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ask scienceline | physics
How could the universe expand faster than the speed of light? That seems impossible!
- asks Paul
In science fiction universes, traveling the galaxy is a snap – just engage the “warp” or “hyperspeed” drive, and off you go, cruising the cosmos at several times the speed of light. But back in reality, we’ve all been taught that the speed of light is a strict traffic law that can’t be broken. This is true, but slightly misleading.
Einstein’s theory of special relativity, first published in 1905, asserted that the speed of light is a constant (300 million meters per second), no matter who measures it. It’s always the same whether you are in motion or at rest. This line of thinking is a lot different than we’re used to experiencing. For example, if you try to measure the speed of an oncoming car from a moving vehicle, you end up getting the combined speed of both cars. This is why cops have to stay parked. Light is different, because no matter what you’re doing it always goes the same speed.
The speed of light affects us more than we realize – it helps us understand the difference between cause and effect. If things could move faster than the light we see them by, we’d be in for weird experiences. If you were a catcher trying to catch a superluminal fastball, you might feel the ball hit your glove even before the pitcher starts his wind-up: The effect before the cause. That’s because the image of object would be traveling at the speed of light, trailing the faster baseball like the slower sound of thunder trails after the image of lightning.
Now that we have a taste for Einstein’s theory, we know that baseballs don’t go faster than the speed of light. But is there anything that can? It turns out that the speed of light is only a limit on objects – like baseballs – as they move through space. The movement of space itself, however, can make the speed of light seem slow.
Right after the Big Bang, the universe had a monstrous growth-spurt called inflation. The whole thing was over in less than a trillionth of a trillionth of a second, but the universe grew exponentially in that brief blip, repeatedly doubling in size. At the end of inflation, although the universe was still smaller than a car, the outer edge had traveled many times faster than the speed of light. Since then, the universe has continued its expansion, but at a more reasonable, steady pace.
This ultra-fast growth seems to contradict what we’ve just discussed, but it makes sense if you understand the distinction between expansion and motion. When astronomers say that the universe is expanding, they’re talking about the rather abstract concept of space-time. Basically, space-time is the three physical dimensions of our existence-length, breadth and depth-combined with the additional dimension of time; think of it as a wire grid that connects every part of the universe to every other part. When we say an object has motion, we’re referring to its change in position relative to the space-time grid. The speed of light is only a constraint for objects that exist within space-time, not for space-time itself.
To better visualize the theory, astronomers often illustrate the expanding universe as a loaf of raisin bread rising in the oven. The raisins are galaxies and the rising dough represents space-time. As the dough expands, the raisin galaxies find themselves farther apart from each other, even though they are not moving relative to the dough between them.
Now let’s imagine that there’s a beetle in the loaf and it starts crawling toward a faraway raisin (don’t worry- we’re not going to eat it anyway). The beetle represents anything within space, such as baseballs, spaceships or photons. When the beetle burrows through the bread, he is moving relative to the dough, and all the other raisins. The speed of light limits how fast the beetle can travel, but not how quickly the bread can rise. Just because the expansion of space can break the speed limit, it doesn’t mean that we can go faster than Einstein said we could.
So, while the speed of light remains an unbreakable barrier for those of us within the universe, it can’t limit the expansion of space-time itself. The universe keeps right on expanding, but the speed of light limits how much of it we can see, and how fast we can move. It may not be fair, but that’s physics.
July 9th, 2007 at 11:55 pm
Einstein, although widely misquoted as doing so, did not assert or hypothesize that the speed of light is a constant. He asserted that it is everywhere the same without respect to the field of reference. This allows for a speed of light that remains uniform throughout the entire universe but not necessarily a constant over time.
There are those of us who are aware that at the beginning the speed of light and the speed of gravity were once equal. Since then the speed of light has decayed greatly and the speed of gravity has not. I refer you to the work of Barry Setterfield; he has a website.
July 10th, 2007 at 3:07 pm
Thomas, great point to bring up- indeed, when I referred to the “constant speed of light,” I meant as measured anywhere in our current universe. Thanks for helping to make the distinction.
August 30th, 2007 at 11:21 am
Hello,
I have a couple of questions on this topic. Here are the facts:
2 flashlights are moving away from each other starting from a fixed point along a straight line.
The flashlights are pointed at each other.
Each flashlight is traveling at 1/2 the speed of light as measured from the fixed point.
The flashlights are traveling away from each other at the speed of light.
The question: If you turn the flashlights on after one minute, will the light ever reach the other flashlight?
If the light will never reach the other flashlight, and the universe is expanding at an ever increasing rate, what happens when the speed of the expanding universe reaches 1/2 the speed of light?
September 5th, 2007 at 8:37 am
That is the point in the first comment. The speed of light is not relative to the motion of the flashlight. The light emmited when you turn on the flashlight will travel at the speed of light towards the other flashlight and reach it at the third minute.
September 25th, 2007 at 6:42 am
Since both flashlights are travelling at 1/2 light speed, after 1 minute both flashlights will be exactly one light minute apart. now the flashlights are turned on. the light from flashlight A will take 1 minute to reach the point when flashlight B was first turned on. Let’s call this point C. Meanwhile, flashlight B was still moving for 1 minute and will be 1/2 light minute further away from that point C. the light from flashlight A will take one more light minute to connect with flashlight B who after one minute will be exactly one light minute away from the point C.
This is only two minutes from the time that the flashlights were turned on. But it is 3 minutes from the start of the experiment.
Please correct the math if I am wrong, but I think it’s right.
October 15th, 2007 at 10:29 pm
so is there something like a photon or anything that can beat the speed of light because if there is i would be delighted to know!
November 28th, 2007 at 3:34 pm
The fact that we will always measure the speed of light as being the same, irrespective of our own speed, seems to have been demonstrated experimenatlly. Also, apparently proven is that clocks keep different time according to how they are moving. However - does anyone actually understand how these apparently logical impossibilties can be? Or why particles should change their behaviour if we look at them or that space is curved or how the whole universe can ever have been concentrated at a point? I once asked a physicist and he answered that no-one understands how these things can be - they just are so. It seems that we must ascribe them to magic?
January 31st, 2008 at 12:56 am
Robert, it is a logical impossibility only because our conception of logic is flawed.
February 4th, 2008 at 2:20 pm
Greg:
If our concept of logic is flawed, then “someone” needs to show where the flaw is. If I´m going at 30mph and another car overtakes at 60, I´ll measure its relative speed at 30mph.. The same applies to a washing machine overtaking me in outerspace or a sound wave on the earth. But not to a photon - I will always measure its relative speed the same, no matter what my speed is. Why???
The Universe is thought by many physicists to have come from nothing - no space, no matter, no energy and no time - you can´t get any more nothigful than that. A quantum fluctuation in the nothing, they say. And what caused this quantum fluctuation? Nothing did - it was random. It´s all pretty unbelievable stuff, which scientists do not understand either. A physicist whom I exchanged emails with said there is a saying in his profession in the context of these impossible questions - keep your head down and calculate.
February 26th, 2008 at 9:03 pm
I don’t think that it is logic that is flawed (here)…
Let’s say two identical spaceships take off from an asteroid at the same time in opposite directions. Both have identical clocks. After accelerating to .9c, Ship A sends a signal. Everyone agrees that this travels the speed of light, and it takes some amount of time. Ship A waits an hour, and sends another signal, but now the two ships are further apart, so it takes longer for this signal to get there. Ship B therefore measures rather more than an hour between the signals and has no choice but to conclude that time is passing slower for ship A.
We know that isn’t true, it just took longer for the signal to get there. As it happens, however, if Ship B sends a signal of its own, it knows that it will travel towards Ship B at c relative to Ship B, which means that it is going at some lesser speed relative to Ship A according to Ship B’s reckoning of time. But Ship A’s time appears to be slower than Ship B’s (according to Ship B), so you have to take that into account, at which point you will realize that the light is still moving at c.
Both ships will be saying that time is passing more slowly on the other, so which is right? Well both… to be able to tell for sure, they’d have to “meet up” again, and to do that, one or both of them would have to accelerate, which changes their frame of reference.
I’m no physicist, but that is my understanding of it.
March 3rd, 2008 at 2:36 pm
I don´t think your example is the same as the peculiarity I´m referring to. If I am travelling at 0.25%c and I´m overtaken by someone travelling at 0.50%c and simultaneously a photon overtakes both of us and we each measure its relative speed, we will both measure its relative speed at c - not 0.75c and 0.50c respectively. (Phew!!) I asked a physicist whether, even though we know it is so, he understood how it possibly could be. (Together with moving clocks keeping different times and a whole lot of other “counterintuitive” things). He replied that the various things I cited are so and we have to accept them - but no-one understands them.
March 12th, 2008 at 11:55 am
Whilst the evidence is convincing that the universe is expanding at an accelerating rate (Supernova Type 1a vs redshift plot), I have seen no evidence that the space-time grid is expanding faster than the speed of light, whether in the past nor in the present. Can anyone help here?
August 26th, 2008 at 12:48 pm
Does anybody really know what time it is? -Chicago