Is there really nothing faster than light?

SvenDahlhaus
Homework Statement:: Einsteins Theorie
Relevant Equations:: Theorie

Hi. My name is Sven Dahlhaus. I'm from Germany and just got my high school diploma. Nevertheless, I am a big fan of theoretical physics. There is a question that bothers me a lot. According to Einstein's theory, nothing can travel faster than light. However, according to the Big Bang theory, for example, the universe should have expanded to the size of our solar system by about 100 seconds when it was created, according to Hawking's theory. Among other things, the stars around our Super Massive Black Light in the center of our galaxy are said to be moving at up to 15,000,000 kilometers per hour. So matter must be able to move faster than light. So Einstein's theory can't be right?
This ist my questions.

vanhees71, PeroK and Delta2

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Among other things, the stars around our Super Massive Black Light in the center of our galaxy
Our what ?
are said to be moving at up to 15,000,000 kilometers per hour. So matter must be able to move faster than light.
Might want to check your reference for the speed of light : it's considerably faster than a measly 15M km per hour.

ohwilleke
SvenDahlhaus
Sorry, my english ist so Bad...

SvenDahlhaus
Our what ?

Supermassive black hole

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ohwilleke
Gold Member
No, your English is actually so good I just didn't know what you meant by "black light", rather than "black hole".

But, there's a large difference between km/hour and km/second.

ohwilleke
Mentor
So matter must be able to move faster than light. So Einstein's theory can't be right?
No, the theory has been verified with a wide variety of techniques covering a broad range of predictions. It is correct: massive objects cannot travel faster than light.

vanhees71
SvenDahlhaus
Light travels 299,792.458 kilometers per second. The sun is on average 149.60 million km away from the earth. Sunlight therefore takes a little more than eight minutes to reach the earth. Hence the real question. How can the universe then have been as big as our solar system shortly after its creation, 100 seconds. How can stars in the center of our Galaxy move at 15 million kilometers per hour (9320567.88 miles per hour) when the maximum speed of everything is 3000000 kilometers per hour? I honestly don't get it.

ohwilleke
SvenDahlhaus
No, the theory has been verified with a wide variety of techniques covering a broad range of predictions. It is correct: massive objects cannot travel faster than light.
Nonetheless, the stars orbiting the supermassive black hole at the center of our Milky Way have been observed to move at about 15000000 kilometers per hour. I didn't want an answer to that. I just can't understand how that can be when nothing can be faster than light.

Gold Member
1 hour = 3,600 seconds.

300,000 km/second = 1,080,000,000 km/hour

1,080,000,000 > 15,000,000

Still, 13% of the speed of light is scary fast.

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ohwilleke and PeroK
Gold Member
. How can the universe then have been as big as our solar system shortly after its creation, 100 seconds.
The Big Bang was not an explosion of mass. Nothing traveled faster than c.
The Big Bang was an inflation of spacetime. The inflation of spacetime is not limited by the speed of light. Essentially, the mass was sort of minding its own business while the distance between massy-stuff grew very big, very fast.

vanhees71, pinball1970 and Delta2
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@SvenDahlhaus it's like a boat that can't go faster than 20mph because that's all its engine can do, BUT ... when the boat is going 20mph along with the current of a river that is going at 40mph, a person on the shore thinks the boat is going 60mph even though the boat itself can't be going more than 20mph. Spacetime expansion is like that. It carries matter along with the expansion so there is no proper motion faster than light.

Grinkle, vanhees71 and pinball1970
bland
@SvenDahlhaus, from what I have been able to glean from the very many cleverly animated and adequately explained youtube videos on this topic, it appears that the limit you refer to is for things traveling within spacetime, but this limit does not apply to spacetime itself.

Gold Member
Homework Statement:: Einsteins Theorie
Relevant Equations:: Theorie

Hi. My name is Sven Dahlhaus. I'm from Germany and just got my high school diploma. Nevertheless, I am a big fan of theoretical physics. There is a question that bothers me a lot. According to Einstein's theory, nothing can travel faster than light. However, according to the Big Bang theory, for example, the universe should have expanded to the size of our solar system by about 100 seconds when it was created, according to Hawking's theory. Among other things, the stars around our Super Massive Black Light in the center of our galaxy are said to be moving at up to 15,000,000 kilometers per hour. So matter must be able to move faster than light. So Einstein's theory can't be right?
This ist my questions.
Hi Sven, welcome to Physics Forums!

As stated above nothing made of matter can travel through space faster than light. However, some non physical things can move faster than light such as a shadow or a spot of light moving across a surface but these carry no matter or information and thus do not violate the Special Theory of Relativity.

Staff Emeritus
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Light travels 299,792.458 kilometers per second. The sun is on average 149.60 million km away from the earth. Sunlight therefore takes a little more than eight minutes to reach the earth. Hence the real question. How can the universe then have been as big as our solar system shortly after its creation, 100 seconds. How can stars in the center of our Galaxy move at 15 million kilometers per hour (9320567.88 miles per hour) when the maximum speed of everything is 3000000 kilometers per hour? I honestly don't get it.
Because there is nothing that is traveling for the Universe to grow larger. The Universe does not grow larger by objects traveling outwards from a point that was the Big Bang (this is a very common misconception). It grows larger by space itself expanding and there is nothing that really limits that. The Big Bang happened everywhere.

And again, update your units …

Doc Al
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Sorry, my english ist so Bad...

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@SvenDahlhaus - "nothing travels faster the speed of light" is a simplified version. A better statement is "nothing overtakes light", and that is always true. In the very early rapidly expanding universe, nothing overtook light. Now, even with stars racing around supermassive black holes, light goes faster.

It may seem like there is no difference between "nothing travels faster than the speed of light" and "nothing overtakes light". In flat spacetime (no gravity), that's true, and in weakly curved spacetime (like the Earth's gravity or the Sun's gravity) there is nearly no difference. But you can detect failures even so - if you look up "Shapiro delay", which relates to radar measurements of the distance to other planets you will find one way of conceptualising it is that light doesn't necessarily travel at 300,000km/s. Still nothing overtakes it. And when you get into situations where spacetime curvature is large, such as the early universe or large distances in our current universe you can easily see superluminal motion - still light will never be overtaken.

The fundamental problem in curved spacetime is that there isn't a way to define distances and times far away from you without distorting them. It's rather like looking at a Mercator projection of the Earth, which suggests that if you are right next to the north pole it should take you as long to walk around the pole as it does to walk around the equator. Both are the same distance on the map, after all. But the map does not (and cannot) represent the distances accurately. So it is with describing speeds in curved spacetime - two things in different places that we describe as doing half the speed of light may be described completely differently by people on the scene.

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Bandersnatch, martinbn, vanhees71 and 3 others
Mentor
Light travels 299,792.458 kilometers per second. The sun is on average 149.60 million km away from the earth. Sunlight therefore takes a little more than eight minutes to reach the earth. Hence the real question. How can the universe then have been as big as our solar system shortly after its creation, 100 seconds.
The constant c (299792458 m/s) is equal to the speed of light in vacuum in an inertial frame. In non inertial frames the speed of light is not necessarily equal to c. The restriction is that a massive object cannot go faster than light, but in non- inertial frames it is entirely possible for massive objects to go faster than c. That does not violate the restriction.

How can stars in the center of our Galaxy move at 15 million kilometers per hour (9320567.88 miles per hour) when the maximum speed of everything is 3000000 kilometers per hour? I honestly don't get it.
As others have pointed out your units are messed up. c is approximately 1 billion kilometers per hour, which is much larger than 15 million kilometers per hour.

pinball1970 and Ibix
“Light thinks it travels faster than anything but it is wrong. No matter how fast light travels, it finds that darkness has always got there first, and is waiting for it.” - Terry Pratchett

Delta2 and Ibix
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“Light thinks it travels faster than anything but it is wrong. No matter how fast light travels, it finds that darkness has always got there first, and is waiting for it.” - Terry Pratchett
A shadow can move faster than light, of course!

pinball1970
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A shadow can move faster than light, of course!
Why of course? Start with a source projecting a continuous stream of photons onto a screen. At some point in time place an opaque object between the source and the screen. The shadow of the object will appear on the screen only after the last photons between object and screen hit the screen. It looks like the shadow cannot overtake light.

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Why of course? Start with a source projecting a continuous stream of photons onto a screen. At some point in time place an opaque object between the source and the screen. The shadow of the object will appear on the screen only after the last photons between object and screen hit the screen. It looks like the shadow cannot overtake light.
There is no upper limit to the speed of a shadow. It can overtake light.

The catch, of course, is that a shadow is just the illusion of motion: the shadow itself is not any physical thing. And no information is being propagated in the direction of apparent motion of the shadow.

vanhees71
Mentor
Which is why the restriction is not "nothing can travel faster than light" it is "massive objects cannot travel faster than light".

There are lots of examples of "things" that are not massive objects that can travel faster than light.

Klystron, vanhees71 and bob012345
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Another example. Let's assume we have a long line of lights spaced ##3 \ m## apart.

If we organise things so that the each light goes ##10^{-9} \ s## after the previous one, then the "light signal" appears to move from light to light at ##3 \times 10^9 \ m/s##.

Of course, we can even organise things so that the lights illuminate simultaneously (in the appropriate rest frame) and then the illusion of motion disappears.

The catch is that no signal is actually moving from light to light. The apparent speed depends only the independent times at which the lights are illuminated. The signals subject to ##c## are the ones from the controlling unit to each light. But, ##c## does not apply to the sequence with which the lights may be illuminated.

David Lewis, vanhees71 and bob012345
cianfa72
The restriction is that a massive object cannot go faster than light, but in non- inertial frames it is entirely possible for massive objects to go faster than c.
I believe the point is that starting from any point in spacetime a massive object moving in a given spatial direction cannot keep up with the process of light propagation in the same direction.

Dale
Mentor
I believe the point is that starting from any point in spacetime a massive object moving in a given spatial direction cannot keep up with the process of light propagation in the same direction.
Or, in more technical terms, the worldline of any massive object is timelike. Which means that at each event on the worldline the tangent vector lies inside the light cone.

vanhees71, PeroK and cianfa72
Gold Member
It seems fast to our everyday experience but I have always been struck by how slow the speed of light is in comparison to the scale of the universe. Or on the scale of the laboratory when accurate time bases might be required across a room when lightspeed is only ~30 cm/ns.

Klystron, vanhees71 and Dale
Paul Thatcher
Cosmic inflation appears to break the light-speed limit, but remember: inflation is not the movement of mass-energy through space-time; it is the expansion of space-time itself, and therefore not limited by the speed of light.

How did this happen? Why did it stop, giving us the expansion observable today? The extreme temperature & density of the early universe gave a high energy density to the "inflaton field". Expansion should have reduced this energy density continuously down to its absolute minimum. But the field "got stuck" in a local minimum ("false vacuum"). Here, at a very high, constant energy density, exponential inflation was still possible.

However, like all quantum fields, the inflaton field is subject to quantum fluctuations. Somewhere in the inflating universe, this happened, allowing the inflaton field to escape the local minimum/false vacuum and reach the true vacuum of minimal energy density. This switched inflation off, but remains enough to drive continued expansion at "sub-light speeds" (only, remember, actually it is not the "stuff" that is moving, but the "space" in between). I am not a physicist, and one source I find very useful and informative is PBS "Space & Time", available on YouTube.

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Gold Member
So why is it "space is expanding", not "everything is shrinking" ?

Mentor
So why is it "space is expanding", not "everything is shrinking" ?
What is the experimental distinction between the two?

Paul Thatcher
It probably is that everything is shrinking, actually - after all, the nature of reality is often the opposite of what we intuit, right? Is the apple falling, or is the Earth rushing up to meet it?
How to distinguish experimentally between the two?
Hmmm, here we are, a lovely spring evening, an open window, a fruit-bowl, passers-by on the pavement below... Great Scott, I have it!

Klystron and PeroK
Homework Helper
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It seems fast to our everyday experience but I have always been struck by how slow the speed of light is in comparison to the scale of the universe. Or on the scale of the laboratory when accurate time bases might be required across a room when lightspeed is only ~30 cm/ns.
Or in relation to modern computing: the PC I am typing this on has 6 cores each of which can multiply a 64 bit float in the time it takes light to reach my eyes from the screen.

bob012345
Gold Member
So why is it "space is expanding", not "everything is shrinking" ?
I know this isn't a serious question but still:
Occam's Razor is pretty useful here.

In the 'space is expanding' scenario, only one thing need change: spacetime.
In the 'everything is shrinking' scenario, multiple things must change - not just matter but also the speed of light.

Gold Member
What is the experimental distinction between the two?
Is there one ? (doable with current knowledge)

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