the universe happen faster than the speed of light?

 

[PeroK]

I read that our telescopes pointing in any direction show light coming from the early days of the Big Bang (like 13.7 billion years ago).
So did the expansion of the Big Bang to "fill" the universe happen faster than the speed of light?

The Big Bang was a rapid expansion of space itself. It wasn't an explosion of matter that filled previously empty space.

Moreover, universal expansion is not measured as a speed, so you cannot compare it to the speed of light. Expansion is a speed per unit distance. So, however small the expansion rate is, if you take two points far enough apart, then the recession velocity between those two points will be greater than the speed of light.

 

[brajesh]

Oh expansion of space? Ok, I understand.

So expansion is speed per unit distance, is that acceleration?

 

[PeroK]

Oh expansion of space? Ok, I understand.

So expansion is speed per unit distance, is that acceleration?

Acceleration is change in velocity per unit time.

Expansion of space is a new concept not found in classical mechanics.

 

[brajesh]

fascinating, thank you @PeroK for these clarifications !

 

[vincenzosassone]

then the recession velocity between those two points will be greater than the speed of light.

I'm really sorry to interfere in this very interesting conversation, but there is something that I want to understand better. I don't know a lot about recession velocity, and even if I have tried to read something about it, I don't understand everything. For this reason maybe I'm willing to say something of very wrong, but I want to know why and where I'm wrong: if those two points are moving far from each other with an incredible speed, then all of them are moving faster than the half of the speed of light. Maybe this is possible, after all the speed that we cannot surpass is that of the speed of light, but it is very weird to me the possibility that an object is moving with this incredible speed. I nearly forgot, why we can't take in account the speed of our universe in expansion as a speed? Thanks early

 

[phinds]

... then all of them are moving faster than the half of the speed of light.

NO. You are not paying attention to what has already been said. The movement is NOT "speed" it is recession. It is not things moving THROUGH space, it is space itself expanding and carrying things with it.

There is no limit on recession velocity and things at the edge of our observable universe are receding from us at about 3c.

 

[PeroK]

I'm really sorry to interfere in this very interesting conversation, but there is something that I want to understand better. I don't know a lot about recession velocity, and even if I have tried to read something about it, I don't understand everything. For this reason maybe I'm willing to say something of very wrong, but I want to know why and where I'm wrong: if those two points are moving far from each other with an incredible speed, then all of them are moving faster than the half of the speed of light. Maybe this is possible, after all the speed that we cannot surpass is that of the speed of light, but it is very weird to me the possibility that an object is moving with this incredible speed. I nearly forgot, why we can't take in account the speed of our universe in expansion as a speed? Thanks early

There is, of course, a postulate of SR (Special Relativity) that no object can move through space at greater than the speed of light. But, it's GR (General Relativity) that governs the universe as a whole and is the basis for cosmology. Within GR, SR applies locally - so that the locally measured speed of an object is always below the speed of light (as measured locally). But, SR has nothing to say about universal expansion: recession velocity is not a velocity through space, but a rate of separation (of distant objects) induced by the expansion of space itself.

 

[vincenzosassone]

NO. You are not paying attention to what has already been said. The movement is NOT "speed" it is recession. It is not things moving THROUGH space, it is space itself expanding and carrying things with it.

I knew that I was wrong... Anyways now I know where is the problem thanks to you. I'm not such an expert in this field and probalby I have already said it, therefore there are many things that I'm not able to catch.

3c.

Sorry, there is only one more thing: "c" is the speed of light, isn't it?
Many thanks to you all @PeroK and @phinds

 

[phinds]

Yes, c is the symbol we use to represent the speed of light.

ACTUALLY, it is not, purely speaking, the speed of light, it is the maximum speed of the universe, a universal speed limit if you will. It is strongly believed, and verified by experiment to a lot of decimal places, that light is massless and therefore travels at that speed limit. IF light were, like neutrinos, found to have a tiny mass, then they would travel at slightly less than c (as neutrinos do) and we would need a new symbol for "the speed of light" (OR a new symbol for the universal speed limit).

 

[Ibix]

IF light were, like neutrinos, found to have a tiny mass, then they would travel at slightly less than c (as neutrinos do) and we would need a new symbol for "the speed of light"

Well, in that case, light wouldn't have a single well-defined speed. You could, in principle, stop a photon just as you can stop an electron. So $c$ would continue to mean the invariant speed and we wouldn't need a dedicated symbol for the propagation speed of light, any more than we have a dedicated symbol for the speed of an electron or whatever.

 

[phinds]

Well, in that case, light wouldn't have a single well-defined speed. You could, in principle, stop a photon just as you can stop an electron. So $c$ would continue to mean the invariant speed and we wouldn't need a dedicated symbol for the propagation speed of light, any more than we have a dedicated symbol for the speed of an electron or whatever.

Good point.

 

[vincenzosassone]

Yes, c is the symbol we use to represent the speed of light.

Alright, many thanks

light is massless and therefore travels at that speed limit.

Now I rembember that there is a law in phisics that claims: all the "objects" with no mass can travel at the speed of light, for instance the gravity, that is massless and effectively is as fast as light. Even the magnetic force I assume...

 

[PeroK]

Alright, many thanks

Now I rembember that there is a law in phisics that claims: all the "objects" with no mass can travel at the speed of light, for instance the gravity, that is massless and effectively is as fast as light. Even the magnetic force I assume...

The propagation of changes to the Electromagnetic field travels at the speed $\frac 1 {\sqrt{\epsilon_0 \mu_0}}$. Where $\epsilon_0, \mu_0$ are the permittivity and permeability of free space, and can be obtained from electromagentic experiments involving charges, currents and magnets. This is embedded in Maxwell's Theory of Electromangentism (1865).

And, of course, when you calculate the quantity $\frac 1 {\sqrt{\epsilon_0 \mu_0}}$, you find it's equal to the speed of light in vacuum. Isn't that something!

 

[Ibix]

Now I rembember that there is a law in phisics that claims: all the "objects" with no mass can travel at the speed of light, for instance the gravity, that is massless and effectively is as fast as light.

I'm not sure it's a law of physics, really, but in relativity mass is the modulus of the four-momentum. Saying something has zero mass is therefore equivalent to saying it has a null four-momentum, which is the same as saying it moves at the speed of light.

Even the magnetic force I assume...

"Magnetic force" isn't really a thing. But changes in the electromagnetic field do propagate at the speed of light, yes.

 

[vincenzosassone]

I'm not sure it's a law of physics

Either is a law or is a theory, now I don't remember. But from your speech I guess to understand that it is a law. Albert Einstein revolutionized all the stuff that we knew about Physics, for this reason I don't know if this was introduced in Physics as a simple theory or a law. Anyways, it doen't matter, if we manage to achive this with the simple and pure thought, this means that is real the things about what we are talking.

And, of course, when you calculate the quantity , you find it's equal to the speed of light in vacuum. Isn't that something!

I don't know, I put it in stake only because is one of the fundamental force of physics...

 

[George Jones]

The propagation of changes to the Electromagnetic field travels at the speed $\frac 1 {\sqrt{\epsilon_0 \mu_0}}$. Where $\epsilon_0, \mu_0$ are the permittivity and permeability of free space, and can be obtained from electromagentic experiments involving charges, currents and magnets. This is embedded in Maxwell's Theory of Electromangentism (1865).

And, of course, when you calculate the quantity $\frac 1 {\sqrt{\epsilon_0 \mu_0}}$, you find it's equal to the speed of light in vacuum. Isn't that something!

And my students do just this. I teach a second-year e&m lab course that performs one experiment per week. In consecutive weeks, the students measure $\epsilon_0$ and $\mu_0$ using

https://media.vwr.com/interactive/p...h_2014/files/assets/basic-html/index.html#289

and then compare to the speed of light. For $\mu_0$, we send up to 20 amps through the metal rods.

 

[aperakh]

no object can move through space at greater than the speed of light.

Very good explanation, but there is a small correction I feel compelled to make, mainly because it is such a common statement even among people who know the subject of Relativity fairly well.
It is NOT true that special relativity states that nothing can move faster than the speed of light.
If you think about it for a moment, since relativity says there is no Universal frame of reference ( usually referred to as absolute space), there isn't even a way so to make such definitive statement because there is no way to definitive way to say how faster something moves at all.

what special relativity actually says is that nothing can ACCELERATE to the speed of light or faster from its own earlier frame of reference.

For even the distinction between velocity and expansion aside, there is no known law that for baked an objects from moving relative to each other faster than speed of light through space so long as they are already doing it.
In fact, physicist even have an name for particles or objects that move faster than the speed of light relative to an observer. They're called tachyons.
And it is one of the Unsolved Mysteries in physics as to why we haven't detected any yet. It is one of the many hopes of the Large Hadron Collider to possibly detect some.

My personal favorite proposed answer to this mystery is the one by Richard Feynman who demonstrated that any particle moving backwards in time (which is the same thing as saying moving faster than light) would be indistinguishable from the same particle's antiparticle. A positron is actually nothing more than a faster-than-light electron, for example.

 

[Vanadium 50]

I feel compelled

You probably should look into such compulsions, because some of what you say is just not so.

there isn't even a way so to make such definitive statement because there is no way to definitive way to say how faster something moves at all.

Um no. There is no boost in which something moving slower than light ever moves faster than light. It is true I can do a boost and the object I am examining will have a new velocity, but it can't be boosted > c.

It is one of the many hopes of the Large Hadron Collider to possibly detect some [tachyons].

The LHC is a poor place to look for tachyons. AFAIK, no results on a tachyon search have been published, nor do I expect any.

My personal favorite proposed answer to this mystery is the one by Richard Feynman who demonstrated that any particle moving backwards in time (which is the same thing as saying moving faster than light) would be indistinguishable from the same particle's antiparticle.

A. This has been known to be incorrect since 1964.
B. By "Richard Feynman" you meant "Ernst Stückelberg", right?

A positron is actually nothing more than a faster-than-light electron, for example.

Sorry, but that's just ridiculous. We can measure the speed of positrons and they are slower than light, not faster.

 

[PeterDonis]

since relativity says there is no Universal frame of reference ( usually referred to as absolute space), there isn't even a way so to make such definitive statement because there is no way to definitive way to say how faster something moves at all.

Wrong.

In fact, physicist even have an name for particles or objects that move faster than the speed of light relative to an observer. They're called tachyons.

Correct, but inconsistent with your wrong statement above. Also irrelevant to the discussion in this thread, since, as you note, no tachyons have ever been discovered, and we are talking about our actual universe, not hypothetical cases that can be worked out mathematically.

A positron is actually nothing more than a faster-than-light electron, for example.

Wrong. Feynman's actual claim was a lot more subtle than what you are claiming.

 

[PeterDonis]

A. This has been known to be incorrect since 1964.
B. By "Richard Feynman" you meant "Ernst Stückelberg", right?

If you're referring to the Feynman-Stuckelberg trick, the trick works fine as long as you don't take it literally.

Feynman also said somewhat similar things about virtual particles--for example, in his Dirac memorial lecture in 1986 [1], he phrases it as "one man's virtual particle is another man's virtual antiparticle". But that was in the course of explaining why quantum field theory implies that there must be antiparticles (given a few reasonable assumptions). It was not in the course of claiming that there must be tachyons.

[1] https://www.cambridge.org/us/academ...emorial-lectures?format=PB&isbn=9780521658621

 

[Vanadium 50]

If you're referring to the Feynman-Stuckelberg trick, the trick works fine as long as you don't take it literally.

That trick conserves CP. In nature (although not for most calculations) CP is not conserved.

 

[PeterDonis]

@aperakh, discussion of tachyons is irrelevant to this thread. You have now been banned from further posting in this thread.

 

[vincenzosassone]

This thread by now has taken a position very hard to carry through because of this stuff about tachyons. I only want to introduce again few things. Firstly, you cannot talk about tachyons here, I have read by wikipedia (I don't know how to inser the link) that tachyons, if exist, have a complex mass, something of very difficult to image...
Secondly, massless "objects" can travel at the speed of light, or better "c", that is the speed of light in the vacuum. Finally, all the other objects have to keep themselves under the speed of light, their mass compell them to undergo this ( I don't think there is an explanation to be shown without put here a good basis in math). I really hope I have been usefull for you all, if there is something that according to you isn't good, talk with me.

 

[brajesh]

Hi OP here :)

This thread has been a fascinating education for me leading me to more exploration.

I was wondering, is there a description of the expansion?

I googled around and it seems that :

-At a billion billion billion billionth of a second the universe was the size of a baseball.
-At 1 second, the expansion was about 100 billion kilometers.
-What was it at 2 seconds?
-What was it at 1 day?
-etc...

Any chart that nicely describes this expansion from the beginning to today?

 

[Ibix]

At a billion billion billion billionth of a second the universe was the size of a baseball.

The universe is infinite in extent and always has been, at least as far as we know. The observable universe, the bit we can see because light has had time to reach us, is finite in extent and is probably what your sources are talking about.

How big you think the early observable universe was depends on what model you're using. I don't think there's yet a complete consensus.

Any chart that nicely describes this expansion from the beginning to today?

Davis and Lineweaver has a number of charts of size versus time, depending on how you choose to measure distance. As I say, the very early universe remains open to discussion, but the rest is pretty solid.

 

[vincenzosassone]

I googled around and it seems that :

-At a billion billion billion billionth of a second the universe was the size of a baseball.
-At 1 second, the expansion was about 100 billion kilometers.
-What was it at 2 seconds?
-What was it at 1 day?

The Universe in the first billionth of second was shaping up. This means that only in the first fraction of second our Universe made something of important. The fundamental particles and the fundamental forces separated to each other and the first atoms were made. After an hour or a day, nothing of important to remember was made and for this reason in the scale there are many holes. The most important event after a second happen only after many thousand of years. Obviously we are talking about the formations of very big objects like galaxies, no more little particles...
I hope to be useful!