# What if the speed of light was slower?

If the maximum speed was say a 10th what it is, would it have any drastic changes in the universe?
I guess there would be more and bigger black holes and light would take longer to reach us, but what would be the main differences in the universe? Would it be much different?

## Answers and Replies

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russ_watters
Mentor
C is one of those constants which if it were much different, would radically alter the structure of the universe. Just off the top of my head, since C is important to the matter/energy conversion going on in the sun (fusion), cutting C to 1/10 would decrease the output of the sun to 1/100 - assuming the rate of fusion is unchanged (it would change).

Integral
Staff Emeritus
Gold Member
Find a copy of Mr. Tompkins in Wonderland by George Gamov.

The real point of this question was to see if there wasn't much difference whether light was slower or not. This was because I wanted to ask this second question:
Instead of the universe expanding at an accelerating speed could it be that the maximum speed of light has been increasing?

HallsofIvy
Homework Helper
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changing the speed of light would not change anything about the universe at all.

this is because the speed of light is a dimensionful number. if you were to change it, all you would do would change the size of the meter. but relative to this new meter, the universe would look exactly the same.

on the other hand, changing the fine structure constant, or changing the electron mass, would change the universe quite drastically.

usually when people talk about a variable speed of light, they are actually talking about a variable fine structure constant.

There is some evidence to suggest that the speed of light has indeed changed somewhat over the course of the universe's development.

Integral
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Originally posted by Adam
There is some evidence to suggest that the speed of light has indeed changed somewhat over the course of the universe's development.
I believe that this is a topic of current research. Some results say yes, others no. It is still undecided as far as I know.

Originally posted by Adam
There is some evidence to suggest that the speed of light has indeed changed somewhat over the course of the universe's development.
there is some evidence to suggest that the fine structure constant has changed.

as i said above, changes in the speed of light are inobservable since it is a dimensionful constant.

so i believe your statement is mistaken.

Lethe

Yes, you're quite right. I just re-read the articles about it. They say the observations suggest the FSC may have been a tad smaller a long time ago.

This concept is really interesting in that if you change the value of c slightly slower, this (obviously) means that c has increased. We've recently discovered, supposedly, that the universe is expanding, and in an accelerating fashion. If the extreme concepts of so-called "string" theory are to be applied, I wonder if C is a factor of the total lenght of the "string," which is expanding.

Nereid
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observational evidence of changes in the FSC

Originally posted by Adam
Yes, you're quite right. I just re-read the articles about it. They say the observations suggest the FSC may have been a tad smaller a long time ago.
While keeping an open mind and always searching for more and more stringent tests is 'a good thing', IMHO the observational data on changes in the fine structure constant (FSC) over cosmological time is overwhelmingly in favour of the hypothesis that it is, truly, constant.

AFAIK, there've been no reported new 'many multiplet method' determinations of changes in the FSC over cosmological time since January, 2003 (or any other observations of such changes). More specifically, nothing better (in the public domain) than the beautifully clear approach (and unambiguous result) of Bahcall, Steinhardt and Schlegel. If only all astronomical research were so straight-forward
http://arxiv.org/abs/astro-ph/0301507

How could there be no difference if you changed the speed of light to, let's say 1 meter per second?

I also don't understand how it being a "dimensionful number" would make a difference because it's ratio with all the other constants would be different.

Originally posted by TheDonk
How could there be no difference if you changed the speed of light to, let's say 1 meter per second?

I also don't understand how it being a "dimensionful number" would make a difference because it's ratio with all the other constants would be different.
if the speed of light were one meter per second, then atoms would be smaller, information would move more slowly across distances about the size of a meter, and a meter would seem extremely huge to us. how huge? in this hypothetical universe, a meter would seem as huge to the inhabitants of that universe as a light-second seems to us in this universe.

the geological processes that shape the earth wouldn t be able to cover distances on the order of meters, and the earth would be much smaller, but that would be OK, because the radius of the atoms that make up the earth would be much smaller.

in fact, if i replace the word meter with light-second, then i would be describing the same universe. all i did was change the units.

this is why changing a dimensionful number has no meaning. our units are defined in terms of the fundamental constants. change the fundamental constants, and you change the units. but you don t change the size and shape of the universe, you have only changed the name of your meter stick.

Alright, all of that makes sense except one thing for me.
Why would the atoms become smaller?
Is it because it takes more energy for their subatomic parts to move around? If so, why does this show that the ratio of the atom's sizes would be exactly the same as the ratio between the light speeds.

I think there would have less potential life on earth because in relativistic approach in explaining why is it that the sun keep on shining for billion of years it rely on einstiens energy equation. So if it happens then there will be less conversion of hydrogen into helium from the sun and the sun rays which sustains life on earth will be lesser so thats it.

Nereid
Staff Emeritus
Gold Member
Universe changed if WHICH constants change?

Originally posted by lethe
changing the speed of light would not change anything about the universe at all.

this is because the speed of light is a dimensionful number. if you were to change it, all you would do would change the size of the meter. but relative to this new meter, the universe would look exactly the same.

on the other hand, changing the fine structure constant, or changing the electron mass, would change the universe quite drastically.

usually when people talk about a variable speed of light, they are actually talking about a variable fine structure constant.
So that's:
$$c$$ ("speed of light"): no change
$$\alpha$$ (fine structure constant): big change
$$e$$ (elementary charge): change (big?)

What about other 'fundamental physical constants'? As you find on the NIST site:
http://physics.nist.gov/cuu/Constants/index.html
particularly hbar, G, k

Staff Emeritus
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Dearly Missed

Originally posted by Nereid
So that's:
$$c$$ ("speed of light"): no change
$$\alpha$$ (fine structure constant): big change
$$e$$ (elementary charge): change (big?)

But since the fine structure constant is a function of c and e:

$$\alpha = \frac{e^2}{hc}$$

any change in either would mean a change in $$\alpha$$

Nereid
Staff Emeritus
Gold Member

Originally posted by selfAdjoint
But since the fine structure constant is a function of c and e:

$$\alpha = \frac{e^2}{hc}$$

any change in either would mean a change in $$\alpha$$
Thanks.

I'm interested to know which set of independent physical constants would make a big change in the universe - if different from what they are here and now.

lethe said "this is why changing a dimensionful number has no meaning. our units are defined in terms of the fundamental constants. change the fundamental constants, and you change the units. but you don t change the size and shape of the universe, you have only changed the name of your meter stick"

What are the constants that really matter?

Originally posted by Nereid
What are the constants that really matter?
the dimensionless constants. this website has a list of the dimensionless constants. i think there are 26 of them, in the standard model.

Nereid
Staff Emeritus
Gold Member
A request of lethe (and others)

Please take us through this, slow step by slow step ...

c is in units of distance and time. The distance scale was based on a couple of scratches on some bar of waste platinum that someone had lying around. The time scale was based on the length of the day (or was it the year?).

To make distance a little less arbitrary, re-write it in terms of atomic spacing in a nice stable crystal, at a temperature tied to the triple point of water (which is independent of the Earth). For time, look up a friendly pulsar (make corrections for the Earth's orbit, SR, etc).

If some analogue of TheDonk, on an Earth-like planet orbiting a Sun-like star in an outer galaxy of a cluster whose central member houses a particularly bright quasar (redshift 5, say) gets hold of our diamond-pulsar definitions of distance and time and repeats them (never mind that the pulsar would be too faint to see), would ve (TheDonk' - sex indeterminate, you see) conclude that c is 300,000 km/sec? If so, why? If not, why not?

What would the universe be like if you changed the speed of light?

This is an essay I recently completed on the topic:

If light traveled at a difference speed than 299,792.458 km/s, the universe would be a very different place. Physicists can only hypothesize at what changes would occur in the universe (some feel, for example, that there would be many more black holes) – but it is nearly certain that there would be a much lesser potential of having life on Earth. If the light could only travel half as fast, energy received from the sun would decrease fourfold.

This, however, would only be the case if you changed the speed at which light travels. In actuality, however, the “speed of light” c is not really a measurement; it is a value that describes the relationship between the universe’s change in time and the change in space. More importantly than stating how fast light travels, the speed of light c means that the change in time is exactly equal to the change in space.

If you were to somehow change the speed of light, its role as the absolute measure of speed would remain unaffected. If you were a “God-like observer outside of the universe” (as described by Robert Bristow-Johnson at www.lns.cornell.edu/spr/2003-11/msg0056602.html), then you would notice the change. If you were to cut the speed of light in half, everything in the universe would move half as fast (and, I think, be only half as big?). From your point of view, a car traveling at 60 mph would instead be moving at 30 mph, and a man of 6 feet and 200 pounds would be 3 feet and 100 pounds. This is assuming (I think?) that the change in the speed of light would not alter any of the other fundamental, dimensionless constants in physics.

To anyone living inside the universe, however, the speed of light would still be c; not c/2. Everything measurable in that perception of reality would have been changed proportionally with the change in c, so nobody would notice any difference. Atoms would be half their “actual” (as far as we know) size. Information would travel half as fast, but across distances that were half as long. The car traveling at 30 mph would not just experience a change in speed, but it would also be only half as big (and be traveling only half as far.) The universe would seem exactly the same to those living within it. To quote an anonymous post from www.physicsforums.com/archive/topic/11694-1.html,[/URL] “Changing a dimensionful number has no meaning [because] our units are defined in terms of the fundamental constants. Change the fundamental constants, and you change the units. But you don’t change the size and shape of the universe; you have only changed the name of your meter stick.”

Meanwhile, the God-like observer looking in on the universe – himself living in an environment with a more absolute reference for length, time, speed, etc. – would see everything in our universe slow down. In debating whether or not such a secondary reality exists, this question ends up being more a discussion of philosophy than physics. In conclusion, however, a change to the speed of light c would not cause any change to the universe in our perception of reality.

Keith Petrower

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